Two Row Coil Piping
Two Row Coil Piping
(OP)
In a two pipe heating system, if I have a two row coil should the fluid inlet be on the inlet to the first coil in the direction of the airflow or the second coil? I have seen it both ways. Any suggestions?





RE: Two Row Coil Piping
What matters most is that the water goes in the bottom port and comes out the top port. Any gases released from the water from bends, twists, velocity changes, and throttling will travel up with the stream naturally versus fighting a bulk stream traveling in a net downward direction.
The air velocity through a coil might be 500 feet per minute or higher. At this speed, if the two row coil is (for example) a foot wide, each air molecule would spend 0.12 seconds between the fins before bouncing around off the fins and leaving with increased kinetic energy. With this short a period of contact for heat transfer, I doubt if there would be a difference in the overall outlet conditions if the inlet portion of the one-foot section were hotter than the outlet portion, or vice versa.
So I say make sure the inlet is at the bottom and the outlet is at the top, and install it in the direction that allows the best maintenance access to the control valve, circuit setter, and strainer.
Evidence might refute this. But this seems sensible... What do you think? -CB
RE: Two Row Coil Piping
RE: Two Row Coil Piping
Generally the inlet is to the second coil from the direction of air flow (by practice). There is a discussion on parallel and counterflow heat exchangers in Heat Transfer Book by Holman. Net effect is almost same.
If the air comes into contact with hotter liquid first, it may not get much heat from the colder liquid afterwards. However I never checked this because all the AHUs I worked with sofar have counter flow coils in them.
Regards,
RE: Two Row Coil Piping
On the other hand, newer coil designs often have a manifold type tube arrangement, which makes them much more forgiving of the piping arrangement.