[Bldg-sim] Chilled Beams

John Weale jweale at gmail.com
Wed Dec 12 12:17:45 PST 2007


The chilled beam is being modeled as a constant volume system and taking a
significant penalty because of this. The fan energy penalty may be real if
you have no mechanism to vary the volume (emphasizing the importance of
getting the system pressure drop down!) or a very annoying EQuest limitation
if you do intend a system with varying air flow rates. The main issue is
that the default induction diffuser system varies coil flow to control
capacity rather than airflow volume, so it does not have a calculation
methodology to reduce fan power during part load conditions.

The baseline VAV system is operating at a part load condition the majority
of the year. So, if it is operating at 60% design capacity, its fan is
turned down to about 35% design pressure drawing in the low 20% of design
power. Meanwhile, your chilled beam system is operating at 100% design power
all the time. If you design the chilled beam to low enough pressure drop and
take credit for the lower airflow volume, you can still show fan savings.
If you design it to 'standard' air handler and ducting pressure drops
without variable air flow volume, the model isn't lying - you've designed an
energy hog.

I currently work this problem with the standard 'Manually Calc Using Ugly
Hourly Outputs In Excel' approach. Someday I dream of pulling together a
User-Defined Default Expression for the fan pressure drop that would allow
me to adjust the fan pressure drop (and therefore the fan power) to a
fractional schedule directly in EQuest. Does anyone actually use those user
expressions, or are they there just to tempt with such 'if onlys' ?

John Weale

On Dec 12, 2007 2:46 PM, Brain, Joshua <JBrain at vanderweil.com> wrote:

> Has anyone compared a VAV system against a chilled beam (CB) system for
> a building located in the Northeastern US using eQuest?  I have modeled
> an office building having a VAV system and then having a CB system and
> have gotten some interesting results.  The CB is saving both cooling and
> heating energy annually, but is being penalized by electric fan energy.
> Might anyone have any input or experience that they could share with me?
>
>
>
>
> FYI:  to simulate the chilled beam I am using an induction unit (2.46:1
> induction ratio) with about 26% minimum outdoor air (there is return,
> this is not a 100% OA system), separate CHW and HW loops for the chilled
> beams.
>
>
>
> Thanks for your input.
>
>
>
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