Page 466 - Mechanical Engineers' Handbook (Volume 4)

7 System Components 455

Figure 15 Typical flooded shell-and-tube evaporator. 3

high-pressure, subcooled liquid to a saturated low-pressure mixture. Other types of flow
control devices, such as pressure regulators and float valves, can also be found in some
refrigeration systems. Discussion of these can be found in Ref. 1. Five types of expansion
devices can be found in refrigeration systems: (1) thermostatic expansion valves, (2) elec-
tronic expansion valves, (3) constant-pressure expansion valves, (4) capillary tubes, and (5)
short tube restrictors. Each is discussed briefly.

Thermostatic Expansion Valve
The thermostatic expansion valve (TXV) senses the superheat of the gas leaving the evap-
orator to control the refrigerant flow into the evaporator. Its primary function is to provide
superheated vapor to the suction of the compressor. A TXV is mounted near the entrance to
the evaporator and has a capillary tube extending from its top that is connected to a small
bulb (Fig. 18). The bulb is mounted on the refrigerant tubing near the evaporator outlet. The
capillary tube and bulb is filled with a substance called the thermostatic charge. This charge
1

Figure 16 Typical spray-type evaporator. 3