How to reduce the resistance of the pulse jet bag filter house?
As the dust collection technology develops, more and more dust collecting methods are invented and improved, because of the advantages of high filter efficiency and stable lower dust emission, the bag style dust filters are the most popular dust filters nowadays, and the pulse jet bag filter house is the most popular bag filters due to the broad adaptability.
As usual, the resistance in the pulse jet bag filter house is at 700~1600 Pa, later operation sometimes increased to 1800~2000Pa, but when compared to the resistance in electrostatic precipitators (about 200 Pa), the later maintenance cost of bag filter houses is quite higher, how to reduce the resistance in the bag filter houses is the big challenge to designers and the end users.
1. The main factors that cause the resistance in pulse jet bag filter house to increase
The construction of the bag filter house
As usual, the resistances are always different when the constructions are different.
For example, as usual, the air inlet design is located at the downside of the bag house and the air rising through the ash hopper; or located in the middle of the bag filter house perpendicular to the filter bags. The first design can make the dust air uniform distribution and prevent the dust air crash directly to the filter bags, and this kind of design always with lower resistance.
Furthermore, the distance between bag to bag is different, and the rising airspeed is different as well, so the resistance is also different.
B. The filter bags.
The air pass filter bags always with a resistance, the initial resistance of new clean filter bags as usual is at 50~500 Pa.
C. The dust cake on the filter bags.
When the bag filter house runs, the dust collects on the surface of the filter bags, which makes the air harder and harder to pass through, so the resistance in the bag filter house will increase, and also different dust cake make the resistance various, mainly from 500~2500 Pa, so the purging / clean works of the bag filter house is crucial to reduce the resistance.
D. With the same construction, air inlet and air outlet, tank size (bag house body), valve size, etc, if the airspeed is different, the resistance is also different.
2. How to reduce the resistance in the pulse jet bag filter house?
A. Choose the most suitable air/cloth ratio.
Air/cloth ratio = (Air flow volume/filter area)
When the air/cloth ratio is bigger, under a certain filter area, that means the dust air from inlet the volume is bigger, sure the resistance will be higher in the bag filter house.
As usual, for a pulse jet bag filter house, the air/cloth ratio better not exceed 1m/min, for some fine particle collection, the air/cloth should controlled even lower in case the resistance increases sharply, but when designing, some designer want to make their bag filter house competitive on the market (smaller size, lower cost), they always try to declare a much higher the air/cloth ratio, in this case, the resistance in these bag filter house sure will on higher side.
B. Control the air rising speed with a suitable value.
The air rising speed means the airflow speed in the space of bag to bag, under a certain airflow volume, higher the air rising speed means the higher the density of the filter bags, i.e. the distance between the filter bags is smaller, and the size of the bag filter house is smaller as well when compared to the suitable design, so higher the rising airspeed which will increase the resistance in the bag filter house. From the experiences, the rising airspeed is better to be controlled about 1m/S.
C．The airflow speed should be controlled well in different parts of the bag filter house.
The resistance in the bag filter house is also affected by the airflow speed at the air inlet and outlet, air inlet distribution valves, poppet valves, bag tube sheet, clear air house, etc, as usual, when designing the bag filter house, we should try to enlarge the air inlet and outlet, use the bigger distribution valves and bigger poppet valves, etc, so as to reduce the speed of the airflow and reduce the resistance in the bag filter house.
Reduce the airflow in the clean air house means the height of the bag house needs to increase, sure that will increase much on the building cost, so we should choose a suitable airflow speed there, as usual, the airflow speed in the clean air house should be controlled at 3~5 m/S.
The airflow speed at the bag tube sheet is proportionate to the value of bag length/bag diameter. The same diameter, the longer the length, the air speed at the bag tube sheet must be higher, which will increase the resistance in the bag filter house, so the value of (bag length/bag diameter) as usual should be controlled not exceed 60, or the resistance should be quite higher, and the bag purging works also hard to process.
D. Make the air distribution equal to the chambers of the bag filter house.
E．Improve the purging works
The dust cake on the surface of the filter bags will cause the resistance in the bag house to increase, To keep a suitable resistance, we have to clean the filter bags, for the pulse jet bag filter houses, will use high-pressure air to pulse jet to the filter bags and make the dust cake drop to the hopper, and the purging work good or not is related to the purging air pressure, clean cycle, length of the filter bags, distance between bag to bag directly.
The purging air pressure shouldn’t be too low, or the dust won’t drop, but also shouldn’t be too high, or the filter bags should be broken sooner and also may cause the dust re-entrainment, so the pressure of the purging air should controlled in a suitable area according to the characteristic of the dust. As usual, the pressure should be controlled at 0.2 ~ 0.4 Mpa, generally, we think only if the pressure can make the filter bags clean, the lower the better.
F．Dust Pre-collection
The resistance of the bag filter house is also related to the dust content, higher the dust content the more dust cake will build up quickly on the surface of the filter bags, sure the resistance will increase much sooner, but if can collect some of the dust before they go to the bag filter house or touch with the filter bags, which sure is much helpful to prolong the cake building time, so resistance won’t increase very soon.
How to do the dust pre-collection? The methods are many, for example: install a cyclone to filter the dust air before it enters into the bag filter house; make the air inlet from the downside of the bag house, so the bigger particles will drop first; if the inlet is located at the middle of the bag filter house, then can install dust removing baffle to lead the air go from downside of the bag house so as to make some bigger particles drop first, also can avoid the dust air crash to the filter bags directly, and can prolong the service life of the filter bags.
Edited by ZONEL FILTECH