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.