In order for a thermostatic expansion valve (TXV) to feed the proper amount of refrigerant into an evaporator, a subcooled (100 percent liquid) refrigerant needs to be fed to its inlet. If a saturated (liquid/vapor) refrigerant enters the valve, it will not be able to feed the correct amount of refrigerant into the evaporator, and the capacity of the system will be reduced.
In a properly designed and operating system, the condenser should remove enough heat from the refrigerant to cause a subcooled refrigerant to leave the condenser, travel through the liquid line, and properly feed the TXV. As the liquid refrigerant exits the TXV, its temperature is reduced to the evaporating temperature by instantaneous boiling or “flashing” off a portion of the liquid into vapor. The latent heat of vaporization involved in the change of state absorbs heat from the remaining liquid refrigerant, lowering its temperature.
The higher the liquid temperature entering the TXV, the more liquid flashes off to reduce the temperature of the refrigerant and the less liquid refrigerant remains to absorb heat from the product within the refrigerated space. In effect, the net refrigeration capacity of the system is reduced. If the temperature of the liquid refrigerant is reduced prior to its entry into the TXV, the capacity of the system can be increased, less liquid refrigerant instantaneously flashes off, and more liquid refrigerant remains to remove heat from the product, thus increasing the net refrigeration effect.
To provide this increase in the net refrigeration effect, a heat exchanger can be added to the liquid line to reduce the temperature of the refrigerant as it travels to the TXV. One popular method is a suction-to-liquid line heat exchanger. Basically, the liquid line is connected to the suction line, so as the cool vapor in the suction line returns to the compressor, it can cool the liquid refrigerant entering the TXV. An added benefit is that at the same time, the cool suction vapor is warmed by the liquid refrigerant to help boil off any liquid that might be flowing through the suction line.
Larger refrigeration systems, such as a rack system used in many supermarkets, can include a separate subcooling circuit to improve the operating efficiency of the entire rack. The refrigerant flowing through the rack’s common liquid line is cooled using one of the rack’s refrigerated circuits. The common liquid line is routed through a brazed plated heat exchanger, which is refrigerated by a TXV fed from the rack’s liquid line header. To regulate the temperature of the refrigerant flowing through the heat exchanger, an evaporator pressure regulator valve is attached to the suction side of the heat exchanger.
Refrigerant subcoolers can also prevent liquid refrigerant traveling through the liquid line from turning back into a saturated state. This can occur if the liquid line travels through an extremely hot location, increasing the temperature of the refrigerant to its saturation temperature. Or its piping causes an extreme pressure drop, causing the pressure to drop to its saturation pressure.
There are many accessories that can be added to an operating system. Some are added to protect the system, while others are used to increase its efficiency, such as subcoolers. Understanding how and why these accessories are used will allow technicians to better maintain, and more importantly, repair refrigeration systems.
Publication date: 6/10/2019