experiment 18 - study of performance of tray dryer

EXPERIMENT 18

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STUDY OF CHARACATERISTICS OF DRYING OF

WET SOLIDS IN A TRAY DRYER

7.1 OBJECTIVES The objectives of this experiment are 1. To study the rates of drying of a wet solid in a tray dryer under constant temperature and

humidity conditions of the drying air. Here, the typical constant rate – falling rate pattern of drying is to be identified with the estimation of the constant rate, critical moisture content and the equilibrium moisture content.

2. To study the behaviour of the drying curve with material beds of different particle size 3. To study the effect of air velocity on drying and 4. To study the effect of air temperature on drying 7.2 THEORY The moisture content (X) of a wet solid is measured in terms of the weight of moisture per unit weight of dry material. When a wet material of initial moisture content Xi is dried in a tray dryer, the typical variation of moisture content with time is shown in Figure 7.1.

D C B

E A Xe Xc Xi TIME (Minutes) MOISTURE CONTENT

FIGURE 7.1 FIGURE 7.2 Typical variation of moisture content with time Rate of Drying vs Moisture content

A plot of the rate of drying i.e. (-dX/dt) vs the moisture content X, shows a pattern as shown in Figure 7.2. Here it can be seen that initially there is a sudden increase in the drying rate along the line ABC followed by approximately a constant rate of drying Rc until a moisture content of Xc is reached at D. Beyond D a remarkable drop in the rate of drying is seen until a point E is reached at which point the rate of drying is nearly zero. The point C is called the critical point and the value Xc is called the critical moisture content. The point E is called the Equilibrium point and the value Xe is called the equilibrium moisture content.

MO

ISTU

RU

RE

CO

NTE

N

-dX/

dt

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For a given material and air temperature, the constant rate of drying is usually a function of the velocity V as given by

Rc = K.Vy (1) where K and y are constants. It has been found that the value of y is approximately 0.8 in most cases. 7.3 APPARATUS The layout of the equipment is shown in Figure 7.3 The apparatus consists of a duct A in which a digital balance B is positioned so that three sample trays T1, T2 and T3 are exposed to the interior of the duct. The trays within the duct can be accessed through an access door C. The duct is clamped on to the floor through clamping D. A blower/fan is controlled through a switch at the control panel E, so that the air flow rate can be set to a desired value. The control panel is also fitted with a heater control switch which can be used to control the power input to the heater, which heats up the air entering the duct. This allows the operator to manipulate the temperature of the air entering the dryer. An anemometer placed at the exit of the duct measures the velocity of the air leaving the duct. An aspirated psychrometer positioned at the upstream of the trays measures the dry bulb and wet bulb temperatures of air upstream of the trays.

FIGURE 7.3 DIAGRAM OF THE LAY OUT OF THE APPARATUS

Figure 7.4 presents a picture of the tray dryer experimental rig used in this experiment.

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FIGURE 7.4 THE TRAY DRYER EXPERIMENTAL RIG

7.4. SERVICES AND MATERIALS REQUIRED Water supply. Power Supply 220V AC A mixed sand sample Fractions of sand sieved approximately 200 –499 a nd >200 7.5 PRE EXPERIMENTAL QUESTIONS 1. Name three industrial applications of drying of solids 2. Write down a short note on each of the following

i. Critical moisture content ii. Equilibrium moisture content

3. Identify the following units in the apparatus. i. Duct ii. Trays iii. Blower/fan iv. Heater v. Control panel vi. Psychrometer

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7.6 EXPERIMENTAL PROCEDURE

1. You must familiarize yourself with the flow paths and apparatus before beginning of the experiment.

Caution for safety:

2. Caution must be taken not to switch on the power before the air fan/blower switch is put

on.

General Experiment

(a) Measurement of Drying Curves 1. Clean the trays with a dry cloth and dry the trays if necessary. 2. Weigh all the three trays empty. 3. Fill all the three trays with sand up to a depth of about 10mm in each tray. 4. Weigh all the trays with sand again. 5. Pour and sprinkle water on the material until the material is completely wet and a thin

layer of excess water is seen over the surface of the trays. 6. Even out the surface of material in the trays with proper care not to spill the material

out. 7. Weigh the trays with the wet material. 8. Position the trays on the balance within the duct. 9. Switch on the fan and set the speed control to a mid position. 10. Switch on the power control position to the maximum. Keep these settings constant

throughout the experiment. 11. Record the total weight of the trays, at regular intervals of time ( say 10 minutes) until

the weight of the trays do not show an appreciable change with time. 12. Note down the air velocity using the anemometer and the wet and dry bulb

temperatures using the psychrometer. (b) To study the effect of particle size on drying 1. Repeat the experiment described under 7.6(a) twice with sand with grades 200 –

499 a nd >200 s e pa ra te ly. 2. Note down the air velocity using the anemometer and the wet and dry bulb

temperatures using the psychrometer. (c) To study the effect of air velocity on drying 1. Repeat the experiment 7.6(b) with sand grade of >200 twice for two new settings

for the air flow velocity. The air velocity can be controlled by controlling the blower fan speed. The heater power output can be controlled if necessary in order to avoid overheating.

d) To study the effect of air velocity on drying 1. Repeat the experiment 7.6(c) with sand grade of >200 twice for two ne w s e ttings

for the air temperature keeping the air velocity at the value as in latter trial of the experiment 7.6 (c). The air temperature can be controlled by controlling the heater power output.

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7.7 EXPERIMENT LOG SHEET Students are requested to fill in the following measurements.

Experiment A: General Drying Characteristics

Particle Grade: Air Velocity = V1 Dry Bulb Temperature = T1 °C Wet Bulb Temperature = °C Weight of Empty Trays (W1)

Weight of Trays with wet san(W2)

Weight of Dry Sand (W3)

Weight of Moisture W4=(W2-W3-W1)

Xi = W4/W3

Time (Minutes)

0

Weight of trays W4* (kg)

Moisture Content X=W4*/W3

Experiment B: Effect of Particle Size

Trial B I Particle Grade: 200 –499 Air Velocity = V1 = (keep constant as earlier) Dry Bulb Temperature (keep constant as earlier) = T1 °C Wet Bulb Temperature = °C Weight of Empty Trays (W1)




Xi = W4/W3

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Time (Minutes)

0



Trial B 2 Particle Grade: >200 Air Velocity = V1 = (keep constant as earlier) Dry Bulb Temperature (keep constant as earlier) = T1 °C Wet Bulb Temperature = °C Weight of Empty Trays (W1)




Xi = W4/W3

Time (Minutes)

0



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Experiment C: Effect of Air Velocity Trial C I Particle Grade: >200 Air Velocity = V2 = Dry Bulb Temperature (keep constant as earlier) = T1 °C Wet Bulb Temperature = °C Weight of Empty Trays (W1)




Xi = W4/W3

Time (Minutes)

0



Trial C 2 Particle Grade: 200 –499 Air Velocity = V3 = Dry Bulb Temperature (keep constant as earlier) = T1 °C Wet Bulb Temperature = °C Weight of Empty Trays (W1)




Xi = W4/W3

Time (Minutes)

0



EXPERIMENT 7

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Experiment D: Effect of Air Temperature Trial D I Particle Grade: 200 –499 Air Velocity = V3 = (keep constant as in Trail CII) Dry Bulb Temperature = T2 °C Wet Bulb Temperature = °C Weight of Empty Trays (W1)




Xi = W4/W3

Time (Minutes)

0



Trial D 2 Particle Grade: 200 –499 Air Velocity = V3 = (keep constant as in Trail CII) Dry Bulb Temperature =T3 °C Wet Bulb Temperature = °C Weight of Empty Trays (W1)




Xi = W4/W3

Time (Minutes)

0



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7.8 POST EXPERIMENTAL QUESTIONS (a) Plot the drying curves X vs. t for all the above cases in a single graph sheet. Label the

temperatures and velocities and particle sizes relating to each curve. (b) Obtain the gradients of several points of each curve and plot the respective drying rate (-

dX/dt) vs X curve. (c) Note down the following for each case

(i) Constant rate (Rc) (ii) Critical moisture content (iii) Equilibrium moisture content.

(d) Discuss the variation of the above parameters with different particle size samples (e) Discuss the variation of the above parameters with different air velocities (f) Discuss the variation of the above parameters with different air temperatures (g) Plot a graph of log (Rc) vs log (velocity of air) and estimate the value of y in equation (1)

experiment 18 - study of performance of tray dryer

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