A hot-water treatment plant was fabricated with locally available materials in the workshop of the Division of Farm Machinery and Post-harvest Process Engineering, Bangladesh Agricultural Research Institute, Gazipur in 2011-2012. The hot water treatment plant is designed with the following consideration:•The capacity of the plant should be half a ton per hour •It should be portable •Fabrication cost should be minimum•Water temperature should be auto controlled 2.1 Development of the hot water treatment plant 2.1.1 Determination of the size of the plantLength (L) of the plant: The plant size was determined based on the available crate size and capacity of the plant. Length of the plant was determined according to the width of the crate. Three crates were used at the same time in operation. Length of it was designed on the width of four crates for attaining the desired capacity.
L=4×width of crate= 4×0.37=1.48 m
The plant was made of available materials such as MS sheet, MS angle bar, MS pipe etc. SS sheet was not used due to higher cost. MS sheet was used to reduce cost. Corrosion of mild steel was prevented by painting the surface. Power was transmitted to the conveyor roller with the use of gear box, pinion, chain sprocket, belt and pulley, self-centered and bearing, etc.Figure 1 Isometric view of the hot water treatment plantThe main functional parts of the plant were: 1) rectangular tank; 2) roller type conveyor assembly; 3) water heating system; 4) power transmission assembly; and 5) stirring assembly. Orthographic views of the plant are shown in Figure 2.2.2 Description of the main functional 2.2.1 Rectangular water tankRectangular water tank was made of MS sheet and MS angle bar, which was welded to a separate frame of MS angle bar. The tank had conveyor rollers, stirring plate, sensor and water. c crates loaded with bananas from feeding end to the delivery end. Motor power was calculated from the following formulaes (Equation (1), Equation (2), Equation (3) and Equation (4)):Pm=Pc/η (1)where, Pm=rated power of motor, W; Pc=power required to move loaded crate, W; η=overall power transmission efficiency, %.η=η1η2η3η4 (2)where, η1= power transmission efficiency of belt pulley, %;η2= power transmission efficiency of gear (there were 4 sets of gear and pinion), %; η3=power transmission efficiency of chain sprocket (there were 22 sets of chain and sprockets), %; η4=power transmission efficiency of nylon bush bearing (there were 22 sets of bush bearing), %.Pc=F×V (3)where,F= force required to move loaded crate, N; V=velocity of plastic crate, ms-1.F=f(m−vρ)g+hρg×A (4)where, f=coefficient of friction of plastic crate surface with the surface of roller; m=mass of plastic crate with banana, kg; v=volume of water displaced by the crate with banana, m3;ρ=mass density of water, 1,000 kgm-3;g=gravitational force, ms-2; h=height of centre of gravity of plastic crate from water surface, m; A=projected area of front side of crate, m-2.The power requirement of the driving motor becomes 128 W. Since the lowest capacity motor available in the market is 380 watts. Therefore, 380W electric motor was selected to operate the roller. 2.1.3 Determination of heater calculationThe electric immersion heaters were designed so that within 2 h time the heater can warm up the quantity of water (466 L) up to desired temperature of 55°C. Electric energy was calculated from the following formulae (Equation (5)):Q=MSΔT(5)where, Q= eat, J; M =weight of water, kg; S=specific heat of water, 4,200 Jkg-1 °C-1;ΔT=difference between temperature (T2−T1), °C; T1= initial water temperature,°C; T2=desired water temperature, °C.2.1.4 Calculation of number of heatersCalculation of number of heaters (Equation (6) and
Width (w) of the plant: Width of the plant was calculated by the addition of the roller pipe length and shaft of the roller on the side.
w=Lr + Lrs×2where,Lr= roller pipe length, m; Lrs=roller shaft length, m. Roller pipe length was taken little bit than plastic crate length due to clearance between the wall of the plant and loaded plastic crate for easy movement. Crate length was 0.54 m, so roller pipe length was taken 0.605 m. Roller shaft length was taken
0.0775 m.w=0.605+0.0775×2=0.76 m Height (H) of the plant: The plant height was determined on the basis of height of plastic crate (hc), height of roller (hr) and free space (hf) so that water could not overflow at full load condition. H=hc+hr+hf=0.30m+0.08m+0.12m=0.50m2.1.2 Motor power calculation The electric motor was designed to produce power required to rotate the rollers at desired speed and convey.