7 replies to this topic

[bob789]

hi i have designed a shell and tube heat exchanger but i am a bit unsure on how to go about calculating the pressure drop in the tube side and shell side. I have been reading around and came across this equatiion which i have attached as a picture because i couldnt get insert the equation in here. The only term i dont understand is the viscosity gradient term. What is ' minus m ' and what are the viscosity terms referring to, as the bottom viscosity has a subscript 'w'. I am sorry in advance if this is a basic question!


thanks

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[ARAZA]

Hi There,

I'm not sure how you stumbled upon this equation; make sure you understand every equation before you attempted to use it. I've seen the viscosity ratio raise to 0.14, hence the value of -m would be 0.14. See if you get satisfactory results.

ARAZA

[bob789]

What is the viscosity ratio of? What are the two viscosities i am supposed to use in this equation?

The only term i dont understand is the viscosity gradient term. What are the viscosity terms referring to, as the bottom viscosity has a subscript 'w'.

[ankur2061]

bob,

The term (μ/μ_w)^0.14 is known as the Sieder-Tate term and the w subscript indicates wall.

Check out this link for calculating pressure drop in tubes:

http://www.wlv.com/p...abook/ch2_4.pdf

For pressure drop in shell check this link:

http://www.wlv.com/p...abook/ch2_6.pdf

Hope this helps.

Regards,
Ankur.

[ARAZA]

BOB,

Whenever the difference in viscosity is high at each end (hot and cold) of the heat exchanger, this equation is used to approximate presure drop and even heat transfer co-efficient. Liquid viscosity varies greatly with temperature, especially if you have a high viscosity liquid to begin with. When a liquid travels thru' a pipe, pipe wall temperature is greater than the temp. in the center of the pipe. Hence, viscosity of the fluid at wall will be lower than the fluid at the center of the tube.

Hence, w refers to the viscosity at the wall and the other viscosity refer to the center.

Pick up any H-T book such as Holman or Kern which explains this subject in detail.

Hope this helps.

[mishra.anand72@gmail.com]

hai bob

Hw r u??

hw u will find viscocity at wall?? Again calculating wall temperature?? Is viscocity of stream and at wall differs so much?? Hw much effect it will cause in aprroximate pressure drop calculation?? tell me in %.

bye

[Art Montemayor]

Bob:

If you are attending a recognized and reputable engineering university in the UK - as I presume you are - then your professors and instructors are going to not only check your design, but they are going to scrutinize your calculation methodology and references. If you have no reference or identification of the equation you submitted (and I also presume that you don't, since you don't tell us) your calculation results are considered not credible and useless. Araza is alluding to the same point when he states the he's not sure how you stumbled upon this equation.

No professional engineer is going to try to justify the investment and costs involved in the design of a heat exchanger (or any other process equipment) without full proof that the calculations are based on practical, proven, and working equations or relationships. It just isn't rational or logical to expect that. And that is something that students should apply as early in their studies as they can because any time or effort spent on equations that can't be identified as credible is time wasted.

I routinely check and review calculations done by other engineers and when I look at your submitted equation, I see things that I can't trust: there is no documented reference as to who originated it; there is no clear definition as to what it is applicable to; the definitions of the terms used are incomplete. I would have to reject any results obtained with this equation as-is.

Basically, the pressure drop through the tube side of a shell & tube heat exchanger is quite easy to calculate as compared to the pressure drop through the shell side. Why haven't you employed the conventional and well-documented relationships found in the references stated by Araza and Ankur? These are readily available and serve as tested and recognized references. I recommend you follow Ankur and Araza's suggestions.

[bob789]

its out of coulson and richardson vol. 6