Problem understanding liquid pressure

[dalin mald]

TL;DR Summary

liquid in mouvement, pressure , Bernoulli equation,

Good morning.

I joined this forum to improve my self and give back what i will learn here for the members of this forum.

I m confused about the notion of pressure and i will explain the point that i don't understand.
since we know that the pressure is the force over surace. and since we know that the liquide pressure is depending to the depth . all is easy now.

so the hard side i did not know is when the liquide starts to move in a long tube for example.
Can you tell me which pressure we mean by the pressure on the tube. i ork in the ship and i always hear that the inlet pressure in fresh water cooler is 4 bars for example , is it the pressure of the up side water or the bottom water on the tube of cooler ?

than what is the pressure we mean in the equation of bernuli. they say the pressure energy exists so i agree because we have a pressure on a surface but is that pressure force is the same on all particuls , why they fix it ?

i really need a help.
i looked

 

[jrmichler]

I think I understand your question, but am not sure. Please post a diagram showing the four pressures in a liquid to liquid heat exchanger, and indicate which ones you want to know about.

 

[russ_watters]

Can you tell me which pressure we mean by the pressure on the tube. i ork in the ship and i always hear that the inlet pressure in fresh water cooler is 4 bars for example , is it the pressure of the up side water or the bottom water on the tube of cooler ?

Just to repeat: we really need to know the system configuration. It's unlikely that at a pressure of 4 bar this has anything to do with Bernoulli's Principle/hydrostatic pressure.

 

[jack action]

Summary:: liquid in mouvement, pressure , Bernoulli equation,

so the hard side i did not know is when the liquide starts to move in a long tube for example.
Can you tell me which pressure we mean by the pressure on the tube.

A tube placed in a horizontal position will always have a higher pressure at the bottom than at the top.

The difference will be $\rho g D$ where $\rho$ is the fluid density, $g$ is the acceleration due to gravity (9.81 m/s²) and $D$ is the tube diameter.

But if we take the example of water ($\rho$ = 1000 kg/m³), the difference is in the order of 10 000 Pa/m or 100 Pa/cm. Thus, for a tube with a 15 cm diameter, the maximum pressure difference is 1500 Pa, which is usually negligible compared to, say, atmospheric pressure (101 000 Pa; about 1.5 % difference). So the pressure is an average value. Just like the velocity of the fluid is an average value as well.

If the difference would be non-negligible, then you would probably not be able to make such simplification to study a liquid in motion.

 

[dalin mald]

thank you friends for help