Modeling fume hood exhausts

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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?

(subsequent alternatives will have variable volume fume hoods).

Peter Simmonds,Ph.D.

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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.

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Other questions -

If you model it as constant volume, do you use full sash height or resticted sash height?

When you model a VAV system, what assumptions do you make about the user's behaviour in closing the hoods, or are you keeping them closed whenever no one is standing in front of them by employing an Automatic Sash Positioning System?

David Sweitzer

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?

(subsequent alternatives will have variable volume fume hoods).

Peter Simmonds,Ph.D.

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Joined: 2011-10-02
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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.
If you know some other way, please let me know also :)

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I just spent a while digging through all the latest docs on modeling
labs for a LEED analysis (90.1-2004, 90.1-2007, addenda, & Labs 21).
This is my interpretation:

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

Let me know if you have a different interpretation....I don't think all
these issues are laid out in detail for modelers yet.

*Aleka Pappas*

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Thanks for all the help. Here is another angle I am coming from.

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.

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?

Peter Simmonds,Ph.D.

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