water to air heat exchanger

woodheatguy

I have a water to air heat exchanger that is smaller then the air duct it is going in.

Should I build a sheet metal "flange" around it ? Or would it be better to center it in the duct with no "flange"?

There is about 3 inch gap around the heat exchanger.



This for supplemental heat from my hot water coils on my wood stove.



Thank You.



Erich.

Brad White

Fit of coil to duct

Funny the slight difference in terminology. In our specifying (consulting engineers) we use the term "in duct coils". No matter and just an observation, I know what you mean.



Do I gather that this is a salvaged coil you are making use of, or was this purchased for a specific duty? In other words, do you know how the coil is rated at certain conditions?



The reason I ask is, unless you know that, you will not know how it will perform thermally or on the air side, how restrictive it is. In coil selection, all factors work together. Change one variable and the others drift a bit.



To your question: You want no gaps around the coil. ALL of the air wants to go through the finned area. With any bypasses and depending on how many rows the coil is, you could get 50% to 75% of the air to go around the coil. What goes through the coil to be heated will again mix with the bypassed air downstream of the coil and you have no idea what that temperature will be.



Does the coil have an internal flange (rolled channel frame)? Or an external/out-turned flange?



If the former, that typically is a "slip-in" coil. One would cut in a slot for it and an access door upstream in a retrofit application. (This door will help you to clean the upstream face over time as well as install it.) Any gaps would be sealed by securing bent sheet metal angles all around, with duct sealer. Think "prevent bypass".



If the coil is an out-turned flange (usually but not always 1.5"), the best way is to remove the entire duct joint and fabricate transitions in and out, with butting flanges. The flanges are sealed with special gasket tape (Ductmate, Ward Duct Connector or TDF system), or brush on mastic, all with screws or bolts.



I prefer this latter method because the transition focuses the airflow into the net finned area and there is no sharp "shoulder" which "forces air to miss the edges".



Back to my airflow concerns: Unless you know the air pressure drop of this coil, you could be compromising your system airflow. If this coil is in one branch and there is another without such a coil, that other branch can have appreciably more airflow and would need to be dampered-down to compensate. Otherwise you would have an imbalance.



Lastly, "normally" we would size an in-duct coil for between 300 and 550 to 600 FPM face velocity. Deeper coils with higher temperature rises we would use a lower face velocity to compensate for the higher pressure drop that comes with more rows. Thus, it is almost always the case that the duct coil will be larger, not smaller than the duct they are in. Thus I am concerned all the more about a restriction in your ductwork.



I hope this helps!

woodheatguy

Very helpful

Thanks Brad for getting back to me so quickly.



The heat exchanger  is an item I picked up cheap. I was planning on building a flange around it to force more of the air through it. I will take it out during the summer so as not to mess with the AC.Thanks for the great info. before I went through all the hassle of building it I just wanted to make sure it was worth my while. Now I know it is.



Thanks Again



Erich.