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Chilled water coil won't condensate

If the chilled water temperature is "scheduled", or reset based on OAT, and it runs at a higher CHW supply temp than originally designed, then the coil may not get cold enough to create condensation.

With an entering air condition (assumed from your post) of 73%RH at 72F, according to the psych chart, you need an average minimum CHW temperature in the coil, and corresponding leaving air temperature of at least 60F to start condensing moisture from that airstream.

If in fact, your CHW entering temps are the usual 44F, to a leaving temp of 56F, you should be condensing like mad. If you are not, then yes, you could have too high an air velocity through the coil and you aren't cooling the air off enough to condense much. Information is required as to what is the entering and leaving air conditions at the coil as well as the average face velocity - should be less than 500 feet per minute depending on coil configuration (4-row, 6 row, how many fins per inch, etc.)

The other part of the issue is if the CHW control valve is modulating only based on space temperature, you may need to run the cooling coil and control valve open more often based on space humidity to accomplish the dehumidifcation function, and then reheat the supply air to maintain space temperature. This is a common Museum/Archives/Art Gallery design- the cooling coil has to be designed and controlled for both space temperature as well as dehumidification, and many time you need reheat after the cooling coil, as the air gets cooled off too much by the dehumidification function.

Comments

  • Dennis KunkleDennis Kunkle Posts: 34Member
    Humidity problems

    The humidity in our museum's basement is 73% at 72F. Our one year old air handler has nothing in the drip tray. Taking off the cover, I can see that the exposed coil returns are very cold and they have droplets of water on them. The engineers first advice was raise the T-stat to 75F. 24 hours later, the area is 75F and the humidity is the same. So yesterday I hooked up a dehumidifier and 15 hours later it has removed about 3 gallons of water. So that thing works! This is a chilled water system with a valve that modulates based on the T-stat. So at any given time, the valve will be about 1/3 open. If I set the stat to 70, the valve will be nearly wide open and the room will cool down, but the humidity stays more or less the same. My conclusion, not the engineer's, is that the velocity of the air is so high that the condensated moisture is just pulled thru the unit. Oh, there are some while circles of mold forming inside. While now out of warranty, he has asked York's rep to tell us what size sheave and pulley is necessary to get the correct CFMs to stop this problem. Your thoughts?
  • Dennis KunkleDennis Kunkle Posts: 34Member


    Checking the CWT at the chiller via the computer, I see a supply of 45F and return of 50F. So the blower coil in the basement should be getting cold enough water. We have a VAV system in our galleries that has steam reheat at each zone. The air handler there delivers 55F supply. The return air humidity up there is currently 51%. For whatever reason, the engineer didn't design a reheat system in our basement which ironically is our main collection storage. (No paper, oil paintings, etc. Just objects, rolled quilts, clocks, radios, furniture, etc) It's too late to redesign the basement system, but it would be nice to remove some humidity. Looking at the balancer's report, the required velocity is 1080 FPM and the actual is 1112 FPM, designed CFM is 2250, actual 2317. Can't find any other measurements
  • GMcDGMcD Posts: 477Member
    Coil face velocity

    That air velocity at the coil seems way too high - I never design coolng coils for more than 500 fpm air velocity.

    On the other hand, there is a contradiction in your data - if the supply air temp is being supplied at 55F, saturated 55F air can only hold so mch moisture, and if that air in the space warms up to 72F, AND no other source of moisture is present, the ambient relative humidity should only be 55% +/-. If your ambient RH is higher, you may have building envelope infiltration problems - warm, humid air leaking into the building.
  • Dennis KunkleDennis Kunkle Posts: 34Member


    The supply air of 55F is for another system upstairs. That one, with reheat, works fine. This one in the basement is using the same chilled water. The basement's dehumidification is just a byproduct of cooling the air. The air is being cooled, it's just that there is little or no condensate. If I put my hand on the "exit fins", so to speak, I can feel moisture. So I still think that the velocity is pulling the condensate back into the space.
  • Dennis KunkleDennis Kunkle Posts: 34Member


    I just emailed the original engineer and he thinks that the balancer's numbers are referring to duct velocity, not coil face, FWIW
  • don_207don_207 Posts: 12Member
    Mr Geoff

    Its good to see you posting again.

    Personally I think you did the right thing.That is adding a
    dehumidifider to the room.
    You have no reheat down there so over cooling is out of the question.Dropping the cfm alittle may help, however that may come back to haunt you at full load.

    I would put my money on the dehumidifier appoach.



  • Dennis KunkleDennis Kunkle Posts: 34Member


    To my dismay, the 3-5 gallons a day is not affecting the RH. I've never considered this basement damp, after 23 years here. It's clean. painted on all surfaces, and no indications of spalting paint, etc. This morning it was 74% The blower coil is still not making condensate. The pan is still dry, save the little bit that falls from U fittings at the ends where there is no real air flow. How else can it be explained, other than high velocity, that 74% air at 72F is passing over a chilled water coil and not condensating? (I would need 4 dehumidifiers to keep up)
  • don_207don_207 Posts: 12Member
    The

    The 5 degree split on the water side alarm me a bit.

    I been to site where the valve show it to be 100% open and its not.Lots of time cartridge replacment is in order on the diverter valves.

  • Dennis KunkleDennis Kunkle Posts: 34Member
    Here's the fix

    Our engineer asked the balancer for the pulley changes to go from 2200 CFMs to 1000 CFMs. The adjustable sheave on the motor is getting 3 turns smaller and the fan pulley will be a new larger one. We are going to do it in-house. Interestingly enough, while looking inside, we've noticed that the motor casing is jammed severely into the coils. Since this would have been as-delivered, photos have been sent to the sales rep. I wonder what he'll have to say.
  • don_207don_207 Posts: 12Member
    If I

    If I had a tale that I could tell you....I would tell you
    full load operation condintions may be coming soon.

    In my area we have not had a summer.Lots of mild temps with high dewpoints.I assume this is a issue for your part of the country as well.Then again at part load operating condintion the latent load is always beating you up.

    Please share the results.
  • tim smithtim smith Posts: 2,279Member
    Only a 5 degree split?? Not removing enough moisture.

    Sounds like too much air on that coil. Pretty easy to do test. Why don't you have a VFD on that fan. You would not be messing around with sheaves then. Also you could get some load to speed benefit also. You could possibly tie to 0-10 humidistat and raise and lower speed based on humidity with min/max settings. Just a thought. Tim
    Ps, here is a link for really well written explanation.
    http://www.udarrell.com/air-conditioning-total-heat-enthalpy-latent-heat.html
  • TechmanTechman Posts: 2,144Member
    Humid room

    How big is this storage room?How does the average of 400 cfm per ton convert over to a chilled wate system?Is there an outdoor air inlet duct/system?
This discussion has been closed.

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