Wood Chips Dryer

[pbag]

I want to increase the capacity of existing Eucalyptus wood chips dryer, which is supposed to dry from 25% moisture to 15% moisture. What are the equations for heat transfer coefficient & equilibrium evaluation.

Kern is mentioning one formula for calculating Volumetric heat transfer coefficient. How this is helpful in predicting performance at higher capacity. I am not able to establish equilibrium similar to S&T exchangers where I simulate the new condition using all 3 energy equations (2 from 2 fluids & 1 for Exchanger i.e. UA LMTD).

I've tried to simulate the situation using Kern Formula, but results are not distinctive.

Any help / discussion.

Thanx
Pawan

 

[m777182]

hello bmclctc,
could you please give us some details about the type of your dryer. Are you using hot air for drying? Do you know the inlet and outlet air humidity and temperature?
regards m777182

 

[pbag]

We can supply hot air varying from temp 65 to 100°C.
Humidity at inlet & outlet are 0.02 & .03 kg/kg.
We want to predict exit air temperature, limiting the chips temp below its fire point. (I was interested in keeping chips temp below 85° as Eucalyptus Oil flash point is ~85°C).

Dryer is a simple (batch type) chamber in which wood chips are dumped on a perforated plate from the top & hot air passes from bottom & leaves at top. after batch completion chips are unloaded from front bottom door.

 

[m777182]

hello bmclctc
You could try to increas the capacity of your dryer by several steps: decrease the size of the wooden chips if possible- you will increase the surface where water evaporates from;use some sort of agitating device to make material more homogenious;employ some time function of the inlet temperature: higher T at start while the surface is still wet enough to prevent rise of surface T and lower T at the end because of the flash point at 85 degC. You can use some steam shocks at start to open the structure of wood, encrease its temperature and rise the difusivity of its moisture.Higher air flow will increase the evaporation rate too.
These are suggestions not based on particular engineering calculations of your system.
You could find a fine engineering approach in books of R.B.Key,Drying,Pergamon Press and a lot of others but you will need some experiments to come closer with predictions.
regards m777182