Improving Coil Performance In The Field
But is there something else contractors should do besides clean the coils? Are there other steps that can be taken to help improve coil performance? We’re glad you asked, because several experts in the field have offered their opinions concerning what contractors can do to optimize coil performance in the field.
New Installation ConcernsWhen an evaporating or condensing coil is first installed, contractors should check a number of different things to ensure optimal performance, said George Western, engineering manager, Astro Air Inc., Jacksonville, Texas. Astro Air manufactures a variety of tube-and-fin-type heat exchangers. The company builds both refrigerant evaporating and condensing coils, as well as water coils for heating and cooling applications.
“In evaporators, make sure that drains are free flowing and free of obstructions,” said Western. “And to promote good, even airflow across the coil, minimize the damage that may be associated with handling the bare coil. If fin damage occurs, use a plastic comb to straighten out the fins.”
When handling evaporating coils, he added that contractors need to be careful not to dent or damage any of the tubes in the coil circuitry. And definitely do not use the manifolds on a coil as handles to situate the coil because one or two dented hairpins or return bends can cause erratic performance due to refrigerant flow that is distributed incorrectly.
On condensing coils, contractors should be aware of the external surroundings. “Coils are meant to have free, open airflow throughout the coil,” said Western. “Debris such as trash, leaves, tree or grass seeds, or cotton-like material can impede airflow to the point where all available performance is reduced to an unacceptable level.”
He also suggested checking for building obstructions or other external airflow impediments, which can cause recirculation of already heated air, leading to loss of condensing performance.
Contractors should also make sure that the coil is correctly oriented, or else it will not perform as designed. “Most coils are designed for counterflow heat transfer. The air should enter the coil on the return or suction line side of the coil, and the suction connection should be located at the bottom of the suction header,” said Jim Leeds, OEM sales manager, Precision Coils, a division of PEF Industries, Somerville, Tenn. The company specializes in manufacturing OEM and replacement coils for the commercial and industrial HVAC market.
In addition, it’s important to use an expansion device that’s the correct type and size for the equipment. Remember that the coil is a heat exchanger, not a heat remover, so an undersized or oversized expansion device will prevent the coil from performing at optimum levels as designed. For example, contractors should make sure that coils with orifice-type expansion devices are sized correctly.
“Also, remember that flow through a sized orifice is dependent upon the available pressure from the condenser liquid,” noted Western. “On higher energy-efficient units, the available pressure drop allowed across the expansion device may be less than what it says in the standard tables. Use the correction factor associated with the condensing unit and anticipated maximum design conditions, and always use the correct charge of refrigerant for the system.”
Damage Over TimeAll coils start losing efficiency over time. The main factor that leads to decreased coil performance is poor airflow caused by dirt and debris on coils. Basically, circulated airborne contaminants degrade the performance of the fin surface, increase the associated air-side pressure loss (drop) across the coil, and reduce available airflow. In some special cases, the airflow can become so choked that the operating temperature of the coil lowers to the point where a freeze-up condition can occur.
To keep dirt and debris under control, Leeds suggested that during routine maintenance or service calls, contractors always check and change the filters, clean the coils, and straighten the fins. “Precision Coils recommends cleaning the coil from the leaving air side to prevent a ‘dirt dam’ in the coil. Do not clean coils with hot water or steam, because this will cause high pressure inside the coil tubing and subsequent damage to the coil. Be sure to only use approved coil cleaners and follow the manufacturer’s directions,” said Leeds.
It’s also necessary to be careful when cleaning the coils because damage to fin surfaces during cleaning can add to the associated pressure drop, said Western. He also said that contractors should look for any leaks around the coil (e.g., loose-fitting service panels, lost air seals) because this can cause a loss in system performance.
“Also be aware of the amount of oil circulated with the refrigerant because it can degrade coil performance if it exceeds the amount needed for proper lubrication of the system to the compressor. This is especially true in small-volume systems, where the performance can be degraded as much as 20 percent to 25 percent just by the addition of as little as 4 ounces of additional refrigerant oil,” said Western.
When servicing a coil, contractors should make sure that there is proper airflow. This can be achieved by determining if the system is producing the correct amount of cfm at each outlet. Also look for frost buildup on the inlet lines, said Western. “This should be minimal or nonexistent on air conditioning systems, but on refrigeration coils, frost buildup is normal. If there is one circuit of a multiple circuit coil that is frosting more than others, it may indicate a blockage or restriction in that circuit.”
He added that a good way to see if the coil circuits are balanced is to measure the outlet temperature at each circuit just before it joins the suction manifold.
“On a properly circuited and load-balanced coil, the leaving temperature of each circuit should be within 2 degrees F of all the others. Use a thermocouple to check each circuit individually or an infrared temperature detector. If using a thermocouple, remember to insulate the contact point from any outside sources of heat. The refrigerant temperature leaving the coil should be superheated or just above the saturation temperature of the pressure at the outlet,” said Western.
If the coil is beyond help, then it might be time to replace it. If that’s the case, then check with the manufacturer to choose a coil that is appropriate for the application.
Publication date: 02/24/2003