Optimization of Gas Cleaning Equipment, Overview
Table of Contents:
Defining the Problem
Let us say you recently installed some wet scrubbing hardware. Now you are focusing on getting that equipment to function as best as possible with your application. Or maybe you have some old equipment that you want to “tweak" to improve its performance and perhaps extend its useful life.
Once equipment is selected, purchased, and installed success often hinges on the optimization of that equipment. Sometimes, the equipment selected does not easily “fit" the process or the process inherently changes its emission characteristics during its operation. In the latter, the air pollution control device, though selected for one condition (say, the maximum emission rate) may not be best suited for other, though "normal" operating conditions. Conversely, a device sized for "normal" conditions may exhibit reduced performance under peak or upset conditions. A sharp reduction in gas volume, for example, may adversely affect the performance of a device that relies upon gas velocity to achieve the desired performance (such as a dry cyclone). Similarly, the droplet separation efficiency of a cyclonic separator or chevron type droplet eliminator may be reduced under certain conditions.
Post Installation Optimization by Equipment Type
Thus, post-installation, air pollution control equipment often needs to be optimized. By "optimized", in the context of air pollution control, we mean that all things that influence the desired performance of the equipment are maximized and all things that detract are minimized. By "maximized" and "minimized" we mean that there will likely be some design or operational parameter that detracts from achieving total 100% performance. Reality adds chaos to most any separation technique. Particles do not always follow expected trajectories. With some particles, during their separation from a gas stream, their contact and interaction can cause size reduction making those smaller particles harder to remove. Gases do not always behave as expected. Liquid does not always distribute and move as either theory or practice predict.
Bounded by the laws of physics, the application of hardware is sometimes like "fitting a square peg into a round hole". Some "sanding and fitting" is often required. In addition, control of the application to which the hardware is applied is really the responsibility of the user. The hardware vendor basically designs and applies that hardware to suit the user's description of the application. The project's success is therefore defined by the proper coordination of the process parameters with the air pollution control hardware. Candid communication between the user and the hardware supplier is thus required.
The reality is that we do not know everything and what we do know is subject to revision as we learn. The optimization of the air pollution control hardware can lead to the success that both the user and the hardware supplier desire. The following chapters provide suggestions as to how to gain the best performance from the hardware you have selected.
Dry Cyclone Optimization
The Dry Cyclone
Of all the "hardware" available to be applied to air pollution control, the dry cyclone often handles the most particulate and performs that task in the simplest of manner. Dry cyclone collectors are often the first device used in a multi-device gas cleaning train. This means the dry cyclone sees the highest concentration and most often, the highest level of erosion and abrasion. As mentioned in the Dry Cyclone chapter (Chapter 4), the target dust that the dry cyclone must separate from a gas stream may also be "friable" (reduced in size as the dust wears on itself and internal cyclone surfaces). Relying on the fact that solid particulate wants to move along a trajectory (unless that path is altered), the mechanical design of a dry cyclone is intended to move that path out of the carrying gas stream for collection. In effect, the particles want to move straight ahead, and the cyclone says "no", you must move in a curved path. Centrifugal force is applied to alter the direction of the particulate towards the vessel wall wherein the particulate moves out of the gas stream. Simple.
A dry cyclones installation is shown in Figure 26.1 (Lundberg). The arrangement shows four (4) cyclones mounted in parallel. The inlet ductwork is to the left of the center of the photo. The discharge ductwork is at the top.
Dry Clone Performance Optimization
Some things can be explored however to optimize the separation efficiency of a dry cyclone.
Some potential problems are as follows:
Dry cyclone. Lundberg.
Importance of Uniform Gas Flow
Optimization efforts to explore may include:
To provide a uniform flow of particulate and carrying gas, the ductwork leading to the cyclone gas inlet should be a straight as possible. Though the installation location may be the final arbiter, if possible, at least two straight duct diameters should be present ahead of the cyclone gas inlet. If the main ductwork is round and the dry cyclone gas inlet is rectangular, any transition should be achieved well before the cyclone gas inlet. Some installations use an eccentric type transition wherein the lower portion of the transition is tangent to the lower portion of the dry cyclone gas inlet. The latter is applied since the particulate tends to flow by gravity more towards the bottom of ductwork than higher up. By using an eccentric transition, the particulate can flow uniformly into the dry cyclone for separation rather than being "kicked" up in the transition.
Minimizing Particle Size Reduction
For some particulate, movement of the particulate in proximity with other particulate can cause a particle size reduction. As the particulate is size reduced, the removal efficiency decreases. Most dry cyclone suppliers will have taken the "friability" (size reduction tendency) of the particulate into account during the sizing of the dry cyclone. Those vendors use empirical data and other experience to alter the geometry of the dry cyclone(s) they provide. During operation, however, conditions can change that affect the particle size. To optimize the cyclone performance, a particle size distribution and gas velocity reading should be taken, and that data is provided to the cyclone supplier. Upon a review of the data, the supplier may recommend changes to the dry cyclone gas inlet. For example, instead of a 90-degree gas inlet, an involute type gas inlet may be suggested. The involute may wrap around the vessel 180 degrees or more thus injecting the gas stream in a less abrupt manner thereby reducing size. They may alternately find that an additional cyclone stage may be needed.
Importance of Vertical Orientation
Dry cyclones in vertical orientation should be truly mounted vertically. This somewhat obvious fact is often overlooked or given settling of the support structure, change over time. The Operation and Maintenance manuals for dry cyclones highlight the need for true vertical orientation. If the cyclone is not truly mounted vertically, the cyclonic gas flow path can be upset. The ascending vortex in a dry cyclone inherently tends to form around a vertical "virtual" axis. If the vessel is not truly vertical, the spinning gases can impinge randomly at the vessel wall thereby upsetting the separation of the particulate from the gas stream. To adjust the position of the vessel shims may be needed at the support lugs or flanges to achieve true vertical orientation. If the application involves hot gases and expansion and contraction produce forces that move the dry cyclone away from vertical, expansion joints may be required in the gas inlet and outlet ductwork. Some installations using hot cyclones hang the dry cyclone vessel from spring-mounted hangers to allow for the expected movement.
Smooth Interior Surfaces
Usually, the supplier makes sure that the interior surfaces of the cyclone are sufficiently smooth for the application. The surface characteristics may range from simple post weld clean up to a polished surface. It is at the interface of the cyclonic gas stream and the vessel wall that inefficiencies can occur. If a dry cyclone vessel has been serviced or repaired, to optimize the performance the interior surfaces should
(if those surfaces are accessible) be brought back to the original level of quality or better. Any appurtenance or rough area should be repaired. Any dents that could impair the smooth motion of the gases should be repaired.
Maintain Control of Dust Level
As the dust level in the dry cyclone dust disengaging hopper (if so equipped) increases, the ascending vortex of the dry cyclone discharge tube can come near the dust. Though the gas stream is spinning, this vortex has a vertical gas velocity component. The vertical component tends to lift the dust back up the gas discharge tube thus reducing the separation efficiency. Typically, the dry cyclone designer allows for an enough gap between the expected highest dust level and the lowest point of the ascending vortex. If the dust level becomes excessive however some of the collected dust can be entrained up and out. To minimize this from occurring, the dust level in any discharge hopper should be carefully regulated. This task can be accomplished in several ways. If the hopper is equipped with a dump type discharge valve (or valves in series) adjustments can often be made to the counterbalances based upon operational experience so that the dust is removed uniformly. If the hope is equipped with a rotary lock, the lock's speed may need to be adjusted. To monitor the hopper dust level, not contact type dust bin monitoring instrumentation may need to be added.
An often-overlooked area that can prevent the dry cyclone from operating in an optimal manner is the effect of the gaskets on particulate build up and discharge. The gasket material should not, in any location, extend beyond the interior vessel wall. If the gasket material is exposed, not only can dust be built up upon the gasket surface and be discharged, but the uniform gas/particulate flow at that point can be disrupted. Some firms optimize the dry cyclone operation by removing any full-face gaskets and replacing the gaskets with rope type wherein the rope is applied within the bolt circle of the flanges. Doing so eliminates the possibility that gasket material may impede the separation efficiency of the dry cyclone. This technique can also be applied to upstream and downstream ductwork.
Minimize Dust Discharge Device Leaks
If a dry cyclone suddenly exhibits a reduction in removal efficiency, the primary cause is likely a leak at the dust discharge device. Air infiltration at this point (assuming the dry cyclone is operated under negative pressure) can cause a drastic reduction in removal efficiency. To check for leakage, if the dust discharge point is accessible a simple smoke test will reveal any inward motion of ambient gases into the dust discharge device. The vertical velocity component of the ascending vortex in the dry cyclone is now supplemented by the vertical velocity of the air stream leaking passed the dust discharge device. If dump or "double dump) dust discharge devices are used, wear can cause leakage. In addition, the dust itself is used to provide a "seal". If the dust hopper has insufficient dust and insufficient seal can result. Sometimes holding the dust discharge valve closed for a period and reducing the counterbalance force and restore the required inventory of dust that provides the seal (refer to the supplier's literature regarding adjusting the dust discharge valve(s)). If the hopper is equipped with a rotary lock, the sliding seals in the lock may need to be replaced or be adjusted. If the lock is variable speed, the rotational speed may require adjustment. Some locks run at excessive speed which not only prematurely wears out the seals but also does not retain enough dust between the lock's internal cavities to affect the required seal. To optimize the dry cyclone operation there should be no inward leakage of air.
Control Static Electricity if Present
A more subtle effect that can reduce dry cyclone performance is static electricity buildup. This so called "piezoelectric effect" is caused by particulate passing over adjacent particulate causing an electric charge to be produced. Not all particulate exhibits this characteristic. Dry cyclone vendors can compensate in their design but can only do so is an accurate description of the particulate is provided during the design or optimization. Sometimes bench scale testing is required. If this charge causes the separation distance between particles to increase, reduced separation efficiency may occur. There exist various techniques to remove this charge but he most basic includes directly ground the dry cyclone vessel. By "directly grounding" it is meant that the vessel is configured with its own, dedicated (not shared) ground. Additional lugs for grounding may be applied to the vessel. Sometimes a grounding probe is added to the dust hopper to remove the static charge. By removing the static charge, the dry cyclone's performance can be optimized.
Adjust for Particle Size Differences
If the particulate is "friable" different dust size characteristics may occur within in the same dry cyclone. The gas inlet, for example, may be populated by large particulate while the zone in the hopper or cyclone discharge may be a mixture of both the captured large particulate plus the smaller, size-reduced particles. To optimize the cyclone performance, the engineering optimization thought process must recognize these size differences and adjust accordingly. The dry cyclone supplier likely has suggestions for optimization based upon size for their specific design.
The following is intended to expand upon and supplement the suggestions that were explored in the Dry Cyclones chapter regarding Operation and Maintenance.
Potential Operating Problems Summary
Some possible operational problems are as follows:
Some optimization techniques to consider are as follows:
Suggestions to explore:
Check the ductwork design both into and out of the device. If turning vanes were used initially, are they still present and functioning? If an inspection reveals disturbances or areas of solids build-up, perhaps a CFD model is warranted. With cyclones, the particulate (dust) tends to channel in a stream that may or may not be the same stream as the carrying gas. If the installation is new, perhaps using longer radius elbows will enhance the flow of gases to and from the device. Often some minor adjustments to the gas flow pattern can improve performance. If dust is building up in the gas inlet ductwork (primarily as the lower portions of the ductwork, cleanouts may be needed to get that dust out of the gas stream prior to the dry cyclone.
Inspect the Hopper If Used
In the zone above the dust in the hopper, the particulate environment might include fine particulate that tends to stay airborne combined with large particulate that wants to settle out. If the dust level can rise excessively, the distance between the captured dust and the discharge zone of the cyclone decreases. The fine particulate might tend to be pulled upward into the discharge of the cyclone(s). Thus, it is important to maintain an adequate separation zone between the dust and the cyclone(s). Various firms offer dust level monitors that sense the dust interface with the gas over the dust. The signal from such a sensor can sometimes be used to modulate the operation of a rotary lock or provide guidance for maintenance personnel to set the dump sequence of a dust discharge trickle valve. In some applications, the zone above the accumulated dust in the hopper is separately ventilated to a shaker or pulse type baghouse for filtration. The externally mounted filtration device discharges the filtered air back into the cleaned air plenum of the dry cyclone. The latter technique is often used where the designer anticipates that a large amount of fine particulate over the large particulate in the hopper can be expected (i.e., where highly friable particulate is suspected). The particles are so tiny that filtration is needed for separation and simple settling cannot be trusted to achieve the desired result.
Use Dust to Augment Sealing
As a corollary to the above, the dust sealing ability of the dust discharge device is often improved by maintaining a level of dust above the discharge device. The manufacturer can provide recommendations as to the depth. As above, a dust/gas interface level sensor can allow the proper control of the dust level.
Check Integrity of All Gaskets
All access doors and flanges should use functioning gaskets and seals. A dry cyclone operated under suction must be fully sealed to prevent ambient air infiltration that whose gas movement could lift dust out of the device and decrease the device's gas cleaning performance.
Double Check the Dust Discharge Device If Fitted
During maintenance inspections, the sealing integrity of the dust discharge device should be checked and repairs, if any are required, should be made. On systems operating under induced draft, a "smoke bomb" type test could be used to determine if air is leaking into the dry cyclone. If you want the best performance, there should be no leaks inward.
Any damage to the body of a dry cyclone should be repaired. During operation, the cyclone body locates the path that the gas stream follows as it spins. Any bump, weld scar, seam etc. can upset this pattern and cause a reduction in particulate removal performance.
With multicyclone type collectors wherein numerous cyclone "modules" are mounted on a tube sheet, any gaps between the cyclone modules and the tube sheet should be eliminated otherwise dust can pass through without being separated. If modules are removed, the tube sheet should be inspected for flatness and any repairs be made prior to replacing the cyclone module.