Many times we will have a customer call in and request a chiller quote from us. When we ask the customer what size chiller he or she needs, the answer is occasionally, “I think a 10 ton chiller?” (We use “10 ton” as an example only). The customer has an idea of what size unit they are replacing but isn’t really sure how that “value” is calculated or derived. We hope to help clarify some of the uncertainty in this post.
One of the biggest misnomers is that a ton of cooling is simply a ton of cooling – no matter what the operating conditions. In theory, yes, this is true. But in real world conditions, a customer’s view of a ton of refrigeration process cooling is actually much different. The actual tonnage for a (nominal) “10 ton” chiller can vary from four tons all the way up to 12 tons based on those pesky design conditions.
You see “nominal tons” are typically calculated at ARI conditions, which is 44°F leaving water temperature, 54°F entering water temperature, and 95°F ambient temperature. What’s that? You say you are using 30% ethylene glycol and operating at 35°F leaving fluid temperature? Well, now the “nominal tons” no longer in the same ballpark as “actual tons”. Using the earlier “10 tons” example, a nominal ten ton chiller at ARI conditions, may only produce 6-7 tons with the conditions just mentioned. Both the glycol and the lower fluid temperature cause the chiller to be less efficient and the output must be derated.
Another common error is referring to the chiller’s compressor HP as being equivalent to the tonnage output. So in other words, a 10HP chiller equals 10 tons of capacity. (Some manufacturers actually base their unit size/model on the compressor HP). The problem is that this is only true in a perfect world and we don’t live in a perfect world. [Hopefully, at this point of the article you aren’t feeling like this guy.]
The truth is the equation for calculating a heat load remains the same no matter what the conditions. It’s the conditions that change.:
Q = m x C x ΔT
Q = Heat Load (BTU/hr)
m = Mass of fluid being cooled
C = Specific heat of fluid (BTU/lb-°F)
ΔT = Change in fluid temperature
Since we know that there are 8.33 lbs in each gallon of water and 60 minutes in an hour, we can convert the “m” value to GPM or gallon per minute (of water). That factor is 499.8. With that, our equation now looks like this:
Q = GPM x C x ΔT x 499.8
Q = Heat Load (BTU/hr)
GPM = Water flow in gallons per minute
C = Specific heat of fluid (BTU/lb-°F)
ΔT = Change in fluid temperature
499.8 = Constant converting lbs to GPM
Further clarifying this to incorporate fluids other than water, the equation changes even more:
Q = GPM x SG x C x ΔT x 499.8
Q = Heat Load (BTU/hr)
GPM = Fluid flow in gallons per minute
SG = Fluid Specific Gravity
C = Specific heat of fluid (BTU/lb-°F)
ΔT = Change in fluid temperature
499.8 = Constant converting lbs to GPM
As a rule of thumb, you need to know at least three of these values in order to calculate the load: Flow rate (& fluid type), Inlet temperature, outlet temperature, and heat load or tonnage required. Going back to our example of a “10 ton” chiller requirement, we will provide some more solid information: Flow = 20gpm; ΔT = 10°F; Fluid is 30% ethylene glycol; Leaving fluid requested at 35°F — Our equation will look like this:
Q = 20gpm x 1.06 (SG) x 0.87BTU/lb-°F x 10°F x 499.8min-lb/gal-hr
Q = 92,183 BTU/hr
Q = 92,183 BTU/hr ÷ (12,000 BTU/ton)
Q = 7.68 tons
So in this case, the correct selection would be our STACT11S (11HP) air cooled scroll process chiller. This unit will produce just under 8 tons at design. There would not be much of a safety factor at these conditions, but it will still handle the duty. Hopefully this helps to clear up some of the confusion in relation to Calculating a Process Load. Thanks for reading.
