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  1. #1
    Join Date
    Sep 2018
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    How to deal with low SHR

    Hi all
    I am new here, actually just a student interested in the HVAC industry and in my final year to obtain my mech eng degree.

    However, I have come across something that I do not understand.

    How does the industry deal with air-conditioning rooms with low SHRs. Let me explain - if we consider that due to recent innovations, designers have been able to reduce sensible heat loads within buildings (better insulation specs, lower u-values, etc.), however not much has been done with latent heat loads. The biggest contributors for latent heat loads is ventilation (which ASHRAE has greatly increased the requirements), infiltration and people. Lets consider a scenario as worst case - in a hot climate like Miami with high outdoor humidity levels. A lecture theatre in a university. Lecture theatres commonly are fully enclosed with no windows, however have a great occupancy. This would probably work out to a very low SHR. Since sensible loads is what is used to determine supply air flow rates, I can see a case where the ventilation rate = supply air flow rate. Thus there would be no return air? Am I on the right track here? Which AC units does the industry use in cases like this where there is no return air?

    Secondly - I was browsing some standards to substantiate my point above, and I found that for lecture halls, the minimum ventilation requirement is 7.5l/s of outside air per person or 2 air changes an hour - depending on which is larger. This seems like a lot of ventilation requirements that will probably just result in energy wastage. Do we have to comply with maximum values?

    Thanks for your assistance.

  2. #2
    Join Date
    Jan 2004
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    PA
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    You would have to comply with the min rates.

    A lecture theater with 500 people in it. Would have a min sensible load from the people of 100,000 BTUs. Then the lighting's sensible load would be added in. SHR would still be well over 60.
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  3. #3
    Join Date
    Mar 2016
    Location
    Dayton Oh
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    It would also depend on the system that is being used. Some systems handle humidity better than others.

    From what I have learned hear (please know this is not my strong area), that a chilled water system using VAV boxes with built in reheat are better at handling this. again, from my understanding, the reheat is able to create a false load on the room thus allowing the a/c to run longer which allows for more moisture removal. I think the main A/C chilled water coil is something like 45 (or is it 55) degrees which will ring out quite a bit of moisture. This cold air is shipped to the room, but if the room does not need much a/c the reheat will heat that 45 degree air to 68 (or something in that range as to not over cool the room).

    That is the way it was explained to me, and my guess would be that is the best method for a commercial room/building.

    Also, it would be my guess that there is almost always return air in these buildings. Even when the room is "satisfied" on load there is still a minimum amount of air being pushed into the room always.

    Some of the much smarter guys that do this type everyday will chime in and clear the mud a little.

    good luck in your studies.

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by vancity33 View Post
    Hi all
    I am new here, actually just a student interested in the HVAC industry and in my final year to obtain my mech eng degree.

    However, I have come across something that I do not understand.

    How does the industry deal with air-conditioning rooms with low SHRs. Let me explain - if we consider that due to recent innovations, designers have been able to reduce sensible heat loads within buildings (better insulation specs, lower u-values, etc.), however not much has been done with latent heat loads. The biggest contributors for latent heat loads is ventilation (which ASHRAE has greatly increased the requirements), infiltration and people. Lets consider a scenario as worst case - in a hot climate like Miami with high outdoor humidity levels. A lecture theatre in a university. Lecture theatres commonly are fully enclosed with no windows, however have a great occupancy. This would probably work out to a very low SHR. Since sensible loads is what is used to determine supply air flow rates, I can see a case where the ventilation rate = supply air flow rate. Thus there would be no return air? Am I on the right track here? Which AC units does the industry use in cases like this where there is no return air?

    Secondly - I was browsing some standards to substantiate my point above, and I found that for lecture halls, the minimum ventilation requirement is 7.5l/s of outside air per person or 2 air changes an hour - depending on which is larger. This seems like a lot of ventilation requirements that will probably just result in energy wastage. Do we have to comply with maximum values?

    Thanks for your assistance.
    We have a similar problem in homes located in green grass climates. ASHRAE suggest a fresh air change in 3 hours. Others suggest a fresh air change in 4-5 hours when occupied. Manual J suggest a/c capacity to adequate to handle the sensible/latent load at he peak load condition. Like you describe, upgraded insulation and excellent windows can reduce the sensible portion of the cooling load.
    Installing a/c contractors also do not set air flow properly resulting a/c coils do not remove enough moisture during the cooling cycle to maintain <50%RH during peak sensible cooling loads. This usually can be adjusted.
    The next problem is that as the sensible cooling loads decline when the sun sets, the latent cooling from infiltration/ventilation plus the moisture from the occupants continues at near the peak rate of the day. The indoor %RH rises by 10 AM and then the cycle repeats. During rainy days with high outdoor dew point and low/no sensible cooling loads, humidity control is dependent on adding supplemental dehumidification which the residential a/c trade has been slow to adapt. Two or Vairiable speed is thought to be a solution. Expensive and complicated with only slight improvement in dehumidification. Still not dehumidification with out a steady a/c cooling load.
    Over-cooling has also caused more moisture movement from outdoors into the home because the home is cooled below the outdoor dew point. Mold in the walls and on attic drywall is slowing up in case of high outdoor dew points and cooling below the outdoor dew points.
    Mold and dust mites are showing up and causing some concern mainly from the effected occupants.
    Regards Teddy Bear
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

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