[Bldg-sim] Accounting for plenums/technical ceilings

[Nick Caton]

1.       Came to learn about plenums.

2.       Learned the phrase "physical verisimilitude."

3.       Will be spending the rest of the day digging for an excuse to slip this phrase into everyday conversation....

Thanks, Joe!

The implications for rooftop radiant exchange are intriguing by the way and could immediately impact a few current projects of mine... Would be interested to learn the results of your investigation!

More to the original topic, I agree there's more than one right answer with respect to liberties of simplification a modeler can reasonably leverage.  So long as you do account for the corresponding perimeter envelope loads and are mindful of the conditioned volume, I think you're on solid ground in many cases to omit the plenums explicitly.  On the flip side (speaking specifically to the eQuest interface), especially if you are using floor multipliers, you'd likely spend much less time actually modeling the plenums.

For further reading, the [equest-users] archives have many threads discussing plenums, interstitial floors, and heat transfer implications.  You might pick up some new ideas and food for thought there ;).

Regards,

~Nick
[cid:489575314 at 22072009-0ABB]

NICK CATON, P.E.
SENIOR ENGINEER

Smith & Boucher Engineers
25501 west valley parkway, suite 200
olathe, ks 66061
direct 913.344.0036
fax 913.345.0617
www.smithboucher.com

From: bldg-sim-bounces at lists.onebuilding.org [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Joe Huang
Sent: Thursday, October 24, 2013 10:44 PM
To: bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Accounting for plenums/technical ceilings

I've been reading the postings on this thread, but have resisted chiming in until now :-)

Although modeling a plenum is appealing from the point of physical verisimilitude, there are actually some drawbacks depending on which simulation program is being used.  With DOE-2, adding a plenum on the top floor converts the ceiling to an internal surface, for which DOE-2 models only conductive heat flows, thus losing the direct radiant exchange between the roof bottom and ceiling top.  I'm still investigating the impact this has on the simulations, but the preliminary indications are that it underestimates the impact of measures affecting the roof reflectivity, such as a "Cool Roof".

The other thought I have is that while the COMNET description quoted below is physically correct, much of it (stratification, leakage through the ceiling, etc.) are ignored in the building simulations.  For all intensive purposes, a plenum in DOE-2 or EnergyPlus is simply an unconditioned or a partially conditioned zone if it contains ducts (and the ducts are modeled).

As far as modeling tricks, adding a plenum to DOE-2 or eQUEST does not add much of a modeling burden if a FLOOR-MULTIPLIER is used to describe a typical floor + plenum.  Similarly, with EnergyPlus, you can create a ZoneList containing all the spaces on one floor plus a plenum above, and then give the Zone List a multiplier.

If you want to omit the plenum, I would still make the exterior wall area include the plenum height, but maybe reduce the air volume of the space as you've described.  I know that's not a problem in DOE-2, but I don't know whether EnergyPlus would allow you to do so, i.e., the input zone volume is less than that enclosed by the surfaces.  I don't think you'll go terribly wrong, but I would still think that having a plenum would be preferable.

Joe

On 10/24/2013 7:39 PM, Patrick Bivona wrote:
Hi,

Thanks for your answers. I found a specific section, 2.2.9, in the COMNET guidelines Shanta directed me to:

(...) Because of the leakage through the ceiling (typically suspended), the temperature of the plenum tracks
the temperature of the space, except that it is generally warmer because of heat stratification and heat
produced by lighting fixtures located at the ceiling or in the plenum.
It is generally recommended that plenums be modeled as separate thermal blocks, but at the modeler's
discretion, they may be combined with conditioned space below for modeling simplicity.

I guess the part I'm interested in is acceptable simplifications. And perhaps modelling tricks, that might be software specific.

Now a bit of context. I'm working on a 40 storey high building. That's quite a few plenums to model, which add a lot of surfaces simply by being there and by breaking up spaces that could have been grouped vertically into continuous thermal zones. I was wondering if there was a better way of approaching this to keep the model simple and the surface count down. Or do I just have to suck it up?

An example of simplification I thought of, with EnergyPlus, is to model space and plenum as a single space but setting the Ceiling Height field of the Zone object to the height below plenum. It's acceptable in terms of air changes in the space according to the documentation. It removes a lot of surfaces and allows to group spaces into continuous vertical thermal zones. But a part of the heat transfers through external walls that would normally affect the plenum will be accounted for in the space itself. Acceptable simplification? Unnecessary optimisation?

Thanks,
Patrick






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