The U-tube head (U) is a very simple design requiring a bundle of U tubes, only one tube sheet, no expansion joints, and no rear-end head at all, allowing easy removal of the bundle. The thermal stress problem is eliminated because each tube is free to expand/contract independently. One disadvantage is, individual tube replacement is not possible except in the outer rows, and an even number of tube passes is required. Some tubes are lost in the center due to HEAT EXCHANGERS 17.7 tube bend limit, and tube side mechanical cleaning of the bends is difficult. Flow-induced vibration could be a problem for tubes in the outermost row. It is the lowest-cost design because there is no need for the second tube sheet.