ASHRAE Journal - November 2013 - 20

technical feature

Figure 5 Efficiency for twin screw compressor operating with ammonia at part-

Figure 6 Comparative performance of both reciprocating and screw compressors

loads over a range of suction pressures.

operating with ammonia in single stage duty including system pressure
drop effects (adapted from Manske, et al.5).

0.7

Compressor Specific Power

6
5 10 psig Suction
4
3
2
25 psig Suction

1
0

40
50 60
Part Load (%)

Reciprocating vs. Screw Compressors
Manske4 and Manske, et al.,5 conducted a detailed
evaluation and optimization of a multi-temperature
level industrial refrigeration system serving a refrigerated warehouse. The installed system used both screw
and reciprocating compressors. For a particular system
being studied, Manske determined operating sequences
to maximize its operating efficiency, and he generalized his findings to extend their applicability to other
systems.
Manske elected to use "specific power" as a key performance indicator in his analysis where the specific
power is defined as the ratio of total power (of system or
compressor) to the capacity where each are expressed in
consistent units to yield a dimensionless quantity. The
specific power is the inverse of the more commonly used
20

ASHRAE JouRnAl

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N ovem ber 2013

Single Screw

0.6

Reciprocating
0.5
-15°F
0.4
-5°F
5°F

100

higher nominal operating efficiency), technology (reciprocating or screw), and load characteristics. In a system
that has a mix of screw and reciprocating compressors, is
it better to base-load with the reciprocating compressors and trim
with the screw compressors? Is it better to run smaller compressors
at full-load and trim with larger compressors? Is it better to equalize
run hours on compressors regardless of their part-load ratio?
These are just a few of the questions that arise when
considering the best approach for sequencing and control of compressors in an industrial refrigeration system.
Let's address some of the major factors that influence
the operating efficiency of compressors and try to generalize some practices for sequencing their operation to
yield efficient system performance.

Low Pressure
Receiver
Temperature

0.3

Refrigerant: Ammonia (R-717)

0.2
0.0

Saturated Discharge Temperature = 85°F
0.1 0.2 0.3

0.4 0.5 0.6 0.7 0.8 0.9
Compressor Part Load Ratio

1.0

Figure 7 Performance comparison for equally sized ammonia screw compressors

including system effects (adapted from Manske, et al.5).

0.40
Aggregate Compressor Specific Power

Efficiency (kWe/ton)

150 psig Discharge

0 psig Suction

Equal Sized Compressors
Saturated Discharge Temperature = 85°F
Low Pressure Receiver Temperature = -5°F

0.39

Crossover Point

0.38

Equal Compressor Loading

0.37
0.36

One Comp. Only

0.35
0.34
0

One Comp. at 100% Load
10

40
50 60
Part Load (%)

100

efficiency measure known as the coefficient of performance (COP).
Specific Power = Power
RE
where Power is the system or compressor power (Btu/h)
and RE is the total refrigeration effect (Btu/h).
Figure 6 shows the comparative performance of an
ammonia screw compressor and an ammonia reciprocating compressor at part-load conditions over a range
of saturated suction temperatures for a fixed saturated discharge temperature where both compressors
include the effects of system piping pressure drop. From
the results shown, the screw compressor has slightly
improved performance compared to the reciprocating
compressor at full-load conditions.

ASHRAE Journal - November 2013

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