REFRIGERANT CAPACITY CONTROLS

In a single-stage installation, one evaporator, one condensing unit, and any one of the five types of refrigerant controls will work. However, in a multistage installation (fig. 14-9) with one condenser, only two types of controls can be used. There are very few lmw-side float systems in actual operation, but you should be aware that there are some units that still use this control. Thermostatic expansion valves are the most commonly used, and on large capacity units, they usually operate a pilot valve that, in turn, operates a larger valve.

The biggest problem associated with capacity controls is using one of the wrong size. When ordering replacements and when making repairs, you should always ensure that the control markings are appropriate for the intended system. Also, ensure the replacement part being ordered is compatible with the type of refrigerant being used in the system.

Automatic Expansion Valve

The automatic expansion valve (AEV) maintains a constant pressure inthe evaporator. Looking at figure 14-22(A), there are five pressures that affect the operation of the AEV. The pressures are p1 (atmospheric pressure), p2 (evaporator pressure), p3 (liquid line pressure), S1 (adjustable spring pressure), and S2 (fixed spring pressure). To adjust the valve, turn the adjusting screw until the desired pressure is obtained in the evaporator.

Automatic expansion valves are installed on systems that have a relatively constant load. Primarily, the AEV is used on domestic refrigerators and small water coolers.

Figure 14-22(A).-Automatic expansion valve.

Thermal Expansion Valve

The thermal expansion valve (TEV) is the most widely used expansion device. The TEV controls the flow of refrigerant to maintain a constant superheat in the tail coil of an evaporator. Referring to figure 1422(B), there are three pressures that affect the operation of this valve. They are p1 (bulb pressure), p2 (evaporator pressure), and p3 (spring pressure).

Figure 14-22(B).-Themal expansionvalve. 14-14

When the pressure of p1 is higher than the combined pressure of p2 and p3, the valve opens. This valve is equalized internally because the evaporator pressure is sensed through an intemal port in the valve. Figure 14-22(C) provides another view showing how a TEV is equalized internally. When a TEV is used on a large evaporator or an evaporator with a pressure drop of 6 to 7 pounds across the evaporator, the valve will prematurely cause hunting (valve fluctuates toward opening and closing). In the case of valve hunting, install a TEV equipped with an external equalizer line. Figure 14-22(D) shows the TEV installed with an external equalizer line. The external equalizer line compensates for a pressure drop from the inlet of the evaporator to the end of the tail coil and eliminates valve hunting. During installation of an equalizer line, ensure that it is located downstream from the sensing bulb.

Air-conditioning refrigeration units come equipped with a metering device that the manufacturer has engineered for the system. You should never change the recommended type of metering device for a system without consulting the manufacturer.

Figure 14-22(C).-Thermal expansionvalve.

Figure 14-22(D).-Externally equalizedTEV.

Figure 14-22(B).-Dial thermometer.

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