SWISS COMBI Belt Dryer - technical details
A belt dryer can be operated with several different energy sources. The most economical way is to use hot water as an energy carrier. Using hot water will minimise costs for piping and for the water to air heat exchangers. Water to air heat exchangers can be operated with one, two or three different temperature cycles all integrated into one heat exchanger.
As an example, the energy for the warm water cycle at a low temperature to preheat ambient air can come from a flue gas condenser. As a second medium temperature cycle, energy from a condensing economiser can be used. Cooling water from a turbine can be used for the high temperature cycle in order to reach the final drying temperature.
The belt dryer can be served with hot water between 50°C and 120°C.
Another possibility as a heat source is saturated steam at pressures between 1.5 - 4 bar absolute. The steam is guided through steam to air heat exchangers and condenses. The condensate can either be fed back to the heat source or it can also be subcooled to gain more energy from the heat source. The condensate subcooling can be used to preheat the ambient air.
With a belt dryer there are different options available to increase its thermal energy efficiency. It has to be considered that the electrical power demand for most options is higher.
Exhaust air recirculation:
Towards the end of the dryer, where the product to be dried is almost as dry as requested, the drying process becomes less efficient due to physical reasons. Therefore, the exhaust temperature rises and is higher than close to the product entry of the dryer. This effect is used for exhaust recirculation. The exhaust air from the back part of the belt dryer is guided towards the entry of the dryer and passes the product layer for a second time. In its second pass through the product layer, the air is further saturated and is released to the atmosphere almost completely saturated.
Ambient air preheating:
The exhaust air contains more thermal energy than the ambient air due to its higher temperature and higher water content. A part of this energy can be transferred to the ambient air before entering the hot water heat exchangers. This helps to reduce the energy demand from the hot water cycle to reduce the requested drying air temperature.
Drying with process gas:
The drying air can be kept in a closed loop to be guided through the heat exchangers and the product layer again. The drying air achieves a high saturation. However, the evaporated water needs to be taken out of the loop. Therefore, a part of the process gas is guided through an air to air heat exchanger where it heats up ambient air which is used as drying air in the pre-drying part of the belt dryer. In this heat exchanger condensate arises. In the pre-drying part of the dryer lower temperatures already lead to efficient drying.
One or two layer system
SWISS COMBI belt dryers are operated with one or two layer systems. The main advantage of the single layer system is low investment costs.
The double layer system achieves - at the same size of the dryer - roughly 5-10% more capacity at the same energy input. In other words, the double layer system increases the energy efficiency of the belt dryer of roughly 5-10%.
In addition, the double layer system also helps to achieve a better product quality because the product can be dried more homogenous due to the mixing which takes place as the second layer is applied on top of the first layer.
Generally said, the higher the drying temperature, the higher the drying efficiency. This means that higher drying temperature leads to lower electricity and heat demand. Additionally, the required drying surface becomes smaller. For the operation of the dryer it is always important to keep the drying temperature as high as possible (max. 110°C). At constant thermal energy availability, more product will be dried if the fan speed is adjusted in order to achieve the highest possible drying temperature.
SWISS COMBI belt dryers are designed with 6m and 7.2m of belt width. The length of the dryers is modularly adjustable. The longest belt dryers which have been installed so far have a drying channel of 60m length. The dryers in total achieve a length of 68m. Also more than one belt dryer can be built either next to or on top of each other.