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Questions about zone mixing and other airflow objects

asked 2015-06-29 13:51:04 -0600

updated 2017-05-03 19:13:04 -0600

OpenStudio is planning on adding ZoneMixing and ZoneCrossMixing objects in the 1.9.0 (September) release. What use cases are these objects most important for? Does anyone have any modeling insights to share regarding these objects? If so, please comment in this thread or on this design document:

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Do people use the ZoneAirMassFlowConservation object? Is that object helpful to preserve conservation of mass when you start doing a lot of exhaust and mixing? Does it work well with air systems?

macumber gravatar image macumber  ( 2015-06-30 10:34:41 -0600 )edit

Just adding link to another question that may be relevant. It involves a building where one zone is like a partially enclose unconditioned corridor or patio. I gave some advise but not sure if it was any good. This doesn't seem like that uncommon of a use-case.

David Goldwasser gravatar image David Goldwasser  ( 2015-07-03 01:22:07 -0600 )edit

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answered 2015-07-10 12:08:02 -0600

Thanks for all the feedback everyone. Based on the conversation so far I am planning on wrapping the ZoneMixing and ZoneAirMassFlowConservation objects but not wrapping the ZoneCrossMixing object. For 1.9.0 these objects will be available in the API only. However, there is a mockup for the design of a future GUI in the linked google doc.

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answered 2015-06-29 18:16:44 -0600

updated 2015-06-30 09:36:19 -0600

That is great to hear as this is a needed feature for labs where zones are negatively pressurized and receive make-up air from other zones. I could see this living at the zone level in the GUI as I most often use this in conjunction with the Zone Exhaust Fan object.

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Thanks @Lincoln! Please feel free to comment on the design doc.

macumber gravatar image macumber  ( 2015-06-30 10:35:22 -0600 )edit

I second the need for this object in labs. There are rather complicated air flow strategies that require transferring air between spaces. The various strategies can have significant energy implications.

jmcneill gravatar image jmcneill  ( 2015-07-01 14:46:08 -0600 )edit

answered 2015-06-30 15:40:18 -0600

Agreed, this will be great to have fully functional in OpenStudio. Regarding use cases:

ZoneMixing: I've used this for quick service restaurants where you have a dedicated outside air supply to various zones, which then flows to a zone where it is exhausted via exhaust fans. I manually ensure that the exhaust flows balance the ZoneMixing flows (otherwise the 1st law of thermodynamics is violated!)

ZoneCrossMixing: I've mainly used in this in E+ for residential modeling for the following scenarios: 1. forced air HVAC system supplies air to one zone, but this zone has no return duct, instead air flows to a different zone then is returned. This is a not an accurate approach because E+ requires return air flow for each zone that has a supply, but at least there is some accounting for the reality that air flows from one zone to the other. 2. Sometimes I have to divide a zone into two zones to satisfy E+'s non-convex zone requirement for the FullInterior solar distribution option, if these really are the same space type I use ZoneCrossMixing to effectively combine these zones into one by fully mixing the air between them. 3. Natural airflow between zones, for example in a passive solar house south facing zones heat up and transfer air with core or north zones.

It would be great to have flexibility with these objects, but also a mechanism to help prevent users from using these objects incorrectly due to the issue with the energy balance being off (violating 1st law)

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answered 2015-07-02 15:50:41 -0600

nigusse gravatar image

The zonemixing object is used for one-directional air transport between two zones, the direction of airflow is defined in this object and the airflow rate is fixed throughout the simulation. Users are required to balance the zones air flow; otherwise, users have to take its impact in the analysis of their simulation results accordingly. Common use of this object that I have seen is in RestaurantFastfood and RestaurantSitdown buildings as described by others. In these two buildings the mixing object represents airflow from the dinning area to the kitchen. It implies that the airflow between these two adjacent zones is triggered by an exhaust fans installed in the kitchen. So, the EnergyPlus, allows users to specify a zonemixing object to represent air drawn from the dinning area to the kitchen and then eventually to the outside via the exhaust fan. If the dinning zone is served with a constant volume system, then one can create a balanced zone air flow using the zonemixing object and the kitchen exhaust fan by specifying a balanced flow rates. It is difficult to balance the zone air mass flow when you have a VAV system serving a building. It is partly for this reason than we added the object "ZoneAirMassFlowConservation" to ensure zone air mass flow balance.

when this object (ZoneAirMassFlowConservation) is specified, EnergyPlus triggers zone air flow balance calculation. In this situation, the zonemixing objects airflow rates are ADJUSTED in order to balance to zone air mass flow. The zone air mass flow balance calculation includes: supply air flow, return air flow, exhaust air flow, zone mixing and infiltration airflow for each time step. There are two methods allowed how the air infiltration flow is treated (adjusted or added) to enforce the zone air mass flow balance (see the IDD for further explanation). A thermal zone can be connected as many zones as (as many zonemixing objects as) the number of adjacent zones. Zone air mass flow conservation is enforced to all zones that used as either a source zone or a receiving zone in the mixing objects. And the air infiltration flow rate adjustment or addition is applied to those thermal zones defined as a source zones in the zonemixing object. The basis of this object need is allowing inter-zone air flow and ensure zone air mass flow balance where air-flow-network is not worth the effort.

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answered 2015-07-01 12:57:58 -0600

Archmage gravatar image

ZoneCrossMixing is not very useful compared to the much more flexible ZoneMixing. Open floor plans where there are not really any walls between zones is one common use case. The other is high exhaust ventilation in kitchens or lab spaces where air is drawn in from other zones.

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Thanks Brent! The one advantage I see in the cross mixing object is that it enforces balanced flow in a single input object level rather than having to keep two objects synchronized. When you are transferring air between two adjacent zones (e.g. perimeter and core) do you suggest using balanced flow across the boundary or not? Do you have any thoughts on using the ZoneAirMassFlowConservation object to correct any imbalances? Finally, do you have an references for good flow values (e.g. m^3/s per m^2 of opening between zones)?

macumber gravatar image macumber  ( 2015-07-01 13:41:40 -0600 )edit

That is the intended advantage of cross mixing, but for the typical 5 zone floor plan, you end up with more than two adjacent zones and so it doesn't really help much For OS, you should be able enforce balance using just ZoneMixing objects.

For the exhaust make up from adjacent zones like with a Kitchen, then there is supposed to be a imbalance in simple airflow and it would be coordinated with zone exhaust flow rate ( suggesting an input that is % of exhaust flow rate.)

I'd recommend an input like air change per hour for the smaller zone, e.g 0.5 ACH, but flow per area is good.

Archmage gravatar image Archmage  ( 2015-07-01 14:49:40 -0600 )edit

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Asked: 2015-06-29 13:51:04 -0600

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Last updated: Jul 10 '15