How to use the DRYIT semi-continuous tray dryer
Practical Action
The efficiency of the dryer
By actually knowing how much water was removed in a given time at a given temperature
using a given fuel it is then possible to calculate the efficiency of the drying system.
Thermal Efficiency
fuel
= kg water evaporated x latent heat / kg fuel used x heating value of
= (Energy actually used / Energy put in to the system) x 100%
How much fuel is used
Using the diesel fired heater it is known that the fuel consumption is 1.6 gallons per hour. As
mentioned earlier, the heater is rarely 'on' all the time. Hence the average fuel consumption is
less than 1.6 gals/hr. From the graph drawn below it can be seen that at 50°C, the average
fuel consumption is at point I (see annex for more details):
Figure 9: Fuel use with drying temperature
Point I = 0.7gals/hr = 0.7 x 4546 cm3 / hr
(4546 is the conversion factor from gals/hr to cm3/hr)
= 3182 cm3 / hr
The density of diesel is: 0.82 g / cm3. Therefore the weight of fuel used per hour is
= 3182 x 0.82 / 1000
= 2.61 kg/hr
To calculate drying efficiency from example drying data
Assume 65kg of product went into the Dryer at 95% moisture. After 41/2 hours the product
was dried to 6% moisture. Hence to calculate total moisture lost:
= 65xO.95 = 61.75kg moisture
24