how this LMTD differs from LMTD of 1-2 shell tbe heat exchanger??
First of all LMTD is only a function of inlet and outlet temperatures, the shell and tube passes do not influence the LMTD. To calculate the log mean temperature difference, just do the regular stuff: = [(Hot fluid inlet - Cold fluid Outlet) - (Hot fluid outlet - Cold fluid inlet)]/ [log(Hot fluid inlet - Cold fluid Outlet) - (Hot fluid outlet - Cold fluid inlet)] with numbers Tube fluid inlet and outlet 100 and 50 , Shell fluid inlet nad outlet 10 and 30 (just some rough numbers) Lmtd = [(100-30) - (50-10)]/[log(70/40)]
For a multiple pass heat exchanger you need to impliment a correction factor. When on eof the fluids does not change temp. as in a feed water heater or other condensation/evaporation envioronment, the correction factor is 1. The correction factor depends on the type of heat exchanger and is almost always given graphically. The graph's I have that show the correction factor are for one shell pass, even number of tube passes and for two shell passes, multiple of four tube passes. The graphs are copyrighted by Tubular Exchanger Manufacturers Association. They might have some published on the web. Or you can google LMTD correction factor and find the forumula you can use, which is more accurate. The graphs can be off by 5%.
