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Help! Stuck on a pitot tube review problem


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#1 dresherm

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Posted 20 January 2011 - 06:09 PM

Greetings,

I'm studying for the exam in April and I've come across a problem in the Chemical Engineering PE Sample Exam 2nd Edition by Das, Additional Problems, #3.33.

The problem is as follows:

Air is flowing in a 20" ID duct at 220 deg F and 750 mm pressure. A pitot tube positioned at the center of the pipe's cross section shows a manometer reading of 4" water column. The pitot tube coefficient is 0.98. Viscosity of air = 0.22 cP. The air flow rate, expressed in scfm at 60 deg F and 1 atm through the duct is most nearly:...

In the answer section I've understood everything up to the point where they calculate an average velocity. They pulled 0.82 from somewhere and I have no clue where they got it from. I've referenced the Pitot tube section in the Chemical Engineering PE License Review 3rd Edition by Das (full of mistakes by the way) pg. 146, Ex. 4.4 and it says "At this value of Reynolds number, Vavg/Vmax = 0.82" and has a reference of Chemical Engineer's Handbook, 5th ed., Perry p.5-11.

I have Perry's 8th edition. I believe they are referring to p.6-11 in my edition which is part of the topic Incompressible flow in Pipes and Chanels. I don't see any chart relating the Reynolds number to a velocity coefficient. I've researched this for about 20 min online and in several other texts and I can't find what relationship the problem is referring too. It's gotten to the point where I'm passed trying to solve the problem and just want to know what they are talking about.

If someone...anyone... could either let me know what the topic is in Perry's 5th edition that the review manuals are referring too, or give me the name of the relationship they are using I'd GREATLY appreciate it.

Thanks

#2 CbusPaul

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Posted 20 January 2011 - 06:27 PM

On page 5-10 of the fifth edition there is a graph with Vavg/Vmax on the y axis and DVrho/mu on the x axis. Velocity ratio vs. reynolds numbers for smooth circular pipes.

#3 snickerd3

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Posted 20 January 2011 - 06:33 PM

I have the 7th ed of perry, i'm not sure if the table numbers are hte same but in mine figure 10-7 in the measurement of flow chapter has that sort of table you reference.

i looked up pitot tube in the index to find this, fyi. so maybe try looking it up that way?

#4 snickerd3

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Posted 20 January 2011 - 06:34 PM

QUOTE (CbusPaul @ Jan 20 2011, 12:27 PM) <{POST_SNAPBACK}>
On page 5-10 of the fifth edition there is a graph with Vavg/Vmax on the y axis and DVrho/mu on the x axis. Velocity ratio vs. reynolds numbers for smooth circular pipes.


this is exactly what figure 10-7 is labled in the 7th edition

#5 CbusPaul

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Posted 20 January 2011 - 06:36 PM

It appears you can use 0.82 for most Reynolds numbers above 10^4. The line really flattens out.

#6 snickerd3

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Posted 20 January 2011 - 06:44 PM

i vaguely remember my professors mentioning to use a value after the Re number got to that point since it levels off like that.

#7 dresherm

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Posted 20 January 2011 - 07:11 PM

QUOTE (snickerd3 @ Jan 20 2011, 12:44 PM) <{POST_SNAPBACK}>
i vaguely remember my professors mentioning to use a value after the Re number got to that point since it levels off like that.


Thanks to everyone!!!! You guys are awesome. Snickerd3, your figure post put me in the area of Perry's I need to be in. The chart is Fig 10-13 in the 8th edition.

I tried the Index. There is not a direct reference for Pitot tube. Was yours a subtopic of something else?

#8 snickerd3

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Posted 20 January 2011 - 08:49 PM

QUOTE (dresherm @ Jan 20 2011, 01:11 PM) <{POST_SNAPBACK}>
QUOTE (snickerd3 @ Jan 20 2011, 12:44 PM) <{POST_SNAPBACK}>
i vaguely remember my professors mentioning to use a value after the Re number got to that point since it levels off like that.


Thanks to everyone!!!! You guys are awesome. Snickerd3, your figure post put me in the area of Perry's I need to be in. The chart is Fig 10-13 in the 8th edition.

I tried the Index. There is not a direct reference for Pitot tube. Was yours a subtopic of something else?

it was a direct listing for pitot tubes with 3 subtopics. interesting on how editions can change

#9 dresherm

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Posted 20 January 2011 - 09:24 PM

Interesting indeed. Thanks again for the help!




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