Handbook of air conditioning and refrigeration / Shan K

Electric Expansion Valves
slugs may chill the sensing bulb and suddenly reduce the refrigerant feed. The overshooting and
sudden reduction cause hunting.
Hunting of the thermostatic expansion valve is determined by the following factors:
● Valve size. An oversized valve aggravates hunting.
● Degree of superheat. The smaller the degree of superheat, the greater the chance of hunting.
● Sensing bulb charge. Cross-charge tends to prevent hunting.
REFRIGERATION SYSTEMS: COMPONENTS 10.55
● Location of sensing bulb. A properly selected sensing bulb location often minimizes hunting. The
sensing bulb should be located at the side of the horizontal section of the suction line immediately
after the evaporator outlet, so that it does not sense the oil temperature or liquid refrigerant at the
bottom of the tube.
Electric, or more truly electronic or microprocessor-controlled, expansion valves can provide more
sophisticated, effective, and energy-efficient refrigerant flow controls than thermostatic expansion
valves. Currently, three types of electric expansion valves have become widely available: step motor
valves, pulse-width-modulated valves, and analog valves.
Step Motor Valve. The step motor is an electronically controlled multiphase motor capable of
moving continuously in forward or reverse direction, as shown in Fig. 10.20a. It can also be discretely
positioned in increments of a fractional revolution. Step motors accept a series of step signals
(such as 200/s). The rotational movement is transformed to linear movement required to open
and close an expansion valve by means of a gearbox and a threaded driveshaft. The gearing is used
to increase the torque. This allows the use of a small motor without overstressing it. One of the
available step motor valves needs only 5.7 W of power. Dolin (1997) reported that a step motor
valve may have 1532 steps of resolution in 0.125 in. (3.2 mm) of travel. Each step can change the
flow rate of HCFC-22 by 0.002 lb/min or 3 Btu/h (0.001 kg/min or 0.88 W).
Pulse-Width-Modulated Valves. These are solenoid valves operated at on/off mode, i.e., either
fully opened or fully closed, as shown in Fig. 10.20b. The resolution (the act, or capability of rendering
the distinguishable parts of an object) of flow rate of refrigerant feed is indicated by the
amount of time, the pulse width, or the duration of valve opening. For instance, a pulse-width-modulated
valve may be pulsed every 5 s. If only a flow rate of 40 percent is required, the valve will
hold open for 2 s and closed for 3 s.
Gears
Valve
pin
Step
motor
Valve
pin
Coil
Valve
pin
(a) (b) (c)
FIGURE 10.20 Electric expansion valves: (a) step motor valve; (b) pulse-width-modulated valve; (c) analog
valve.
Coil