September 02, 2008

Easily calculate LMTD orally

You may feel frustrated when somebody suddenly ask you about LMTD for the given heat exchange condition. That time you feel screamed if you do not have calculator with you.

Also, when you just begin using your calculator somebody answered the LMTD value and you feel surprised & wonder how he did it so quickly?

Now you can also do it.

LMTD can easily be calculated manually by the difference of airthematic means of fluid temperatures. Got it or Not....NO Problem. Here is an Example.

Suppose Hot fluid is entering at ~140°C & leaving at ~110°C. So its Airthmatic mean is 125°C.

Now if Cold fluid is entering at 40°C & leaving at 60°C its mean (average) is 50°C

Now the difference of two mean i.e. 125 - 50 is 75°C.
So your LMTD should be ~75°C.

Let us check it with conventional formula.
Hot side difference = 140 - 60 = 80°C
Cold side diff = 110 - 40 = 70°C

So LMTD = 10 / Ln(80/70) = 74.88°C

So an error of <0.2%. Usually it is less than 1%.

Hope all of you find it useful.

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This method is fantastic. I'll pass this method to my friends too.

kiran said...


Tom Long said...

Wow, that's so useful! Thanks for sharing it.

zandroid said...

This won't work all the time, because the denominator is supposed to be in kelvin. Or else you have an issue with negative or, God forbid, zero temperatures.

Wesly said...

Nice "Thumb Rule". Thanks.

Anonymous said...

what stand Ln for in this formula?

profmaster said...

Dear Ln is natural log.

Anonymous said...

U make it so Simple! Well done

Anonymous said...

Intelligent way of quick answering and very close to calculation, Well Done

Anonymous said...

Good one !!

Anonymous said...

It Dosent work on

195-> 70
80<- 60

Techkasamba Agarwal said...

Yes it may not as it doesn't guarantee it. But it is very clear from this type of arbitrary data that even though it is possible but not a good idea to design it as one side very large flow is there and other side is higher delta T. This means your No of passes will increase & hence correction factors will change.

Hope it is clear.

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