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Evaporative Cooler turns off in summers

asked 2016-03-18 12:32:38 -0600

Rehan Khalid gravatar image

updated 2017-05-02 12:40:55 -0600

Hi,

I am modeling a hybrid DX-Evaporative Cooler system with the location set to Phoenix Arizona. However, my evaporator turns off during the peak summer months of July and August, when the outdoor Dry-bulb is 42 deg. C and the wetbulb is around 22 C.

I have no limits on the operation of the evaporative cooler; however, the outdoor air controller is set to have a maximum of 27 C for the economizer mode and the OA mixing box inlet node is set to have a setpoint that varies between 15 and 35 deg C. Can anyone offer a possible explanation and a fix. I'd greatly appreciate the help.

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Comments

Are you able to upload your file? or email it to me so I can review the setup.

rraustad gravatar imagerraustad ( 2016-04-06 19:24:03 -0600 )edit

Thank you for offering the help Sir. I will email you the file.

Rehan Khalid gravatar imageRehan Khalid ( 2016-04-07 10:13:32 -0600 )edit

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answered 2016-04-07 20:47:33 -0600

The OA controller is turning off the air flow. The minimum and maximum outdoor air flow rates are autosized. Note also that the OA controller input fields stop at the "Minimum Outdoor Air Schedule Name" field and there are more inputs that can be used to control outdoor air.

Controller:OutdoorAir,
  Data Center OA Controller,  !- Name
  autosize,                   !- Minimum Outdoor Air Flow Rate {m3/s}
  autosize,                   !- Maximum Outdoor Air Flow Rate {m3/s}
  FixedMinimum,               !- Minimum Limit Type
  MinOA_Sched;                !- Minimum Outdoor Air Schedule Name

The result of sizing the autosized inputs yields a 0 minimum (I used Miami weather so your results may be different).

Component Sizing Information, Controller:OutdoorAir, DATA CENTER OA CONTROLLER, Maximum Outdoor Air Flow Rate [m3/s], 6.83914
Component Sizing Information, Controller:OutdoorAir, DATA CENTER OA CONTROLLER, Minimum Outdoor Air Flow Rate [m3/s], 0.00000

The Minimum Outdoor Air Schedule is constant at 1, however, 1 * 0 = 0.

Schedule:Compact,
  MinOA_Sched,             !- Name
  Fraction,                !- Schedule Type Limits Name
  Through: 12/31,          !- Field 1
  For: AllDays,            !- Field 2
  Until: 24:00,            !- Field 3
  1;                       !- Field 4

If you change the OA controller inputs to place the min OA schedule name in the "Minimum Fraction of Outdoor Air Schedule Name" input field, the evap cooler will operate all year.

Controller:OutdoorAir,
  Data Center OA Controller,  !- Name
  OARelief Node,           !- Relief Air Outlet Node Name
  Supply Equipment Inlet Node,  !- Return Air Node Name
  Unitary Package Node,    !- Mixed Air Node Name
  OAInlet Node,            !- Actuator Node Name
  autosize,                !- Minimum Outdoor Air Flow Rate {m3/s}
  autosize,                !- Maximum Outdoor Air Flow Rate {m3/s}
  DifferentialDryBulb,     !- Economizer Control Type
  ModulateFlow,            !- Economizer Control Action Type
  28,                      !- Economizer Maximum Limit Dry-Bulb Temperature {C}
  64000,                   !- Economizer Maximum Limit Enthalpy {J/kg}
  ,                        !- Economizer Maximum Limit Dewpoint Temperature {C}
  ,                        !- Electronic Enthalpy Limit Curve Name
  0,                       !- Economizer Minimum Limit Dry-Bulb Temperature {C}
  NoLockout,               !- Lockout Type
  FixedMinimum,            !- Minimum Limit Type
  ,                        !- Minimum Outdoor Air Schedule Name
  MinOA_Sched;             !- Minimum Fraction of Outdoor Air Schedule Name
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Comments

Thank you Sir. I get your point and understand what I had been doing wrong.

Rehan Khalid gravatar imageRehan Khalid ( 2016-04-07 21:28:09 -0600 )edit

I am now wondering how this change will affect the economizer settings. If the minimum = maximum, there will be no economizer.

rraustad gravatar imagerraustad ( 2016-04-07 21:56:51 -0600 )edit

Sir, if you look at the output file, doing the above switches off the mass-flow rate within the zone, which causes CPU CPU/Fan power to go to zero and the PUE values to shoot up. This was not happening previously; however, then the evaporative cooler was not functioning properly.

Rehan Khalid gravatar imageRehan Khalid ( 2016-04-07 22:18:58 -0600 )edit

I plotted all mass flow rate and power variables in the output file and none of those show 0. Remember to specify exactly in the outputs where you are seeing an issue. Report variable name, time of simulation, column number, row number, etc. That way we can see what you see.

rraustad gravatar imagerraustad ( 2016-04-08 08:18:50 -0600 )edit

I understand Sir. Comment followed and email sent highlighting the problem.

Rehan Khalid gravatar imageRehan Khalid ( 2016-04-08 10:22:00 -0600 )edit

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Asked: 2016-03-18 12:32:38 -0600

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Last updated: Apr 07 '16