5 replies to this topic

[SeanK]

Hi to All,

I would like to know how to size the highly viscous lines, what would be the viscosity ranges and how about the velocity limits.

As far as i know i can take the velocity limits for liquids as 1 to 4 m/s, can i use the same limits for viscous liquids as well or any limits?

Please clear my doubts

Thanks
Sean

[ankur2061]

SeanK,

Can you quantify what do you mean by highly viscous by telling us what value of dynamic viscosity are we talking about? Most high viscous flow such as for molten polymers is laminar in nature, with Reynolds number much below 2000. Velcoities are very low (< 1 m/s) in such case and the flow is also called viscous flow. Polymer melt dynamic viscosities are of the range of 250,000-300,000 cP.

Viscous flow in pipes for molten polymers is defined by a modified form of the laminar flow equation of Hagen-Poiseuille. In an early post I have provided the equation for polymer melt flow in pipes. Below is the link:

http://www.cheresour...h__1#entry39676

Hope this helps.

Regards,
Ankur.

[SeanK]

Ankur,

Many thanks for your reply, i am talking about the molten polymer lines and your answers and the links are very useful and cleared my doubts as well.

Still i have a doubt, how to calculate the line size, can i use the Eq. Q=V*A, if so what about the velcity, can i use the Vel.max. as 1 m/s, please clear my doubts.

Regards
Sean

[ankur2061]

SeanK,

In most polymer applications you have pressure drop limitations for the uninterrrupted operation of equipment. For example in textile applications the inlet pressure to the spinning machine / manifold is known. The discharge pressure from the polymer source (polymer pump) is also known. Also in most applications the downstream unit is designed for a given capacity (kg/h or T/day). Using the delta P and the flow rate in the equation provided in the link you can calculate the inside diameter of the pipe and thus the pipe size.

This is normally the method to size polymer pipelines. Assuming a velocity and calculating the line size for a given flowrate can give erroneous results. The pressure drop method based on upstream and downstream pressure is much more reliable in providing the correct line size simply because these parameters are much more readily fixed for a given polymer plant operation.

Hope this helps.

Regards,
Ankur.

[SeanK]

Ankur,

Many thanks for the prompt reply, now i have clearly understood the method to size the lines in the polymer applications.

Regards
Sean

[ankur2061]

SeanK,

I have posted a spreadsheet on polymer line sizing on my blog entry "Polymer Melt Flow through Piping" at "Ankur's _Chemical_Engg_Blog" on Cheresources forum.

Would be happy to get comments on the blog entry from engineers like you in the polymer field.

Regards,
Ankur.