The thermostatic-expansion valve, used primarily in commercial refrigeration and air-conditioning, is a refinement of the automatic-expansion valve (see Fig. 11-3). A bellows or diaphragm responds to pressure from a remote bulb charged with a substance similar to the refrigerant in the system. The bulb is attached to the suction line near the evaporator outlet. It is connected to the expansion valve by a capillary tube.
In operation, the thermostatic-expansion valve keeps the frost line of the unit at the desired location by reacting to the superheat of the suction gas. Superheat cannot be present until all liquid refrigerant in the evaporator has been vaporized. Thus, it is possible to obtain a range of evaporator temperatures by adjusting the superheat control of the thermostatic-expansion valve.
The prime importance of this type of metering device is its ability to prevent the flood-back of slugs or liquid through the suction line to the compressor. If this liquid returns to the compressor, it could damage it. The compressor is designed to pump vapors, not liquids.
The automatic-expansion valve controls liquid flow by responding to the suction pressure of the unit acting on its diaphragm or bellows (see Fig. 11-2). When the valve opens, liquid refrigerant passes into the evaporator. The resulting increase in pressure in the evaporator closes the valve. Meanwhile, the compressor is pulling the gas away from the coils, reducing the pressure. This pressure reduction allows the expansion valve to open again. In operation, the valve never quite closes. The needle floats just off the seat and opens wide when the unit calls for refrigeration. When the machine is shut down, the pressure building up in the coils closes the expansion valve until the unit starts up.
Of the several types of metering devices. the hand-expansion valve is the simplest (see Fig. 11-1). Used only on manually controlled installations, the hand-expansion valve is merely a needle valve with a fine adjustment stem. When the machine is shut down, the hand-expansion valve must be closed to isolate the liquid line.
usually found at the top of the regulator. In an electrically compensated regulator, turning the stem to obtain different refrigerant pressures and temperatures in the evaporator is accomplished by a small electric motor. This motor rotates the adjusting stem in accordance with temperature variations in a thermostatic bulb placed in the medium being cooled as it leaves the evaporator. The adjusting stem, spring, and controlling diaphragm have been separated from their positions at the top of the regulator. They have been placed in a small remote unit mounted on a common base with the motor and gear drive. This compensating unit may be located in any convenient place within 20 ft of the main regulator. The unit is connected to it by two small pipelines. These convey the pressure changes set up by the control diaphragm.
The total arc of rotation of the motor and the large gear on the motor acting through the smaller pinion on the adjusting stem of the diaphragm unit will rotate the stem about two turns. This is sufficient to cause the regulator to vary the evaporator pressure through a total range of about 13 lb (see Fig. 10-27).
A standard regulator is reset by manually turning the adjusting stem, which increases the spring pressure on top of the diaphragm. In an air compensated regulator, a change of pressure on top of the diaphragm is accomplished by introducing air pressure into the airtight bonnet over the diaphragm. As this air pressure is increased, the setting of the regulator will be increased. This will produce like changes of evaporator pressure and refrigerant temperature. The variations in air pressure are produced by the temperature changes of the thermostatic remote bulb placed in the stream of the medium being cooled as it leaves the evaporator.
Temperature changes in the medium being cooled over the remote bulb of the thermostat will cause the thermostat to produce air pressures in the regulator bonnet within a range of 0 to 15 lb. This will cause the regulator to change the evaporator suction pressure in a like amount. A more definite understanding of this operation is obtained by assuming certain working conditions for the purpose of illustration. In cases where a larger range of modulation is required, a three-to-one air relay may be installed. This will permit a 45-lb range of modulation (see Fig. 10-26).
The upper portion of the valve head is similar to a standard pressure regulating head. On the lower portion of the head another diaphragm is connected to the main diaphragm by a push rod. As the thermal bulb warms, the liquid in it expands, pushing up on the rod and opening the regulator. Because this is accomplished by an outside power source, the pressure drop through the head is reduced considerably. The valve head will function in connection with the regulator on a 1/2- to 3/4-lb overall pressure drop. The point at which the modulation or compensation takes place may be adjusted by turning the adjusting stem. By turning the stem in, the product temperature is increased. By turning the stem out, the product temperature is decreased. The back-pressure valve will remain wide open, taking advantage of the line-suction pressure until the product being cooled approaches the temperature at which modulation is to begin. The valve head will hold the temperature of the product to within ±1/2°F (0.28°C) of the desired temperature. In the case of failure of the thermal element, the valve head can be used as a straight back-pressure valve by readjusting it to the predetermined suction pressure at which the system is desired to operate (see Fig. 10-25).
In a refrigeration system designed to maintain a predetermined temperature at full load, any decrease in load would tend to lower below fullload temperature the temperature of the medium being cooled.
To maintain constant temperatures in applications having varying loads, means must be provided to change refrigerant temperature to meet varying-load requirements.
Refrigerant temperature is a function of evaporator pressure. Thus,the most direct means of changing refrigerant temperature to meet varying-load requirements is to vary the system pressure. This variation of system pressure is accomplished by adjusting the setting of a back-pressure regulator. A number of back-pressure valve controls are
available. Some of them are mentioned here.
The following valves and controls are used in the hot-gas defrost systems of ammonia-type evaporators:
Hot-gas or pilot-solenoid valve. The valve is a 1/8 in. ported solenoid valve. It is a direct-operated valve suitable as a liquid, suction, hotgas, or pilot valve at pressures to 300 lb.
Suction-, liquid-, or gas-solenoid valve. The suction-solenoid valve is a one-piston, pilot-operated valve suitable for suction-, liquid-, or gas-lines at pressures to 300 lb. It is available with a 9/16 in. or 3/4 in. port.
Pilot-operated solenoid valve. The valve is a one-piston, pilot-operated solenoid valve used as a positive stop valve for applications above ?30°F (?34°C) on gas or liquid.
Pilot-operated two-piston valve. The solenoid valve is a rugged, pilotoperated, two-piston valve with spring return for positive closing under the most adverse conditions. It is used for compressor unloader, suction, liquid, and hot-gas applications.
Gas-powered solenoid valve. The gas-powered solenoid valve is a power-piston type of valve that uses high pressure to force the valve open through the control of pilot valves. Because of the high power available to open these valves, heavy springs may be used to close the valves positively at temperatures down to ?90°F (?68°C).
Dual-pressure regulator valve. The dual-pressure regulator valve is designed to operate at two predetermined pressures without resetting or adjustment. By merely opening and closing a pilot solenoid, either the low- or high-pressure setting is maintained.
Reseating safety valve. The reseating safety valve is generally used as a relief regulator to maintain a predetermined system pressure. The pressure maintained by the valve is adjustable manually. Back-pressure regulator arranged for full capacity. The back-pressure regulator is normally used where pressure control of the evaporator is not required—as in a direct expansion system. A pilot solenoid is energized, allowing pressure to bypass the sensing chamber of the regulator holding the valve wide open. De-energizing the pilot valve allows the valve to revert to its function as a back-pressure regulator maintaining a preset pressure upstream of the valve. The valve performs both as a suction solenoid and as a relief regulator.
Differential relief valve. The differential relief valve is a modulating regulator for liquid or gas use. It will maintain a constant preset pressure differential between the upstream and downstream side of a regulator.
Reverse-acting pressure regulator. The reverse-acting pressure regulator is used to maintain a constant predetermined pressure downstream of the valve. When complete shutoff of the regulator is required, a pilot valve is installed in the upstream feeder line. When the solenoid valve is closed, the regulator closes tightly. When the solenoid valve is open, the regulator is free to operate as the pressure demands. With the solenoid installed as described above, this becomes a combination reverse-acting regulator and stop valve.
Gas-powered check valve. The gas-powered check valve is held in a normally open position by a strong spring. Gas pressure applied at the top of the valve closes the valve positively against the high system pressures. A manual opening stem is standard.
Check valve. The check valve is a spring-loaded positive check valve with manual opening stem. It is used to prevent backup of relatively high pressure into lower-pressure lines.
In-line check valve. The in-line check valve is used in multiple-branch liquid lines fed by a single solenoid valve. This check valve prevents circulation between evaporators during refrigeration. The in-line check valve is also used between drain pans and evaporators to prevent frosting of the drain pan during refrigeration.
These valves and controls are necessary. They cause defrosting operations to take place in large evaporators used for commercial jobs. Some manufacturing operations also call for large-capacity refrigeration equipment.
Adual-pressure regulator is shown in Fig. 10-24. It is used on a shell-andtube cooler. The dual-pressure regulator is particularly adaptable for the control of shell-and-tube brine or water coolers, which at intervals may be subjected to increased loads. Such an arrangement is shown in Fig. 10-24.
The high-pressure diaphragm is set at a suction pressure suitable for the normal load. The low-pressure diaphragm is set for a refrigerant temperature low enough to take care of any intermittent additional loads on the cooler. In this case, a thermostat affects the transfer between low and high pressure. The remote bulb of the thermostat is located in the wateror brine-line leaving the cooler. A temperature increase at this bulb indicating an increase in load will cause the thermostat to open the electric pilot and transfer control of the cooler to the low-temperature diaphragm. Upon removal of the excess load, the thermostat will cause the electric pilot to close the low-pressure port. The cooler is then automatically transferred to the normal pressure for which the high-pressure diaphragm is set. The diaphragms may be set at any two evaporator pressures at which it is desirable to operate. Any electric switching device responsive to load change may be used to change from one evaporator pressure to the other.
The constant liquid control system is a means of increasing the efficiency of a refrigeration system that utilizes air-cooled, atmospheric, or evaporative condensers (see Fig. 10-23). This is accomplished by automatically maintaining a constant liquid pressure throughout the year to assure efficient operation. Constant liquid pressure on thermal expansion valves, float controls, and other expansion devices results in efficient low-side operation. Hot gas defrosting, liquid recirculation, or other refrigerant-control systems require constant liquid pressure for successful operation. Liquid pressure is reduced by cold weather and extremely low wet-bulb temperatures with low refrigeration loads.
To compensate for a decrease in liquid measure, it is necessary automatically to throttle the discharge to a predetermined point and regulate the flow of discharge pressure to the liquid line coming from the condenser and going to the receiver. Thus, predetermined pressure is applied to the top of the liquid in the receiver. The constant liquid-pressure control does this. In addition, when the compressor “start and stop” is controlled by pressure-stats, the pressure-operated hot-gas flow-control valve is a tight closing stop valve during stop periods. This permits efficient “start and stop” operation of the compressor by pressure control of the low side.
The three valves in the system shown in Fig. 10-23 are
? The reverse acting pressure regulator
? The pressure-operated hot-gas flow-control valve
? The relief check valve
The function of the control system is to maintain a constant liquid pressure (A). The reverse acting pressure-regulator valve accomplishes this, which is a modulating-type valve. It maintains a constant predetermined pressure on the downstream side of the regulator. To maintain a constant pressure (A) it is necessary to maintain a discharge pressure (B) approximately 5 psi above (A). This is accomplished by the hot-gas control valve, which will maintain a constant pressure (B) on the upstream or inlet side of the regulator. Due to the design of the regulator, a constant supply of gas will be available at a predetermined pressure to supply the pressure regulator to maintain pressure (A). Excess hot gas is not required to maintain a fill flow into the condenser.
The relief check valve prevents pressure (A) from causing backflow into the condenser. When the compressor shuts down, the hot-gas flow control valve closes tightly and shuts off the discharge line. This prevents gas from flowing into the condenser.
The check valve actually prevents the backflow of liquid into the condenser. Thus, liquid cannot back up into the condenser in extremely cold weather. Sufficient low-side pressure will be maintained to start the compressor when refrigeration is required.