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Re: [EnergyPlus_Support] Humidity Issues
I suggest you revert back to a single cooling coil. Manually size this
coil if autosizing does not give you the performance you desire. You
won't get the same performance as a single coil. You must also add a
humidistat if you want to use humidity control and change the
Dehumidification Control Type to CoolReheat and the Run on Latent Load
input to Yes in DXSystem:AirLoop. I believe you will also have to change
to Coil:DX:MultiMode:Empirical using
CoilPerformance:DX:CoolingBypassFactorEmpirical objects for this to
work. Just use 1 cooling stage and 1 dehumidification stage. As for your
last question, the wet-bulb temperature should not increase across the
second cooling coil in your example file. If the second coil is off, the
wet-bulb temperature should be the same as that leaving the 1st cooling
coil. If the second coil is on, the wet-bulb temperature should always
be lower than that leaving the 1st cooling coil.
DXSystem:AirLoop,
1E DX, !- Name of DX Cooling Coil System
On, !- Availability schedule
1E Supply Fan Outlet, !- DX Cooling Coil System inlet node name
1E Cooling Coil Outlet, !- DX Cooling Coil System outlet node name
1E Cooling Coil Outlet, !- DX Cooling Coil System Control Node
COIL:DX:MultiMode:Empirical, !- Cooling coil type
1E DXSystem Cooling Coil, !- Cooling coil name
CoolReheat, !- Dehumidification Control Type
Yes, !- Run on Sensible Load
Yes; !- Run on Latent Load
Add the new objects for the multimode DX coil.
Coil:DX:MultiMode:Empirical and
CoilPerformance:DX:CoolingBypassFactorEmpirical.
You also need to add Set Point Manager:Single Zone Max Hum object to set
the correct DX coil outlet humidity ratio.
You should also consider entering values for fields 13-16 below (default
suggestion from InputOutputReference.doc) in the
CoilPerformance:DX:CoolingBypassFactorEmpirical object. These fields
simulate latent degradation or re-evaporation of moisture from the
cooling coil when the compressor only runs a portion of the time step.
You should really always use these fields even when not simulating
humidity control.
COIL:DX:MultiMode:CoolingEmpirical,
1E DXSystem Cooling Coil, !- Coil Name
On, !- Availability Schedule
1E Supply Fan Outlet, !- Coil Air Inlet Node
1E Cooling Coil Outlet, !- Coil Air Outlet Node
, !- Crankcase Heater Capacity {W}
10.0000002649095, !- Maximum Outdoor Dry-bulb Temperature for
Crankcase Heater Operation {C}
1, !- Number of Capacity Stages
1, !- Number of Enhanced Dehumidification Modes
CoilPerformance:DX:CoolingBypassFactorEmpirical, !- Normal Mode
Stage 1 Coil Performance Object Type
1E Coil Standard Mode-Stage 1, !- Normal Mode Stage 1 Coil
Performance Object Name
CoilPerformance:DX:CoolingBypassFactorEmpirical, !- Dehumid Mode 1
Stage 1 Coil Perf Object Type
1E Coil SubCoolReheat Mode-Stage 1; !- Dehumid Mode 1
Stage 1 Coil Perf Object Name
I think you could actually use the same CoilPerformance object for each
stage but gave them unique names here just to be safe.
CoilPerformance:DX:CoolingBypassFactorEmpirical,
1E Coil Standard Mode-Stage 1, !- Coil Performance Specification Name
autosize, !- Rated Total Cooling Capacity (gross) {W}
autosize, !- Rated SHR
3, !- Rated COP
autosize, !- Rated Air Volume Flow Rate {m3/s}
0.0, !- Fraction of Air Flow Bypassed Around Coil
1E DX Coil Cap-FT, !- Total Cooling Capacity Modifier Curve
(function of temperature)
1E DX Coil Cap-FF, !- Total Cooling Capacity Modifier Curve
(function of flow fraction)
1E DX Coil EIR-FT, !- Energy Input Ratio Modifier Curve
(function of temperature)
1E DX Coil EIR-FF, !- Energy Input Ratio Modifier Curve
(function of flow fraction)
1E DX Coil PLF, !- Part Load Fraction Correlation (function
of part load ratio)
ContFanCycComp, !- Supply Air Fan Operation Mode
1000, !- Nominal Time for Condensate Removal to
Begin {s}
1.5, !- Ratio of Initial Moisture Evaporation Rate and Steady-state
Latent Capacity {dimensionless}
3, !- Maximum ON/OFF Cycling Rate {cycles/hr}
45; !- Latent Capacity Time Constant {s}
Performance:DX:CoolingBypassFactorEmpirical,
1E Coil SubCoolReheat Mode-Stage 1, !- Coil Performance
Specification Name
autosize, !- Rated Total Cooling Capacity (gross) {W}
autosize, !- Rated SHR
3, !- Rated COP
autosize, !- Rated Air Volume Flow Rate {m3/s}
0.0, !- Fraction of Air Flow Bypassed Around Coil
1E DX Coil Cap-FT, !- Total Cooling Capacity Modifier Curve
(function of temperature)
1E DX Coil Cap-FF, !- Total Cooling Capacity Modifier Curve
(function of flow fraction)
1E DX Coil EIR-FT, !- Energy Input Ratio Modifier Curve
(function of temperature)
1E DX Coil EIR-FF, !- Energy Input Ratio Modifier Curve
(function of flow fraction)
1E DX Coil PLF, !- Part Load Fraction Correlation (function
of part load ratio)
ContFanCycComp, !- Supply Air Fan Operation Mode
1000, !- Nominal Time for Condensate Removal to
Begin {s}
1.5, !- Ratio of Initial Moisture Evaporation Rate and Steady-state
Latent Capacity {dimensionless}
3, !- Maximum ON/OFF Cycling Rate {cycles/hr}
45; !- Latent Capacity Time Constant {s}
noelcrrl wrote:
> Dear All-
>
> We are modeling a school building in the humid climate of Houston.
> Our HVAC system consists of constant volume rooftop units that have
> 2 DX cooling coils (2 cooling coils are necessary in EnergyPlus to
> achieve 55F air temperature after the cooling coil) and a gas
> heating coil. The system does not have any reheat coils or
> perimeter units. With this system we have had a problem controlling
> the humidity in the zones.
>
> The zone air relative humidity routinely exceeds 50% during the full
> weather file simulation even though the air temperature after the
> cooling coil remains relatively constant at 55F. In addition, it
> seems that the wet bulb temperature increases from the end node of
> the cooling coil (1E DX Outlet) to the air loop outlet node (1E Air
> Loop Outlet). What is causing this rise in wet bulb temperature?
>
> The idf file has been posted and is named C1C_1.idf. Thank you for
> your help.
>
> Noel Corral
>
>
--
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
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