20 replies to this topic

[ayan_dg]

I am little bit confused on defination of "temperature approach". Consider a specific example a hot fluid enters at 95 C & exit at 40 C whereas the cold fluid enters at 25 C and leaves at 35 C in counterflow shell & tube exchanger.
According to some there are two approaches
Outlet temperature of hot fluid - inlet temperature of Cold fluid= (40-25) C= 15 C &
Inlet temperature of hot fluid- outlet temperature of cold fluid= (95-35) C= 60 C

Whereas some other literature states that it is the difference of the lowest temperature attained by hot stream & the highest temperature attained by the cold stream

That is : Outlet temperature of hot stream- outlet temperature of cold stream= (40-35) C= 5 C

Which one is correct ?

[Art Montemayor]

Without specifically stating the reference temperatures or explaining which terminal is being compared, no one is correct in this case. The matter of stating a “temperature approach” without making reference to which temperatures one is applying the “approach”, is simply sloppy and inept engineering because it can cause errors, confusion, or simply misconception of what is meant. This is the stuff out of which bad communications is made of. It is analogous to mentioning “standard conditions” without referencing specifically what are the so-called conditions. This continues to be a sore point because many engineers simply are lazy, don’t understand what they are talking about, or just plain don’t care if they are understood or not.

This type of stuff should be immediately “snuffed out” and outlawed at the academic level in order to spare engineering students a lot of future grief and problems in trying to understand just what is being explained. Unfortunately, this is not the case and sloppy teaching and text book writing continues to create and foment sloppy engineering. This is the same kind of communications skills that caused the Mars explorer project to spend countless millions of dollars only to discover that the Mars rover vehicle was programmed in the wrong units and rendered useless. Another example of laziness in communications.

Do not waste your valuable time in trying to believe that there is an “International Standard” on the definition of temperature approach – and that it is respected and practiced by 100% of all engineers. It doesn’t exist – and even if it did, many would not pay the slightest attention to it. Always work under the concept that you must demand a specific explanation or definition of what other engineers mean when they throw such verbiage at you – things such as “standard conditions”, acronyms, code assertions, etc., etc. Always demand specifics when you don’t know or don’t understand. It is not a crime nor a sin not to know what is in other people’s minds. If they want to communicate with you, it is their obligation to see to it that you receive the accurate and complete information. If you don’t receive it, then ask for it.

There are two temperature terminals in each normal heat exchanger. There can be more. There are also parallel flow and counter-current flow. But the point is that we can’t know for sure which “approach” is meant unless we are told.

[djack77494]

Bad communication is the bane of engineering, science, and all fields where precision and exactness are requirements (i.e. life). It is easy to avoid if you will discipline yourself to mean what you say and say what you mean (or write). All technical people should be instructed on the importance of this simple principle.
Just my humble opinion (and vast amounts of personal encounters with sloppiness),
Doug

[siretb]

As already stated there is no universal, general definition of "temperature approach".
It often makes sense, to use the minimum temperature difference between the cold stream and the Hot Stream.
In theory you have to check everywhere along the heating/cooling curve.
In practice, very often the minimum temperature approach will occur at either end. But not always.
So in your exemple case an educated guess would be to use 15°. Since the hot-inlet <-> Cold outlet difference is much greater than the cold-inlet <-> hot outlet T difference it is likely that the mimimum temperature difference alond the heat curve will be 15°

Although you posted more than one and half years ago, I'd like to answer your question. Please refer to the definition of " Temperature Apporch":

Page 106 of "Fundamentals of heat exchanger design" By R. K. Shah, Dušan P. Sekulić: "Temperature Apporch in counterflow heat exchanger as Absolute{(Temperature hot side inlet - Temp. cold side outside),(Temp.hot side outlet - Temp. cold side inlet)}=ABS{(95-35), (40-25)}=15"

[shan]

The approach temperture for counter current flow heat exchanger is 40-25 = 15 ^oC
The approach temperture for co-current flow heat exchanger is 40-35 = 5 ^oC

Please notice that it is not necessary that approach temperature is at the hot end/cold end of a counter flow heat exchanger. If a phase change is involved during the heat transfer process, the approach point may be in the middle heat exchanger some place. As shown on the attached typical steam generator curve, the approach point is B rather than highest temperature point A or lowest temperature point B.

Attached Files

•  Heating Curve.xls   22KB   459 downloads

[fallah]

shan:

The minimum temperature approach is considered to avoid temperature cross,and in any case (cocurrent/countercurrent) would be as:


Outlet temperature of hot stream- outlet temperature of cold stream

[shan]

fallah:

The first potion of your statement is correct but the second potion and the formula are incorrect. As indicated in the attached Chart 4, although Outlet temperature of hot stream - outlet temperature of cold stream = 0 in a counter current flow heat exchanger, there is no temperature cross at all.

The approach temperature is the minimum temperature difference between hot stream and cold stream in a heat exchanger.

Attached Files

•  Heating Curve.xls   25KB   252 downloads

[fallah]

shan:

Contrary to your thinking,your explanation confirms my statement:

You mentioned in chart 4 approach=0 and there is no temperature cross.It is right and usually 5 C is considered as minimum approach for e.g. S&T heat exchanger.

Your second statement "The approach temperature is the minimum temperature difference between hot stream and cold stream in a heat exchanger" also confirms my above equation to define approach,because minimum temperature difference in co/counter current flow would be between outlet temperatures of cold and hot streams.

[shan]

fallah

Then, how will you explain attached Chart 5 and Chart 6? There is no temperature crossover even if Hot Outlet Temperature – Cold Outlet Temperature < 0 in Chart 5. There is temperature crossover although Hot Outlet Temperature – Cold Outlet >0 in Chart 6. Also, as shown on Chart 6, the miminum temperature difference is not always at outlet of cold and hot streams.

Attached Files

•  Heating Curve.xls   29.5KB   219 downloads

[fallah]

shan:

A temperature cross situation is: "where the cold stream leaves at a temperature higher than the outlet temperature of the hot stream", thus contrary to your statement about chart 5 there is temperature cross in this chart.

In the charts you have attached to your post,the horizontal axis should indicate the "exchanger length" not "duty".Based on this,the situation presented in chart 6 couldn't be acceptable,and two streams won't being interrupted.

[shan]

fallah:

Your definition of temperature cross is for co-current flow heat exchanger. If there is no problem even if the temperature cross situation happens in the exchanger as shown on my Chart 5, why do you need to concern it?

Chart 6 is typical steam boiler curve. The horizontal axis is "Duty" or "Duty Transferred". The horizontal potion is cold stream (water) at the saturation condition (duty increase without temperature change).

Truth is still truth. It doesn't matter you accept it or not.

[fallah]

shan:

My definition about temperature cross occasionally is applicable for countercurrent situation.

I have no concern about temperature cross. Pure (actually almost pure) countercurrent service results in temperature cross situation.

Horizontal axis should be "exchanger length".

I agree with your statement as: "Truth is still truth".

[shan]

fallah:

The whole discussion here is about approach temperature (>0) to avoid heat exchanger temperature cross. If you don't care about temperature cross, you should not jump in the discussion in the first place.

As indicated in the attached Fig 9-2 from GPSA, temperature vs. duty chart is a common practice for a heat exchanger analysis. If horizontal axis is "exchanger length", what is the length? Shell length or Tube length? What should you do for a multi- pass heat exchanger? How should you take length for a Plate Frame heat exchanger?

Of cause, you have the freedom to make any charts you want and to define any terms you like.

Attached Files

•  20090923112829551.pdf   94.78KB   355 downloads

[fallah]

shan:

The discussion is about confusion over "temperature approch" and i think every body should be allowed to jump in the discussion to submit his/her viewpoint.

When we present some charts for general discussion about "temperature Approach" or temperature variation in heat exchanger tube/shell length,as mentioned e.g. in Kern/Holman heat transfer books, usually the horizontal axis indicates exchanger length.

[djack77494]

The use of the term "temperature approach" is purely a matter of definition and convention. I have always heard the term being used to describe the difference between the two terminal temperatures; i.e. Hotside T out - Coldside T out. This seems to be a conventient definition and is useful when dealing with certain ideal situations. For example, you can say that a positive temperature approach is required for a cocurrent exchanger. Most S&T exchangers are NOT ideal; they are neither cocurrent or countercurrent. Temperature crosses are theoretically possible, but usually very difficult (read impractical). Positive approaches of about 10 degrees F (+ or -) are convenient limits that I mentally apply in thinking about S&T exchangers. Sometimes smaller or larger approaches may be more practical, but 10 degrees is my "alarm point". For air coolers, the approach would be higher because it's more difficult to reject heat to air than (say) cooling water. If you don't like this approach (pun intended) or if you prefer an alternate definition, kindly explain your reasoning, if only for my benefit.

[kkala]

Page 106 of "Fundamentals of heat exchanger design" By R. K. Shah, Dušan P. Sekulić: "Temperature Apporch in counterflow heat exchanger as Absolute{(Temperature hot side inlet - Temp. cold side outside),(Temp.hot side outlet - Temp. cold side inlet)}=ABS{(95-35), (40-25)}=15"

WATER IS NOT CLARIFIED, UNLESS IT GETS TURBID FIRST
(Turkish proverb)

A real interesting hot thread, noted by me after some delay. I will also need some more time to "digest" it!

Anyhow, in my work we use the term "approach" according to above definition. Term "Absolute" is better expressed as "Minimum" in my opinion. A value of 5 0C - 10 0C is recommended as minimum for an "economic" exchanger, but this is not a "must". Of course process may dictate a higher value.
In case of co-current flow (where no real possibility of temperature cross) it is the temperature difference of outgoing fluids.

Edited by kkala, 10 January 2010 - 05:57 AM.

[Socratesofeast]

Very useful and interesting thread, Thanks for contributions

[sheiko]

I agree with Art, Fallah and Doug.

[Raj Mehta]

Its a very confusing topic or I guess it has been made confusing by various view points of few professionals. 
 
I feel Kkala has summarized the temperature approach part in his last post. 
 
I am attaching my learning & understanding about the temperature cross, which was also a reason of confusion.

 

What I feel is temperature cross should be avoided because the cold fluid at one point starts behaving as a hot fluid i.e. initially the moment when the cold fluid was taking in the heat, after it enters temperature cross region it starts behaving as a hot fluid and now instead of taking in the heat it starts giving of heat because it is  at a higher temperature now. 

 

Kindly correct me if I am wrong. 

 

Attached Files

•  Temperatrure cross.jpg   13.4KB   69 downloads

[thorium90]

Oh, how so? The hot fluid (the upper line) is always above (hotter) the cold fluid (lower line). So how does the heat flow in the other direction?