New compressors screw down air-conditioning costs
Replacing reciprocating compressors in a chiller serving the London office of Rolls Royce with screw compressors has not only solved the problems of repetitive compressor failure but also reduced power consumption by 30 to 40%, paying for the upgrade in about 18 months. An improvement in performance of 27% at full load and 42% at part load has been achieved. The work was carried out by ThermaCom.
If the cost of remanufacturing the reciprocating compressors is compared with the extra cost of the new screw compressors, the payback is much quicker — less than eight months.
The chiller was installed in 1991 and had six reciprocating compressors in a compact unit, with plate heat exchangers for condensers and evaporators. The refrigerant is R407C.
Among the problems revealed by an inspection of the plant were the following.
• A wide variation in load, with considerable variations in the demand for chilled water over a short period.
• Low refrigerant charge, which was also understated on the manufacturer’s data plate.
• The resulting system instability caused slugs of liquid to return to the compressor — which presents serious problems for reciprocating compressors.
In addition, reciprocating compressors operating with R407C are particularly prone to valve failure due to reduced upper-cylinder lubrication by the polyolester lubricants that are necessary with R407C.
Only two of the reciprocating compressors had failed and they were replaced with Hanbell parallel-shaft screw compressors. Because no heat-load data was available, the compressors were sized to provide a displacement volume equivalent to the swept volume of the reciprocating compressors being replaced.
The new compressors were positioned to one side of the bedplate of the chiller, with appropriate repiping.
The reciprocating compressors had a single stage of capacity reduction to 75%, which was simply wired to the 75% step unloader position of the screw compressors.
Although the chiller has six circuits, the limitations of the site power supply prevent two of them being used, and a large and oversized centrifugal chiller has to used to meet peak loads. Replacing all the reciprocating compressors with screw compressors will enable all six circuits to be used to satisfy the building load throughout the year.