ASHRAE Journal - November 2013 - 24

technical feature

COMPREHENSIVE COVERAGE OF
DISTRICT HEATING AND COOLING
SYSTEM DESIGN

Figure 9 Booster compressor (ammonia) performance during fixed speed and vari-

able speed operation.

110
100

Compressor Power (kWe/ton)

second compressor trimming or unloading both compressors to maintain equal part-load ratios.
Figure 7 (Page 20) shows a performance map for both of
these operating scenarios. The left-most curve illustrates
the compressor specific power for a screw compressor operating. The most efficient operating point is at
full load (which coincides with a system load of 50%).
Whenever the system part-load exceeds 50%, two compressors are needed. The two curves show the specific
power for base-loading one machine and trimming with
the second as well as equal part-load operation for both
compressors.
At high system loads, the most efficient operation is
attained by base-loading one of the compressors and
trimming with the second compressor. However, there
comes a point at which the combined specific power
for both compressors will increase above that of equal
compressor part-load operation. This "cross-over point"
occurs at a system part-load ratio of approximately 65%,
which corresponds to a part-load operation at 30%
capacity for the trim machine.

Compressor C-2
Fixed Vi = 2.6

Fixed Speed

80
70
60

Variable Speed

50
40
30
20
10
0.0

0.1

0.2

0.3

0.4 0.5 0.6
Part Load Ratio

0.7

0.8

0.9

1.0

Referring back to the results shown in Figure 5 for the 10
psig (69 kPa) suction pressure case, it becomes clear that
the precipitous decrease in part-load efficiency below
the 30% of full-load capacity is responsible for diminishing the performance of the entire system. As the system
load decreases further, it is more efficient to transition
to an equal part-load operation for each compressor
until the 50% system load operating point is reached, at
which point one machine would be shut down.
Keep in mind, this is one example for specific compressors and their associated operating performance
in a relatively high compression ratio application. The
decrease in screw compressor efficiency at part load
is less severe in lower compression ratio applications.
Because of the diversity of operating conditions found in
industrial refrigeration applications, each specific situation warrants its own evaluation.

Fixed Speed vs. Variable Speed Screw Compressors

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24

ASHRAE JouRnAl

ashrae.org

N ovem ber 2013

As the capital cost for variable frequency drives continues to decrease, engineers are increasingly applying this
technology throughout industrial refrigeration systems
including: condenser fans, evaporator fans, and compressors. Figure 4 showed the comparative energy efficiency for both a fixed speed and variable speed screw
compressor, but the results did not include inefficiencies associated with the motor or variable speed drive or
the effects of suction/discharge line pressure drops during operation.
To evaluate these effects, field tests were conducted
on a twin screw compressor operating in booster duty
shown in Figure 8a. In a somewhat unique situation,

ASHRAE Journal - November 2013

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