Editor's Note: CR4 would like to thank Dick Cawley of GEA Consulting for contributing this blog entry.

It is important that a system as discussed here - or any system for that matter - be allowed sufficient runtime to approach equilibrium after startup. What is equilibrium and how long to reach it? If a compressor is started, transients will occur for a period of time before the lubricant and refrigerant streams settle to a constant, or near constant pace.

To be certain that equilibrium is obtained, the very minimum run time after startup can be calculated by dividing the system refrigerant charge by the refrigerant flow rate. For example, if we have a system with refrigerant charge of 20 lb and a flow rate of 900 lb per hr (typical for 5 ton R22 system), the very minimum runtime should be 20/900, or 0.022 hours (1.32 minutes). A safe minimum runtime in this case might be 2 or 3 minutes.

Minimum run time is usually determined by the unitary system manufacturer and programmed into the control system; but, if not, this is one way of calculating it. Runtime is usually ensured by a timed on control or some space thermostats. Note that the time required will be system dependent. Most unitary systems apply scroll compressors. Runtime information and many other subjects are treated in Copeland Bulletin AE4-1331 R3.

(Stay tuned for more)

- Dick Cawley