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CBECC-Com HVAC: Modeling Multi-zone MUA systems and Controls #29

Open joesinger12 opened 7 years ago

joesinger12 commented 7 years ago

We were modeling a building with multiple commercial kitchens and found that there are numerous issues with how a baseline kitchen exhaust system is modeled in CBECC-com v3c. I understand that this is older version of the software but there has been nothing mentioned relating to kitchen exhaust in the “new to this version” in new software versions. The result of these issues caused a huge swing in overall compliance of the project, in the range of 12% overall TDV. I have attached the ap.xml & ab.xml for your reference. The following were issues were found:

  1. The baseline make-up air units are incorrectly being sized based on the cooling load and are sized significantly smaller than the required exhaust air flow. In the example baseline XML file, Thermal Zone: Z-L03KITC24 has a kitchen exhaust of 5390 CFM and a make-up air unit of only 767 CFM. This caused a huge penalty for the proposed design because the baseline is not properly conditioning enough make-up air. Please refer to the following section in the Non-Residential ACM:

5.6.5.1 VAV Air Flow

“For kitchen MAU systems, the design airflow is the greater of the airflow required to meet the design cooling loads and the total exhaust CFM.”

  1. The baseline system is by default sized under the assumptions that the proposed exhaust air is over 5,000, even if that is not the case. Please refer to Thermal Zone: Z-L03KITC25 which used the exhaust flow schedule: “RestaurantExhaustHoodGreaterThan5000cfm” even though the exhaust air flow is only 3750 CFM.

5.6.5.3 Zone Exhaust

“Same as the proposed design but not above the maximum standard design flow rates listed in Appendix 5.4A. Exception for kitchen space types with over 5,000 cfm of exhaust: the baseline is a function of the Kitchen Exhaust Hood Length, Kitchen Exhaust Hood Style and Kitchen Exhaust Hood Cooking Duty, and is determined by Table 140.9-A.”

  1. Minimum hood CFM is not properly being maintained in the proposed model. The terminal units on the make-up air unit are modeled to have a turn down of 20%; however, they if you look to the OpenStudio model the turn down is actually only 30%.

Please advise us on these software issues and what modeling work-arounds we should be using in the interim.

Reported by: joesinger12

Original Ticket: cbecc-com/tickets/1892

joesinger12 commented 7 years ago

Original comment by: joesinger12

joesinger12 commented 7 years ago

9/21 DR - responses below:

  1. Basically the baseline system design fan/OA flow will not track the design exhaust CFM, but rather the design outside air (ventilation) flow for the zone(s). In your model, the ventilation air specifications for the kitchen zone has not been elevated to reflect what is the (likely) proposed OA flow to the zone from the MUA unit. If you review the AirSystem screen for MAU-1 (below), you can see that the sum of zone OA is low. For MUA units, the OA typically is equal to the design fan flow (i.e. 100% OA). Basically, if you want the system to provide OA to a zone, you have to increase the ventilation flow to reflect this. For units w/ MUA units, I generally recommend just specifying the design OA flow in the 'Per Space' definition, maintaining use of the 'Maximum' method. As mentioned, the baseline will track the proposed ventilation MUA ventilation flow, which depending on the design, may or may not match the design exhaust flow if you are relying on make-up air drawn from surrounding zones

  2. The name of the schedule is referring to the building total kitchen air exhaust flow, not the zone air flow. If the building total commercial kitchen exhaust air flow is > 5000cfm, this will be the exhaust schedule for all baseline kitchen exhaust systems.

  3. Can you provide an example snipit of the OS/IDF model input file that illustrates this issue? I generally recommend reviewing/verifying simulation inputs in the IDF, since this is what is ultimately simulated and avoids potential misdirection from what happens between OS and IDF translation.

When I ran the model provided in v3c, the building does comply by about 17%. Did you submit the correct model to us? However, there is an error in your model that was not identified in v3c, but is called out in v3d, and that is the lack of child fan objects defined for the ParallelFanBox terminals. For each terminal, a fan object should be defined, though the fan power, in W/cfm, is defined at the terminal level (fan BHP and other inputs are automatically populated to reflect the value defined at the terminal)

Original comment by: joesinger12

joesinger12 commented 7 years ago

9/28 User:

  1. The approached described above does not fix the issue that we are having with CBECC. The design MUA unit serves multiple variable speed exhaust fans. By overriding the minimum ventilation flow (per space cfm) to match the exhaust flow total, it prevents the ability of turn down in the terminal units of the MUA and makes them constant volume. This override value is the MINIMUM outside air that is required to the space. See images below for when we try to input the minimum OSA rates.

When these minimums (Min. Primary Flow) were hardcoded to the equivalent minimum zone ventilation flow, the effect in compliance is shown below.

When opening the baseline model in OpenStudio, the zone fan is fixing the minimum air flow to exactly half of the hardcoded minimum zone vent flow. For example, the zone L03KITC54 has a hardcoded exhaust of 20,790. Shown below are the 2 OpenStudio models: • the original CBECC created baseline (sent to you before) on the left, • the model with your suggested edits in the OA per Space and the required additional hardcode input (in terminal unit minimum flow) to prevent the errors shown above on the right The model in both cases is allowed to drop below the minimum (shown below). The proposed OSA has a minimum air flow rate in the terminal fan set at 20,790 constant volume. The baseline has the same OSA rate but allows the fans to turn down to 50%.

Overall the approach of controlling exhaust air with the minimum OSA does not work. Please advise on an additional work around and potential software update.

  1. It is unclear why there is a distinction of using building total kitchen exhaust over individual space kitchen exhaust. Based on our interpretation of the NACM there appears to be no mention of the total building exhaust flow rate is made.

Our interpretation is that this schedule should be based on the total exhaust per kitchen hood, not per total building kitchen exhaust as you have referenced.

  1. Below shows the correct fixed minimum air flow rate (20% of the 20,790) while the constant min fraction is .3, which overrides the hardcoded min flow rate.

Original comment by: joesinger12

joesinger12 commented 7 years ago

9/28 DR - Can you forward your revised model? In addition to making changes to your vent rates, did you also add child fan objects to your parallel boxes? We'll review and get back to you on this and your comments

9/28 User - Attached is the XML file for the revised model. We have not updated the parallel fan powered boxes as there here are the exhaust system used VAV boxes with no reheat for the terminal units. The fan powered boxes are only located under a different AHU.

What do you think about modeling the exhaust fans as the return fan on the MAU?

9/29 DR - Just to confirm, what your are trying to describe in your model is a multi-zone VAV MUA system, that is controlled to provide OA flow that balances the variable volume kitchen exhaust systems in each zone?

9/29 User - Yes that is correct

Original comment by: joesinger12

joesinger12 commented 7 years ago

9/30 DR - Unfortunately, CBECC can't directly support what you want to do here by describing the MUA system as a multi-zone VAV system that is controlled to track the zone exhaust air flow.

The closest workaround I can provide you is to break your one VAV MUA system into multiple SZVAVAC systems, with each system serving one kitchen zone via an 'Uncontrolled' terminal unit. Leave the ventilation rate as you have it now specified, and define the SZVAV fan to have capacity equal to the ventilation air flow so it is 100% OA, and specify the supply fan minimum flow to be equal to the minimum flor your system supports. Distribute heating and cooling capacities proportional to the flows you have. Attached is modification of your model illustrating the concept.

I also had to change the Design SAT values to reflect roughly the range that the system will supply air at. This may not match your design exactly, but having a cooling design temp that is greater than the heating design temperature results in some undesired control behavior.

This workaround is not perfect since even with these changes, the MUA unit air flow will not track the exhaust flow. However, it will allow you to reflect reducing your MUA down to what your design reflects.

As for the interpretation of the exhaust system for the standard design. Section 140.9 indicates special prescriptive requirements for kitchen exhaust systems, and the trigger for these requirements is "A kitchen/dining facility having a total Type I and Type II kitchen hood exhaust airflow rate greater than 5,000 cfm shall..." I realize the NACM Section 5.6.5.3 does not exactly reflect this, but hopefully this explains why it has been implemented this way in CBECC. We'll make note to clarify the language in the next release of the NACM.

Your model/design is a good test case for some new features I am going to recommend we add to the program to better support multi-zone MUA systems and their controls.

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