Question: It is desired to specify the dimensions of a heat exchanger whose function is to heat 3 kg/s of air at 1 atm from 18°C to 90°C using an available supply of CO₂ at 4 kg/s, 3 atm pressure, and 125°C. The pressure drop for both of the fluids is to be 3 percent. That is, the pressure drop through the heat exchanger for the air will be 3 percent of 1 atm, and that for the CO₂ will be 3 percent of 3 atm. Plate-fin construction is to be used, and the heat exchanger is to have a single-pass cross-flow arrangement. Any two of the plate-fin surfaces described in this chapter may be used (or both sides could be made from the same surface geometry). The thickness of the plates separating the surfaces should be 0.005 mm. The material should be aluminum. An iterative solution will be necessary in order to satisfy both the thermal requirements and the pressure drops. One of the objectives of the problem should be to find which pair of surfaces provides a minimum-volume heat exchanger, and which pair provides a design having the most "compact" design, i.e., the design that minimizes the largest dimension.