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Re: [EnergyPlus_Support] Re: Humidity control in an Autosized AHU



You will have to manually size the cooling coil to meet the latent load. 
Sizing to meet the latent load is scheduled to be added to EnergyPlus 
for the next or following release.

Elmer Morrissey wrote:

> Hi Richard,
>
> I tried what you recommended and it seems I do have a capacity
> problem. However I have used the following system sizing:
>
> SYSTEM SIZING,
> A3:Loop, !- name of an AIR PRIMARY LOOP object
> sensible, !- type of load to size on
> autosize, !- Design (minimum) outside air
> volumetric flow rate {m3/s}
> 1, !- minimum system air flow ratio
> 5, !- Preheat design set temperature {C}
> 5, !- Central cooling design supply air
> temperature {C}
> 16, !- Central heating design supply air
> temperature {C}
> noncoincident, !- Sizing Option
> Yes, !- Cooling 100% Outside Air
> Yes, !- Heating 100% Outside Air
> 0.0025, !- Central cooling design supply
> air humidity ratio {kg-H2O/kg-air}
> 0.008, !- Central heating design supply
> air humidity ratio {kg-H2O/kg-air}
> Design day, !- cooling design air flow method
> , !- cooling design air flow rate {m3/s}
> Design day, !- heating design air flow method
> ; !- heating design air flow rate {m3/s}
>
> I can maintain around 55% RH during peak hours but I can't get down
> to 50%. The SYSTEM NODE SETPOINT TEMP after the cooling coil becomes
> as low as -55C during these hours. The `System Node Humidity Ratio
> Max' displays similar swings in setpoint.
>
> During the winter, I able to maintain 50% RH most of the day but 4
> of the hours display values around the 41% range for some unknown
> reason. I checked things like infiltration and I can't seem to
> account for it.
>
> I am using one central AHU for zone control. It supplies and
> extracts air from the room. It contains a mixer, a CC, a HC, a
> humidifier and a supply fan. Finally, it contains a terminal reheat
> coil.
>
> I appreciate your help.
>
> Kind regards
>
> -Elmer
>
> --- In EnergyPlus_Support@xxxxxxxxxxxxxxx 
> <mailto:EnergyPlus_Support%40yahoogroups.com>, Richard Raustad
> <RRaustad@...> wrote:
> >
> > Are you saying you don't have enough capacity to maintain RH, or
> that
> > the system is not actively controlling to the RH set point? I
> assume you
> > already have a humidistat in the control zone. What type of system
> are
> > you using? If you are using a chilled water coil, the simple
> > controller's control variable should be set to TEMPandHUMRAT and a
> > single zone max hum set point manager is required to place the set
> point
> > at the cooling coil outlet node. Report out the temperature set
> point
> > after the cooling coil, when the RH rises above the set point, the
> > temperature set point should start to drop. The higher the
> humidity
> > rises above the set point, the lower the temperature set point
> after the
> > cooling coil. If this happens, the system is actively controlling
> to the
> > RH set point and it is a capacity issue. If you are using DX
> > System:AirLoop object, the Run on Latent Load field should be set
> to YES
> > and the dehumidification control type field should be set to
> COOLREHEAT.
> > A single zone max hum set point manager is also required to place
> the
> > humidity set point at the cooling coil outlet node. If you are
> using a
> > Furnace or Unitary System, all that is required is setting the
> high
> > humidity control field to YES in the furnace or unitary system
> object.
> >
> > Elmer Morrissey wrote:
> >
> > > Hello all,
> > >
> > > I am having major difficulty maintaining 50% RH in a single zone
> > > supplied by a simple AHU.
> > >
> > > Fresh air- -> Fan -> CC -> HC -> Hum -> -----------TRH -> Zone->-
> ----
> > > ------exhaust
> > >
> > > The max humidity ratio is assigned after the cooling coil while
> the
> > > minimum humidity ratio is set after the humidifier. I am looking
> at
> > > a typical design day in a British climate and it should not be
> > > overtaxing.
> > >
> > > Unfortunately, the RH is not maintained during the summer months
> > > (even though I oversized the system sizing object). The average
> RH
> > > is around 57%. During the winter, RH values slip around mid-day
> for
> > > a four hour period that I cannot explain.
> > >
> > > I've taken a look at the setpoint output from the CSV file for
> the
> > > max and min humidity ratios and it seems to me like they just are
> > > not be applied within the model. I am aware that humidity
> control is
> > > not the strongest area in E+ but is there any way of creating a
> file
> > > that would work for an autosized run?
> > >
> > > Can anyone help or point out some things I may have overlooked?
> > >
> > > Kind regards
> > >
> > > -Elmer
> > >
> > >
> >
> >
> > --
> > Richard A. Raustad
> > Senior Research Engineer
> > Florida Solar Energy Center
> > University of Central Florida
> > 1679 Clearlake Road
> > Cocoa, FL 32922-5703
> > Phone: (321) 638-1454
> > Fax: (321) 638-1439 or 1010
> > Visit our web site at: http://www.fsec.ucf.edu <http://www.fsec.ucf.edu>
> >
> > UCF - From Promise to Prominence: Celebrating 40 Years
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
>
>  


-- 
Richard A. Raustad
Senior Research Engineer
Florida Solar Energy Center
University of Central Florida
1679 Clearlake Road
Cocoa, FL  32922-5703
Phone:   (321) 638-1454
Fax:     (321) 638-1439 or 1010
Visit our web site at: http://www.fsec.ucf.edu

UCF - From Promise to Prominence: Celebrating 40 Years



[Non-text portions of this message have been removed]



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