[Equest-users] Multiple DOAS in Series

Tue Oct 19 09:49:08 PDT 2010

Kathryn,

I may be mistaken, but I think you're causing yourself to run in circles by trying to think of DOAS systems as they exist in the real world instead of how we can account for in eQuest.  I did this for a long time before "getting it..."

To answer one of your first questions, take another look at the second screen where you define ERU components under "cond/frost control."  You can explicitly model pre-heating the OA before the ERU wheel/HX with either electricity or HW to either a floating or fixed setpoint.

Carol is cautioning you to avoid specifying too many systems - and you may have inadvertently hit the nail on the head with your last words "...come up with an equivalent."  

While your real world system looks something like:

DOAS preconditioning box à ERU box with a wheel à big AHU box ààà multiple Fancoil boxes

Your eQuest equivalent to the above is ideally:

One system, with most components/airflows defined at the system level and a some at the zonal level.

Trying to make the "real world" distinction between the various parts of your system will require you to jump through hoops to ensure the correct quantities of OA and EA are going through the various components.  It's conceptually possible with a dummy zone/loads/scheduling and a lot of coffee, but if you really try to model that may distinct systems and make them all dance in a line for a single group of zones, you're bound to lose sleep and grow some gray hairs in the process.  This has been my experience.

I think you'll also find eQuest can indeed model desiccant energy recovery very well, with a lot of flexibility, so take heart!

~Nick

NICK CATON, E.I.T.

PROJECT ENGINEER

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From: equest-users-bounces at lists.onebuilding.org [mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Kathryn Kerns
Sent: Tuesday, October 19, 2010 11:08 AM
To: Carol Gardner
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Multiple DOAS in Series

Carol, I am not sure I understand what you mean. Can you draw a picture or explain further? If I have one DOAS, how do I go about specifying entering conditions for its ERV other than outside air and exhaust?

I am also wondering just exactly what happens to DOAS supply air. According to the literature it appears that the outside air is fed through the DOAS, which then heats it and cools it via the DOAS entered heating and cooling supply temperatures, and then the supply air is passed to the associated space air (which I assume has the normal space temperature range of 70 to 78) which it then transmitted to the downstream terminal units as their primary air. At least I think this is what I think I read.

Does anyone else have a different interpretation? Maybe we are passing the DOAS supply air to the terminal units at the supply heating and cooling temperatures the same way the DOAS supply air gets passed to its associated space?

If I figure this out, I will share the results. Desiccant systems like the one I am modeling are being used more and more often as energy savers in humid climates. Somehow we need to figure this out or come up with an equivalent.

________________________________

From: Carol Gardner [mailto:cmg750 at gmail.com]
Sent: Mon 10/18/2010 7:45 PM
To: Kathryn Kerns
Cc: equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Multiple DOAS in Series

Hi Kathryn,

Just a couple of quick thoughts. Do you need to physically specify 2 DOAS units? Could you just specify one with an ERV and with the entering conditions of the first unit and the leaving conditions of the second unit? I'm not thinking of any downside to doing it that way but I'm not deep in it like you are. Let me know what you think.

Cheers,

Carol

On Mon, Oct 18, 2010 at 2:06 PM, Kathryn Kerns <kathryn.kerns at bceengineers.com> wrote:

Has anyone ever tried to model 2 DOAS units in series? I have successfully modeled a single DOAS/multiple fan coil system, but I have never tried to string two DOAS units together by feeding the supply air output of the first DOAS into the outside air input of the second DOAS. The reason I ask is that I am looking for a way to model a DOAS that has an ERV device located downstream after a heating and a cooling coil.  Does anyone know whether DOE-2 would treat the system exhaust air coming back through the second DOAS ERV any differently than the exhaust air going back through a single DOAS with an ERV unit? 

Assuming that this works, does anybody know how to add heat to, or manually raise the temperature of, the ERV exhaust air? 

I am thinking if I can model a multiple DOAS with an elevated exhaust temperature, I might be able to mimic a desiccant system. According to James Hirsch in California, the energy balance used in the desiccant tabs is not correct and should not be used, so I am out of luck there.

If anyone has another method for modeling a desiccant system, I would love to hear from you.

Kathryn Kerns

Systems Specialist

BCE Engineers, Inc.

| Ph: 253.922.0446 | Fx: 253.922.0896 | 

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Carol Gardner PE

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