Annual Walk-in Energy Factor (AWEF) is an energy standard by the Department of Energy (DOE) that measures electrical energy input versus its cooling capacity. All commercial refrigeration equipment manufacturers should comply with the AWEF rating specified by the DOE. HVAC equipment of 3000 ft² or less must conform to this new requirement. All new walk-in installations should be with an AWEF compliant unit. Furthermore, anytime a replacement unit is installed, it needs to comply with AWEF.
There are several ways original equipment manufacturers (OEMs) can achieve the AWEF rating. These include adding an electric expansion valve, oversizing the condenser, on-demand defrost, among others. However, the most economical way to achieve AWEF is to lower or float the system head pressure.
Traditional head pressure settings are anywhere from 180 psig to 295 psig, depending on the system refrigerant. This setting maintains the system's high side temperature and pressure like a summer condition year-round. OEMs are now using head pressures as low as 123 psig for medium-temperature applications and 100 psig for low-temperature applications. This lower head pressure setting significantly impacts expansion valve sizing. In the winter, the cold temperatures allow for more compressor capacity. However, less capacity is available from the expansion valve counteracting the compressor capacity.
An expansion valve's capacity is determined by the liquid temperature and pressure drop available. The colder the liquid temperature, the more available capacity the valve has; the greater the pressure drop, the greater the available capacity. In winter, the expansion valve has a colder liquid temperature, but there is very little pressure drop available to the valve due to reduced head pressure resulting in a much lower available capacity. The result is a larger than expected expansion valve needed in AWEF applications.
In the below example, look at what happens in the system before lowering head pressure. Using R-407A, 100°F condensing temperature (238 psig), 5°F subcooling, and -10°F (15 psig) saturated suction temperature, there is 192 psi available to the valve in this scenario, assuming an appropriately sized distributor with about 31 psi drop across it.
If the head pressure gets floated down to 100 psig, there is much less available capacity. At the colder temperature, the compressor now has approximately 2.7 tons. The cold liquid temperature makes the distributer oversized, so we only get 14 psi across it, resulting in an available pressure drop of 71 psi across the expansion valve.
To counteract this problem and make sizing valves easier for our customers, Parker Sporlan has researched compressor efficiencies, distributor sizing, and valve sizing at the maximum summer condition and the minimum winter condition for AWEF applications. Bulletin 500-10-AWEF provides expansion valve capacity information for AWEF applications. The bulletin helps the contractor and wholesale counter employee to be able to accurately size and select the proper expansion valve for AWEF applications. The values presented in 500-10-AWEF provide maximum and minimum BTU/hr load at the unit's rated condition. Most times, this rating is at 105°F condensing temperature and 96°F liquid temperature. Recommendations in Bulletin 500-10-AWEF are per AHRI 1250. This bulletin helps provide thermostatic expansion valve selections for AWEF applications. Download Parker Sporlan Bulletin 500-10-AWEF.
View how to properly size an expansion valve for a DOE AWEF compliant system in this short video.
HVACR Tech Tip Article contributed by Jason Forshee, application engineer, Sporlan Division of Parker Hannifin
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