When a TEV or a capillary tube is used as a metering device, what is the effect of a low charge on head pressure?

When a thermal expansion valve (TEV) or a capillary tube is used as a metering device, a low refrigerant charge can decrease the head pressure in the system. This occurs because the amount of refrigerant available for circulation is insufficient to adequately maintain the necessary pressure levels in the system.

In a refrigeration or air conditioning system, head pressure is primarily influenced by the amount of refrigerant present and the system's ability to dissipate heat. A low charge can lead to reduced saturation temperature in the evaporator, resulting in less vaporization of refrigerant and, consequently, less pressure buildup in the compressor discharge side. As a result, the condenser may not receive enough refrigerant to maintain optimal pressure, leading to a decrease in head pressure.

This phenomenon illustrates the importance of maintaining the correct refrigerant charge in a system, as both overcharging and undercharging can affect system performance, efficiency, and lifespan.