Hot Water Coil/Air Handler

Jim Davis_3

oops?

Jim Davis_3

23gpm vs 79 gpm

I have a friend at a High School that has received engineering specs that state they can get 800,000 btus out of a hot water coil using only 23gpm. The coil is rated for 913,000 btu and 79 gpm and they are using 25470 cfm in both senarios. Think we had this discussion before but not quite at this radical difference. I say it can't be done.

Brad White

Entirely Possible

Depends on Delta-T, always.

The paradox of flow to output is rather neat: Cut the flow in half and get about 90% of capacity. Why is that?

Say you have a coil at 180 degree F. entering water, leaving at 160 F. Say it is 1.0 GPM and 10,000 BTUH for easy math.

Average water temperature is...170 F. correct?

Now, cut the flow in half. 180 F entering water, that does not change. Leaving water temperature is 140 F. correct? Half the flow, twice the delta-T.

Average water temperature is... 160 F.

You have only dropped your AWT by ten degrees.

You would still get about 9,000 BTUH and the actual delta-T would settle at an in-between factor because of laminar flow and other variables, but close enough.

In your real example, at 913 MBH and 79 GPM, you are running a 23 degree water delta-T. (Say, 180 in and 157 out for argument's sake.) Average water temperature would be about 168.5 F.

Now take 23 GPM against 800 MBH. That is a 69.6 almost 70 degree delta-T on that water. Your 180 F. water would return at about 110.4 degrees. (Hope you can protect the boiler here!).

But the average water temperature is now about 145.2 degrees, not terribly departed from the 168.5 F. prior average. Less capacity for sure, but when you may be heating air entering the coil in the 50's, F. range, you still have sufficient delta-T to extract heat.

See how seriously over-pumped we are in this country?

I would say that if so rated, that coil will perform pretty much as expected (less any coil fouling of course).

If this is coupled to a boiler, beware of "shock and Awwwwwww!". If supplied from a steam to water heat exchanger, enjoy the condensing!