Keep Energy Use in Control by Maintaining HVAC Equipment
By Dan Greenberg, Associate Director, E Source -- Foodservice Equipment & Supplies, 9/26/2008 7:09:00 AM
In a typical restaurant, heating, ventilation and air conditioning (HVAC) account for more than 30 percent of annual energy consumption. That being the case, it makes sense for foodservice operators to keep their HVAC tuned up and operating at peak efficiency. Some maintenance steps, such as identifying and repairing refrigerant leaks, or maintaining fan motors require a trained professional who has the proper equipment. But the following are three easy do-it-yourself maintenance ideas.
1. Clean condenser and evaporator coils. To maintain efficiency in a vapor compression cooling system, it is important to keep condenser and evaporator coils clean.
Dirt on the evaporator coil causes two problems: It reduces system airflow, and it directly degrades the coil's heat-transfer efficiency, which significantly cuts cooling capacity. Foodservice operators should inspect the evaporator coil at least once a year by shining a flashlight through it to make sure that the filters have been doing their job. If there’s visible debris or built-up grime on the coil fins, it’s time for a cleaning.
Unlike the evaporator coil, the condenser coil is exposed to unfiltered outdoor air — so it suffers much greater degradation due to dirt. The performance penalty of a dirty condenser makes this task one of the most cost-effective energy-efficiency practices available for rooftop unit maintenance. A dirty coil reduces the cooling ability of the air blowing across the condenser coils. If the condensing temperature is raised from 95°F. to 105°F. as a result, then cooling capacity will decrease by 7 percent and increase power consumption by 10 percent, with a net compressor efficiency reduction of 16 percent.
The best tool foodservice operators can use for this maintenance job is a power washer that feeds cleaning solution into a high-pressure water flow. Some companies specialize in performing this type of cleaning for a competitive price. They typically use tank trucks and custom self-contained equipment. Spray-on cleaning solutions that are intended to be used with a brush and a hose will not do a good enough job of cleaning the coils, even though they may brighten the outer surface. Power washing, if done improperly (for instance, using the wrong spray angle or excessive pressure), can damage coils by bending the fins, so use the minimum pressure necessary to remove dirt.
2. Lubricate supply fan bearings and keep fan blades clean. Supply air moves through rooftop units by belt-driven, forward-curved centrifugal fans.
A supply fan's two key maintenance requirements are bearing lubrication and blade cleaning. Fans in older rooftop units have sleeve bearings, which are simply oiled metal-to-metal running surfaces. These should be lightly oiled two or three times per year with the recommended lubricant. A label near the bearings should indicate the lubrication interval, lubricant type, and perhaps a log of past service.
Newer fans are equipped with self-lubricating bearings, which are sealed-cassette ball-bearing cartridges preloaded with grease. There is no way to re-grease these bearings, so when they finally fail — typically after several years of service — the bearing cassette must be replaced. Warning signs of impending failure are excessive noise, vibration or heat emanating from the bearing.
Conventional greased ball bearings are found occasionally in rooftop units. The most common problem with these bearings is overgreasing — the service technician connects a grease gun to the fill fitting and pumps in grease until it flows out of the bearing seals. Overgreasing can be as damaging as undergreasing for a variety of reasons. The proper procedure is to open the drain plug and inject grease through the fill fitting until clean grease comes out of the drain. If it is possible to do so safely, re-grease the bearings while the motor is running to help ensure a complete grease exchange. Take care not to get grease or oil on the pulley wheels or belt, because that will cause slip-stick action that will jar the system.
The impeller blades on a forward-curved fan are scoops that can fill up with dirt. Good filtration helps keep dirt out of the fan, but an annual visual inspection still makes sense. If impeller blades are coated with dirt, fan efficiency will suffer. Cleaning the blades will often require removing the fan from its housing, which in some cases can be a time-consuming process. Be sure that fan power is disconnected before attempting a cleaning.
3. Clean and lubricate outside air damper. Perhaps the most common rooftop unit malady is improper damper operation.
Outside-air dampers on a packaged rooftop endure a continual flow of dirty air that fouls the pivot points and actuator mechanism. The coarse prefilter only keeps leaves and birds out of the unit. If operating properly, the dampers will prevent the compressor from running when outside air temperature is less than about 60°F. Unless they are kept clean and well-lubricated, however, they can stick in place and rob the unit of free cooling potential, if stuck closed, or bring in excess outside air, if stuck open, that then must be cooled or heated. This problem can have major energy consequences in regions that otherwise could take advantage of economizer operation; it can also have potentially serious indoor air-quality impacts in all climates.
During damper servicing, clean and lubricate all moveable surfaces. The cleaning can be done with a power washer or with a bucket of soapy water and a brush. As long as a service technician is already on the roof, this cleaning and testing should take about 15 minutes.
