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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%?

Thanks in advance

simeon
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  • Welcome to Engineering, simeon. At the moment, your question seems to be getting at a lot of topics, so it's hard to figure out what problem it is that you're trying to solve. On a first read, I thought you were trying to speed up how long it takes to cool off your bread. On a second read, I thought you were trying to figure out how large of a dehumidifier you need to reduce the relative humidity from 90% to 60%. On a third read, I thought you were trying to dry the bread instead of cool the bread - there's a difference. – Chuck Jul 17 '19 at 03:20
  • Hi Chuck, the actual purpose is to cool the bread to a target temperature and target moisture with the room operating at a lower humidity. This is based on the logic that drying with lower ambient humidity levels impacts the distribution of moisture within the product. That said - I've realized that I have over-complicated my question and a simple mass balance appears to give the appropriate answer. I will write up what I've done for others. – simeon Jul 17 '19 at 03:29
  • @Chuck Is evaporation proportional to (1-RH) if temperature and air velocity is kept constant for the purposes of rough estimates. For example - will it dry 4x faster at 60% RH vs 90% RH? – simeon Jul 17 '19 at 04:37
  • Actually I should use humidity ratio like here: https://www.engineeringtoolbox.com/evaporation-water-surface-d_690.html – simeon Jul 17 '19 at 04:48

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