8 replies to this topic

[ayan_dg]

I read for the design of Shell & Tube Heat exchanger , the minimum "approach" should be 10 C . What is "approach" here . Whether it is
1) Hot fluid outlet temperature - cold fluid outlet temperature

Or

2) Hot fluid outlet temperature- cold fluid inlet temperature

or

3) Hot fluid inlet temperature- cold fluid outlet temperature

[Art Montemayor]

Ayan:

Didn't you read my reply to your previous post in this Forum?

I thought I took the necessary time and words to explain to you that you have little to worry about or to try to understand when others don't explain themselves clearly to you.

The "approach" in a heat exchanger requires a definition or a reference as to what terminal temperatures define the approach for that particular conversation or discussion.

There is no "universal" and accepted definition to the term approach. That's why it should be defined or explained each time it is used.

Putting it quite bluntly, there is no Universal Court of Chemical Engineering that determines or defines these terms nor is there a Chemical Engineering Armed Agency that enforces the non-existant Court's ruling. And what's more..... there never will be!

[ayan_dg]

Thanks Art Montemayor for your valuable insight. The problem is I was reading about the thumb rules in Chemical Engineering posted in this forum & there I got to know about the practical limitation on tempreature approach on design of shell & tube HX. The author have not explicitly mentioned the defination of temperature approach he is talking to & this is haunting me. If you can tell me about the practical limitation on "tempreature approach" I will be more than delighted. Once again thanks

[harsh]

Hi,
ayan_dg

I think, Temperature approach is the minimum driving force required for carry out the necessary heat exchanging between hot fluid and cold fluid in the heat exchanging network, so that minimum heating and/ or cooling utility is used. 10ºF is the minimum temperature driving force assumed by author. This is used because, practically it is not possible to transfer all the heat from hot fluid to cold fluid or cold fluid cannot receive (or absorb) all the heat which is provided. This I learned from the Book name Conceptual Design of Chemical Process by James M. Douglas. International Edition 1988, Chapter no. 8 Heat Exchanger Network, he talks about First Law analysis and second law analysis....Very interesting book read it. You will come to know. Thanks, Good luck

[Zauberberg]

I cannot agree with the previous statement without knowing the complete set of data required for heat exchanger design and/or rating. By saying only "10F is minimum driving force for transferring the heat", one could think of dozen of different configurations and heat transfer technologies. So it cannot be stated as a "general rule of thumb", as pointed out by Mr Montemayor.

Regarding the literature term itself, my guess is that "minimum temperature approach" is defined by the minimum temperature difference of hot and cold fluid when looking at their respective heat curves for a given heat exchanger service (with known configuration and heat balance). And nothing more than that.

[harsh]

I Agree with you.. and I have mentioned that 10ºF is "assumed" by the author, we can assume 20ºF also. Thanks

[Purpurato]

Temperature difference depends on the type of heating required. This is certainly related to both thermodynamic and practical limitations. From other sources I saw that temperature differences between hot and cold fluid could be

5ºC for criogenic processes
10ºC for exchange at near ambient temperature
20ºC for exchanging with fluids at T>300ºC

I want to know where this rule of thumb come from.

I am evaluating the possibility of heating a process fluid to 390ºC with a thermal fluid utility at 400ºC. I believe this is not practical (following this rule of thumb). However I want to confirm the rule, and to know also what's the reason of it.

[djack77494]

Guys,
You can get too carried away trying to come up with and apply these magical "rules of thumb". Any such rules tend to be applicable to narrowly defined situations. For example, I might have a rule of thumb which says to limit my temperature approach to 10 degrees F. Well, that could be well and good when cooling a liquid hydrocarbon stream with cooling water. On the other hand, if I'm exchanging heat between two low pressure vapor streams, it's very unlikely that I can economically achieve a 10F approach. Here I might change my rule of thumb to 20 degrees F. If I switch to brazed aluminum or plate and frame or spiral heat exchangers, I'll find myself looking for new rules of thumb. There's no magic here, just some common sense thinking. Consider your fluids, consider your equipment and how it performs, and the answer will probably jump out at you. That's my rule of thumb.

[Padmakar Katre]

Dear,
I will explain based on what I know about this term. The term 'Approach' definition varies with the configurations of heat exchanger.
In a single shell of a shell and Tube heat exchanger the temperature approach will be Hot Stream outlet temperature minus the cold stream outlet temperature and the maximum one can have zero (industrially impossible), whereas in case of more than one shells in series the definition will change to hot stream inlet temperature minus the cold stream outlet temperature. Any approach value can be feasible but not economical. Lower the value of approach the higher the number of shells in series and higher the economics. I have taken approach values ranging from 5 degrees to 25 degree while simulating and subsequent designing the shell and tube heat exchangers.
I will suggest you the readings from a Book named "Chemical Process Design and Integration" by R. Smith. Indeed a good book to go through.