Probably the most widely used flow control device in general refrigeration practice, including air conditioning, is the superheat-controlled expansion valve. The valve is popularly called a thermostatic expansion valve, a name that implies controlling a fixed evaporating temperature. The valve does no such thing, as will be explained later, so we shall call the valve a superheat-controlled valve, although another acceptable name is thermovalve. The valve is also called a TXV by some refrigeration professionals. Even though the superheat-controlled valve is used so widely, one objective of this section is to distinguish where the valve can be applied effectively in industrial refrigeration and where it is not recommended.
A schematic diagram of the basic superheat-controlled expansion valve is shown in Fig. 11.14. The essential objective of the expansion valve is to regulate a flow rate of refrigerant to the evaporator that matches the rate boiled off. The valve accomplishes this objective by controlling the amount of refrigerant superheat leaving the evaporator.
The valve stem is positioned by the pressure difference on opposite sides of the diaphragm. The pressure under the diaphragm is provided by the refrigerant at the entrance to the evaporator, and the pressure on the top side of the diaphragm by what is called the power fluid. In the basic expansion valve, the power fluid is the same refrigerant used in the system. It is in vapor form, except for a small amount of liquid in the sensing bulb.
A slight force exerted by the spring on the valve stem keeps the valve closed until the pressure above the diaphragm overcomes the combined forces of the spring and the evaporator pressure. For the pressure above the diaphragm to be higher than the evaporator pressure below the diaphragm, the power fluid temperature must be higher than the saturation temperature in the evaporator. The suction gas must, therefore, be superheated to bring the power fluid up to the valve-opening pressure.
The superheat-controlled expansion valve is a type of proportional controller in that the response of the valve stem is proportional to the error between the sensed and set values. The stem position thus bears a relationship to the superheat at the evaporator outlet as shown in Fig. 11.15. Conceivably the valve could be adjusted to operate with no superheat when just closed and thus hold about 3°C (5.4°F) superheat when completely open. But such a setting provides no protection against a surge of liquid moving through and out of the evaporator more rapidly than the valve could respond. As a precaution, then, the application of a precompression on the spring assures that even at its nearly closed position, the valve provides some superheat leaving the evaporator, as is the case in Fig. 11.15.
If the amount of superheat at the evaporator outlet is to be truly regulated by the valve, the stem position should be controlled by the pressure difference of the power fluid and the pressure at the outlet of the evaporator, rather than at the inlet as is true of the basic valve of Fig. 11.14. Because of the pressure drop between the inlet and outlet of the evaporator of perhaps 15 to 40 kPa (2 to 6 psi) which varies as a function of evaporator load, the performance of the basic expansion valve would be distorted. To correct this problem, a standard expansion valve is available that incorporates an external equalizer, as shown in Fig. 11.16. This valve is provided with an additional connection which permits the pressure at the outlet of the evaporator to be applied to one side of the diaphragm.
We stated earlier that the name thermostatic erroneously suggests that this type of expansion valve controls the evaporating temperature. Instead, the evaporating temperature and pressure of a refrigeration system served by a compressor of a constant displacement rate will ride up and down, as shown in Fig. 11.17. Two balance points are shown, one at a heavy refrigeration load and the other at a low load. The valve admits a flow rate of refrigerant equal to that evaporated, and at low loads the compressor/valve combination settles on a lower evaporating pressure and temperature than at heavy refrigeration loads.