I designed an air handler replacement for a hospital last year. 14,000cfm, 100% fresh air, 65F delta T in heating mode. It has been installed and works great between 0-30 and 45-100. It has a steam preheat coil with face/bypass dampers, chilled water coil and a humidifier downstream. We are constantly overshooting the temperature when the ambient temp is 31-44F ish. With the dampers closed and valve at 100%, we have 10-15F rise past the preheat coil. The manufacturers published info says there should be 2-3F override. They informally told us initially to expect less than 5F. Now 4 weeks into this problem they say 10-15 is perfect normal. I have looked around and can find noone that says more than 5F is normal. We assumed the increase was due to damper leakage. We sealed the front of the dampers with foil tape and ran steam to 100%. Zero affect on temperature rise. We had 11F rise with or without the tape. So I guess we are abandoning the leakage avenue. We also have stratification problems that we are dealing with by installing a blender. I haven't found any guidelines for designing a blender and have never done it, but the contractor says they have built one successfully before, so I guess I'm going to let them have at it. The next thing to try as far as controlling the temperature is installing a 1/3, 2/3 valve arrangement to utilize in this temperature range. Enough to keep the coil from freezing, but hopefully reduce the heat output. Any thoughts or suggestions would be greatly appreciated.
Steam problems
- Thread starter goodal
- Start date
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The steam valve is 100% closed or open? I'm assuming open with that type of Delta T
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MikeGlass1969
Well-known member
Was there a visual confirmation that the face dampers were open the bypass dampers closed?
Yes, the valve was open 100% during the test. Yes, I went inside the AHU during the test to see the dampers position. Yes, we have enough sensors on this thing to choke a mule. I can know everything about the air at any stage going though the AHU before and after too. The manufacturer stands by their statement that 10-15 is normal. We have a quote from their competitor to replace the coil. We gave the competitor enough information to quote the coil and then asked what the override should be. He said less than 5F and more like 2-3F without doing anything special to the coil. The conclusion we have come to is that the coil and dampers are junk and the manufacturer won't own up to it. Still not sure what we are going to do just yet.
MikeGlass1969
Well-known member
Doesn't sound like an equipment problem to me. Sounds like a control problem. If the coil can supply enough heat 30F and below the coil is sized large enough and the fan should have enough pressure to draw the air througas well. I would like to see the control programming....
The control valve is at 10% alot of the time and its still overshooting the temp. The contractor keeps pushing for a 1/3 2/3 valve installation saying the one valve is too large to turn down that far.
MikeGlass1969
Well-known member
So the valve is oversized? Why not just change it?
Have you completely shut the valve? Steam is not linear (as I'm sure you know). I would shut the valve all the way and see if you get better control. If you do, treat the damper and valve as 1/3 (damper) and 2/3 valve to maintain a coil leaving temperature of 50 degF at all times. If the control guys ask what that means tell them to use ONE PID loop and control the damper from 0-33% and the valve from 33 - 100%.
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