time to cool dry granules using cool air 1

[Micole]

Hi All, my manager asked me to calculate the cool dry yeast in the dryer from 45 degrees C to 20 degrees C using air at inlet temperature of 11 degrees C and mass flow rate of 18 000 kg/hr. The dry yeast batch is 160 kg. I'm not sure which formulas to use? Fourier's law and the heat energy Q = mcdT and how to use them? Please assist on how to go about? I'm under pressure to answer this question and I've tried to check on Coulson and Richardson etc. but still not getting an example that can assist? I understand how to calculate the drying time but not sure about cooling time.

 

[IRstuff]

It should be similar in nature; you have a certain amount of heat energy to be removed by the air, so this is a forced convection situation. The bigger questions are what is the exposed surface area and how well the yeast conducts heat. Look up Biot number in Wikipedia to determine whether you can treat this as lumped parameter case, or whether some sort of distributed parameter case. The convection part deals with how fast the air is moving over the surface(s) exposed, which determines the heat transfer coefficient.

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[Micole]

Thank you for your assistance and for the quick response The surface area is 3.14 m2 as the dry yeast will be at the bottom of the dryer with a diameter of 2m and air needs to be forced to go through it. The particle size of the dry yeast is about 1 mm thick and these dry particles will be on a sieve plate at the bottom of the dryer were air will be entering through at 18 000 kg/hr and at 11 degC. If it's a convection situation, does it mean I must use Newton's law of cooling to calculate time of cooling?

 

[IRstuff]

I suggest that you avail yourself to Google:

https://www.google.com/#q=forced+convection+through+bed

rather than making arbitrary assumptions./

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homework forum: //

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[Compositepro]

Heat transfer coefficient is likely not to be limiting factor In this problem. Try doing a simple heat balance to start with.

 

[EdStainless]

160kg of powder, the surface area of the yeast is going to be huge.
Average particle size leads to particle surface area and mass, figure out how many particles and work the math.
This will not be heat transfer limited.

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