In an air handling unit system, there is no other HVAC part that can create more inefficiency than the cooling coil. Not even the filter upstream of the coil can make such a negative performance impact on an HVAC system. Filters can be readily changed, and any problems quickly remedied – not true with the multi-row cooling coil.
Here are some of the major reasons for coil inefficiency:
- Air side plugging
- Freeze event damage
- Inadequate air flow
- Inadequate water temperatures
- Inadequate water volume
- Tube side fouling
The following looks at these reasons in more depth.
Air side plugging
We’ll start with inadequate filtration upstream of the coil. The quality of the filtration or the lack of changing filters can cause air borne particles to seep into the coil cores downstream. All systems are not the same. The “office unit” is completely different from the “ICU unit”. Environments and the infusion of fresh air into the system are also different. The more outside air – the more foreign materials!
Once a coil core is plugged, it’s very difficult to get the core cleaned. You’ll also have to overcome the problems created when units have little or no access to both sides of the coil. From experience, it’s been determined that you can clean 2 rows on each side of a cooling coil – provided there is access. With 6 to 10 row coils, you have a problem before you even start the cleaning process.
The “plugged coil” reduces the capacity through reduction in the air flow. The capacity is also further reduced, because the foreign material in the coil core acts as an insulator between the heat exchange mediums.
Damaged freeze event
A coil that has a “freeze event” has split holes in the tubes and/or return bends that need to be repaired before the coil can go back into operation. Many times, the leaks are in the inner core and cutting out “circuits” is the way it’s often fixed. When the coil goes back into service, it’s lost a lot of capacity. Why? When you pinched the tube closed, you may have taken 6 to 10 other tubes out of the water circuit.
Many times, the fin surface is removed, and cut or folded over to fix a leak. Once again, this reduces capacity because air travels to the path of least resistance. If you have cut large swaths out of the coil core, the air will move accordingly. This means you will not be using the entire effective area of the coil surface to cool.
Inadequate air flow
Many systems are starved for air because of design static pressure problems. If a system must provide downstream outlets with a total of 10,000 CFM @ 2.8” total static pressure and the pressure has never been below 3.3” total static pressure, it means the total CFM may have dropped to 9,500 CFM.
Inadequate water temperatures
From the start, many systems do not deliver the specified and required water temperature to air handling units. Due to a lack of insulation or chiller capacity during peak periods, some units will have insufficient capacity. Example: Take a system that requires 58 degrees F leaving air temperature to cool a room based on 45-degree F entering water temperature. Instead of 45-degree F. , the coil received 48-degree F water. The 58-degree F leaving air could become 61- or 62-degree F leaving air and with far less dehumidification.
Inadequate water volume
The same thing can happen to coil capacity when it doesn’t receive the specified GPM. Once again, this can be a systematic design problem or can be caused by increased water side resistance built up over the years from faulty valves or pumps and water side fouling in the piping. The further away the air handler coil is from the source, the more this inadequate volume can be exacerbated.
Tube side fouling
This usually happens over a period of years, and the inside of the coil tubes can have a caking of the tube walls that severely reduces coil capacity. It may be from the years of chemical use during the treatment process or the use of raw untreated water. The deposits act as an insulator to the heat transfer between the colder fluid and the hotter air.
Southwest Coil represents USA Coil & Air exclusively in the Southwest Region. Together, we have 4+ decades of experience, and our sales professionals have seen it all. We can normally pinpoint your problems by asking the right questions and then analyzing the information. We’ve built every design since the inception of the HVAC industry, and we have all types of shipment programs, including expedited shipments, to meet your needs and budget. Take advantage of our experience and knowledge, and call Southwest Coil for your next coil replacement.