Heat-Source Systems

Equation (2-18) expresses the critical-radius-of-insulation concept. If the outer radius is less than the value given by this equation, then the heat transfer will be increased by adding more insulation. For outer radii greater than the critical value an increase in insulation thickness will cause a decrease in heat transfer. The central concept is that for sufficiently small values of h the convection heat loss may actually increase with the addition of insulation because of increased surface area. 

Calculate the critical radius of insulation for asbestos [A = 0.17 W/m °C] surrounding a pipe and exposed to room air at 20°C with h = 3.0 W/m2 °C. Calculate the heat loss from a 200°C, 5.0-cm-diameter pipe when covered with the critical radius of insulation and without insulation. 

Without insulation the convection from the outer surface of the pipe is 

the addition of 3,17 cm (5.67 - 2.5) of insulation actually increases the heat transfer by 25 percent. 

A number of interesting applications of the principles of heat transfer are concerned with systems in which heat may be generated internally. Nuclear re- 

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In an ICES that serves both small and large buildings, the surplus internal heat from the large buildings can be used to provide source heat to smaller ones. An ICES in areas with moderate winter temperatures may use air as a heat source. Systems that use lakes or reservoirs rely on the natural collection of heat by these water sources throughout the year. 

The pinch represents the boundary between the balanced heat sources (system below the pinch) and heat sinks (system above the pinch). In the optimum case, no heat transfer occurs across the pinch. The grid diagram (Fig. 59) is used in the development of economical heat recovery systems using the pinch technology. 

McGinnis, D.S., Automated line-heat source system for the measurement of thermal conductivity and diffusivity. Can. Agric. Eng., 29 201-207 (1987). 

The AEG sponsored research ia the program known as Systems for Nuclear AuxiHary Power (SNAP) as early as the 1950s. Most of the systems developed iavolved the radioisotope plutonium-238 as a heat source for a thermoelectric generator. Such electrical suppHes permitted radio transmission to earth from spacecraft such as Pioneer and l qyager. 

Evaporative crystalli rs generate supersaturation by removing solvent, thereby increasing solute concentration. These crystallizers may be operated under vacuum, and, ia such circumstances, it is necessary to have a vacuum pump or ejector as a part of the unit. If the boiling poiat elevation of the system is low (that is, the difference between the boiling poiat of a solution ia the crystallizer and the condensation temperature of pure solvent at the system pressure), mechanical recompression of the vapor obtained from solvent evaporation can be used to produce a heat source to drive the operation. 

If an enclosure may be divided into several radiant-heat sources or sinks Ai, A2, etc, and the rest of the enclosure (reradiating refractoiy surface) may be lumped together as A at a uniform temperature Tr, then the total interchange area for zone pairs in the black system is given by... 

The sampling system consists of a condensate trap, flow-control system, and sample tank (Fig. 25-38). The analytical system consists of two major subsystems an oxidation system for the recovery and conditioning of the condensate-trap contents and an NMO analyzer. The NMO analyzer is a gas chromatograph with backflush capabihty for NMO analysis and is equipped with an oxidation catalyst, a reduction catalyst, and an FID. The system for the recovery and conditioning of the organics captured in the condensate trap consists of a heat source, an oxidation catalyst, a nondispersive infrared (NDIR) analyzer, and an intermediate collec tion vessel. 

Evaporators require a source of heat to operate. This heat may be supplied from a boiler, gas turbine, vapor compressor, other evaporator, or a combination of sources. Multiple effect evaporators are very popular when cheap, high pressure steam is available to heat the system. A Mechanical Vapor Recompression evaporator would use electricity or a gas turbine to drive a compressor that recycles the heat in the evaporator. 

Technology Description Infrared radiators can be used as the heat source in the destruction of hazardous waste. This system (Figure 35) is made up of a primary chamber consisting of a rectangular carbon steel box lined with layers of a light weight. 

Proper selection and sizing of ventilation systems require knowledge of emissions from internal contaminant and heat sources and an understanding of the mechanisms and characteristics of air and contaminant movement. 

Heating and cooling load calculation for HVAC system design is based on the heat balance principle. For the given building, room, or independent building zone, heat balance components should be established and analyzed. The ma or heat sources and sinks in industrial buildings are ... 

The use of a natural ventilation system assumes temperature stratification throughout the room height. Air close to heat sources is heated and rises as a thermal plume (Fig. 7.105). Part of this heated air is evacuated through air outlets in the upper zone, and part of it remains in the upper zone, in the so-called heat cushion. The separation level between the upper and lower zones is defined in terms of the equality of and G, which are the airflow rate in thermal plumes above heat sources and the airflow supplied to the occupied zone, respectively. It is assumed that the air temperature in the lower zone is equal to that in the occupied zone, and that the air temperature in the upper zone is equal to that of the evacuated air,... 

The application of the strategies in system selection can be illustrated by using a simple example. Let us think about an industrial hail with some internal heat sources but without any remarkable internal movement that would disturb the stratification. 

Rossiter (1986) demonstrated the procedure for the production process of crystalline common salt from brine. It was found that the optimal median size is determined by the entrainment limit in the crystallizer. The crystallizer had to be operated at maximum allowable temperature and the slurry density measured for quality constraints. It was also suggested that cost discontinuities should be imposed based on temperatures of the available heat sources, possible materials of construction and other intrinsic properties of the system. 

Reflux Failure (a) At top of distillation column, capacity is total overhead vapor [10], (b) when source of heat is in feed stream, capacity is vapor quantity calculated in immediate feed zone [3], (c) when reboilers supply heat to system, capacity is feed plus reboil vapors [3]. Each situation must be examined carefully. 

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