Capacity Issues for Shop Evacuation Systems

It is important that all readers note the title of this presentation: Capacity Issues for Shop Evacuation Systems, not to be confused with Baghouse Capacity Solutions. There is no magic wand that could resolve what is in reality an issue of the two “M”s: Maintenance and Money.

Foundry baghouses have traditionally been one of the last places any Company wants to spend capital dollars, with good reason. Baghouses are expensive to build and operate, and the expansion and optimization of this equipment does not necessarily track capital expenditures in the productive, profit-making parts of a plant. As a result, baghouses are often bottlenecks that limit furnace operations, fail to prevent environmental problems and elevate the over-all costs of production. .

Early on, during the development of the melt shop fume evacuation systems, a baghouse that handled 25,000 cfm was considered to be a big system and 10 ton heats were large. Today, baghouses that handle >700,000 cfm are normal and 45 ton or much larger heats are standard. Between the early days and now, there is one major piece of steel mill equipment that has not kept pace with furnace productivity gains: the baghouse.

Most melt shop operators fit into one of two categories: those with baghouse capacity issues; and those about to have baghouse capacity issues. We must recognize that baghouses are not only pollution control devices but also critical components of the shop evacuation system. Absent a well-operating baghouse, no furnace shop can meet the legislative requirements nor present the employees with a healthy working environment.

Even in the newest facilities, melt practices and furnace expansions have created conditions that exceed the nameplate capacity of their baghouses, either in air handling, temperature limitations or grain loading, often all three at once. Plants originally commissioned at 100,000 tpy are producing at a rate of 175,000 tpy without commensurate expansion of the dust evacuation system.

As a rule of thumb, for the last several years the metals melting industry has used the rate of 32 lbs. of dust generated per ton of metal melted. We have computed the ratio of dust per ton of melted metal recently, and it exceeds 40 lbs. at many locations. At least one domestic producer is currently generating in excess of 50 lbs. of dust per ton of metal. The increase from 32 lbs. to >40 lbs. is a massive 25% increase, impacting the ability of the baghouse to handle the load, and highlighting the inability of the hopper evacuation system to keep up with and evacuate the increased amount of deposited dust.

There are several reasons for the current situation in many mills with insufficient baghouse capacity and inadequate shop evacuation. Among these are: increasing energy rates in the furnaces; enhanced melt practices; declining quality of scrap; closed-shop designs; shorter tap-to-tap intervals. It has been our experience that the majority of the melt shops we work with experience all or most of these circumstances, and are choking at the baghouse and failing to evacuate the shop properly.

If you factor in the reduction in permit levels for newer baghouse installations, the situation becomes even more critical. The permit levels have gone from a standard of <.0050 gr/dscf to .0032, then to .0018 gr/dscf. The trend is certainly in a direction that forces this Industry to pay closer attention to the shop evacuation system. In addition to the decreased permit levels, the Enforcement Agencies are in many instances forcing mills to install single-point-exhaust systems, or negative-flow baghouses with stacks. Layer onto this the EPA’s Office of Enforcement and Compliance Assurance (OECA) and the MACT Initiative, and the issue of filtration capacity approaches critical mass.

There are four (4) components within all baghouse systems that must be considered when addressing capacity issues:

• Gathering the air and dust

• Transporting the air and dust

• Separating the air and dust

• Disposing of the dust

Problems in any one of these four components will manifest themselves in poor baghouse performance. Most shops do have adequate control of the gathering process, or at least have the equipment installed for that purpose. I have to say that in many of the shops I visit, it is not unusual to find that damper controls, either on the canopy or direct-evacuation duct, are locked into position and not modulating as designed. Correctly operated canopy and 4th hole dampers are essential for good shop evacuation.

The transporting ductwork also is generally in place, but often is flow-restricted by deposited and accumulated dust. Periodic vacuuming of the ductwork should become standard and required practice. In addition, extension of the water-cooled segment or proper use of evaporative cooling can lower 4th hole duct gas temperature and decrease the amount of tempering air needed from the canopy for cooling.

Most capacity issues are more apparent at the interface of the dust-laden gas and the filtering media, i.e. the filter bags. High-pressure drop, spark holes, plugged thimbles, filthy shop environment, dust plumes…..All of these and more are often traced to the filter bags. Separating the air and dust is the crux of baghouse operations, and the place where the action is most aggressive. Let us review for a moment what we have: a furnace operating in excess of 2000° F, 8 ga. water-cooled ducts, 800+ HP fans, and at the end of it all stands a tubular piece of fabric, weighing less than the weight of your greens. Where do we think the problems will show up? Filter bag failures and the failure modes are more often than not indications of system problems, not specifically bag problems.

The fourth component, disposing of the dust, is perhaps the forgotten element of the system. This includes removal of the dust cake from the filter, depositing it in the hopper and evacuating it from the baghouse. When a melt shop has, over the years, increased the productivity of the furnace, it many times has not kept pace with the baghouse system modifications. If your tap-interval has decreased, the tap-size has increased or your scrap quality has gone down, then the baghouse must keep pace. Your system is may be moving the same amount of air, but the dust concentration is vastly elevated. Keeping the dust entrained until in enters the hopper then providing an adequate evacuation system to take the dust away from the hoppers is critical.

In addition to many others, one rule of successful baghouse operations cannot be overstated: hoppers are not inventory storage devices! If the dust-to-air concentrations are elevated, the rate of removal, or the dust output, must stay up with dust input. This should include constant operation of the screws and rotary locks, since dust is deposited into hoppers constantly, not only during cleaning. In addition, a near-empty hopper allows for a dropout chamber for the particulate. The filters then do not have to handle all of the dust and some of the heavier particles will drop into the empty hopper. Speeding up or up-sizing screws and rotary valves are ways to address this issue. Near-empty hoppers are essential to successful baghouse operations.

As part of your system evaluation to address baghouse capacity problems, do not overlook the cleaning cycles. Most plants have a modified version of the OEM’s cycle, but do not tailor them to specific baghouse conditions. One distinct advantage of changing all of your filters at one time is the ability to adjust the cycles for optimization. New filterbags require less aggressive cleaning than older filters, so periodic cleaning cycle adjustments are required.

There are several things that can maximize baghouse capacity, the primary one being maintenance of the current system. Keep all dampers working properly, whether canopy, 4th hole, inlet, isolation, pulse systems or reverse-gas. Improper damper motion equals loss of air flow control. Periodic cleaning of ducts, cleaning cycle adjustments and hopper maintenance are all vital elements.

Let me address the filtration media for a moment. At best, a standard media change can realistically offer 7% to 10% airflow enhancement. In most systems that amount of increase is available by optimizing your current equipment. However, the ePTFE membrane media in either polyester or high-temperature fiberglass offer enhanced dust-cake release properties and improved airflows. Unless the system, flange-to-flange, is maintenance-current and fully capable of handling the evacuation of the dust load, the benefits of these expensive filter bags will not be fully recognized.

Lastly, let us not forget that the calendar most people use to evaluate bag life is outdated. The critical element in bag life is not the calendar….it is the load. If you have increased your annual productivity by 40%, often 100%, your equipment, including baghouse and filters, are working harder. A baghouse designed for 25-ton heats every 75 minutes with a dust load of 43 lbs. of dust per minute may now be handling a 45-ton heat every 62 minutes. This increase in dust loading must be handled by the filters and the hoppers. Accommodation must be made for this change in work load, and recognition given to the impact on bag life.

Pleated elements, available for pulse-cleaned (pulse jet) baghouses, are yet another way to enhance the over-all baghouse performance. Relatively new to the market place, they offer increased capacity due to greater filtration surface; longer life by a factor of two or more; higher operating efficiencies to 99.99% and lower element operating pressure drop. Since pleated elements also better address the issue of the inlet abrasion in filter bags, as well as reduce downtime and maintenance cost, more and more baghouse operators are considering these products as a money-saving option, which also positively affects the capacity of their equipment.

System maintenance, system optimization and capital expansion are the avenues available for baghouse capacity solutions; in that order. Written by Don Council, Metals Accounts Manager, Midwesco Filter Resources, Inc. 800-336-7300
http://www.midwescofilter.com/

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