Condenser Subcooling in Compression Chillers

The necessity of subcooling in compression chillers

  1. The potential intrusion of refrigerant vapor bubbles reduces the capacity of the expansion valve of the chiller, thus restricting the liquid refrigerant input to the evaporator. Therefore, to prevent this phenomenon, the liquid refrigerant is cooled down to below the saturation temperature.

    As a result, subcooling, which is establishing a temperature difference between the liquid refrigerant leaving the condenser and the refrigerant saturation temperature, is essential for ensuring gaseous refrigerant bubble do not infiltrate the expansion valve.

  2. When the superheated refrigerant vapor enters chiller condenser at high temperature and pressure, the temperature of the condenser coil exceeds its surrounding air. The temperature difference between the coil and the ambient air promotes heat transfer from the condenser coil to the ambient air, thus cooling down the refrigerant vapor running in the coil from superheat to the saturation temperature. The refrigerant begins to distill upon reaching the saturation temperature.

    The refrigerant gradually moves to the end of the condenser coil, completing its transformation into liquid by transferring its latent heat and superheat to the ambient air in the condenser of the water cooled compression chiller (although, the liquid is still hotter than the ambient air at this point).

    Then, the liquid refrigerant begins to lose its sensible heat and cool down. At this point, where the temperature of the liquid drops below the saturation point in proportion to the condenser pressure, the refrigerant is said to be subcooled (undercooled).

  3. Subcooling is, in fact, cooling the liquid refrigerant down to below the saturation temperature. When the system is overcharged, the condenser fills with the refrigerant, which reduces its capacity. The reduced capacity is due to the extended presence of the liquid refrigerant in the system, which implies its further cooling.

  4. A small temperature difference between normal and subcooled conditions makes it difficult to measure the temperature accurately. If discharge pressure exceeds the normal level, the subcooling temperature must be measured. Subcooling it most evident at full load. In air-cooled systems, the gas charging is determined by the increase in the subcooling temperature of the refrigerant in the condenser.
    Under normal working conditions, the temperature difference remains between 4 and 7 °C.