µm = micrometer=1/1000 mm
Dust can occur while processing or loading or unloading material. This hampers specifically the work for the working people around. Furthermore, the environment is polluted, and neighbors suffer from the polluted air.
Generally, fine particles are hygroscopic which means that they cannot be bound or mixed with water.
However, with the help of water fog, particles can be attached to the surface of the water drop. The fog “coats” the fine particles, hence the weight is heavier, so it sinks to the floor faster.
At the same time, the water consumption should be held to the lowest possible.
The system has the following requirements:
- Creating the biggest surface as possible while using as little water as possible.
- Guarantee a long lasting “fog effect” (it should stay in the air as long as possible)
These requirements can be fulfilled with the creation of a fine water fog. (drop size should be around 10-20 µm)
We have developed a calculation to determine the droplet size. This method shows that the water droplet size depends on the amount of water and the size of the area which needs to be covered. The following cases shall give insight into the calculation.
If 1-liter water is divided into drops, with a diameter of 0.1mm, an area with 60m2 can be covered.
Therefore, an area with 400m2 requires 7-liters of water. The rate of descent of the 0.1mm-sized drops is 0.3m/s.
If the size of the drops is only 15µm at 1 liter used water, the area which can be covered is 400m2. The rate of descent of 15µm drops is 7mm/s. Hence, the 15µm drops stay 40 times longer in the air to absorb the dust.
All in all, dust suppression of smaller drops (15µm) is 280 times more efficient than of bigger drops (0.1mm). The drop diameter highly depends on the used water pressure.
In general, it can be said: the higher the water pressure à the smaller the drop diameter (not too small) à the higher the dust suppression effects à the smaller the water consumption.
The diameter of the drops depends on the type of spray nozzles. This diameter should not be exceeded at increasing pressure.
Also called evaporation chill
Adiabatic cooling occurs during the phase transition of water from liquid to gaseous. The cooling effect can be achieved through depriving the heat of evaporation which occurs during the evaporation. Hence the liquid itself and the surrounding environment is cooled down.
The following circumstances foster the effect:
- Dry ambient air (low relative humidity)
- Big active surface
- Long evaporation time
Sufficient air change is a prerequisite for adiabatic cooling. Due to the steadily rising humidity, cooling can only work with air circulation.
Energy is drawn from the ambient air during evaporation which implicates a cooling. The following questions need to be kept in mind:
- How much energy is drawn from the ambient air?
- How does it impact the temperature?
The evaporation chill for water accounts for 2.2MJ per kg of vaporized water. Therefore, each vaporized gram of water drawns 2.2kJ from the ambiance.
The specific heat capacity of air accounts for 1kg/kg therefore, one can say that each gram vaporized water implicates a reduction in temperature of “1K” which means that temperature drops by 1°C.
The ambient air always consists of steam hence, it is not fully dry. How much water can be absorbed by the air depends on its temperature. Warm air can absorb more, and cold air can absorb less water. The absorbed air is measured in gram per cubic meter (g/m3) or in gram per kg of air (g/kg).
The absolute humidity indicates the amount of water contained in the air.
The relative humidity indicates how much percent of steam of the possible total (=saturation content) is really contained in the air. The maximum possible amount of water depends on the temperature of the air.
At 100% relative humidity, the air is saturated with steam and cannot absorb any more humidity.
At 50% relative humidity, the air is saturated with steam by half.
If the temperature of air falls, its saturation content falls too, while the absolute humidity does not change. Following, the relative humidity increases. At last, the saturation content lowers due to critical temperatures, also called dew point. If temperature falls even more, the contained steam will be exuded in the form of fog.
The difference in temperature can be perceived especially in summer and winter.
Due to the low air temperature, relative air humidity in winter is higher and saturation content is lower. Saturation content is 0°C or 5g/m3. Hence the possibility of applying adiabatic cooling is limited. Fortunately, dust emission by nature occurs less in winter which implicates that the amount of water can be reduced. Furthermore, the cooling effect is appreciated in summer, in winter however, it is not desirable. The heating would need to compensate the effect which would also need more energy.
On the other side, in summer the cooling effect is a pleasant side effect. The general principle is the drier the air, the better and more the cooling effect of the adiabatic cooling. The saturation content at 30°C air temperature is 30g/m3. This allows further humidifying, except at muggy weather, and the concomitant adiabatic cooling until 15°C.