Estimating latent load in a grow room

[jmk0407]

I am currently engineering an HVAC system for a plant growing warehouse (marijuana, in a legal state). The client wants to maintain 72 deg F at 40% RH in the grow rooms. I am trying to estimate the latent loads in one of the grow rooms (around 400 square feet) so I can ensure the system is sized correctly and I have adequate leaving air temperatures. I'm looking at using DX packaged units with digital scroll compressors and hot gas reheat, if possible. What are the best ways to estimate latent loads in this space?

 

[jmk0407]

Also, it is a recirculating system. No outside air will be introduced into the space.

 

[BronYrAur]

Sounds like a basic load calculation but you need more info. Exposures, glass, etc. What kind of vapor barrier do you have? Door seals? 72 deg @ 40% could be difficult to maintain in the summer (depending on location) if surrounded by unconditioned spaces. You might want to consider positively pressurizing it.

 

[jmk0407]

I know about the load calcs, but my big question is wuantifying the moisture generation in the space due to the plants. Thanks for answering!

 

[chicopee]

I would suggest that you do some research on plant biology since plant transpiration will increase the moisture content within the warehouse. Per BronYrAur suggestion on basic load calculation of the building without the plants for the heating and cooling season will be a starting point, then you can address the moisture problem during the plant growing cycle.

 

[Wilbur55]

Interesting question.

Some searching led here:

https://en.wikipedia.org/wiki/Evapotranspiration

which led here:

https://en.wikipedia.org/wiki/Penman_equation

Other pages focused on the total leaf area as an important parameter:

http://passel.unl.edu/pages/informa...rmationmodule=1092853841&topicorder=4&maxto=8

Other important factors seem to be lighing level and air motion.

Perhaps there is something similar to this for plants:
[link Kleiber's law]

https://en.wikipedia.org/wiki/Kleiber's_law

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[Wilbur55]

http://meetings.copernicus.org/

http://www.cosis.net/abstracts/EGU05/07181/EGU05-J-07181.pdf

Transpiration of plants is directly proportional to biomass production

Maybe plant yield or growth in terms of pounds per day could be a useful parameter in estimating latent heat generation. But I don't know what constant to use if q = k * (lbm/day) is even in the ballpark. And k would seem to be a function of several variables, including: (leaf area/mass), lighting, temperature, humidity, air speed. Though those values may be relatively stable when the greenhouse is in full operation and the proportions of plants at each stage of growth are relatively constant.

You might ask whether they have any information on how much watering they do per day and the yield in mass per day. Any water that doesn't go down a drain in a steady-state operation will turn into latent heat. Perhaps their current temperature and humidity are comparable to what they want to maintain and the current watering rate will be informative.