By: Christopher Duranceau, CCP, Project Engineer
Horizon Engineering Associates, LLP
With the summer temperatures gearing upwards, it is important for facility operators to be proactive with their building’s HVAC cooling systems. Most large HVAC systems rely on mechanical cooling. This could be a chilled water system or perhaps a closed loop direct expansion (DX) system. Each system presents its own unique set of maintenance requirements before being able to fully operate during the cooling season.
This article will quickly summarize the needs of a chilled water system and DX system.
Chilled Water: Chilled water systems rely on a closed loop refrigeration cycle to cool an open loop water system. This type of system relies on tubing bundles immersed in refrigerant to cool water that is then supplied to the HVAC cooling system. Facility operators should ensure that the tubing bundles are kept clean and free of scale. Each chiller is given a performance number when manufactured; this performance number is usually expressed in Kw/ton. The performance number is based on a fouling factor. The fouling factor indicates the amount of scaling that can occur before the performance of the chiller is compromised.
Facility operators should be taking water samples prior to operating the chilled water system to ensure the water quality in the loop is properly treated with chemicals that inhibit scaling of tube bundles. Facility operators should also ensure chemical feeders, and side stream filters, are operating and properly cleaned before operating the system. Pumps and valves should be operated and checked to ensure bearings are properly greased and valves are properly actuating. Finally, alarm points should be checked on the chiller local controls along with building automation controls (BAS). Ensuring that alarm points activate and shut down the system properly ensures that the system will not operate in dangerous or catastrophic conditions that could possibly damage the system or be hazardous to human health.
Direct Expansion: DX systems usually rely on air cooled condensing units and indoor evaporator coils to provide cooling to air handling systems. Ensuring that coils are clean prior to operating the system is essential for proper operation of the system. If outside condensing unit coils are fouled and dirty, the refrigerant will not be sub cooled enough to properly provide cooling to the spaces. On the other hand, if the indoor evaporative coil is fouled, air flow will not be allowed to pass through coils. This may cause high discharge pressures at the compressor, which causes poor cooling performance.
Once the coils are cleaned, air filters should be replaced and the system should be operated for a minimum of 2 hours before allowing the system to go into max cooling mode. Max cooling mode is usually triggered by high discharge air temperatures to spaces, or by averaging space temperatures and programming the air conditioner to stage compressors to maintain load. In either case, technicians should be vigilant during this time, as this is usually the first time compressors have run for over six months. Once the system is at max cooling, technicians should verify suction and discharge pressures along with suction and discharge temperatures. Some units provide this information via on board controls, other units may require a set of refrigeration gauges to determine suction and discharge pressures. This information provides the technician with valuable performance information on the system and can be used to determine if the system is charged properly, coils need to be cleaned further or confirm that the compressors are performing properly.
By taking a proactive approach to HVAC cooling systems building owners and operators can ensure that the systems will be performing when needed most during the hot muggy summer season.