Scroll Compressors — a Primer


Recently, I have received questions about scroll compressors and how they work.  With the aid of the Copeland web site and some materials I have gathered over the years, here is a “primer” on scroll compressors.  Hopefully, this will help explain the functionality of scroll compressors. The only difference between Copeland and Bristol that I am aware of is where the fixed and moving scrolls are located.  Otherwise, the principle of operation is the same — components are just located in different spots.

A variety of compressors are used in  air conditioners and heat pumps. Some units utilize basic reciprocating compressors with “reed” valves.   While still others utilize the Scroll compressor. All of these compressors are referred to as HERMETICALLY SEALED compressors, meaning they are enclosed in a can that is welded shut.  All components are internal to the sealed can. The reason some manufacturers use a scroll or a recip in their units is simply the fact in certain designs, a recip or a scroll will give the unit better efficiency and the manufacturer goes with the one that gives it the “efficiency edge” for their product.

But this post is on scroll compressors so we will begin the discussion here. Below is a cut-away of a scroll compressor.  Both Copeland and Bristol make scroll compressors.  We will use this cut-away of a Copeland Compressor to explain how it works.

scroll

Suction gas, oil, and droplets of liquid refrigerant enters the compressor through the larger of the two stubs located ¾  of the way up on the side of the shell.  As the gas enters, the entrained oil and heavy liquid will tend to drop over the motor windings providing some cooling effect before draining through the slots around the stator of the motor, finally ending up in the sump.  Suction gas and small droplets of liquid will be pulled into the scrolls and is compressed into a high pressure discharge gas exiting the domed area of the compressor.  The complete chamber at the top of the compressor is under high pressure/high temperature before the gas leaves through the outlet or discharge tube. scroll cut away

At the top of the compressor, in the dome (not pictured here), is a capped cavity where a thermostat or sensor is located.  Copeland supplies a thermostat set to open at 280 F +/- 10 to monitor discharge temperature. Also notice in the dome is a check valve that opens when the compressor is running and closes when it shuts down.  There is also a pressure relief valve in the dome as an added safety. If pressure or temperature starts to build up in these compressors, the internal relief valve opens and “dumps” hot discharge gas into the shell causing the compressor to shut off on internal overload.  The internal reliefs are set at [+/- 50 lbs] R410A – 600 lbs,  R22  -450 lbs ‘Differential” pressure.  (see my post  —Compressor Valves / Internal Relief Diagnostics for more detail)

Scroll compressors have a spring-loaded fixed scroll laying directly over the moving or orbiting scroll to assure that no gas bypasses the chambers during compression.  If the suction gas has liquid refrigerant present as it enters the chamber, it will cause hydraulic pressure to exceed 1000 PSIG during compression.  The spring-loaded fixed scroll acts as a safety release valve and the liquid bypasses the scrolls without damaging the compressor.  The “rattling” noise that is heard for a few seconds is actually liquid refrigerant Passing through the check valve as it exits the dome.

Pscroll fixed and orbiting1

The scroll compressor derives its name from the two identically shaped scroll forms made of heavy cast steel.  The top scroll in the shell is fixed inside the compressor.  The orbiting scroll fits inside the other and is attached to the motor which is rotating at 3500 RPM.

There are 3 important characteristics of scroll compression:

  1. Six pockets of gas are being compressed at the same time making it a very efficient compressor.
  2.  The compression process requires only a very simple orbiting motion of about 3/8” and eliminates the many moving parts of a piston type compressor.
  3. The compression process is continuous with about 2 revolutions to complete one pocket cycle.  This feature makes it a very quite running compressor since it has no gas pulsation that are apparent in a piston type compressor.

As a review, it should be noted that centrifugal force from the orbiting scroll during the compressor operation ensures very low leakage between the scroll halves.  More importantly, the radial compliance design allows the scroll parts to separate in the event that any debris or liquid refrigerant finds its way in between the two scroll members and quickly re-seals once the particles have passed. You could call this action “spring loaded” in a sense.  An additional benefit of this actual compliance is that the scroll is self-compensating for wear.  It actually allows the compressor to “wear in” and improve performance over time.

Lastly, contrary to “public opinion” that says Scroll compressors  do not  need hard start kits, this is not always true.  Because of the operating characteristics of scroll compressors, they do start easier and may not need start kits.  However, this does not eliminate the need for start components in application with non-bleed TXV’s, operating at 208 volts (or any low voltage), or when there is long line applications. In these cases, hard start kits should be applied to the compressors.

Important:  unlike reciprocating compressors that utilize two pistons attached to a crank-shaft driven by a motor, Scroll compressors are directional.  This is especially important to understand when working with 3-phase scroll compressors.  Rotation must always be checked on 3-phase scroll compressors since they will not function and will continue to trip the overload if allowed to run in reverse.  This is noted by increased noise from the compressor, suction and liquid pressures are almost equal, and the compressor is drawing low amps. Failure to check rotation of a 3-phase scroll compressor can eventually damage the motor of the compressor causing compressor failure.

(Thanks to Copeland for the pictures and for some of the information in the post that was taken from the Copeland web site)

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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