Jack Hilbert, SME, and Julien Mathieu Morel, guest author | Hatch
In this issue, guest author Julien Mathieu Morel, bulk material handling engineer at Hatch, offers his perspective and recommendations on the interactions between a pneumatic conveying system and the downstream particulate filtration device.
When we think about a pneumatic conveying system, we typically think about process pieces such as blowers, piping, and rotary airlocks, but one piece of equipment that’s often left out is the type of filtration equipment at the end of the conveying line. Pneumatic conveying systems and filtration equipment are often put together with less thought or planning and more so with cost as the driving consideration. This can lead to a range of issues including insufficient system performance, excess energy consumption, material damage, equipment damage, operational downtime, or limited flexibility for process or associated system changes.
The selection of the filtration equipment depends on certain factors: The pneumatic conveying system type, the overall layout, and the material receiver type as well as the number of equipment pieces that need to have an extraction point. Here’s a typical description of the most common filtration equipment that‘s used in conjunction with a pneumatic conveying system.
Dust collector. Typically, a dust collector is a self-contained, free-standing unit used when several equipment pieces need to be dedusted at the same time or the location of the final filtration point needs to be located away from the particulate source point.
Filter-receiver. A filter-receiver is basically a receiver unit into which a pneumatic conveying system directly discharges both material and air. The material discharges from the filter-receiver’s bottom via an airlock while an integral dust collector section in the filter-receiver’s upper portion is used to capture the fine particulates before the conveying system air is discharged downstream to atmosphere (in the case of a pressure system) or to the vacuum blower (in the case of a vacuum system).
Bin vent. A bin vent is a dust collector without a hopper that’s installed on a bin, silo, or hopper into which the pneumatic conveying system discharges both material and air.
All these equipment types need to be considered with the same level of importance during the pneumatic conveying system design. In this article, design considerations of both pneumatic conveying systems and filtration equipment will be looked at to show how one can affect the other.
As previously presented in other “Pneumatic Points to Ponder” columns, a pneumatic conveying system design needs to follow design considerations regarding the system type, conveying mode, velocity profile, pressure drop, air-to-cloth ratio, and the conveyed material’s physical and chemical properties with respect to the filtration equipment to ensure that the entire system will operate adequately. All these considerations and design decisions could have an impact on the filtration equipment’s design and performance. The final filtration selection and its installation can lead to pneumatic conveying system performance issues if not implemented correctly. For this reason, each consideration is important and needs to be carefully evaluated before selecting the filtration equipment.
System type. The first consideration is the pneumatic conveying system type. Will you choose a pressure system or vacuum system? The mix of air and material will go through the filtration equipment in both systems. For this reason, the same design parameters that deal with the filtration equipment performance need to be considered while designing filtration equipment for either system.
The primary difference to keep in mind is that in a pressure system, the end of the conveying line — where the venting and filtering take place — will be at near atmospheric conditions because the pressure blower is upstream of the filtration equipment, and the back pressure created on the filter housing is from the air moving through the filter elements. Back pressure is the resistance against the airflow through the filtration equipment. Whereas in a vacuum system, because the vacuum blower is downstream of the filtration equipment, the filter housing must be designed to withstand the conveying system’s full operating pressure.
Conveying mode. The second consideration is the conveying mode in which the material will be transported. This has a significant influence on the filtration equipment design because — depending on the conveying mode chosen — a surge factor over the pneumatic conveying system’s design airflow rate needs to be considered. The surge factor is the multiplier used on the pneumatic conveying system’s design airflow rate. The main reason for using the surge factor is that as the solids-to-air ratio increases and the airflow becomes denser, the more likely the slug (amassed material and air moving in a wave formation, as shown in Figure 1) in the pipeline will grow and result in intermittent discharge of the dense slugs with less dense periods of airflow. When this happens, there’s an instantaneous amount of air and material that discharge into the filtration equipment, which could overload the filter elements, creating a greater-than-anticipated back pressure.
If the filtration method is a remote dust collector, this surge factor concern is mitigated as the primary receiver, such as a cyclone or receiving bin, will dampen the airflow so that there’s less of an impact on the airflow to the remote dust collection point.
Typical surge factors to apply to the system’s design airflow rate are as follows:
- Dilute-phase mode: 1.0
- Two-phase mode: 1.5 to 2.0
- Dense-phase mode: 2.5 to 3.0
Then, that number is corrected for the actual pressure and temperature conditions in the filtration equipment, and the resultant number is the final design airflow rate. Taking the surge factor into consideration will ensure that the filtration equipment’s capacity is adequate for the equipment’s purpose.
As an example, the very common pressure differential truck-to-silo unloading system is very prone to this condition of high instantaneous air volume because the pressurized truck volume is rapidly released as the truck empties. That puff of dust from the bin vent on top of the truck-unloading silo is a sign of pressurizing the filtration equipment when that instantaneous air bubble enters the filter with too low of a filtration area. This is why giving consideration to a pneumatic conveying system’s surge factor is critical to proper operation.
Velocity profile. The third consideration is the velocity of the air and solids leaving the conveying line and the subsequent can velocity in the filter unit. Can velocity is the velocity of the air and solids right before they encounter the filter media, as shown with red arrows in Figure 2. Can velocity needs to be carefully selected based on the application and the material properties. While there aren’t any real consequences of a too-low velocity in a filtration system, a too-high velocity could wear out the final filter-receiver and could also lead to premature wear of the filtration media.
Interstitial velocity is another velocity that needs to be considered when designing and selecting an appropriate filtration unit. Interstitial velocity is the upward velocity of the air and solids in the open areas of the filter section between the filter elements, as shown with green arrows in Figure 2. This velocity takes the filter elements into consideration. Interstitial velocity is calculated by taking the final design airflow rate as previously mentioned but using the cross-sectional area of the filter section minus the cross-sectional area of the filter elements themselves to get the true interstitial velocity between the filter elements.
Similar to can velocity, there aren’t any consequences for having a too-low interstitial velocity. However, if the interstitial velocity is too high, when the filter elements are pulsed during the cleaning cycle, the dust won’t fall into the hopper or bin below but will simply be re-entrained on the filter elements when normal airflow through the elements is returned. This situation can lead to increased differential pressure across the filter elements, which will likely lead to an increase in the cleaning cycle duration, frequency, or both, which can result in a shorter filter life.
The increased back pressure in the filtration equipment can lead to conveying system performance issues as well as particulate emissions to atmosphere if a vacuum or pressure relief valve is being used with a low release pressure setting.
Undersized filtration equipment can choke when the pneumatic conveying system discharges into the material receiver. This situation normally causes a back pressure to the system that wasn’t considered in the design. This back pressure could reduce the pneumatic conveying system capacity if the system was designed at the limit or could cause the pressure relief valves on the material receiver to open, emitting fugitive dust around the material receiver environment. In another case, the back pressure could also influence the bin weight if load cells are installed on the bin, which would influence the overall production accuracy.
Pressure drop. Pressure drop is the pressure difference between the two filter sides (before the filter and after the filter) in an airflow system. The filtration equipment at the end of the system has its own specific pressure drop in regard to which filtration system type is chosen. No matter what system is chosen, the filtration pressure drop becomes part of the total system pressure drop.
Air-to-cloth ratio. The fourth consideration is to calculate the filtration area required for the filtration equipment using the air-to-cloth ratio. Divide the final design airflow rate (the pneumatic conveying system’s airflow rate multiplied by the appropriate surge factor and corrected for actual filtration equipment conditions) by the air-to-cloth ratio to get the total filtration area in square feet. Once this number is determined, the appropriate filtration unit size can be selected.
The air-to-cloth ratio to be used highly depends on the material’s properties and the filtration media type, a couple of which are shown in Figure 3. Typical air-to-cloth ratios to be used are as follows:
- High-pressure, reverse pulse bag filter type: Use 5-to-1 to 10-to-1
- Cartridge filter type: Use 2-to-1 unless the material being handled requires a different ratio
Picking the correct ratio is important. If the ratio is too low, you’ll end up with an oversized filtration unit, having spent more money than necessary on the equipment and installation for a very conservative design. However, if the ratio is too high, you’ll have an undersized filtration unit that can create the type of pneumatic conveying system back pressure previously mentioned, and the plant would likely experience high maintenance with a frequent need to replace the filter elements.
Material properties. The last consideration is the material’s physical and chemical properties, including temperature, friability, abrasiveness, toxicity, and any hygroscopic tendencies. Making certain that the material won’t have a negative effect on the filtration media is mandatory to ensure the filtration equipment’s life expectancy. Different filtration media have different resistance to continuous or surge temperature, and material acid, alkali, and abrasion. The filtration media needs to be carefully selected based on the application and the filter media’s rating with respect to the individual parameters mentioned.
Now that the pneumatic conveying system’s design consideration in relation to the filtration equipment has been given, let’s consider a hypothetical example of a dense-phase pressure system that’s designed at 1,200 cfm with a bag-type bin vent filter on the receiving bin. Applying the recommended dense-phase multiplier (surge factor) of 2.5 to 3.0 to 1,200 cfm, an airflow volume ranging from 3,000 to 3,600 cfm needs to be considered for the filtration equipment’s design and used to calculate the filtration area. To calculate the filtration area, divide each airflow amount (3,000 cfm and 3,600 cfm) by the air-to-cloth ratio of 7.5-to-1 (3,000/7.5 = 400 and 3,600/7.5 = 480). The totals suggest a required filtration area of 400 to 480 square feet for an adequate design. However, if the surge factor isn’t considered, the filtration area selected could be as low as 160 square feet (1,200/7.5 = 160), but this would very likely lead to some performance-related issues.
The filtration equipment is an equally important part of the pneumatic conveying system as is the line charger, air supply, or any other component selection. The filtration equipment needs to be designed accordingly to prevent any issues and reworked once the entire system is installed. The next time you’re designing or reviewing a new system, take a few moments to place a sharper focus on each step, thinking about how a pneumatic conveying system is a whole and not individual parts. By incorporating some of the suggestions made in this article, you can avoid running into later troubles.
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Julien Mathieu Morel (www.hatch.com) is a bulk material handling engineer for Hatch, based in Montreal, QC. He has more than 6 years of experience working exclusively in bulk material handling. He’s a graduate of École de Technologies Supérieure with a bachelor’s in mechanical engineering.
Jack D. Hilbert, PE (610-657-5286) is an expert bulk solids pneumatic conveying consultant for Hatch in Schnecksville, PA. Prior to that, he was the principal consultant for Pneumatic Conveying Consultants. He holds a BS and MS in mechanical engineering from Penn State University, State College, PA. He has more than 45 years of experience in the application, design, detailed engineering, installation, and operation of pneumatic conveying systems.
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