[BLDG-SIM] Displacement ventilation and airflow modeling in eQuest

Mike Tillou miket at etcgrp.com
Tue Feb 21 12:55:31 PST 2006


Renee,

If Peter's suggestion doesn't help the problem may be in the way eQuest handles user specified airflows.  eQuest assumes that the airflow you have entered is at sea level conditions and it automatically adjusts the airflow rate to account for the elevation of the project.  I know of two ways to deal with this, one ignore the fact the project is at elevation and enter a project elevation of zero or calculate the density correction factor and divide your design cfm by this amount before you enter it into equest. I suggest the later.

You can't really model displacement ventilation in equest because you cannot simulate the temperature stratification that occurs.  However with some effort you can create a model that gives reasonable results.  I suggest you read the paper written by Marlin Addison on the subject which I think is available on the www.DOE2.com website.

Mike 

   

Michael Tillou, PE
etc Group, Inc.
PO Box 7, Williamstown, MA 01267
ph. (413) 458-9870 fx. (413) 458-9875





-----Original Message-----
From: BLDG-SIM at gard.com [mailto:BLDG-SIM at gard.com] On Behalf Of Renee J. Azerbegi
Sent: Saturday, February 18, 2006 5:44 PM
To: BLDG-SIM at gard.com
Cc: 'Linda Morrison'
Subject: [BLDG-SIM] Displacement ventilation and airflow modeling in eQuest


Greetings,

For a middle school in Colorado, I'm modeling a constant volume displacement ventilation system and comparing this to a conventional variable air volume with reheat overhead system.

I cannot seem to set an exact total AHU airflow rate that has been calculated by the mechanical design engineer. When I do this, the eQuest program adds about 15% of airflow to the AHU airflow rate I specified. I specified at the zone and at the system level and each time, it increases the total airflow rate, even when I remove the exhaust airflow. Has anyone had this happen to them before? I did set the altitude to zero since that is what you are supposed to do if you want to put in exact airflow rates.

The number of hours outside the throttling range for the displacement system is very high since the temperature entering the space is a constant 68 F and the setpoint for the room is 70 F. The baseboard sizes have been entered and are maximized for the exterior wall. If I change the plenum height to anything above six feet being plenum, then part of my windows will be in the plenum which eQuest has a problem with. I have put in all the possible mechanical design parameters of the two types of systems into eQuest, except the fact that one is entering in overhead and one system is entering into the space at the floor level. I'm not sure if it is reasonable to assume that it is modeled correctly with many unmet hours of heating. But perhaps it is due to the limitations of modeling this type of system? Has anyone effectively estimated the performance of displacement ventilation in eQuest such that it is as designed without many unmet heating hours?

Also, just for some background, there is no cooling - cooling is provided only by an economizer cycle as they have summers off and the school is in the mountains.

Any suggestions to either of these issues would be greatly appreciated!

Thank you,

Renée




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