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The Difference Between Nominal and Actual

Last updated: 03-10-2021

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The Difference Between Nominal and Actual

One of the biggest misconceptions when purchasing a chiller is that a ton of cooling is always a ton of cooling no matter the operating conditions. In a perfect world, yes, that’s true. But in the real world, the actual tonnage of a chiller is often quite different from its nominal tonnage. So what’s the difference between those two values?

First, the basics: When an air conditioner is rated as 100 tons, it’s not a description of the weight of the unit. Rather, tonnage is a measurement based on how much heat a unit can remove from a space in one hour. The measurement of a ton originally came from the early days of refrigeration when cooling was provided by large quantities of ice blocks. A load that melted 1 ton of ice in a 24-hour period was 1 ton of load. In more modern terms, 1 ton of cooling capacity is equal to 12,000 British thermal units (BTUs) per hour. (A BTU is defined as the energy required to increase one pound of water by 1 degree Fahrenheit (1˚F), so if you wanted to heat 10 pounds of water by 15 degrees, that would require 150 BTUs of energy.)  

With an understanding of how tonnage is calculated, it’s important to know why a chiller might not have the assumed capacity — even when perfectly designed, installed and maintained. That’s where nominal tonnage and actual tonnage come in. The cooling capacity of a chiller, especially a packaged air-cooled chiller, is often described in terms of its nominal tonnage value, which is a rough approximation of the tonnage at “typical” conditions. However, nominal tonnage is really only useful as an order-of-magnitude metric for the chiller’s size. Nominal tons are often thought of as being at AHRI standard conditions, which for an air-cooled chiller are 44˚F leaving water temperature with 10˚F delta T and 95˚F ambient outdoor temperature.

But, in reality, there is no industry standard or hard-fast rule that dictates nominal tonnage values. Instead, each manufacturer looks at nominal tonnage differently. And the difference can be significant compared to the real tonnage of the machine, even at AHRI conditions. Further compounding the variable is that as application and jobsite conditions deviate from standard conditions, the chiller’s capacity will change, sometimes substantially. For example, a nominal 100-ton chiller that is making 20˚F glycol instead of 44˚F water may only produce 60 to 70 tons of cooling. Think of it this way: If you can normally walk about 3 MPH on flat ground, you may not be able to walk that same speed uphill due to the extra effort of the incline. When a compressor is working against larger differences in temperature than normal, it cannot output as much capacity. 

The thing to remember is that operating conditions matter when assessing the size chiller needed for a building or process. It is always best to work with a professional engineer who can calculate the real required capacity and properly specify equipment based on field performance instead of nominal capacity. Always take actual tonnage into consideration to ensure that a chiller is appropriately sized and has enough cooling capacity for the application.  

For additional information and support, get in touch with a Daikin sales representative who can rate HVAC equipment to provide the needed capacity at the specified conditions.

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