There are specific applications in which helical coil heat exchangers are advantageous over straight tubes. The foremost of these is for high temperature applications where long straight tubes may pose severe problems due to thermal expansion , which can be minimized in coiled tubes. Another advantage of coiled tubes is that they can be used to pack more surface area in a small volume and are therefore used for applications with small temperature difference or high volume heat rating. Coiled tubes also have better heat transfer coefficient and residence time distributions and are therefore used in compact heat exchangers. The main disadvantages of coiled tube heat exchangers are that they are more costly than the straight tube type to manufacture, they cannot be cleaned easily and are therefore not suitable for concentration or crystallization type of applications.