7 replies to this topic

[AFFAN]

Hi friendz

Pressure drop calculation inside tubes and pipes is perhaps the most frequently done calculation in Engineering practices.

But pressure drop calcualtions at inlet and exit of tubes or pipes is some what not common at least bcz in these you would have to take account of turbulence the pipe roughness and shape factors and angles as well.

I am looking for some authentic corelation in this regard, usually as a rule of thumb we add definite no of velocity heads to take in to count inlet and out let pressure drops, but that is not specific it is just for appoximation,

looking forward for some good literature reference

regards

Affan

[AFFAN]

Hi Mates

Still waiting for any Positive Comments

Regards

Affan

[pleckner]

I'm not sure where you coming from. Every fluids problem starts with a beginning and an end. The beginning is usually from a vessel of some type and the end is again a vessel of some type. The beginning is an entrance loss, the end is an exit loss. The determination of these losses are just as common as determining the losses experienced within the pipe/tube.

The use of velocity heads is the correct way to characterize the losses so again I don't know where you are coming from.

If you want references, just look at any book on fluids. You can look at CRANE TP 410, Perry's Chemical Engineering Handbook, etc.

[AFFAN]

I am not too sure about ur communication skills, but to me these need lot of improvement.

look when flue gases from reversal chamber enter in the flue gas path there is sudden pressure drop at the entrance, due to lot of swirling and turbulence, then when these gases move out of it and go in to Eco, again sudden pressure losses due to change in area, shape factors, and angle at whcich it has to move, (this is not at all any vessel) so plz open up ur mind fluid dynamics is not confined to vessels only, also velcocity head is only for rough approximation not exact calculation, pressure drop always take into account inertial forces as well as frictional forces, in case of exchangers we always calculate inlet nozzle pressure drop separatey as well as at exit (no vessel come into action).

u might well guess by now where i am coming from...

Regards
Affan

[JoeWong]

AFFAN,
Well...veloctiry head or K-factor or equivalent length are some common way of getting the pressure drop on entrance of exit of vessel or pipe. These methods have been adopted in many application and believe to provide a good engineering value to engineer.

Your "exact calculation" seem unclear. Does it mean a calculation has considered all factors such as
- interaction between fluid with the pipe wall
- interaction between fluid with the atmosphere
- air humidity, air temperature, air pressure, etc
- temperature changes along the pipe till the exit point
- heat input from solar
- heat input from surrounding heat source e.g compressor, pump, fire heater, etc
.
.
.

To answer this question by considering above factors that i have listed, i believe it will be few PhD thesis...

In real world of engineering, engineer is there to analyse and made good justification & assumption and work out a simple and reliable approximation method. Thus, veloctiry head or K-factor or equivalent length are commonly applied.

If you really think it is necessary from engineer pespective, may be you can consider to engage some specialist or Professor to conduct the detail modeling and/or use CFD.

[AFFAN]

Hi

thks for valueable advises, yes i have made clear by counting the factors which can effect inlet pressure drops.


Any how thks for your time, also it would be valueable if u can comment on Silencer Pressure drops at choke conditions (see my next topic)


Affan

[Art Montemayor]

AFFAN:

I am responding to your request for "positive comments" with the attached workbook. Please study it carefully. I have taken the time and effort to do this because I find the subject a simple one in Fluid Mechanics and one that I think I fully understand. Therefore, I believe it important to make some corrections of what seems to be erroneous engineering logic or bases on your part.

First and foremost, I share Phil Leckner's problem in not understanding why you make the assertion that the entrance and exit losses of fluids are mere approximations and not accurate. They are used throughout all the engineering calculations that I have seen, checked, and reviewed and give good and acceptable results. They certainly are not approximations. In that assertion you are definitely wrong. Empirical values are used for the velocity heads; empirical values are not approximations. They are as realistic and accurate as you can get - well ahead of theoretically-obtained values.

Note the formal derivation of the value for the discharge loss coefficient. It is proven to be exactly 1.0. This has been borne out in the field time-after-time. You are not correct when you state "pressure drop calcualtions at inlet and exit of tubes or pipes is some what not common ". I have been involved with this type of calculation for the past 47 years and have found it to be quite common - and accepted in the professional ranks. You can find that any Fluid Mechanics book or reference will substantiate what I claim. Friction, change of direction, turbulence, and physical shape all play a part in determining head loss in Fluid Mechanics. You are certainly correct in stating that. However, the loss of a fluid upon entering or exiting a pipe or circular conduit is expressed correctly by the material I am presenting in the workbook and it backsup the use of velocity heads as appropriate for head loss calculations in fluid flow applications - whether a vessel is involved or not.

I hope you do not find my position on this subject critical of you or your theory. This is not my intent and, I am certain, not Phil Leckner's intent either. You have requested professional comments on this subject and that is exactly what we have furnished - at our expense and time. We are not trying to compete here. We contribute our knowledge and experience as we are requested in order to possibly help others. If our comments are not found to be in accordance with those of others, we are not guilty of anything but being charitable. We are engineers - and not actors in a theatre that try to out-perform or up-stage each other for applause or personal gain.
 Pipe_Entrance_and_Exit_Losses.zip   238.74KB   558 downloads

[AFFAN]

Hi

Thks for your time and skills.


affan