When selecting a pneumatic conveying system, whether it be for dilute-phase or dense-phase applications, the primary consideration lies in the characteristics of the material being conveyed. This encompasses factors such as particle size and shape, bulk density, moisture content, abrasiveness, friability, cohesiveness, hygroscopicity, static charge, explosivity, toxicity, and melt point, among others.
Pneumatic conveying is a prevalent technique for transferring powders, granules, and other forms of dry bulk materials. However, mastering this conveying technology can pose a challenge. This article aims to address frequently asked questions about pneumatic conveying in the context of dry bulk materials processing and handling facilities.
Regardless of whether your facility currently utilizes pneumatic conveying, you may still have inquiries about this transfer technology. Below are responses to some common questions pertaining to pneumatic conveying.
A pneumatic conveying system facilitates the transfer of powders, granules, and other dry bulk materials through an enclosed pipeline. The driving force behind this transfer is a combination of pressure differential and the flow of a gas, typically air or nitrogen (hereinafter referred to as “air” for simplicity). The fundamental components of the system include a motive air source (also known as an air mover, like a fan or blower), a feeder or similar device for introducing material, a conveying pipeline, a termination vessel (such as an air-material separator), and a dust collection system.
While common materials conveyed by pneumatic systems include flour, cement, sand, and plastic pellets, the technology can accommodate a wide range of chemicals, food products, and minerals. Generally, these materials should be relatively dry, as slurry or paste-like substances are not typically suitable for pneumatic conveying.
How Pneumatic Conveying Systems Operate
Operating a pneumatic conveying system necessitates energy, akin to turning a bolt which requires torque. Torque, measured in foot-pounds, signifies the blend of force (pounds) and distance (feet) required for mechanical work. Similarly, moving materials through a pneumatic conveying system demands energy, albeit in the form of pressure differential (measured in pounds per square inch) and airflow (measured in cubic feet per minute).
The air mover within the pneumatic conveying system modifies the air pressure in the conveying line, creating either pressure or vacuum. The location of the air mover dictates its function: at the start, it pushes air through the system (operating under pressure); at the end, it pulls air (operating under vacuum). By managing the pressure, vacuum, and airflow, the system effectively transports materials.
Advantages of Pneumatic Conveying Systems
In contrast to mechanical conveying systems, which often run in straight lines with limited directional changes, each requiring its own motor and drive, pneumatic systems offer notable advantages. Mechanical systems may be open, generating dust, and possess numerous moving parts necessitating frequent maintenance. They also consume significant plant space.
Conversely, pneumatic systems leverage a compact, small-diameter pipeline that can be flexibly arranged with bends to navigate around existing equipment. Their enclosed nature and minimal moving parts contribute to their reliability.
Disadvantages of Pneumatic Conveying Systems
Despite their advantages, pneumatic conveying systems typically require more horsepower than equivalently sized mechanical systems. Altering air pressure for conveying consumes substantial power and is less efficient than mechanical transfer. For the same transfer rate and distance, a pneumatic system may demand 10 times the horsepower of a mechanical one.
Additionally, pneumatic systems necessitate larger dust collection systems due to the need to separate conveyed material from the conveying air at the system’s end.
Certain materials, such as large-particle, high-density substances like gravel or rocks, and sticky materials like titanium dioxide, can be challenging to convey pneumatically. These materials often lead to pipeline blockages in pneumatic systems, whereas carefully selected mechanical systems might handle them more effectively.
Types of Pneumatic Conveying Systems
Pneumatic conveying systems are categorized into two types based on their operating principle: dilute phase and dense phase. Both types can operate under pressure or vacuum.
Dilute-phase pneumatic conveying, also referred to as stream-flow conveying, can be aptly compared to the experience of walking through a parking lot on a windy day. Imagine the scenario: after work, you’re making your way to your car and are hit by a swirl of dust and dirt lifted off the ground by gusty winds. The intensity of the wind determines the amount of debris that strikes you; faster winds entrain more and even larger particles. Think of the massive debris carried by tornadoes or hurricanes—these storms exemplify dilute-phase pneumatic conveying on a grand scale. Similar to how wind lifts dust, a dilute-phase conveying system uses the velocity of an airstream to pick up and suspend particles throughout the conveying line.
The point where particles initially gain velocity, known as the material feed point, is crucial in the system. Here, the air is at its slowest speed, and the material, transitioning from a static state, must quickly become entrained. The airspeed needed to pick up the material varies based on particle size and density, typically ranging between 3,000 and 8,000 feet per minute (fpm). Additionally, the air mover must compensate for the frictional loss caused by the air and material rubbing against the conveying line’s interior wall.
To conceptualize a dilute-phase conveying system, imagine it operating at high velocity with a relatively low pressure differential. Designing such a system requires meticulous mass calculations (i.e., the ratio of material weight to air weight), taking into account factors like ambient air temperature, humidity, and altitude at the installation site. Moreover, to ensure an optimal air-material mixture, the material must be metered into the system at a controlled rate.
Dense-phase pneumatic conveying can be aptly compared to the process of making sausage, where high pressure is applied to push ground meat into a casing. Ideally, a dense-phase conveying system would propel material with sufficient pressure to travel as a continuous, elongated piece throughout the pipeline, akin to a sausage’s continuous meat filling. However, with dry bulk materials such as powders and granules, this is often unachievable due to the material’s high friction against the pipeline’s inner wall. Instead, the material and air flow through the pipeline in various patterns, including two-phase flow and slug flow.
Despite the existence of various dense-phase conveying systems, they all operate under a relatively high pressure differential with a low air velocity. The most prevalent type of dense-phase system employs a transporter (also known as a blow tank or pressure tank) for batch transfer. In this system, material from a storage vessel gravity-feeds into the transporter. Once full, the transporter’s inlet and vent valves close, and compressed air is metered in to push the material out through the conveying line to its destination. After the transporter and line are empty, the compressed air stops, and the transporter is reloaded. This cycle repeats until all required material is transferred.
Certain dense-phase systems feature supplemental air injectors (or air boosters/assists) along the conveying line. These injectors help clear any plugs caused by insufficient air volume or pressure, thereby avoiding the need to dismantle the line. By reducing the air volume required for reliable conveying, these injectors also decrease the system’s air volume safety factor, which is typically included in the design to ensure adequate air volume. While this extra air enhances reliability, it also increases energy consumption.
Air injectors can also gently restart material flow when material remains in the line after a conveying cycle, which is particularly beneficial for abrasive, friable, or blended materials. Without supplementary air, higher-speed flow can cause premature wear on the line and material-contact components, damage friable materials, and deblend mixtures of different particle sizes and densities. Supplementary air not only prevents these issues during unexpected shutdowns but also offers greater design flexibility for applications where material is intentionally left in the line between cycles.
Air injectors must incorporate fail-safe check valves to prevent conveyed material from entering the compressed-air supply. Material slugs (or pistons) forming within the conveying line can create overpressure conditions, especially before line bends, which can force particles into the compressed-air supply if the injector lacks a check valve.
When considering a pneumatic conveying system for your application, it is important to take into account the various characteristics of your material, such as particle size and shape, bulk density, moisture content, and more. It should be noted that there is no industry standard for measuring the operating phases of these systems, so the presence of certain components does not necessarily indicate whether it is operating in dilute phase or dense phase. However, there are general guidelines you can use to determine the operating phase, such as air pressure and velocity, and material velocity. It’s important to communicate clearly with the supplier when discussing material velocity, as some may use interchangeably with air velocity.
When considering the installation of a pneumatic conveying system, it is important to take into account the system’s size and planned routing, as well as any physical constraints in the plant area. Additionally, the system should be configured to allow for easy maintenance access to components. Other factors to consider include the material transfer rate needed to meet application requirements and the type of conveying system, such as dilute phase or dense phase. Pilot-plant testing can assist in designing the system, allowing for simulations of field conditions and verification of conveying rates and air consumption. It is crucial to define specific criteria for the tests, such as acceptable levels of material attrition, in order for the supplier to accurately size the system for the plant installation.