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Re: [EnergyPlus_Support] unit ventilator woes
I've uploaded the simulation file in case anyone wants to take a look at it.
Many thanks,
Josh
On Jan 18, 2008 4:47 PM, josh_miarch <jeemang@xxxxxxxxx> wrote:
> I've created a simple one-zone model, in which the single zone is
> conditioned in the following manner:
>
> -a constant volume of ventilation air is supplied during occupied
> hours from the central AHU (via a DIRECT AIR object)
> -there is a unit heater to heat the space during unoccupied hours, and
> make up any additional heating load not covered by the central AHU
> during occupied hours
> -there is an exhaust fan that runs in the middle of occupied days,
> removing a volume of air from the room in excess of the air supplied
> from the central AHU
> -there is a makeup air unit to supply a volume of air to the zone
> equal to that removed from the zone by the exhaust fan when the
> exhaust fan is running. I've attempted to simulate this with a UNIT
> VENTILATOR object, where I've set the maximum air flow rate, the
> maximum OA air flow rate, and the minimum OA air flow rate to a volume
> equal to that specified in the ZONE EXHAUST FAN, chosen the FIXED
> AMOUNT outside air control type, and assigned the object the same
> component availability schedule as the exhaust fan it is meant to be
> compensating for.
>
> I'm having a couple of problems with this system.
>
> 1. I'm getting an air imbalance when the exhaust fan/MUA combo is
> running. As I understand it, EnergyPlus calculates a zone air balance
> strictly by subtracting the sum of the flow through the zone exhaust
> nodes from the sum of the flow through the zone inlet nodes and
> assigning the difference, if any, to flow through the zone return
> node. However, I'm running into a problem where the sum of the flow
> through the exhaust nodes exceeds the sum of the flow through the
> inlet nodes.
>
> Looking into this further, I saw that the problem seems to lie with
> the UNIT VENTILATOR. I've set the unit's fan volume flow rate to an
> amount equal to the minimum OA flowrate specified in the UNIT
> VENTILATOR description, so presumably when the unit runs, it should be
> drawing air entirely from its OA node. However, what actually happens
> is that it draws an amount equal to it's fan volume flow rate from
> both the OA node AND the zone exhaust node to which it is connected.
> As well, it outputs air into its zone inlet node at this same rate --
> meaning that a volume of air equal to the unit fan's volume flow rate
> is basically disappearing.
>
> I guess I'm wondering if I've specified a part of the file wrong, or
> if my understanding of how these components work together is somehow
> flawed.
>
> 2. The UNIT VENTILATOR I've specified has no cooling coil, but in
> situations where there is a cooling load, I'd like it add to the zone
> a volume of outside air at the outside ambient temperature to make up
> for the air leaving the zone via the zone's exhaust fan. According to
> the Input/Output manual, when a UNIT VENTILATOR is in a COOLING/NO
> COIL/FIXED AMOUNT scenario (which I've presumably described), "The
> unit ventilator fixes the outside air flow rate as defined by the user
> and sets the maximum and minimum outside air flow rate to be equal in
> order to avoid the variation of outside air flow rate between the
> maximum and minimum values. Since a coil is not present to further
> condition the supply air, the zone simply receives whatever
> temperature air results from the outside air controls."
>
> That is pretty much what I want to happen, assuming that the maximum
> and minimum outside air flowrates, when made equal, are set to the
> amounts that I've entered in the UNIT VENTILATOR description (which
> are equal). Unfortunately, this is not happening; when there isn't a
> heating load, the UNIT VENTILATOR doesn't run. Again, I'm wondering if
> anyone knows what I might be doing wrong.
>
> Sorry for the amazingly long post, and thanks for any help I may receive.
>
> Josh
>
>
>
--
Josh Kjenner, EIT, LEED AP
Manasc Isaac Architects Ltd.
10225 100 Avenue :: Edmonton, AB T5J 0A1
780.429.3977
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