# What exactly does pressure drop tell you?

1. Mar 27, 2015

### jrivers010

Hello, I am currently a chemical engineering major and i have a question.

What exactly does pressure drop tell you?
i know the equations to solve for pressure drop for various situations (using ergon equation for a bed, or skin friction for pipes, using friction factor, etc etc ( is there more?) ) but I dont exactly know what a high pressure drop or low pressure drop means (physically)

Thank you

Last edited: Mar 27, 2015
2. Mar 27, 2015

### Baluncore

Welcome to PF.

Pressure drop represents a loss of potential energy.
The loss of potential energy could be to kinetic energy where a fluid accelerates, or it may be heating of the fluid due to flow turbulence.

3. Mar 27, 2015

### jrivers010

thx for the welcome .

So high pressure drop mean high energy loss (potential or kinetic ??) but what does that tell you if you have high energy loss?

also, if there is high pressure drop in a system why would the cost of it be high?

4. Mar 27, 2015

### 256bits

That is not exactly what Balancore said.

5. Mar 27, 2015

### Baluncore

Pressure is potential energy. Energy is not lost, it is conserved by conversion to other forms.
If pressure falls then that reduction in potential energy will be converted to say an increase in the thermal or kinetic energy.

It depends entirely on what system you are considering.

6. Mar 27, 2015

### Staff: Mentor

It tells you the power of the pump you need.

Chet

7. Apr 4, 2015

### jrivers010

I am still very very confused lol. still not understanding the physical aspect of it.

So lets say i have a system where i am pumping out water to a home from a water tank.
after doing bernoulli's equation to solve this problem for pressure drop, (assuming i have all other terms already such as height, friction values, etc etc)
what would high pressure drop and low pressure drop tell you?

another example,
in a fluidized bed, using erugon equation, what would the pressure drop here tell you? (in high drop and low drops)

8. Apr 4, 2015

### Baluncore

You must define where you are measuring the differential pressure drop.
Are you considering pressure drop due to flow or also due to change of head?

If there is a high pressure drop in a pipe, then the pipe is too small for the flow. It was cheap to install but it will cost more to pump water through that pipe over time.

If there is a low pressure drop then the pipe is bigger than needed for the flow. You were planning for the future, or could have used a smaller diameter pipe, with a lower initial cost.

9. Apr 5, 2015

### jrivers010

thank you this kinda helped me out a bit. but could you explain a bit more?
like why does low pressure drop indicate the pipe is bigger than needed, or why the smaller pipe would cost more to pump water through it.

also, what do you mean by "change of head" ??
and as for a real example, what about in these videos, exactly what are the pressure drops telling you about the system? (other than the obvious answer that there is a pressure gradient between the points)

Last edited: Apr 5, 2015
10. Apr 5, 2015

### Baluncore

11. Apr 5, 2015

### Staff: Mentor

It sounds like you are a ChE, is that correct? Are you asking within the context of designing a new system, or improving the operation of an existing system (including the possibility of increasing the process throughput rate)?

Chet

12. Apr 5, 2015

### Staff: Mentor

With regard to your Ergun equation question, suppose the reaction in your packed bed reactor involves a gas. Is the rate of a gas phase reaction a function of pressure?

Chet

13. Apr 7, 2015

### jim hardy

The ubiquitous "Crane Manual" is the quintessential introduction to fluid flow.

It's known as "Crane TP410 Flow of Fluids"
it's the orange pamphlet in most engineers' bookcase

try a search engine, and check your school library.

14. May 3, 2015

### L3g3nd4ryN1nj4

Just want to point this out - the pressure drop could also be related to a phase change, like the flashing of a liquid to a vapor. Adding to what Baluncore said, keeping in mind that energy isn't lost is important, because conservation of energy is needed for flash calculations.

15. Jan 8, 2016

### jsmbm7

I work in the sulfuric acid catalyst industry. I think I can give you a better explanation of what pressure drop is because it is something that pops up frequently.

Pressure drop is the change in pressure from one part of a system to another. For instance, if you have a gas like SO2 flowing into a bed of catalyst, the catalyst blocks the path of the SO2. The result is the pressure before the catalyst bed > the pressure after the catalyst bed; this is pressure drop. In this industry, like many I presume, you want to maximize the surface area of the catalyst without loosing too much pressure after the bed (you might need that pressure later on in the process). You could use a fine powder for fantastic surface area, but there would be so much pressure drop because it is hard to push the SO2 through the catalyst. So, the industry standard is a ribbed ring ( imagine a cylinder with a hole through it, but instead of a circular external surface it is a star shape). This provides good surface area and minimal pressure drop.

Why do you want little pressure drop? Well, someone said it before. When you have a lot of pressure drop you need a more powerful pump to get the desired pressure later on in the system (the catalyst bed in this case). More powerful pumps are more expensive.

This is fundamentally what pressure drop is, and you can now take the idea of losing pressure because something is getting in the way of a fluid (catalyst and SO2) and think of the friction of the wall of the pipe "getting in the way" , or pipe joints like elbows creating pressure drop.

16. Jan 15, 2016

### Aleena786

Pressure drop implies the change in pressure from high to low. If we look at fluid moving in glass horizontal tube, a smooth wall wouldn't slow down the particles i.e. 'no resistance' and hence pressure=force exerted/Area of particles would stay same. Now later fluid may hit a rough wall and the particles will slow down due to the added 'resistance' by the wall and hence low pressure i.e. P=F/A will occur. That is change in pressure from initial state.

17. Jan 15, 2016

### Staff: Mentor

This is not correct. Please don't speculate on fluid dynamics until you have had a course in fluid mechanics. To find out the correct situation, google "non-slip boundary condition."

18. Jan 15, 2016

### Aleena786

But pressure drop in a tube would be dependent on velocity, friction factor f and geometry of pipe and fluid properties? The friction factor itself is dependent on characteristics of pipe. e.g. for laminar flow and Newtonian fluid darcy weisbach equation. Would this be incorrect? Thanks.

Last edited: Jan 15, 2016
19. Jan 15, 2016

### Staff: Mentor

This is correct, but not the part about there being no pressure change in a glass tube with a smooth wall.