Percent per Degree Rule of Thumb for Refrigeration Cycle Improvement

Abstract

A value of 1-1.5% power reduction per degree Fahrenheit (F) has been used successfully for years to estimate the effects of either lowering condenser temperature or raising evaporator temperature in a mechanical cooling system. This concept shows up in a variety of energy conservation measures, all with the goal of reducing power requirements. Reviewing the underlying science will allow confident use of this rule of thumb and explain the range of values given.The principles involved with this rule of thumb include lift, heat exchanger approach, coefficient of performance, refrigeration cycles, and sources of error. Since the work involves differences, pressure and temperature units do not have to be in absolutes. These terms and a Mollier Diagram (pressure-enthalpy, or p-h) were used to evaluate this rule of thumb.Manufacturer's data is the most accurate source of power reduction from a change in operating conditions because it captures all the various influences in a bottom-line live test. A Mollier (p-h) diagram can be used with before/after system conditions for good results. The rule of thumb can be used for reasonable accuracy, especially when incorporating the baseline lift (system lift before the change is made). Using the traditional 1-1.5% rule of thumb can overstate savings, but is ‘safe’ at the 1% level.