10 replies to this topic

[Jedax]

Hi everyone. I am a Mechanical Engineer and I'm involved in an Air conditioning Services where I have to design the piping layout. That includes the fittings and other components needed in the whole system. To be honest I am newbie in designing such thing.

The design:
I have Fan Coil Units that are using chilled water as a medium. Now at some boundary of the entire layout of pipes, I begin to examine the flow characteristics of chilled water in a pipe.Please check the attached document.

Question:
1.) Q3 = Q1+Q2 (which is correct). But what is its V3 when D1=D2=D3?


I can't get V4 if V3 is not known. Getting V4 is simply by following the continuity equation. Not that there is no information of pressure here. Please correct the information on that pdf if I missed out something in there. Your help is very appreciated. Thank you

Attached Files

•  sample01.pdf   10.3KB   77 downloads

[Profe]

Hi Jedax

Review the fundamentals of flow, and the continuity equation:

V1A1+V2A2=V3A3=V4A4=Qtotal
I think that is the clue, then V3 = 6 ft/s

Good luck in your work

[Art Montemayor]

Jedax:

Notice how simple the calculations and the logic are expressed in an Excel spreadsheet, as attached to this post.

Note that I get different answers for your average pipe velocity when I assume schedule 40 pipe. You didn't specify your pipe schedule, so I have to assume what is normally used.
 Velocity_in_a_Pipe_Rev2.xls   463.5KB   143 downloads

[Jedax]

Art Montemayor,

Yeap it is a schedule 40 pipe but the sizes are assumed to be as I.D... In the catalogue, it is actually a nominal size but the way I see in your excel sheet, the I.D. is increased from 1" to 1.049". I would not agree on that instead you could have lowered it.

If Q=AV where Q is directly proportional to either A or V. Then how come V3 is increased when D1 ,D2, D3 are equal? I know that the V3 or D3 should be increased to get its desired Q3 but what if we are not putting any expander, can you still say that the V3 is twice of that V1 & V2?

The reason I have been questioning this because on the drawing where the pipe layout shows, I'm going to mark the information on every branch and line of the pipes with their respective velocities, flow rates and pipe sizes. So I need to be accurate on this so that when our pipe fitters are going to start installing the pipes, at least they have an idea where to put those fittings and other components based on the information I have put on the layout.

Thanks guys for all your help.

[Art Montemayor]

Jedax:

You are having trouble in making sense out of simple fluid flow because you are missing out on several basic facts.

All USA commercial or industrial pipe is fabricated with a fixed Outside Diameter (O.D.) – regardless of its Schedule Number. For example, if you read a pipe table – like that found in Crane's Tech Paper #410 – you will find that 1" pipe is really 1.315 inches O.D. and its Schedule 40 has an Internal Diameter (I.D.) of 1.049 inches – as clearly marked in my spreadsheet calculations. The so-called "size" of a pipe (such as the term 1") is merely its NOMINAL SIZE and has no bearing or relationship to the actual dimensions it has. Therefore, you are wrong in stating that the 1.049" dimension should be "lowered". This is totally erroneous thinking as it will result in an erroneous, higher velocity for the water. It is the I.D. that fixes the internal flow area and, consequently, the average fluid velocity.

Now to address your specific questions or comments:

1) "If Q = AV where Q is directly proportional to either A or V. Then how come V3 is increased when D1, D2, D3 are equal?" That's easy to explain. Once again, refer to my schematic drawing and note that although D1, D2, D3 are equal (they are all 1.049" I.D.), the flow going through D3 (V3) is the SUM OF D1 and D2 (in other words, Q3 is TWICE Q1 or Q2). If the flow rate is TWICE and the diameter is constant, then the resulting velocity has to be DOUBLED.

2) "I know that the V3 or D3 should be increased to get its desired Q3 but what if we are not putting any expander, can you still say that the V3 is twice of that V1 & V2?" WRONG! You are not obtaining a "desired" Q3. Q3 is simply the sum of Q1 and Q2. Q3 is FIXED, regardless of what you desire it to be. The velocity of Q3 (5.53 ft/sec) is fixed by the I.D. of the 1" pipe it is flowing in. This has nothing whatsoever to do with the downstream expander. The effect of introducing the expander is not felt until AFTER the expander – NOT BEFORE.

Note that your resulting velocities are listed in my calculation tables. This is the reason for working in an engineering spreadsheet. Everything is clearly shown as well as the resulting calculations. Your resulting velocities make good common sense. The principal reason for introducing the 1" x 1-1/4" expander approximately 2 to 4 feet before the pump inlet nozzle is to lower the velocity of the water going into the pump suction and avoid excessive turbulence there. This is good engineering practice. The 1-1/4" suction line should be a straight, level run into the pump without elbows or obstructions.

You are very correct in taking care to be accurate in your piping diagrams and sizing. The care and attention you give the future installation now will pay back in better and more consistent operation of the pump and related piping.

I hope you can now understand what I have tried to depict in my spread sheet in a detailed and listed manner. If you have any concerns, comments or questions please let us know and we will do our best to explain and try to help out. Please go back and note that I have revised the workbook now that I know for certain that you are planning to use Sch 40 pipe. The velocities shown are accurate with respect to the flow rates you predict.

[Profe]

Hi Art.

Very good and simple explanation

or that speaking spanish people we say "Muy buena y simple explicación, te pasaste"

Saludos Cordiales.

[Jedax]

Art,

Ok I admit that you have more experience than I am regarding the pipe sizes. I know that they are manufactured at fixed O.D. but what I do not know is their actual I.Ds. I am only telling that the nominal size is neither O.D. nor I.D. so when you stated the 1.049" I.D., I reacted because what I know is that the I.D. is below nominal and O.D. is above nominal regardless of its actual sizes. Now I have a better understanding on this particular sizes when you mentioned that Crane Tech Paper #104 that states their actual sizes. Thanks for this one and it helped a lot actually.

Regarding the principle of fluid flow, I do understand the theories but there are things that I need to clarify because I'm into the real world of designing. Ok shall we say that we are removing the expander on that system. Can you tell me the output of Q3 and V3?

Thanks for the help Art, really.

[fallah]

Please note that for sizes from 14 inch and more,the OD is equal to Nominal Diameter.

Regards

[Qalander (Chem)]

Dear Jedax

Hello, Although you have got advises and info(definetly correct, accurate, and very useful), I just somehow add that it is a usual practical observation about most of the commercially available pipelines that sizes above 12 inches diameter are considered "Nominal Dia" term for OD and vice versa for smaller sizes i.e. ID.

That has to be kept in mind always by us the process/design engineers; as this helps a lot.

Best Regards
Qalander

[Art Montemayor]

Jedax:

All I have tried to do is to help you out in what I consider to be a rather simple and direct application. If you feel I am using my 48 years of engineering experience to overwhelm you, I can only assure you that I have no intent on that. While I refuse to apologize for my age and my experience, I can only add that all that you have been asking is answered, repeated, and calculated in an Excel workbook that we have posted for free downloading within the Student Forum on this Website. You will find it at the very beginning of the Student Forum. If you download and read through this workbook, you will find many similar – if not identical problems resolved. You will also find detailed listings of the commercial Pipe Tables, with all the important pipe details carefully listed for your use. Additionally, you will also find a copy of all the "official" Hydraulic Institute water tables that give you the velocity and pressure drop for water in commercial piping. The answer to your problem(s) is directly found in this table.

As I've stated, I have been in the "real world" of designing for 48 years, so I can identify easily with your problem. You now propose to remove the 1x1-1/4" expander in the pump suction line (which I think is foolish, but anyway I'll go along ….). This scenario now becomes what I've added to the Excel spreadsheet as "Schematic & Calculations(2) worksheet. You should now refer to the revised workbook attached to this thread. The answers to your specific new questions are:

1) "Can you tell me the output of Q3 …."
This, again, is easy. Q3 remains the same as before (0.94 L/s) since it still is the sum of Q1 (0.47 L/s) and Q2 (also 0.47 L/s)

2) "and V3?"
Since you have now removed the expander, I can only presume that the diameter of the pump suction line is now also 1", schedule 40 commercial pipe. That being the case, the velocity of the water entering the pump suction is now DOUBLED (5.53 ft/s) – since the flow rate has doubled within the same pipe I.D. (1.049"). As I've stated, all this fluid flow information is explained and detailed in my free Crane Calculations workbook.

I hope this helps you out.

[Jedax]

Jedax:

All I have tried to do is to help you out in what I consider to be a rather simple and direct application. If you feel I am using my 48 years of engineering experience to overwhelm you, I can only assure you that I have no intent on that. While I refuse to apologize for my age and my experience, I can only add that all that you have been asking is answered, repeated, and calculated in an Excel workbook that we have posted for free downloading within the Student Forum on this Website. You will find it at the very beginning of the Student Forum. If you download and read through this workbook, you will find many similar – if not identical problems resolved. You will also find detailed listings of the commercial Pipe Tables, with all the important pipe details carefully listed for your use. Additionally, you will also find a copy of all the "official" Hydraulic Institute water tables that give you the velocity and pressure drop for water in commercial piping. The answer to your problem(s) is directly found in this table.

As I've stated, I have been in the "real world" of designing for 48 years, so I can identify easily with your problem. You now propose to remove the 1x1-1/4" expander in the pump suction line (which I think is foolish, but anyway I'll go along ….). This scenario now becomes what I've added to the Excel spreadsheet as "Schematic & Calculations(2) worksheet. You should now refer to the revised workbook attached to this thread. The answers to your specific new questions are:

1) "Can you tell me the output of Q3 …."
This, again, is easy. Q3 remains the same as before (0.94 L/s) since it still is the sum of Q1 (0.47 L/s) and Q2 (also 0.47 L/s)

2) "and V3?"
Since you have now removed the expander, I can only presume that the diameter of the pump suction line is now also 1", schedule 40 commercial pipe. That being the case, the velocity of the water entering the pump suction is now DOUBLED (5.53 ft/s) – since the flow rate has doubled within the same pipe I.D. (1.049"). As I've stated, all this fluid flow information is explained and detailed in my free Crane Calculations workbook.

I hope this helps you out.

Now that one helps a lot. I appreciate your help Art and others also who shared their knowledge and experience.