ZERO LMTD for Shell and Tube exchanger
ZERO LMTD for Shell and Tube exchanger
(OP)
Hi,
I am performing exchanger design and found that my LMTD is zero as my heating and cooling flow are same.
Does it mean that the design is not correct?
Thanks!
I am performing exchanger design and found that my LMTD is zero as my heating and cooling flow are same.
Does it mean that the design is not correct?
Thanks!





RE: ZERO LMTD for Shell and Tube exchanger
Regards,
SNORGY.
RE: ZERO LMTD for Shell and Tube exchanger
RE: ZERO LMTD for Shell and Tube exchanger
Do I have to consider parallel flow?
Another question is,
How do i know when to use LMTD OR arithmetic mean temperature difference?
RE: ZERO LMTD for Shell and Tube exchanger
rmw
RE: ZERO LMTD for Shell and Tube exchanger
I believe the correct temperature difference to use in this case is 30F.
RE: ZERO LMTD for Shell and Tube exchanger
RE: ZERO LMTD for Shell and Tube exchanger
Regards,
SNORGY.
RE: ZERO LMTD for Shell and Tube exchanger
Regards,
SNORGY.
RE: ZERO LMTD for Shell and Tube exchanger
The definition of log mean of a and b implies a≠b, and if a = b, then log mean = a.
RE: ZERO LMTD for Shell and Tube exchanger
http://antwrp.gsfc.nasa.gov/apod/astropix.html
RE: ZERO LMTD for Shell and Tube exchanger
We can solve the limit as GTTD->LTTD by LeHopital's rule which says that limit f(x)/g(x)= limit f'(x)/g'(x).
You can correct my math, but I think this comes out as:
lim(x->c) of (x-c)/ln(x/c)=
lim(x->c) of d(x-c)/d(ln(x)-ln(c))=
lim(x->c) of dx/(dx/x)=
lim(x->c) of x = c
or for your case
LMTD = GTTD = LTTD = 10
best wishes,
sshep
RE: ZERO LMTD for Shell and Tube exchanger
LMTD= ΔT1= ΔT2
Where
ΔT1= Thot in – Tcold out
ΔT2= Thot out – Tcold in
In the OP's specific case: LMTD = 30 °C
RE: ZERO LMTD for Shell and Tube exchanger
The LMTD is actually 30C.
If you use for instance your cold inlet Temp of 24.999 C instead of 25.00 C, you'll get the 30C LMTD. The result would be the same for each of the other streams....For instance, if you take 74.99C for your Hot-in, instead of 75.00, the LMTD would again be 30.
I really wonder what HTRI would do! It probably would not converge unless one of the temperatures is adjusted.
RE: ZERO LMTD for Shell and Tube exchanger
GTTD=LTTD is a fairly common calculation "problem"- as would be encountered when a stream recovers heat from itself before and after a simple unit operation. This has nothing to do with any "temperature inaccuracy", it is just a mathematical qwerk.
HTRI or any other heat exchanger design program will NOT have difficulty with such a problem since these programs will use localized temp differences, and any "integration" will be numerical. Even Aspen-plus in shortcut mode has no problem, the programing caters for it. It is only hand calcs and excel that will fail with divide by zero errors.
best wishes,
sshep