1. Introduction One of the important experiments is drying experiment. There are several drying equipments such as vacuum-shelf indirect dryers, continuous tunnel dryers, rotary dryers, drum dryers, spray dryers and tray dryers. Tray dryers are used for removing water from material which can be lumpy solid or a pasty solid. Spreading uniformly solid on a metal tray which has depth 10 mm to 100mm is put in cabinet of dryer. Water is removed by fan which recirculates heated air over the surface of the trays. Air is heated by electrical energy and 10-20 % of air in cabinet is fresh air, remaining is recirculating air. In this experiment, tray dryer is used. [1]
Fig.1 Tray dryer [2]
In this kind of drying, one of the important parameters is wet bulb temperature. When gas coming from outside and a lot of water contacts each other, saturation is achieved. The adiabatic saturation temperature is the steady state temperature in such situation. If small amount water and a gas inlet is contacted under adiabatic conditions, a temperature which is nonequlibrium and steady state is reached. This temperature is called wet bulb temperature. In adiabatic saturation, temperature and humidity of gas are changed but in this situation, temperature and humidity of gas are not changed because of small amount of water.[1] In measuring wet bulb temperature, thermometer which is shown in figure 2 is covered with wet cloth which called wick. Air at dry bulb temperature T and humidity H comes and evaporates some water on wick. Due to this evaporation, temperature
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T is seen as Tw on thermometer. The difference in T and T w is due to latent heat of evaporation.
Fig. 2 Wet bulb temperature [2]
If making a heat balance on wick is wanted, amount of heat lost by vaporization is [1]: q=MA NA
λw
A
(1)
Where q is amount of heat lost by vaporization (W, J/s), M A is molecular weight of water, NA is mol of water evaporating/ s.m 2,
λw
is latent heat of vaporization at Tw
(J/kg H2O) and A is surface area (m2).[1] Then the flux is: NA= k y (yw-y)
Where
k y
(2)
is mass transfer coefficient (mol/s.m2), y w is mole fraction of water vapor in
the gas at the surface and y is mole fraction of water vapor in the gas. There is a relation between humidty H and y which is [1]: y= (H/MA) / (1/MB + H/MA)
(3)
Where MA is molecular weight of water and MB is molecular weight of air. Since H is small, then [1]: y= HMB/MA
(4)
Substituting Eq. (4) into Eq. (2) and after substituting into Eq. (1) [1] q= MB k y
λw
(HW - H) A
(5)
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Rate of convective heat transfer from T to Tw is [1]: q= h (T - Tw) A
(6)
Substituting Eq. (5) into Eq. (6) gives: (H – HW) / (T – Tw) = - (h/ MB k y) / λw
(h / MB
k y)
(7)
is called psychrometric ratio. In addition the adiabatic saturation lines can
be also used for wet bulb lines with reasonable accuracy but for only water vapor. Therefore, humidity of air water vapor mixture measure can be done by using wet bulb determination.[1] Knowledge about the basic mechanisms of rates of drying, obtaining some experimental measurements of drying rates is necessary in most cases. Therefore, data of loss in weight of moisture during drying at different time intervals should be taken without interrupting the operation. In addition data should be converted to rate of drying curves.[1] Xt= (W – Ws) / Ws
(8)
Where Xt is moisture content (kg total water/kg dry solid), W is weight of the wet solid and Ws is weight of dry solid in kg. Then, the equilibrium moisture content X* (kg equilibrium moisture/ kg dry solid) is determined. After that, the free moisture content X (kg free water / kg dry solid) is calculated for each Xt.[1] X = Xt – X*
(9)
Fig. 3 Free moisture vs time graph [2] 3
Then, free moisture X vs time t is plotted in figure 3. [1] To draw rate of drying curve from this plot, the slopes of the tangents drawn to the curve at given values of t. Then rate R is calculated for each point with equation [1]: R= - ( Ls X) / (A t)
(10)
Where R is drying rate (kg water/h.m 2), Ls kg of dry solid used and A exposed surface area for drying (m 2). [1]
Fig. 4 Rate vs free moisture content [2]
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2. Experimental Making material balance over tray dryer apparatus with using psychrometric chart and determining performance of tray dryer with respect to moisture content and drying rate curves at different operation conditions is the objective of the experiment. The tray drying unit consists of mainly an air duct, electrically heater element and digital balance. Air duct blows hot air with an axial flow fan impeller. The velocity of air can be arranged up to 1.5 m/s and the temperature can be arranged up to 80
.
Weight of the sample is measured by in-situ monitor and air flow rate is measured by a vane anemometer. Figure 5 shows experimental setup. During experiment, in time intervals, weight of sample which is being dried, dry bulb temperature, wet bulb temperature is measured. Temperatures are measured before and after trays. [3]
Fig. 5 Experimental Setup [3]
In experiment procedure, firstly dry sample was weighed. Dry sample was wetted with water and then wet sample was weighed. Samples were put into trays and were weighed again. Wick of wet bulb temperature was wetted and reservoir was filled with water for necessary measurements. Tray dryer started operation, air flow was arranged to ? and air temperature was arranged to ?. In every 5 minutes, weight of sample, dry and wet bulb temperature was recorded. With these data, drying rate vs moisture content and moisture content vs time figures plotted. Data are shown below in table 1. [3]
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Reference [1] Geankoplis “Transport Processes and Separation Process Principles”, 4/e. [2]http://www.eng.auburn.edu/users/tplacek/courses/4860/Lab%20Notes%20for%20
Drying.pdf [3] Lab manual Tray Dryer
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Nomenclature q= amount of heat lost by vaporization (W, J/s) MA= molecular weight of water NA= mol of water evaporating/ s.m2 λw=
latent heat of vaporization at Tw (J/kg H2O)
A= surface area (m 2) k y=
mass transfer coefficient (mol/s.m2)
yw = mole fraction of water vapor in the gas at the surface y= mole fraction of water vapor in the gas MA= molecular weight of water MB= molecular weight of air Xt =moisture content (kg total water/kg dry solid) X* = equilibrium moisture content (kg equilibrium moisture/ kg dry solid) X = free moisture content (kg free water / kg dry solid) R = drying rate (kg water/h.m2) Ls = dry solid used (kg) A = exposed surface area for drying (m2)
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