Enhancement of heat and mass transfer in silica-expanded graphite composite blocks for adsorption heat pumps: Part I. Characterization of the composite blocks

Composite blocks for adsorption heat pumps of water/silica gel systems were manufactured by mixing expanded graphite powders and silica gel powders, then by consolidating them via compressive molding. Optimum expansion condition was determined as 600°C for 10 min by investigating carbon and sulfur contents during expansion. Amounts of water adsorption, gas permeability and thermal conductivity were measured for the composite blocks prepared with various compositions and molding pressures. Expanded graphite in the composite blocks had no effects on the equilibrium adsorption amount of water on silica gel and increased the rate of adsorption. The composite blocks of 20–30% graphite fraction under 4–40 MPa molding pressure showed good permeability of 3 to 40×10−12 m2. Permeability increased with graphite fraction at constant molding pressure. Thermal conductivity of the composite blocks was 10–20 Wm−1 K−1 depending on the graphite bulk density (ρgr) in the block. It is a much higher value than 0.17 Wm−1 K−1 in silica-gel packed bed.