By using interchangeable novel end caps, heat exchangers can be designed for operation with a variable number of tube passes. Such flexibility can allow significant savings in operating costs, as it allows exchangers to be tuned to actual process conditions. Since the pass partitions in the end caps take up minimal space, the tube bundles contain virtually the full number of tubes of a single-pass unit: an advantage that becomes more significant as the number of tube passes increases. Other advantages include improved arrangements for gasketing, which reduce the possibility of hazardous leaks as well as reducing maintenance costs. These improved features are discussed in connection with actual exchanger applications