Mechanical Engineering

Created by tomas omalley on 14 June, 2018

Real Application for reducing Pressure Drop?

Hello guys, I have few questions. I am interested to know the real application where the pressure drop has to minimized. Also can you tell me where the pressure drop also plays a major role. Does the heat exchanging capacity increase with the pressure drop or not.

Accepted answer

In diesel fuel injection systems, there are lots of carefully planned pressure drops.

Two examples: discharging the high pressure fuel line for old injection pumps (rotary, or inline). The relief valve/delivery valve has a special shape to make it travel a long distance until on or off. So when the injection is finished, the hi pressure from the line must be ruduced a lot in ored to prevent the injector accidentaly oppening. (relief valve at pump 700Bar, while injector opening pressure 250Bar.. this is to fully open quickly when the high pressure comes). The fuel is cut off by this relief valve that will first travel a bit and close the line, and then will trave around 2-3mm more increasing the volume in the line creating an abrupt pressure drop from 700bar to around 100 or even 50bar. Look at delivery valve action part.

Another use is for diesel common rail injectors. Theese have the needdle held close by the fuel pressure from the ramp (constant pressure in the ramp 1500-3500bar). So the needle has a conical surface near the tip that wants to lift the needle starting the injection, and on the other end is flat and held shut by the same pressure. Force is more than double, so the injector stays shut. The flat side is in the control chamber. This chamber has a very small volume and has a very small inlet hole (from the ramp let's say 2000bar) of about 20-50microns diameter. The outlet of this chamber is closed by a ball bearing held shut by springs. An electromagnet will lift the ball bearing and open the control chamber with a hole of more than 0.5mm diameter. The outlet is much bigger in diameter than the inlet, so in this control chamber can't remain 2000bar even if the inlet is still open, it's not possible to gain pressure as the chamber has a much bigger discharge hole opened. So the pressure drops in the control chamber (where the flat side of the injector needle is), now the needle will move up opening the injector as the constant 2000bar was pressing at the bottom on that conical surface near the tip. so it shoots up opening the injector.
When the ball is reseated closing the discharge hole, the control chambber will rise pressure and reseat the needle tightly shut. In order to close the injector very rapidly, this control chamber has a very small volume and is already filled with diesel at about 10-20bar return pressure. A very small amount of diesel will have to enter in order to create 2000bar again.

2 Other answers

hmmm. . . everywhere. unless you're looking to design a pressure drop, pressure loss is bad. I can think of a water main running through a city, you'd be pretty annoyed if you live on the outside of town and your fossetts tricked do to pressure loss over the entire run. A fast expansion can cause a pressure drop, it also causes turbulent flow eddy current and possible cavitation. Overall, in my mind, if you're losing pressure, you're losing energy (if you're not converting it into another form of energy like pumping uphill to get potential energy later (water tower)), you're losing energy, you're losing efficiency, which means you're wasting power/money etc. As for the heat exchange capacity, I think you should research the Cp charted by NIST-JANAF. it's faniating when the US did in the 60's-80's.

Like Thomas Larson said, pressure drop is something you try to avoid most of the time. Unless you have a very very specific reason for wanting a pressure drop, you are otherwise wasting energy.

For example, in an oil pipeline, pressure drop is your #1 enemy. Imagine the cost of pumping oil in a pipeline over a thousand kilometers with a bad pipe that creates a pressure drop. You would need a huge quantity of pump stations to get the oil to the end of the pipe.

The same goes for water in pipes!