How to Calculate Backpressure

Backpressure refers to the force exerted on a fluid moving through a closed system such as a pipe or hose that causes it to slow down, usually due to an obstruction such as a bend in the pipe. It is not actually a pressure at all. Calculating it involves the use of Newton's Second Law of Motion, which states that the backpressure is equal to the change in the momentum of a fluid. This means that in order to calculate backpressure you must know the rate of momentum as your water enters and leaves your system.

Things You'll Need

• Paper
• Pen
• Calculator

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Instructions

• 1

  Determine the mass flow rate of the fluid as it enters and leaves a section of solid pipe where an obstruction occurs, such as a bend. The mass flow rate is the mass of the water as compared to time. For example 5 kg/s is the mass flow rate of water entering your pipes at a rate of five kilograms per second.

• 2

  Find the speed of the water as it enters and leaves your system. In other words, how fast was the water moving at both points in the system.

• 3

  Compare the two sets of numbers to each other. You should see a change in the mass flow rate, the velocity, or both. This change in value signifies the presence of a force obstructing the flow of the fluid.

• 4

  Convert your units so that each mass flow rate is in kilograms per second and each speed is in meters per second. This will allow you to find your answer in Newtons, which is the unit that measures backpressure.

• 5

  Set up your equation. The backpressure is equal to the mass flow rate of the fluid entering the system times the speed of the fluid entering the system minus the mass flow rate of the fluid exiting the system times the speed of the fluid exiting the system. So, with backpressure defined as P while mass flow rate is defined as m and speed is defined as v, your equation would look like this

  P=mv(entering)-mv(exiting).

• 6

  Subtract the values of the mass flow rate times the speed at each end of the system to find the backpressure in Newtons. A Newton is a kilogram times a meter per second squared. This is your backpressure.