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Using Performance Curves as a Service Tool
by icemeister
Often when servicing an HVACR system the technician finds a situation where the
compressor's apparent performance is questionable. Any number of possible
scenarios and causes may lead to this such as a higher than expected suction
pressure or possibly an excessive amp draw. In any case, after performing all the
standard system checks and tests the question often becomes “Is the compressor
doing what it's designed to do?
To get to this point the tech may believe he has exhaust all of his resources and
feels stumped as to why this pump isn't operating as he feels it should. There's
one additional tool he is overlooking. The Compressor Performance Curve.
Compressor manufacturers publish standardized performance information in
several forms. The most basic of these are those for performance at standard ARI
rating conditions. These typically only state a single capacity rating for a standard
condition. Heat pump compressors are usually rated and two standard conditions.
Neither is of much real use to the field service technician.
Manufacturers also publish expanded ratings which may be in the form of catalog
capacity ratings charts where the compressor performance data includes capacity
(Btu/hr) and power input (KW) but still this doesn't offer much for the service
tech.
This brings us to yet another form of published compressor ratings offered by
manufacturers. It's the performance curve.
The term “performance curve” comes from the method which has been used for
generations by plotting compressor performance in a graphical format and
relating it to saturated suction temperature vs. saturated discharge temperature.
Typically the plots include curves for capacity, power input and amperage at a
given voltage.
The amperage rating is precisely what the field service technician need to answer
his question.
Take a look at a classic performance curve from 1966 on p.2. This is for a 3-phase
medium temp Copelametic running on R22 refrigerant.
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Note how the performance curve can be used to determine what the compressor
should be doing at any given condition. You can see that if the suction and
discharge temperature (or pressures) are known, it's a simple task to read off
what the expected amperage is at those conditions.
One point to consider when using the curve for checking amp draw is the curves
show amperage at a specific voltage. Notice that the Amps curve as shown is for
a 230V supply voltage. In the upper left corner, there are correction factors for
460V and 575V. If the actual or measured supply voltage is different the expected
amp draw will change in direct proportion to the voltage.
If for example the measured supply is 210V. What would be the expected
amperage? To determine this simply multiply the amperage indicated on the
curve by the ratio of the rated voltage / measured voltage:
Expected Amps = Curve Amps x (Rated Voltage / Measured Voltage)
Also you will see that the performance curve lists some fixed parameters like
Return Gas Temperature, Liquid Subcooling, Ambient and Hz
Operation(Frequency).
Return gas temps will have a minimal effect on amp draw. Ambient, as used in
the curves is not to be confused with the ambient temperature entering a
condenser. It's basically only is referring to the air temperature the compressor
sees and also has little effect on performance. The frequency of course is what
determines compressor speed or RPM and would have a significant effect on
performance but for the most part we really don't have to be concerned as our
power supplies are fairly fixed at 60 Hz. The one to be most concerned with is the
liquid subcooling rating as many technician may be confused with how subcooling
affects amp draw in a system.
An increase in liquid subcooling will increase the refrigerating effect dramatically.
It doesn't affect the amperage however. If you're looking for expected capacity of
a compressor, the liquid subcooling factor is necessary. For determining expected
amp draw, subcooling really doesn't matter.
A typical Copeland performance curve from 1982 is shown on pages 4 & 5. Note
that the plotted curve on p.4 now only includes capacity and power input. Again,
this may be fine for a system designer but next to useless for a service tech. The
expected amps are now indicated on chart on p.5.
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Using the chart on p.5 to determine expected amperage at a give set of
conditions usually requires interpolating between the listed ratings points. You
can plot this on some graph paper or even with a computer program for plotting
points, but a simple math calculation or straight line interpolation isn't all that
difficult...although in this case you have to interpolate three times.
You'll notice that the difference between the listed evaporator temperatures is 5ºF
and the difference in condensing temperatures is 10ºF. As an example, let's
consider a condition of 18ºF evaporator and 114ºF condensing temperature.
The first step is to interpolate for the evap temperatures. 15ºF and 20ºF are
listing amps of and we want to know what the amps would be at the condensing
temperature of both 110ºF and 120ºF, so we have to calculate the amps at those
two conditions.
From the chart:
A @ 15ºF/120ºF = 27.09
A @ 20ºF/120ºF = 28.11
1. To find what the amperage is at 18ºF:
A @ 18ºF/120ºF = (18 – 15) / (20 – 15) x (28.11 – 27.09) + 27.09
or... A = (3 / 5) x 1.02 + 27.09 = 27.70 amps @ 18ºF/120ºF
2. Next solve for A @ 18ºF/110ºF:
A @ 15ºF/110ºF = 26.28
A @ 20ºF/110ºF = 27.20
A = (3 / 5) x (27.20 – 26.28) + 27.20 = 27.75 amps @ 18ºF/110ºF
The difference is only .05 amps so we don't have to go further...just call it 27.7
amps. But in some case it may be significant, so here's how to do it:
3. Find the amps at 18ºF/114ºF:
A @ 18ºF/120ºF = 27.70
A @ 18ºF/110ºF = 27.75
A = (114 – 110) / (120 – 110) x (27.75 – 27.70) + 27.70 = 27.72 amps
But wait...we're not quite done yet. Don't forget to factor in the actual supply
voltage. For this example, let's say the measured voltage at the compressor
terminals is 242 volts (averaged). Going back to p.3 and using the amperage
correction equation:
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Expected Amps = Curve Amps x (Rated Voltage / Measured Voltage)
So for 242 volt and inserting our interpolation results:
Expected Amps = 27.72 x (230 / 242) =
26.35 amps @ 18ºF/114ºF/242 V
Now you have a reliable prediction of what this compressor should be doing at
these conditions. Your measured amperage should be within 10% +/- of this
value. If all external factors have been checked and eliminated and the measure
amperage is outside of this range, the performance curve is telling you to look to
internal mechanical or electrical problems.
So far a bit of the past and a lesson of how to use performance curves to your
advantage in the field have been presented. Hopefully this has added a new
diagnostic tool for those tech who haven't used one before and perhaps a
refresher for those who do.
I would be remiss not to mention The Copeland Compressor Performance
Calculator. This is a wonderful tool to add to your arsenal. It is available for free
download at http://www.emersonclimate.com/en-US/Pages/home.aspx
in their Online Product Information page. You must register and log on (which is
fast and easy), download it to you PC or Laptop and you'll be able to generate
performance data for all compressor made by Copeland.
See a screenshot of Copeland's Compressor Performance Calculator with the
results for the example we just did on p.8.
Some compressor models allow for a little extra tool within the Calculator. Its a
Check Amps feature where all you need to is enter the compressor model,
measured voltage, suction press, discharge pressure and it gives you the
expected amperage directly. (For whatever reason this is available for
Copelawelds and Scrolls but not for Copelametic and Discus compressor.)
I hope this has been helpful and I welcome all comments, corrections and
discussion on what has been presented.
Thanks,
icemeister
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(You'll note that the compressor model is updated, but it's the same compressor
as the one in the earlier example. The expected amp draw at 230V shows 27.2
amps. Pretty cool, eh?)