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26 TRG-TRC010-EN
notes
period three
Compressor Capacity Control
As the load and the corresponding refrigerant flow rate continue to fall, the
radial velocity (force) drops, too. At some point, the radial force becomes
smaller than the generated static pressure, letting the pressurized refrigerant
vapor flow backward from the diffuser passages into the impeller. This
instantaneously reduces the pressure within the passages below the radial
force and the compressor is able to re-establish the proper direction of
refrigerant flow.
This condition is known as surge. So long as this unstable load condition
exists, the refrigerant alternately flows backward and forward through the
compressor impeller, generating noise and vibration.
These curves represent the performance of a typical 2-stage compressor over a
range of inlet vane positions. The pressure difference between the compressor
inlet (evaporator) and outlet (condenser) is on the vertical axis and the
refrigerant flow rate is on the horizontal axis. The dashed line represents the
V
V
r
r
V
V
t
t
Surge
V
V
r
r
< static pressure
< static pressure
R
R
Figure 37
Compressor Map
25
25
75
75
10
10
14
14
63
63
pressure difference
pressure difference
refrigerant flow rate
refrigerant flow rate
s
u
r
g
e
s
u
r
g
e
vane position
vane position
(degrees)
(degrees)
90
90
36
36
51
51
Figure 38
trgtrc010_book.book Page 26 Wednesday, September 1, 1999 10:39 AM