[Typical Drying Curve][Download Excel Energy Costs per Piece Calculation]

Heat required to dry a mould or piece

To dry water or other volatile from material like plaster of Paris we need to know the following information: -

What is the base material – will it allow the volatile to permeate through it?

What is the volatile – will it readily evaporate?

If we consider a simple case, i.e. that of drying the water from a plaster-of-Paris mould of wet weight = 20 kg and water content = 10%. Shop temperature = 20 deg.C.

The heat required consists mainly of two parts.

A. Sensible heat.

This is required to raise the base material and the water to a temperature when water evaporation will readily take place.

For reasons, discussed later, this temperature is assumed to be 55 deg C.

The water (2000 gm.) takes 1 calorie / gram to raise it’s temperature 1 deg.C.

(i.e. Specific Heat = 1)

Therefore heat =  2000gm x SH x 35 degree C rise x 4.18 j/W  =  81.27 Watt Hours

The base material (18000 gm) takes 0.5 calorie / gram.  (Specific Heat 0.5)

Therefore heat = 18000gm x 0.5 x 35 degree C rise x 4.18 j/W  =  365.75 Watt Hours

B.  Latent Heat

This is the heat required to evaporate the volatile (water) without raising the temperature of the mould or water.   It takes 540 calories / gram to evaporate the water.

There are 2000 gm. of water

Therefore  heat  =  2000gm x 540 c/gm x  4.18 j/W  =  1254 Watt Hours

Total Heat Required

Sensible Heat (water)  + Sensible Heat (mould)  + Latent Heat (water)

    =  81.27 + 365.75 + 1254 = 1701 Watt Hours

Given the conversion efficiency is 75% then this would require 2.268 kWh from the mains.

The use of slightly heated (+ 8 deg.C.) shop air, which would have a reduced Relative Humidity, to remove the moisture which the microwave drives to the surface would reduce this requirement.

The working temperature of 55 deg. C. for the mould is maintained by the balance between the power input and the amount of airflow. 

By choosing this figure the chances of damage to the mould or piece is very much reduced.

Typical Drying Curve

Area “A”
In this area the microwave energy is mainly providing the Sensible Heat required to raise the temperature of both the base material and the volatile.  Practically no evaporation of the volatile takes place.

Area “B”
At this point the temperature of the whole is such that the volatiles are beginning to evaporate.  The energy used to evaporate the volatiles (Latent Heat) slows down the rate of temperature rise.

Area “C”
Further increase in evaporation rate further reduces the rate of temperature rise.

Area “D”
Over this area the evaporation (L.H.) consumes most of the microwave energy and a balance is reached.
This temperature may change by +/-2 or 3 deg. C depending on the Relative Humidity of the shop air supply.

Area “E”
Here the volatiles have nearly all been evaporated and the microwave energy is supplying the sensible heat
(S.H.) to raise the temperature of the block.
* At this point an infra-red temperature monitor could switch off the microwaves
 i.e. at 60 ^oC

Note
The sensible heat (S.H.) of the base material is usually about 0.5 cal. /gm. and for water is 1 cal. /gm.
In the case of pure water the latent heat of evaporation (L.H.) is 540 cal. /gm.

Energy Costs per Piece Calculation

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