We mentioned earlier that the TXV regulates the amount of refrigerant to the cooling coils. The amount of refrigerant needed in the coils depends, of course, on the temperature of the space being cooled.

The thermal control bulb, which controls the opening and closing of the TXV, is clamped to the evaporator coil near the outlet. The substance in the thermal bulb will vary depending on the refrigerant used. The expansion and contraction (because of temperature change) transmit a pressure to the diaphragm. This causes the diaphragm to be moved downward, opening the valve and allowing more refrigerant to enter the cooling coil. When the temperature at the control bulb falls, the pressure above the diaphragm decreases and the valve tends to close. Thus, the temperature near the evaporator outlet controls the operation of the TXV.

EVAPORATOR

The evaporator consists of coils of copper, aluminum, or aluminum alloy tubing installed in the space to be refrigerated. Figure 16-4 shows some of this tubing. As mentioned before, liquid R-12 enters the tubing at a reduced pressure and, therefore, with a lower boiling point. As the refrigerant passes through the evaporator, the heat flowing to the coils from the surrounding air causes the rest of the liquid refrigerant to boil and vaporize. After the refrigerant has absorbed its latent heat of vaporization (that is, after it is entirely vaporized), the refrigerant continues to absorb heat until it becomes superheated. The amount of superheat is determined by the amount of liquid refrigerant admitted to the evaporator. The amount of liquid refrigerant, in turn, is controlled by the spring adjustment of the TXV.

Figure 16-4.-Evaporator tubing.

A range of 8 to 12F of superheat is considered desirable because it increases the efficiency of the plant by ensuring that all of the liquid entering has changed state to a vapor before it leaves the evaporator. Evaporation of all the liquid prevents any liquid carry-over into the compressor. Remember, the compressor is designed to pump a gas-not a liquid.