Trane CVGF manual Why Centrifugal Compressors Surge, Two-Stage Compressors Surge Less and Later

CTV-PRC001-E4

1 - Vr = f (Q)

2 - Vt = f (D, RPM)

3 - V = Resultant

4 - rpm

5 - D

6 - Q

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 fl ow Q.

• The tangential velocity component Vt,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, Vt is fi xed 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 fl ow it draws in and modifi es 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.

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.

In Trane centrifugal chillers, gas prerotation vanes ahead of the compression stage improve impeller aerodynamic effi ciency, 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.				1	- Load Line
	1		- Load Line		2	- Surge Line
	2		- Surge Line		3	- A
	3		- A		4	- B
	4		- B		5	- 20%
	5		- 40%		6	- 90°
	6		- 90° Vanes		7	- 80°
	7		- 100%		8	- 70° Vanes
	8		- Compressor Head		9	- 100%
		9	- Refrigerant Gas Flow		10 - Compressor Head
Typical single-stage compressor				Typical two-stage compressor	11 - Refrigerant Gas Flow
performance curve				performance curve

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