Chemistry Laboratory Model

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I am about to begin an energy model for a large university lab project that includes about 150 VAV chemical fume hoods. The building systems will include a true VAV exhaust system, not a bypass system. Please advise how this can be modeled using eQuest. Thank you in advance.

Nick Kugler, LEED AP

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Nick,

The way I would approach this:

1. Establish what the minimum flow rate for each hood is (as a
percentage of maximum). Most chemical hoods have a minimum required cfm
rate even when closed that flows through the reagent shelf below to keep
it ventilated.

2. Establish a diversity schedule for the hoods

3. In eQUEST v3.63 and above you can specify airflow tracking for
exhaust air streams on a thermal zone basis - there's a good help
section on what the different tracking options do. It's my understanding
that this was put in specifically for fumehoods. Set your max flow rate
here and set up a fraction schedule to determine flow rate.

One thing you might want to check up on is the fumehood density. If the
lab is very fumehood intensive, then it might make sense to go with the
VAV hoods. However, if it's a ventilation driven lab (as opposed to an
exhaust driven lab), you might as well have constant volume hoods.

For instance, if your EHS requires 6 air changes per hour in a 10 foot
high 10,000ft2 lab, your supply cfm = 10,000 cfm. Lets say you have 10 x
6' wide fumehoods with 800cfm at max sash height (18"). That would give
you 8,000 cfm. So having VAV controls on the hoods is not going to
reduce your airflow unless you ramp down to below 5ACH during unoccupied
hours.

Hope this helps

Vikram Sami, LEED AP

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I'd like to add to Vikram's comment about CAV hoods. I'd encourage you to advocate for VAV hoods if the zone's hood CFM is 2ACH or greater. There has been a lot of progress in recent years in working towards lower occupied and unoccupied ACH minimums, and 2 ACH may be within reason for many clients in the coming years. If VAV hoods are in place, and the capabilities of the air valves allow, the system would be able to turn down to a lower ACH minimum and garner your client savings.

Paul Erickson LEED(r) AP

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That's a good point Paul

Vikram Sami, LEED AP

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Oops - hit send before I finished typing.

What I meant to say was - that's a really good point Paul. You can set
up your CAV hoods to be 2 steps - for unoccupied and occupied hours too
if that is the situation. We end up doing that on a lot of projects.

Vikram Sami, LEED AP

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Nick,
Are you considering/planning to use heat recovery through system-level
variable flow exhaust system(s)? If so, I don't think it will work if
you model zonal exhaust air flow tracking. (Someone please correct me if
you have determined otherwise.) One way you can model variable hood
exhaust and allow for energy recovery is to use a MIN-AIR-SCH at the
system level to vary the percentage of outside air, in combination with
MIN-FLOW-SCH at the zone level, provided your system is VAV. I would add
total system supply flow and O/A percentage to your hourly reports, so
you can see what the system is doing throughout the year.
Regards,

William Bishop, EIT, LEED(r) AP

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Thank you all for your input. The building exhaust systems will be separated into 4 components: general exhaust, fume hood exhaust, snorkel exhaust and biological safety exhaust. The reason for this is for cross contamination considerations in the non-general exhaust systems and the goal to achieve total energy recovery with a energy recovery wheel. In addition, the systems will have different static pressure requirements. Yes, I am implementing airside heat recovery for the general exhaust system, and considering application for the fume hood system depending on feasibility of a system with no cross contamination as required by the owner.

Bill - I am planning on having variable flow at the system level for both the general exhaust and fume hood exhaust systems. This is precisely why I was concerned with the software's ability to model this situation. Thank you for your advice.

Nick Kugler, LEED AP

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