How to add humidity control with direct evaporative cooling

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1) Does the Outside Air Damper open to 100% when the direct evaporative cooling is enabled? This is typically a real-world control strategy to control RH.

• No not automatically. If you want 100% OA then you need to schedule the min oa fraction with a value of one.

2) Open Studio has humidity set-points that the user can set at the zone level. This was the approach I first took, however the software to me appears to either have a bug or I am incorrectly using it. When I enter a set-point schedule into the fields and the tab refreshes, the schedule disappears. Any help?

• I don't know of a bug in the humidity set points, although we have had a few growing pains since we introduced the grid view so it is possible. In any case those humidity set points won't impact the operation of the direct evaporative cooler. The evap cooler model just isn't setup to control itself according to a humidity limit.

3) What are some output variables to report to better understand the direct evap operation?

• Without looking up the exact variable names, I think you want to look at the zone RH, as well as the temperature and humidity ratio of the nodes around the humidifier. Those node conditions will tell you exactly what the humidifier is doing to the air stream. @aparker has a nice Reporting Measure that will plot these node conditions on a psychometric chart, but I don't think he has published it to the BCL yet.

4) It would be great to hear any other suggestions you have when modeling direct evaporative cooling.

• I believe a common technique is to precede the direct evaporative cooling module with an indirect module thereby limiting the lift (and moisture) that the direct evap introduces.

Does your system have an air-side economizer? You may be able to find a work around by playing with with the high-limit shutoff value of the economizer and controlling the economizer based off of wetbulb instead of drybulb. Haven't tested this but it may be an option.