Meaning of horsepower ratings on compressors

[Gautam]

This a topic that's always bothered me, and seems to confuse me more the further I try to make sense of it.

I've read some people quoting that you can convert about 7000 btu to 1 hp, and others 12000. According to google (http://www.google.com/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=g93&q=horsepower+to+btu%2Fhr&btnG=Search) and some other sites, its only about 2500. :confused:

However, even barring the apparent inconsistency in conversion factors, how can a capacity rating on a compressor possibly mean anything unless its tied to a temperature?

For example, looking at the data sheet (http://rc.danfoss.com/TechnicalInfo/literature/manuals/06/nf11fx_steel_r134a_115v_60hz_12-2005_ed402g122.pdf) for the NF11FX, its rated at over 4500 btu/hr at +45f, translating to nearly 2 hp, but at -30, a significantly tinier rating of 500 btu/hr or only 1/4 hp.

If one looks at minimum rated temp, the inferior NF9FX (http://rc.danfoss.com/TechnicalInfo/literature/manuals/06/NF9FX_R134a_115V_60Hz_07-03_Cd43r822.pdf) actually looks superior, because its minimum rated temperature is 10f higher, but its capacity at this temperature is also higher.

Which rating has more meaning, if any? And which do people talk about when quoting horsepower figures?

Sorry for this newbie question, but I'm thoroughly confused here.

[Blaster]

one thing is the motor electrical power and other thing is the refrigeration capacity

they are 2 different things

[[XC] gomeler]

I go by how much power the motor is rated for and how much it draws from the wall, 746 watts to the HP. So a 400 watt compressor is roughly a 1/2HP compressor, just working around from there.

[wdrzal]

you need be be sure that is in the "electrical data chart" and not "thermodynamic chart", it can be in watts too,or btu's, joules ,calories etc etc

[DetroitAC]

how can a capacity rating on a compressor possibly mean anything unless its tied to a temperature?


You've hit it right on the head! Ratings for cooling capacity and motor power (horsepower) rating only applies to the point at which that compressor is rated, the "nominal" conditions. If you have an R-134a compressor that is rated at 1/2hp at air-conidtioning conditions (something like 5C evaporating temperature, 45C condensing temperature), then the 1/2 hp is BALLPARK the amount of motor power used. The cooling capacity of the compressor is entirely a different figure (how much cooling the compressor will produce when operated correctly in a refrigeration cycle), and again depends on the conditions.

If you take the same compressor and operate at -40C evaporating, 30C condensing, the compressor motor will consume way less than 1/2 hp, and the cooling capacity will be very tiny.

The hp rating of the compressor just gives a rough ballpark number to tell you what size the compressor is, and as such is useful.

[Clemmaster]

The power the constructors give to us is, in general, the motor's power consumption when it works alone. If you take a look at the specifications you'll find the real power consumption at different evaporating temperatures of the gas the compressor is designed for. Which means that the power doesn't mean anything, lots of phase-change maker still ask why their systems aren't able to carry any load, if they just looked to the specification they'll understand they aren't using their compressors well (such as using a MBP compressor (piston) in vaccum......... :|).

If you compare the nl11f (1/3hp) and SC12G(1/2hp), the first one is able to carry more than 100w @ -35° (R134a in CECOMAF conditions), the other one isn't able to carry more than 65w in the same conditions, but it's still a 1/2 compressor.....

[Big SturL]

Also, don't think anyone uses HP to give figures to cooling capacity, of course, one could.

[Clemmaster]

As DetroitAC said, the indications (1/2hp, 1/3hp ...) are just used to classify compressors that belong to the same category (LBP, MBP, HBP). You can't compare a LBP compressor to a MBP compressor, that's totaly weak.

The power consumption is also variable, a compressor doesn't consume the same if it's working @-35° than @-15°.

[Xeon th MG Pony]

As an example:

20F Evap @ 130F Cond:
R-12
3/4HP
6,650 BTU/h (1.95Kw)
120V 1ph W:1.135Kw FRA:11.6 LRA:68

That is the spec of compressor that I am retrofitting for a walk in cooler and it is medium temp See how the ratings are given with the condensing temp AND evap temp?