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can anyone explain why absolute (14.7)has to be addedto suction & discharge pressure to find the compression ratio .I was asked this question by my boss & did not know the reason why.

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It is a function of math that requires you to change to absolute pressure in order for the ratio to be meaningful. Some teachers claim that it is to keep the calculations out of a vacuum. That is incorrect!

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Originally posted by rix870
can anyone explain why absolute (14.7)has to be addedto suction & discharge pressure to find the compression ratio .I was asked this question by my boss & did not know the reason why.
rix,

At sea level with an ambient temperature of 70 degrees F with dry air , Barometric Pressure exerted on everything, (including the refrigerant in your circuit) is 14.696 pounds per square inch absolute. (Correct your boss on his use of 14.7 if you dare!)Barometric Pressure is the weight of air pushing down on the earth.

I'm not sure any more what the formula is for calculating the compression ratio....if you know it please let me know.

4. because we can not read atmospheric pressure (14,7 psI) with our gauge set...read zero when not connected to a pressurized system......you have to add it!!!!

5. I'm not sure any more what the formula is for calculating the compression ratio....if you know it please let me know.
(psig discharge + 14.7) divided by (psig suction + 14.7) = compression ratio

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Its all relative

0 psig is relative to atmospheric pressure which will read 0 psig when the pressure guage is open to the air.

When we use absolute pressure, which is relative to an absolute vacuum, then 0 psi is indeed zero. Even if a vacuum is part of the calculation the value will still be a positive value.

In theory a perfect vacuum is impossible to attain so any presence of an atmosphere will be a positive value.

Since compression ratio is Discharge/Suction and division by zero is undefined we must keep the suction pressure a positive interger even if it is in a vacuum.

Best regards...

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