MEC 422 - Energy Recovery from an Electrical Generating Facility
1. MEC 422
Design of Thermal Systems
Project 28
Jackie Chen
Matthew Stevens
Luis Lituma
2. Problem 64: Energy Recovery from an
Electrical Generating Facility
This problem was taken from Design of Thermal Systems, 3rd Edition.
Four 185 kW diesel engines are used to generate electricity at a facility that
is located where power from the utility company is very expensive. It may be
cost effective for the facility to generate its own power, and so it is desirable
to have as little waste as possible. The proposed layout of the system is
shown in figure 4.
As indicated, the engine-generator unit are spaced 2 m apart, although this
spacing can be changed if necessary. All are fed from a common heard that
brings fuel from a tank located at an unknown (at this time) distance L away.
The exhaust gases from these engines are to be used to heat water to as high
a temperature as possible. The water flow rate is that typically supplied by
the city, although a pump can be used if a higher flow rate is desirable. Any
type of heat exchanger can be used.
It has also been suggested to recover the waste heat at the radiators as well.
The water that circulates through the four radiators would be used to heat
water in one or more heat exchangers, again to as high a temperature as
possible. The water being heated by the exhaust gases.
3. Figure 1 - Layout of diesel engines set up for heat recovery project
4. Product Design Criteria
The cost of the project should be minimized, and at least
must be justified by economic gains from efficiency boosts.
Energy recovery system should maximize harvested energy
from exhausted heat.
The system should be adjustable and require minimum
technical expertise to improve its potential for retrofitting.
System should be durable and robust.
System should be easily maintained.
Frictional losses in the piping system and thermal losses in
the exchanger should be minimized.
5. Product Design Specifications
Tubing and manifold should be made of easily fitted materials.
Cooling fluid reservoir will come from municipal sources, and be pumped through the
system.
Product should be able to transfer heat available from the exhaust into water in parallel
or series flow arrangements.
Payback period for system must be below 20 years.
System should be resistant to corrosion from water, diesel fuel, and waste gases
produced by diesel engines.
System should use the water heated from radiator jacket as the cooling fluid of the main
gas-liquid heat exchanger.
System should be simple to install, clean and maintain.
The outlet temperature of the cooling fluid must be as high as possible without phase
changing.
6. To be designed, selected, or determined:
1. What typically is the fuel requirement for a 185-KW diesel engine?
2. What is the power delivered by a 185-kW engine? Design the manifold
and fuel inlet line to each engine. Specify the material to be used.
3. How much of the energy in the fuel is used to generate electricity?
How much energy is exhausted from these engines, and at what
temperatures? How much energy is rejected at the radiator and at
what temperature?
4. The heat available in the exhaust from four engines can be transferred
to water in a parallel or series flow arrangement within one or more
heat exchanger. The same can be said of the heat rejected by the
engines at their radiators. Select flow arrangements for increased heat
recovery and determine an optimum water heating scheme. Keep in
mind that the objective is to obtain the highest possible outlet
temperature of the water. City water is to be used with or without a
pump. What is the typical pressure and flow rate delivered by the city
and at what cost?
5. Calculate the cost of the fuel piping system and the cost of the heat
exchangers. Devise a calculation for estimating the savings involved in
the heat recovery system.
8. What typically is the fuel requirement
for a 185-KW diesel engine?
Typical fuel requirement for a 185-kw
diesel engine is an average of 13.38
(gal/hr) at full load. This number was
obtained using the data from engine
spec sheet and the interpolation
technique.
9. What is the power delivered by a 185-
kW engine?
Nominally, the mechanical power delivered by a 185 kW
engine is 185 kW, and is reduced by engine, generator,
and power transformation inefficiencies.
10. Design of manifold and fuel inlet
line to each engine.
In order to feed the four Cumming engines with diesel
from the fuel tank we need to design an intake
manifold. The diesel will be transported though a single
point injection manifold. A pump will be added to feed
the diesel into the engines.
11. It was found by assigning the pipe sizes to each port and
applying the modified Bernoulli that :
Due to the exceptionally low flow rate of the diesel in
the fuel inlet line, frictional effects and inertia forces
are low enough to be neglected.
As a result, volume flowrates in each of the manifold
ports are equal to one another. That is, split between
the four ports, the fuel flowrate of the diesel in each
port is expressed by:
12. Materials and Design
The distance travelled by the diesel fuel from the tank
to the engine is no more than 30 feet, and prospective
problems involving interrupted or inadequate flowrates
due to distance are avoided. The pipes are 4 nominal
schedule 40 galvanized “black iron” (mild steel) tubes,
and are joined together by welding. Black iron was
chosen over copper, zinc, and iron for its resistance to
corrosion in the presence of sulphur particulates in the
diesel fuel, and over stainless steel for its lower cost
and ease of joining by welding.
The main manifold pipe and the four ports will be
joined by three T-intersections and a 90 degree elbow.
The ports are all also 4 nominal schedule 40 galvanized
black iron.
The manifolds for the outlet lines are the same make,
material, and sizing as those for the inlet lines.
13. How much of the energy in the
fuel is used to generate
electricity?
To calculate how much energy in the fuel is used to
generate electricity per engine/generator, the following
formula is used:
That is, 32% of the total Lower Heating Value of the
diesel is used to generate electricity.
14. How much energy is exhausted
from these engines, and at what
temperatures?
This applies to one engine. From reference 3 heat combustion
is 43.4 MJ/kg and the fuel consumption rate is 0.0126 kg/s.
The formula to obtain the energy rejected at the radiator is:
The energy rejected at the radiator is recovered with a water
flow before being sent to the cooling fluid inlet line in the
heat exchanger.
The percentage of heat not used for work and exhausted by
the radiator are taken from typical values found for diesel
engines.
15. What is the typical pressure and
flow rate delivered by the city
and at what cost?
For this case the assumption of the power generating
plan is made to be on Suffolk County Long Island.
According to reference 5 the flow rate is 21ft3/s.
According to the Suffolk County water authority the
average water pressure is 50 psi and the price is $20.73
service charge per quarter .
These values were obtained from the Suffolk County
Water Authority website.
16. Market Research
To analyze the market for existing solutions, three
different models of heat exchanger capable of
accommodating gas to liquid heat transfer were taken
under consideration.
17. Model1
Spirax-Sarco Heat Pipe Heat
Exchanger
The Spirax-Sarco model is our choice for
best design, as it can be retrofitted to a
wide variety of industrial applications,
using a number of materials and both
water and gas as working fluids. It has
consistent documented success in both
cooling processes as well as recovering
energy. It is designed specifically for
hazardous environments, and is easy to
clean and maintain. While Spirax
solutions have been developed for an
aforementioned wide range of
applications, Spirax also vows to engineer
the correct solution for an application
should it not exist. The Spirax Heat Pipe
heat exchanger can also be easily
modified post installation, with the
ability to easily change heat pipe
configurations and add additional
modules as required.
18. Model 2
Bowman Exhaust Gas Heat
Exchanger
The Bowman Exhaust Heat
exchanger is our choice for worst
design, as it is limited to small
industrial applications. While we
were initially intrigued by the
small and simple stature of the
design, ultimately this will not
prove useful in recovering heat in
industrial applications handling
large volumes of exhaust gases.
19. Model 3
Cain Industries Heat
Recovery Heat Exchanger
The HRS Radial waste heat
recovery silencer is a module
configuration packet. It has 176
standard modules available. It
has a full exhaust bypass, full
heating surface access, factory
insulation, hard shell exterior.
This product is designed to
receive the total exhaust liquid
flow from a single source and
control exit temperatures.
20. Modifications
To adapt the chosen heat exchanger to fit the requirements outlined
in our PDS and in the project problem statement, we propose the
following modifications:
A redesigned shell, cylindrical and tall rather than rectangular and
wide, to increase exposure of internal vertical cooling fluid pipes to
exhaust gases and permit the addition of baffles.
The addition of multiple baffles to force the exhaust gases into
repeated crossflow with the cooling fluid tubes and thereby
improving heat transfer.
The addition of a maintenance port in the shell wall, to retain the
original design’s advantage of easy access to internal heat transfer
tubes for cleaning and inspection.
The consolidation of multiple heat transfer “cassettes” into a single
group of cooling tubes to reflect the need for maximized heat
transfer rate and the lesser importance of modularity in our case
scenario.
21. Assumptions
In order to model the whole system and proceed
with calculations it was necessary to base our
calculations on a simple case scenario. To emulate
the simple case scenario the following assumptions
were made.
22. The heat exchanger operates under steady-state conditions.
Heat losses to or from the surroundings are negligible.
There are no thermal energy sources or sinks in the exchanger
walls or fluids.
Over the cross section in counterflow the temperature of the
each fluid is uniform.
For the entire exchanger the wall thermal resistance is
distributed uniformly.
There is no phase change in the fluid streams.
On the walls longitudinal heat conduction in the fluids is
negligible.
The individual and overall heat transfer coefficients are
constant.
23. The thermophysical properties of each fluid can be represented
by averages throughout the length of the heat exchanger.
For large surfaces the overall extended surface efficiency 𝜂 𝑜 is
considered uniform and constant.
The heat transfer surface area A is uniformly distributed on each
fluid side.
For the plate-baffles the temperature rise is considered to be
small compared to the total temperature rise in the exchanger.
The temperature and velocity at the inlet of the heat exchanger
on each fluid side are uniform over the flow cross section.
The working fluid rate is uniformly distributed through the
exchanger on each fluid side in each pass.
Flowrate differences in manifold ports are near zero when
frictional forces are negligible.
24. The individual and overall heat transfer coefficients are
constant.
The specific heat of each fluid is constant throughout the
exchanger.
For large surfaces the overall extended surface efficiency n_o is
considered uniform and constant.
The heat transfer surface area A is uniformly distributed on each
fluid side.
For the plate- baffled the temperature rise is consider to be
small compared to the total temperature rise in the exchanger.
The temperature and velocity at the inlet of the heat exchanger
on each fluid side are uniform over the flow cross section.
The working fluid rate is uniformly distributed through the
exchanger on each fluid side in each pass.
31. Calculating the cost of the fuel
piping system and the cost of
the heat exchanger.
For the Spirax-Sarco heat exchanger there is a lack of
information on pricing schema, and its cost changes
depending on customer needs. The price is given on a
project demand and complexity basis. In this case, we
have taken the price of 30,000 USD as an estimate for
the cost of a standard heat exchanger. The price is
based from comparisons with a similar product, the
CDTM-40 Heat Exchanger manufactured by China
Thermal Heat.
32. Payback Period
Total Project Cost=Heat Exchanger + Materials & Labor
Total Project Cost=30,000+3,711.39= 33,711.39
Years to payback exchanger = Total Project cost /cost
savings
Years to payback exchanger = $33,711.39 purchase
cost /$2,793.57 yearly savings = 12 years
33. References
Janna, William S. Design of Fluid Thermal Systems. 3nd ed. Boston: PWS Pub., 1998.
Print.
American Institute of Chemical Engineers, Effectively Design Shell-and-Tube Heat
Exchanger. Chemical Engineering Progress, February 1998.
"Generator Set Data Sheet." Cummins Power, 1 Jan. 2007. Web. 20 Nov. 2014.
<http://www.cumminspower.com/www/common/templatehtml/technicaldocument/Spec
Sheets/Diesel/na/d-3221.pdf>.
"Approximate Diesel Fuel Consumption Chart." Industrial Diesel Generators: New & Used
Generator Sets – We Buy/Sell. Diesel Service & Supply, 1 Jan. 2013. Web. 20 Nov. 2014.
<http://www.dieselserviceandsupply.com/>.
Water Flow Rate. National Water Authority. Web. 20 Nov. 2014.
<http://www.nwda.gov.in/>.
"SCWA." Water Pressure Facts. Suffolk County Water Authority, 1 Jan. 2013. Web. 20 Nov.
2014. <http://www.scwa.com/search/?keywords=pressure>.
"Heat Exchanger For Power Plant /heat Exchanger Production Bases In China - Buy Heat
Exchanger For Power Plant,First Brand Heat Exchanger For Power Plant Product on
Alibaba.com." Www.alibaba.com. Web. 20 Nov. 2014. <http://www.alibaba.com/product-
detail/china-first-brand-heat-exchanger-for_1337739734.html>.
