Refrigerant subcooling in a chiller is the difference between the liquid refrigerant temperature at the outlet of the chiller’s condenser and the gas condensation temperature. This can be expressed using the following formula:
Subcooling = Refrigerant outlet temperature – Condensation temperature
From this formula, we can also deduce:
Outlet temperature = Condensation temperature – Subcooling
Calculating Subcooling of Air-Cooled Refrigerant
Let’s consider an example of a chiller with 3°C subcooling operating in an ambient temperature of 30°C. The condensation temperature of a refrigerant is typically 10°C to 20°C above the ambient temperature. For this example, we will use an average of 15°C:
Condensation temperature = Ambient temperature + 15°C
Condensation temperature = 30°C + 15°C = 50°C
The refrigerant outlet temperature is calculated as:
Outlet temperature = Condensation temperature – Subcooling
Outlet temperature = 50°C – 3°C = 47°C
Importance of Refrigerant Subcooling in Chillers
- Prevention of vapor phase:
Higher subcooling ensures the refrigerant remains farther from the vapor phase at the condenser outlet. - Improved condenser performance:
Greater subcooling indicates better condenser performance, usually resulting in lower high-side pressure in the chiller and, consequently, reduced energy consumption. - Efficiency at the expansion valve:
- The liquid refrigerant leaving the condenser flows to the expansion valve, where it undergoes a sudden pressure drop. With higher subcooling, less refrigerant evaporates in this valve (flash vaporization).
- Reduced evaporation at the expansion valve improves system efficiency since liquid refrigerant is responsible for absorbing heat in the chiller’s evaporator.
- Reduced superheating:
Less refrigerant evaporation at the expansion valve leads to lower superheating, which increases the chiller’s energy efficiency. - Unwanted evaporation:
Refrigerant that evaporates in the expansion valve returns to the compressor without contributing to system cooling, which negatively affects efficiency.
How to Improve Refrigerant Subcooling in Chillers
- Maintain clean coils:
Keep the air-cooled condenser coils clean at all times. - Check airflow:
Ensure proper airflow from the fans cooling the condensers. - Maintain water-cooled condensers:
Perform proper maintenance of the water circuit in the cooling tower to prevent scaling and contamination that can affect heat exchange and condenser cooling. - Control air flow rate:
Maintain the correct air flow rate and ensure the cooling tower is in good condition in air-cooled systems.
