I am working on a problem where I need to calculate the flow rate of the gas. I was given the pipes diameter, type of fluid, and the back pressure. But flow rate equation has Pressure Drop (delta P) and I have back pressure. So, What is the difference between pressure drop and back pressure? Can I consider back pressure as pressure drop and find the flow rate?
No you can’t consider back pressure (say PB) as pressure drop (say PD) because the two are different. While PD is the pressure loss (or energy loss as pressure is a form of energy) in a flow for example in a pipe flow, the PB is a pressure at the end of the pipe controlled by you.
Consider you are causing a pipe flow by providing higher pressure P1 at the inlet and a lower pressure P2 at the exit. PD (pressure loss or energy loss) in a flow is caused by various agents like sudden expansion, sudden contraction and most commonly by friction between the fluid and the pipe wall. Obviously whatever pressure (energy) you lose you need to replenish that by either increasing P1 or decreasing P2. Generally P2 is varied to change the flow, for example mass flow through a nozzle is controlled by varying the P2. And in such practical applications like nozzles, wind tunnels etc. where flow rate is controlled generally by varying P2, they prefer to call it back pressure (PB) instead of P2. Generally P2 is also called exit pressure, PExit but if it is controllable it is called PB.
So the two are different. Pressure drop is caused by agents inside whereas back pressure is controlled by you from outside.
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