To increase the cycle efficiency and reduce power input in a water-cooled chiller, a sub cooling circuit, also called a thermal economizer, maybe built into the condenser. The condensed liquid refrigerant is circuited to a special section in the bottom of the condenser called the sub cooler.
In the sub cooler, the refrigerant is subjected to the coldest entering condenser water. The liquid refrigerant is sub cooled by 10 F to 15° F below the saturated condensing temperature. A sub cooler is an economical way to increase refrigeration cycle efficiency without adding work on the compressor.
The typical method of sub cooling +6 as described earlier simply cools the liquid below its saturation temperature in a tube bundle that cold condenser water circulates through. This might also be described as sensible sub cooling, because no refrigerant has flashed or changed state (latent heat transfer).
Another method is called “flash sub cooling”. In the flash sub cooler a portion of the refrigerant is allowed to flash back into vapor further cooling the remaining refrigerant and effectively lowering the saturated condensing temperature. The flash sub cooler is not really a simple sub-cooler but a lower saturation temperature condenser. Thermodynamically, the net effect of a properly sized water chiller sensible sub cooler can be the same as that of a flash sub cooler in that the amount of heat being rejected by the refrigerant to the condenser water is the same.
The “flash sub cooler” has two cost advantages over a sensible sub cooler:
- It requires less refrigerant charge, since vapor is less dense than refrigerant and most of the refrigerant side volume in a flash sub cooler is vapor while it is liquid in a sensible sub-cooler.
- It has a very good refrigerant-side heat transfer coefficient (two-phase flow has a better heat transfer coefficient than a pure liquid).