# Estimating rate of drying by comparing identical process with different ambient humidity

We have a drying process where we take a thin bread product and cool it in a room at 11 C and 90% relative humidity. The product comes in at 80 C and comes out at 22 C. There is air blowing across the product. Because of the design of the room (nearly a sealed box) it is hard to estimate the water gain of the air before and after passing the product.

I would like to size a dehumidifier to keep the cooling room at 60% relative humidity - however I need to estimate how much water I need to removing from the air. We would adjust the temperature to ensure that the product is 22 C at the end of the cooling process.

I can estimate how much evaporation is occurring at 90% relative humidity by doing a simple mass balance - however this will be higher at 60% relative humidity.

I was wondering what the best approach is to take data from the 90% relative humidity condition and make a back of the envelope estimate of an evaporation rate at 60% relative humidity?

At the moment I'm thinking of assuming the rate limiting step is mass diffusion between the air at the surface of the wrap (100% relative humidity) and the ambient humidity. If I do that then maybe the rate is merely proportional to the water kg of the air at the ambient humidity vs 100%?