Minimum allowable condensing pressure

Liquid injection oil cooling

One of the methods of oil cooling for screw compressors is a liquid refrigerant injection. Usually, liquid refrigerant from the receiver is injected into the compressor at an early stage of compression. TXV regulates the flow of liquid to maintain the desired oil temperature. Compressor manufacturers often specify a minimum condensing pressure around 125 psig to ensure adequate injection performance.

To keep oil temperature at a desired level, we have to balance demand and supply. Demand is an oil cooling heat rejection. Supply is a liquid refrigerant flow. When we reduce condensing pressure, demand and supply are decreasing, but at different rates. Reduction of supply is going down faster than reduction of demand. At certain lowered condensing pressure we will have an imbalance and oil will overheat.

To keep the balance of demand and supply, we have to increase the capacity (supply) of TXV:

There are several methods to increase the capacity of TXV:

1. Increasing size of TXV. This method required 2 different settings of TXV for high condensing pressure and for low condensing pressure.
2. Subcooling of liquid refrigerant before TXV. DX heat exchanger can cool refrigerant before TXV and capacity of TXV will increase up to 20%.
3. Reducing outlet pressure of TXV.  Sometimes screw compressors have dual-port liquid injection oil cooling. Internal pressure inside screw compressor is lower for low Vi (compression ratio) port and it is higher for high Vi port. To increase capacity of TXV at lowered condensing pressure, we have to use low Vi port regardless of compression ratio.
4. Increasing inlet pressure of TXV. There are 2 ways to increase inlet pressure of TXV:
   • increasing condensing pressure
   • increasing liquid refrigerant pressure by liquid pump at lowered condensing pressure.

Every refrigeration plant is unique, and every refrigeration plant has a unique solution for liquid refrigerant oil cooling.

Regrettably, majority of companies are using the least efficient method, such as increasing condensing pressure.

Hopefully, this newsletter has shown you other methods to increase capacity of TXV. Implementation one of these methods will give you opportunity to reduce the minimum allowable condensing pressure lower than 100 psig.