Many production facilities have a significant number of dust collectors. Many have continuing problems with short bag life and low-pressure problems at the farthest points from the central air system. They often run on timers. When they try to run on demand control, they often get extreme short cycling, which causes even more bag problems. On at least half of the dust collectors, most have gauges at the entry; some of the gauges are even operational. Often, the compressed air feed lines are the same size as the connector opening.
Proper operation of dust collectors is critical to minimizing cost and maximizing system effectiveness. There are many types, and sizes and many use a pulse of compressed air to clear the bag or filter. The pulse is usually controlled by a timer which might have an auxiliary demand control. The timers are generally set by the operators to what they believe is appropriate for proper cake removal and bag life.
Pulse jet dust collector
In a pulse jet dust collector, the dust is collected on the bag or fingers, and, when the cake of dust is of appropriate thickness and structure, a pulse or pulses of compressed air hit or shock the bag and knock the cake off. This pulse may sometimes be accompanied by physical shaking and even reverse air flows, depending on design.
When the cake is removed correctly from the dust collector, the system removes dust from its assigned environment and has a normal bag life. When the cake is not removed efficiently, the dust collector doesn’t remove dust effectively from its assigned environment and the bag life can be significantly shortened.
Dust collection system designs specify the compressed air inlet pressure to the manifold and pulse valves necessary for effective dust removal. The pulse valve sends a given volume or weight of air to the bag at a predetermined velocity to strike and clear the cake. The actual amount of weight of air is dependent upon the pulse nozzle being fed compressed air at a predetermined and steady pressure. The dust collector must receive the correct pressure, or close to it, and a steady repeatable pressure level for each pulse, particularly if timers are used to control the pulses. The operator may experiment to find the right timing sequence at a desired compressed air inlet pressure. However, if this pressure varies, then performance will not be consistent or satisfactory.
Installation considerations for proper compressed air supply
Short bag life usually comes from the pulsers hitting the bag when the cake is not ready to flake off or the cake has gone too long between pulsing and grown too thick and heavy to clean effectively. This causes not only short bag life, but very poor performance. There are usually several basic causes for this.
- Incorrect timer settings for the operating conditions. The actual requirement for the optimum timer setting may well change as various product runs change or even seasonally. These settings have to be set carefully to begin with and have to be monitored regularly.
- Lack of sufficient storage or compressed air supply near the inlet manifold to supply the required pulse air without collapsing the inlet pressure. With too low of an inlet pressure, the mass weight of the air pulse is too low, which then becomes ineffective in removing the cake.
- Too small of a feed line to the dust collector entry. This will have the same effect as lack of air supply.
- Too small or an incorrect regulator. This will make it unable to handle the required rate of flow required by the dust collectors.
Pulse jet dust collector operation
Improper compressed air delivery and supply may create an ineffective pulse.
- Use proper line size to handle rate of flow without high pressure loss.
- Use of storage to supply air without pulling down feed to receiver/collector.
- Monitor inlet pressure and drop at pulse.
- Monitor flow.
One very common installation or system situation causes restricted air flow. They occur because, prior to the installation or prior to some operational change, the proper rate of flow wasn’t identified for the dust collector cleaning action. Feed line sizing, regulator sizing, and air supply all require an identified rate of flow. You can’t use average flow rate.
Rate of flow
Flow rate is the average flow of compressed air in ft3/min either required by a process or delivered to the system. Rate of flow is the actual rate of flow of compressed air demand expressed in ft3/min, regardless of duration. Even relatively small air demands in ft3 can have a very high rate of flow, if they occur over a very short time period. Dust collectors have this characteristic.
Sequence controllers can have a very significant impact on the required rate of flow. For example, pictured below is a dust collector system which has six pulsing valves that use 3.5 ft3 over a half-second for each pulse. When this is a problem, appropriate storage and piping can be a very effective correction when properly implemented (Table 1 and Table 2).
The impact of two different rates of flow would show similar differences in regulator sizing if they are used on the feed line flow. The high flow velocities entering the manifold and controls for the pulse valves will create extra pressure loss through the nozzle, affecting the performance of the pulse cleaner. The same effect would show up in air receiver sizing to minimize system and feed line pressure drop if that is a question (Figure 1).