Refrigeration Cycle of Centrifugal Chiller

What are the Refrigeration Principles?

1. Heat always moves from a warm area to a cooler area.

2. To cool an area, heat must be removed from it.

3. Heat can be removed from an area by evaporating a liquid, such as a refrigerant, or by compressing a gas to a liquid.

4. Heat transfer can be increased by increasing the temperature difference between the two areas.

5. To create a cooling effect, the temperature of an area must be lower than the temperature of the surrounding environment.

6. The efficiency of a refrigeration system depends on the type of refrigerant and the design of the system.

Fig: Refrigeration Cycle System Diagram for Centrifugal Chiller

What is the Refrigeration Cycle of a Centrifugal Chiller?

Centrifugal Chiller: A centrifugal chiller is a type of air conditioning system that uses a rotating impeller to compress a refrigerant gas and then circulate it through a condenser coil to cool air. The chilled air is then circulated throughout a building or industrial process. Centrifugal chillers are commonly used in large commercial and industrial applications because of their efficient operation and ability to cool large volumes of air quickly.

The description of the refrigeration cycle is perfectly represented in the p-h chart or Pressure-Enthalpy chart. 

This is the formal way to define the refrigeration cycle which is shown in the figure. 

But understand very first and easily, let’s analyze the components of a 2-stage centrifugal chiller in the framework of the refrigeration cycle as shown in the above schematic diagram. Here we considered a 2-stage compressor centrifugal chiller to describe the refrigeration cycle.

If we follow the refrigeration vapor flow route we find that refrigerant vapor leaves the evaporator and flows to the compressor.  

Vapor is compressed to a higher pressure and temperature in a compressor. 

The high-pressure refrigerant vapor 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 vapor enters the economizer. 

Here, the vapor 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.

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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 vapor enters the evaporator. 

In the evaporator, the liquid refrigerant boils as it absorbs heat from the water, and the resulting vapor is drawn back to the compressor to repeat the cycle.

Fig: Typical P-H Chart for Chiller Refrigeration 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 expands directly to evaporator conditions, producing a smaller refrigeration effect (B to A). Some chiller designs may sub-cool the liquid refrigerant in the condenser (moves to the left) to increase this refrigeration effect.

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Also, in a chiller without an economizer, all of the refrigerant vapor must go through both stages of compression to return to condensing conditions. 

In a chiller with an economizer, refrigerant vapor that flashes in the economizer bypasses the first stage of compression, resulting in an overall energy savings of 3 to 4 percent.

Refrigeration Cycle of a Centrifugal Chiller

The refrigeration cycle of a centrifugal chiller follows a similar basic principle to other refrigeration cycles, but it has some specific characteristics due to its centrifugal compressor. Here's a breakdown of the refrigeration cycle of a centrifugal chiller:

Compression: The cycle starts with the centrifugal compressor, which is driven by an electric motor. The compressor draws in low-pressure refrigerant vapor from the evaporator and compresses it to high pressure and temperature. The centrifugal compressor uses rotating impellers to increase the velocity of the refrigerant and then converts this velocity into pressure through a diffuser.

Condensation: The high-pressure and high-temperature refrigerant vapor from the compressor is then passed through a condenser. In the condenser, heat is transferred from the refrigerant to a cooling medium, typically water or air. As a result, the refrigerant condenses into a high-pressure liquid.

Expansion: The high-pressure liquid refrigerant leaving the condenser is throttled or expanded through an expansion valve. This valve reduces the pressure and temperature of the refrigerant, causing it to partially vaporize and enter the evaporator as a mixture of liquid and vapor.

Evaporation: The partially vaporized refrigerant enters the evaporator, where it absorbs heat from the chilled water or the air being cooled. The refrigerant evaporates fully into a low-pressure vapor, and the chilled water or air is cooled down.

Return to Compressor: The low-pressure vapor from the evaporator is then drawn back into the centrifugal compressor to begin the cycle again. The compressor boosts the pressure and temperature of the vapor, and the cycle continues.

Throughout the cycle, a refrigerant such as R-134a or R-123 is used to transfer heat from the chilled water or air to the condenser, providing cooling. The cycle is continuous, with the compressor continuously drawing in and compressing the refrigerant vapor, and the chilled water or air being continuously cooled in the evaporator.

It's worth noting that the specific details and components of the refrigeration cycle can vary depending on the design and manufacturer of the centrifugal chiller, but the general principles outlined above apply to most centrifugal chillers.