Understanding Your Cooling Capacity
To do that, the gradients of temperatures in large rooms should be considered. The air coming directly off the evaporator is the coldest, while the air filtering out through the room is somewhat warmer, and the air coming back onto the coil is the warmest.
Using average room temperature when analyzing a refrigeration space sounds logical, but it’s also essential to understand how coils are selected and rated. Applying incorrect ratings can result in the purchase of undersized air coolers that appear to satisfy the calculated refrigeration load. In that scenario, lower-than-expected operating suction temperatures will increase energy consumption.
“Selecting an evaporator comes down to making sure you understand the impact it will have on the resulting room temperature and what that means in terms of your operating cost and product quality,” said John Kollasch, vice president of industrial refrigeration marketing at EVAPCO, Inc. “With the TD method, the equipment first cost will be higher but the operating cost will be lower. Using the DTM method at the same temperature difference will result in smaller evaporators, but they will have to run longer or will require a lower suction temperature to maintain the design room temperature.”
“If you select the DTM method you must be willing to accept that the design room temperature is in reality the average as opposed to the maximum temperature,” he said. “The difference between the temperatures impacts the performance of your operation and what is being stored or processed. The warmer temperature resulting from the DTM method will have an impact. The TD method defines room temperature as the air entering the coil which is the warmest temperature in the room. The payoff for the end user who purchases coils rated for the TD method is the assurance of maintaining room temperature at the design suction with the fewest number of hours of operation.”
The rating system commonly used in the United States is the TD method, which measures the air temperature entering the coil face minus the saturated refrigerant temperature to establish the rating temperature difference.
DTM rated capacity at the same temperature difference may be higher than the TD rated ratings for the same air cooler operating under the same conditions.
And that’s because the effective initial temperature difference the evaporator coil encounters is higher by approximately one-half of the air temperature change. Therefore, the same evaporator can appear to produce more cooling capacity simply by changing the definition of the temperature difference.
In that way, confusing ratings can result in a decrease in expected capacity. For example, if both rating methods use a 10-degree temperature difference, the air entering the coil is -10 degrees for a TD-rated coil, but only -5.8 degrees for a DTM coil. When a DTM-rated coil is confused with a TD rating, the difference in capacity can approach 50 percent.
And energy savings vary depending on the size of the equipment and the room being cooled. Although air coolers using DTM ratings are smaller and less costly for the same cooling load and temperature difference than those using TD ratings, it stands to reason that DTM rated coolers will operate with a lower suction temperature for the same cooling load. This results in greater power consumption. The incremental return on investment on reduced power consumption using TD ratings can be as high as 156 percent for a single-stage compression system and 52 percent for a two-stage compression system, depending on the room temperature, resulting in payback as quickly as eight months.
The bottom line, said Kollasch, is that misunderstanding cooling ratings can result in the failure of a refrigeration system to perform as expected.
