Understanding Scroll Protection Devices

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Scroll compressors have been used in the HVAC industry since the late 1980’s. As government regulations push system efficiency standards higher, scroll compressors have become the standard for compression found in current OEM air conditioning systems. The development of this type of compression relies on tight machining processes, which are capable of achieving the close clearance/tolerances needed to minimize re-expansion. By reducing this potential leakage, the scroll compressor is able to achieve overall better system performance than conventional reciprocating compressors. This improvement in system efficiency is indicated recently by the sales of scroll compressors overshadowing reciprocating compression in both OEM and service markets.

Therefore, it is extremely important for the HVAC technician to understand the design and operation of scroll compressors. One key area I would like to highlight in this article is the internal protection devices common to scroll compressors. If proper service practices are not followed, severe system damage or improper system troubleshooting could result.

Protection Controls
Scroll compressors incorporate a variety of internal protection controls. It is important to understand these protection features because they actuate the internal line break motor protection. Protection features that you could find in an air conditioning scroll less than 7 tons may be:

The TOD (or Therm-O-Disc) – this is a bimetallic disc that senses the discharge temperature and opens at approximately 270*F internal plentum temperature.
The IPR – (Internal Pressure Relief), opens at approximately 400 +/-50 pounds differential for R-22. If this pressure differential is exceeded, the IPR will open. On a ZP scroll for R-410A, the IPR differential is set higher, 550 to 625 PSID.
The floating seal separates the high side from the low side, but also can prevent the compressor from drawing into a deep vacuum and damaging the Fusite electrical terminal.
The internal motor protector is an inherent protector, sensing both internal temperature and amperage. A compressor with a tripped motor protector will exhibit a high shell temperature. Allow the compressor to cool at least 90 minutes in order to reset the motor protector.

Summary
By being aware of these protection controls on the scroll compressor you should be able to avoid improper troubleshooting and accidentally causing severe system damage.

Have you heard any stories of a fellow contractor misdiagnosing a system problem due to being uninformed about these protection controls?

Understanding Scroll Protection Devices