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Numerical simulations of a high temperature shell and tube heat exchanger and chemical decomposer (thereafter — heat exchanger) with straight tube configuration have been performed using Fluent 6.2.16 code to examine flow distribution on the tube side. The heat exchanger can be a part of sulfur iodine thermochemical water splitting cycle which is one of the most studied cycles for hydrogen production. Uniformity of the flow distribution in the heat exchanger is very critical because the flow maldistribution among the tube or shell sides can result in decreasing of chemical decomposition and increasing of pumping power. In the current study the flow rate uniformity in the heat exchanger tubes has been investigated. Simulations of the straight tube configuration, tube configuration with baffle plate arrangement and with pebble bed region inside the tubes were performed to examine flow distribution on the tube side. It was found the flow maldistribution along the tube direction is very serious with the simple tube configuration. An improvement of the header configuration has been done by introducing a baffle plate in to the header section. With the introduction of the baffle plate, there was a noticeable decrease in the flow maldistribution in the tubes. Uniformity of flow was also investigated with catalytic bed inside the tubes. A significant decrease in flow maldistribution was observed with this arrangement. But if the catalytic bed zone is created on the shell side, then the improved header configuration with a baffle plate is best suitable to avoid flow maldistribution.