Direct expansion. In what is referred to as direct expansion, liquid refrigerant enters the expansion valve and only vapor leaves the evaporator, as shown in Fig. 6.9. One of the most popular types of expansion valves that facilitates this control is the superheat-controlled valve, which is also called a thermo-valve, thermostatic valve, or a TXV. More on this type of control will be covered in Chapter 11 on valves and refrigerant controls, but it is sufficient at this point to explain that the valve controls the flow rate of refrigerant such that the vapor leaves the evaporator superheated by from 4 to 7°C (7 to 12°F). Direct expansion is limited to evaporators where the refrigerant evaporates in the tubes.
If the sensing bulb detects a higher-than-setpoint superheat at the evaporator outlet, the valve opens further. The evaporator fed by a superheat-controlled expansion valve is probably the lowest in first cost of the three methods described here. It is used widely with halocarbon refrigerants at moderate refrigerating temperatures, but its use is limited in low-temperature applications and for ammonia. More on the possibilities of direct-expansion coils with ammonia will be found in Sec. 6.27.