Knowing process chiller loads is crucial for ensuring the system operates efficiently and reliably. However, industrial chiller users sometimes don’t know the chiller capacity they need or how to calculate it. The process is also not as simple as estimating that you need a 10-ton chiller, for example, and figuring out the appropriate capacity.
Accurate calculation is a key step in designing, selecting and operating a process chiller to meet specific cooling needs. In this guide, we’ll break down the calculation process and all the various steps involved to help you find a chiller with the capacity to support your operations.
Key Factors and Equations for Accurate Chiller Load Calculations
Chillers use refrigeration to remove heat, and the process is measured in terms of capacity. Chiller capacity is measured in tons of cooling or refrigeration tons to Btu/hr. Additionally, the tonnage of a chiller can vary based on cooling temperature and percentage ratings.
Another essential factor in calculating chiller capacity is the fluid used as a coolant since it can impact the system’s heat transfer properties. By accurately assessing these variables, you can ensure the chiller selected is capable of meeting the specific cooling needs of your process. Plus, optimal chiller loads also help improve performance and energy efficiency.
The Difference Between Nominal Tons and Actual Tonnage
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 much different.
The actual tonnage for a (nominal) “10-ton” chiller can vary from four tons to 12 tons, depending on the design conditions.
Nominal tons are typically calculated at ARI conditions, which are 44°F leaving water temperature, 54°F entering water temperature and 95°F ambient temperature.
What’s that? You say you use 30% ethylene glycol and operate at 35°F leaving fluid temperature? Well, now the “nominal tons” are 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 make the chiller less efficient, and the output must be derated.
The Difference Between Compressor HP and Tonnage Output
Another common error is referring to the chiller’s compressor horsepower (HP) as being equivalent to the tonnage output. So in other words, a 10HP chiller equals 10 tons of capacity. (Some manufacturers 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. While tonnage output refers to the chiller’s cooling capacity, compressor HP is about the power of the compressor, which is a vital part that plays a fundamental role in the operation of the chiller. Compressor HP tells you how strong the system is, which is directly related to its cooling power and efficiency.
[Hopefully, at this point, you aren’t feeling like this guy.]
Calculating Chiller Capacity
Calculating process chiller loads involves a few basic details that must be considered beforehand. Some of the information you need to perform the calculation includes water flow rate, inlet and outlet chilled water temperatures and the specific heat capacity and density of the water at an average temperature.
The Equation for Calculating a Heat Load
Understanding the heat load produced by the equipment and the environment helps determine the chiller’s cooling capacity requirements. The equation for calculating a heat load remains the same no matter the conditions.
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
How to Calculate Chiller Tonnage — Formula
The final step to convert your answer into tons of chiller capacity is to divide your result, Q (BTUs per hour), by 12,000 (the number of BTUs in one ton of cooling capacity). This will give you, in tons per hour, the chiller capacity that is needed to handle the process heat load.
Calculate tons of cooling capacity equation: Tons = BTU/hr, ÷ 12,000
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; and 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 will help to clear up some of the confusion about choosing a chiller system. Still, looking for help on how to best calculate process chiller loads? If you don’t have a chiller tonnage calculator, check out our technical library or contact Smart Family of Cooling Products today!
Contact Us For Chiller Tonnage Capacity Help
Smart Family of Cooling Products knows how to calculate chiller tonnage to get you the right size. Talk to one of our experts or contact us to get started on your next project!
