Modeling fume hood exhausts

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Peter,
I think as Vikram mentioned, it depends on the total exhaust (cumulative exhaust
of all the fume hoods). If the total exhaust exceeds 15,000cfm, then the
baseline needs to have AT LEAST one of the following:

1. VAV hood exhaust till a 50% min turn down over the total CFM. Hence you could
only take advantage of a lower VAV set point on the actual equipment.
2. Make-up air. Separate make-up air with supply temperature very close to room
temperatures.
3. Heat recovery . If the building has both VAV and HR, then you should be able
to take full benefits of heat recovery or vise-versa.

Hence if your total fume hood exhaust is lower than 15,000 cfm, then you should
be able to take advantage of VAV fume hood exhaust over constant volume
exhaust.

Does this make sense?

Gerald

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Peter et.al,

Sorry for the delay in responding to the string. I've tried to summarize the questions/answers below and have combined responses and added my own content in places I think appropriate...

1) Can anyone tell me when modeling a lab system with fume hoods as to whether the baseline systems are to be modeled as constant volume?

VIKRAM: I think that depends on the total volume of exhaust. I don't have it in front of me, but I think if your exhaust is more than a certain amount you have to have either energy recovery to fifty percent or vav flow.

VARUN: I think it also depends on the software you are using to model a fume hood. Because if exhaust is specified more than the supply due to the hood, the software may disregard the lower supply and equate the higher value of exhaust to the supply. I am also trying to model a VAV exhaust fume hood and facing a lot of problems doing it. So you have to figure out a way to control you exhaust by altering your VAV supply.

ALEKA:

o If the building has fume hood exhaust over 15,000 cfm, the Baseline system is VAV, and exhaust is turned down to at least 50% for unoccupied hours. I am modeling the unoccupied turn down rate the same in the Baseline as in the Proposed models, because how low it can go at night depends on the type of lab (ie: the project doesn't get more credit for turning down to 20%). This is from Labs 21, and 90.1 (2004 or 2007) section 6.5.7.2.

o A project can claim energy savings for demand controlled fume hoods during occupied hours if the controls are automatic (Labs21 has some schedules for guidance in modeling this).

o Energy recovery is not modeled in the Baseline (see 90.1-2007 G3.1.2.10 Exception h)

GERALD: I think as Vikram mentioned, it depends on the total exhaust (cumulative exhaust of all the fume hoods). If the total exhaust exceeds 15,000cfm, then the baseline needs to have AT LEAST one of the following:

1. VAV hood exhaust till a 50% min turn down over the total CFM. Hence you could only take advantage of a lower VAV set point on the actual equipment.
2. Make-up air. Separate make-up air with supply temperature very close to room temperatures.
3. Heat recovery . If the building has both VAV and HR, then you should be able to take full benefits of heat recovery or vise-versa.

Hence if your total fume hood exhaust is lower than 15,000 cfm, then you should be able to take advantage of VAV fume hood exhaust over constant volume exhaust.
PAUL:
This is my response for 90.1-2007 (which incorporates addenda to 2004 and Labs21 guidance):

o If the building laboratory space [interpreted as system] (per G.3.1.1 exception d) has exhaust over 5,000 cfm, the Baseline system is VAV, and exhaust is turned down to at least 50% of design for unoccupied hours. I am modeling the unoccupied turn down rate the same in the Baseline as in the Proposed models, because how low it can go at night depends on the type of lab (ie: the project doesn't get more credit for turning down to 20%). This is from Labs 21, and 90.1 (2004 or 2007) section 6.5.7.2. Whether or not your hoods provide the 50% turndown or if a reduction in general exhaust is also required will be determined by the type of hood specified and the minimum air change rate in your lab zones. the exhaust is turned down. In either case, credit can be taken in the Proposed if it has a turndown lower than 50%. We have had this approved on projects. However, the LEED Reviewers have been particular about the minimum outdoor air ventilation requirement (G3.1.2.5) and insist that the difference between your Proposed and Baseline airflows (typically 100% OA) be treated as Return Air. This is not a typical practice in laboratory design for good reason, but there has been insistence from the Reviewers. This poses "issues" for strategies such as cooling/ventilation decoupling strategies, dynamic ventilation based on air quality sensing, and fume hood controls/ECMs (as well as the realities of actually modeling it accurately). Have others seen instances of this Reviewer feedback?

o A project can claim energy savings for demand controlled fume hoods during occupied hours if the controls are automatic (Labs21 has some schedules for guidance in modeling this). We agree with this. Labs21 developed a fume hood modeling schedule which is now incorporated into the 90.1 User's Manual. However, it assumes certain hood parameters that should be adjusted to those of the hoods on your project. It also assumes perfect sash management based on the Labs21 occupancy schedules. We take credit by modifying the fume hood schedule based on industry/client data on % of time people are actually in front of the hoods and the response time of the sensors/control strategies. Perfect sash management at night is still considered in our approach.

o Energy recovery is not modeled in the Baseline (see 90.1-2007 G3.1.2.10 Exception h). 90.1-2007 no longer offers an either or as was the case in 90.1-2004.

2) Assuming the base case were to be a traditional fume hood exhaust system which used constant volume fans in order to create the 3000 fpm discharge velocity. If the fume hoods have sash control the there is an outside air by-pass on the fan header, so down stream VAV and upstream constant volume exhaust fans. I know these constant volume fans can be staged to form a 'quasi' variable exhaust. So if this were the base case then one could apply the list of various innovations on how to apply variable exhaust flow to the proposed model. If lab system had <5000 cfm of exhaust then you can have constant volume hoods in the Baseline. If your spaces are not hood driven at any time, then modeling hoods as constant volume is pointless as you don't need to model the hoods. If your hoods do drive the space ventilation and the former two items aren't true then they will need to be VAV. Your question really gets to the matter of exhaust fan modeling if I read it correctly, and the ability to see savings there due to strategies for reducing zone/system exhaust. Currently there is no defined Baseline exhaust fan modeling specified in 90.1. We have submitted many projects with constant exhaust fan bhp in the Baseline case to simulate variable exhaust with constant velocity discharge using OA bypass. We have taken credit for fan staging and variable velocity discharge (based on wind tunnel guidance for the turndown). The ASHRAE Continuous Maintenance process considered adopting this as an addendum to 90.1-2007 but as I understand it the direction taken was to specify some level of fan staging as the Baseline. This is not yet published to my knowledge.

3) The ASHRAE 90.1 6.5.7.2 isnt really a solution, however Aleka you mention Lab 21, have you had any luck in getting this accepted by the LEED meanies? Hopefully my comments above clarify what we've attempted and what's been accepted.

Perhaps others see it differently and would like to comment. Good discussion.

Regards,

Paul Erickson LEED(r) AP

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