Two Row Coil Piping

[krb]

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?

 

[ChasBean1]

(This might also be a good question to post in the heat transfer/thermo forum...)

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

 

[krb]

After some research into the subject, ASHRAE and the manufacturer say to pipe the supply into the bottom of the second coil (or third, fourth, fifth..if that is the case). I do have a good understanding as to why. I know we are doing it correctly. Anyone else have any ideas?

 

[quark]

KRB!

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,

 

You want the piping arranged for counterflow. While it probably doesn't make much of a difference on a 2-row coil, it can have a noticeable effect on deep, multi row installations, since the temp difference approaches zero at the coil exit side.

On the other hand, newer coil designs often have a manifold type tube arrangement, which makes them much more forgiving of the piping arrangement.