Complex Engineering Problem (CEP) Descriptive Form. Simultaneous Heat and Mass Transfer. The concentric tube heat exchanger is replaced with a compact, plate-type heat exchanger that consists of a stack of thin metal sheets, separated by N gaps of width a. The oil and water flows are subdivided into N/2 individual flow streams, with the oil and water moving in opposite directions within alternating gaps. It is desirable for the stack to be of a cubical geometry, with a characteristic exterior dimension L. (a) parallel flow (b) counter flow, A counter flow, concentric tube heat exchanger is used to cool the lubricating oil for a large industrial gas turbine engine. The flow rate of cooling water through the inner tube (Di - 25 mm) is 0.2 kg/s,.