I found out the area. Is it L=pi*D*L or L= n*pi*D*L and what is n?I have to find this for a) for one shell pass, 2 tube passes(b) for one shell pass, 3 tube passes(c) for two shell passes, four tube passes
You'll need the number of tubes. The area of a shell and tube exchanger is the outside surface of all the tubes. The outside area of a tube is pi * D * L. n looks like the number of tubes. Shell and tube exchanger tubes are frequently 20' long, sometimes they are shorter, 16'. Rarely I've seen longer ones except for some air coolers.
Starting from scratch you need the overall heat transfer coefficient and the approximate velocity that was used for that calculation. You also need the amount of heat to be transferred and the log mean temperature difference for the two streams. This is all basic heat transfer and heat exchanger design information With that information you can calculate a surface area. You have to select a tube size to determine the area per foot of tube length. From that you can get the total length of tubing you need. Next select the number of tubes so the velocity in the tubes will be in the range of that used to determine the overall heat transfer coefficient. With the number of tubes and the total tube length you can come up with the tube length needed to satisfy these conditions. Hint, the approach on the heat exchanger is typically 5 to 10 degrees F and a very common tube size is 3/4. Tubes typically come in increments of 10 feet up to about 60 to 100 feet. If you have multiple pass exchangers, you figure the tube length for a single pass and divide the length by the number of passes.