Importance of ACR in design of PJBH/RABH
Cement manufacturing involves several processes where the raw materials like limestone, coal, gypsum, fly ash, slag and other materials are transported, ground, weighed, burnt and calcined. These processes use air for moving/burning the materials. After the purpose is accomplished, air is to be removed from the circuit. Several pieces of equipment like cyclones, multi-cyclones, electrostatic precipitators, ventury scrubbers, etc, are used for separating the process dust/ particulate from the gas/air. But one equipment that has caught the fancy of the plant owners for the separation of dust particles from air/fumes, is the bag house. Be it pulse jet or reverse air, a physical filtration device ensures almost one hundred per cent filtration efficiency without being affected by the input conditions.
The pulse jet bag house (PJBH) / reverse air bag house (RABH) are preferred equipment in a cement plant. Their installations have increased year after year and today more than a hundred units are installed in a modern cement plant. However, the awareness about the equipment and its usage has remained limited to a few people. A cement plant process/maintenance engineer is keener in learning the nuances of production equipment like the kiln, cooler, raw mill, cement mill, coal mill, and simply ignores the PJBH/RABH, depending on external agencies for its operation and maintenance.
Unavailability of pertinent literature has also contributed to this comparatively lower awareness. Today, the situation is improving and many cement plants are appointing separate environment engineers who are responsible for the running and upkeep of the equipment. With growing awareness about emission and its ill effects, and the commitment of cement industry to become a green industry with minimum discharge of harmful materials into the environment, the situation will be very different.
What is air- to -cloth ratio?
It is the ratio of gas volume/air flow to the filtration area provided in the PJBH/RABH.
ACR = Gas Volume / Filtration Area
ACR = m/min
Gas Volume = m3/min
Filtration Area = m2
Normally, it is the application engineer who is designing the PJBH/RABH who decides on what shall be the ACR for a particular PJBH/RABH. However, at several times, the plant owner´s consultant also specifies the ACR to be considered for designing the PJBH/RABH. Who decides what should be ACR is not an important issue here, how to decide ACR is of critical importance. ACR determines the size of the equipment. If you become too conservative then your equipment costs rise very high, whereas if you are too aggressive and design equipment with high ACR, there may be performance issues. Therefore, it is of critical importance that a proper balance is kept while choosing the ACR.
The various aspects to be considered for choosing the right ACR are as follows:
Criticality of application: Is the PJBH/RABH is going to be used as a production and/or product recovery equipment? If yes, then a sufficient design margin must be incorporated and a lot of information should be obtained before deciding the ACR. Operating temperature: The higher the operating temperature, the more critical the filtration. The media used for high temperature filtration are generally very expensive and therefore, sufficient margins must be considered.
Chemical composition of dust: If you are filtering a chemically active dust which is likely to react with any of the components of the flue gas or the filter material, a bigger size filter will help.
Chemical composition of flue gases: If the flue gases carrying the dust to be filtered are corrosive in nature, choosing a conservative ACR will enhance filter life and life of other components. Chemistry coupled with temperature makes it a very critical application.
Presence of moisture: If the flue gases contain water vapour or any other vapour, then having a little more filtration area will only help.
Particle size analysis: Most of the PJBH/RABH are used in industries. The processes generate different particle sizes of the dust. If more than 20 per cent of particles are between 1-5 microns, it is considered fine dust. Filtering fine dust is difficult.
Inlet dust load: If the dust load at inlet to PJBH/RABH is high (> 100 gm/NM3) then using a pre-collector is advisable.
Large particles: If the process is going to have a large part (more than 50 per cent) of the particles above 100 micron in size, then installing a pre-collector can be very useful. If a pre-collector is not installed, abrasion of the filters and casing/hopper is possible.
Cost of product collected: Cost of the product collected is of critical importance. Sometimes, a few days or months` operation can pay for the entire equipment. As a general rule of thumb, a conservative (low) ACR will give better collection efficiency. Therefore, if the bag filter is being installed to collect precious dust, then a lower ACR will pay back very quickly.
Location of the plant: The public awareness about emissions has gone up in recent years and every industry wants a good relationship with its neighbouring community. Therefore, if the plant is close to a city/village and any stoppage of the APC unit even for a few hours, is going to make a big impact on this relationship, then choose the ACR carefully. Equipment with lower ACRs have a tendency to give good life to filters and need fewer replacements.
PJBH/RABH designed using optimum ACR will give the following benefits: Less frequent cleaning: Filters requiring lesser clearing cycles last longer. Each cleaning cycle damages them.
Lower compressed air consumption: Compressors are low efficiency equipment. Usually, they have 30-50 per cent efficiency. PJBH uses compressed air for reverse cleaning of filter bags/cartridges. More cleaning cycles consume higher compressed air and operating costs go Up.
Lower operating pressure drop: PJBH/RABHs designed using optimum ACR yield lower operating pressure drop and thus saves on fan power without sacrificing the filtration efficiency or emission levels.
Useful filter life: It is observed that RABH/PJBH filters with conservative ACR results in long filter life and fewer change of filters over its useful life. This reduces the downtime of equipment, saves the associated parts which are damaged during filter changes and saves the expense on manpower for change of filters, too. I have witnessed more than ten years of filter life for RABH bags and more than six years of filter life for PJBH in process filters where a conservative ACR was used.
The air- to- cloth ratio plays a major role in the performance of PJBH/RABH and it is of utmost importance to specify the ACR you want for your PJBH/RABH. Often, the plants unnecessarily draw more air though the PJBH/RABH due to unavailability of devices for controlling the air flow. Be aware of the flow for which the RABH/PJBH is designed and control it to the designed value, to obtain best performance.
Power saving methods to keep ACR in control
Use on demand cleaning: Use a DP switch to measure the differential pressure drop across the PJBH/RABH and adjust your cleaning cycle accordingly. Maintaining operation within a close range of DP across the plant will lead to stable operating and lower cost of power as well as efficiency.
Use dryer for compressed air: The moisture present in atmospheric air condenses when the air is allowed to expand inside the bag filter. This may make the filter wet at the neck and up to a certain length which then reduces the useful filter area and increases ACR. Therefore, use a dryer for compressed air. The compressed air going into the filter must not have oil/moisture more than 10 ppm.
Use dampers at fan inlet: Process variations results in variation in vent volumes. If there are no dampers to control the flow, the centrifugal fan may draw excess air and push the ACR to very high levels. Dampers are used to create an artificial pressure drop and maintain flow within limits.
Use variable frequency drives: Use variable frequency drives if the process varies frequently. VFDs regulate fan speed and reduce air flow through the unit when it is not desirable to draw large gas volumes. They save energy as well as keeps the ACR within limits.
PJBH/RABHs are positive filtration devices and when designed properly, give you perfect solutions for product recovery/ air pollution control. By specifying or knowing the designed ACR for a particular unit and maintaining the same, you can reap great benefits and derive optimum performance from these units. Insist on knowing these values and have measuring devices in the plant so you can ensure that the operating ACR is as per design. This will help you assess if there are any problems in your process and can also help you save precious energy.
If you become too conservative then your equipment costs rise very high, whereas if you are too aggressive and design equipment with high ACR, there may be performance issues.
Rajendra Kumar Yadav,
Managing Director, Gulmohar Filtech.