To understand what an unstable conveying zone is, let’s look at a phase diagram. A typical example of this familiar diagram is shown in Figure 1 for a hypothetical material. In the diagram, an air-only line is plotted on a logarithmic scale, making it a straight line. This line is typically used for determining the conveying line pressure drop for air only.
The U-shaped curve on the diagram is one of an infinite series of curves that could be drawn for this material when the conveying system is operating at different feedrates.
As the feedrate (and, hence, material loading) increases, the curve will rise, indicating that more pressure is required to convey more material. The vertical line on the diagram indicates the material’s saltation velocity. Any part of the U-shaped curve to the right of this vertical line is above the saltation velocity, indicating that the system is conveying in dilute phase. The curve portion to the left of the vertical line indicates that the system is conveying in dense phase, moving the material in a pulsating flow of slugs or pistons through the conveying line.
Now look at the portion of the curve labeled “unstable conveying zone.” Here, conveying is in two-phase fluidizable flow — that is, material flows in dilute phase in the line’s upper section and in fluidized flow in the bottom section. The advantage of conveying in this zone is that it requires the least system pressure for conveying at a given capacity. And because the required airflow is slightly less than that for the saltation velocity, the system will require less horsepower and produce less material degradation and line wear.
Yet problems frequently experienced in such systems lead many users to ask, “Can I design a system to operate dependably in this zone?” Problems in unstable-conveying-zone systems often stem from differences in their predicted performance (based on laboratory tests) and actual production performance. Results of tests in a laboratory pneumatic conveying system can be used to produce a curve like the one in Figure 1 for any material. However, the test system’s operation won’t correspond perfectly to a production system’s operation. This is because the tests typically run for a short time and the laboratory system is purged or cleaned out after each test, essentially resulting in batch operation. When these laboratory conditions are duplicated in a plant in a continuously running unstable-conveying-zone system, occasional line plugs, inadequate conveying capacity, or other unsatisfactory operating conditions can result.
But this doesn’t mean that you can’t design a system to operate satisfactorily in the unstable conveying zone. In fact, a material such as cement is almost always pneumatically conveyed in this zone.
Understanding your material’s characteristics is key to designing a system that will operate well in the unstable conveying zone. A fluidizable material will work best in the system. A small volume of air flowing upward through a bed of fluidizable material, such as cement, fly ash, or wheat flour, eliminates the interparticle friction, causing the bed to expand and the material-air mixture to behave like a fluid. The material also retains air after the fluidizing air supply is turned off, remaining in a fluid-like state for a time — this is called the air- (or gas-) retention time. A fluidizable material with a long enough air-retention time will remain in a fluid-like state through the entire conveying system, enabling you to confidently design the system to operate in the unstable conveying zone.
But if your material is coarse and has a uniform particle size and thus can’t be fluidized, or if it has a short air-retention time, it can be difficult to convey in a system operating in the unstable conveying zone. To solve this problem, you can operate the system in batch mode, which requires using a pressure tank to feed material to the system at intervals. This enables the conveying line to clean itself out as the airflow increases at the end of each conveying cycle, allowing you to dependably convey the nonfluidizable material or material with a short air-retention time. Utilizing one of the numerous technologies that exist for distributing air to multiple points along the convey line to prevent the formation of slugs or waves of a length, which could result in conveying line blockages, is another way to move materials in an unstable-conveying-zone system that operates continuously.
As we’ve said many times, one of the very first discussions you want to have when thinking about a pneumatic conveying system for your plant is the discussion with your material! You need to understand how that material wants to be conveyed and how it does not want to be conveyed. The finest system design and best engineered equipment won’t compensate if you’re trying to get a material to be conveyed in a mode or mechanism that’s incorrect.