When do I choose a pneumatic system over a mechanical system?
When do I use a pressure system versus vacuum system?
What is airlock "blow by air"?
When is my airlock worn out?
When do I specify a pressure vessel versus an airlock system?
When do I specify dense phase versus dilute phase?
Question: When do I choose a pneumatic system over a mechanical system?
Answer: Each type of system has their advantages and disadvantages. Pneumatic Versus Mechanical Systems:
- Most pneumatic systems are totally enclosed dust tight systems. That makes them more environmentally acceptable than are the mechanical systems, which can be dusty and dirty.
- A mechanical system simply could not be used because of the routing required for some conveying systems, whereas, a pneumatic system can convey a product any place a pipe line can be run.
- Mechanical systems require less energy to convey a product and are therefore used when possible for high rate conveying.
- Mechanical systems usually are less expensive than their pneumatic counterpart.
- Practically all powders like wheat flour, portland cement, fly ash, or talc are conveyed pneumatically because they create too much of a mess in most mechanical systems.
Question: When do I use a pressure system versus vacuum system?
Answer: The short answer to the question is that it is possible to go longer distances at higher rates with pressure than with vacuum systems. The reason this is true is because mother nature's limit on a full vacuum is 29.4 inches of mercury (14.7 psi) and a full vacuum is a complete lack of air. But air is what we are using to convey with. The practical maximum vacuum we can go to before the convey rate starts dropping off, or line plugging takes place, is 12.5 to 13 inches of mercury (6.5 psi).
Pressure systems are not limited by mother nature. The limits of pressure available to us are man made. Airlock systems are limited to about 15 psig but some go as high as 50 psig. There is almost no limit on how high pressure vessel systems could go but for practical reasons very few go over 60 psig internal vessel pressure. Plant compressed air systems usually make air available at 100 psig, but air can be compressed to over 1000 psig. Vacuum systems are used extensively in applications where it is not possible or it is difficult to use a pressure system.
A couple of examples are:
- To convey out of a barrel or gaylord the easiest thing to do is to vacuum the product out using a vacuum wand.
- Frequently hopper type rail cars are unloaded by vacuum because the discharge outlet on the car is so close to the ground that the easiest way to get at the product is by a vacuum pickup; then conveyed to a filter receiver where the product can be fed by gravity into a pressure system and conveyed the rest of the way by pressure to the destination, usually a storage silo. If the storage silo is next to where the rail car is being unloaded it may be practical to use a complete vacuum system.
Question: What is airlock "blow by air"?
Answer: An airlock is, or should be, a precision machined device where the clearances between the rotor and the housing are 0.004 to 0.005 inch. The rotor is divided into a number of pockets by the rotor vanes (eight vanes would be typical). Product from the hopper above flows by gravity into each rotor pocket as it passes the hopper to which it is attached. The pockets rotate to the bottom and the product drops out of each pocket in succession into the conveyed air stream and through the convey line. The pockets completes the rotation to the top filled with the compressed convey air which expands into the airlock inlet hopper. Therefore, some convey air is lost into the airlock inlet hopper. I call convey air lost in this manner "airlock displacement blow by air."
Convey air can also be lost a second way, because of the required clearances between rotor and housing as mentioned above. I call the convey air lost in this way, "the orifice factor loss of the airlock." Every airlock has its own orifice factor, which is a function of the clearance amount and the size of the airlock.
Therefore, the total blow by air loss of the airlock is the sum of the displacement loss and the orifice factor loss.
Question: When is my airlock worn out?
Answer: The quick answer is when the convey line plugs or when product will no longer feed into the airlock.
What happens is that the 0.004 to 0.005 clearances originally built into the airlock increase with use due to abrasive wear by the product. As the clearances increase the "blow by air" orifice factor losses increase. Eventually the clearances become so great that too much convey air is lost through the airlock and there isn't enough air left to convey the product, or the "blow by air" loss through the airlock in so great that it prevents product from entering the airlock in the first place. When an airlock is badly worn by abrasive wear it is much better to replace it than it is to repair it.
Products vary significantly in their abrasiveness. All the way from not being a factor to being too abrasive to be conveyed using an airlock.
Experience is the best way of telling how well an airlock will withstand abrasion. The silica content of a product is a good indicator of its abrasiveness. Fly ash for example has a high silica content and cannot be handled successfully by an airlock if long life is expected.
Question: When do I specify a pressure vessel versus an airlock system?
Answer: Systems which use pressure vessels are often referred to as "Dense Phase" systems and systems which use airlocks are referred to as "Dilute Phase" systems. This generalization is true about 75 percent of the time. The way an airlock functions to introduce product to the convey line in a pneumatic conveying system has been covered in the answers to the previous questions: "When is my airlock worn out?" and "What is airlock blow by air?"
An airlock system is a continuous flow system while a pressure vessel system is a cyclic process. First the vessel must be filled with the product to be conveyed, then it must be pressurized up to convey pressure, the product is now conveyed through the convey line, and finally, the convey line must be purged of product and the vessel depressurized to prepare it to receive another batch.
One advantage pressure vessels have over airlocks is that they do not depend upon closely machined parts to operate. This advantage makes them inherently more adapted to handle abrasive products. The inability of airlocks to handle abrasive products has been somewhat overcome in recent years by airlocks constructed of abrasion resistant materials like NiHard castings and Stellite tipped rotors.
The inherent ability of pressure vessels to operate at higher pressures makes it possible for them to convey products longer distances.
As a general rule systems incorporating pressure vessels are more complicated and expensive than systems incorporating airlocks. The continuous flow nature of airlock systems is sometimes an advantage over the batch flow nature of systems using pressure vessels. In airlock systems the average convey rate is the same as the instantaneous convey rate. In pressure vessel systems the average convey rate is usually less than half the instantaneous convey rate. This is because, as previously explained, pressure vessel conveying is a batch type process involving four distinct steps. Actual conveying of the product takes place in only one of these four steps.
There are several pneumatic systems in which degradation of the conveyed product is a factor. When this is the case pressure vessel type systems are the best choice.
Question: When do I specify dense phase versus dilute phase?
Answer: In order to answer that question I think we should first define the difference between dense and dilute phase. Dense phase is a high (dense) product to air ratio and dilute phase is a low (dilute) product to air ratio. If the definition were to end there the question would be at what product to air ratio does dilute stop and dense begin. No two people will agree on the product to air ratio. The best definition I have heard to define the two phases is: Dilute Phase flow is when the air velocity in the convey line is high enough to keep the product being conveyed airborne (non of the product being conveyed laying in the bottom of any horizontal portion of the convey line).
When the air velocity falls below the point where all the product being conveyed is airborne it is known as the "saltation velocity."
At air velocities below the saltation velocity, the product is being conveyed via dense phase flow. At velocities below the saltation velocity the product being conveyed lays for periods of time in the bottom of a horizontal line and the product sometimes flows through the convey line in slugs. This is the case usually with a fluidizable product.
Air will percolate through and flow through the interstices between granules of a granular product. To be successful with a granular product there should not be many fines to fill up the interstices between granules, otherwise air cannot percolate around the granules. The air velocity has to be high enough to push the product on through the convey line while the air is percolating through the conveyed product. It is important to remember that the lower the flow rate of the product through the convey line the lower the convey rate will be.
In order to dense phase convey a product, the product should be fluidizable or percolatable. Practically any product can be dilute phase conveyed.