[BLDG-SIM] Variable flow laboratory exhaust systems.

[John Weale]

I think the 'right' way to do this is to assign your exhaust fans to a
schedule.  You may even be able to assign the fan to an appropriate VFD
load curve.  Of course, you've probably already found that VDOE doesn't
have a nice wrapper to do this, so direct editing of the input file (.iXX
for VDOE I think) would be required.  Hopefully the gurus on the list come
up with an easy way to do this.

A 'close enough' solution could be to model the laboratory exhaust as a
return fan in the laboratory system.  Then manipulate the laboratory
system's OA schedule as appropriate to accurately model the exhaust flows.
This should also allow modeling the return/exhaust fan as a VFD fan,
dependent on the type of system you use.  In some situations (such as an
induction-diffuser or chilled beam cooled space), you will have to assess
the impact of modeling phantom recirculation airflow.  Note that in
VisualDOE the OA schedule is defined in the HVAC Systems Editor, not the
Room occupancy schedule.

The return fan approach is not as satisfying as just inputting exhaust
fans using the Zone-Exhaust fan option, but can be flexible enough to work
for many lab configurations.  It breaks down if your laboratory system
already has a return fan, for example a combined office/lab system (which
can sometimes offer some slick indirect cascade ventilation).  This
approach would probably require (at least) a bit of fidgeting and 'reality
checking' of a few hourly outputs, OA ratio and system flow in particular,
to reach a satisfactory model.

Hopefully I haven't missed the mark too much - its Friday for me too!

Good luck,

John Weale

On Fri, 8 Apr 2005, Peter
Simmonds wrote:

> OK , its Friday so I am allowed to ask for advice and guidance.
>
> Firstly I am using VisualDoe, I am modeling the energy performance of a
> 200,000 sf building which has 60,000 sf of laboratories. The labs all
> have fume hoods so the supply air is balanced to the exhaust air (100%
> OA).. Now I have the following problem: a fan system is either on or
> off, therefore the lab fans are on 24 hours a day. I would like to
> reduce the exhaust flow during unoccupied periods to exhaust air from
> the fume hoods at their minimum flow of 40%. Reducing the flow would
> also reduce the make up of outside air to balance the flow and obviously
> reduce energy consumption.
>
> Am I missing something here or does anyone have a magic trick that will
> allow me to accomplish my goal. Simply said HELP
>
>
>
> Peter Simmonds Ph.D.

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