The Nodaway Valley School District in Greenfield, IA, installed the APR Control manufactured by Rawal Devices, Inc. on all their unit ventilators to prevent the problems they had been experiencing because their direct expansion (DX) air conditioning systems were oversized for the spaces they were servicing. The Rawal controls addressed the immediate problems and also provided protection against other potential malfunctions.
Unit ventilators are designed to provide precise temperature control for classrooms by keeping the air circulating in the room while adding fresh air and maintaining a comfortable temperature. However, like all DX equipment, these units are sized based on peak design conditions (which include variables such as occupancy, solar load, thermal gain, and ventilation). Changing any of these variables, such as occupancy, or renovating a space can result in the system being oversized. Oversized air conditioning systems create a number of problems, the most important being poor indoor air quality.
Facility renovations at the school district created just such complications with their existing R-22 (DX) unit ventilators. The ventilators began to short cycle, resulting in poor indoor air quality, high humidity levels, and generally an uncomfortable learning environment.
In addition, noise from the unit ventilators exceeded the acceptable ASHRAE standard of noise criterion (NC) 30 (the equivalent of 40 A-weighted decibels [dBA], or conversation volume inside of a home). To address this problem, fan speed controls were installed to reduce airflow. Reducing fan speed eliminated teachers’ complaints about noise in the classroom because the air was no longer whistling through the grates, and it allowed the district to be in compliance with the ASHRAE noise standards. However, the reduced fan speed resulted in a lower discharge temperature off the evaporator coil causing the (inactive) hot water heating coils to freeze and burst.
Replacing the units would have been a large financial undertaking, consuming a substantial portion of the district’s renovation budget. Installing Rawal’s APR Control gave the system “continuous capacity modulation” and the ability to match the capacity of the system to the changing load conditions in a space.
By installing the APR Control on all of their unit ventilators, the school district extended the units’ runtime while keeping the active portion of the coil below dew point for better dehumidification. The APR Control stabilized the discharge air temperature, preventing the inactive hot water coils from freezing and bursting. Furthermore, the APR Control prevented the evaporator coil frosting that was caused by the fan speed controls (which were installed to meet ASHRAE NC requirements). The APR Control is expected to prevent a broad spectrum of possible complications that can arise from using DX unit ventilators, such as coil frosting, liquid slugging, and system failure.
For more information, visit www.rawal.com.
