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How to keep air loop flow rate as the one imposed by schedule? and not automatically varied according to cooling load

asked 2019-04-16 15:32:50 -0500

Jore's avatar

updated 2019-04-19 14:48:11 -0500

Hello everybody,

I have been trying to model something specific these days and since I did not manage to achieve it and did not find relevant information, I thought of asking here. I would be grateful if somebody can help me out.

I have an AirLoop which provides ventilation to various thermal zones of a school, based on a schedule (low ventilation during night, high during day). I want to keep this schedule but I also need to add a cooling device (DX single speed) of certain capacity and try to see whether thermal comfort can be achieved based on these two parameters (cooling capacity, ventilation rate). The DX single speed is activated based on a setpoint manager "Warmest zone" (activated when the highest temperature of the served thermal zones reaches a specific temperature).

My problem is that when the cooling load cannot be satisfied, the ventilation rate is increased and does not follow the schedule that I have imposed. Consequently, I cannot know what the conditions would be under the real mass flow rate.

Any thoughts or ideas that could help me? Thanks for your time

Cordially, Jore

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Have you tried modeling it as a DOAS system?

mdahlhausen's avatar mdahlhausen  ( 2019-04-17 11:01:06 -0500 )edit

Hello, thanks a lot for your response. To my understanding, DOAS is a sizing option, not a strategy one. I have tried both but it still changes the mass flow rate. Am I missing something?

Jore's avatar Jore  ( 2019-04-17 15:32:22 -0500 )edit

3 Answers

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answered 2019-04-17 11:06:29 -0500

You might try using the min/max schedules in the outdoor air controller to replicate your schedule. Use a fractional schedule (OA flow / Max OA flow) to determine the flow rate needed then use this schedules name for both the min and max schedule name inputs.

  A12, \field Minimum Fraction of Outdoor Air Schedule Name
       \type object-list
       \object-list ScheduleNames
       \note schedule values multiply the design/mixed air flow rate
  A13, \field Maximum Fraction of Outdoor Air Schedule Name
       \type object-list
       \object-list ScheduleNames
       \note schedule values multiply the design/mixed air flow rate
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Hello, thanks a lot. To my understanding the fraction of outdoor air has to do with the mixing of outdoor air and recirculated air. I have tried it and confirmed it (checking exhaust, admission and supply mass flow rates). In my case, we need 100% outdoor air, so this fraction should be always 1. Thank a lot anyways. Any other ideas?

Jore's avatar Jore  ( 2019-04-17 15:52:43 -0500 )edit

Did you enter a schedule name with values of 1 into these 2 fields?

rraustad's avatar rraustad  ( 2019-04-18 08:28:18 -0500 )edit

Just in the minimum fraction of outdoor air. That keeps the mass flow rate 100% outdoor air, while following the schedule I introduced in absolute values of mass flow rate.

Furthermore, I checked the activity of the economizer and it is zero (I was suspecting that this desobedience for the mass flow rate was caused by the economizer). Now I suspect the DX coil

Jore's avatar Jore  ( 2019-04-18 12:42:34 -0500 )edit

So the question is then why is the air loop air flow rate increasing (which will increase OA flow) when the load is greater than the coil capacity. Your using a single speed coil, so what else in your inputs allow supply air flow rate to increase? Is this a VAV system?

rraustad's avatar rraustad  ( 2019-04-18 12:52:48 -0500 )edit

Thanks a lot for your involvement! Yes, it is VAV. Now, I have a setpoint manager at the end of the supply which is scheduled. So the VAV is activated based on time. In this case the air flow rate is the one I impose, so up to here fine, I can see the classroom temperatures based on the real airflow rate.

However, the control is not realistic (cooling will be activated when the temperature in the classrooms is high, not based on time). So I implemented a setpoint manager "Warmest zone supply air temp set", and cooling thermostats in the classrooms. This is when the air flow is being modified

Jore's avatar Jore  ( 2019-04-18 13:21:50 -0500 )edit

answered 2019-04-22 13:11:54 -0500

Do you have an economizer in your Controller:OutdoorAir object? If so, try changing it to NoEconomizer.

Also, sometimes you need to specify a Controller:MechanicalVentilation within the Controller:OutdoorAir object to fully control the ventilation flow rate.

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Thanks a lot Eric. The economizer of the Controller:OutdoorAir is deactivated. I will have a look on the Controller:MechanicalVentilation that you suggest.

I have set up the case in OpenStudio, is it possible that some features of EnergyPlus are not included in OpenStudio?

Jore's avatar Jore  ( 2019-04-23 02:39:49 -0500 )edit

Yes, not all EnergyPlus objects and fields are available through Openstudio. I don't use Openstudio very much so I'm not sure if these fields/objects are available in Openstudio.

ericmartinpe's avatar ericmartinpe  ( 2019-04-23 09:10:52 -0500 )edit

Thanks Eric! I have changed to EP, but Controller:MechanicalVentilation does not seem to have any option that will serve me. The inclusion of thermostat makes my air flow rate to change and thus not respect the ventilation rate I impose

Jore's avatar Jore  ( 2019-04-24 08:04:19 -0500 )edit

Do you have DesignSpecification:OutdoorAir objects? The Controller: MechanicalVentilation should maintain the sum of the design ventilation flows specified in the DesignSpecification:OutdoorAir objects.

ericmartinpe's avatar ericmartinpe  ( 2019-04-24 10:13:03 -0500 )edit

answered 2019-08-28 06:47:48 -0500

Have you try hard sizing your AirTerminal:SingleDuct:VAV:Reheat? You can set a maximum airflow and set the minimum air flow method to "scheduled" and input your night ventilation schedule. This should limit how much airflow your fan will send to your zone(s).

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Asked: 2019-04-16 15:32:50 -0500

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Last updated: Aug 28 '19