Pressure drop is an important physical and financial consideration when working with subatmospheric and vacuum applications. Pressure drop is the loss of line pressure caused by frictional resistance in the flow path. Everything causes some degree of frictional resistance on the fluid flowing, such as a valve, fittings and tubing, and this results in the loss of pressure. By determining how much pressure drop each part causes, you can calculate how much pressure you need to run your process. The lower the total pressure drop of the system, the less gas is needed to run it, which saves you money.
How Pressure Drop Works
Static pressure affects the amount of pressure drop across an Alicat mass flow meter, and this is important to consider when choosing a device for use in sub-atmospheric pressures. Alicat devices calculate flow rates by measuring the differential pressure across a laminar flow stack. Since the flow is laminar, we can use the Hagen-Poiseuille equation to calculate the pressure drop caused by an Alicat device. The equation is notated as follows:
Where: ΔP = pressure drop L = length of pipe η = viscosity of the fluid Q = volumetric flow rate r = radius of pipe π = mathematical constant Pi
Since Alicat mass flow meters measure the pressure drop internally, L and r are constant for each flow device. Assuming the gas viscosity (η) remains the same, the pressure drop increases proportionally to the volumetric flow rate.
ΔP ∝ Qη
In a previous blog post, we explained that decreasing your static line pressure increases the volume of gas flowing through your system and therefore your volumetric flow rate. With this thought in mind, the relationship above shows that increasing volumetric flow (as a result of our decrease in static pressure) also increases pressure drop.
For the sake of simplifying this concept, I am going to pretend temperature remains constant or does not exist in the following example. Say I fill up a balloon that has been blown up to perfectly fit through a tube. I then make a road trip up into the mountains with the same balloon and tube, and attempt to fit the balloon through the tube. Since I am at higher elevation, there is less pressure compressing the molecules in the balloon, resulting in the balloon increasing in size and volume. I can still fit the balloon through the tube, but I have to apply more force since it now takes up more volume and generates more resistance against the tube walls. Pressure drop increases as static pressure decreases.