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  1. #1
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    Suction Pressure Control

    Hey Guys,
    I have an AAON RTU that I am programming and the specs call for suction pressure control to dehumidify. The unit has a VFD on the fan and a VSD on the compressor for first stage. The compressor also has an unloader. There is no hot gas reheat on this unit and the sequence is calling for humidity control but doesnt specify how to accomplish it. I have never programmed this type of scenario and I was wondering how does the suction pressure sensor factor into dehumidification...along with the vfd and vsd?

  2. #2
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    Feb 2005
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    Quote Originally Posted by Control Freek View Post
    Hey Guys,
    I have an AAON RTU that I am programming and the specs call for suction pressure control to dehumidify. The unit has a VFD on the fan and a VSD on the compressor for first stage. The compressor also has an unloader. There is no hot gas reheat on this unit and the sequence is calling for humidity control but doesnt specify how to accomplish it. I have never programmed this type of scenario and I was wondering how does the suction pressure sensor factor into dehumidification...along with the vfd and vsd?

    I don't know the specifics about your suction pressure but suspect you would use that to load/unload the compressor when in dehumidification mode. As far as the dehumidification your option would be to decrease the discharge temperature of the unit and slow the fan speed down in order to not subcool the space. As you slow the fan down your DX load would decrease so your suction presssure would drop. You would then unload the compressor to keep the coil from frosting up.

    I've used the fan routine before on chilled water unit but not DX. I would control the actual airflow stpt of the unit via space temp. The units chilled water valve would be controlled via DA reset or humidity worst case/highest signal. The fan would slow down as space temp decreased. In my application it worked well. I would let the units go down to 40% air flow and more or less idle there keep the humidity down without using reheat. My AHUs where in the core areas of a school where the load would change drasticly as kids would enter and exit the hall ways, cafateria and auditorium. Hard to dehumidify when there isn't much cooling load without using reheat. This school was also at about 10% capacity of what the equipment was sized for. It was set up as a special needs school but equipment was designed that it could be used as a regular high school if it's purpose changed.

    Since you have DX I would control the DX leaving DA stpt instead of just letting the compressor go faster. I would then also let the suction pressure limit how much the compressor would load up when slowing the fan down. With the chilled water coil I didn't have to worry about tearing up compressors so I would just let the humidity drive it directly.
    It took actually learning something to realize how much I had to learn.

  3. #3
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    Awesome...thanks for the advice I'll give it a try

  4. #4
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    Willf650....
    Another question i have that may be a stupid question but I'm having a hard finding the information is what is the suction pressure sensor range? Are there different types of sensors or are all the same range?

  5. #5
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    Quote Originally Posted by Control Freek View Post
    Willf650....
    Another question i have that may be a stupid question but I'm having a hard finding the information is what is the suction pressure sensor range? Are there different types of sensors or are all the same range?
    I know you were asking Willf this question, but I'll throw in my two cents. Generically, I will say 0-200 PSi. It will kind of depend on what type of refrigerant the machine is running. If it's running R-22 you could use 0-100 to get better range on your sensor, but if it's R-410a you will need to use 0-200 psi range.
    "It's not that I'm smart, it's that I stay with the problem longer”
    Albert Einstein

  6. #6
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    Great...thanks for responding xarralu

  7. #7
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    In dehumidication mode aaon uses the suction pressure transducer and loads compressors based on calculated coil temp. In cooling mode it would load based on supply air temp.

    Once programmed properly there should be no need to adjust fan speeds or rig up stuff.

  8. #8
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    Just FYI for anyone following this thread... The pressure transducer had a range of 0 - 500. Also, I dont have a coil temp input on my board. All I have is SAT which is not a true coil temp reference. It will get me close but not actuall coil temp. Also, it doesnt say in my SOO to use suction pressure control during normal operation, only during humidity control. Without any kind of reheat I dont see how dehumidification is possible. I set my fan at minimum and set my compressor at only 5% loaded and watched it for a minute. It still dropped my space temp by 2 tenths of a degree every minute and this is a large room. So I guess I just dont understand how I can dehumidify with no reheat without subcooling the space...

  9. #9
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    Also...the fan is controlled by a VFD that is controlled based on space temp. The SOO says that the fan is to run continuosly in normal operation and in my experience that adds to the humidity problems because when cooling or heating is not being utilized your bringing in outside air and water off the coil. I would rather cycle the fan during normal operation so to not bring in unnecessary moisture

  10. #10
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    Quote Originally Posted by Control Freek View Post
    Just FYI for anyone following this thread... The pressure transducer had a range of 0 - 500. Also, I dont have a coil temp input on my board. All I have is SAT which is not a true coil temp reference. It will get me close but not actuall coil temp. Also, it doesnt say in my SOO to use suction pressure control during normal operation, only during humidity control. Without any kind of reheat I dont see how dehumidification is possible. I set my fan at minimum and set my compressor at only 5% loaded and watched it for a minute. It still dropped my space temp by 2 tenths of a degree every minute and this is a large room. So I guess I just dont understand how I can dehumidify with no reheat without subcooling the space...
    If you are using Aaon controls (vcm vcm-x) it uses the suction pressure transducer. Converts the pressure to a temp reading and uses that for coil temp in dehumidication.

  11. #11
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    Sicofthis...thank you for your input. What i did was go online and get a suction pressure chart for 410a. That told me at x pressure the coil temp would be y. So the problem isn't setting anything up anymore. The problem that i have now is even at the lowest capacity on both the fan and compressor i am subcooling the space too fast, about .2 a minute and not getting any dehumidifaction to really speak of. The risk/reward ratio (subcooled space/dehumidification) is unsatisfactory in my opinion. AAON claims that this strategy should work without hot gas reheat but i am just not seeing the results expected when implemented. I have pretty much conceded to the idea of just cycling the fan in normal operation and cycling in and out of dehumidification mode on a regular basis. Damn value engineering...pay the extra money and get a unit that is capable of producing the atmosphere you desire...

    Thanks again to everyone who has contributed...

  12. #12
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    I am controlling the unit with a mach pro zone...Reliable Controls

  13. #13
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    Apr 2012
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    Louisiana
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    Yeah my experience would be with the aaon/watt master controls. Glad to hear you have it sorted out.

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