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Re: The chilling truth about cold horsepower

I called my HVAC vendor today to ask him about this question. He uses 1 HP = 1 Ton Refrigeration =12,000 BTU/hr but also said that the HP rating is only used in the really big units. In our conversation he also stated that they use 3200 BTU/hr. as the heat generated by each 1 HP of electric motor that is operating when calculating heat loads in rooms to size them for air conditioning. This could be a rounded up figure from the 2450 BTU/hr to allow a safety factor for them. Could it be that the reference tables are using the 2450 BTU/hr conversion for 1 HP as the heat they generate when operating and not be referring to the output power of the motor?

My understanding has been that the HP rating on the larger units is intended more for sizing of the electrical circuits supplying the units rather than having to do with their air conditioning ability. All the larger units give the HP rating for the electrical loading and the tonnage for the air conditioning ability. One HP = about 746 watts, and that's RUNNING load, starting load can be much more since a motor will use a lot more current to start than to run. And then there are induction motors, capacitor start/run motors, and the three phase motors that all load the circuit differently...

The HP -> BTU conversion might refer to the work done by the motor. It's possible to convert various units of energy, which let's you do things like say you have a 3/4 HP computer or whatever (goofy, but possible). It is thus possible to specify rotational energy from a motor in BTUs instead of the more usual HP, but I think it's safe to say that would be a non-standard way to rate a motor. The BTU number you're seeing as a conversion for HP is probably the total energy INPUT to the motor (all electrical losses and work done by the motor in rotating a load), and thus all the heat that that motor could put into a space that an air conditioning unit would need to remove. At work we calculate tonnage of air conditioning needed in a given space to keep a datacenter at a certain temperature, and the calculations involve the total wattage of all equipment used in the room, the draw down we need temperature wise, and a few other things. We can arrive at a tonnage requirement for the air conditioning system that way.

When I was looking at various articles in the World Book Encyclopedia I read the one on Horsepower. In it it gave an example of the rating of gasoline engines. "Indicated Horsepower" is an estimate based on work done on the pistons by the gases produced by combustion. "SAE" Rating is another estimation method used. "Brake Horsepower" is not an estimate but a measurement of the power applied to the crankshaft. The example they gave rating the same engine resulted in 36.4 Indicated Horsepower, 19.6 SAE Horsepower, and 29 Brake Horsepower. The article also defined 1 Boiler Horsepower as the ability of a boiler to change 34.5 lbs. of water at 212 F into dry steam at 212 F in one hour. There is obviously more than one type of Horsepower depending where it is being used.

Gasoline engine HP doesn't directly equate to electric motor HP, but it's due to the fact that electric motors produce a "smoother" rotational energy and gasoline engines produce rotational energy in bursts as the pistons fire (with a flywheel to smooth it out). That's a bit rough but hey, I'm an EE, not an ME :-)

In a catalog, www.mcmaster.com, search for Liquid Chillers. They are on page 438. I'd give you a link but I don't know how to do it. They rate the units they sell as Compressor Horsepower and BTU/hr. I don't know what is meant by Compressor Horsepower and if it is another type of Horsepower.

Thinking waaay back, I remember a book talking about a compressor duty motor being required to produce more torque at lower speeds. A compressor rated motor is usually an induction or three phase motor that can run at a very heavy load without overheating. There is probably more to it than that, but basically a compressor duty motor is going to be able to start with a full load better than a more conventional motor.

I have a friend who is an ME with Trane (a big manufacturer of both air conditioners and compressors). I'm going to see if maybe he can answer some of the questions posed in this thread.


Well this is long and it is late.

Jerry Smith in Bloomingdale, NJ

Waveform Technology
UNIX Systems Administrator