9 replies to this topic

[ankitg009]

Dear Fellow Engineers

 

Kindly guide me on this.

 

This question is really Baffling me.

 

Say, there is a vessel of capacity 20 m3 in a 20 TPD plant. Now suppose i have to estimate the size of same vessel for a 10 TPD plant. Can i say that, if capacity is reduced by a factor of 2 , the vessel volume should be reduced by the same factor. ?? Is it the right methodology??

 

Secondly , for heat exchangers (Shell and Tube , Air Cooler etc) , can i use the same approach to estimate the heat transfer area.??

 

"Reason for applying this methodology: Both Volumetric flow rate and Heat transfer rate are directly proportional to mass."

 

Thanks in advance.

[S.R.Shah]

It may not be directly in proportional.There are many factors too like Fluid properties,standard equipment sizes and others.For example, For Heat exchanger for viscous liquids cooling like Sulfuric acid , half rate may not exactly the Half and of same design option.It may be more than half.Without complete data one can not comment.

[MTumack]

You also need to look at operational factors; maybe you need 12" of vertical distance for a level switch to operate smoothly, maybe you need more room for nozzles around certain key points in the vessel for instrumentation.

 

I agree with S.R. Shah.

[ankitg009]

@ S.R. Shah: Won't the fluid properties will remain constant , as it is the flow which is varying not the temperature and pressure.?? If we talk about basic design of vessel, doesn't we see only the volumetric flow rate and residence time to calculate the volume of vessel(properties of fluid are fixed at particular temp. and pressure).

 

@ M. Tumack: I completely agree with you.That instrumentss and nozzles will affect the size .

 

On a completely different note, can you help me with these standards like you have mentioned "12" of vertical distance for a level switch to operate smoothly" . Can you refer me some text, where i can get familiar with standards like these?

[Teknas]

Hi Ankit,

 

When you are scaling up or scaling down a plant, the equipment size will not change linearly. I will list out a few reasons to guide you as a start because stating all will be exhaustive and beyond my capabilities:

 

1. Knockout drums : Some minimum distances never change inspite of reduction in flowrate, for example gaps between the LLLL, LLL, HLL and HHLL i.e. Low Low to High High Liquid Levels, so even if you reduce flowrate these gaps will not change and hence the equipment size may not reduce at all. Only your holding time will increase. Usually these are described as minutes of residence time or absolute numbers as 150 mm / 300 mm, whichever is greater. Check in the project philosophy.

 

2. Surge Vessels : Minimum distance from Tan Line / Weld lines are standard and will never reduce with respect to flowrate.

 

3. Exchangers : Since the tube diameters are standardized, the overall hydraulics will be entirely different even if you reduce the total mass flow either on tube side or the shell side. Because to maintain same velocities, overall coefficients etc. the shell geometry can be radically different.

 

But then there are always cases where you get lucky to get a linear pattern....

 

Hope above examples throw some light on your predicament.

[MTumack]

@ S.R. Shah: Won't the fluid properties will remain constant , as it is the flow which is varying not the temperature and pressure.?? If we talk about basic design of vessel, doesn't we see only the volumetric flow rate and residence time to calculate the volume of vessel(properties of fluid are fixed at particular temp. and pressure).

 

@ M. Tumack: I completely agree with you.That instrumentss and nozzles will affect the size .

 

On a completely different note, can you help me with these standards like you have mentioned "12" of vertical distance for a level switch to operate smoothly" . Can you refer me some text, where i can get familiar with standards like these?

 

No particular Standard, just knowledge that comes with experience using particular types of instruments; Float type level switches are kind of finicky and have (in my opinion) kind of mediocre reliability numbers (~95%), and require a certain amount of love, as an example.

 

Each instrument has its own kind of quirks as well, talk with your suppliers and see what they say is their experience with their product types.

[ankitg009]

Thanks Teknas.. Got what you are trying to convey.

 

@ MTumack.. Thanks for adding this to my knowledge.

[Linda1978]

Hi, 

 

maybe as a rough estimation you can divide every thing by using the 2 factor but for sure it is not correct as an accurate design. The best thing that you can do (if you don't want re-designing your system and you are sure about the previous sizes) is to scale down the previous design. 

for the vessel keep L/D constant and repeat the calculation. for heat exchangers you need to modify the size by scaling down (keep required heat transfer area to flow rate constant), however, you may need to adjust the size after because ID and shell length are usually based on standard sizes.

 

good luck

[S.R.Shah]

Thermal design of heat exchanger shall not not be half. It depends on flowr rates and properties. For example reducing flow rate to half amd HTA to half, for viscous fluids flow pattern may change to turbulent to laminar for which Heat transfer coefficient shall reduce.So one has to redesign exchanger.

Edited by S.R.Shah, 09 April 2015 - 01:28 AM.

[gegio1960]

It seems the ancient art of prorating and its meaning/applicability have been lost :-(

Edited by gegio1960, 09 April 2015 - 03:15 AM.