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Decreasing humidity level in office

  1. Oct 28, 2007 #1
    Hi all, i have been assigned a task to decrease humidity level in a office environment.

    Since the office is located in tropic areas, therefore the surrounding air humidity is always around 80-90%.

    However, the office is equipped with centralized air conditioning, ACMV system.

    Current reading from hygrometer is 75% , is there anyway that i can lower down the humidity level by just manipulating the ACMV settings? (ie. fan speed, chilled water supply and return... etc)

    My current idea would be increasing the blower motor speed in AHU room and at the same time lowering the chilled water return rate for sufficient time for proper heat transfer in the system)

    Please comment.
  2. jcsd
  3. Oct 29, 2007 #2


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    Science Advisor

    Can you not just use a dehumidifier? How big is the office?

    What is the temperature of the chill water across the coils? If it's not below the dew point of the air then you'll have extra humidity coming out. Maybe you just need to lower the chill water temperature below the dew point.
  4. Oct 29, 2007 #3
    sorry i was not allowed to use a dehumidifier. only limited to manipulation of ACMV settings.

    by the way, i obtained the readings of chiller plant from fellow technicians.

    5AM readings


    Condenser Water
    Return Temp: 31C,
    Supply Temp: 35C
    Return Pressure: 48bar
    Supply Pressure: 33bar

    Chilled Water
    Supply Temp 4C
    Return Temp 10C
    Return Temp 6.5bar
    Supply Pressure 6.0bar


    Chilled Water Supply 5.4C
    Chilled Water Return 12.8C
    Supply Pressure 2.5 bar
    Return Pressure 2.5 bar

    AHU Fan motor running (4pole): running at 50hz, appx 1500rpm
  5. Oct 30, 2007 #4


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    Science Advisor

    I don't think lowering the chill water return rate would help you at all. It probably will make the problem worse. If you restrict the flow of chill water out of the system (which I would imagine would also decrease the flow rate into the system), you won't get the same cooling capacity.

    It appears from your readings that the chill water is cool enough. Is the office subjected to repeated exposure from the outside air (like maybe a door opening frequently)? If so, you might try increasing the total volume of air exchanged by the system.

    Russ_Watters is the moderator of the engineering forum. One of his specialties is HVAC systems. If he doesn't see this thread then you might try a PM to him and ask for his advice.

    Hope this helps.

  6. Oct 31, 2007 #5


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    Staff: Mentor

    Hi, sorry I missed this thread before. Office building humidity problems are very common and my company does a lot of work on them.

    The basic cause of high humidity in a system such as this is supply air that is too warm. To remove the proper amount of water from the supply air, you need the supply air to be 13 C. If you trace across on a psychrometric chart, when saturated air at 13 C is warmed up to room temperature, you get about 50% rh. 75% RH implies (by matching it on the pschrometric chart) an average of 19C supply temperature, assuming a 24C space temperature.

    You have chilled water and presumably a modulating valve to control space temperature. You said the supply fan is at 50hz - is it on a VFD? Typically people hold airflow constant and vary the valve position to meet space temperature. What you need is to vary the valve position to maintain 13C and vary the airflow to meet space temperature.

    If you don't have a VFD and your load is relatively constant (or you can verify that supply air temperature under heavy load conditions), you can decrease the supply air quantity. The space conditions you gave imply that you have twice the supply air you need. So if you cut the supply air in half, to meet your space temperature the air will have to be cooled twice as much - and thus also dehumidified better.
  7. Nov 1, 2007 #6
    Thank you for the suggestion, so far i have tried to shut off the outside air intake in AHU room (since outside air humidity around 80%). Shutting down improves humidity level a bit. but the drawback is we keep circulating the air in office.

    anyway i calculated the size of office.. its approximate 388m2 and the volume is appx 1151m3 for 3m high ceiling
  8. Nov 2, 2007 #7


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    Staff: Mentor

    I don't know where you live, but every mechanical code I know of requires the use of ventilation air for comfort/health reasons.

    Shutting the outside air off will also only help the situation but not solve it - water vapor still infiltrates into the building, just at a slower rate.

    Bottom line - you really need to fix the problem instead of applying a band-aid that improves one symptom a little bit. Unfortunately, it may not be a simple fix, but you really should start with having an engineer do a small study, calculating the cooling loads and seeing if a constant volume system is even workable for your application. If not, variable speed drives and capacity control will be required.

    Be forewarned - it is typical and understandable for building owners to want to try simple and inexpensive solutions for what can be an expensive problem to fix. But in so doing, you'll ensure that it will take you years to fix it and when you do finally get it fixed, it'll cost twice as much as if you do it correctly now.

    I have a client, currently involved in a lawsuit, who did not follow our advice 3 years ago and only just this summer got their buiding humidity under control - and then they screwed up the temperature control. They are at $175K and counting, whereas our estimate for the cost of the necessary upgrades was $100K.
    Last edited: Nov 2, 2007
  9. Jan 28, 2008 #8
    Try a DOAS

    I was a Mechanical Contractor for six years building mostly schools and institutional buildings. I ran into this same problem constantly. I'd have to agree with the post that suggests that you need to have varriable capacity and digital controls to handle the humidity problems you are having. If you wnat a simpler solution, I'd go with a dedicated outdoor air system. This system is decoupled from your existing buidling load and is completely focused on providing ventilation air. Basically they have more coil capacity, bigger drain pans, stronger fans for better filtration etc. I just saw one at the ASHRAE show in NY that looked really good. It was called a Seirra DCV. You might want to give those guys a call and see what they can do. www.sierrafas.com
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