basic explanation of tonnage to a non-Refrigeration person 2

[ozmosis]

I wonder if somebody could explain, in quite simple terms, (or point me to a decent informative website)how the term 'tonnage' is derived with respect to chiller refrigeration. I found the statement that "A refrigeration ton is the power required to cool 1 short ton of water by 1 °F" but am having difficulty relating this. I come from the motor control business and as I hear tonnage quite often talking to chiller folk, I just want to brush up on the basic knowledge.

 

[Wolfie82]

Tonnage is the unit of measure used to describe cooling capacity. It is a term that goes back to the beginning of the air conditioning industry when they used ice for cooling buildings. One ton of cooling is based on the amount of heat needed to melt one ton (2000 lbs.) of ice in a 24 hour period. One ton of cooling is equal to 12,000 Btu/hr. The term "ton" is easier to use instead of talking about hundreds of thousands or millions of Btu/hr.

Regards,
Wolfie82

 

[willard3]

The 12000 btu/hr number includes sensible heat to change the water temp and the heat of fusion/formation (latent) for turning ice to water. 144 bu/lb latent, by the way......

This is in case you start with numbers trying to figure it out as I did as a young Engineer.

 

[ozmosis]

Thanks for your info. I've had difficulty relating a unit of mass to, what is in effect, a unit of energy.

 

[katmar]

There is no sensible heat involved in the definition. It is the power required to melt 1 short ton (2000 lb) of ice at 32 deg F to water (also at 32 deg F) in 24 hours, i.e. it is the latent heat of fusion only.

Using Willard3's 144 btu/lb latent heat the conversion is
144 btu/lb x 2000 lb / 24 hr = 12000 btu/hr


Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[Yorkman]

To get an idea of what one ton of a/c represnts. The avergage window a/c unit ranges from about 9000 BTU/Hr .75 tons to about 18000 BTU/Hr for a large window unit or about 1.5 tons. The first chiller I ever set went about 1200 tons and being a young cub I though my G-D that is one heavy piece of equipment. Little did I know they were refering to capacity.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[Yorkman]

For the metric folks thats 12000 BTUs equal about 3.5 KW

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[SAK9]

I ton is the cooling effect produced by 1 ton of ice when it melts to water at 0 C

 

[ozmosis]

yorkman
although I'm not a young pup, I had the same thoughts on "wow, that doesn't look 400tons...there must be something more to this tonnage idea.." and thanks for the metric conversion, I can certainly relate to that a little better. However, tonnage seems to be universal whether you are metric, imperial or ambidextrous.

 

[ozmosis]

another question, on the same topic. Is there a rule of thumb guideline between tonnage/btu/kw and electrical rating (kW/hp)for the compressor?

 

[25362]

One can add that the TR, ton refrigeration, is a unit for the rate of heat flow equal to 200 Btu/min or ~3516.85 watts. Aka standard ton.

 

[quark]

That is quite difficult and depends upon no. of parameters viz., type of compressor, condensing temperatures, load and capacity control etc.

Generally, you can consider the following ranges.

Centrifugals (above 200TR) - 0.5 to 0.75 kW/TR
Screws (above 100TR) - 0.7 to 0.9 kW/TR
Recips (below 100TR) - 0.9 to 1.2 kW/TR

 

[Yorkman]

sed2developer,
When you asked for a rule of thumb did you mean: tons/H.P.?
For general a/c compressors, positive displacement; it is about 1 H.P. per ton, open drive centrifugals for typical 45 degree chilled water systems, that still holds pretty true. When you go to medium and low temp. refrigeration the ratio goes to heck because the specific volume of the gas goes up and so does your compression ratio. You will need more horses for that same ton of refrigeration.

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.

 

[willard3]

snip
another question, on the same topic. Is there a rule of thumb guideline between tonnage/btu/kw and electrical rating (kW/hp)for the compressor?
snip

"Rules of thumb" are the one method guaranteed to make troublesome HVAC systems because the "rule of thumb" is subsituted for thorough analysis.

 

[Yorkman]

I agree and I would not use a rule of thumb guideline to make an analysis of a system that appeared to be out of adjustment or not producing the expected result.
On the other hand, sometimes a rule of thumb can send up a red flag when I see a system performing outside an expected norm. It will cause me to look a little closer to what pressures and tempratures are indicating and try to resolve the cause of the discrepancy.
A mentor of mine once said, "If you can't verify a problem with accurate instrumentation and, or prove it through mathmatical analysis you really haven't identified the problem". So is the life of a refrigeration service mechanic!

I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int.