Thermal resistance heat exchangers

Thermal applications of polymers include heat shrinkable tuhing, heat resistant parts, oven parts, oven grills, cooling systems, expansion tanks, heating systems, heat exchangers, thermal protections and high heat applications. 

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

PWRs operate differendy from BWRs. In PWRs, no boiling takes place in the primary heat-transfer loop. Instead, only heating of highly pressurized water occurs. In a separate heat-exchanger vessel, heat is transferred from the pressurized water circuit to a secondary water circuit that operates at a lower pressure and therefore enables boiling. Because of thermal transfer limitations, ultimate steam conditions in PWR power plants ate similar to those in BWR plants. For this reason, materials used in nuclear plant steam turbines and piping must be more resistant to erosion and thermal stresses than those used in conventional units. 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

Figure 10-40B. Fouling resistance for various conditions of surface fouling on heat exchanger surfaces. Thermal resistance of typical uniform deposits. Note that the abscissa reads for either the inside, r or outside, r , fouling resistance of the bulidup of the resistance layer or film on/in the tube surface. (Used by permission Standards of Tubular Exchanger Manufacturers Association, 6 Ed, p. 138, 1978. Tubular Exchanger Manufacturers Association, Inc. All rights reserved.)...

Convective heat transmission occurs within a fluid, and between a fluid and a surface, by virtue of relative movement of the fluid particles (that is, by mass transfer). Heat exchange between fluid particles in mixing and between fluid particles and a surface is by conduction. The overall rate of heat transfer in convection is, however, also dependent on the capacity of the fluid for energy storage and on its resistance to flow in mixing. The fluid properties which characterize convective heat transfer are thus thermal conductivity, specific heat capacity and dynamic viscosity. 

Above this size, the flow of air over the condenser surface will be by forced convection, i.e. fans. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 1 and 10 1. 

Table 9.15. Thermal resistance of heat exchanger tubes...

The advantages of the steel foundation pile to be the ground heat exchanger are its high water-tightness and low thermal resistance due to high thermal... 

The error due to the simplification becomes significant when the thermal resistance in the solid PCM-layer (Figure 129) leads to a significant reduction below Tph at the heat exchanger surface. If the thermal resistance of the heat exchanger itself is neglected, then the driving temperature difference for the heat transfer is reduced by the ratio of the thermal resistances... 

When glass tubes with the same dimension, instead of stainless tubes, are used in the liquid-liquid heat exchanger of Example 5.1, estimate LI based on the outside tube surface, neglecting again the resistance of the dirty deposit. The thermal conductivity, k, of glass is 0.63 kcal h m °C. As for the heat transfer resistance of a round tube, see Problem 2.1. 

The temperature controller consists of a simple electrical resistance if only heating is required. Otherwise a heating/cooling jacket, or a heat exchanger inside the cell is used (see Fig. 9.10-2). This is the most widespread method, as the metallic thermal inertia is very high, and does not permit a suitable temperature-control in the other cases. 

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