"Collection of dust - vapor - smoke" has many practical meanings for both home cleaning, public hygiene, and industrial hygiene. Dust collection is more extensive than the field of material transportation "aerodynamic transport" for the production stages - it is very meaningful in the pharmaceuticals, agriculture, food production, cement production, manufacturing tobacco, ... In addition, collecting dust has the meaning of protecting the environment, protecting human health.
The simplest dust collector is a mini dust collector - cleaning dust for homes, offices, buildings - that we often see in existence. These include only a small vacuum motor, a dust collector and a suction pipe. During the house cleaning processes, a larger, more convenient system has been developed than the central vacuum cleaner system. The system also includes a larger, conveniently placed vacuum motor and plastic pipes to the rooms of the building. This system is very modern because the system works on demand when in use, which means it operates and turns itself off, and adjusts its capacity according to usage requirements.
In cleaning in industrial plants, nowadays we often use a central dust collection system. This system has many advantages over small machines. The central system can save space, save energy, filter dust, and be more professional. But this system often requires a high initial investment. This central system consists of exhaust fan system or vacuum pump, pipe installation is usually twisted corrugated iron pipe or stainless steel pipe and dedicated suction vents for each type of dust and field.
Over the years in the “Air Pollution Control” field, we would like to summarize a few important considerations in the design of a dust collection system. Hope these notes bring many useful things to you. This discussion includes sections:
Part I: Overview of dust collection system:
I. The parameters to note when choosing and designing a Vacuum system.
- Air to cloth coefficient of air to cloth:
- Is the amount of air passing through 1 unit of filtration area in one minute. (m3 / 60 / m2 or CFM / ft2).
- Based on the types of dust we choose this coefficient accordingly to let the dust collector work well.
- In many cases users will give an A / C coefficient they want or based on experience or practical tests to come up with the appropriate A / C coefficient.
As we know Q = A x V
- Q is the dirty air flow to be filtered and the dust pen. (m3 / s)
- A (Area) = the area of the membrane material to be arranged for the dust collector (m2)
- V (Velocity): Wind speed through the filter (m / s)
The Q parameter is usually fixed according to the requirements of the system, so we calculate two parameters A (Arae) and V (Velocity) to have the best dust collector operation.
If the parameter A (area) filter is larger, the better the operation and the higher the cost, and vice versa.
If the V parameter is larger, the faster the machine damage, the lower the price and vice versa.
Therefore, V is the decisive factor for the cost or performance of the dust collector.
Based on the types of dust we choose this coefficient to be suitable for the dust collector to work well. Normally, V selected at 0.6 -0.7 m / min is good performance.
2. Dust extraction rate of suction nozzle and dust particle size.
- Light dust and not moving: 0.25-0.5 m / s
- Light dust and not high spreading speed: 0.5-0.1m / s
- Scattering dust: 1-2.5 m / s
- Dust disperses with high speed: 10m / s and more
The principle that the suction speed must be equal to and greater than the dust dispersion rate
3. Wind speed to transport dust in the pipeline:
Usually 10 - 25 m / s for the dust collector
- Gas: 5-10 m / s.
Mist, smoke: 10-15 m / s
- Light dust (cotton, cotton): 12.5-15m / s
- Dry dust (powder, dry dust): 15-20 m / s
- Average industrial dust: 15.2-20 m / s
- Heavy dust (heavy or moist): 22 - 45m / s or more
4. Mass Efficiency Collector:
- Cartridge Camfil farr filters 99,999% for dust particles 0.5 micron
- DOP test method according to American standard STD 282 or MPPS according to European standard EN 1822
5. Filter performance and dust cake
How much dust formation is appropriate to assess its effects on filtration performance is a matter of consideration. As the dust cake grows on the surface of the filter material, it actually regulates all filtration. Therefore, it is really important to establish and maintain an appropriate layer of dust cake for all filter media.
6. Some other issues:
- The amount of dust per unit volume of air (the concentration of dust in the air). When the dust concentration is high, large dust particles need to apply "central vacuum" vacuuming technique;
System maximum operating pressure;
Humidity, air flow temperature,
- The gas stream has an odor, is acidic or other impurities;
- Whether the collected dust is harmful to humans and the environment, whether or not to accumulate flammable substances;
- Friction and abrasion caused by dust.
- The anti-static problem for the transport system.
II. Several types of dust collector.
1. Type of cyclone
- Low cost, simple manufacturing
- Dust collection in dry form, dust particles> 5 m
- The resistance is almost fixed and not large (250 - 1500N / m2);
- The effect is not dependent on the dust concentration change.
- Poor operating efficiency when dust with fine dust particles less than 5 m in size
- Can not recover adhesive dust
- Dust collection in xiclon takes place under the effect of centrifugal force.
2. Vibration type
3. Cartridge dust collector type
The advantages of this type are:
- Gather all types of dust
- Light weight and low profile
- Energy saving
- High filtration efficiency
- Reasonable price
Part III: Design of dust collection system in accordance with safety standards:
Some safety guidelines for dust collection are as follows:
- Protects fire and dust explosion from production in accordance with NFPA 654
- The blast hole is manufactured according to NFPA 68 standard
- The dust extraction system is grounded against electricity according to NFPA 77
- Design, manufacture, install, operate, inspect and maintain gas, dust and steam transport system according to NFPA 91.
- Comply with NFPA 61, NFPA 69 standard ... with special dust with high Kst (metal dust, pharmaceutical dust, chemical dust).
I. Basics of Dust Explosions
There are five factors for the dust explosion to occur, represented as the vertex of a pentagon.
In which there are 3 most basic factors are:
FUEL (Dust Explosion) + FIRE SOURCE + OXY IN THE AIR
Rated value Kst (bar.m / s) Rated through:
St 0: 0 - not explosive
St 1:> 0 and <= 200 - weak explosion
St 2:> 200 and <= 300 - strong explosion
St 3:> 300 - extremely strong explosion
The parameters of dust explosion, mJ priming energy, oxygen concentration to occur dust explosion under atmospheric pressure are detailed in the above standards. (nfpa.org)
II. Select equipment for dust explosion protection
There are three basic principles for choosing dust explosion-proof equipment:
Reducing Pressure Quenching Isolation
Treatment flowchart when choosing the most suitable explosion proof system:
1. Dust Test:
- Understanding the dust calculation will choose the right equipment
- Kst - dust explosion index.
- Pmax - maximum pressure at which dust is detonated at a favorable condition.
- Ordinary dust has a Kst index of less than 200 (bar.m / s), so we use explosion-proof equipment with Kst = 200 bar.m / s
- With Kst> 200 bar.m / s, customers can self-check dust at authorized organizations, or supply dust to us.
- For dust in pharmaceuticals should choose Kst value 300 bar.m / s for safety.
Camfil Farr has designed for hundreds of customers and also has dust tested on customers, so Farr has gathered hundreds of dust records.
Some common dust in agriculture
2. "explosion vent" hole
Definition: A hole opens from a vacuum cleaner to relieve pressure during explosion.
Diagram for calculation of a blast hole is instructed by NFPA 68
Farr dust collector has a thickness of 3.5 - 4.1 mm so there is no deformation after hole explosion.
3. Deviation Plate For The Railroad
- For the safety of people and equipment at the rail of the explosion, it is necessary to design a shield in front of the explosion.
Design deflection plate according to NFPA 68_2007 standard
4. Safe Pipeline
- For safety when home appliances need a duct called a vent duct
- The maximum vent duct length is calculated using the following formula: (NFPA 68-2007)
5. Fire Extinguisher:
- NFPA 68 allows for a flameless design to dissipate fire when the appliance is placed indoors (sections 6.9 and 10.6).
- The flameless device is independent of vent duct but is not required for hazardous dusty applications such as pharmaceutical dust because it can release smoke in the room.
- Note: Hazardous dusts often go in bag-in-bag out systems.
6. Explosive Extinguisher
- Extinguishing system consists of a detecting device (sensor) and chemical quenching before a dangerous explosion is crushed.
- Blasting pressure will increase rapidly within 50 milliseconds if not quenched, extinguishing systems can detect and react up to 1 millisecond and then extinguish.
- Quenching system meets global standards: ATEX - NFPA69 - CSA
* Cases to consider for using explosion extinguishing equipment:
- Equipment is arranged in the house;
- Dust with a high Kst value or mixture (gas and dust);
- Insufficient space for blasting on design calculation;
- Dangerous dusts cannot be released into the environment;
- There is not enough safe space for the blast hole;
- Fire spreads through the pipeline system.
* Using more than one device can cost more than using an extinguishing system:
ISO 9001- OHSA directive - NFPA69_654 - FM Global
7. Explosion Isolation
Explosive fire isolation system against the spread of fire on the front panel for quick fire extinguishment. These are chemical isolation valves or barriers
* Isolation system by sprinkler system
The system includes sensors (spark / heat / pressure detection type ...), programmable processor (1 zone or many different zones) and water spray.
8. Some Other Important Equipment
- Burst indicators: explosive indicators: each blast plate is usually equipped so that during an incident it stops the whole system.
- Console (PLC): the microprocessor receives the signal from the sensor and immediately gives an alarm signal (in the form of a tone or display indicator) and can turn off the power for the fan.
- Spark detector (spark sensor): light sensor type electromagnetic sensor.
III. Fire and explosion prevention
Removes static electricity from NFPA dust 7
- Considerations for elimination of static electricity (chapter 6- NFPA 77)
Electrostatic priming is eliminated by:
Connect the devices and ground them
The total resistance of the grounding system should be equal to or less than 1 mega ohm (10 ^ 6 ohms).
- The resistance of each part of connecting the devices should be less than 10 ohms.