Most heating coils are either a preheat or reheat coil mounted in a unit. Also, reheat coils can be mounted downstream of a unit in a supply duct. This article will address hot water and steam heating coils. There are numerous reasons why a heating coil may not perform to its expected capacity. The following are the major ones.
Improper Coil Selection
Heating coils can be selected at higher air velocities, because there isn’t a problem with moisture on the fins and carry over downstream. These higher velocities produce high air resistance which can reduce the air flow volume. With additional resistance to the air, your CFM can be reduced so much that you lose 10% to 20% of air flow, which can correlate to the same reduction in BTUH capacity. It’s imperative that both the selection specialist and the customer understand the ramifications of high resistance and what it could do to the overall capacity of the coil.
Many heating coils are mounted in the supply duct. Remember that most ducts are sized at 800 to 1200 FPM (feet per minute) and that your selection of coil needs to be no higher than 700 FPM to keep the coil air resistance at an acceptable level. This means the coil face will be larger than the duct, and there will need to be a transition from the smaller duct (rectangular to rectangular or round to rectangular) to the coil.
You must have “the in direction of air flow” distance from the start of the transition to the entering face of the coil and from the leaving face of the coil to the start of downstream duct. If not, you can have what’s known as “air stratification”. This means that the volume of air will not be flowing evenly across the entire effective face of the coil. In most cases, the air will be “punching a hole” in the middle of the coil and most of the perimeter area will receive far less air. This can cause 20% to 40% less capacity and uneven pressure drop across the coil.
Hot Water Distribution
Many hot water heating coils have problems with either the volume of water delivered to a coil or the temperature not being per the coil design. This happens because of systematic problems or decisions that are made that affect these coils indirectly.
If a hot water coil has less flow (GPM), the capacity will be reduced. This happens in systems where the pump is undersized to the total volume versus system pressure drop. If you have 20% less GPM to a coil, that may reduce coil capacity by 10%. This is usually a 2 to 1 ratio. Another reason for lack of capacity is the addition of glycol. We see many systems where glycol has been added, and the coils were not sized for a water-glycol mixture. Thirty percent propylene glycol can reduce the capacity of a coil by 25% – 30% and add 20% – 40% to the fluid side resistance as well.
Temperature to a coil is a problem as well. If a coil is sized for a higher entering water temperature and it’s drastically reduced, the capacity is affected. We see many systems sized for 180 degrees F entering water temperature and it’s been reduced to 160 or even 150 degrees F because of operating costs. This will affect the BTUH capacity of your coils.
Many steam heating coil systems do not remove condensate in the coils as quickly as it’s formed. Since the condensate is sitting in a coil, steam trying to enter can’t do so evenly. Matter can’t take up the same space at the same time. This systematic problem not only causes reduction in capacity but also causes freezing of the coil and steam hammer.
Condensate is removed from a coil when there is positive pressure at the inlet to the trap. If there is negative pressure, condensate will stay in the coil and capacity issues will persist. Many systems operate with a modulating control valve. Negative pressure begins when this control valve starts to reduce the steam flow during intermediate load conditions. You will need to have a working vacuum breaker mounted between the valve and inlet to the coil. This will allow the inside of the coil to never be at a negative pressure. However, it doesn’t mean it will be a positive pressure, just never worse than zero.
The trap needs to be sized for 3 times the condensate lbs./hr. rate of coil and be located 18” below the coil outlet. This 18” is ¾ of a pound of positive pressure. It will be necessary to run full size piping from the outlet of the coil to the inlet of the trap and only reduce the size of piping after the trap.
Steam system piping and accessory selection is very important to steam coil performance and its ability to not freeze or be destroyed with steam hammer over time.
USA Coil & Air has been in the heating coil business for over 4 decades. We understand the problems with heating coil performance and have the knowledge and experience to assist you in solving your problems. USA Coil also has the shortest lead time and exceptional service. Make USA Coil your “go to” source for all your heating coil needs, you won’t regret it!