9 replies to this topic

[ashton46]

I know what the LMTD is and the AMTD.  When can we use AMTD instead of LMTD?  Is there a certain kind of heat exchanger that you can use AMTD for?

[Art Montemayor]

What is "AMTD"????

 

In 53 years of engineering, I never came across this acronym.  Please enlighten me.

[ashton46]

(AMTD) arithmetic mean temperature difference.  I am trying to resize a heat exchager and I thought it might be easier if I didn't have to use the log mean temp difference.  

[chocobang]

Heat duty is always done using LMTD. If possible, always use this for double-pipe and 1-1 heat exchangers. Add a correction factor if multipass heat exchanger.

 

That is, Tlm = ((Thi - Tco) - (Tho-Tci)) / ln((Thi-Tco)/(Tho-Tci))

 

 

From my humble understanding of heat transfer, if your Thi-Tco = Tho-Tci (both temperature approach is the same). Your LMTD can be calculated because ln 1 = 0

 

Just use ((Thi-Tco) + (Tho-Tci))/2 and calculate the heat transfer

 

ashton46, is this what you mean by AMTD? I never heard this term either.

 

Choco

[Bobby Strain]

Try both and convince yourself. And, don't forget the efficiency factor.

 

Bobby

[samayaraj]

Ashton,

 

Arithmetic mean temperature difference is used when there is constant temperature difference across heat exchanger at each interval is maintained. If not, you need to use LMTD. LMTD is nothing but you are finding out temperature difference across heat exchanger at fixed intervals and finding out the average of these difference.

 

LMTD = (Δt1 + Δt2 + .... Δtn)/n

 

Where Δt1, Δt2... Δtn is temperature difference across heat exchanger at various intervals and 'n' is the number of intervals.

 

This is mathematically given as LMTD = (ΔT1 - ΔT2)/ln(ΔT1/ΔT2)

 

Look at the attached sketch where I have shown various profiles for easy understanding.

 

 

Attached Files

•  Heat exchanger profiles.pdf   34.28KB   33 downloads

[shantanuk100]

Dear Ashton,

 

The AMTD is not a term we use regularly for heat exchangers.

Simply put, it is nothing but the arithmetic mean of temperature differences at corresponding points on the shell side and tube side.

AMTD = (Delta T1 + Delta T2 + ...... ) / N,

where n is the number of intervals

But it is not used in any calculations for the heat exchanger duty etc.

 

The Lmtd is the fundamental parameter, because the LMTD is a measure of the Driving force in the heat exchanger.

The larger the LMTD, the more the heat that is transferred in the exchanger.

It tells us the amount of heat force that exists for exchange to take place, and is defined as :

 

LMTD = (Delta T1 - Delta T2)   /   ( Ln (Delta T1) - Ln (Delta T2) )

where 1 and 2 are inlet and outlet cross sections on the heat exchanger, with Delta at each cross section being difference in Tube and shell fluid temperatures.

 

Regards,

Shantanu

Edited by shantanuk100, 15 April 2016 - 02:11 AM.

[srfish]

If you want to see the derivation of The LMTD equation, refer to that old process  heat transfer standby:  Kern,

[Mahdi1980]

Ashton

 

AMTD may occur at special conditions as friends explained.

If you see how the heat balance equation has been derived throughout the Heat Exchanger, you will find that the LMTD is a real term which is obtained from a differential equation. So, you are not allowed to use arithmetic mean of temp. difference to achieve a precise design.

I suggest you to not use it never.

 

Mahdi

[Napo]

Ashton,

 

In the book: "Examples and Problems to the Course of Unit Operations of Chemical Engineering", by Pavlov K., Romankov A., Noskov A., Mir Publisher, 1979, Moscow, we have:

 

If the ratio Delta Thigher/ Delta Tless < 2, we can use the following equation with quite accurately:

 

Delta tm = (Delta Thigher + Delta Tless) / 2

 

Napo.