As the cost of producing cement increases, owners, operators and designers’ plants are increasingly looking at whole of life solutions.
With the occupational safety and health moving up the agenda, dust control becomes a more urgent issue for many businesses.
Often it results in installation of more and larger bag houses type dust collectors, but they are expensive to own and operate. Selecting the right type of dust collector for your application and project can save you valuable time and money.
Within a range of different types of bag house dust collectors currently on the market, there are different bag cleaning systems available. One of the oldest methods of cleaning bags is reverse air cleaning (RAC).
Over the years this “old” style of dust collector fell out of favour, with the “newer” pulse cleaning system being preferred. While both systems have their strengths and weaknesses, recently the RAC system has come back into vogue as a viable and often preferred option. Let’s see why.
Advantages of RAC Dust Collectors include:
- Lower energy consumption
- longer bag Life
- often higher collection efficiency over an extended period
- reduction in moisture related issues
- often smaller size
Lower Energy Consumption
The reverse air system uses a fan to blow clean air back against the main dirty air flow. A pulse system uses a compressor to blow clean air back against the main dirty air flow.
The RAC fan operates at typically 2.5kpa, most compressors are operating at 620kpa. While the volume of cleaning air used by the RAC dust collector is slightly higher the cost of making this pressure outweighs the cost of the air volume substantially. In short, the cost of running a small fan is very much lower than the cost of running a compressor.
The fan is also almost maintenance free when compared to the compressor.
Typically, RAC dust collectors also run at a lower total differential pressure, this converts to a lower cost to run the main fan
Longer Bag Life
Pulse cleaned dust collectors clean the bags by means of a blast of compressed air being shot down the bag tube. The bag is suspended open by means of being wrapped around a cage. When the bag is blasted it snaps open to its full and or tight diameter. Then the remaining air permeates the cloth and dislodges the dust from the inside out.
This process wastes compressed air because the first air is actually used just to open the bag, see the comments above regarding this. However, it also introduces a process of mechanical fatigue as the bag opens and then at the close of the pulse closes back on the cage.
The RAC system is equipped with a traveller mechanism in its clean-air plenum. The traveller directs low pressure air into the filter bags from the clean-air side, blowing the collected product off the outside of the bags. To prevent migration of dust from one bag to another during the cleaning process, the bags on either side of the bag being cleaned are blocked off. This creates an area of static air around the bag so the dirt can easily drop off and fall directly into the hopper. Low pressure air only is needed to gently clean the bags as a result of this static air.
A feature specific to the Mideco range of RAC filters is the bags are suspended under tension. This means the cloth is taught at all times. This results in no air being wasted due to having to open the bag. It also means if the bag cloth is not moving there is no mechanical fatigue.
Eliminating mechanical fatigue in cloth prolongs its life in almost all circumstances. It also reduces the chance of its indices opening and thus allowing the dust to blind it.
Often higher collection efficiency over an extended period
As a result of the cleaning method and bag tension not being as arduous on the cloth or protecting it, the cloth maintains its as new characteristics longer. The transfers into better performance over the extended period
The RAC system provides a very high efficiency of dust collection. The RAC special features can guarantee a collection efficiency of 99.9% of all particulate matter down to 0.5 microns in size. This generally equates to an emission lower than 10 mg/Nm 3 for types of dust so small that it borders on being a fume.
A reduction in moisture related issues
Cement manufacture involves heat and therefore Dew Point can be an issue. In certain conditions cement is hygroscopic. Compressed air when released can carry moisture or at least cools. Dust collector filter media tends to become plugged with material when dust is hygroscopic / absorbs water. The dust-liquid mixture clogs the pores in the filter media, creating an impermeable coating that can resist most types of filter cleaning systems.
In this application this can lead to substantial issues including wetting and setting the cement inside the Bag House. RAC dust collectors use ambient air that can even be heated if required to clean the bags. Therefore, they can combat or even reduce the risks associated with moisture in the plant.
Are Often Smaller
RAC dust collectors do not use round bags, they tend to use enveloape bags. The envelope bag is a much more efficient method of arranging square metres or a quantity of cloth than round bags. The result is they are the most compact dust collectors externally.
To sum it up, RAC cleaning system is extremely efficient while being gentle on the filter fabric. This preserves the filter bag and increases the system durability. In some cases, 10 to 20 years on lead barton pots and carbon black banburies have been easily achieved.
Mideco is one of the few companies experienced in the design and manufacture of RAC systems.
We installed a RAC dust collector at Birkenhead, South Australia for the Adelaide Brighton Cement. The dust control system was implemented back in 1976. The inspection early this year confirmed that after 40 years in use the RAC dust collector is performing well and providing adequate dust control.
The reverse air filter dust collectors are a unique high-efficiency solution for addressing dust emissions in the cement industry. They can help businesses protect their staff and provide a long-term dust control solution during various stages of cement manufacturing.