Horsepower = Tonnage

[acman]

How does horsepower relate to tonnage? I must have missed that day at school! Or I'm getting old!

[Dan]

It's a coincidence. In air-conditioning, the horsepower rating of the compressors approximates the tonnage. In refrigeration, the relationship does not exist. It causes a lot of confusion when people who are used to air conditioning step into the realm of refrigeration and refer to "tons" of refrigeration.

Even "horsepower" as we use it only approximates horsepower, for that matter.

[Prof Sporlan]

If we are simply converting units of power, we have:

1 ton refrigeration = 4.712 horsepower, electric

But when we try to relate compressor horsepower to tons refrigeration, we are not simply converting units. The work of the compressor is being used to move energy for the most part, and not change it as you would have with a resistance heater.

So if we have a 16 EER a/c unit, which is to say we get 16 Btu/hr cooling for every watt of power used, and we note another conversion:

1 watt = 3.412 Btu/hr

We see that our 16 EER a/c unit is providing 16 / 3.412 = 4.69 watts of cooling for every watt of power used. This is also referred to as the coefficient of performance (COP).

Some confuse this issue as getting something out of nothing, but this is hardly the case. Again, we are moving energy around, not converting it.

So we get the following relationship with our 16 EER unit:

1 hp, electric = 4.69 / 4.712 = 0.995 tons refrigeration

Thus the one horsepower to one ton relationship with a/c units.

As we try to achieve lower evaporator temperatures with refrigeration systems, more power is required to move the same Btus, and we require more horsepower to achieve a ton of cooling.

[Dan]

Good post, Professor. It reminds me that horsepower and amperage may have a relationship. Permit me to use the word "draw" if I may.

I recall seeing compressor curve charts where a 10 hp compressor and a 20 hp compressor have the same amperage draw under certain conditions. Is this possible?

[acman]

ok,so how do we determine the horsepower of a compressor in refrigeration,as it is not on the nameplate? ( besides phoning the supplier):confused:

[Dan]

Professor, your observation regarding the coincidence between the horsepower and tonnage thingy, was the first thing that came to my mind, when I observed the thread title....

Jeesh. I never get credit for anything. It was I who used the word "coincidence." The Professor merely began to address the issue properly.

[Dan]

ok,so how do we determine the horsepower of a compressor in refrigeration,as it is not on the nameplate? ( besides phoning the supplier)

Amperage. Amperage describes work. Voltage describes power.

Like blow and suck. Voltage blows and amperage sucks, for example.

There are are many ways to determine the horsepower of a compressor that has no nameplate. But a motor with no load and the same motor overloaded has a different horsepower.

Nominal horsepower is what we are addressing. Nominal means "in name only."

Horsepower and tons are probably some terms we need to abandon. Especially since we do not use melting ice for refrigeration and horses to deliver it. Clip clop.

[acman]

I seem to remember 1 hp = 746 watts...If we convert that to btu/h can't we determine tonnage? It doesn't seem to add up!!!

[Prof Sporlan]

I recall seeing compressor curve charts where a 10 hp compressor and a 20 hp compressor have the same amperage draw under certain conditions. Is this possible?
Compressor horsepower, of course, is simply a nominal rating point, no different than say a "10 ton" TEV.

Actual TEV capacity will vary from its nominal rating, depending on the operating conditions. We could have a situation where a 10 ton and 20 ton TEV have the same rating, if appropriate operating conditions are selected for each.

One would not expect a 10 and 20 hp compressor to be drawing the same amps at the same voltage if both were operating at the same conditions. But perhaps we could have a situation where they might operate very similarly if, say, the 20 hp compressor had a more efficient motor, and both compressors were operating at low suction pressures, near or outside their design envelope, with the 10 hp compressor delivering greater capacity per horsepower. :)

[Prof Sporlan]

I seem to remember 1 hp = 746 watts
This is correct


If we convert that to btu/h can't we determine tonnage? It doesn't seem to add up!!!
You need to account for the system's COP. See the Prof's example with the 16 EER a/c unit.