clarification on race-pressure-restrings

[Iraz]

Bye to all,
I write here because I have a dilemma about a question of pressures/ports and I am a little rusty on the topic (or rather, I was accustomed to seeing "unique" conduct without particular shrinkages or other and now I have a different situation).

I am drawing a system to cool components; usually in the company we pass copper pipes with water inside; There are not so many calculations, indeed, this has been empirical for years.

Today I want to put some problems on the plate I'm asking myself several questions and here falls the stage.

We usually use diam tubes 8 or 10 depending on the encumbrance mainly. today's machine is more complex with ducts that branch upstream and gather downstream.

you want to start with a 3/8" tube, I don't know that prevalence has the pump but it is estimated that it has about 7 pressure bars (?).
the path at a certain point is issued a 3 pipes of diam. 8mm and then gather in a single passage with 8mm tube too.
Afterwards from that only 8-tube goes back to 3 by 8 and then you can 3/8".

what I ask myself is: what is the diameter that "command"?

because if I have a pump it means that I have a constant external load, but the sections change and therefore change pressure and speed of the fluid.
But I can't think that the fluid just burns to the rub, I'll have limits. and here I am doing a thousand paranoias not being efferted in plant matter (although I have also worked in the middle of "hydraulics").

all this because at some point I don't know if I agree (for parts/space issues) switch from 3/8 to a 1/4 shrinkage and then enter the 3 8mm tubes and then into the unique 8 (and then symmetrical return route).

I imagine that many will smile, but unfortunately not handling such calculations for a long time I don't feel confident in evaluating water behaviour to the various strokings.

Hello everyone

 

[paulpaul]

I would say that first would be useful a circuit diagram, with internal diameters, lengths of the various diameters, position of shrinking, curves, valves and other accidentalities. then they would serve the differences, and maybe even the materials of the pipes, if different. of course also the position of the pump.
then the expected (or measured) temperatures in the various points, or at least an average to understand if we talk about 10, 50, 80 °c.
finally pump datasheet, with the characteristic curve.

these are the minimum data to set a rational calculation of the circuit, and determine flow rate, speed and pressure in the various points (maybe with the help of some free program that you also find on the network).

Obviously calculations of this type can be very simple (approximate) or very complex: it depends on the importance of the application!

Hi.

 

[Iraz]

thanks paulpaul for the answer.

below a quick box.
View attachment Schema idraulico.pdfyour comments are correct.
Unfortunately, I don't know the pump.
My problem is to have a maximum estimate of what happens, I don't need to determine the hydraulic losses of the whole route.
what I was wondering is what it takes to have a 3/8" input and then narrow it down to a diam section. 8 mm to the user.

I wanted to present to the customer some details (including this) so as to optimize the finished machine and possibly understand if there are problems caused by the 8-threading (which unfortunately exists and cannot change).

 

[paulpaul]

ok! council to download pressure drop (http://www.pressure-drop.com/): the free version, although with some limitations, allows you to make numerous calculations of this type. the pump characteristics are indispensable, how come you don't have them? Don't you have to choose her? or if you already have it, can't you ask the manufacturer or search the net?

I proceeded this way, with pressure drop or similar program:

1. imposed as fluid water at a certain temperature (even an average of the whole circuit, you don't need what precision: only to have a sufficiently approximate calculation of its viscosity, which then enters subsequent calculations);

2. hypothesize a couple of "tentative" flow values (you still don't know the flow rate, but you're interested in getting the track feature, i.e. loss of load according to the flow rate)

3. for each of the above flow values, retrace your entire circuit by selecting from the list the corresponding circuit element (tubo, enlargement, narrowing, etc.) by inserting geometric characteristics, and calculations with programming the load loss for each flow value;

4. on a plane having on the axis x the flow and on the axis y the pressure bring back the two points found, and you build (with excel) the characteristic (parabolic, polynomial of second degree, there is the appropriate interpolating function) that passes for these two points and for the origin.

5. at this point, on the same chart you carry the characteristic of the pump. the intersection point of the two is your flow rate (and pump pressure).

 

[MBT]

Unfortunately, like it or not, fluids follow the rules, even those that we do not know or do not care to apply.
given a certain pressure available at the entrance (which should be known...) the flow of water is according to load losses.
Load losses are based on the diameter of the pipes (to be more precise than the speed of the fluid, then the flow divided the section) and their trend.
in a way, the snake bites the tail until it finds its balance
In fact, you don't know the scope but you wonder what speed the fluid has to know the load losses... However, speed is the function of the flow you do not know so... You're screwed!
programs like pressure drop that indicated you paulpaul are excellent in these cases.
schematize the circuit, take a flow rate (say the nominal pump) and see that load losses you have.
if necessary repeat the calculation a couple of times with different courses (pressure drop however allows you to calculate a diagram of the losses according to the flow) until you find "balance"
you also need to remove heat. I don't know how much this heat is, but I can suggest you pay attention to the speeds of the fluid. if they are too high (in addition to having unnecessary load losses) you could worsen the heat exchange. vice versa if they are too low you may have stagnation zones with risk of formation of steam bubbles (if the heat load is high)

 

[paulpaul]

mbt quoto: there is extreme ignorance in the field of hydraulic calculations (I don't refer to you iraz, which rightly placed the problem, but to so many people with whom I collaborated and who don't even think about it).
in the field of heat exchangers I have happened cases in which the flow of water is calculated more or less correctly necessary for the disposal of the thermal power (with that deltat), but then there was no concern at all of the load losses, especially those of the circuit: being the iterative calculation, it can be non banal when dealing with plants of a certain complexity, certainly not immediate and rather boring. But then we are surprised if the water "my shore and the pump makes it of Cioc" (locution in dialect parmense to say "no water comes and the pump makes noise (cavity)" - sorry but it makes good)

 

[Iraz]

Thank you very much for the answers.
beyond my technical gaps (despite studies... :frown: ) I find myself with "artisanal" solutions that sometimes work instead are "zoppicanti".

I have just arrived in the company where I am now and on the one hand I have to learn new things but on the other side quesiti being still "virgin" of mind and therefore I realize some details that until now or were not considered or underestimated.

in the current case the characteristics of the pump I do not have and despite having asked I am not supplied...(the pump is of the customer, it works well,...etc etc but no number).

as you point out mbt there are some physical rules that we may neglect or substimate that lead to proper functioning or not.
Now I'm not planning the space shuttle (eh, maybe...) but at least I would like to have two numbers on which to reason and then eventually decide if all my thoughts are just useless complications or if I can optimize the system.

thanks again to all