Key conclusion: The interdependent relationship between discharge pressure and liquid supply pressure.
In the actual operation of screw refrigeration machines,The higher the discharge pressure, the higher the liquid supply pressure tends to be.This is because the discharge pressure (condensing pressure) and the liquid‑supply pressure (the pressure upstream of the expansion valve) both reside on the high‑pressure side of the refrigeration system, and they are physically interconnected via the condenser, receiver‑accumulator, and liquid‑line piping. When the overall pressure on the system’s high‑pressure side rises, the liquid‑supply pressure naturally increases in tandem.
Basic Concepts of Discharge Pressure and Liquid Supply Pressure
To understand the relationship between the two, one must first clarify their respective positions and functions within the refrigeration cycle:
- Exhaust pressure: Refers to the pressure of the high-pressure refrigerant gas discharged from the compressor as it enters the condenser. It is primarily influenced by factors such as the condensing temperature, the temperature and flow rate of the cooling medium (water or air), and others.
- Supply pressure: Refers to the pressure of high-pressure liquid refrigerant after it has passed through the condenser and receiver‑drier, but before it reaches a throttling device such as a thermostatic expansion valve or an electronic expansion valve.
In an ideal scenario, if the frictional pressure losses in the liquid line are neglected, the supply pressure is approximately equal to the condensing pressure, which is essentially at the same level as the discharge pressure.
Why does an increase in discharge pressure lead to a rise in liquid supply pressure?
The refrigeration system is a closed, integrated loop in which pressure transmission on the high‑pressure side is continuous. When the discharge pressure rises, it typically indicates that the condensing pressure within the condenser is also increasing. After the high‑pressure refrigerant vapor condenses in the condenser, it flows into the receiver‑accumulator or is conveyed directly through the liquid line. Because the gas–liquid two‑phase region or the all‑liquid phase within the receiver‑accumulator and the liquid line is connected to the condenser, any increase in condensing pressure will directly elevate the static pressure throughout the entire high‑pressure side. Consequently, as the delivery point at the end of the high‑pressure side, its pressure inevitably increases along with the discharge pressure.
Potential Risks and Impacts of Excessive Stress
Although an increase in discharge pressure does cause the liquid supply pressure to rise, this does not mean that “the higher, the better.” Operating at high pressures beyond the design limits can lead to a host of adverse effects:
- Compressor power consumption increases: Excessive discharge pressure increases the compressor’s compression ratio, causes a significant rise in shaft power, reduces the system’s coefficient of performance, and elevates operating costs.
- Throttling element adjustment is difficult.Excessive liquid supply pressure can alter the pressure differential across the expansion valve. If this differential becomes too large, it may cause the valve’s flow characteristics to deviate from the design curve, leading to fluctuations in the refrigerant flow rate, uneven evaporator frosting, or uncontrolled superheat at the compressor suction.
- System security vulnerabilitiesLong-term operation at excessively high pressure will accelerate the aging of seals, increase the risk of leaks at pipeline joints, and, in severe cases, may even trigger a high-pressure protection shutdown.
Recommended Practices for Maintaining Stable System Pressure Operation
To ensure the efficient and safe operation of screw chillers, the discharge pressure and refrigerant supply pressure should be kept within the limits specified in the equipment’s technical data sheet. The following measures can be implemented during routine maintenance:
- Optimize condensation performanceRegularly clean the condenser heat exchange tubes to remove scale or dust; ensure that the fans of cooling towers or air-cooled condensers are operating properly, and maintain appropriate flow rates and temperatures of the cooling medium.
- Inspect the system for impuritiesCheck whether non-condensable gases (such as air) have entered the system, and if necessary, promptly perform vacuumizing and venting.
- Monitoring pipeline resistanceRegularly inspect the filters on the liquid supply line for clogging and verify that the valves are fully open to minimize unnecessary frictional pressure losses and ensure stable supply pressure.