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post #41 of 74
OK, good discussion.

I will have to disagree with the presentation although I appreciate the perspective. I am just more practical in thinking I suppose. If you want me to design and select a pump, I will determine:

System pressure drop curve (Pressure Drop vs Flow Rate)
Pump PQ curve (Total Dynamic Pressure Head vs Flow rate)
Pump power consumption (watts or amps vs Flow Rate)
Use general flow rate guides of 1GPM or better preferred.

Each of which includes critical "pressure" related data. So when someone says pressure is not created, that implies to me that you do not use pressure in design and that it is not important. I agree that you need resistance before pressure developes, but blocks do not also create the pressure by themselves, in the real world we always have some restriction and pressure is a critical measure when defining a pumps performance.

To me.. pressure couldn't be more important in loop design and I will continue to promote pressure thinking rather than promoting flow thinking. New users who are encouraged flow thinking will simply grab the max flow rate specs off a pump and assume that their system flow rate should match. That is of coarse very wrong

I want to help people design a loop and pressure is without a doubt critical to do that. I guess that is why I got wrapped up in the discussion, I feel it may cause more confusion and does not adequately help or provide design guidance for selecting pumps. What happens at the molecular level is not going to help me or anyone select a pump.

Our audience here is a very broad range of users, so presentation and discussing in terms that result in good guidance and help the user select and design loops is MORE important than technical correctness at the molecular level. I suppose these are things Universities do not educate well enough. I have had both University interns and Technical school interns, and I have always had the best employees come from the technical schools. The technical students just seem to focus on what needs to be done where the university grads are off trying to derive a new equation and never finish..lol I guess I have been out of school too long now too, going on 15years since I finished my degree in Civil so I guess I am just old and practical now.

I will disagree that pressure is not energy and that pumps do not create pressure energy. That just doesn't make any sense to me, but I am old and perhaps a bit stubborn anymore.

Cheers!
Martin
    
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post #42 of 74
Thread Starter 
Quote:
Originally Posted by Martinm210 View Post

OK, good discussion.

I will have to disagree with the presentation although I appreciate the perspective. I am just more practical in thinking I suppose. If you want me to design and select a pump, I will determine:

System pressure drop curve (Pressure Drop vs Flow Rate)
Pump PQ curve (Total Dynamic Pressure Head vs Flow rate)
Pump power consumption (watts or amps vs Flow Rate)
Use general flow rate guides of 1GPM or better preferred.

Each of which includes critical "pressure" related data. So when someone says pressure is not created, that implies to me that you do not use pressure in design and that it is not important. I agree that you need resistance before pressure developes, but blocks do not also create the pressure by themselves, in the real world we always have some restriction and pressure is a critical measure when defining a pumps performance.

To me.. pressure couldn't be more important in loop design and I will continue to promote pressure thinking rather than promoting flow thinking. New users who are encouraged flow thinking will simply grab the max flow rate specs off a pump and assume that their system flow rate should match. That is of coarse very wrong

I want to help people design a loop and pressure is without a doubt critical to do that. I guess that is why I got wrapped up in the discussion, I feel it may cause more confusion and does not adequately help or provide design guidance for selecting pumps. What happens at the molecular level is not going to help me or anyone select a pump.

Our audience here is a very broad range of users, so presentation and discussing in terms that result in good guidance and help the user select and design loops is MORE important than technical correctness at the molecular level. I suppose these are things Universities do not educate well enough. I have had both University interns and Technical school interns, and I have always had the best employees come from the technical schools. The technical students just seem to focus on what needs to be done where the university grads are off trying to derive a new equation and never finish..lol I guess I have been out of school too long now too, going on 15years since I finished my degree in Civil so I guess I am just old and practical now.

I will disagree that pressure is not energy and that pumps do not create pressure energy. That just doesn't make any sense to me, but I am old and perhaps a bit stubborn anymore.

Cheers!
Martin

Good discussion indeed.

Yes, the pressure from blocks is a reciprocal pressure rather than an inherent pressure.

I guess I did not explain myself clearly enough, trying to be brief at the expense of intuitive reading. It makes sense to me, but then I am very familiar with the principals.

I will put an edit in the first post that people should read the entire discussion for the full picture.

I'm not gonna carry on the energy argument anymore, if it makes sense to you to think of it that way and you still get the right results then I guess carry on, but should you ever get a few minutes to yourself just look over the units and equations and get back to me smile.gif.

Other than that, all the best bro. xD
 
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post #43 of 74
Thread Starter 
Ive posted an edit, guys, if I've forgotten anything that I needed to be clearer about, please let me know and I will update the edit.
 
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post #44 of 74
Quote:
Originally Posted by YowZ View Post

GingerJohn, compressed air doesn't store energy the way a spring does, if you read back I already explain that the work done compressing a gas is converted to heat. Even if the gas is cooled and some of that energy is lost it is still under pressure and as it expands it cools, that energy is converted to mechanical energy by the turbine is passes through, air tool etc. either way it dissipates some of its heat during expansion, but does not store potential energy strictly speaking. Its ability to do work is no less significant of course.


I think you are confusing the use of the pV=nRT equation here.

Go and grab an aerosol can, is it warm? Is it cold? No, it is at ambient temperature.

Now, is it at ambient pressure? No. Therefore the fluid in it must be at a higher energy level than the atmosphere, but clearly this energy difference is not stored as temperature.

Temperature and pressure can be independant, you can have a air tank at 2bar and 0°C, but you can also have that same tank at 2bar and 30°C, it will just contain a lower amount of air. In the pV=nRT equation you are changing the n and the T to maintain pV. p and T and not dependant.

Now it is true that if you take that aerosol and heat it up the pressure will rise this is in accordance with the pV=nRT equation as n, R and V are maintained, as T increases so must p.


Let us take the pV=nRT equation and do a dimensional analysis. You should have done this during university and it is a great fall back if you ever need to classify a term in an equation.

p=nRT/V

n in mols
R in Joules / (mol Kelvin)
V in m³
T in Kelvin

so p = mol * J / (mol K) * K / m³

Cancel out the mols and K terms

p = J / m³

So pressure can be termed as energy per cubic meter, or energy density.

Pressure is a form of energy.

Quote:
Originally Posted by YowZ View Post

Yeah, dynamic pressure was great fun when I was doing a module of jets and turbofans at uni. Creeping in my equations and causing all kinds of trouble xD.

Yeah, it was fun when I used the knowledge from my masters degree in mechanical engineering to to the thermodynamic and aerodynamic design steam turbines for 3 years.

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post #45 of 74
Thread Starter 
GingerJohn,

very interesting evaluation of pV=nRT. i shall have a think about it in the capacity of energy density, although the first thought that come to mind is that it is an expression of the internal kinetic energy a fluid has. this would make sense in the context of this equation.
I never disputed that fluids store energy, only that they don't store in mechanically the way a spring does.

your point about an aerosol. when you discharge an aerosol, its temperature drops quite considerably, as energy leaves the gas doing work pushing the propellant and aerosolized fluid out.
 
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post #46 of 74
Quote:
Originally Posted by GingerJohn View Post

p=nRT/V

n in mols
R in Joules / (mol Kelvin)
V in m³
T in Kelvin

so p = mol * J / (mol K) * K / m³

Cancel out the mols and K terms

p = J / m³

Thank you for breaking down the equation for those of us unfamiliar with it (although as soon as you explained the terms I remembered hearing it before.)

Yowz please don't modify your original content, or anyone else for that matter please. EDIT: I mean this in the sense that it is useful for those willing to read through it al, i.e me

Martinm210 I like the practical approach for ease of starting a build but I also like the technical correctness because then I can be as exact as I want once I learn.

I love this thread, it has helped quite a bit in my understanding.
post #47 of 74
Thread Starter 
dw, i haven't deleted anything. just added it in as an edit xD
 
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post #48 of 74
Epic discussion!
Got to page 3 - will read more after!
I'm not an engineer, not someone THIS technical, nor a guy that watercools - but this discussion is just intriguing and great for my understanding!
post #49 of 74
for those of us that don't have a degree(in anything), is this correct in laymans terms

if my pump was a car,
flow= horsepower
head= torque

????
post #50 of 74
Thread Starter 
uhhh, flow being more like revs.
 
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