evelopment of combined infrared and hot air dryer for vegetables

References and further reading may be available for this article. To view references and further reading you must purchase this article.


H. Umesh Hebbar, , K. H. Vishwanathan and M. N. Ramesh

Department of Food Engineering, Central Food Technological Research Institute, Mysore 570 020, India


Received 9 October 2003; accepted 18 February 2004. Available online 9 April 2004.

Abstract
Application of combined electromagnetic radiation and hot air is gaining momentum in food processing. A combined infrared and hot air heating system was developed for drying of vegetables. A conveyorised drying system having three chambers was fitted with mid-infrared (MIR) heaters for radiative heating. Through-flow hot air circulation was also provided for convective mode heating. The system was designed to operate under infrared, hot air and combination mode independently. The performance evaluation studies indicated that combination drying of carrot and potato at 80 °C with air at a velocity of 1 m/s and temperature of 40 °C reduced the drying time by 48%, besides consuming less energy (63%) compared to hot air heating. Combination drying also gave better results over infrared heating alone. The energy utilization efficiency of the dryer was estimated to be 38% for both carrot and potato drying.

Author Keywords: Drying; Electromagnetic radiation; Hot air; Infrared; Heating chamber; Specific energy

Nomenclature
Nomenclature
A
total outer surface areas of the chamber walls, m2
AV
total volumetric air flow, m3/min
Ad
total drying area, m2
C1i
specific heat value of the vegetable, kJ/kg °C
C3
specific heat value of water vapor, kJ/kg °C
C4a
specific heat value of the chain carrier, kJ/kg °C
e
emissivity of the chamber wall outer surfaces, dimensionless
F
utilized capacity of heating source, kW
Gli
weight of vegetables entering the chamber per hour, kg/h
G2
water evaporation per hour, kg/h
G4a
total weight of the chain carriers entering the chamber/hour, kg/h
hc
convection heat transfer coefficient, kJ/m2 °C
Ke
unit conversion factor, DIMENSIONLESS=71
K1
correction coefficient of voltage fluctuations, DIMENSIONLESS=1
K2
power reserve coefficient, DIMENSIONLESS=1.5
L
loading density, kg/m2
Mi
initial moisture content, % wet basis
Mo
final moisture content, % wet basis
P
installed capacity of the electric heating source, kW
Q1
heat absorbed by the vegetables, which results in increase in their temperature, kJ/h
Q2
heat absorbed for water evaporation, kJ/h
Q3
heat absorbed by water vapor for chamber heating up to the chamber temperature, kJ/h
Q4
sum of the heat losses, kJ/h
Q4a
losses through the chamber band including all types of chamber carriers as they travel outside the oven chamber and emit the thermal energy to the lower temperature atmosphere, kJ/h
Q4b
heat losses through the walls, kJ/h
q′
heat of evaporation of water, kJ/kg
t
drying time, h
T1
absolute temperature of the heating chamber wall outer surface, K
T2
absolute temperature of the heating chamber, K
t1
average temperature measured practically at the outer surfaces of the walls, °C
t2
temperature of the heating chamber atmosphere, °C
Δt3
difference in temperature of water vapor from 100 °C to the temperature of the heating chamber, °C
Δt4a
temperature difference of the conveyor before and after heating, °C
ΔT
change in air temperature, °C
Δt
difference in the temperature of the material before and after drying, °C
λ
latent heat of vaporization, kJ/kg
η
heat utilization efficiency, %
 
!! نوشته شده توسط پرهام سهولی | 5:1 بعد از ظهر | جمعه 19 تیر1388 •

RSS