Heat Exchanger Thermal Cycling One possible issue is thermal cycling of the heat exchanger. If the heat exchanger is heated to design and then shut down over regular periods, thne gasket between the cast iron heads and steel tube sheet may drip. The gasket's "seating force" may drop due to expansion and contraction. As the heat exchanger operates, the head, tube sheet, bolts, and gasket materials are affected by the temperature. That is why the contractor is asked to e-tighten the bolts once the