14.01.2010 | Author / Editor: R. Sims / Marcel Dröttboom
Quite often, only little attention is paid to the design of dust collection systems, simply because they are not part of the primary production process. But a properly designed system does not only affect safety and environment, it also contributes to the revenue.
Baghouses and filters are typically the last pieces of equipment that are considered when planning a new facility or plant expansion. Filtration equipment is typically not directly involved in the production process and, thus, does not contribute to the revenue stream, or does it? Revenue is dependent on safe, pleasant working conditions, minimal down time, and a low cost of operation. A properly planned and designed dust collection system can contribute to all of these variables to enhance the bottom line of every application. There are many different baghouse styles available on the market today as well as a wide variety of media products possible for each baghouse. In addition to process related decisions, protection of the dust collection system, and the baghouse itself from the hazard of dust explosions and fires must be considered in the overall system design. All of these variations make it difficult to decide which solution is the best choice for your specific application. Not everyone is a dust collection expert, so when designing or purchasing a system, it pays to work with someone who is.
In addition to requiring a safe dust collection system for a process, one would also want it to run as economically as possible. Two factors that heavily affect the total cost of operation are the compressed air usage for the cleaning mechanism and the pressure drop across the baghouse. Traditional ‘pulse jet’ baghouses are the most commonly used cleaning technology throughout most industries and have been an efficient workhorse for decades. They typically use high pressure compressed air (6 to 7 bar), have multiple diaphragm type pulse valves, and are cleaned based on a set duration between pulses. This does the job, but at what cost?
Many industries are now turning to more modern cleaning methods. One of the most efficient options is to utilise pulse jet cleaning with the exclusion of medium pressure air (0.5 to 1 bar) instead of the traditional high pressure. Studies have shown that medium pressure/high volume and high pressure/low volume are technically comparable in cleaning efficiency, however, there are several advantages to using medium pressure air .
The first advantage is found in the amount of power required to generate the cleaning air volume necessary to clean the filter bags. Common sense tells us that it will take less energy to compress air to 1 bar than it does to compress it to 10 bar. Studies have shown that a traditional high pressure pulse jet baghouse, sized to handle 34000 m3/h at an 8:1 air to cloth ratio will require a 11.2 kW air compressor with a drier and filter to prevent an accumulation of condensation inside the filter’s air header. The same size medium pressure pulse jet filter will require a 3.7 kW positive displacement blower and no drier. Air on the medium pressure unit is not compressed and heated enough to cause condensation. In addition to the 60 percent energy savings, the power required to heat high pressure compressed air cleaning headers in cold climates to prevent freezing is not necessary. Adding ‘on demand cleaning’, which only allows the timer to pulse the bags when the differential pressure indicates they require cleaning, can further enhance the savings of using medium pressure technology.
In addition to the medium pressure pulse jet units with the multiple diaphragm valve, there are round units available that may only have one or two large valves that do the same work as 20 or 30 smaller ones. This design may greatly reduce maintenance cost and still provide the advantage of less power consumption.
Medium pressure power savings could also pay off in a way the user may not be aware of. There are, for example, some areas in the U.K. where incentives are available from state governments and power companies to use the most energy efficient equipment available. There have been cases where these incentives have paid for the cost of the new equipment.
The second factor in baghouse energy savings is the pressure drop required to move the air through the baghouse. The two most common features that contribute to this are the inlet design and the filter media. There are many different inlet designs, such as tangential, involute, radial, etc, each having specific advantages for various applications. Each inlet design will require different energy levels to move the process air though them. The differences may only be 12.5 to 25 mm of water column, but that pressure difference on a 150 kW fan can have a significant impact on its amperage draw every hour of every day that the fan is running. Therefore, it is absolutely important to make sure that the most efficient inlet for the application has been selected and not just the cheapest alternative.
Filter media is the other most significant air restriction in the baghouse. As mentioned previously in this article, there are many types of media available. Each media has a specific purpose for a specific application and a specific dust. The various media also have different flow rate characteristics and air cleaning efficiencies. The most effective way to determine the correct media for the application and dust is through testing. Acquiring a particle size analysis or even having the dust ran through a small scale simulation is the most positive way to determine what the best media choice would be. The same media efficiency required for carbon black will probably not be needed for wood shavings.
It is fundamental, that the baghouse is sized so that the air to media ratio is not so high as to cause high differential pressure. Bag life does vary by application but as a rule of thumb, if a baghouse is sized correctly, the bags should last a minimum of 1 to 2 years.
In summary, spending a little more time in the planning stage of a project and a little more money on the capital equipment can return huge dividends during the life cycle of the equipment. The most cost effective way to solve a dust collection opportunity is to work with someone who is very knowledgeable with both the regulations and the technology that is available and right for your particular application. Preliminary testing is a very inexpensive method to get it right the first time, to find out what will work, what will not, what is the most cost effective, and what will meet the required emissions standards. Testing in the field can be very expensive and time consuming.
 Hilbert, J.D.: Silo dust collector comparison. Pneumatic Conveying Consultants, Schnecksville, PA, 2008.
Clyde Materials Handling Ltd.
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