ï»¿               # Theory

A screw heat exchanger causes an indirect heat transfer. The amount of heat that is transferred can be calculated with the following formula: Formula 1

The level of the heat transfer coefficient U, amongst others, is determined by the thermal and mechanical characteristics of the product that needs to be treated in the screw heat exchanger. One of the most important thermal parameters is the specific heat of the product. If the specific heat of a product is high, it will be necessary to add or remove more energy to change the temperature of the product by 1?C compared to a product with a lower specific heat.

Also, the conductivity coefficient of bulk goods is very important for the level of the heat transfer coefficient. The conductivity coefficient of bulk goods is determined by the conductivity coefficient of the product, the specific heat and the granule size.

Surface A is the available heat transferring surface in the screw heat exchanger. It is clear that if the surface is twice as big the transferable amount of heat is also twice as big with U and the temperature difference remaining equal. The available surface depends on the type of screw heat exchanger. Figure 2 shows the available surface per type.

Formula 1 shows that the logarithmic temperature difference dTln is an important factor in the amount of heat transfer in the screw heat exchanger. If the difference in temperature becomes bigger, the amount of heat transfer will also increase. The logarithmic temperature difference can be calculated with formula 2: Formula 2

Symbols

 U - Heat transfer coefficient [W/m2.K] Q - Capacity [W] A - Heat transferring surface [m2] Tln - Logarithmic temperature difference T - Temperature[°C] m - Medium p - Product   