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Filtration and Vacuum Specialists since 1976

www.vacuumpumpusa.com




A Scientific Review of Dust Collection - Part 6

High Pressure Reverse Fan Cleaning Collectors

Reference material by: Scientific Dust Collectors

Because our newsletters are a service to our valued customers we have decided to share some important and educational information on Scientific Dust Collection. Over the next several months we will be focusing on the use of Dust Collectors. We felt that the extensive information and it's importance in the industry would be very useful in helping our customers make an informed decision on their needs for dust collectors in their businesses. Because the information is extensive we will be spreading it over several months.

Development of a Continuous Cleaning Collector with High Pressure Reverse Air


The next step in the evolution of the equipment was a type of collector that was able to continuously clean during the ongoing industrial process and without any special compartments. Also, this collector style utilized either a large pyramid hopper or trough hopper, and because of the capability to clean during the industrial process, this type of collector became known as an "on-line" cleaning collector as illustrated in Figure 6-1. This fan pulse collector utilizes a traveling manifold that traverses back and forth across the mouth of the envelope filter bags. The reverse air circuit only cleans one bag or one row of bags at a time.

There are some important elements to this design when compared to mechanical shaker collectors. For example, there are no requirements to keep the gags stretched during the cleaning cycle and the filter bags are pressurized from the cleaning manifold. The first important reverse air cleaning principle is developed in this collector design. This principle is the filtering capacity of the bag in the cleaning mode is related to the cleaning flow. Also, on larger collectors, they are able to operate at much higher air to cloth ratios than the shaker collectors that they replaced in identical processes. These collectors were very effective on a wide range of applications.

Image of a Shaker collector


Figure 6-1

Cleaning Process Analysis


If we take a collector with sixteen bags (regardless of cloth area and maintain some limits of filter media permeability) and size it to filter 100 cu. ft. per minute (CFM) per bag, the total flow would be:

100 cu. ft./min. x 16 bags
=
1600 cu. ft./min.

To clean a single bag, we need a reverse flow:

100 cu. ft./min. x 3
=
300 cu. ft./min.

To maintain 1600 cu. ft. per minute through the system, the exhaust fan must draw:

1600 + 300
=
1900 cu. ft./min.

In selecting a fan, the reverse airflow is treated as from another source in the system. If the negative pressure drop in the collector is less than 10 inches w.c., an adjusted slide gate is placed in the reverse air cleaning circuit.

On these types of continuous cleaning designs, the bags that are next to those being cleaned must be blocked for cleaning, the agglomerated dust falls vertically from the targeted bag into the collection hopper. The traversing manifold is powered by a chain driven gear motor which is located in the clean air plenum. In a typical collector, 3% of the collector is cleaned so the cleaning or reverse airflow must be provided at about three times the pressure drop. The power requirements are as follows:


3% x 3 times pressure drop
=
9% power consumption

A variation of this continuous cleaning collector (Figure 6-2) consists of a cylindrical housing and filter bags that are arranged in a radial pattern. This arrangement features a rotating arm with traveling manifolds. Also, the rotating arm extends to the adjoining bags in order to also block their airflow during the cleaning phase.

Application Details

A common application is venting wood floor and general wood dust. The dust loads in the vent exhaust stream are about 10-15 grains per cubic foot. The vent stream is normally positive and the addition of several positive pressure blowers can also vent into one collector. The positive blowers are less efficient with their paddle fan wheels than the backward inclined designs. However, the backward inclined designs must be mounted on the clean air side of the collector. In general, the positive pressure blowers have some advantages:
  • Multiple fans can divide the branches of the vent system with low and high pressure drops to reduce the power draw between the various branches. The system fans energize only when the specific branches are active while back draft dampers prevent the dust from entering the inactive branches.
  • Modifications are easier to make to the branches as demonstrated by the change in airflow which is accomplished by only changing the fan drive (belts and sheave) for the individual branch.
  • The dust is collected at one point for ease of disposal.
  • The conical hopper makes the flow of product to the hopper outlet smoother than the trough or pyramid hopper when comparing the same width of collector.
  • Often these collectors mount to the same structures that support the low pressure cyclone collectors.
  • When upgrading to more efficient fabric collectors, the previous system duct work connects with very little changes.

Figure 6-2

Low pressure cyclones operate with no rotating seal on the cyclone hopper outlet. The hopper outlets are at atmospheric pressure on positive pressure systems; therefore, the dust falls freely into a collector container. Generally, the hopper openings are between 16 and 24 inches in diameter so that the wood dust falls easily into a container and does not bridge across the opening of the hopper outlet.

When fabric dust collectors were initially introduced and applied to the woodworking applications, the fan air unit was located on the clean air side of the collector. An air lock device was required to maintain a vacuum inside the collector.

Conversely, with a positive pressure venting system, a less expensive welded housing is needed and wear on the airlock's wiper blades is low since the fans air pressure is less than four inches w.c. The flow capacity of the collector can be increased by adding more flow through the reverse air fan, which made the positive pressure systems very versatile.

These collectors are especially effective in applications such as grain collection and other similar processes. In many of these systems, the process gas is close to the dew point and the dry bulb temperature, both the system and cleaning fans contribute a drying effect on the dust that is collected. Also, more heat can be added to the reverse air circuit when an additional temperature spread is needed between dry bulb and dew point temperature.

Fan Pulse Collectors

From these reverse air fan collectors, the first major modification was to pulse the cleaning airflow. It was found that almost all the dust would be ejected from the bags during the first tenth of a second that the bag was being cleaned. During the cleaning air pulse flow, the opening and closing of the dampers were usually accomplished by the rotation of the arm on the cylindrical collectors. In Figure 6-3, the reverse air pressure blower is mounted outside on the shell of the collector.

To start and stop cleaning airflow, some rectangular designs had solenoids that opened and closed the dampers. The typical damper opening was for 1/2 second, and the total collector could be cleaned in 3 to 6 minutes. For instance, if the collector had 30 radial rows, the power consumption is calculated to clean the bags as follows:

The reverse airflow requires an instantaneous cleaning flow of nine percent which is the same as the continuous fan cleaning system. The new power requirement can be calculated:

(30 radial rows)
(9%) x (0.5 seconds
= .75%
(3min.) x (60 sec./min.)

The cleaning airflow from the reverse air fan is trying to flow continuously even though the dampers are closed most of the time except during the cleaning cycle. It is a mistake to estimate the cleaning air power consumption by merely noting the horsepower of the motor. The reverse fan pulse cleaning collectors used less cleaning air to clean the bags than the earlier collectors. The fan pulse units had the same advantages as the continuous cleaning (fan) collectors with the additional feature of much lower power consumption.

Figure 6-3

Advantages of Reverse Air Cleaning Collectors

The cleaning flow gradually increases with the reverse fan air cleaning collectors so that the dust leaves the bag at low velocity and gradually increases to a velocity of approximately 10,000 feet per minute. Because the fine dust leaves the bags at the lowest possible velocity, the dust is not subject to the higher cleaning forces. For grain and other food applications, this is preferable.

Disadvantages of Reverse Air Cleaning Collectors

The main disadvantages of reverse air fan pulsing are:

  • Practical capability and manufacturing costs limit the use of fans with both high positive air pressure and at high air flow rates.
  • Damper cleaning arrangements are inherently slow in operation and can be expensive to purchase and maintain.
  • The reversing air fan motor is operating continuously in order to provide instantaneous pulsed air for cleaning.

Air Pump Fan Pulse Variations

The next major improvement in fan pulse collectors consists of increasing the pressure of the cleaning arm to approximately 7 1/2 psig and the use of a positive displacement air pump. This design has different design features (Figure 6-4):

  • The reverse airflow pulsed into 8 to 12 inch diameter diaphragm valves which are able to open and close faster than the dampers.
  • The exhaust velocity exits fast enough from the openings on the rotating arms so that the openings could be placed inches from the clean air plenum. The extra flexibility in the location of the openings allowed versatility in mounting various types of bags and provided easier top access with the clean air plenums.
  • The clean air pumps could be placed on the ground next to the collector with little pressure drop losses because of the high pressure of the cleaning system.

Figure 6-4



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