The Preliminary Choice of Fan or Compressor

GBH Enterprises, Ltd.

Engineering Design Guide:
GBHE-EDG-MAC-1030

The Preliminary Choice of Fan or
Compressor

Information contained in this publication or as otherwise supplied to Users is
believed to be accurate and correct at time of going to press, and is given in
good faith, but it is for the User to satisfy itself of the suitability of the information
for its own particular purpose. GBHE gives no warranty as to the fitness of this
information for any particular purpose and any implied warranty or condition
(statutory or otherwise) is excluded except to the extent that exclusion is
prevented by law. GBHE accepts no liability resulting from reliance on this
information. Freedom under Patent, Copyright and Designs cannot be assumed.

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Engineering Design Guide:

The Preliminary Choice
Of Fan or Compressor
Type

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

METHOD FOR PRELIMINARY SELECTION
OF COMPRESSOR

3

PROCESS DATA SHEET

3

5

5.1
5.2
5.3

6

Essential Data for the Completion of a
Process Data Sheet
Gas Properties
Discharge Requirements

4
4
5

PRELIMINARY CHOICE OF FAN AND
COMPRESSOR TYPE

5

6.1 Essential Data for Preliminary Selection

5


7

FAN AND COMPRESSOR APPLICATIONS

6

7.1

Fans

6

7.2

Centrifugal Compressors

6

7.3

Axial Compressors

6

7.4

Reciprocating Compressors

8

7.5

Screw Compressors

8

7.6

Positive Displacement Blowers

9

7.7

Sliding Vane Compressors

9

7.8

Liquid Ring Compressors

9

8

PROVISION OF INSTALLED SPARES

9

9

PRELIMINARY ESTIMATE OF COSTS

11


APPENDICES
A

B

DETAILS OF OPERATING LIMITS FOR FANS
AND COMPRESSORS

13

DETAILS OF PROCESS DATA SHEET FOR
COMPRESSOR

15

FIGURES
1 SELECTION SEQUENCE FOR FAN OR
COMPRESSOR TYPE
2 GRAPH SHOWING OPERATING LIMITS OF FANS
AND COMPRESSORS
3

4

4

13

PROCESS DATA SHEET FOR COMPRESSOR
- SHEET 1 OF 2

15

PROCESS DATA SHEET FOR COMPRESSOR
- SHEET 2 OF 2

16

TABLES
1 COMPRESSOR/FAN OPERATING LIMITS

7

2 LIKELY INSTALLED SPARES BASED ON FAN AND
COMPRESSOR TYPE AND DUTY

10

DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE


0

INTRODUCTION/PURPOSE

This GBHE Engineering Guide has been prepared to assist in the selection of
Fan or Compressor types and to prepare information for an enquiry to be issued
to a supplier.

1

SCOPE

This GBHE Engineering Guide defines the way in which the type of fan or
Compressor, and the number of units required for a particular duty, may be
determined from knowledge of the process parameters.
This Guide shall be read in conjunction with the relevant engineering and
purchase specification for the specified fan or compressor type.

2

FIELD OF APPLICATION

This GBHE Engineering Guide is intended to be used by personnel responsible
for the selection of rotating equipment for chemical process plant.

3

DEFINITIONS

For the purposes of this Engineering Guide, the following definitions apply:
There are no terms which are specific to the subject which require definition.
With the exceptions of terms used as proper nouns or titles, those terms with
initial capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.

4

METHOD FOR PRELIMINARY SELECTION OF COMPRESSOR

Having obtained the process data, the first step in the selection process is the
preliminary choice of fan or compressor type. The choice is constrained by the
operating limits which are a characteristic of each machine type. Further
selection constraints arise from considerations of the plant operation such as
desired turndown and freedom from contamination of the product by the machine
lubricating oil or corrosion debris from the materials of construction, as well as
the required availability of the machine.

Once the preliminary choice of fan or compressor type has been made, refer to
either Clause 6, 7 or 8 for more details of fan and compressor applications.
The sequence for preliminary selection is shown in Figure 1.

5

PROCESS DATA SHEET

The purpose of the process data sheet is to describe the normal duty, including
all operating cases, all possible combinations of gases, pressures, flow and
temperatures. It should not, at this stage, contain any margins on the flowrate or
pressure.
A typical 'Process Data Sheet' is shown in Appendix B.
FIGURE 1: SELECTION SEQUENCE FOR FAN OR COMPRESSOR TYPE


5.1

Essential Data for the Completion of a Process Data Sheet
Specify the normal process duty in the following terms:
Mass flows

-

kg/hour

Gas composition

-

Molar percentage

Gas pressures

-

bars absolute

Temperatures

-

°C

Actual volume flow -

5.2

m3/hour.

Gas Properties

The following gas properties are to be given for the inlet to each stage:
(A)

Gas compressibility factor ‘Z'.

(B)

Adiabatic exponent ‘k'.

(C)

Minimum and maximum gas molecular weight attainable in any
process operating condition.

(D)

Corrosive constituents by quantity and type.

(F)

Particle solids content by quantity, particle size and particle
hardness.

5.3

Discharge Requirements

Specify whether oil free discharge is required.


6 PRELIMINARY CHOICE OF FAN AND COMPRESSOR TYPE
Operating limits for fans and compressors are summarized in Table 1. Figure
2 shows a fan and compressor selection chart which shows the operating
ranges and areas of application.
Calculate the essential data required for preliminary selection from 6.1 and
use Table 1 and Figure 2 for making the preliminary choice. It may be
necessary to refer to Clause 7 which contains application guidance for
various types of fan and compressor.

6.1

Essential Data for Preliminary Selection

The following essential data will be required to make the preliminary selection
from Table 2 and Figure 2.
(a)

Pressure
Inlet pressure
Discharge pressure

bars absolute =

Pressure differential

(b)

bars absolute =

bars =

Volume flowrate

The actual volume flowrate Q1 at the inlet can be calculated from the
expression:


(c)

Power


7

FAN AND COMPRESSOR APPLICATIONS

This clause provides guidance on the application of the various types of fans and
compressors available. The information should be read in conjunction with Table
1.
7.1

Fans

Centrifugal and axial fans are suitable for low pressure applications over a wide
range of flowrates. Special designs are available for high temperature duties.
Although fans are normally specified with a modest aerodynamic duty, care
needs to be taken with material selection and fan fabrication standards. Fans
which handle arduous duties (e.g. high tip speeds or contaminated fluids) will
need careful design specification.
Care should be taken with the selection of fans where a wide range of gas
densities is to handled. For example, extreme atmospheric variations will have a
significant effect on the fan absorbed power and driver power rating. To minimize
fan costs the selection may be made for normal ambient conditions, accepting
that in the occasional extreme condition the fan may fail to meet plant
requirements.

7.2

Centrifugal Compressors

For efficiency reasons centrifugal compressors should only be specified if the
discharge volume flow is greater than 100 actual m3/hour. This limitation does
not apply to some special compressor designs which operate with close running
clearances.
As for fans care should be taken with the selection of centrifugal compressors
where a wide range of gas density is to be handled, particularly when driven by a
fixed speed driver.
The discharge temperature should not normally exceed 200°C without specific
analysis of the compressor design and material selection. However, special
compressors with a discharge temperature of 400°C are commercially available.
For some high alloy compressor casings the maximum differential temperature
between inlet and discharge may be limited to 120°C.


A suction catchpot may be required to collect liquid droplets. Although centrifugal
compressors are generally tolerant to atomized liquid carryover, liquid entrained
with the gas can cause erosion and other severe damage.

7.3

Axial Compressors

Axial compressors are generally used for high flows and low discharge pressure
applications, achieving high compression efficiencies. They should only be used
where the process gas is clean and dry.
As with fans and centrifugal compressors they should be avoided where a wide
range of gas density is to be handled.


TABLE 1:

COMPRESSOR/FAN OPERATING LIMITS


7.4

Reciprocating Compressors

Reciprocating compressors are well suited to applications involving low flow rates
or high pressure ratios. They can accommodate a relatively wide range of gas
compositions, pressure ratios and flow rates, whilst maintaining a good
efficiency. Turndown in rate can be achieved by either individual valve lifters, or
by bypassing the stage or compressor. The latter is less efficient.
A disadvantage of the reciprocating compressor is the requirement for routine
maintenance which, in many cases, necessitates an installed spare. This
requirement should be considered when comparing costs with other compressor
types.
Oil lubricated reciprocating machines may subject the process to oil
contamination. The sensitivity of the process to oil contamination needs to be
checked. An important safety issue is the build up of carbonaceous deposits in
the compressor discharge system. This can lead to self ignition and fire or
explosion. Preventative measures include selecting the correct type of
oil, regular cleaning and temperature trip protection.
Reciprocating compressors are not ideally suited to dirty or polymerizing gases,
and cannot tolerate liquid droplets in the suction flow without causing erosion
damage.

7.5

Screw Compressors

A screw compressor is likely to have a higher initial cost than a reciprocating
compressor for the equivalent duty. However, screw machines offer high
efficiency, high availability, moderate operating costs, and do not normally
require an installed spare.
Screw compressors should be considered when the gas composition and
pressure ratio is likely to vary.
Dirty and polymerizing gases will require careful specification and specific
applications should be discussed with a manufacturer.


7.5.1 Oil-free Screw Compressors
Oil free screw compressors are ideal for a wide variety of gases where the duty
requires low pressure ratios at constant volume flow. Typical gases include:
Methane, Ethylene/Ethane, Propylene/Propane, Acetylene, Butadiene, Cokeoven gas, Lime kiln gas, Steam (typically for vapor recompression cycle).
They should be considered when liquid is entrained with the gas.
The capacity turndown is inefficient.
Oil free screw machines are relatively inflexible and will not tolerate excessive
pressure differentials or discharge temperatures.
Noise levels will be high [typically 95-105 dB(A) at 1 meter].
They are frequently used in vacuum applications with suction pressures as low
as 0.1 bar absolute.

7.5.2 Oil Injected Screw Compressors
Turndown capacity is stepless, hence are more flexible and with higher efficiency
than the oil free types.
Noise levels are lower than the oil free machines [typically 85-95 dB(A) at 1
meter].
They should not be used where oil carry-over cannot be tolerated in the process
(typically 5 ppm), or where oil/gas compatibility
They may be used on applications for low molecular weight gases, most
hydrocarbon gases, refrigerants and corrosive gases.


7.6

Positive Displacement Blowers

Positive displacement blowers are a less efficient method of compression and
hence normally limited to low power, low pressure applications e.g. pneumatic
conveying of particulate material.
When fitted with shaft seals they are suitable for a variety of clean and dry gases.
Turndown is inefficient, generally requiring bypass or blow off arrangements
Noise levels can be high [typically 90-100 dB(A) at 1 meter].
They can operate as a blower working between atmospheric pressure and
above, or as an exhauster working from below atmospheric inlet to atmospheric
pressure.

7.7

Sliding Vane Compressors

Vane compressors are limited to low power, low pressure applications. They
generally operate at low speed and therefore noise levels are usually low.
Where vanes are oil lubricated, oil carryover into the process needs to be
considered.
For continuous duties reliability may be affected by vane wear.

7.8

Liquid Ring Compressors

Liquid ring compressors are generally limited to low power, low pressure
applications. The cooling effect of the sealing liquid means that discharge
temperatures are low, and in some applications discharge aftercoolers can be
eliminated. Since oil is not used to lubricate the compressor internally, the
discharge process gas is oil free.
Liquid ring compressors are suitable for dirty gas duties, however it is important
to ensure that the process gas is compatible with the circulating liquid.


8

PROVISION OF INSTALLED SPARES

Whether to install a spare machine is a key decision in the selection process, not
only because of the effect on the total cost, but also because the choice of
multiple machines may allow several smaller machines to be used. Three factors
will influence this decision.
(a)

Reliability
Rotating machinery is intrinsically more reliable than reciprocating
machinery. For example, centrifugal compressors can be specified for
continuous operation for periods of up to 3 years. Therefore, a single
unspared centrifugal machine can be used, whereas the same duty may
require at least two reciprocating machines.

(b)

Availability requirements
In considering the likely availability of the plant to produce its final product
rather than the reliability of the compressor installation, repair time is a
significant factor. The definition of availability is:

This effect of repair time on availability can either be eliminated by having
installed spare machines, or mitigated by carrying major items of spares.
(c)

Part-load performance
The required part-load performance of the plant shall be considered, and
for an installation with more than one machine, a machine failure may
allow the plant to continue in operation with reduced output.

As a preliminary guide to the provision of installed spares, Table 2 should be
used for estimating the number and size of installed machines.


TABLE 2:

LIKELY INSTALLED SPARES BASED ON FAN AND
COMPRESSOR TYPE AND DUTY


9

PRELIMINARY ESTIMATE OF COSTS

The total installed cost of the machine will be affected by many of the factors
listed below. This list is not intended to be a detailed list of all cost factors, but
only those which are likely to influence the preliminary choice of compressor
type.
(a)

Equipment costs
(1)
(2)

Gearbox between machine and driver.

(3)

Testing in manufacturers works.

(4)

Pulsation dampers, separators, intercoolers.

(5)

Instrumentation and control system.

(6)

Inlet filtration.

(7)

Lubrication and seal systems.

(8)

Driver.

(9)

(b)

Driven machine.

Baseplate.

Installation costs
(1)

Foundations.

(2)

Area classification.
standard packaged machines are not suitable for installation in
some areas of process plant.

(3)

Noise control.


(c)

Operating costs
(1)

Efficiency at duty points.

(2)

Efficiency at turndown.

(3)

Maintenance requirements.

(4)

Major spares

Provided that an acceptable estimate of costs has been obtained, the preliminary
choice of fan or compressor can be confirmed. Then refer to the appropriate
GBHE Engineering Design Guide to specify the principle characteristics of the
chosen type.
An unacceptable estimate of costs will require a recycle to Clause 6 to establish
other feasible options, or to Clause 5 to revise the proposed process duty.


APPENDIX A: DETAILS OF OPERATING LIMITS FOR FANS AND COMPRESSORS
FIGURE 2: GRAPH SHOWING OPERATING LIMITS OF FANS AND COMPRESSORS (These are nominal limits check with individual manufacturer limits)


APPENDIX B: DETAILS OF PROCESS DATA SHEET FOR COMPRESSOR
FIGURE 3: PROCESS DATA SHEET FOR COMPRESSOR - SHEET 1 OF 2 –


APPENDIX B: CONTINUED
FIGURE 4: PROCESS DATA SHEET FOR COMPRESSOR - SHEET 2 OF 2


DOCUMENTS REFERRED TO IN THIS GBHE ENGINEERING GUIDE
This GBHE Engineering Guide makes reference to the following documents:

GBHE ENGINEERING GUIDES

GBH Enterprises

Glossary of Engineering Terms (referred to in Clause 3)

GBHE ENGINEERING PROCESS DATA SHEETS
COMPRESSOR Sheet 1 of 2 (referred to in Appendix B)
COMPRESSOR Sheet 2 of 2 (referred to in Appendix B)


The Preliminary Choice of Fan or Compressor

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