Compressor Capacity Control chiller trane

Compressor Capacity Control chiller trane

 

chiller trane

Inlet vanes:

The capacity of most centrifugal compressors is controlled by vanes at the inlet of the compressor impeller.

While a survey of other centrifugal compressor designs shows that there are various methods of capacity control, many of them function in a manner similar to the inlet vanes presented in this period.

Inlet vanes, installed ahead of the impeller, are a common method of modulating the capacity of the compressor over a wide range of load conditions (refrigerant flow rates). These vanes “preswirl” the refrigerant before it enters the impeller. By changing the refrigerant’s angle of entry, inlet vanes lessen the ability of the impeller to take in the refrigerant. As a result, each inlet vane position creates a new compressor performance characteristic without changing the rotational speed of the impeller.

multistage centrifugal compressor:

multistage centrifugal compressor

 In a multistage centrifugal compressor, the operating characteristics of each impeller are modulated by the impeller’s own set of inlet vanes

This example shows 2 impellers in series. These impellers share the task of compressing the refrigerant. Centrifugal water chillers are generally available with 1, 2, or 3 impellers.

The forces that act on the refrigerant vapor within the centrifugal compressor impeller can be broken down into 2 components. One component acts to move the refrigerant away from the impeller in a radial direction. This component is
called radial velocity (Vr).
The second component acts to move the refrigerant in the direction of impeller rotation. This component is called tangential velocity (Vt).
Together, these components generate the resultant velocity vector (R), the length of which is proportional to the amount of kinetic energy in the refrigerant. Recall that kinetic energy is converted to static energy, or static pressure.

 

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.



 
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