[Equest-users] sizing return air flow for an autosized supply air flow - follow up for return air flow greater then 90%

[Kevin Kyte]

Hello,

I have a few zones where the FX must be greater than the outside air venting.  OA is between 6-8% supply air flow.
I am trying to automate the return air flow to equal that of supply flow less outside air.  Is there anyway to set FX to equal outside air flow?

What I have tried, without success, is to set Return-cap-ratio equal to (supply-OA)/supply.
Such as, (Local("SUPPLY-FLOW")-ChildRef("OUTSIDE-AIR-FLOW"))/ Local("SUPPLY-FLOW").


Do supply-flow and outside-air-flow need to be entered numbers to achieve?
I am trying to autosize supply flow based on 20F coil-to-space temp so I do not want to enter a number there.

Thanks for the insight.

Kevin



------------------------------

Message: 3
Date: Mon, 11 May 2009 12:47:43 -0700 (PDT)
From: eric wang <shaojieworld2006 at yahoo.com>
Subject: Re: [Equest-users] sizing return air flow for an autosized
        supply  air flow
To: "equest-users at lists.onebuilding.org"
        <equest-users at lists.onebuilding.org>,   Kevin Kyte
        <kkyte at robsonwoese.com>
Message-ID: <775227.71864.qm at web58101.mail.re3.yahoo.com>
Content-Type: text/plain; charset="utf-8"

Kevin.
?
Under the tab of Flow Parameters, you?may use Return-Cap-Ratio to specify the return air flowrate. Also,?you can check the definition of this parameter to make sure that's what you want.
?
Hope this helps.

?
Shaojie
RETURN-CAP-RATIO
For systems that can have a return fan, specifies the ratio of the return/relief fan capacity to the supply capacity. Applies only when a return fan exists. In many buildings, the return fan is deliberately sized 10% to 20% smaller than the supply fan (RETURN-CAP-RATIO = 0.8 to 0.9) so that the building is pressurized whenever the fans run. The difference between the supply and return air flow exfiltrates from the building (or is exhausted through zone exhaust fans). The default value of 1.0 implies that no building pressurization occurs. The amount assumed to exfiltrate is
FX = [SUPPLY-FLOW] * (1.0 - [RETURN-CAP-RATIO])
In other words, the amount exfiltrated due to building pressurization (or zone exhaust) is assumed to be constant all hours of operation. In the hourly simulation, the return/relief fan operates only when the actual return air flow is greater than (FX - zonal exhaust). The return/relief fan moves only the amount of return air greater than FX. If FX is greater than the outside air ventilation requirements, as defined through other keywords, then FX will override any other outside air requirements including those specified through scheduled values. If you have specified the return fan size, a check will be made to see if any building pressurization is occurring. The difference between the supply flow and return flow is compared to the zonal exhaust quantity. If the zonal exhaust quantity is less than this difference, then the balance is assumed to exfiltrate. Note that infiltration is still assumed to occur, as infiltration is calculated in the LOADS
 module. For this reason, the infiltration schedules in LOADS should be coordinated with the fan schedules in SYSTEMS.
?


--- On Mon, 5/11/09, Kevin Kyte <kkyte at robsonwoese.com> wrote:

From: Kevin Kyte <kkyte at robsonwoese.com>
Subject: [Equest-users] sizing return air flow for an autosized supply air flow
To: "equest-users at lists.onebuilding.org" <equest-users at lists.onebuilding.org>
Date: Monday, May 11, 2009, 2:39 PM








I have seen that something like local(?Supply-Flow?)* .9 will work if the supply air flow is already entered as an input.?
If supply air is autosized, I want return air to be supply air minus outdoor air or 90% of supply air whichever is larger.
?
Is eQuest capable of generating a return air flow number that is a percentage based on an autosized supply air flow?
?
Thank You.
?
?
?
Kevin Kyte, EIT, LEED?
Junior Engineer
RobsonWoese, Inc.
D:? 716.636.1800 x211
?


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