TRAY DRIER
TRAY DRIER
1.0
Introduction
A large number of manufacturing processes necessitate the drying of product or material and the
equipment used in these industrial operations depends on the particular process and type of material
being dried. For example, the requirements for food processing are more stringent than for agricultural
fertilizer materials and the design of driers for these purposes will be dictated by the relative importance
of such factors as heat sensitivity, porosity, bulk density and particle size of the dried material. In fact,
since the dried solid is generally a valuable product. Its shape, color, stability, stickiness and hence its
overall sale ability, all depend upon the drying processes to which it has been subjected.
Although there are many different types and operating characteristics of industrial driers, the SOLTEQ
Tray Drier (Model: BP 772) has been designed to provide an experimental facility based on one of the
most fundamental designs. Drying involves the transfer of liquid from a wet solid to an unsaturated gas
phase such as air and the solid itself can exert a considerable influence on the drying process. The
equipment enables the basic principles of drying to be investigated and students of process engineering
may examine the problems of fluids mechanics, surface chemistry, solid structure and mass heat
transfer associated with general drying behavior.
2.0
Theory
2.1
SUMMARY
The drying rate of wet solid in air changes throughout the drying period since the controlling factors are
different for each major section of the drying rate curve. However, many wet different for each major
section of drying rate is essentially constant and:
Rc α hv ( Tv – Ti )
Where,
Rc = drying rate during constant rate period
hv = total heat transfer coefficient (essentially convective)
Tv = temp. of the drying gas (dry bulb)
Ti = temp. of liq/gas interface (wet bulb)
Extensive experimental result (Sebord 1993) suggest that for air drying:
hv α Gv 0.8
where G = air mass velocity
The drying rate of a wet solid in air changes throughout the drying period since the controlling factors
are different for each major section of the drying rate curve. However, many wet solids exhibit a period
during which the drying rate is essentially constant, and:
Rc
hv ( Tv – Ti )
Where,
Rc = drying rate during constant rate period
hv = total heat transfer coefficient (essentially convective)
Tv = temp. of the drying gas
Ti = temp. of the liq-gas interface
The temperature of the drying gas (Tv) is normally measured dry bulb temperature. At steady state, the
temperature of the liquid-gas interface (Ti) is equal to the wet bulb temperature of the drying air. Thus
the drying rate is proportional to the different between the dry and wet bulb temperature of the air.
2..2
DESCRIPTION OF THE EQUIPMENT
The equipment comprises of an air duct mounted on a floor standing frame to give a comfortable
working height for the operator. Air is drawn into the duct through a mesh guard by a motor driven axial
flow fan impeller whose speed can be controlled to produce a range of air velocities up to 2.0 m/sec in
the duct. The air passes over an electrically heated element controlled by a power regulator to provide a
variation in air temperature up to a maximum of 60ºC at low air velocities.
The air passes into the central section of the duct where trays of material to be dried are suspended in
the air stream. The trays are carried on a support frame, which is attached to a digital balance, mounted
above the duct and on which the total weight is continuously indicated. The trays are inserted or
removed from the duct through a latched side door with a glass panel for viewing purpose.
After passing over drying trays, the air is discharged at atmosphere through an outlet duct. Relative
Humidity and Dry Bulb temperatures of the air are measured using the Rotronic Sensors fitted at both
upstream and downstream of the drying trays.
3.0
EXPERIMENT
This experiment is divided into 2 sub-experiments:
Experiment A = Effect of air temperature
Experiment B = Effect of Air Velocity
EXPERIMENTAL PROCEDURE
Experiment A = Effect of air temperature
Learning outcomes:
To investigate the influence of air temperature on the drying rate of a wet solid
in air at a fixed velocity.
Procedures
1.
Sufficient dry sand to fill to the tray to depth of 10 mm each must be accurately weighed before
being saturated with water in a container. The sand should be removed from the container and
drained of excess ‘free’ water before being loaded evenly and smoothly into the drying trays,
taking care to avoid any spillage. The total weight of the wet sand should be noted before
drying commences.
2.
Set the heater power control to ‘OFF’.
3.
Switch on the fan till point 1
4.
Set the heater power control to point A.
5.
At some arbitary time, (t=0), when the value of the heater power control stable, Measure the
dry bulb and wet bulb temperature and relative humidity of the air upstream of the sand tray.
6.
Record the total weight of sand in the trays at regular time intervals until drying is complete.
7.
Repeat the experiment f with the setting heater power control to point B. It is important to keep
the air velocity constant and to use the same weight and distribution of sand in each of the
tests.
Results & Discussion:
Results
Air velocity
= ________________________ m/sec
Weight of dry sand
= _______________________ kg
Dry bulb temp., Tv ( C)
Relative humidity,(%)
Wet bulb temp., Ti ( C)
Tv – Ti ( C)
Time (min)
Wet sand weight (kg)
Moisture content, Xe’
X 'E
Weight of liquid
Weight of dry sand
Weight of wet sand Weight of dry sand
Weight of dry sand
From the result, plot the drying curve relating moisture content and time for each test.
Differentiate data from this curve to produce the drying rate-moisture content curve.
Experiment B = Effect of Air Velocity
Learning outcomes:
To investigate the influence of air velocity on the drying rate of wet solid in air of fixed temperature
and humidity.
Procedures:
1.
Sufficient dry sand to fill to the tray to depth of 10 mm each must be accurately weighed before
being saturated with water in a container. The sand should be removed from the container and
drained of excess ‘free’ water before being loaded evenly and smoothly into the drying trays,
taking care to avoid any spillage. The total weight of the wet sand should be noted before
drying commences.
2.
Set the heater power control to ‘OFF’.
3.
Switch on the fan till point 2
4.
Set the heater power control to point B.
5.
At some arbitary time, (t=0), when the value of the heater power control stable, Measure the
velocity using the digital anemometer.
6.
Record the total weight of sand in the trays at regular time intervals until drying is complete.
Results & Discussion:
Air velocity
= ________________________ m/sec
Weight of dry sand
= _______________________ kg
Dry bulb temp., Tv ( C)
Relative humidity,(%)
Wet bulb temp., Ti ( C)
Tv – Ti ( C)
Time (min)
Wet sand weight (kg)
Moisture content, Xe’
X 'E
Weight of liquid
Weight of dry sand
Weight of wet sand Weight of dry sand
Weight of dry sand
From the result, plot the drying curve relating moisture content and time for each test. Differentiate data
from this curve to produce the drying rate-moisture content curves.
REFERENCES
1. Geankoplis, C. J., Transport Processes and Unit Operations, 3rd Edition, Prentice Hall, 1995.
2. McCabe, W. L., Unit Operations of Chemical Engineering, 5th Edition, Prentice Hall PTR, 1993.