A Lesson on Compressor Displacement, Horsepower

June 5, 2000
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This incident actually happened.

A low-temperature, central rack system was short on capacity. Two people decided to replace a 10-hp compressor with a 15-hp model. They had monitored the original amp draw. When the compressor changeout was accomplished, the rack was still short and the amp draw was unchanged. Why?

First, consider the compressor. It was a semi-hermetic. (By the way, did you know that calling a compressor “semi-hermetic” is a misnomer? Is it a true hermetic whose insides are accessible? “Accessible hermetic” is a better term, but both it and semi-hermetic are used interchangeably.)

The important thing is that the compressor is limited to only one speed. That is a function of the number of motor poles present and the frequency of the electricity provided. For 60-Hz power, the speed is usually 1,800 rpm. This is the synchronous speed. The motor is subjected to some slip, however, and the actual speed could be 1,750 rpm.

Did you know that the same compressor could be used on 50-Hz power? The capacity will be five-sixths of the 60-Hz capacity. If the 60-Hz voltage is 480, then the 50-Hz voltage must be 400. (If this reads like some elementary, boring tome — it is. But it’s good for you, so keep reading.)

Dimensionless Parameters

Compressor displacement can be determined by the total area of the pistons x the stroke length x the rpm x a constant, which is 2 x pi. Pi — as you know — is 3.1417. And there are 2 pi radians in one revolution.

Forgot what a radian is? It is a dimensionless angle measurement obtained by determining the arc length of the angle and dividing the value by the radius of the arc.

The value of dimensionless parameters cannot be underestimated. We deal with them every day. Whenever you convert feet to inches, you are multiplying feet by the dimensionless quantity unity or one.

Since 12 in. equal 1 ft, there are 12 in./ft that is equal to one. Actually, the dimensions (inches, feet, and pounds) can be cancelled in an equation, the same as numbers.

The realms of heat transfer, aerodynamics, and gas dynamics are loaded with numerous dimensionless parameters to provide insight into scientific areas where relatively little hard, indisputable theory exists.

With respect to compressor displacement, various horsepower capacities are available for compressors with the same displacement. What this means is that the potential for horsepower is available.

The actual horsepower delivered is that which is required, not desired. A 5-hp load is a 5-hp load. A 10-hp compressor will deliver 5-hp and no more until the load increases.

The difference is that the 10-hp compressor will not overheat and the motor windings will not burn out on a 10-hp load. The 5-hp compressor might meet the 10-hp load, but not without harmful overheating.

Why not make all compressors meet the heavy load condition? Some compressor manufacturers actually do this. That’s not a bad thought from a reliability point of view. In this age of expensive power, sometimes convenience and conservatism has to give way to higher-efficiency motors.

Heavy loading, reduced copper windings, and reduced steel in the motor fields can accomplish this. What is the answer? You make the call!

Let’s Bring Refrigerants Into The Picture

In refrigeration, we commonly deal with medium- and high-pressure refrigerants. A typical medium-pressure refrigerant is R-134a. For high pressure, we have R-22.

High-pressure refrigerants have one definite advantage: You can pack more horsepower and refrigerating capacity into a smaller space. This is of vital importance with respect to a multiple-compressor, central refrigeration rack system. The rack’s price is mainly determined by its size and how many compressors are on it.

Compressor displacement is costly; horsepower is not. Why use medium-pressure refrigerants anyway? Why indeed.

With respect to automobiles, the answer is obvious. Even with R-12, high-side pressure can soar to more than 300 psig. In the realm of refrigeration, the answer is not so obvious.

Consider this: R-12 was the first popular halocarbon that happened to be a medium-pressure refrigerant. It had one endearing quality that proved to be its most important attribute: It loves oil throughout its whole operating range. This made it the most forgiving refrigerant with respect to oil return. Sloppy piping practice would still provide an operating system.

Now, however, all bets are off. In short order, R-12 is meeting its demise and no new refrigerant can meet the prime function of superb oil return. Therefore, it would seem that for refrigeration, high-pressure refrigerants should be used due to the smaller displacements required.

Now For an Example

Consider R-22 for a medium-temperature application. A popular Copeland Discus, in-line compressor is the 3DS1500. For a low-temperature R-22 application, the compressor need only be a 3DS1000. This equates to a one-third drop in horsepower potential.

Why so, if the refrigerant is unchanged?

The answer is that the low-temperature condition utilizes a thinner gas. In other words, its specific volume has increased, which results in a density decrease.

Listen to a central system rack when it comes out of defrost. The higher-pressure, warmer suction gas will produce a very noticeable sound difference as the compressor loads up. A compressor amp reading verifies the result.

And now back to the incident that prompted this — the low-temperature, central rack system that was low on capacity. As you have probably figured out by now, the 15-hp compressor changeout resulted in the identical displacement substitution that had no effect whatsoever on the refrigerant.

Indeed, the compressor’s actual function was to be a medium-temperature application. The changeout needed a compressor with increased displacement as well as horsepower potential.

A number of open-drive compressors have been converted from R-12 to -22 with respect to air conditioning applications. The compressors were originally direct driven at 1,750 rpm. One cannot provide a direct R-22 substitution, since low suction and motor overloading will result. The compressor needs to be slowed to l,140 rpm through the use of belts and pulleys.

By cutting the base and relocating the motor, one can obtain the desired 1,140 rpm with the motor at 1,750 rpm.

Any questions?

Batista works with Hussmann Corp., 12999 St. Charles Rock Rd., Bridgeton, MO 63044-2483; 314-291-2000.

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