Reprinted with permission from RSES Journal
The Importance of Clean Coils In Commercial Equipment
Keeping coils clean helps equipment operate in near-new conditions, which keeps customers happy, saves them money and reduces techs’ headaches
BY MARK STEFURA, CM
Once again warm weather is upon us and our industry’s services are in high demand. Knowing this, it is important to focus attention on one of the most neglected and overlooked aspects of the refrigeration cycle – coil cleanliness.
The basic refrigeration cycle is based upon absorbing heat where it is not wanted and transferring it to another place where it is unobjectionable. When coils are dirty this heat transfer process is affected, but it is not the only repercussion. A decrease in system reliability and an increase in power consumption often are the results.
A study on a typical 3-ton rooftop unit with a fixed-orifice metering device showed that if the evaporator airflow is restricted by 36 percent, the capacity drop is 19.4 percent. This changes the 3-ton unit to a 21/2-ton unit. On the other hand, when the condenser coils are 56 percent restricted, the capacity drops 10.9 percent.
It should be noted that although capacity drop is not as great with dirty condenser coils as it is with dirty evaporator coils, the coefficient of performance (COP), or energy efficiency, is severely affected. Simply stated, the unit has to work harder and longer to remove the heat and, therefore, will use more power. These times of greater electricity demand, and with power companies trying to meet these demands, should prompt both technician and customer alike to ensure that equipment operates efficiently.
Again, reliability also is an issue. A decrease in compressor life expectancy often is a result of liquid refrigerant returning into it. This may be due to improper airflow across the evaporator coil because of partial blockage or overcharging. Compressors are designed for vapor compression and the introduction of a liquid into that equation sometimes can be catastrophic. This is just one example on the low side of the refrigerant cycle.
On the high side, the compressor also is affected. The increased condensing temperature resulting from the dirty coils causes an increase in head pressure, which also increases the oil
temperature. When oil reaches its critical point it no longer serves the purpose for which it is intended. Worse yet, oil breaks down into acid-forming compounds when it reaches its critical point, which will destroy the system.
Is a coil cleaning in order?
Now that we know why coil cleanliness is important, it’s time to focus on how to properly clean coils. When approaching a unit, especially for the very first time, the first thing to check for is a dirty coil. If the above-mentioned 3-ton unit was to move 400 cfm/ton across the evaporator, the same unit would move approximately 1,000 cfm/ton across the condenser coil. This is because the condenser not only removes the heat absorbed by the evaporator, it removes all the heat generated by the power input to the compressor.
Condenser coils are good targets for thorough cleaning because they typically do not use any means of air filtration. The fastest and simplest way to determine if coils need to be cleaned is by measuring condenser T. This is the temperature split between the air entering the condenser coil and the air leaving the condenser coil. Simply take your pocket thermometer and measure the temperatures.
If a condenser T of 30 F or higher is measured, a coil cleaning would be in order. Record this reading along with the other standard measurements a technician would check for proper system operation. Document all these readings because you are going to use them in future calculations.
Once the coils are cleaned you will gather and document more information. You often may find that by cleaning the coils a system that may have had the appearance of being fully charged will show that the charge was altered to compensate for the reduction of heat transfer ability of the coils.
If the condenser coils were dirty, the charge may have been reduced to bring the head pressure down without regard to subcooling. If the evaporator was partially blocked, the charge may have been removed to bring the reduced superheat back up to better reflect the amount called for on the equipment’s charging chart.
By now the necessary data has been gathered and it has been determined that the coils are dirty and need to be cleaned. What’s the next step? Depending upon coil design and the size of the unit, different approaches can be used to do the cleaning.
A successful method of cleaning commercial coils is the use of a pressure washer and the correct type of coil cleaner. Self-rinsing, detergent-style cleaners for evaporator coils and condensate pans are popular. The air usually is filtered and a stronger solution usually isn’t required. For condenser coils, use an alkaline-type cleaner. Both acids and alkaline cleaners will burn your skin. Acid burns are much more difficult to treat, which leads to safety issues.
When handling chemicals, wearing gloves and safety glasses is a must. Long sleeves and pants are better options than shorts and short sleeves. Also, wearing a full-face shield is a good idea, especially if there is a breeze that may cause the foam to become airborne. Read the labels and inquire at your wholesaler for Material Safety Data Sheets (MSDS).
Maintaining safety and respect for chemicals is highly recommended. Take the necessary precautions to protect people and property. Plastic tarps and duct tape may be required to deal with overspray. Take the time to look over the equipment. Sometimes the condenser fans are so close to the coils that water penetrates the air ports. If this occurs, cover the ports with duct tape until the cleaning is completed.
The same holds true for the evaporator coil cleaning procedure. The fan motor may need to be protected. The procedure itself is pretty simple: wet, wash, rinse and repeat. This sounds like a trip to a hair salon, but this is how to battle dirt. Wet the coils down with a pressure washer. This gets the cleaning solution in deeper on multiple-pass coils. Wash the coils with the cleaner using the built-in chemical feeder on the pressure washer while following the manufacturer’s instructions.
Let the cleaner do its job by letting it set for the recommended time. Most condenser cleaners will foam out the dirt. Rinse thoroughly. This probably is a good time to point out that the phrase "more is better" doesn’t apply. A chemical reaction takes place when the cleaner is applied. Condenser cleaners, when applied at higher-than-recommended strength, can cause premature failure of coil fins. At times, one application is enough and at other times multiple applications will be required.
Experience with different manufacturers’ equipment is the best teacher. Some units require more attention than others. For instance, thick coils may need to be cleaned from both sides. Also, some manufacturers use condenser coils that are sandwiched together. This can be determined by looking at the return-bend end of the coil. Metal clips usually hold each pass together. The manifold-end tube sheet is one solid piece so it can’t be separated at this point.
Be creative on how to support the top panel while opening the coil passes to clean each pass. Dirt that makes it past the first pass gets trapped on the face of the second, which then looks like a clothes dryer lint trap. Sometimes brushing the coils is required to remove big clumps of dirt. A nylon brush works well, and remember to brush in the direction of the fins. Don’t flatten them. Cut off the end of an ice scraper and use the snowbrush portion for this task and to clean the condensate pan.
Once you have cleaned the coils and condensate pan, flushed the trap and drain line and put the unit back together, add some means of condensate pan treatment. Another time-saving method when working on rooftop units is to dry-fit one of the pvc fittings. Purposely leave one fitting unglued to allow easy removal of the trap during a maintenance inspection for cleaning. This comes in handy with multiple rooftop units.
When dealing with split systems and chilled-water air handlers, install some form of cleanout, usually a T-fitting to flush water through to keep the trap and drain clean. If the T-fitting is installed between the trap and the air handler, it must be capped or the trap will not maintain a water seal. Remember that keeping the trap clean helps to keep the pan clean. It only makes sense to protect the customer’s investment.
Installing clean air filters also is a must. It would be an injustice to reinstall the same dirty filters.
Once everything is back together it is time to gather more data. Record the outdoor ambient temperature and all the readings as stated earlier and compare what the system is doing now vs. before the coils were cleaned. Make necessary adjustments and document final readings.
Knowing that you helped restore the equipment to its original operating conditions should have a positive effect on you and your customer. And remember, a contracting company is only as good as the tech it puts on the job.
Mark Stefura, CM, is a member of ASHRAE with 18 years of experience as a commercial hvac service technician.