Refrigeration Cycle of Centrifugal Chiller
Description of refrigeration cycle is perfectly represented in p-h chart or Pressure-Enthalpy chart. This is the formal way to define the refrigeration cycle which shown in the figure. But understand very first and easily, let’s analyse the components of a 2-stage centrifugal chiller in the framework of the refrigeration cycle as shown in above schematic diagram. Here we considered 2-stage compressor centrifugal chiller to describe refrigeration cycle.
If we follow the refrigeration vapour flow route we find that refrigerant vapour leaves the evaporator and flows to the compressor. Vapour is compressed to a higher pressure and temperature in compressor. High-pressure refrigerant vapour then travels to the condenser where it rejects heat to water, and then leaves as a saturated liquid. The pressure drop created by the first expansion device causes part of the liquid refrigerant to evaporate and the resulting mixture of liquid and vapour enters the economizer. Here, the vapour is separated from the mixture and routed directly to the inlet of the second-stage impeller. The remaining saturated liquid refrigerant enters the second expansion device.
The pressure drop created by the second expansion device lowers the pressure and temperature of the refrigerant to evaporator conditions, causing a portion of the liquid refrigerant to evaporate. The resulting mixture of liquid and vapour enters the evaporator. In the evaporator, the liquid refrigerant boils as it absorbs heat from water and the resulting vapour is drawn back to the compressor to repeat the cycle.
The change in enthalpy from C to A that occurs during the refrigeration cycle is called the refrigeration effect. This is the amount of heat that each pound [kg] of liquid refrigerant will absorb when it evaporates.
The benefit of the economizer can be demonstrated by comparing the refrigeration cycles with and without an economizer.
Without an economizer, refrigerant from the condenser ➅ expands directly to evaporator conditions ➉, producing a smaller refrigeration effect (B to A). Some chiller designs may sub-cool the liquid refrigerant in the condenser (➅ moves to the left) to increase this refrigeration effect.
Also, in a chiller without an economizer, all of the refrigerant vapour must go through both stages of compression to return to condensing conditions. In a chiller with an economizer, refrigerant vapour that flashes in the economizer bypasses the first stage of compression, resulting in an overall energy savings of 3 to 4 percent.