The element charge (i.e., the fluid in the power element of a TX valve) is usually one of three types:
1. GAS CHARGE
2. LIQUID CHARGE
LIQUID CHARGES, like gas charges, use the same refrigerant in the element as is used in the refrigeration system. The difference between the liquid and gas charges is that the liquid charge has a volume such that there will always be some liquid in the bulb regardless of the amount of superheat in the suction line.
Because liquid is always present in the bulb, the thermostatic element always has control of the opening force on the valve. Therefore, unlike the gas charge, the liquid charge can contribute to flood-back during startup.
Furthermore, liquid charges tend to cause more hunting than other charges (hunting is presented later in this module). Also, liquid charges should be avoided in low-temperature systems.
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