AirflowNetwork - set up a limit for ventilation

asked 2019-10-04 06:24:18 -0500

Roccan
63 ●1 ●1 ●4

updated 2019-10-08 12:05:46 -0500

Jason DeGraw

2208 ●6 ●10 https://github.com/jasondegraw

Hello!

I used the AirflowNetwork objects to simulate a naturally ventilated Greenhouse. First results are in and everything works fine appart from the fact that I cannot seem to limit the maximum ventilation rate. At some points I get infiltration air change rates of 136 1/h. I would like to limit the air change rates to a maximum of 30. Is that possible? Should I do it via schedule or another object? In the objects that I've used the only defineable upper limit that I could find is the Enthalpy difference. I don't really know if control through enthalpy is the right way?

Thanks in advance!

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answered 2019-10-07 09:19:54 -0500

Jason DeGraw

2208 ●6 ●10 https://github.com/jasondegraw

Most of the controls that are there are there to control whether a window is opened or closed, and not about how much flow there is. There may be a way with EMS to provide the exact control you're asking for, there are other questions/answers about that.

When flows are too large then it is likely that either

there isn't enough flow resistance built into your network or
the wind pressure forcing is too high

Or it could be both. The first problem can be addressed by adjusting flow coefficients (it depends on which object you're using) or using the "Window/Door Opening Factor, or Crack Factor" field of the "AirflowNetwork:MultiZone:Surface" object. Assuming you're using one of the window objects, using an opening factor of 30/136 limits the window opening to a quarter of what it could be, so you should (hopefully) get about a quarter of the flow. If there aren't any wind issues and that reduces the flows, you'll still probably have to play with that some to get what you want. To check out potential wind problems, check your weather file to make sure the wind speeds aren't crazy and check that there isn't a wind pressure coefficient problem (if your Wind Pressure Coefficient Type is "Input").

It'd help to know which objects you're using, but generally those are the things to look at.

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Thank you for the comprenhensive answer! I will take a look in the weather file, but I assume that it's fine as it is averaged. High air change rates are understandable, as the greenhouse has ~50% openable surface. So wind is basically blowing through. The peak air change rates would not happen when the temperature is at it's highest, so it is not temperature driven. Anyways, I am using a detailed opening object and have tried immediately to limit the window opening. It needs some play, as you said, but it does lower the peak. I was looking to avoid EMS, but I guess that is the only way to go.