Dust collection systems require routine maintenance and periodic evaluation.
Dust control is an important element in a grain facility. It consists of all means used to combat dust, including mechanical dust collection and manual housekeeping, as well as all measures to minimize dust generation.
Dust collection systems, a significant element of dust control, have five basic components: exhaust fans, capture enclosures, ductwork, dust separation devices and storage bins.
The most common dust-producing points in a grain facility in need of aspiration include the receiving pits, grain cleaners and scalpers, grain conditioning equipment, the mixing system, elevator legs and distributors, belt conveyors, baggers and loadout areas. If any such points are added to a facility, the existing dust system needs to be reevaluated.
"As the grain industry continues to change and adjust with the times, it often becomes necessary to evaluate existing dust control systems," said Delmar Mains, sales engineer, dust collection, for The Boone Group, Ames, Iowa, U.S. Mains was one of the speakers at a workshop on dust control sponsored last year by the Grain Elevator and Processing Society.
"Many of the dust systems that are still in use today were installed 20 to 30 years ago," Mains said. "For the most part, the basic technology of a dust system has changed very little in that time, but the elevators and other equipment have changed tremendously. In the 1970s, a 10,000-bushel-per-hour leg was very adequate, but now 40,000-bph legs and conveying equipment are not at all uncommon. More and more, enclosed conveyors are being used, and larger truck receiving pits are the norm."
Kevin Miles, president of The Boone Group, said dust collection for receiving pits has been one of the most common requests in recent years.
"That’s what a facility’s neighbors will see, and employees are standing right next to the pit and all the dust," Miles explained.
A solution for such a request is not usually easy or cheap. "Many existing pits are not designed properly for a good dust collection system," Miles said. "Often, a pit will have to be enlarged somehow to make room for the hood collection unit, which can get expensive."
All dust collection systems require a fairly large investment, Miles said. "A lot of smaller facilities don’t even look at them unless the insurance agency or regulatory agency forces them," he said.
For facilities that have a dust collection system, it is important to make sure the system is not outdated because of increased capacities in the facility, whether in the receiving pits, conveyors or loadout stations.
Capacity changes mean that the aspiration rates for dust control have also changed, Mains explained. He said he is often asked, "Can I still use my old filter and fan on this updated equipment and have a good dust control system?"
To answer that question, Mains suggests to first determine whether all dust collection equipment is operable. If not, determine the cost to get it operable. Next, determine all the areas in the facility that need to be aspirated.
Also evaluate the baghouse. Does it have explosion-vent panels? Does it need any repairs?
Check the fan, which may need a higher horsepower motor, and the ductwork, which usually needs to be rerouted when a high-capacity pit and conveyor are installed.
Look at what your facility needs, Mains said, then consider these questions:
•How permanent is the configuration of equipment we now have in our facility?
•Are we willing to invest the amount of money that a marginally effective system will cost?
• Or would we prefer to spend the money on a dust control system that will perform well?
It is a good idea to consult a dust collection system expert to help you further evaluate your needs.
SORTING IT OUT.
If a new dust collection system is needed, there are a few basic equipment options.
"A major consideration in baghouse selection is that of which bag cleaning method will be used," Mains said. The main options include reverse-air and pulse-jet filters, which are quite similar in how they filter the dust-laden air.
"The dirty air enters the baghouse in the lower area, and the air is filtered as it goes to the inside of the bags leaving the dust on the outside," Mains explained. "The clean air is then exhausted out the top of the unit and the dust falls to the bottom of a hopper and is discharged, usually by means of an airlock. These types of baghouses are designed so that filtration and bag cleaning can go on simultaneously for long periods of time before the bags need to be changed."
According to dust collection consultant Steve Palmer of B/P Associates, Ames, Iowa, U.S., reverse-air filters use a low-pressure air source to clean the filter bags. "The low-pressure air is forced back through the inside of the bags and the reverse air flow blows the collected dust off of the bags," Palmer said. "The reverse air is generally provided by a medium-pressure fan or a positive-displacement pump. This type of filter is usually round with the bags arranged in concentric rings from the center to the outside of the filter housing."
Pulse-jet filters, however, use high-pressure compressed air to clean the filter bags. "The high-pressure air is released back through the inside of the bags, and the pulse of compressed air is distributed through a main header and then through a series of pipes mounted directly above the bags," he said. "A row of bags is pulsed at intervals. The compressed air is provided by an industrial air compressor with an air dryer to eliminate moisture from the compressed air. This type of filter is usually square or rectangular with bags arranged in rows."
Cyclone collectors, another option, use centrifugal force to separate dust from the air stream. Miles said cyclones are less commonly used because they contain only 85% to 90% of the dust that enters the system, while the reverse-air filters and pulse-jet filters, also known as baghouses, contain 99.9% of the dust.
According to Palmer, cyclones are sometimes used before a baghouse to separate the larger particles before the air goes through the baghouse filter.
Although dust emission regulations vary from country to country, all types and styles of dust collection systems are available in nearly any country, Palmer noted.
MAKE IT LAST.
"A well-designed dust collection system can easily last for 20-plus years," Palmer said. "Average life, however, is closer to 10 years due to poor maintenance, upkeep and employee training."
Palmer suggests routine maintenance to include:
•Proper training of employees that operate and maintain the system.
• Management and employee awareness of what the system was designed to accomplish.
n Proper operation of the reverse-air and pulse-jet cleaning mechanisms and electrical controls.
• Know the significance of and how to figure the filter media differential pressure.
Differential pressure is a static pressure reading comparing the dirty side and clean side of the filtering media. This is commonly read using a magnehelic differential pressure gage and is expressed in Static Pressure Water Gage. A new filter bag normally has between .5 and 1 inch SPWG for the air to pass from the dirty side to the clean side.
As dust is collected, a cake, or layer of dust, accumulates. This "cake" seasons the filter media and makes it more efficient. Usually when the "cake" has developed, the differential pressure is between 1.5 and 3 inches SPWG. As additional dust is collected, the differential pressure will raise until the system fan can no longer pull or push air through the filter media. The periodic or continuous cleaning of the bags using reverse-air or compressed air keeps this from happening.
Watching the magnehelic gage will tell a trained employee whether the dust collection system is operating correctly. A rise in differential pressure could mean:
•The bags are not being cleaned. Check the bag cleaning mechanisms or controls.
•Off-time cleaning may be required. This involves shutting down the system fan and allowing the cleaning mechanisms and controls to operate for a period of time until the differential pressure reading drops to an acceptable level.
•Bags need to be replaced.
•The mechanisms that take away the collected dust may have failed and the collected dust has built up inside the filter housing.
• Make sure the filter remains water-free. Moisture inside the filter is one of the most damaging to the filtering media. Moisture and dust make mud. Mud clogs or "blinds" the filter media and in most cases will not be cleaned out using the reverse-air or compressed air cleaning methods.
Moisture enters a filter from four common situations: pulse-jet cleaning with compressed air that does not have a dryer in the system; rain or melted snow on the vacuum-side ducting that is sucked in to the system; leaks in the filter housing or fan housing; or collection of damp or moist dust from an aspiration point in the system.
• Normal maintenance of the system fan, rotary airlock and other components based on the manufacturer’s recommendation.
• Watch, listen and smell. If it doesn’t look right, sound right or smell right, something is probably wrong.