RE: [EnergyPlus_Support] OutdoorAir:Mixer questions

[YuanLu Li <yli006@xxxxxxxxxxx>]

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.

================
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.
======================================

Hi There,
I am struggling a bit to understand how OutdoorAir:Mixer works.