4 replies to this topic

[chemUW]

Hi all,

This is my first post, and it's a question (oh, and I'm still in my first year of Chem. Eng)...

We recently did an experiment using a Double Pipe heat exchanger (shell and tube I believe is another name) and we repeated the experiment for two different oil flow rates (while keeping the steam/water flow the same). The high oil flow rate resulted in a high Overall heat transfer coefficient (HTC) while the low oil flow rate resulted in a low Overall heat transfer coefficient (HTC). What is the reason behind the change in the HTC? The difference between the oil flow rates were 68%, which resulted in a change of 73% in the HTC.

What would have caused this change? Can someone please help as I would really aprreciate it. If you have any further questions, please ask.

Thanks in advance

[Adriaan]

Higher flow rate would give you a greater overall temperature difference I suppose (because the extra oil isn't heated as much). Could also be that increased flow causes increased turbulence which in turn improves heat transfer (thinner layer of non moving oil on the heated pipe, less buildup of pollutants etcetera).

[aliadnan]

Hi ChemUW

Welcome to this Forum. First I want to correct you, Shell and Tube Heat exchanger is not the other name of Double Pipe Heat exchanger. Both are different, in Double pipe heat exchanger, also known as concentric pipe exchanger, you have only two pipes. One fluid passes through the inner pipe while the second fluid passes through the space between inner and outer pipe, know as Annulus. On the other hand, in shell and tube heat exchanger, there is one big shell in which a number of tubes are present. Since your in first year only, so I will not go into the detail of differences between these two exchangers. I am sure you will get the differences and uses of both when you study the basic subject of Heat Transfer.

Now coming to your question, when we increase the flowrate, Reynold's Number increases and higher the Reynold's number higher will be the turbulance and thus it increase the Overall heat transfer coefficent. That is why to get Higher Overall Heat Transfer Coefficient higher velocity through the pipe is favorable but higher velocity has its disadvantages like it causes errosion of pipe.

I hope this helps.

Regards,

Ali

[Art Montemayor]

I can add very little or nothing to an excellent discussion by Ali. This response ranks as one of the best to a basic and good heat transfer query on this forum.

I know from the thorough and detailed description that Ali gave that he also knows the following, but I'll add it for consideration by all. A higher Reynolds Number inevitably leads to a higher film coefficient on the respective side and a higher Overall Heat Tranfer Coefficient ("U") as well. Together with the higher Reynolds Number, be ready to accept a larger pressure drop through that side of the heat exchanger.

Another feature of Shell & Tube heat exchangers is that they - unlike double pipes - have baffles as direction and turbulence promoters. This feature also has a marked effect on an increased heat transfer coefficient - and increases the pressure drop as well. All this leads to the identification of pressure drop as the price one has to pay for increased heat transfer through turbulence.

Double pipe heat exchangers are notorious for very small Reynolds Numbers and are justifiable only in special, small applications. However, they serve as excellent tools for heat transfer investigation and key learnings.

[chemUW]

Thanks for all your replies...that answered my question well...

Thanks again and this is an excellent forum..