Studies on the Forced Air Cooler (Part 1)

Design, Construction and Performance of Stage type Forced Air Cooler

Abstract

This paper reports design, construction and performance of Stage type Forced Air Cooler, which uses the principle of exposure of the surface of individual products to a cold air blast as the products move on a conveyor.
1) To achive an efficient system design and also a desirable product environment, the precooler was divided in three stages and designed so that the temperature of air and products were progressively reduced from the entrance (1st stage) to exit (3rd stage) of the cooler. At each stage the standard cooling temperature was 10°C (1st stage), 0°C (2nd stage) and -10°C (3rd stage) respectively, and each was adjustable within ±5°C.
2) It has a capacity to cool up to 400kg of products in bulk from 25°C to 4°C in an hour.
3) Each unit is equipped with a limited load fan which has a variable-speed drive and a capacity to deliver from 100 to 160m³ per minute.
4) The conveyor is constructed of stainless wire mesh having 150kg per m² strength and equipped with sideplates to accomodate loose products up to 20cm in depth. Drive mechanism consists of a 4 to 1 ratio variable-speed drive to provide a cooling tunnel with retention time of 15 minutes to 1 hour.
5) For citrus Unshu whose average diameter was 5.5cm, the value of m (hr^-1), the gradient of logarithm on cooling curve, was 2.17 (hr^-1) with the air velocity of 3.4m per sec.
6) The difference in citrus Unshu cooling rates between upper and lower layers of bed (15cm-products depth) was small, but for Oranda Endo the difference was large when static pressure did not exist between the product bed (10cm).
7) We discussed the procedure of determining adjustable elements using the value of m (hr^-1), such as cooling temperature, air volume and conveyor speed when the cooler is operated for various products.