This editorial submission has been provided by MACS member Bergstrom
The British Thermal Unit (BTU) ratings for mobile air conditioning systems are a confusing and often misleading issue. Ratings in the marketplace currently range from a few thousand BTU per hour to as high as 18,000 and even 30,000 BTU per hour.
These discrepancies arise because there is no governing body that sets and enforces an industry standard for the testing and rating of cooling and heating capacities. As a result, some manufacturers manipulate and inflate BTU per hour numbers to entice buyers. Bergstrom has developed this article to help trucking executives create an even playing field and make intelligent decisions when comparing mobile air conditioning systems.
Existing Standards & Recommended Practices
With the lack of an enforced industry standard, many manufacturers develop internal standards that are derived from a combination of ratings recommendations issued by industry organizations. These include:
If a company tells you they have a 30,000 BTU per hour evaporator, be sure to ask them how it is rated. Don’t be shy about asking the ratings questions, and if in doubt, ask for clarification or comparison. If they can’t tell you, chances are they have manipulated the numbers in one of the methods listed below.
Increasing air inlet temperature
A higher air inlet temperature will result in higher capacity and ratings. For example, at 100 ºF or above, the higher temperature difference between the air and the refrigerant causes more heat to flow out of the evaporator. As a result, this is one of the variables that can be manipulated to indicate higher capacities or BTU per hour.
Manipulating refrigerant temperature
One method of “increasing” performance is to operate a system with an excessively low refrigerant temperature. However, remember that an air conditioner has an absolute low end for cooling. Do not accept a rating with evaporator air outlet temperatures below 32ºF, because at that condition the moisture removed from the air will freeze in the coil and block the airflow. With no airflow, no heat is added to the passing refrigerant and it will not evaporate. Liquid refrigerant returning to the compressor could cause damage or failure.
Using blower manufacturer’s airflow stats to measure capacity
Another rating deception, often used by companies who don’t own their own test facilities, is to use the blower manufacturer’s stated airflow at a zero-restriction condition. These companies then use that airflow in a coil sizing computer program to publish results. This coil capacity data is obtained from wind tunnel testing of a coil – by itself – with a uniform airflow across its face. In reality, airflow from a blower assembly is delivered with anything but a uniform velocity. Furthermore, using the blower manufacturer’s stated performance at zero restriction is very misleading. The coil itself, along with other system restrictions (such as filters, inlet ducts, outlet ducts and cab pressure), reduces the blower assembly’s output to a lower CFM value.
Rating performance at maximum air temperature and humidity
Many people would like to rate evaporators at 110 ºF and 100% humidity. In theory, this would provide an impressive capacity. However, these people fail to take into consideration that the other components that make up a mobile air conditioning system cannot support those values. To do so would mean that the compressor must pump enough refrigerant, the condenser must remove enough heat and the blower assembly must provide enough airflow across the evaporator to introduce that much load. None of these 3 components can do this.
Neglecting to show time unit of measure
A new trick in manipulating BTU per hour performance numbers is to fail to show a time unit for measure. Most performance numbers are measured in BTU per hour. If no unit is displayed, make sure you follow up with the manufacturer to determine what unit of time they are using to measure BTU per hour.
Bergstrom’s ratings system
With the lack of a governing body that enforces an industry standard for BTU per hour testing and rating, Bergstrom has developed an internal standard that is derived from the ARI 310/380, TMC RP-432 and IMACA 200 recommended practices.
All NITE systems are tested at 80ºF/50% humidity indoor, and 100ºF outdoor. As our capacity has increased, we have also begun to test at 90ºF/50% humidity indoor and 100ºF outdoor in order to illustrate capacity during a pull down situation. While this rating condition shows elevated capacity it is a situation that rarely occurs. Also, Bergstrom only rates its NITE systems based on actual calorimeter test data from our test and development lab. We do not develop any BTU per hour ratings based on theoretical calculations.
We prefer to simulate the downstream restrictions and to mimic the as-installed conditions of the system to ensure proper design requirements are met. Coils, blowers and housings used in testing are the same as those used in real life conditions. We believe this is as close to the actual performance you can get.
To learn more about BTU per hour ratings, don’t hesitate to contact the Bergstrom team at 1.866.204.8570 or firstname.lastname@example.org.