Trane CVGF - Manual (Page 9)

9 CTV-PRC001-GB
Two-Stage Compressor
Widens the Application
Range
Why Centrifugal Compressors Surge
Centrifugal compressors produce their
pressure differential (head) by
converting the kinetic energy of the gas
leaving the impeller into static
pressure. The velocity of this gas is the
result of two components:
• The radial velocity component V
r
,
which is directly proportional to the
refrigerant gas flow Q.
• The tangential velocity component V
t
,
which is a function of both impeller
diameter D and the rotational speed
rpm.
The length of the resultant vector V is
proportional to the kinetic energy
available for conversion to static
pressure in the volute. Consequently,
for a given compressor, V
t
is fixed and
V
r
varies with the cooling load. With the
chiller unloading, the pressure
differential between evaporator and
condenser decreases. The compressor
matches the new load and the lower
“head” by closing the inlet guide
vanes.
This reduces the gas flow it draws in
and modifies its direction. Component
Vr decreases accordingly, the vector
diagram shifts and at some point, the
balance of forces breaks down.
As pressurized gas rushes backwards
through the impeller, the pressure in
the gas passages falls, allowing the
compressor to restore the balance of
forces. If the process repeats itself, the
compressor is said to surge.
1 - V
r
= f (Q)
2 - V
t
= f (D, RPM)
3 - V = Resultant
4 - RPM
5 - D
6 - Q
Two-Stage Compressors Surge Less
and Later
To produce the same head as a single-
stage compressor, two-stage machines
use two small diameter impellers.
Component V
t
is the same as on each
stage, though V
r
is the same as on a
single-stage compressor. This results in
a better balance of forces at low loads
and produces a machine with a wider
unloading capability.
In Trane centrifugal chillers, gas
prerotation vanes ahead of the
compression stage improve impeller
aerodynamic efficiency, resulting in
smoother unloading and reducing
power consumption.
The curves show that two-stage
compressors surge less and later than
single-stage machines. Intersection
point B, when the load line meets the
surge area, corresponds to a higher
part load for the single-stage
compressor than would be the case
with a two-stage compressor. Two
stage machines, therefore, have a
wider range of applications.
Typical single-stage compressor
performance curve
1 - Load Line
2 - Surge Line
3 - A
4 - B
5 - 40%
6 - 90° Vanes
7 - 100%
8 - Compressor Head
9 - Refrigerant Gas Flow
Typical two-stage compressor
performance curve
1 - Load Line
2 - Surge Line
3 - A
4 - B
5 - 20%
6 - 90°
7 - 80°
8 - 70° Vanes
9 - 100%
10 - Compressor Head
11 - Refrigerant Gas Flow
Features and
Benefits