Originally Posted By: George Bucklin
This post was automatically imported from our archived forum.
I believe that the training slide for Air Conditioning Capacity – Titled “One Ton of Cooling” (Slide 1215) – is incorrect (in part).
While text items 1. cooling is typically measured in tons
2. one ton of cooling is equivalent to 12000 BTU/hour
are correct in general context, they tend to be misleading. This is shown
by the third text item.
3. 12000 BTU's is the amount of heat required to melt one ton of ice.
This doesn't really make sense if we think about it. One BTU is amount of energy to raise one pound of water by one degree fahrenheit. By this reasoning 12000 BTU would raise 6 tons of water by one degree f. (= 6 times 2000 pounds).
Of course, it takes more energy to convert one pound of ice at 32 degrees f to water at 32 degrees f than it would to raise one pound of water fro 32 degrees f to 33 degrees f. For a unit volume in the metric system it takes 80 calories to convert one gram of ice to water, while only taking 1 calorie to raise water by one degree centrigrade. Converting this to the British Fahrenheit system we multiply by 9/5, which yields 144 calorie units to go from ice to water.
NOW the misleading part of this slide is in the definition of a TON. A ton of air conditioning is the amount of energy required to melt a ton of water over a 24 hour period. So the actual energy is 12000 BTU/hour times 24 hours = 288,000 BTUs. If we divide this by the pounds/ton, we get
288000/2000 = 144 BTUs/pound, which matches the units in the paragraph above.
So the end result of all this math is that the third text bullet is incorrect. It should be 12000 BTU is the energy required to melt 2000 pounds/ 24 hours = 83 1/3 pounds of water. Or it should read 12000 BTU/hour for 24 hours is the energy required to melt one ton of water. Or it should read 288,000 BTUs is the energy required to melt one ton of water.
Hope this helps. The intent here is to give a realistic sense of the ton rating. Otherwise, we are off by a factor of 24.
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