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  1. #1

    Question 7.5 Package Unit, Feasible to use VFD to Control Humidity and More?

    Hello there. I own a small commercial building in northern IL, it's a newer single stage 7.5 ton package unit with a single speed belt drive blower, and (as a lot of people have) I have a bad part load humidity problem. I was considering different options to try to correct this problem. I understand replacing the unit with a 2 stage unit, or a unit with dehumidification (hot gas bypass) would help, but I would rather not replace the unit.

    I know slowing down the blower speed will increase my dehumidification, but I do not want to freeze my coil or overheat the heat exchanger when winter comes, so I was thinking of a more versatile approach then then just statically setting the blower speed lower. After, do a lot of reading, I was wondering how feasible is it to install a VFD to the blower motor? It's a fairly small motor, and VFD pricing is not so bad for loads this size. Could this be used to control the speed of the blower motor successfully, taking logic factors into consideration (obviously by adding other devices to the system) such as call for cooling, humidistat signal, coil temperature? Also, could it even be taken a step further to allow for soft starts which should help with motor and belt life, and even perhaps slower operation of the blower for fan only calls?

    Thanks in advance.

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,470
    Consider adding a 200 pint per day whole building dehumidifier like an Ultra-Aire 205. These units are able to maintain <50%RH without any cooling loads. An a/c is unable to operate without significant cooling loads. The basic setup of your a/c should have a 45-50^F coil temp to provide <50%RH during peak cooling loads. The dehu will maintain <50%RH whenever there is not enough cooling loads for the a/c.
    Regards TB

    http://ultra-aire.com/index.php?opti...=74&Itemid=121
    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"

  3. #3
    Funny you should mention it, I actually am looking into a whole building dehumidifier as well. But, would my original idea be a worth while venture too? The VFD and other hardware seems to be low cost, I talked to the HVAC contractor I usually call for my properties, and he said they use them on a lot of larger units for energy savings. Could I not benefit from the same energy savings just on a smaller scale? Would I experience improved dehumidification? Plus the other possible advantages I was contemplating as well such as reduced blower speed when called for fan only (could be interesting for fresh air without pushing so much air volume into the space), would they be worth while? Or is this all just a pipe dream on paper?

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,470
    Slowing the motor will decrease the watts by a small percentage. Turning of the fan when the compressor is not operating will save much more. Also the moisture left on the cooling coil will evaporate back into the space slower. These are small measures. Reheat is the biggest waste of energy. The dehus remove 6 lbs. of moisture per kw. A/c remove 2-3 lbs. per kw.
    Regards TB
    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"

  5. #5
    Join Date
    Dec 2000
    Location
    Flat Rock, NC
    Posts
    463
    use the dedicated dehumidifier. Compressor run time won't be adequate for humidity control w/ single stage cooling unit during mild weather. Idiot single compressor unit, I bet its a trane.

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,470
    Quote Originally Posted by codgy View Post
    use the dedicated dehumidifier. Compressor run time won't be adequate for humidity control w/ single stage cooling unit during mild weather. Idiot single compressor unit, I bet its a trane.
    The multi-speed including vs speed a/c are better than single speed. The data logging of the multi-variable speed acs indicates that all of these a/cs are unable to provide <50%RH with low/no cooling loads.
    There are many weeks of high outdoor dew points and low sensible cooling to make statement that you can maintain <50%RH without supplemental dehumidification or the energy hog reheat.
    Many are customers are being promised <50%RH with vs a/c. Long term no-way.
    A whole house dehumidifier will maintain <50%RH regardless the cooling load. Turn the a/c off when unoccupied and have <50%RH for a faction of the operating cost. Include a small whole house dehu like an Ultra-Aire with any of the a/cs and reduce operating cost and avoid over cooling and high %RH.
    Regards TB
    Regards TB
    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"

  7. #7
    Join Date
    Dec 2010
    Location
    Down by the river
    Posts
    1,664
    The VFD on the fan is a good idea. My only ? to figure out would be how is it gonna be controlled? The bad...like TB suggest during low loads you are running a 7.5 ton compressor just for humidity control. I think payback would be better on the dehum unit.

    Re heat is a waste in my opinion, I work on so much with reheat in small RTU's and the engineering just makes me mad.
    It's hard to stop a Trane. but I have made one helluva living keeping them going.

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