DDDF Systems - Humidification

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Hello everyone,

I've been digging into the humidification calculations for a DDDF (Dual Duct Dual Fan) system because I was getting what seemed like an enormous energy consumption for the humidification end-use for this system. I believe that I may have found what seems to be an error. So I wanted to report my findings and was wondering at the same time if anyone else was aware of the problem.

The problem seems to be in the mixed air humidity ratio calculation. See attached spreadsheet for calculation details.
eQuest seems to calculate it with the following formula:
Mixed air humidity ratio = (Total CFM * OAratio * OA Hum Ratio + Total CFM * (1 - OAratio) * Ret Hum Ratio) / Total CFM

In any single duct system, this formula would be totally fine. However, for a DDDF system, the OAratio parameter is defined relative to the cold duct air flow instead of the total air flow. The mixed air humidity ratio thus seems to be calculated with more outside air than it should be. To me, the formula below would make better sense:
Mixed air humidity ratio = (Cold duct CFM * OAratio * OA Hum Ratio + (Total CFM - Cold duct CFM * OA Hum Ratio) * Ret Hum Ratio) / Total CFM

Did anyone else ever see or delve into this?

Also, does anyone know if there's a good workaround to make the minimum humidity check to be before the return fan rather than after it? In the designs I encounter, the humidity sensor is always located before the return fan. Since eQUEST uses the temperature after the return fan to make the minimum humidity check, the temperature rise of the return fan increases the minimum humidity ratio calculated which results in an increase in humidification energy output. It would sure be nice if in later versions of eQUEST, we could choose where the sensor is located, but right now I can't seem to find a good workaround (outside of doing an external calculation). Anyone has any good idea?

[Bouthillette Parizeau]

Patrick Lapierre_ing. BEMP
plapierre at bpa.ca | www.bpa.ca | t: 5143833747x2807

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