Stenter exhaust heat recovery for combustion air preheating

In this project, the option of preheating the air using the
exhaust heat has been studied. An Air to Air heat exchanger
has been designed and fabricated for experimental
purposes. The experimental results have resulted in the
design for a techno economically viable heat recovery unit.

Project Report

Indian Institute of Technology

Bombay

Heat Transfer Laboratory

6/6/2005

1

Chapter 1

Introduction

The term “stenter” can be said to have achieved the importance of magic word in the world of
textiles. The presence of one or more stentering machines, or simply stenters, in a textile dye
house indicates the “status” of the outfit. Although in good old days, this word was not known to
the cloth bleacher and the dyer, yet, he was fully aware that unless the cloth is stretched to its full
width, it would be very difficult for him to have a satisfied client. In due course, the significance
of stenters in a textile machine has been recognized to such an extent that currently it has been
termed as the “heart” of the textile processing house. If the stenter fails, the entire process comes
to a standstill. The versatility of this machine certainly justifies the importance attached to it by
the textile processor. In recent years, several economic forces have been at work that has
changed the structure of the economy. Factors such as globalization and trade liberalization,
among others, have intensified competition resulting in the reallocation of resources among
sectors in all over the world. To be internationally competitive in this rapidly evolving and
dynamic trading environment, textile producers are striving for improvements in productivity and
efficiency. So the technicians have applied their minds to various improvements in stenters and
have now elevated the status of the stenter to become the most important machine, having an
unquestionable position in the textile processing department. Several techniques are being
devised aiming at improvements in stenters. This project deals with such a technique in detail.

2

1.1 Opportunity for Energy Saving

Textile stenters have two main purposes – convection drying so as to remove the
moisture in the fabric and secondly to provide for fabric width control. Drying is
achieved by impinging high velocity air jets uniformly across the full width of the fabric
on both sides. The air being used is heated to a temperature of about 90 to 160oC. The
hot air is recirculated and a certain amount of air is continuously removed from the
system through exhaust fans so as to avoid buildup of excessive humidity. To that extent,
the system is supplemented by fresh air. So the major source of waste heat in the stenter
is the hot humid air from the exhaust. In the case of stenters with thermic fluid heaters,
the exhaust temperature can be as high as 160oC. High exhaust temperatures provide the
opportunity to recover this heat in order to reduce the operating cost. Heat in stenter
exhaust being a major loss of textile industry, installation of heat recovery systems will
result in substantial energy savings.

1.2 Objective

The aim of this project is to design a heat recovery and exhaust air cleaning system that
will recover the stenter exhaust heat and utilize it as per the requirements in the industry.
This heat shall be used to preheat the combustion air which is to be supplied to stenter
burner. In both the cases, heat recovery system condenses pollutants out of the exhaust
air. The various objectives of this project are as follows:
(a) Studying the conventional systems for stenter exhaust heat recovery, their
advantages and limitations
(b) Designing a system for recovery of exhaust heat for preheating of combustion air
(c) Simulation of the performance of this system and analyze its economic viability
(d) Fabrication of a prototype of this system and carry out the experiments to validate
the design

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1.3 Overview of the Project

The project has been planned to accomplish in three stages. These stages are:
(a) Literature survey
(b) Design of the proposed system and calculation for economic viability
(c) Experiment on new system, error analysis and detailed economic study

This report opens with an introduction. This chapter starts with a brief note stating the
importance of stenters in textile industries, then it deals with the objective of project and
further discusses the scope of energy conservation in these stenters. Finally a layout of
the report is presented.

The second chapter presents an evolution, construction, basic parts, classification,
working principle and the modernization of the stenters. This chapter emphasizes the
various functions that stenters are performing these days in textile processing houses and
also the changes that have been brought in the stenters for higher production and
efficiency.

The third chapter starts with a brief introduction of heat exchangers and then discusses
three important heat exchangers which are being used for stenter exhaust heat recovery in
various industries at present. This chapter emphasizes the various advantages and
limitations of the existing systems and also suggests a new design with the methods to
overcome those earlier limitations.

The fourth chapter presents the detailed design and fabrication of a prototype of the air to
air heat recovery unit. The experimental results have been used for final designing of the
heat exchanger. Further, economic viability of such a system is looked into.

The fifth chapter concludes the report. It also describes the work that need to be done in
future.

4

Chapter 2

Stenters: Basic Functions and Modernization

In good old days, in the wool industry, the metallic frames on which the woolen fabrics were
stretched, used to be called “wool-tenters”. When the stretching operation was applied to the
cotton industry, this term was transformed into “Stenter”. The early stentering machines were
used as width equalizing stenters where a small amount of heat was given at the selvedges by
arranging finned steam pipes below the stenter chains. These machines are now retained as “the
batching stenter” for the purpose of cropping and shearing. At the intermediate stages of
development the machine was used as an equalizing and drying stenter where blowers and fans
were used to dry the fabric in addition to equalizing of fabric width. The modern stentering
machines known as the finishing, equalizing and drying stentering ranges are required to do a
wide range of finishing operations besides the conventional ones like drying and equalization of
the fabric width.

2.1 Utility of a Stenter

After preparation, washing or coloration, fabrics need to be dried. This is usually carried out in
two stages. The first stage is mechanical dewatering using centrifuges, mangles and vacuum
slots. Mangling is the most cost effective way of removing water mechanically but water

5

retention levels are still quite high. Centrifuging can only be used for relatively small batches of
fabric. It is more effective than mangling but costs almost twice as much in terms of the energy
required per kg of water removed. It is a method more commonly associated with the wool
industry. Suction or vacuum slots are the most effective way of mechanical dewatering (except
for woolen fabrics where water removal under suction is poor) but they are the most expensive.
Improved drying rates alone may not be sufficient to justify the expense. An alternative use of
vacuum slots is the recovery of chemicals from pad finishing operations. It is sometimes the case
that they are bought for the chemical savings alone.
The second stage involves heating the fabric and removing the remaining water by evaporation.
This is done using either drying cylinders (intermediate drying) or stenters (final drying). Drying
cylinders are basically a series of steam-heated drums over which the fabric passes. It has the
drawback of pulling the fabric and effectively reducing its width. For this reason it tends to be
used for intermediate drying. On the other hand, the stenter is a gas-fired oven, with the fabric
passing through on a chain drive, held in place by either clips or pins. Air is circulated above and
below the fabric, before being exhausted to atmosphere. So drying with stenters avoids any
drawback of reduction in fabric width. As well as for drying processes, the stenter is used for
pulling fabric to width, chemical finishing and heat setting and curing. It is a very versatile piece
of equipment.

2.2 The Sections of the Stentering Range

The main sections of the stentering range can be classified as follows:
a The entrance zone of the stenter in which the width equalizing and adjusting take place.
It is narrower at the actual entrance and tapers into a wider width till the end of the section
i.e. up to the heating chambers. The fabric is pulled during the passage of the fabric in
this section
b The intermediate zone, consisting of the fabric compensating device, weft correcting
system, crease removing and selvedge uncurling devices, fabric selvedge guiding and
introduction zone.
c The finishing zone, consisting of the finishing padder and its component parts. The fabric
is introduced into the finishing padder, squeezed to a planned retention percentage.

6

d The drying zone, consisting of the number of drying chambers according to the drying
process planned. Each of the drying chambers consists of suitable positioned blower fans
with fan motors, nozzle sets for blowing the hot air on the fabric and heating registers
which heat the air coming from blower fans and achieve the desired effect.
e The delivery zone, consisting of fabric releasing system, cooling device, plaiting device
and the type of batching system which may have been selected. This zone permits the
cooling of the fabric before it is stripped off the pins or clips at the take off point. In
some stenters air blower is arranged after the drying section for adequate cooling of the
fabric. Fresh air to bring down the temperature is blown onto the fabric with the help of
suitable ducting and nozzles.

2.3 Performance

The stenter has a dual function to perform. It is required, as mentioned earlier, not only to
equalize the fabric width, but also to dry the materials at ideal drying conditions, at a pre-
determined production speed and to impart to the fabric under process, the desired and well
expected final finish. Special attention is being paid to the temperature during drying, the
composition of the drying medium, the resultant moisture content, and ultimate dimension and
the finish of the fabric.

Depending on the types of fabric being processed on the machine, the processor has to decide as
to whether the material should be processed through the gripping system, i.e., the clip type stenter
or the non-gripping pin type stenter. Generally, woven goods are run through the clip links, with
the exception of the very delicate materials. Superfine fabrics and knit goods as well a crepe cloth
are treated on the pin stenters. For the sake of convenience it is generally preferred to install a
stenter having a pin and clip type execution.

The fabric is introduced to the finishing padder, squeezed to a planned retention percentage and
then is passed on to the stenter through a systematic crease removing and guiding system to the
chain links pair. The fabric is held at the selvedges by two endless chains, which convey the
same through the body of the stenter, in which hot air is blown over the cloth and also from

7

below. The cloth is released from the grip at the end of the machine. An attempt is made to
retain in the cloth an optimum moisture content equivalent to the regain of the fiber concerned.
After emerging from the outlet the cloth is rolled in a batch or plaited down as per convenience.
The endless chains, because of their endless character, turn round at the exit end, after releasing
the fabric selvedges, and run back through the drying zones, back to the starting point. At this
starting point, the chains again take up the selvedges of the newly entering fabric, which is
running continuously, and repeats the process through the drying zones.

2.4 Type of Heat Sources

A variety of heat energy sources are used to heat the drying medium from which the heat is
distributed through heating registers or heat exchangers.

a Steam: High pressure steam from the boiler mains is fed to the heat exchangers located in
the drying chambers. The steam pressure is regulated at a point, which is in the vicinity
of the stenter in order to maintain a continuous and consistent supply of steam at the
desired pressure and temperature to the heating registers. The temperature reached by
steam at reasonably high pressures is around 165°C, which is adequate for drying, curing
and cross-linking the chemical reactants. Therefore the use of steam is limited to drying
purposes only. Circulation of hot air, which follows the exchange of heat, is effected by
means of suitably designed, motor driven blower fans. Highly super heated steam is
usually not used due to operational difficulties at higher pressures.
b Steam and electrical heating: Electrical heating systems for attaining higher temperatures
required for heat setting of fabrics are also used. These systems are used along with
pressurized steam for normal drying purposed. The main problem with this system is
cumbersome design features and maintenance of both type of heating arrangements.
However, exclusive electrical heating arrangements are made in small production houses
where boilers could not be installed. This process is slightly costlier because of high cost
of the electrical energy compared to gas and oil.

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c Thermic fluid or hot oil: Hot oils or thermic fluids are also circulated through heating
registers when higher temperatures are required. One of the most redeeming features of
this system is that the same thermic fluid can be heated and recirculated through the
stenter or any other drying machines. The whole circulation is effected under normal
pressure, with suitable control valves for each of the drying chambers. After flowing
through the hot oil registers throughout the dry and heating zones, the oil is pumped back
to the storage cum expansion tank. From the expansion tank, it flows to the oil-heating
tank, is brought to the required temperature and is then again circulated through the
drying machine.
d Direct heating by oil burning: In this system, each stenter chamber has it’s own oil
burner arrangement. The flames are discharged on one side of the heat exchanger. The
oil burner design and position is arranged in such a manner that the system can impart
sufficient heat recovery for one standard drying chamber, along with necessary oil spray,
air mixing device, and igniting device. Sometimes, oil combustion products are directly
fed into the chamber but are not preferred because of oil spots and soot formation owing
to incomplete combustion.
e Direct gas firing: Some stenter chambers are designed to be heated by direct gas firing
with natural gas being used as the fuel. The heat in the gas flame is taken up by the air
from the blower fans and passed on to the fabric surfaces. It is possible to control the gas
flame in such a manner that the hot medium temperature is adequate for drying, and when
needed, for the thermo-setting process, as the case may be. Indirect firing though a heat
exchanger can also be used for gas-produced heat but they are usually avoided because
the arrangement is very costly and the drying efficiency goes down by 20 to 30 %.

2.5 Modern Stenters

Towards the latter half of the seventies, the mounting cost of energy and other utilities had a very
adverse effect on the textile industry, because of its highly vulnerable nature in respect of the
costs of power, water, labour and raw materials. Attempts were therefore made by textile

9

machinery manufacturers all over the world to develop new models of stenters which would
enable savings in costs, with special reference to energy.

2.5.1 Features of Modern Stenters

Advanced stenters with special features are now available. Some of the features are highlighted
below.
a Padding Mangle: Before entering the stenter, mechanical squeezing of fabric is done to
remove moisture. This decreases the thermal load on the stenter. A device known as
padding mangle with large diameter rollers provides even squeeze across the width of the
fabric. The speed of the padding mangle is synchronized with the speed of the stenter.
b Weft Straightener: Fabric straightening equipment is integrated in the stenter entry for
reliable correction of skew or bow distortions before gripping or pinning-on of the fabric
a great advantage with tension-sensitive knit fabrics. The combined bow/skew
straightening unit comprises two bow rollers with two downstream skew straightening
rollers. Separately driven rollers permit adaptation to the requisite fabric tension.
c Overfeeding: The overfeed assemblies permit reliable pinning-on of even the most
delicate fabrics. The selvedge uncurlers or edge spreaders feed the fabric to the overfeed
assemblies directly from behind, for uniform pinning on. Adjustment of the mechanical
overfeed input is possible from -15% to +40%. In special cases, this can be increased to
give a higher rate of shrinkage. The edge tension can be adjusted for each side separately.
d Edge Gumming and Trimming: The selvedge trimmers cut precisely and reliably with a
minimum loss of fabric, even at high speeds. Unpinning of tension-sensitive fabrics takes
place without stretching the edges. Powerful injection blowers extract the trimmed
selvedges from the machine.
e Fabric Edge Heating: Infrared heated hot air dryers are provided directly in front of the
first drying chamber to dry the gummed fabric edges. This ensures that the fabric edges
are not damper than the rest of the fabric, permitting an increase in the production speed.
f Moisture Control: Control systems are provided to measure and control the moisture on
the fabric leaving the stenter. Moisture level is continuously compared with a pre-set
value and accordingly the stenter speed is regulated, automatically.

10

2.5.2 Energy Conservation Aspects in Stenter

Several aspects need to be incorporated in the new stenters for achieving energy efficiency.
Utilization of full width of the stenter should be ensured to avoid wastage of energy; otherwise
the specific heat consumption in the case of narrow width will be higher. It is recommended to
utilize at least 75 to 80% of the width of the stenter, to optimize on energy. The stenters should
be equipped with specially designed heat transfer system and nozzles. This should ensure that
the hot air is circulated more number of times than the conventional stenters and maximum
moisture is removed before exhaust. Heat recovery systems should be installed to extract heat
from outgoing vapours and to pre-heat the input air. Thermic fluid heating/steam heating can be
used in stenters depending on temperature requirements. If steam is used 100% condensate
recovery should be ensured. Flash steam recovery system may also be considered. The flash
steam may be used in any low-pressure machines viz., jiggers, recuperators of mercerizing
machines, soapers, washing machines, etc. Periodic cleaning of filters is necessary since clogged
filters will impair drying efficiency, resulting in energy loss. The chamber doors should be leak
proof so that hot air is not lost to the atmosphere. Adequate insulation of top, bottom, sides, etc.
should be ensured. The blower motor should be interlocked with main drive such that when the
machine stops, fan blower motors are also stopped. Automatic moisture measurement and
control system should be provided to avoid over-drying of the fabric.

2.6 Heating and Circulation of Drying Medium

The drying medium utilized in the stentering machine is moist air. For the purpose of being used
as drying medium, the air heated by some convenient heat transfer system in which it is
necessary to utilize heat energy in some easily procurable form. The device should be able to
heat the air up to 160° C in case of normal drying and 210° C in case of thermosetting stenters.
As shown in figure 2.1, the hot gases are forced into pairs of tapered ducts, extending across the
width of the fabric and then discharged onto the fabrics through specially designed nozzle sets.
In some stenters, the ducts have nozzles, running full width to provide uniform distribution of air
medium i.e. hot air. The drying medium being blown over the fabric from top and bottom has the
same temperature and in most cases, have the same heating source. Other designs have circular
nozzles arranged to strike the fabric at a particular angle. Usually in fabric drying, slightly more

11

air is forced into the bottom ducts, to support some of the weight of the fabric and because the
bottom of the fabric is wetter than the top. Supporting the weight of the wet fabric helps to
prevent excessive sagging, particularly in knitted fabrics. The hot drying medium, after being
forced against the fabric surface makes its way towards the side of the stenter where it is
collected by one fan, and then gets re-circulated.

Exhaust

Heater Fan Damper

Fabric

Fresh Air

Figure 2.1: Airflow Pattern (www.greenbusinesscentre.com)

At the top of the compartment, a damper is placed to regulate the rate at which, exhaust gases,
including steam, are removed into an exhaust duct. The damper has to be appropriately regulated
and opened to an optimum level so that the composition of the medium is maintained. This will
enable to adjust an economic drying rate and also keep a control over the fuel cost. Generally,
10% steam is allowed to be exhausted from the damper.

12

There are two kinds of drying chambers based on the airflow patterns and the construction of fans
and blowers.
a The ‘one-and-one’ countertype drying chamber: Each drying chamber compartment
comprises of two identical sections, one being the mirror image of the other. Section wise
construction of the one-and-one counter type chamber also enables transport of the
machine in pre-assembled condition.
b The ‘two-and-two’ countertype drying chamber: Each compartment has two fans, located
on one side of the compartment. The compartments, identical in design, are lined up,
facing each other in pairs so that one compartment is positioned with its two fans on the
right and next one with its two fans on the left.

2.7 Modification in Design

The design aspects of the stentering machine and the drying chambers have now been modified
in the following manner to improve production and efficiency.
a The design of the drying chamber has now been modified in such a manner that the total
volume of the hot air medium handled at one time is about 15 % less than the erstwhile
models.
b The moisture content of the drying chamber is constantly measured and setting is done for
optimum moist content. Substantial saving of heat energy can be achieved by following
this technique.
c Spring-loaded telescopic curtains are arranged at the entrance and delivery end of the
drying zones. This also helps saving the heat energy.
d The angle of the first set and the last setoff nozzles is rearranged in such a manner that the
airflow is directed inside the drying zone and no portion of the blown medium escapes
outside the drying zone. This arrangement helps in energy saving to some extent.
e The packings arranged at the sliding doors and panels are thoroughly rechecked for
thickness, dimensions and resilience, ensuring that there is no leakage from the drying
zones from these parts.
f The inlet of fresh air inside the drying zone in such a manner that the proportion of stem:
hot air is not disturbed at all.

13

g Enclosure of the stenter chain positions along the rails and the moving clip links, in the
entrance zone and the delivery zones in order to minimize the heat losses due to
dissipation.
h Construction of partitions in between the standard drying chambers and the thermosetting
chambers to retain the maximum heat energy within the drying chambers owing to
differences in the temperatures of the drying medium between the drying chambers and
heat setting chambers.

14

Chapter 3

Heat Exchanger Technology

Heat Exchanger is a device which is used to transfer heat from one fluid to another through a
separating wall. Although there are many different sizes, levels of sophistication, and types of
heat exchangers, they all use a thermally conducting element; usually in the form of a tube or
a plate; to separate two fluids, such that one can transfer the thermal energy to the other.

3.1 Energy Saving in Stenters

As discussed earlier, one of the means of improving the thermal efficiency of the stenter is by
recovering some of the heat, which will, otherwise be wasted. Waste heat recovery from the
stenter is possible in two ways:
(a) Waste heat recovery from the hot condensate and flash steam
(b) Waste heat recovery from the exhaust gasses

Waste heat recovery from the condensate is easier. As the condensate comes from steam
traps, flash steam is produced. The flash steam may be condensed in a direct contact
condenser where cold water will extract its heat. Whereas the hot condensate may be returned
to the boiler feed water tank, by using a centrifugal pump. On the other hand, recovering heat
from the exhaust air probably offers the greatest potential reducing fuel consumption.

15
Air to Air Heat Exchanger

The hot stenter exhaust gas at up to 160°C is transported by the existing waste air fans
through the bare tube heat exchanger into the atmosphere. The cold, clean fresh air
(approximately 50% of the waste air volume) is drawn in by the fresh air fan in counter-flow
to the waste air, through the heat exchanger and into the stenter. As it passes through the heat
exchanger, this air is preheated to approximately 100°C. This hot air can be used in the
burner of the stenter itself so as to lessen the requirement of fuel in the burner. The only
problem is that the burners will have to be modified in order that they are capable of accepting
the air at high temperature. This process is illustrated with colour coding in the following
figure.

Figure 3.1 Schematic of a stenter with and without Air to Air Heat Recovery Unit
(www.monforts.com)

16

3.2 Prior Art
Presently, there are three kinds of heat recovery systems which are using waste heat from
stenter exhaust gases to preheat combustion air for the stenter or to heat water for other
processes within the factory, such as fabric dyeing. The existing systems are as follows

3.2.1 Shell and Tube Heat Exchanger: It consists of a bundle of parallel tubes that
provide the heat-transfer surface separating the two fluid streams. The fresh air or water
passes axially through the inside of the one inch tubes; and the exhaust gas passes over the
outside of the tubes. Baffles external and perpendicular to the tubes direct the flow across the
tubes and provide tube support. Tubesheets seal the ends of the tubes, ensuring separation of
the two streams. The thermal performance of such a heat exchanger is high but it can be
further increased by using some efficient fins.

The use of large diameter tubes increases the size of this heat exchanger. Also it usually has
to be located at a place where there is room to open it up for cleaning purposes. It can be
made more compact by using small diameter tubes. This will also increase the heat transfer
coefficient slightly. A better arrangement of tubes will result in high heat transfer coefficient,
low pressure drops and reduction in volume of heat exchanger. The separation and discharge
of condensate of exhaust gas over the tubes also make this a complicated system.

3.2.2 Z-Duct Plate Heat Exchanger: Developed and marketed by Des Champs
Technologies, it is a unique system designed specifically to permit the recovery of large
amounts of energy from stenters that would otherwise be wasted. Counter flow of fresh air
and exhaust gas streams are brought into close proximity, separated by one continuous,
dimpled and folded sheet of aluminum. The heat transfer surface is formed into a matrix with
two completely separate and distinct air passages. The ends of the matrix are sealed for
minimum leakage, virtually eliminating cross-contamination. The spacing between plates can
be varied to allow for optimum energy saving effectiveness and may be different for each air
stream in the event of unbalanced air flow through the heat exchanger. The Z-Ducts are
highly efficient, low cost heat exchangers with no moving parts to break down or replace. In
addition to being very effective at energy-transfer, the Z-Duct is able to operate at a relatively

17
low internal pressure drop. This is a result of the Z-Duct being constructed entirely of
primary heat-transfer surface.

The heat-transfer matrix consists of formed and folded 8 mm thick aluminum with truncated
dimples formed into the plates to effect plate-spacing that can be varied separately for each
flow stream. Variable plate-spacing allows maximum performance for each application. The
ends are sealed with refractory cement leading to a heat exchanger with virtually no air
leakage between air streams. Series 85 heat-exchangers have high sensible efficiency, low
maintenance, and good serviceability.

Figure 3.2 Z-Duct Heat recovery unit (www.deschamps.com)

The disadvantage of this type of heat exchanger is that it does not work efficiently if the
temperatures are very high so it can not be used if the stenter is being used for heat setting
process. Apart from that, it is suitable only for small flow rates so it can not be scaled up
efficiently to make it suitable for big industries.

3.2.3 Bry-Air Heat pipes: Developed and marketed by Bry-Air Inc. U.S.A., Bry-Air heat
pipe is a self-contained, passive energy recovery device. One of the good features of the heat
pipes is that they have no moving parts and hence require minimum maintenance. They are
completely silent and reversible in operation and require no external energy other than the
thermal energy they transfer. Heat pipes are robust and can withstand a lot of rugged

18
handling.

From construction point of view, the Bry-Air heat pipe comprises of three elements, a sealed
container, a capillary wick structure and a working fluid. The capillary wick structure is
integrally fabricated into the interior surface of the container tube which minimizes heat loss
across fin tube bond.

Figure 3.3 Integral finned construction (www.bryair.com)

The Bry-Air heat pipe transmits thermal energy by evaporation and condensation of the
working fluid. The working fluid inside the heat pipe is in equilibrium with its own vapour as
the container tube is sealed under vacuum. Thermal energy applied to the external surface of
the heat pipe causes the working fluid near the surface to evaporate instantaneously. Vapour
thus formed absorbs the latent heat of vaporization and this part of the heat pipe becomes an
evaporator region. Due to the pressure gradients thus created within the heat pipe by the rapid
generation of vapour near the surface, the excess vapour is forced to a remote area within the
heat pipe having low temperature and pressure. The vapour then travels to the other end of the
pipe where the thermal energy is removed causing the vapour to condense into liquid again,
thereby giving up the latent heat of the condensation. This part of the heat pipe works as the
condenser region. The condensed liquid then flows back to the evaporator region to be reused,
thus completing a cycle.

Heat is removed from the external surface of the condenser region by conduction, convection
or radiation. The heat pipe works continuously in a close-loop condensation/evaporation cycle
and thus, the capillary pumping force is established within the wick structure that returns the

19
working fluid from the condenser region to the evaporator region. The transfer efficiency
level of each heat pipe is 99%.

Figure 3.4 Functioning of a Heat Pipe (www.bryair.com)

Bry-Air industrial and commercial heat pipe heat exchanger contains a number of heat pipes.
These heat pipes are placed horizontally across the width of the exchanger and pass through a
center seal partition to avoid cross contamination. The exchanger is installed across two side-
by-side air ducts. The exhaust air and the supply air are discharged in counter flow direction
across the exchanger through the ducts to facilitate the maximum energy transfer. The heat
pipes pick up the thermal energy from the exhaust (evaporator region) and transfer it to supply
air (condenser region).

Figure 3.5 Heat Pipe heat exchanger (www.bryair.com)

3.3 The Proposed Design Methodology
Keeping in view the existing designs, a heat exchanger is being aimed at that is supposed to
overcome the limitations of these designs. Hence, it will prove to be a better alternative for

20
heat recovery from stenter exhaust. This puts certain requirements on the design if it has to
prove better than its present counterparts.

3.3.1 Requirements of the design
(a) Low cost: Should have a payback period of lesser than 6 months.
(b) High thermal effectiveness and low pressure losses: These affect an efficient heat
recovery process.
(c) Minimum weight and volume: The weight is directly proportional to material cost
and a small volume of the unit makes it easier to handle. These requirements may be
fulfilled with small hydraulic dimensions of primary heat transfer surfaces.

3.3.2 Design Parameters
The design parameters for the system are as follows

(a) Feed air inlet temperature, Tfi 30°C
(b) Feed air exit temperature, Tfo 80 - 90°C
(c) Exhaust air inlet temperature, Tei 90 - 160°C
(d) Exhaust air exit temperature, Teo 50 - 80°C
(e) Mass flow rate of feed air 8000 kg/h
(f) Mass flow rate of exhaust air 8000 kg/h
(g) Line Pressure ~ 1 bar
(h) MOC GI (thermally stable up to 420°C, UTS 500 MPa)

Tei = 90 - 160 oC Teo = 80 - 50 oC
me = 8000 kg/h
Tfo = 80 - 90 oC Tfi = 30 oC

Figure 3.6 The Design Conditions

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3.3.3 Heat Transfer Geometries
The next stage in design of the heat exchanger is the selection of the appropriate geometry for
the heat transfer surfaces of the heat exchanger. The various heat transfer geometries that are
available are as follows
(a) Primary Surface Geometry: These surfaces have passive enhancement of heat
transfer process as opposed to active techniques such as surface vibration etc. In the
passive techniques secondary flows structures are created by means of curved and
interrupted duct surfaces which disturb the insulating near wall layers and thus
improve heat transfer process in the duct.
(b) Plate-fin Geometry: Plate fin recuperators consist of flow separating metal sheets
with supportive offset strip fin secondary surfaces between them. The efficiency of
heat exchange to the separating metal sheets from the secondary surfaces depends on
the fin height, material conductivity, etc. An advantage is the capability for operation
with a high internal pressure. This configuration has higher mass of the recuperator
unit than the primary surface types of surfaces.
(c) Tubular Geometry: It consists of thin walled small diameter tubes. While thin-
walled small diameter tubes have high cost, tubular geometries have excellent pressure
containing capability. Also they come with the option of enhancement of heat transfer
coefficient with the incorporation of tightly wound spring acting as fin.

By having a look at the design conditions one can easily determine that the most suitable
geometry for the system would be primary surface geometry. This choice arises from the fact
that the line pressure is 1 bar which removes the requirement of the capability for
withstanding high internal pressure. Also primary surface geometry offers improved heat
transfer and low cost and hence it wins over other options.

The next step is to choose from three different primary heat transfer geometries, namely the
Cross Corrugated (CC) surface, the Cross Undulated (CU) surface, and the Cross Wavy (CW)
surface. A comparative study of these surfaces shows that the CC surfaces having the
smallest P/Hi ratios result in the smallest matrix volumes and lowest weights. Also such
surfaces are simple and easy to fabricate. Therefore, Cross Corrugated surface is chosen for

22
the heat exchanger element of the unit to be designed for stenter exhaust heat recovery for
combustion air preheating.

Figure 3.7 Cross Corrugated (CC) surface Figure 3.8 Cross Undulated (CU)
surface

Figure 3.9 Cross Wavy (CW) surface

3.3.4 The Initial Proposed Design

The heat exchanger that is being designed is going to be modular in nature. Such a design
will facilitate easy scaling of the system according to the requirements of different types of
stenters having different capacities. Initially a prototype of about one-hundredth of the
required size will be targeted.

23
The proposed design utilizes corrugated GI sheets. The arrangement shown in Figure 3.10
is just a module of such a heat exchanger having 15 sheets. The sheet thickness of 0.5 mm
would suffice the purpose here and also provide with cost benefits. The width of the sheet
would be 0.76 m and length would be 1 m. The sheets are put on top of one another at an
angle of about 2 degrees so as to form flow passages from within the corrugated surfaces as
shown in figure 3.10. Thus, a stack of sheets with flow passages between them is obtained.
The feed air and exhaust air can be made to pass through alternate flow passages along the
corrugations of the sheets thereby exchanging heat.

Top View

Front View

Perspective View
Figure 3.10 The Proposed Design

24

These sheets need to be held in place with the help of fasteners at two or if required all four
corners. Simple nut and bolt arrangements can be used as fasteners. With the use of
fasteners, each of the sheets can be maintained at the required distance and angle from other
sheets. The benefit of such an arrangement is that it will lead to turbulence in the flow of air
streams. We know that in a turbulent flow, the boundary layer formation does not take place
as in the case of laminar flow. Also formation of boundary layer is undesirable because it
inhibits heat transfer across itself. Therefore, present arrangement helps in better heat transfer.

Various heat exchanger parameters are presented in table 3.1.

Table 3.1 Heat exchanger parameters

Parameter Value
Surface Area of each sheet 0.816 m2
No. of Sheets in the stack 96
Total Heat transfer Surface Area 156.67 m2
Volume of the stack 0.59 m3
Surface Density 266.27 m2/m3
Total Area of Flow Cross Section 0.59 m2
Mass Flow rate of exhaust air 8000 kg/h
Volume flow rate of exhaust air 2.45 m3/s
Velocity of flow for exhaust air 9.3 m/s

3.3.5 The Modified Design

The proposed design discussed in section 3.3.4 may suffer from lack of support on the sheets
which may cause the heat exchanger element to become disoriented as flow passes through it.
Also, the air flow velocity through the design is very high which might lead to high pressure
drops within the heat exchanger element. Also it might be possible to save upon material cost

25
by going for thinner sheet. Thus some modification is required in design which can
overcome these shortcomings.

A modified design is proposed as follows. A sheet thickness of 0.15 mm is taken to reduce
the material cost. The material of construction is changed to Stainless Steel (Grade SS 304)
instead of Galvanized Iron. This grade of steel has excellent temperature withstanding
capabilities and is virtually corrosion free in the operating conditions of the present case.

In addition to this, the angular shift between successive sheets is increased to affect a better
heat transfer by breaking the boundary layer to a greater extent. Also the need for spacers is
virtually eliminated in the design so proposed. The spacing between sheets is maintained by
the cross corrugated arrangement itself.

It was decided to seam weld the cross corrugated sheets being stacked to form the heat
exchanger by taking them pair by pair. This ensures minimum risk of cross contamination.
One pair of such sheets makes up a heat transfer cell. Such cells are stacked on top of each
other to form the heat exchange matrix. The cold stream of feed air is to flow from within the
cells formed by seam welding of sheets. The hot stream, which is nothing but the flue gases
from the stenter exhaust, is to flow over these cells and in the process, exchange heat with the
cold stream. This flow pattern ensures that the heat exchange matrix is accessible for
cleaning and removal of the impurities that might deposit over the surface due to the flow of
exhaust gases. If the hot stream was to flow from within the cells, they might get clogged up
thereby affecting the performance of the heat exchanger.

With these considerations and some heuristics in mind, a design is arrived at. This design is
shown in Figure 3.11. The heat exchanger is to consist of several stacks of the cell shown in
the figure. This design has been fabricated and subjected to testing, and based on these
experimental results, final designing of heat exchanger is to be proposed.

26

Projected area on
one side 787.5 cm2
Heat Exchange area on
one side 1.12 x (787.5) = 882 cm2

Perimeter 1.136 mm

Figure 3.11 The Modified Design

The accompanying calculations and analysis is as follows,

.
Mass flow rate of exhaust gas, m eg = 8000 kg/h
= 2.2 kg/s

For the purpose of designing this heat exchanger, we may use simple corrugated parallel SS
304 sheets of 0.15 mm thickness. Now, for the design conditions,

160 90 80 30
LMTD = °C … (3.1)
160 90
ln
80 30
= 59 °C
The given flow is estimated to behave similar to a flow through parallel plates of similar
dimensions. Thus for such a flow,

hD
NuH = 8.235 = … (3.2)
k

27
D = 2 W = 11 mm
k = 0.0479 W/m-K
h = 39.5 W/m2-K

Now say we are designing for a heat duty of 186.7 kW which gives us
.
Q
Aht = … (3.3)
U LMTD
186.7 1000 2
= m
20 59.4
= 157.1 m2

Further,

a,120°C = 0.898 kg/m3
.
m
va = … (3.4)
a Acs
= 3.87 m/s
va D
Re = … (3.5)

= 929

Thus, the flow through the sheets would be laminar. The pressure drop for this flow can be
estimated using the following relation,

2
f l va
p = … (3.6)
2 g D
64
f = … (3.7)
Re
p = 597.13 Pa
= 0.006 bar

Thus the flow encounters a pressure drop of only about 0.6% of line pressure. Various
parameters of this design are presented in table 2.2

28

Table 3.2 Heat exchanger parameters for the modified design

Parameter Value
Surface Area of each cell 0.176 m2
No. of cells in the stack 900

29

Chapter 4

Fabrication and testing of Heat Exchanger

The heat recovery unit for stenter exhaust heat recovery is to be made out 0.15 mm thick
sheet of stainless steel (SS 304). The fabrication process incorporates several steps
starting from cutting the sheet to corrugation of the sheets to seam welding and headering
of the heat exchanger cells. All these steps are described in this chapter. Also included is
the testing procedure for the prototype and calculations for performance of the same.

4.1 Fabrication of Heat Exchanger Cells

The fabrication process for making the heat exchanger element starts with shearing pieces
of SS 304 to the required dimension. The pieces so cut are then going to be subjected to
the process of corrugation. The pieces are then to be washed to remove any impurities on
their surface. After corrugation, the sheets are to be seam welded in pairs to form cells.
Following the seam welding, the cells are to be subjected to multiple passes of shearing
and seam welding to arrive at the required shape. After the cells are ready, they are to be

30

headered to allow for the passage of air through them. The cells can then be assembled
one on top of another to form the heat exchanger element.
The processes are listed as follows in their order of occurrence:

a Shearing pieces
b Washing and drying
c Corrugation
d Seam Welding and Shearing of cells
e Pinching of cells
f Welding of headering arrangement
g Preliminary leak testing

Each of these processes shall be taken up individually now and discussed in detail.

4.1.1 Shearing pieces

It has been observed that during corrugating, a rectangular piece of sheet shrinks along
one diagonal and hence the dimensions of the corrugated sheet change. This has to be
born in mind before determining the dimensions of the SS 304 sheet to be cut.

It has been experimentally found that for achieving a width of 230 mm in a length of
about 480 mm, one has to cut a sheet of 480 mm by 275 mm. Another consideration
before the sheet gets cut into pieces is that it must be done in such a fashion that the sheet
is corrugated perpendicular to the grain. Also the corrugations are to be done in a way so
that they are almost parallel to the longer edge of the piece being corrugated. Therefore,
the sheet to be cut for corrugation has to have its longer edge perpendicular to the grain
of the steel roll. This consideration results in the orientation for the sheets to be cut as
shown in Figure 4.1

31

Figure 4.1 The sheet to be cut from the sheet roll
After being cut from the roll in accordance with the consideration above, the sheet is
subjected to the process of corrugation. As mentioned earlier, one of the diagonals of the
rectangular sheet becomes shortened owing to corrugation. This phenomenon is
demonstrated in Figure 4.2

Figure 4.2 Shrinkage of SS sheet due to corrugation

32

As shown in the figure, the rectangular sheet cut from the roll takes the shape of a
parallelogram following corrugation. According to the design, the heat exchange cells are
to be cross corrugated, thereby implying that the corrugated sheet shown in Figure 4.2 is
to be aligned with another similar sheet as shown in Figure 4.3. However, due to the
shape, the sheets do not match. Therefore, it is necessary to cut such a shape off from the
roll of stainless steel that after corrugation gives matching pairs.

Figure 4.3 Misalignment of corrugated sheets

In view of this consideration, the diagonally opposite edges of the rectangular pieces are
cut to an extent that would result in perfectly matching corrugated pairs of sheets. It was
observed that the corrugated sheet misalignment was coming out to be 15 mm. Therefore,
same amount of material needs to be cut from diagonally opposite corners. This leads us
to arrive at the final shape of sheets that are to be cut from the roll of SS 304. The shape
is shown in figure 4.4.

33

A B

C D

Figure 4.4 Final shape of SS 304 sheet to be cut from roll

This shape is arrived at by first cutting off a width of 275 mm from the roll as shown in
Figure 4.1 and then placing a template according to Figure 4.4 made out of polypropylene
sheet onto the rectangular piece so cut and shearing the corners off. The four corners of
the sheet are also named A through D in the order shown. These names are marked onto
the sheets with the help of permanent markers. This helps in the process of corrugation
to avoid wrong way of corrugating.

4.1.2 Washing and Drying

The pieces of sheets that are cut as discussed in the previous section are taken directly
from a steel roll. As such, they might be contaminated with several impurities that can
hinder the quality of the final product. For example, the seam welding operation can
result in a lot of sparking, burn holes and consequently weak weld if the workpiece is not

34

clean. To cater to this need, the pieces of stainless steel sheet are washed with soap and
water and dried to ensure a clean surface.

4.1.3 Corrugation

The washed and dried pieces are then to be corrugated. This process is carried out by
feeding the sheets into the rollers designed for the purpose. The roller assembly consists
of two meshing rollers having involute tooth profile. As the sheet passes through them, it
bears the impression of the teeth of the rollers and hence becomes corrugated. The teeth
profile determines the pitch and depth of the corrugations that have impression on the
sheets. The roller assembly is driven by a motor that powers the top one of the rollers.
The second roller is driven by the movement of the first roller. The rollers are made to
the dimensions listed in table 4.1

Table 4.1 Attributes of the corrugating roller
S no Attribute Value (mm)
1 Addendum 90
2 Dedendum 87
3 Pitch 10
4 Pitch Circle Diameter 88.5
5 Length 600

As discussed before, the cells are to be made in a cross corrugated fashion. This is
achieved by corrugating the sheets at an angle so that when they are placed on top of one
another with their respective corrugations pointing in different directions, cross
corrugated profile is achieved. This is demonstrated in Figure 4.5. The sheet becomes
shortened along one of its diagonals after corrugation. So it is important to feed the
correct side into the rollers.

35

Figure 4.5 Cross corrugated sheets
The angle for cross corrugation has been kept as 20o. This means that each sheet has to
have the corrugations making an angle of 10o with the longer edge. Though heat transfer
in cross corrugated passages is known to be maximum at around 55o the value cannot be
adapted in this case. The angle cannot be lesser than 4o or greater than 29o, the former
limit coming from the fact that the sheets might fall into each other and the latter coming
from the fact that the flow will encounter more resistance from the corrugations leading
to increased pressure drop. Another prime consideration in this case is that the seam
welding is affected by the angle of corrugations. If the welding is to be done
perpendicular to the corrugations, it leads to the pinching of corrugated ends and leads to
improper weld. So the present angle of 20o is decided upon and taken up for corrugating.
To ensure that the corrugations are at an angle with the edge of the sheet, the feeding is
also done at the required angle. The sheet is fed into the roller with a tilt of 10o with the
corrugating teeth of the same. The roller assembly is shown in Figure 4.6.

Figure 4.6 Roller assembly

36

While feeding the sheets into the roller assembly for corrugating, following precautions
are to be observed,

a The sheets shrink along the diagonal that is fed first into the roller. Therefore,
always the uncut corner should be fed first. All the sheets are to be corrugated by
first feeding the corner marked “D”. Precaution should be taken not to feed any
other corner into the corrugating roller.
b The roller assembly has a provision to adjust the spacing between the rollers at
both ends to accommodate various sheet thicknesses. It should be ensured that the
spacing on both ends of the rollers is equal. In other words, the roller teeth should
be meshing to the same extent on both ends before carrying out the process of
corrugation.
c The rollers and the sheets to be fed should be clean. Any impurities present shall
be pressed against the surface of the sheet due to the force of the roller. This
might cause imperfections in the corrugations. Also, contaminated surfaces are
not desirable during subsequent fabrication.
d It should be ensured that after getting corrugated the sheet is coming out of the
roller properly. Care should be taken to avoid the buckling of the sheet which may
spoil its profile.
e The rollers are driven by a high torque. Therefore it is advisable to not wear loose
clothes while operating them and also to take care while feeding the sheets so as to
not let the hands get hurt by the rollers.

4.1.4 Seam Welding and Shearing

The sheets are to be made into pairs with cross corrugations as shown in Figure 4.5.
They are then to be seam welded from all sides leaving only opening for headering
arrangement. The process of seam welding is carried out on SMW 50 resistance seam
welding machine.

37

Figure 4.7 SMW50 Resistance Seam Welding Machine

There are several parameters of welding that come into play at this point. They are listed
as follows.

4.1.4.1 Welding Parameters
The following settings are for a sheet thickness of 0.15 mm
a Electrode Speed: The electrode speed is governed by a voltage variac that
governs the input voltage to the DC motor driving the electrode. The related
attributes are as follows

Unit % age of maximum value
Range 0 – 100

38

Set Value 19-20%
Calibration 100% = 1m/min

b Weld Force: Weld force is an important parameter for obtaining satisfactory weld.
It is delivered by the hydraulic cylinder mounted on top of the driver electrode.
Weld force is governed by the pressure of the air line fed into the machine.

Unit kg/cm2
Range 0 -10 kg/cm2
Set Value 3 kg/cm2

c Pulsation: The pulsation switch is located on the electronic control panel. If
pulsation is required for the welding process, this switch should be on. The switch
being on indicates that the weld cycle shall occur in accordance with the heat cycle
and cooling cycle settings of the electronic panel.

d Weld / No-weld: The weld / no-weld switch is located on the electronic control
panel beside the Pulsation switch. This switch can be used to set the welding cycle
to weld or no weld. No-weld cycle implies that no current passes through the
electrode at all and hence no welding takes place. No-weld cycle can be used to
flatten the workpiece before actual welding to minimize the arcing and ensuring a
satisfactory weld. If this switch is on indicating weld, current passes through the
electrodes as per the pulsation settings and welding takes place accordingly.

e Squeeze time: Squeeze time is set on the electronic control panel by adjusting the
first two digits shown on the panel. The two digit number corresponding to the
squeeze time of weld cycle corresponds to the number of squeeze cycles occurring.
Squeeze time is the time during which no current is passed through the electrodes
and no movement of the work piece takes place. Squeeze time occurs in the
beginning of the welding process.

39

Unit Cycles (Two digits)
Range 0 – 99 Cycles
Set Value 50 Cycles
Calibration 1 cycle = 1/50th of a second

f Heat cycle: Heat cycle is set on the electronic control panel by adjusting the third
digit from left. Heat cycle setting determines the number of cycles during which
current passes through the electrodes and heating of the workpiece occurs due to
resistance to the flow of current.

Unit Cycles (One digit)
Set Value 3 – 5 Cycles

g Cool cycle: Cool cycle is set on the electronic control panel by adjusting the fourth
digit from left. Cool cycle setting determines the number of cycles during which
no current passes through the electrodes and no heating of the workpiece occurs.
Cool cycle accommodates the expansion of the workpiece due to heating cycle. A
heat cycle of 3 and a cool cycle of 1 indicate that heating time is to cooling time
ratio is 3:1 for the respective welding cycle.

Unit Cycles (One digit)
Set Value 1 Cycle

h Hold time: Hold time is set on the electronic control panel by adjusting the last
two digits from the left shown on the panel. The two digit number corresponding
to the hold time of weld cycle corresponds to the number of hold cycles occurring.
Hold time is the time during which no current is passed through the electrodes and

40

no movement of the work piece takes place. Hold time occurs at the end of the
welding process.

Unit Cycles (Two digits)
Set Value 15 Cycles

i Percent heat: The percent heat is set using the knob at the bottom of the electronic
control panel. It determines the amount of current that flows through the
electrodes during the time of weld. The greater the current, the more is the heating
that takes place during the welding process.

Unit % age of maximum value
Range 0 – 100
Set Value 5 - 9%
Calibration 100% = 12800 Amperes approx.

The effects of different welding parameters on the weld have been thus described. Each
setting of the seam weld machine corresponds to a combination of all the welding
parameters. Several settings were tried before arriving at the present setting of the
machine that delivered leak proof seam weld. These are listed in table 4.2

41

Table 4.2 Settings for SMW50 Seam Welding Machine

S no Welding Parameter Setting 1 Setting 2 Setting 3 Setting 4

1 Electrode speed (% of max. value) 21 19 19 19
2 Weld force (kg/cm2) 3.5 3.5 3.0 3.0
3 No Weld passes (nos.) 1 1 1 0
4 Weld passes (nos.) 1 1 1 2
5 Squeeze cycles (nos.) 50 50 50 50
6 Heat cycles (nos.) 3 3 5 5
7 Cool cycles (nos.) 1 1 1 1
8 Hold cycles (nos.) 15 15 15 15
9 Percent heat (% of max. value) 5 10 7.5 7.5
Weak Holes in Leakage Good
10 Remarks about welding quality
Weld Weld from Weld Weld

The cells are then seam welded according to the setting 4 described above. There are
four welds to be done onto the cells. Two of the welds are along the longer edge of the
cells. The other two welds are at an angle with the shorter edge. An opening of 100 mm
width is left for the flow to enter the cell. After welding the extra material is to be
removed by shearing. Due to welding, the cells might become a little twisted causing
them to lose their flatness to some extent. Therefore a mask has been designed for
marking off a length of 410 mm in the welded cell for shearing. The welding and
shearing process takes place in the following order.

1 The sheets cut to size and corrugated are held in welding frame A which is
made for the purpose. The frame is shown in Figure 4.8

42

Figure 4.8 Fixture for Seam Welding the Long Edge

2 Edge AB is welded according to the settings described above
3 Edge DC is welded according to settings described above
4 The welded cell is taken out of the frame and the extra material is sheared off
as shown in the following figure

Figure 4.9 Heat Exchange Cell with Long Edges Welded

5 After shearing the extra material out of the welded seams, the cell is held in a
mask and marked for shearing to a length of 410 mm as shown below

43

Figure 4.10 Mask for Marking Length
6 The length of 410 mm is sheared off
7 The cell is then held in the frame B as shown in Figure 4.11 and the slant
edges are welded

Figure 4.11 Fixture for seam welding the slanting edge

8 The extra material is sheared off

The cell is complete with respect to welding and shearing after the above procedure

44

4.1.5 Pinching of Cells

The cells so formed are now to be provided with an arrangement for flow to be admitted
into them. Such an arrangement has to be leak proof to avoid the mixing of the two
streams exchanging heat. Initially it was thought to seal the alternate layers of corrugated
sheets with metal adhesive to make them leak proof. However, the metal adhesive did
not adhere to the smooth surface of stainless steel and hence the idea had to be dropped.

Figure 4.12 Initially proposed headering arrangement

Another headering arrangement was thought of that comprised of joining three SS 321
tubes onto the opening left for flow admission. This seemed favourable owing to several
considerations such as the fact that tube arrangement can be made absolutely leak proof
by brazing them. The tubes have good pressure sustaining capabilities. It was then
required to ‘pinch’ the opening to form the grooves required for entering the three tubes
into the cell. After getting pinched, three tubes fit perfectly into the opening of 100 mm.

For the purpose of pinching, an aluminium die is fabricated. The die consists of two half
inch aluminium bars into which three holes are drilled with a diameter of 11 mm and
their centres separated by 25 mm.

45

The initial arrangement for carrying out the pinching operation is described henceforth.
Both the halves of the aluminium die so formed are welded onto one inch aluminium
angles to allow the die to be fixed onto a bench vice. Now the die is ready for pinching.
Three tubes of 3/8” diameter and 1½” length are inserted into the opening of the cells up
to ½” length. The cell with these tubes jutting out is held in the die and bench vice is
tightened onto the cell. The tubes press against the matching holes in the die from within
the cell opening and the latter comes to bear the impression of the tubes.

Figure 4.13 Initially fabricated die for pinching

However, there were certain shortcomings of the pinching arrangement described above.
Firstly, there was no provision for locating the sheets while pinching. Therefore the
pinching was non uniform. Secondly, there was no way to ensure that the tubes are
homogenously inserted to a depth of ½” in all the cases. In view of all this, the pinching
arrangement was modified as follows. A stainless steel frame was made to hold the
sheets while pinching was carried out. The aluminium die was fixed onto this frame
itself. The frame was also provided with an arrangement for ensuring that the tubes go
into the cell opening uniformly to a depth of ½” in all cases.

46

Figure 4.14 Pinching Frame

The following procedure was adopted for pinching the cells in the new frame,

1 The welded and sheared cell is placed into the frame
2 The three 1½” tubes are inserted into the cell openings on each side
3 The dies are mounted onto tubes such that the cell skin is held between the
tube and the die
4 The frame is tightened to hold the sheet in place
5 The frame is held in the bench vice and dies are pressed against each other
6 After fully tightening, the vice is let loose and the frame is disengaged

Figure 4.15 Pinched cell and tubes to be inserted

47

Shown above are the pinched grooves for holding the headering tubes that are formed
onto the cells as a result of pinching.

The sheets are pinched according to the following scheme to ensure that when they are
stacked one on top of another, the headered tubes do not get in the way of them being
apart by the required distance. A hexagonal arrangement of tubes is desirable to ensure
this which places a requirement onto the pinching orientation. This is demonstrated in
Figure 4.16,

Figure 4.16 Stacking Arrangement of cells

4.1.6 Welding of headering arrangement

The tubes are to be welded onto the cells and the gaps between the tubes are to be sealed
off to render the cells leak proof. The gaps between the tubes are sealed by seam welding
them. Since the tubes are only ½” inside the cell opening, ½” long seam welds are
sufficient to seal the gaps. Tubes are inserted into the grooves made by pinching and
then the cell is fed into the seam welding machine as shown below.

48

Figure 4.17 Seam welding the pinched openings

Following this operation, the tubes come to be set in their respective positions. The task
only remains to silver braze their sides to render them leak proof. This has to be done
skilfully. The flame from the brazing torch should not directly hit the cell, it should only
hit the headering tube and the heat from the flame should be used to melt the filler
material onto the cell surface. A small mask is used to ensure that the tubes remain intact
in their positions while brazing. This is shown in the accompanying figure.

Figure 4.18 The mask used for brazing the cells

Following these operations the headering provision is complete. The cells can be now
stacked one on top of the other according to the arrangement shown in Figure 4.16

49

4.1.7 Preliminary leak testing

The cells so formed are now tested for leaks. This is done by sealing off all the headering
tubes except one. The cell is submerged in a bath of water and then air is blown into the
cell from the one open header tube to check if there are any leaks. The leaks, if any, can
be cured depending upon where they occur. The several probable sites for leaks to occur
are seam welding, brazing, and corrugated sheets. The leaks occurring in seam welding
can be cured by re-welding locally at the site of the leak. The imperfections in brazing
can be overcome by depositing more material onto the site of the leak. Any leaks due to
holes in corrugated sheets can be overcome by sealing with loctiteTM silicone adhesive.

4.2 Assembly of cells

The cells fabricated according to the procedure outlined in section 4.1 are subsequently
assembled to form a heat exchanger matrix. Five cells are taken for fabrication of the
prototype of the actual heat exchanger to be tested. These cells are stacked one on top of
another as shown previously in Figure 4.16 and headered to form the heat exchanger
prototype.

Figure 4.19 Heat exchanger prototype

50

After that, the cells are fitted with 2.3 mm thick plate with holes corresponding to the
headering tubes on both sides of the cell stack. The tube ends are inserted into the plate
up to an extent that they just come out of the other end. These ends coming out are then
welded to fix them to the plates.

Figure 4.20 Welded tube ends

Subsequently, an arrangement for directing flow onto the headered tubes is to be made.
This is done by providing triangular ducts at the ends where the tubes are brazed. The
triangular ducts are fitted with tubes to admit flow. The assembly is shown in Figure
4.21; the cold stream enters the cells through the triangular ducts whereas the hot stream
flows from over the cells.

After providing ducts for the flow passage, the prototype to be tested is put in an
enclosure made of SS 321 stainless steel. The enclosure is provided with a duct for flow
at one end of the cells and the other end is kept open. The heat exchanger assembly is
shown as follows

51

Figure 4.21 Heat exchanger prototype assembly

4.3 Performance Evaluation of Heat Exchanger

This section outlines the various tests and calculations required to assess the performance
of the heat exchanger.

4.3.1 Tests to be Performed

Following two tests are to be performed for evaluating the performance of this system.
a Calibration of Thermocouples: This is a pre-requisite for the performance of
any experiment, so as to obtain an error estimate of the data. A calibration
procedure was adopted over a range. The thermocouples and digital indicator used
are quite new and in good condition.

52

b Calibration of Flow meter: The flow meters being used to determine the flow
rate of the hot stream and the cold stream is to be calibrated. The coefficient of
discharge is to be found out for the flow meters. This is done by passing water
flow through them and maintaining similar Reynolds number as the air flow. The
water passing through the flow meter is collected and actual flow rate is obtained
by dividing the amount of water collected by the time it took to collect. The flow
shown by the flow meter is calibrated against the actual value of flow so obtained.
c Performance Evaluation of Heat Exchanger Unit: The performance of the heat
exchanger so fabricated is to be evaluated by the virtue of experimentation. This is
done by admitting two streams of hot and cold air through the respective flow
passages and determination of the heat transfer taking place from one stream to the
other.

4.3.1.1 Thermocouple Calibration

Calibration of thermocouple wires is a primary step for conducting the experiments.
Thermocouple can be calibrated using ice point, steam point or calibration over a range
of temperature. In this experiment the range of temperatures of operation of fluids is well
within 3oC to 100oC. So all thermocouples wires have been calibrated between this
temperature range of 3oC to 100oC using water as discussed in Appendix I.

4.3.1.2 Calibration of flow meter
The flow meter used for determination of flow through the prototype is a venturimeter.
The dimensions of the same are shown in Figure 4.22

53

Figure 4.22 Venturimeter for flow measurement

Aim
To calibrate the flow meter for air flow by maintaining similar flow conditions for water
stream

Apparatus
Venturimeter, pressure gage, digital indicator, water flow line, vessel, stopwatch

Experimental Setup
a Venturimeter is connected in a flow line to admit water
b Tappings are provided for reading the flow in the venturimeter
c Vessel is placed at the end of flow line to collect the flowing water
d Tappings are connected to the pressure gage

Procedure
a Flow is admitted into the flow meter and then it flows into the vessel
b Digital indicator is switched on
c As soon as the venturimeter shows a steady value of flow, vessel is emptied and
placed again in place to collect water
d Stopwatch is switched off when vessel is full

54

Observations
The observed values are tabulated as follows

Table 4.3 Observations for venturimeter calibration
Pressure Drop Time taken to fill the
S No Coefficient of Discharge
pd vm (bar) vessel w.c (s)

1 0.0045 101 1.307
2 0.012 62 1.306
3 0.017 51.73 1.326
4 0.023 43.9 1.333

Sample Calculation

Corresponding to the first observation,
Pressure drop across venturimeter, pd vm I = 0.0045 bar

Time taken to fill 33.5 kg water, w.c = 101 s

Now, ratio of throat diameter to inside diameter of the venturimeter is given by

d throt.i
= …(4.1)
d vt .i

d throt.i = 0.019 m

d vt.i = 0.038 m

= 0.5
Mass flow rate of water as measured by venturimeter is given by

55

1
mvt = d 2 throt .i 2 pd vm w …(4.2)
1 4 4

For the present case, substitution of values in the expression yields,
m vt = 0.276 kg/s

= 16.6 kg/min

Actual flow rate of water measured by collecting 36.55 kg of water

36.55kg
mactual = …(4.3)
w.c

= 0.362 kg/s
= 21.7 kg/min

The coefficient of discharge is defined as the ratio of actual flow rate to that measured by
venturimeter
m actual
Cd = …(4.4)
mvt
= 1.307
Result
The venturimeter was calibrated against actual flow values and coefficient of discharge
was obtained as the average of measured values. The value of coefficient of discharge
obtained is 1.318.

This value is cross checked by connecting the venturimeter to the line that goes through
the air heater assembly. The electric power consumed by the heater is converted into
thermal energy. So by knowing temperature change to be from 45 oC to 160 oC, we can
estimate the mass flow rate which can verify that shown by venturimeter.

pd vt = 0.014 bar

m vt = 0.015 kg/s

56

112.07587kWh 111.71902kWh
Pheater = …(4.5)
9 h
60
= 2.379 kW
= m actual C p.a Ta …(4.6)

This gives,
mactual = 0.0197 kg/s

The discharge coefficient is given by equation 4.4
m actual
Cd =
mvt
= 1.313

Which is in good agreement with the one obtained before.

4.3.1.3 Performance Evaluation of Heat Exchanger Unit

A prototype of five cells is taken for testing. The results are then to be extended to the
design of the entire unit.
Aim
To evaluate performance of heat exchanger and obtain the experimental values of overall
heat transfer coefficient.

Apparatus
Thermocouples (K type) accuracy +/-1oC, temperature indicator accuracy +/- 0.1oC,
Venturimeter (2 nos. ), digital pressure transducer, digital indicator, 9 kW electric heater,
centrifugal blower (2 nos.)

Experimental Setup
Important features of the setup are the following:

57

1 The 9 kW capacity heaters are connected to the system so that hot air can be
supplied to it.
2 Air streams are admitted into the system with the help of two centrifugal blowers.
3 One thermocouple each is used for measuring the temperature at cold air inlet and
exhaust. Three thermocouples were used to measure hot air exhaust. Hot air inlet
was governed by PID controller to a value of 160 oC.
4 The digital pressure transducer is connected between the cold air inlet and outlet
to get the cold air side pressure drop. The setup is shown below

Figure 4.23 Setup for prototype testing

5 The apparatus is insulated using blocks of calcium silicate and then additionally
covered with heatlon sheet.

58

Figure 4.24 Insulation of test module
Procedure
1 Electrical connections to the blowers, heaters, pressure indicator and temperature
indicator were made.
2 The heater was turned on and the flow rate was adjusted to get the desired hot air
flow
3 Temperature and pressure drop readings were registered after they stabilized.
4 Based on the temperature and flow readings obtained, heat duty and the overall
heat transfer coefficient of the system was calculated.

Observations
Zero error in measurement of pdvt.ha. = 0.0055 bar

Zero error in measurement of pdvt.ca. = 0.0140 bar

The temperatures together with the pressure drops across venturimeters and across the
flows on both sides are measured and tabulated as follows

59

Table 4.4 Performance of Heat Exchanger Unit
S. tha.i. tha.o.1 tha.o.2 tha.o.3 tca.i. tca.o. pdvt.ha. pdvt.ca. pdha pdca
o o o o o o
No. C C C C C C bar bar bar bar
1 29.2 28.3 34.3 27.5 27.4 28.1 0.0180 0.0270 - -
2 160 70.5 60.4 68.2 46.3 71.8 0.0195 0.0280 0.0020 0.023
3 160 77.9 66.6 75.1 47.3 77.5 0.0195 0.0280 0.0020 0.023
4 160 80.7 71.2 78.4 47.6 79.4 0.0195 0.0280 0.0020 0.023
5 160 85.7 74.2 84.0 48.1 82.3 0.0195 0.0280 0.0020 0.023
6 160 89.5 78.8 86.7 48.9 83.6 0.0194 0.0280 0.0020 0.023
7 160 92.8 81.6 89.9 49.1 84.1 0.0195 0.0280 0.0020 0.023
8 160 93.9 83.4 91.0 49.4 84.5 0.0195 0.0280 0.0020 0.023
9 160 97.8 89.4 94.8 50.4 86.6 0.0196 0.0280 0.0020 0.023
10 160 98.9 90.1 95.0 51.2 86.9 0.0195 0.0280 0.0020 0.024
11 160 100.1 91.4 95.7 51.3 87.7 0.0195 0.0280 0.0020 0.024
12 160 99.9 91.8 96.1 51.4 88.1 0.0195 0.0280 0.0020 0.023
13 160 100.3 92.1 96.4 51.4 87.6 0.0195 0.0280 0.0020 0.023
14 160 100.3 92.3 96.5 51.3 87.7 0.0195 0.0280 0.0020 0.023
15 160 100.3 92.4 96.5 51.4 87.7 0.0195 0.0280 0.0020 0.023

Precautions and sources of error

1 Precautions are to be taken to ensure that no leaks develop during the assembly of
the heat exchanger, because any leaks will give faulty results experimentally.
2 It must be ensured that the heaters are not overheated. The PID controller display
should be used to determine whether the heaters are getting overheated.
3 Heat loss should be minimized by insulating the apparatus.

Calculations, results and discussion

Hot air inlet temperature, tha.i = 160 oC
Hot air exit temperature, tha.o = ( tha.o 1 + tha.o 2 + tha.o 3) / 3 …(4.7)

60

= 96.4 oC

Cold air inlet temperature, tca.i = 51.4 oC
Cold air exit temperature, tca.o = 87.7 oC

eff = 33.42 %

Air flow rate, ma = mvt C D …(4.8)
= 19.35 g/s

160 96.4 87.7 51.4
LMTD = °C …(4.9)
160 96.4
ln
87.7 51.4
= 48.75 oC

Hot side heat duty, Qh = m a C p .a t ha.i t ha.o …(4.10)

= 0.01935 kg/s 1.05 kJ/kg oC 63.6 oC
= 1.27 kW

Cold side heat duty, Qc = m a C p .a t ca.o t ca.i …(4.11)

= 0.01935 kg/s 1.05 kJ/kg oC 36.3 oC
= 0.737 kW

Aht = 0.882 m2
Overall heat transfer coefficients can be evaluated as,
Qh
Uh = …(4.12)
Aht LMTD

= 29.54 W/ m2-K
Qc
Uc = …(4.13)
Aht LMTD

61

= 17.17 W/ m2-K

The difference between the heat duty on the hot side and that on the cold side is 0.53 kW.
This is a representative of heat losses from the apparatus that remain despite the
insulation provided. There is a need to provide a better insulation that can be achieved by
covering the apparatus with glass wool. A reduction in heat loss can improve the
performance of the heat exchanger prototype being tested.

The pressure drop on the cold air side is at a high of 0.023 bar. In addition to the drop
across the heat exchanger prototype, this also owes heavily to certain sites where the flow
is undergoing sudden expansion or contraction. Also the headering tubes are 1.2 mm
thick. If this thickness can be reduced to say 0.5 mm, the pressure drop is expected to
come down by a factor of 2.5 approximately due to increase in area of flow to the header
inlet.

The heat duty on the cold side is at a low of 0.737 kW which can be improved upon by
minimization of heat loss. Anyhow, even if the present loss is to be considered, the value
of heat duty can be doubled by increasing the area of heat transfer to twice its present
value. In such a case, the cells will have to be made longer by 342.4 mm. Therefore, the
new cells will be 752.4 mm long instead of a present value of 410 mm. This will give a
heat duty of 1.474 kW on the cold side which is the productive heat duty.

The flow rate through the test module of 5 cells is 19.3 kg/s which is 114 times less than
the maximum allowable flow through the complete heat exchanger. So, the heat
exchanger has to be arrived at by scaling the prototype through a factor of 114. The
number of cells to be used for the heat exchanger comes out to be 570. These cells can
be arranged in three stacks of 190 cells each. In the complete heat exchanger, the heat
duty on the cold side would then be again the duty for the test module scaled by 114,
giving a value of 168 kW.

The attributes of the final design are summed up in table 4.5

62

Table 4.5 Heat exchanger parameters for the final design

Parameter Value
Surface Area of each cell 0.352 m2
No. of cells in the stack 570

4.5 Economic Analysis

In this section, with the help of calculation, economic viability of the modified design
will be assessed. Each cell would weigh about 400 grams of SS 304. Taking the cost of
SS304 sheet to be about Rs. 300/- per kg, the cost of heat exchanger made out of 0.15
mm thick sheet comes out to be about Rs. 60,000/-.
Considering about twice of this to be the installation cost, we arrive at a cost figure of
roughly Rs. 1,20,000/-

Now to calculate the saving in fuel consumption,

Type of fuel = furnace oil,
Calorific Value = 9650 kcal/kg
= 40337 kJ/kg
Price of fuel = Rs.13.8 per kg

63

Efficiency of Boiler = 80 %

Use of stenter is in three shifts for, say, about 300 days per year

Energy delivered by fuel = 80% 40337 kJ/kg
= 32269.6 kJ/kg
= 8.96 kWh/kg
The cost of this energy is obtained by incorporating the price of the fuel as Rs. 13.8 per
kg

Cost of energy produced = Rs. 1.539/kWh

The saving brought about per hour is obtained by multiplication of this cost with heat
duty,

Saving per hour = Rs. 1.539/kWh 168kW
= Rs. 258.55/h
Total annual saving = Rs. 258.55/h 24 h/day 300 days/year
= Rs. 18, 61,574.4
Cost of heat exchanger = Rs. 1,20,000/-

Simple Payback Period less than a month

So it is observed that if the system of proposed design is installed, cost and volume of
heat exchanger, both will become much lower than all the existing systems for same
purpose.

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