water coil btu transfer

Empire_2

It is always best to stay with the manufacturers specs as far as flow goes for that particular coil. With a 10 to 15 Del,T on the water side it needs to remain at a certain flow. More does not mean better performance. Brad, I think that is what you said....;-)

Mike T.

Helgy_3

water coil btu transfer

I'm currently working on a system with a water to water geo paired with a air handler w/ water coil in cooling. My question is, is it better to slow the water down going through the water coil to abtain a high delta across the water going into the coil and coming out. Or is it better to let the water run as fast as the pump will pump it. In other words does more flow equal more btu transfer or slow down water for more contact time in the water coil equal more btu transfer.

Let me know what you think.

Thanks, helgy

Brad White_9

More Flow Means.....

marginally greater capacity. Essentially you can run about 50% flow through most coils and get 90% capacity on the heating side of things. (The nation is seriously over-pumping!)

On cooling the key function is your "apparatus dew point" (ADP). The effect of water flow is more subjective here and is a balance point of temperature and flow; more critical than in heating because you are removing moisture -or not. That extra factor goes dramatically to effectiveness.

Here is my experience on water cooling, flow rate to capacity, in general terms:

If you double the flow rate (ignore the quadrupling of pressure drop just this once), you will halve the delta-T which means your average coil temperature will move to the 25 percent mark of your full range starting point (average being 50 percent of course).

Example: 10.0 GPM chilled water at 44 degrees, leaving at 54 degrees. Your average coil water temperature will be 49 degrees. Cool.

Double the flow to 20.0 GPM, entering at 44 degrees and leaving at 49 degrees. For all of that effort you have reduced your coil average water temperature to 46.5 degrees.

A coil at 46.5 degrees has a much greater ability to condense out moisture than one at 49 degrees, all things being equal such as identical entering air conditions. If you are in a muggy place this will have a greater effect than if you are in a desert someplace. Still welcome, not as necessary.


Reverse this example. Same 10.0 GPM and 44 degree entering water. Halve the flow to 5.0 GPM, leaving at 64 degrees. Your average coil temperature would be 54 degrees, barely below the dewpoint of your space air if 75 degrees and 50% RH, yet you are doing more sensible cooling. Not nearly as much moisture removal.

Obviously these are dynamic equations, you are really not doubling the BTU output for the conditions and heat transfer rates slide along with temperature changes... but for illustration purposes, you get the idea. Colder means more dehumidification. More flow means colder. Lose temperature, lose humidity control first and foremost.


Hope this helps.

Brad