Dilute Phase Pneumatic Conveying
Material is suspended in the air while being moved through the pneumatic conveying pipe. In order to keep the materials in the air, dilute phase conveying requires a high velocity of around 3,400-4,000ft/min and 4,800-5,000ft/min for heavier materials. Because of the high velocity at which materials are pneumatically conveyed, dilute phase conveying is best suited for less abrasive materials like flour, saw dust, or coffee grounds. Conveying materials that are too abrasive such as metal shavings can result in wearing down the conveying line. 85% of all pneumatic conveying systems are Dilute Phase conveying.
Dilute phase uses both regenerative and positive displacement (PD) blowers. The term dilute phase notates a higher ratio of air to product mix during conveying. The exact blend of air to product is determined by calculations that are derived from formulas that depend on pneumatic conveying factors specific to the material that is to be conveyed. Distances (horizontal and vertical), number of elbows, and line size are also used in the calculations to determine:
- Blower air requirements
- Blower pressure requirements
From these requirements, a blower size and horsepower can be selected. Typical factors used for calculating pneumatic conveying systems are:
- Fa – Acceleration Factor
- Fd – Total distance in system
- Fe – Total degrees of bends or elbows in system
- Fv – Total distance of vertical pipe in system
Safety Factors are placed on final calculated values of up to 30% to insure that the convey line does not plug.
Dilute Phase Pneumatic Conveying: Pressure
Dilute phase pressure conveying is the process where a positive displacement blower is set at the beginning of the pneumatic conveying line. This blower moves the air through the conveying line while material is then released from a bin via a rotary airlock.
A benefit to utilizing a pressure conveying system is that the material that is moving can be transferred to different lines through the use of diverter valves. For example if you are moving grain to several different silos, you could split the conveying line into two different lines using a diverter valve. Using multiple diverter valves will allow you to distribute the grain to as many silos as you need. The location of the blower can create a lot of heat around the area where material is dropped into the line from the bin. Because of the heat, heat exchangers can be equipped in a pressure system to keep heat sensitive material such as plastic pellets from melting. If the extra equipment isn’t in the budget, then a vacuum conveying design would be your best option, as the blower is placed at the end of the conveying line.
Dilute Phase Pneumatic Conveying: Vacuum
Another option for dilute phase pneumatic transportation is dilute phase vacuum conveying. A difference in the dilute phase vacuum design is that the positive displacement blower is placed at the end of the conveying line. The power from the blower draws in air from the beginning of the conveying line, resulting in air being pulled through the line.
A major asset when using a vacuum system in pneumatic conveying is that the material can be picked up from multiple locations. For example if the material is being brought in by various railcars and the conveying line can be attached to the multiple railcars. While the material can be retrieved from multiple locations using a vacuum design, it can only be distributed to a single location. Vacuum conveying is also used for conveying toxic materials such as lead oxide. Since air is being pulled into the conveying line and not pushed out, should a leak occur in the line the vacuum design won’t allow the toxic material to escape as easily as a pressure system would.
Rotary Airlock Valve
Dense Phase Pneumatic Conveying
Unlike in dilute phase, materials conveyed in dense phase pneumatic transportation systems are not suspended in the air because they are extremely abrasive and/or extremely heavy. Dense phase is also operated at a much lower velocity than dilute; typically around 700-1,500ft/min. Materials that are more than 55-60lbs per cubic foot are good candidates for dense phase conveying. However there are exceptions such as cement. While cement is 95 lbs per cubic foot, it is also a fine material that moves well in an air stream. Similar to dilute phase, dense phase conveying can use a vacuum or pressure system design.
Semi-Dense Phase Pneumatic Conveying
As the term implies, semi-dense phase pneumatic transportation characteristics fall between dilute phase and dense phase. Conveying velocities are typically in the range of 1,500 to 3,500 feet per minute and the material in the convey line is only partially suspended.
Is Dense or Dilute phase pneumatic conveying best?
Many different factors come into play when deciding between dilute and dense phase pneumatic conveying, and a pressure or vacuum conveying system. Luckily the engineers at IAC can help make that decision much easier. Our expert engineers can assess the material that needs to be conveyed, the conditions at which it’s being conveyed in and help you determine which pneumatic conveying design works best.