With the summer temperatures starting to climb, facility operators need to be preemptive 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 exhibits its own unique set of maintenance conditions before being able to work during the cooling season fully.
This article will quickly run down the needs for a chilled water system and DX system.
Chilled Water: This type of system relies on tubing bundles immersed in refrigerant to cool water that is then supplied to the HVAC cooling system. Chilled water systems rely on a closed loop refrigeration cycle to cool an open loop water system. Facility operators should confirm that the tubing bundles are kept fresh and free of scale. Each chiller is given a performance number when produced; this performance number is usually stated in Kw/ton. The performance number is created on a fouling factor. The fouling factor reveals the amount of scaling that can happen before the operation of the chiller is compromised.
Water Samples should be taken by Facility operators prior to running the chilled water system to ensure the water quality in the loop is correctly treated with chemicals that inhibit the scaling of tube bundles. Ensuring chemical feeders, and side stream filters, are operating and properly cleaned before operating the system, is also among the tasks the Facility operators should handle. Pumps and valves should be operated and verified to ensure that bearings are correctly greased, and valves actuate correctly. Finally, alarm points should be tested on the chiller local controls along with building automation controls (BAS). Ensuring that alarm points activate and shut down the system properly safeguards that the system will not operate in critical or tragic conditions that could possibly damage the system or be harmful to human health.
Direct Expansion: DX systems generally count on air cooled condensing units and indoor evaporator coils to provide cooling to air handling systems. Ensuring that coils are fresh prior to operating the system is essential for the correct operation of the system. If outside condensing unit coils are fouled and dirty, the refrigerant will not be sub cooled enough to correctly provide cooling to the areas. Alternatively, if the indoor evaporative coil is fouled, air flow will not be able to pass through the 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 at least 2 hours before allowing the system to go into max cooling mode. Max cooling mode is typically activated by high discharge air temperatures to areas, or by averaging area temperatures and programming the air conditioner to stage compressors to maintain load. In either case, technicians should be alert during this time, as this is typically the first time compressors have run for over six months. Once the system is at max cooling, technicians should confirm suction and discharge pressures along with suction and discharge temperatures. Particular units provide this data via onboard controls, other units may require a set of refrigeration gauges to verify suction and discharge pressures. This data provides the technician with valuable performance data on the system and can be used to determine if the system is charged correctly, coils need to be cleaned further or confirm that the compressors are performing properly.
A proactive approach to HVAC cooling systems building owners and operators can guarantee that the systems will be performing when needed most during those hot muggy summer days and nights.
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