[bldg-sim] Package VAV w/reheat Versus CHW VAV w/reheat

[Glenn Haynes]

John,

I am assuming your packaged system is DX.  If not, the next few comments
might be a little presumptuous, especially if you are already doing
them.

Is it possible you are letting DOE2 over-specify the chilled water and
condenser water pumps?  I found that it was necessary to go through some
careful design steps to accurately specify pumps (flows, heads,
efficiencies) in my water-cooled chiller models to obtain valid results.
Also, you need to be careful in specifying the right two-way chilled
water coil valve(s) in the secondary chilled water loop if you are using
variable capacity pumping.  The same design rigor may be needed for the
cooling tower specification.

If the CHW system is running at 2/3 of summer kW usage during the time
the package system is practically nil, then it must be due to auxiliary
loads, and not the chiller, assuming the system schedules and control
strategies for both systems are achieving the same effect within the
cooled spaces.

If DOE2 is allowed to default on most specifications, it can sometimes
be difficult to trace the real cause of an obvious flaw in the results
back to what you actually specified within the model.  It's probably
best to nail down everything if you need to accurately compare such a
simple system to one so complex.

Good luck with this issue.  Please let us know if and when you identify
the real problem.  Could be educational for some of us.

Glenn Haynes, PE
Senior Consulting Engineer
RLW Analytics, Inc.
860-346-5001x202

-----Original Message-----
From: bldg-sim at gard.com [mailto:bldg-sim at gard.com] On Behalf Of Aulbach,
John
Sent: Monday, January 29, 2007 4:37 PM
To: bldg-sim at gard.com
Subject: [bldg-sim] Package VAV w/reheat Versus CHW VAV w/reheat

 
Hi all:

I swear this continued comparison will send me to an early grave.

ASHRAE 90.1 requires a building less than 5 stories and between 75K -
150K floor area to have a package VAV/reheat as its system. There are
those designers that like the CHW VAV system better. There is a penalty
of having a running CHW pump in that case.

However, I just did an eQuest comparison between the two systems, all
things equal (I hope). The CHW system space cooling used 60% MORE energy
than the package system. In the dead of summer, the CHW beats the
package. But in the winter, there seems to be a false load the chiller
is seeing, as the package cooling is almost nil, but the CHW system is
still has 2/3 of the summer usage.

I THINK I have CHW reset correctly programmed for the CHW system.

ANY IDEAS ???

John R. Aulbach, PE, CEM
Project Manager
Nexant, Inc.
100 North Barranca, Suite 820
West Covina, CA 91791 USA
Phone: 626-430-9054
Fax: 626-430-9060
email: jaulbach at nexant.com
 

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