In air conditioning and refrigeration applications, the ability of the thermostatic expansion valve (TEV) to match refrigerant flow to the rate at which refrigerant can be vaporized in the evaporator makes the it the ideal expansion device for most HVACR applications. The TEV controls the flow of liquid refrigerant entering the direct expansion (DX) evaporator by maintaining a constant superheat of the refrigerant vapor at the outlet of the evaporator.
Superheat is the difference between the refrigerant vapor temperature and its saturation temperature. To measure the superheat the TEV controls, the difference between the actual temperature at the sensing bulb and the saturation temperature corresponding to the suction pressure at the sensing bulb location is determined. By controlling superheat, the TEV keeps nearly the entire evaporator surface active while not permitting liquid refrigerant to return to the compressor.
The purpose of the external equalizer is to sense the pressure in the suction line at the bulb location and transmit it to the TEV diaphragm. This usually means installing the external equalizer immediately downstream from the bulb. This ensures the correct pressure is signaled to the TEV.
In some situations this “ideal” location may not be possible. In these cases, an alternate location, such as at B or C (see diagram), could be used. However, the pressure at these locations must be nearly identical to the pressure in the line where the bulb is located.
In other words, locations B and C are acceptable as long as these pressures are essentially the same as A when the system is operating at full load. In the past there has been concern about installing the external equalizer “up-stream” from the bulb. This was due to the possibility of refrigerant leaking past the TEV push rods, passing through the equalizer line and into the suction line, thus falsely influencing the TEV bulb temperature.
Today, with Sporlan’s TEV design, this possibility is virtually eliminated.
HVACR Tech Tip Article contributed by Jason Forshee, Product Manager, Sporlan Division of Parker Hannifin