Hi Dr. Li
in 1zone evapcooler example OutdoorAir:mixer is set up (via controller:Outdoor Air) in such a way that minimum and maximum outdoor air flows are equal (2 m3/s), economiser cycle is off and Fan which is connected to a mixed air node (OutdoorAir:Mixer, mixed air node) is constant flow (2 m3/s).
I assumed that return air from the zone would be completely relieved and mixed air node conditions would be the same as outdoor air node conditions(because return air is relieved before it mixes with outdoor air).
In other words all the air that comes to the Fan inlet would be outdoor air.
But when I look at the results of the simulation that is not the case. Air temperature at fan inlet (which is same node as outdoorair:mixer mixed air node) Differs from the outdoor air temperature.
I have attached 1zoneevapcooler example and results file. The only thing I changed in the original file is to include few other nodes in the result and I ran it with Auckland, New Zealand weather file
Thanks
On 23.09.2011 05:32, YuanLu Li wrote:
Hi, you are not only one confused with the OA mixer operation. There have been many discussion on this topic.
The OA mixer box actually has a splitter in it as well. The environment is a large air tank. The OA box relief air node air is feb back to the OA inlet node via the air tank. The used air becomes the fresh air. The OA inlet node has a damper.
Because of this air tank connection, the OA relief air amount is always equal to the OA intake amount, ignoring the temperature, pressure variation.
The mixer is described in the document. The OA inlet air is mixed with part of the return air. The output of the mixer is the mixed air dode.
The splitter is not documented. The input node is the return air node. The two split paths are the relief air node and the bypass path to the mixer. I use the name bypass path, because you all know about the bypass path in the plant loops. This bypass path bypasses the environment air tank.
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1. Are relief air conditions always equal to the zone return air conditions for OutdoorAir:Mixer?A splitter does not change the temperature, therefore the relief air node temperature should be the same as the input return air temperature.2. How do you achieve 100% fresh air? If the air flows are balanced like in the above example does that mean that flow is 100% outdoor air?If you want 100% fresh air, the amount of return air through the bypass path is zero."Note that in the above example flows are balanced. Relief air kg/s equals outdoor air (.) , mixed air, and return air kg/s and economiser cycle is off."This statement is not very clear. The first condition is corrent. The rest do not make sense.3. If the No. 2 is true why are then mixed air conditions in 1zone evaporative cooler example different from Outdoor air conditions?I do not know your exact air balance conditions. If the (minimum ) air flow of the OA inlet is the same as the return air flow, all the air will leave through the relief air node.======================================If the OA inlet flow is zero, all the return air is channel back to the air loop via the mixed air node. Mixed air node temperature is the same as the return air temperature.If the OA inlet flow rate is the same as the fan flow rate, 100% fresh air enters through the mixed air node. However, how do you make the OA inlet flow rate the same as the fan rate? There is no control to cut off the bypass path. This is another question. On paper, it is quite simple. Make the two flow rates equal. In practice, it is not so simple without defining what you really meant by 100% fresh air to the zones..
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The four nodes of a zone have the similar configuration.
Two input paths to a mixer into the zone node. The zone air is split into two outlet paths, exhaust and return air nodes. The thermostat is connected to the zone, and the internal load is added to the return node. This is the way I think of it, and is not the official version.
Dr. Li
To: EnergyPlus_Support@xxxxxxxxxxxxxxx
From: dulen@xxxxxxxxxx
Date: Thu, 22 Sep 2011 14:51:09 +1200
Subject: [EnergyPlus_Support] OutdoorAir:Mixer questions
Hi There,I am struggling a bit to understand how OutdoorAir:Mixer works.From reading the i/o manual it appears that outdoorair:mixer will calculate relief and mixed air conditions based on mixing of outdoor air inlet and return from the zone. If that is the case than the relief air and mixed air should have the same temperature.If you run the 1zone evaporative cooler example you will notice that that is not the case. Relief air has the same temperature as return air from the zone (which makes sense). That would imply that mixed air should have same properties as outdoor air (because air flows are balanced) but again that is not the case. Mixed air temperature is higher than outdoor temperature. I would assume that, because all of the return air is relieved, there is nothing to mix with outdoor air?Note that in the above example flows are balanced. Relief air kg/s equals outdoor air, mixed air, and return air kg/s and economiser cycle is off.So the questions are?1. Are relief air conditions always equal to the zone return air conditions for OutdoorAir:Mixer?2. How do you achieve 100% fresh air? If the air flows are balanced like in the above example does that mean that flow is 100% outdoor air?3. If the No. 2 is true why are then mixed air conditions in 1zone evaporative cooler example different from Outdoor air conditions?Thanks
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