Arnav Mahapatra, APC Consultant, Ruedo Enginering
A variety of material is available for the use in bag house of a cement plant. Arnav Mahapatra has attempted to look into the technical aspects and the merits and demerits of nonwoven needle felt filter fabric.
What is nonwoven needle felt filter fabric used for?
For the last 40 years, needle punching is considered as the state-of-the-art technology for producing filter media for pulse jet as the gas filtration is practiced all over the globe. The production of nonwovens take place in three stages, although modern technology allows an overlapping of the stages. In some cases, all three stages can take place at the same time.
Needle felting is using metal needles of different shapes and gauges for making nonwoven felt. Basically the woven fibre yarn is used as scrim to provide mechanical strength. Both side of the scrim consist of dust side batt and clean side batt, and the fibre sizes varies from micro denier to standard size of 5-denier staple fibre to make 350 gsm to 600 gsm in general.
What is nonwoven spun-laced filter fabric used for?
Spunlacing technology for heavy filter media structures (300 to 600 g/m2) has been available for a decade only and is currently used by a small number of manufacturers in globe. Spunlaced structures use high pressure water jets instead of needles to make the nonwoven filter fabric. Here no such scrim is used as in needle felt. Inorganic layers are used some times as per demand.
The modern, very sophisticated spunlacing technology improves the fibre distribution in the nonwoven structure, keeping the dust particles more at the surface. This results in a lower dust leakage and ensures a lower pressure drop (dP) during its life time.
Modern spunlacing technology allows production of up to 600 g/m2 structures with or without scrim support that offers the following advantages versus classic needle felt structures
What would be the design aspect of spun-laced fabric for making filter bag for dust collection system?
The selection of weight of the spun laced filter fabric strictly depends upon the manufacturer of the fabric. Generally 400 gsm spun-laced fabric can work equivalent to 550 gsm needlefelt fabric.
What will be emission level difference in both types of filter fabric?
Due to the metal needles used to make the needle felt, it shows under the microscope many grooves where the fine dust particles may enter into the core of the felt thus creating a higher dust leakage and tends to increase the pressure drop over a period of filtration time. So spun-laced filter media has lower emission levels.
Why is offline dust collector´s performance not good enough?
Many designers feel offline dust collectors can handle better pulse cleaning for fine dust laden air stream. In fact the offline dust collector costs more than the online ones. Actual filter layer, i.e., the dust cake on the filter bag never falls down on pulse jet even in offline pulsing except mechanical cleaning of the filter bag. Dust cake is highly recommended for proper filtration effect and to control emission level as well as product recovery. In offline pulsing, the high pressure jet opens up the porosity of the filter fabric resulting penetration of fine dust for escaping through filter fabric and create dust particle to stay inside the thickness of the fabric and creates premature blinding of the filter bags.
Why using cyclone mechanical dust collector (MDC) is bad if used before dust collector?
Pulsejet dust collector can handle the mixture of fine and average dust load of 30 gm/m3. If the dust load is higher, say about 100gm/m3, the pre-collector (Drop Box) combined with dust collector is of more help than cyclone or MDC. Cyclone has the collection efficiency of 70 per cent of the dust size more than 1 mm, also consume 70 mmwg of pressure drop resulting energy losses in the motor. Cyclone reduces the higher size dusts in the gas stream, which is entering the dust collection system. The fabric dust collector experiences more pressure drop and less pulse cleaning efficiency. The non-operation of pulsejet dust collector creates a lot of process problem and production.
How to handle the mistakes that come out while operating/designing the Pulsejet dust collector?
Air to cloth ratio should be <70 m3/hr/m2 for cement dust for filter fabric of needle felt virgin fibre of 2.2 dTex measuring 550 gsm should be selected for getting low pressure drop of 60 to 90 mm wg in operation, emission level <40 mg/m3, opera¡tional life of more than two years.
CAN Velocity ( Face Velocity mtr / sec across the Tube sheet except holes´ area ) should be <1 mtr/sec for Cement. It designates the gap between two consecutive bags in which the pulse cleaned dust of the bags from one row should not be adhered to the nearest bags of other rows which are in suction mode.
Clean Plenum (Height between Tube sheet and Top Cover) Height should be minimum of 800 mm, and the outlet wall should be 80 per cent open to connect a transition piece reduced to dust size of connected blower.
Inlet dust entry should not be in hoper, should be casing entry, i.e., horizontal entry or down flow design with pre-collector combined with dust collector for proper dust settling in hoper and uniform cleaning of filter bags in low pulse pressure. We found in many places that rotary air lock discharge is connected to the screw conveyor, but the rotary air lock (21 rpm) should be placed below the screw conveyor (31 rpm) for proper discharge of dust.
For commissioning the pulsejet dust collector, pre-coating with CaCO3 dust should be done on filter bags by opening the inspection door in suction mode through blower (by stopping inlet dust entry and pulsing). The pressure drops across the dust collector with new bags is 35 mmwg. Arranging pre-coating of CaCO3 on filter bags ensures abrasion resistance and chemical protective layer and dust cake formation. Pulsing should not start for approximately 40 hours in the system after commissioning the total process. Pulsing will start only when the pressure drop reaches 150 mmwg across the tube sheet (200 mm above and 400 mm bellow the tube sheet) during operation. If pulsing takes place immediately after dirty air entry to the dust collector then the protective shield dust cake will not be there to get better operation life, filtration efficiency, etc.
Changing of filter bags in the dust collector is a big question to end user. If dust is found in the chimney, than the dust collector has to be stopped immediately to find the faulty filter bags by more dust deposition around the faulty filter bags. If the change of bags is possible in early leakage period then it is well and good, and if not, then pulsing has to stop till the shutdown time within 3-4 hours of leakage. Then finding of faulty bags and installing new bags to be carried out. If pulsing is on after having leakage in the system, the dust will be forced to settled inside the bags during pulsing, which causes easy penetration of dust inside the thicken fabric wall resulting premature the bags after certain days. And same leakage will be found after few days. So conclusion is if nothing is controlled after leakage in dust control, one has to change all bags for cost-effectiveness in non-stop production point of view especially for cement or else while stopping the production immediately after leakage, one has to change few faulty bags only.
Sometimes pressure drops and it creates a lot of problem in operation. The user has to troubleshoot the system by experts. Instead of troubleshooting, the system user open the dust collector bag and cleans the dust cake manually by mechanical means. It should be absolutely stopped otherwise there will be a lot of health hazard, loss of maintenance cost, loss of filter bag cost and wrong implementation of filtration technology.
Arnav Mahapatra is a Consultant of Ruedo Engineering. He is in environmental science for the last 18 years, and is a specialist of air pollution control devices.