Cold Link Africa VOL. 01 - No.01 | September / October 2015 | Seite 37

TALKING TECHNICALLY
INCORPORATING COLD CHAIN

Number of
cylinders in
compressor

Number
of
controlled
cylinders

2

Capacity, % of full load capacity
100

87½

83⅓

75

66⅔

62½

50

37½

33⅓

25

Power requirements, % of full load
100

90

86

80

74

71

60

50

45

38

1

1

-

-

-

-

-

-

-

-

-

3

1

3

-

-

-

2

-

-

-

-

-

4

3

4

-

-

3

-

-

2

-

-

1

6

4

6

-

4

-

4

-

3

-

2

-

8

6

8

7

-

-

-

5

-

3

-

2

12

8

12

-

10

-

8

-

6

-

4

-

Capacity control by cylinder unloading:
compressor capacity and power
requirements.

causes a reduction in crankcase pressure
whilst maintaining the evaporator
pressure close to design value. As the
crankcase or suction pressure decreases,
the density of the vapour entering the
compressor decreases. The refrigerant
mass flow is reduced giving the desired
result of lowering compressor capacity.

This method gives good control but is
inefficient. It is, in fact, used more for
evaporator pressure control to maintain a
constant evaporating temperature.

Cylinder unloading.

Multiple compressors
This provides a very good capacity control
and is extensively used. A system with two
compressors can run at 100% and 50%
capacity, a system with three compressors
can go down to 33,33%, etc. Very often
one of the compressors is fitted with
cylinder unloading which provides even
more steps of capacity control.
The added advantage of the
multiple compressor system is the
‘standby’ capacity provided. If one
of the compressors breaks down, the
system can still operate on part load.
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