Trane CVGF Two-Stage Compressor Widens the Application Range, Why Centrifugal Compressors Surge

Features and Benefits

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 Vr, which is directly proportional to the refrigerant gas flow Q.

•The tangential velocity component Vt, which is a function of both impeller diameter D and the rotational speed rpm.

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 Vt is the same as on each stage, though Vr 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.

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, Vt is fixed and Vr 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 - Vr = f (Q)

2 - Vt = f (D, RPM)

3 - V = Resultant

4 - RPM

5 - D

6 - Q

1 - Load Line

2 - Surge Line

3 - A

4 - B

5 - 40%

6- 90° Vanes

7- 100%

8- Compressor Head

9- Refrigerant Gas Flow

InTrane 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 - 20%

6 - 90°

7 - 80°

8 - 70° Vanes

9 - 100%

10 - Compressor Head

11 - Refrigerant Gas Flow

Typical two-stage compressor performance curve

CTV-PRC001-GB 9