ASHRAE Journal - November 2013 - 18

technical feature

Figure 4 Part-load efficiency for twin screw compressor with slide valve and VFD/

Figure 3 Relationship between capacity and speed for a twin screw compressor.

slide valve.

100
6.0

Compressor Only

5.5

Twin Screw Compressor

70
60

4.5

Slide Valve Control

4.0
3.5
3.0
2.5

2.0
0

40
60
Percent Compressor Speed

100

when we consider the part-load efficiency of screw compressors and guidance for maintaining efficient operation at part-load conditions.
Because each rotation of the screw's rotor translates to
a defined volume of gas being compressed and expelled
out the discharge port, changing the shaft speed translates to a direct proportional change of gas flow through
the compressor. Figure 3 shows the capacity of a typical
twin screw compressor directly varying as a function of
shaft speed. Screw compressors have a limited range of
capacity modulation using speed control based on their
design. Most compressor designs will accommodate
turndown to 50% of full speed, but some compressor
designs can accommodate lower speeds (as low as 20%).
In cases where a compressor retrofit from fixed speed
to variable speed is considered, it is essential to consult
with the manufacturer to establish the minimum speed
for a given machine. If additional capacity modulation
on a VFD-equipped machine is required, the slide valve
can be used in conjunction with speed control. Efficient
operating strategies for this tandem approach for capacity control is discussed later in this article.
Figure 4 shows the efficiency of a smaller capacity twin
screw compressor in single-stage duty equipped with
slide-valve as the only form of capacity control and
the comparative efficiency of the same machine VFDequipped. As noted in the figure, the results are for the
compressor-only and do not factor in the mechanicalelectrical conversion efficiency associated with the
electric motor or variable speed drive and system effects
such as pressure drop in piping are omitted.
18

Saturated Suction Temperature: -20°F
Saturated Discharge Temperature: 90°F

5.0

Efficiency (hp/ton)

Percent of Full Load Capacity

ASHRAE JouRnAl

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N ovem ber 2013

VFD and Slide Valve Control
0

Refrigerant: Ammonia (R-717)

40
60
Capacity (Percent of Full Load)

100

In both cases, the best efficiency point (lowest hp/ton)
is at full load. For the screw compressor equipped with a
slide valve for capacity control, the efficiency decreases
as the machine unloads. The specific character of
degraded performance will vary depending on the size
of the screw compressor, its design details, and operating suction/discharge pressures but all screw compressors exhibit this general trend.
The part-load penalty shown in Figure 4 is for a compressor operating in a relatively high compression
ratio situation. For lower compression ratio operation,
the efficiency penalty at lower part load is not as dramatic. When equipped with a variable speed drive, the
degradation of the compressor's operating efficiency
can be held in check until low part-load conditions
are reached at which time the performance begins to
rapidly decrease with further capacity reduction. The
lower hp/ton for the variable speed case might suggest
that it would be advantageous to equip all compressors
on an operating suction pressure level with a variable
speed drive, but that is certainly not the case as will be
discussed.
So far we have ignored other factors that affect the
operating efficiency of a compressor once integrated
into the system. As the load on a compressor decreases,
the gas flow into and out of the compressor will
decrease, resulting in lower suction and discharge line
pressure losses. Lower suction and discharge line pressure loses will help compressor performance.
Another factor ignored so far is the efficiency of
the conversion process from primary energy in the

ASHRAE Journal - November 2013

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