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8. ENERGY PERFORMANCE ASSESSMENT OF
COMPRESSORS
8.1

Introduction
The compressed air system is not only an energy intensive utility but also one of the least energy efficient. Over a period of time, both performance of compressors and compressed air system reduces drastically. The causes are many such as poor maintenance, wear and tear etc. All these lead to additional compressors installations leading to more inefficiencies. A periodic performance assessment is essential to minimize the cost of compressed air.

8.2

Purpose of the Performance Test
To find out:

Actual Free Air Delivery (FAD) of the compressor

Isothermal power required

Volumetric efficiency

Specific power requirement

The actual performance of the plant is to be compared with design / standard values for assessing the plant energy efficiency.

8.3

Performance Terms and Definitions
Compression ratio

: Absolute discharge pressure of last stage
Absolute intake pressure

Isothermal Power

: It is the least power required to compress the air assuming isothermal conditions.

Isothermal Efficiency

: The ratio of Isothermal power to shaft power

Volumetric efficiency

: The ratio of Free air delivered to compressor swept volume Specific power requirement: The ratio of power consumption (in kW ) to the volume delivered at ambient conditions.

8.4

Field Testing

8.4.1

Measurement of Free Air Delivery (FAD) by Nozzle method
Principle: If specially shaped nozzle discharge air to the atmosphere from a receiver getting its supply from a compressor, sonic flow conditions sets in at the nozzle throat

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8. Energy Performance Assessment of Compressors

for a particular ratio of upstream pressure (receiver) to the downstream pressure
(atmospheric) i.e. Mach number equals one.
When the pressure in the receiver is kept constant for a reasonable intervals of time, the airflow output of the compressor is equal to that of the nozzle and can be calculated from the known characteristic of the nozzle.
8.4.2

Arrangement of test equipment
The arrangement of test equipment and measuring device shall confirm to Figure 8.1.

8.4.3

Nozzle Sizes

The following sizes of nozzles are recommended for the range of capacities indicated below:
Capacity (m3/hr)
3–9
9 –30
27 –90
60 – 170
130 – 375
300 – 450
750 – 2000
1800 – 5500
3500 - 10000

Nozzle size (mm)
6
10
16
22
33
50
80
125
165

Flow Nozzle: Flow nozzle with profile as desired in IS 10431:1994 and dimensions
8.4.4 Measurements and duration of the test.
The compressor is started with the air from the receiver discharging to the atmosphere through the flow nozzle. It should be ensured that the pressure drop through the throttle valve should be equal to or twice the pressure beyond the throttle. After the system is stabilized the following measurements are carried out:

Pressure and temperature before the nozzle
Pressure drop across the nozzle
Speed of the compressor kW, kWh and amps drawn by the compressor

The above readings are taken for the 40%, 60%, 100% and 110% of discharge pressure values. •

Measuring instruments required for test
Thermometers or Thermocouple
Pressure gauges or Manometers
Differential pressure gauges or Manometers
Standard Nozzle

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8. Energy Performance Assessment of Compressors

Psychrometer
Tachometer/stroboscope
Electrical demand analyser

P1

T1

P2

FILTER

AIR COMPRESSOR

P3

P4

T3
THROTTLE
VALVE

Nozzle
P3 – P4

DISCHARGE TO
ATMOSPHERE

FLOW
STRAIGHTENER

BY-PASS

Figure 8.1: Test Arrangement for Measurement of Compressed Air Flow

8.5 Calculation Procedure for Nozzle Method
I.

1/ 2

T ⎛ 2( P3 − P4 )( P3 x Ra ) ⎞

Free air delivered , Q f (m / sec) = k x x d x 1 x ⎜

4
P ⎜
T3
1 ⎝

s

k d T1
P1
P3
T3
Ra
P3-P4

:
:
:
:

:
:
:
:

π

2

Flow coefficient – as per IS
Nozzle diameter M
Absolute inlet temperature oK
Absolute inlet pressure kg/cm2
Absolute Pressure before nozzle kg/cm2
Absolute temperature before nozzle oK
Gas constant for air 287.1 J/kg k
Differential pressure across the nozzle kg/cm2

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8. Energy Performance Assessment of Compressors

II.

Isothermal Efficiency

=

Isothermal power (kW ) =
P1
Qf r Isothermal power / Input power

P x Q f x log e r
1
36.7
Absolute intake pressure kg/ cm2
Free air delivered m3/hr.
Pressure ratio P2/P1

=
=
=

III. Specific power consumption at rated disch arg e pressure =

IV. Volumetric efficiency =

Power consumption, kW
Free air delivered , m3 / hr

Free air delivered in m3 / min x 100
Compressor displacement in m3 / min

Compressor displacement =

π
4

x D2 x L x S x χ x n

D
L
S χ =
=
=
=

n

Cylinder bore, metre
Cylinder stroke, metre
Compressor speed rpm
1 for single acting and
2 for double acting cylinders
No. of cylinders

=

8.6 Example
Calculation of Isothermal Efficiency for a Reciprocating Air Compressor.
Step – 1 : Calculate Volumetric Flow Rate k : Flow coefficient (Assumed as 1) d : Nozzle diameter : 0.08 metre
: Receiver Pressure - 3.5 kg / cm2 (a)
P2
: Inlet Pressure - 1.04 kg / cm2(a)
P1
T1
: Inlet air temperature 30oC or 303oK
: Pressure before nozzle – 1.08 kg / cm2
P3
T3
: Temperature before the nozzle 40oC or 313oK
P3 – P4 : Pressure drop across the nozzle = 0.036 kg / cm2
Ra
: Gas constant
: 287 Joules / kg K
1/ 2

T ⎛ 2( P3 − P4 )( P3 x Ra ) ⎞

Free air delivered , Q f (m / sec) = k x x d x 1 x ⎜

4
P ⎜
T3
1 ⎝

s

Free air delivered , Q f (m s / sec) =1 x

π

π
4

2

1/ 2

x (0.08) 2 x

= 0.391 m3/sec
= 1407.6 m3 / h.

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303 ⎛ 2 x 0.036 x1.08 x 287 ⎞ x⎜ ⎟
1.04 ⎝
313

8. Energy Performance Assessment of Compressors

Step – 2 : Calculate Isothermal Power Requirement

Isothermal power (kW ) =

P x Q f x log e r
1
36.7

P1 - Absolute intake pressure

= 1.04 kg / cm2 (a)

Qf -Free Air Delivered

= 1407.6 m3 / h.

Compression ratio, r =

3.51
= 3.36
1.04

Isothermal power (kW ) =

1.04 x1407.6 x log e 3.36
= 48.34 kW
36.7

Step – 3 : Calculate Isothermal Efficiency

Motor input power
Motor and drive efficiency
Compressor input power

= 100 kW
= 86 %
= 86 kW

Isothermal efficiency

= Isothermal Power x 100
Compressor input Power
= 48.34 x 100 = 56%
86.0

8.7 Assessment of Specific Power requirement
Specific power consumption = Actual power consumed by the compressor
Measured Free Air Delivery
In the above example the measured flow is 1407.6 m3/hr and actual power consumption is
100 kW.
Specific power requirement =

100
1407.6

= 0.071 kW/m3/hr

8.8 Measurement of FAD by Pump Up Method
(Note: The following section is a repeat of material provided in the chapter-3 on Compressed
Air System in Book-3.)
Another way of determining the Free Air Delivery of the compressor is by Pump Up Method
- also known as receiver filling method. Although this is less accurate, this can be adopted where the elaborate nozzle method is difficult to be deployed.

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8. Energy Performance Assessment of Compressors

Simple method of Capacity Assessment in Shop floor

Isolate the compressor along with its individual receiver being taken for test from main compressed air system by tightly closing the isolation valve or blanking it, thus closing the receiver outlet.
Open water drain valve and drain out water fully and empty the receiver and the pipeline. Make sure that water trap line is tightly closed once again to start the test. Start the compressor and activate the stopwatch.
Note the time taken to attain the normal operational pressure P2 (in the receiver) from initial pressure P1.
Calculate the capacity as per the formulae given below:
Actual Free air discharge

Q =

P2 − P1 V
× Nm3 / Minute
P0
T

Where
P2
P1
P0
V

=
=
=
=

T

=

Final pressure after filling (kg/cm2 a)
Initial pressure (kg/cm2a) after bleeding
Atmospheric Pressure (kg/cm2 a)
Storage volume in m3 which includes receiver, after cooler, and delivery piping
Time take to build up pressure to P2 in minutes

The above equation is relevant where the compressed air temperature is same as the ambient air temperature, i.e., perfect isothermal compression. In case the actual compressed air temperature at discharge, say t20C is higher than ambient air temperature say t10C (as is usual case), the FAD is to be corrected by a factor (273 + t1) / (273 + t2).
EXAMPLE

An instrument air compressor capacity test gave the following results (assume the final compressed air temperature is same as the ambient temperature) – Comment?
Piston displacement
:
16.88 m3/minute
Theoretical compressor capacity
:
14.75 m3/minute @ 7 kg/cm2
Compressor rated rpm 750
:
Motor rated rpm : 1445
:
7.79 m3
i.e., pipe / water cooler, etc., is
:
0.4974 m3
Total volume
:
8.322 m3
Initial pressure P1
:
0.5 kg/cm2
:
7.03 kg/cm2
Final pressure P2

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8. Energy Performance Assessment of Compressors

Atmospheric pressure P0

:

Compressor output m3/minute

:

1.026 kg/cm2,a
(P2 − P1 ) × Total Volume
Atm. Pressure × Pumpup time

(7.03 − 0.5) × 8.322

= 13.17 m3/minute
1.026 × 4.021
Capacity shortfall with respect to 14.75 m3/minute rating is 1.577 m3/minute i.e.,
10.69 %, which indicates compressor performance needs to be investigated further.
:

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8. Energy Performance Assessment of Compressors

QUESTIONS

1)

What is meant by Free Air Delivery?

2)

Describe the method of estimating flow by nozzle method.

3)

Describe the method of estimating flow by pump up method.

4)

Define the term isothermal efficiency and explain its significance.

5)

Define the term volumetric efficiency and explain its significance.

6)

How is specific power requirement calculated?
REFERENCES

1.

IS 10431:1994: Measurement of airflow of compressors and exhausters by nozzles. 2.

IS 5456:1985 code of practice for testing of positive displacement type air compressors and exhausters

3.

Compressor performance – Aerodynamics for the user by M Theodore GreshButterworth Heinemann.

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