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20 points : A tray dryer needs to be designed to dry a batch of 500 kg apricots from their 90% (w/w) moisture content to 50% (w/w) moisture content...

  1. 20 points: A tray dryer needs to be designed to dry a batch of 500 kg apricots from their 90% (w/w) moisture content to 50% (w/w) moisture content (wet basis) in 24 hours. The temperature of apricots should not exceed 55 °C at any time. Design and estimate the following for this dryer:

To what temperature would you heat the inlet air (20 °C dry bulb temperature and 50% RH) before passing it through the dryer? (You have the freedom to choose that temperature knowing the requirements above are met)

What would be the flow rate of the inlet air (kg/s)

What would be the temperature and RH of the outlet air?


  1. 20 points: A block of tofu (5 cm × 2.5 cm × 1.25 cm) needs to be sterilized to make it shelf stable. Typically holding this product at 120 C for 2.5 minute is adequate to sterilize this product. This is accomplished by keeping the product in high pressure retort containing steam at 121 °C. The initial temperature of tofu is 4 °C. Calculate the process time for this operation if the convective heat transfer coefficient of steam is 200 W/m2°C, and thermal conductivity of the tofu is 2 W/m °C. The density of product is 1300 kg/m3 and specific heat is 3800 J/kg C.

Condition: You cannot assume a block of tofu as an infinite slab of one characteristic thickness. You must assume a finite slab



  1. 20 points: Feta cheese is prepared by immersing fresh cheese in brine solution containing 200 g/L salt (sodium chloride) and holding it in the brine solution until the salt concentration reaches 5 g/100 g product. If the moisture content of feta cheese during brining process remains constant at 50% (w/w), calculate the time required for this process if a single block of cheese is a cube of 5 inch. The diffusivity of sodium chloride in water is 1.5×10-9 m2/s. Feta cheese does not have any salt before brining and the brine concentration does not change with time.


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