HVAC Year-Round-Automatic Constant Liquid Pressure Control System

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.

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