Temperature and elevation's impact on pneumatic conveying systems
Q: How do temperature and elevation influence pneumatic conveying system design?
When designing a dilute-phase (high-velocity, low-pressure) pneumatic conveying system to convey material from one process to another, the following design parameters are needed: conveying line length (horizontal and vertical), number of bends (elbows and angle bends), pipe size, inlet cubic feet per minute (icfm) or inlet airflow rate, system pressure drop, material bulk density, material particle size, solids flowrate, and pickup velocity.
Typically, you can determine the inlet airflow rate, system pressure drop, and optimal pickup velocity experimentally in a pneumatic conveying system test center. The mathematical models developed to predict the system pressure drop and pickup velocity may not be accurate because of the various unknowns in the formula, particularly the solid friction factor in the models. Oftentimes, the solid friction factor is experimentally determined to make the model work.
How temperature and elevation help determine the inlet airflow rate
Apart from these parameters, temperature and elevation play an important role in determining the appropriate inlet airflow rate at the blower to achieve the required airflow rate at the material pickup point after accounting for the compression ratio, temperature expansion ratio, and the air density effect caused by the elevation. The intake airflow is relative to the absolute conditions of the actual site. As altitude increases, the absolute pressure decreases. The effect of the compression ratio is more pronounced at higher elevations. As air density decreases, the inlet airflow rate requirement increases. That means, to achieve a specific pickup velocity to convey the same material, the inlet airflow rate requirement will be higher for higher-elevation locations compared to lower-elevation locations. So, for a pressure system to achieve a 4,000 fpm pickup velocity and a 5-psi pressure drop (determined experimentally for that bulk solid material) in a 4-inch schedule 10 pipe, we need 396 cfm airflow rate at the pickup point. However, when we include the compression pressure ratio (air is 5-psi compressed) and the temperature expansion ratio (air expands because of the temperature increase caused by the compression heat of 5 psi), the inlet airflow rate requirement (without feeder leakage) in Houston, TX, will be 468 icfm whereas for Denver, CO, it will be 494 icfm when the air temperature is 70°F. The air densities at 70°F (21.1°C) for Houston, TX, and Denver, CO, are 0.075 and 0.062 lb/ft3, respectively. Therefore, the airflow requirement changes because of the change in air density from locations at a lower elevation to those at a higher elevation.
The compression heat in the pressure system reduces the inlet airflow rate requirement. However, in a vacuum system the inlet airflow rate requirement to the blower is higher for the same conveying parameters (pickup velocity and pressure drop) compared to a pressure system. This is because the air density decreases as negative pressure increases below the atmospheric pressure, which, in turn, increases the inlet airflow rate requirement. The inlet airflow rate of a blower for a vacuum system will be higher than a pressure system of equal line pressure because there's no heat of compression from the blower causing air to expand. This expansion reduces the amount of the inlet airflow rate from the blower. In vacuum systems we don’t have the heated air within the transfer, and thus the temperature correction ratio will be equal to 1. For example, to achieve a 4,000 fpm pickup velocity and a 5-psi (or 10 inches of mercury) negative pressure drop in a vacuum system with 4-inch schedule 10 pipe, we need the same 396 cfm at the pickup point. However, when we include the suction pressure ratio (air suction causes a negative 5-psi pressure difference in the system) and no temperature expansion ratio (heat compression occurs at the blower with no effect on the air at the pickup point), the inlet airflow rate requirement (without feeder leakage) in Houston, TX, will be 600 icfm whereas in Denver, CO, it will be 678 icfm when the air temperature is 70°F at either location.
The air requirement for a pneumatic conveying system changes as the temperature changes. For the same elevation, as the temperature changes the air density changes. The air density at the Houston location for the temperatures 50°F (10°C) and 90°F (32.2°C) (representing winter and summer temperatures) are 1.24 and 1.15 kg/m3 (0.078 and 0.072 lb/ft3), respectively. The inlet airflow rate for these temperatures 50°F (10°C) and 90°F (32.2°C) for the Houston elevation are 467 and 470 cfm. Similarly, for the Denver location, the air density for the temperatures 30°F (-1.1°C) and 80°F (26.7°C) (representing winter and summer temperatures) are 1.07 and 0.97 kg/m3 (0.067 and 0.06 lb/ft3), respectively. The inlet airflow rate for these temperatures 30°F (-1.1°C) and 90°F (26.7°C) for the Denver elevation to achieve a 4,000 fpm pickup velocity and a 5-psi pressure drop in a 4-inch schedule 10 pipe are 489 and 495 cfm, respectively. This clearly shows that as the temperature increases, the inlet airflow rate requirement increases, which will reduce the pickup velocity at the existing inlet airflow rate. If this inlet airflow rate pickup velocity falls below the saltation velocity (minimum velocity to keep the solid material suspended in the airstream) that will lead to particle settling in the pipeline, which will eventually end up blocking the pipe. Therefore, for the most effective pneumatic conveying system design, the highest potential operating temperature at your plant needs to be used for proper blower sizing.
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