1.6 Fluid Type 1 A gas-to-gas heat exchanger requires a significantly greater amount of surface area than that for a liquid-to-liquid heat exchanger for a given heat transfer rate. This is because the heat transfer coefficient for the gas is Tb to that of a liquid. The increase in surface area is achieved by employing surfaces that have a high heat transfer surface area density 3. For example, fins are employed in an extended surface heat exchanger, or a small hydraulic diameter surface is employed in a regenerator, or small-diameter tubes are used in a tubular heat exchanger. Plate heat exchangers (of the type described in Section 1.5.2) are generally not used in a gas-to-gas exchanger application because they produce excessively high pressure drops. All prime surface heat exchangers with plain (uncorrugated) ates are used in some waste heat recovery applications. The fluid pumping power is generally signifi cant and a controlling factor in designing gas-to-gas exchangers