Chiller Cooling Capacity Control

How do chillers control their cooling capacity? One of the most common ways, for centrifugal type chillers, is to use vane guides. Vane guides sit directly in the flow path of the refrigerant and can be controlled to change their position.
The chiller produces chilled water by removing the thermal energy, from the “chilled water loop” within the evaporator, which was picked up from the building. The amount of cooling generated needs to vary to match the load, otherwise too much heat can be removed and this will cause the chiller to freeze. This is very bad for chillers, if they are not designed to do this, and will result in material fatigue, where pipes burst internally and refrigerant is then able to leak.
The most common method, within centrifugal compressors, is to use vane guides. As mentioned, the vane guides sit in the flow path of the refrigerant. They are located between the suction line and the compressor.

The vane guides are a triangular shape and multiple vane guides sit radially in the cross sectional area. The modulate their position between fully open and fully closed.

The vanes are aerodynamically profiled to minimise the pressure drop across them from the flow of refrigerant.

The purpose of these vane guides is to change the flow direction of the refrigerant. When the vanes are fully open the refrigerant simply passes straight over them, this occurs at full capacity to provide maximum cooling. When the vanes modulate their position and are either partly or fully closed, the refrigerant hits into the vane guide which in turn changes the direction it was flowing in.

This change in direction means the refrigerant will hit the compressor impeller at a different angle. The lesser the angle, the less momentum the impeller will impart of the refrigerant. The more closed the vane guide, the greater the angle of entry into the impeller. If the angle of entry is changed, then the refrigerant flows slower through the compressor and volute, this reduces the kinetic energy and pressure.

You can imagine this like placing your hand out flat, through the window of a moving car. As you move the angle of your hand, you will feel the air flow over the top or bottom of your hand and there will be a resulting force pushing against your hand depending on its angle.

The position of the vane guides can be adjusted, typically based on the leaving water temperature of in the evaporator. There are many methods used to change the position, some use oil pressure from the lubrication system, others will with a belt driven motor, a gear ratio.

To find out more, please watch the video below.