RE: [EnergyPlus_Support] Re: SetpointManager:MixedAir and Economizer question

The sizing to a larger value of course would help.

I got this problem in 2008 with an Australia real VRV - VAV installation, and your answer was the CBVAV at that time for simulation purposes.

I did not post this to the help desk, because I would never use this VAV system myself, because over cool and reheat combination is not really energy efficient.  It may have running cost and comfort advantage.
===========================
The maximum air volume is imposed by the sum of the active zone air volume, and not by the fan.  The zones have different demands on different days. When the over cool level is at 12.5°C, the problem is less obvious. When the air loop temperature is set to say, 18°C instead of 12.8°C setpoint, and without reheat, many warnings appeared as setpoint value not met.

I know that this is not the EPlus intented way the system should be designed, but the users are doing it.

I like to set the air loop temperature fixed in between the cooling and heating dual band temperature.   12.5°C is too low as the air loop duct temperature, but can be the coil temperature.

Dr. Li

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: RRaustad@xxxxxxxxxxxx
Date: Wed, 30 Nov 2011 13:58:43 -0500
Subject: Re: [EnergyPlus_Support] Re: SetpointManager:MixedAir and Economizer question

I have read a lot of your posts and you obviously know what you are talking about. So I am a little confused.

What I do not understand is why we would want to try to get better performance out of a system that is obviously undersized? Of course if you increase the load to be greater than the system can handle, the zone temp will rise. Instead of using a bypass, how about sizing the system correctly in the first place.

I must not fully understand what you are trying to say.

Let me also add that I usually respond only to questions on the help desk. I do not typically answer questions on this forum but sometimes I can provide an answer very quickly and it doesn't take much of my time.

On 11/30/2011 1:24 PM, YuanLu Li wrote:

Hi, Richard

We are back on the VAV problem again.

I test the system by increasing the internal load in one zone, to a value higher than that for sizing. This would make the damper to be fulling open and the fan at maximum volume.

From this point on, increasing the internal load would cause the set point ref. temperature to rise. This is when I suggested that a by-pass path from the ref. node to the return air node of the OA mixer would solve the supply capacity bottle neck problem. Fan volume would increase, and the coil would absorbe the extra load in the return air from the zone.

Fan position does not matter.

Dr. Li

To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: jeannieboef@xxxxxxxxx
Date: Wed, 30 Nov 2011 18:09:05 +0000
Subject: [EnergyPlus_Support] Re: SetpointManager:MixedAir and Economizer question

Dear Richard,
I have a blow through unitary cooling system (dx) followed by an extra hot water coil as the last componant on the supply side of my Airloop (meaning the last node has a setpoint applied as required by the hot water coil controller). So earlier, you suggested using an uncontrolled node for the reference node, and suggested the fan outlet or exhaust node.
1) Would it matter if the fan outlet node is contained in the unitary system?
2) Should I insert a Duct object as the last componant on the supply side, before opting for the exhaust node (this may be important for DOAS).

--- In EnergyPlus_Support@xxxxxxxxxxxxxxx, Richard Raustad <RRaustad@...> wrote:
>
> 1) put the reference node on any node that will not be controlled, use
> the return node if you have to, or the fan outlet would also work
> 2) don't worry about upstream and downstream, just enter the fan
> inlet/outlet node names and the manager will do the rest
>
> What you are basically doing is calculating a set point at the reference
> node, and then using the mixed air manager to adjust that calculated set
> point by the fan heat. So when all is said and done, the outlet of the
> air loop is at the same temperature as the reference node set point
> temperature
>
> On 11/29/2011 11:50 AM, Jean marais wrote:
> >
> > Thanks James. A few more questions, if you'll indulge me...
> >
> > The referance node:
> > I assume one must be quite careful as to what setpoint is used on the
> > reference node (usually the last node on the supply side). For
> > example, if my last componant is a heating coil, I can't assign a
> > fixed schedule 32degC with a setpoint manager on the htg coil exit
> > node, if I plan heating and cooling all year round. I need to use, in
> > this case a setpoint manager that can decrease the setpoint on this
> > node during times of cooling. Right?
> >
> > The other setpoint nodes in the list:
> > How does the MixedAirSPmanager "know" on which node the fan sits, to
> > deturmine upstream and down stream from it? I don't have it here. Is
> > it an input field? I ask, because not just the fan takes or adds to
> > the air stream. Maybe, it just looks at the setpoint on each node in
> > the list (not the airstream temp), checks if it must be applied (eg.
> > clg sp not applied if in heating mode), and estimates the energy put
> > in or out at these nodes. Using this information, it sets the setpoint
> > at the exit of the mixing box, then the controller tries to control
> > the damper to match. Right?
> >
> > Please explain "exception 1". My guess is that if the downstream htg
> > coil exit node was included, the manager would see it as a second fan
> > and (if outside had cold air) possibly introduce more than nessecary
> > ODA, which the poor htg coil would have to heat up. This all becomes
> > hellishly complicated considering econimizers and other factors.
> >
> >
>
> --
> Richard A. Raustad
> Senior Research Engineer
> Florida Solar Energy Center
> University of Central Florida
> 1679 Clearlake Road
> Cocoa, FL 32922-5703
> Phone: (321) 638-1454
> Fax: (321) 638-1439 or 1010
> Visit our web site at: http://www.fsec.ucf.edu
>
> UCF - From Promise to Prominence: Celebrating 40 Years
>

-- 
Richard A. Raustad
Senior Research Engineer
Florida Solar Energy Center
University of Central Florida
1679 Clearlake Road
Cocoa, FL  32922-5703
Phone:   (321) 638-1454
Fax:     (321) 638-1439 or 1010
Visit our web site at: http://www.fsec.ucf.edu

UCF - From Promise to Prominence: Celebrating 40 Years

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