Issue link: http://winesandvines.uberflip.com/i/101495
WineEast Understanding heat-transfer coefficients W hen contemplating any task that involves protection against heat loss or gain, it is impor- tant to understand some of the mechanics and the differences in magnitude of heat transfer for the materials involved with this phenomenon. In general, thermal conductance is the quantity of heat passing through the material of a unit area in a specified unit of time. In a similar manner, the effect of heat transfer is the heat-transfer coefficient, which is defined as the quantity of heat passing through a unit area of material, of a unit thickness, in a specific unit of time, when one side differs by 1�� Kelvin. In practical terms, this is equiva- Vance_Jan11.qxp 11/23/10 9:29 AM Page 1 lent to 1��C. Stainless steel transfers heat more than 30 times faster than an equivalent piece of polyethylene, and its transfer is about 350 times as fast as fiberglass or polyurethane foam. The question then becomes: Why didn���t either polyethylene or fiberglass allow the heat-transfer system to achieve cold stability in the Flextank? Good insulating properties require protection against moisture. For every gram of water that condenses on the surface of the tank or in the insulation, it transfers about 540 calories into the tank of wine. That heat is transferred directly into the wine in the tank and then must be removed by the heat-transfer fluid. Thus, you need to have an insulating material that will prohibit water from condensing on the surface, and the only way to meet that task cost effectively is to use foam that will not let water migrate to the tank surface. Condensation transfers heat into the tank, thereby reducing the cooling effect of the heat-transfer fluid. R.C. Win es & Vin es JA N UA RY 20 13 151