# Compressor tonnage = horsepower

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• 08-01-2008, 06:30 AM
The Doctor
Compressor tonnage = horsepower
What is the conversion from tonnage to horsepower? Or, where can one find out this information?
• 08-01-2008, 06:44 AM
beenthere
Generally 12,000 BTUs = 1 HP.

But don't expect to find a 3 HP compressor in a 3 ton A/C.
• 08-01-2008, 07:46 AM
The Doctor
Quote:

Originally Posted by beenthere
Generally 12,000 BTUs = 1 HP.

But don't expect to find a 3 HP compressor in a 3 ton A/C.

Now a conversion from kW to hp I understand, more or less.

But this matter of 3 does not equal 3 I don't get...:confused:
• 08-01-2008, 07:52 AM
ISOTHERMAL
Because the more efficient units use a smaller compressor. Example; 3 ton unit may have a 30,000 BTU compressor
• 08-01-2008, 08:03 AM
Special-K
The tonnage-horsepower-BTU correlation isn't so cut and dry anymore. Most compressors now are rated for multiple refrigerants and temperature ranges and all that factor into this. A 3 ton r-2 compressor may be rated at 36'000 BTU in air conditioning, but the same compressor in a refrigeration application could be rated at half that. All you can really go by is the BTU rating for the refrigerant/application/temperature range the compressor will be used at.

One of the ways newer A/C equipment achieves their higher seer ratings is to operate at lower condensing temperatures. That allows the compressor to have an actual BTU output that is higher than it's nominal rating. For example....a 3 ton high efficiency unit may only have a 2.5 ton compressor in it. But at the lower condensing temperature that [nominal] 2.5 ton compressor can put out around 36'000 BTU which give the unit a 3 ton rating. In an extreme example, I've seen a 3 ton water source heat pump that only had a 1.75 ton compressor in it. The very low condensing temps produced by the water cooling gave that tiny compressor a 3 ton output.

Clear as mud? :D
• 08-01-2008, 09:42 AM
powell
Quote:

Originally Posted by The Doctor
What is the conversion from tonnage to horsepower? Or, where can one find out this information?

There isn't a conversion. A ton of refrigeration is 12,000 btuh at any given designed SST.

So, you can just determine the systems designed temperature: cooler, freezer. A/C whatever. Then determine the designed SST, look up the btuhs for that SST and divide by 12,000 and you will get the tonnage.

For systems with additional subcooling you just add that extra capacity btuh to the above btuh.

Or you can just take what the mfg says the btuh are and divide by 12,000 to get the tonnage.

Bottom line, the HP of a compressor, by itself, does not reflect the tonnage of a system.
• 08-01-2008, 04:49 PM
gruntly
Things that make you go Hmmm.

I saw this thread this morning on my way out and did not have the time to post...

The higher the compression ratio, the more horsepower required per ton. A walk in freezer requiring the same tonnage as an A/C unit requires a lot more horsepower because of the compression ratio required to maintain a low temp evap.
• 08-01-2008, 07:13 PM
Tool-Slinger
I'll add this in case it helps:

Same 5hp compressor with multiple tonnage ratings as follows:
5 tons hi-temp
3 tons medium temp
2 tons low temp

That is from memory on an old copeland cast-iron. I think the HP per ton is generally consistent along that formula. That is old-school info., previous exceptions noted.
• 08-01-2008, 07:48 PM
The Doctor
I love this site. Thanks for the replies. Each one made it more interesting, as it should.
Right before I walked out the door I read on Cleaver-Brooks website how a boiler horsepower is not quite what you might think of as a "horsepower". I was looking for a link to the line of boilers that use the water as the electrical neutral, and have nothing in the vessel that is hotter than the water.

Be that as it may, I work with someone who speaks of (refrigerant)compressors in terms of horsepower, and it is a new concept to me, more or less. I know that IR's air compressors are rated in both (kW and horsepower)right on the label, so I was wondering on the topic in a roundabout way.
• 08-01-2008, 09:15 PM
beenthere
Boiler HP is a whole story in itself.
33,479 BTUs to the HP.
• 07-15-2016, 12:07 PM
benatwhodotnet
My only reason for searching out this thread is to figure out how much start capacitor is needed for different tonnage units. The KickStart brand has a TO5 for up to 5hp. Their KS-1 is for everything else? And if I use 745.7 Watts per horsepower then is a 22.8 RLA Copeland really UNDER 5hp? I just found this at

http://www.onlineconversion.com/motor_horsepower.htm

And does a start capacitor really care what the HP is? I would think the start amps would be way more indicative of the need.
• 07-15-2016, 12:27 PM
pecmsg
Quote:

Originally Posted by benatwhodotnet
My only reason for searching out this thread is to figure out how much start capacitor is needed for different tonnage units. The KickStart brand has a TO5 for up to 5hp. Their KS-1 is for everything else? And if I use 745.7 Watts per horsepower then is a 22.8 RLA Copeland really UNDER 5hp? I just found this at

http://www.onlineconversion.com/motor_horsepower.htm

And does a start capacitor really care what the HP is? I would think the start amps would be way more indicative of the need.

Only use factory start parts.
• 07-15-2016, 01:06 PM
benatwhodotnet
Quote:

Originally Posted by pecmsg
Only use factory start parts.

Most of the systems I see in AZ are builder installed. I call them "contractor specials" because they have no start cap, no pressure limit switches, no delay-on-break timer, no fan run detection, and will sit there for days popping the thermal check in the compressor every minute when they can't start. It is sad but these units burn up the compressor when the outdoor fan motor run capacitor fails. If there were factory start capacitors on these I would repair them with matching parts.
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