Hi Rafael

May be I can't give you the direct equation for your air flow calculation, but I can provide the analytical approach to calculate it.

Increase in humidity level inside greenhouse is affecting the drying, so you have to keep amount of air to maintain the humidity of initial level,

with the temperature increase inside the greenhouse you can calculate the rate of evaporation and change in humidity level with time (by knowing the volume of air present in side you close box, humidity = mass of water carried by volume of air).

so with evaporation rate and to maintain initial humidity level volume of air required can be calculated.

I can do the calculation for you if you can provide me the data required, otherwise I hope the methodology will help you to calculate the fresh air volume required.

You need to know:

Initial humidity of your air (absolute and relative)

Initial percentage of water inside grain

Percentage of water inside grain you want to achieve

Calculate Mass of water inside grain that you need to evaporate with a mass balance:

Mev= (Mass of initial water)-(Mass of final water)

This initial and final quantities you can calculate from initial and final percentage of water inside grain

Of course, this evaporated mass flow will go in your air flow, and it will change humidity of air (and its temperature)

Air dryed mass flow you would need it is costant for a fixed shift of humidity, what changes is the fresh air you need (because it has water inside)

M (dryed air) = (evaporated water flow from grain)/(Difference of final and initial humidity in air)

Use a psychrometric to find final absolute humidity above, you need to know the temperature drop after drying grain and you need to move on a isoenthalpic line in the diagram.

Now you know M (dryed air required), from initial humidity calculate mass of water inside air feed, add this value on dryed air value, you have the mass air flow required.

As you can see, mass flow required will change if atmospheric humidity changes.

I hope it will be clear, have a nice day

I suspect you could tackle this question using a technique very similar to that used for growing greenhouses. Instead of modelling the transpiration of the plants, you need to model the evaporative process of your grains etc (whether they are on trays, piled up on the floor, or whatever). Growing greenhouses are optimised for plant growth; yours would be optimised for drying rate, so conditions might be a bit different, eg perhaps higher temperature. On the other hand, you will have similar constraints such as keeping humidity low to prevent growth of mould etc.

Some references that I had to hand:

• Lake, J.V., et al., Seasonal variation in the transpiration of glasshouse plants. Agricultural Meteorology, 1966. 3(3): p. 187-196.
• Thermal Energy Use In Greenhouses https://pub.epsilon.slu.se/12037/1/maslak_k_150407.pdf

I would say important factor is Vapour-pressure deficit (see Wikipedia). Essentially a factor reflecting relative humidity of the air coming in contact with the object that needs to be dried. For example in the coastal area, when the relative humidity is close to 100%, how much ever fresh air you bring in, it will not help in drying. Unless you increase the temperature of the object (seed/painted surface...) (OR you preheat the air to higher temperature) so that relative humidity of the air coming in contact with the object will fall below 100% that will facilitate drying.