How to reduce the resistance in the pulse jet bag filter houses?
As the dust collection technology develops, more and more dust collecting methods be invented and improved, because 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 compare to the resistance in electrostatic precipitators (about 200 Pa), the later maintenance cost of bag filter house is quite higher, how to reduce the resistance in the bag filter houses is the big challenge to designers and the end users.
1.why the resistance in bag filters increase?
A. 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 down side 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 avoid the dust air crash directly to the filter bags, and this kind of design always with a lower resistance.
Furthermore, the distance between bag to bag is different, the rising air speed is different as well, so the resistance 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 running, the dust collected on the surface of the filter bags, which make the air harder and harder to pass through, so the resistance in 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), valves size, etc, if the air speed is different, the resistance also different.
2.How to reduce the resistance in the pulse jet bag filter house?
A．Choose a most suitable air/cloth ratio.
Air / cloth ratio = (Air flow volume / filter area)
When the air/cloth ratio bigger, under a certain filter area, that means the dust air from inlet the volume is bigger, sure the resistance will 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 particles collection, the air/cloth should controlled even lower in case the resistance increase 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 air flow speed in the space of bag to bag, under a certain air flow 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 compare to the suitable design, so higher the rising air speed which will increase the resistance in the bag filter house. From the experiences, the rising air speed better to be controlled about 1m/S.
C．The air flow speed should be controlled well in different parts of the bag filter house.
The resistance in the bag filter house also affected by the air flow speed at 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 air flow and reduce the resistance in the bag filter house.
Reduce the air flow in the clean air house means the height of the bag house need to increase, sure that will increase much on the building cost, so we should choose a suitable air flow speed I there, as usual, the air flow speed in the clean air house should be controlled at 3~5 m/S.
The air flow speed at the bag tube sheet is proportionate to value of bag length/bag diameter. The same diameter, longer the length, the air speed at bag tube sheet must be higher, that will increase the resistance in the bag filter house, so the value of (bag length/bag diameter) as usual should 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 sure will cause the resistance in the bag house to increase, for keep a suitable resistance, we have to clean the filter bags, for the pulse jet bag filter houses, it 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 couldn’t too low, or the dust won’t drop; but also couldn’t 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 also related to the dust content, higher the dust content the 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 be 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 enter into the bag filter house; make the air inlet from the down side of the bag house, so the bigger particles will drop first; if the inlet located at the middle of the bag filter house, then can install a dust removing baffle to lead the air go from down side 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.