1. A method for reducing the coefficient of performance to a design value from measured temperature conditions of a vapor compression type refrigeration system of the type having series arranged compressor, condenser and evaporator units and an electric motor for driving said compressor unit, said system further having a condenser water supply line and evaporator chilled water supply and return lines, said method comprising the steps of: storing a value (CWF) for the chilled water flow through said condenser; sensing the condenser water supply temperature (T.sub.con), sensing the evaporator water supply and return temperatures (T.sub.chw,in and T.sub.chw,out), sensing the power input (.phi.) to said motor; calculating the cooling load (Q.sub.c)* from the expression Q.sub.c =CWF*(T.sub.chw,in -T.sub.chw,out); storing the manufacturers' performance data for said system comprising the condenser supply water design (T.sub.con,D) and the evaporator return water design temperature (T.sub.chw,D); storing a value (.alpha..sub.temp) which is the ratio of the required change in condenser temperature to a unit change in chilled water temperature to maintain the same rated chiller capacity; calculating the equivalent temperature (T.sub.r) which compensates for variation from the given design temperatures from the expression EQU T.sub.r =(T.sub.con -T.sub.con,D)/.alpha..sub.temp -(T.sub.chw -T.sub.chw,D), storing experimentally determined coefficients b.sub.1, b.sub.2, and b.sub.3 for the quadratic expression EQU .beta.cap=b.sub.1 +b.sub.2 *T.sub.r +b.sub.3 *(T.sub.r).sup.2 wherein .beta..sub.cap is the ratio of the available capacity (CAP) at the actual chilled water and condenser water conditions to the design capacity (CAP.sub.D); calculating said .beta..sub.cap and said CAP; calculating the part-load ratio .GAMMA..sub.load from the expression .GAMMA..sub.load =Q.sub.c /CAP, storing experimentally determined coefficients a.sub.1, a.sub.2 and a.sub.3 for the quadratic expression EQU .delta..sub.pow =a.sub.1 +a.sub.2 *.GAMMA..sub.load +a.sub.3 *(.GAMMA..sub.load).sup.2 wherein .delta..sub.pow is a fraction of full-load power, calculating the full-load power (PFL) from the expression PFL=.phi.*.delta..sub.pow ; and calculating said COPA from the expression COPA=CAP/PFL.