Heating medium

The use of an unnecessarily hot utility or heating medium should be avoided. This may have been a major factor that led to the runaway reaction at Seveso in Italy in 1976, which released toxic material over a wide area. The reactor was liquid phase and operated in a stirred tank (Fig. 9.3). It was left containing an uncompleted batch at around 160 C, well below the temperature at which a runaway reaction could start. The temperature required for a runaway reaction was around 230 C. ... 

If a heat transfer intermediate is to be used and the cooling/ heating medium is fixed, then the cooling/heating medium should... 

Heat Pumps. A heat pump is a refrigeration system that raises heat to a useful level. The most common appHcation is the vapor recompression system for evaporation (qv) (Fig. 14). Its appHcation hinges primarily on low cost power relative to the alternative heating media. If electricity price per unit energy is less than 1.5 times the cost of the heating medium, it merits a close look. This tends to occur when electricity is generated from a cheaper fuel (coal) or when hydroelectric power is available. 

Higher temperatures result in permanent degradation. The amount of this irreversible loss in mechanical properties depends upon moisture content, heating medium, temperature, exposure period, and, to some extent, species. The effects of these factors on modulus of mpture, modulus of elasticity, and work to maximum load are illustrated in Figures 6—9 (6). The effects may be less severe for commercial lumber than for clear wood heated in air (Fig. 10). The permanent property losses shown are based on tests conducted after specimens were cooled to - 24° C and conditioned to a moisture content of... 

For extended, noncyclic exposures, it can be assumed that the entire piece teaches the temperature of the heating medium and is, therefore, subject to permanent strength losses throughout the piece, regardless of size and mode of stress application. Because dry wood is a good insulator, it often does not teach the daily extremes in temperature of the air around it in ordinary constmction thus, estimates of long-term effects should be based on the actual wood temperatures experienced by critical stmctural parts. 

Steam-Heated Exchangers Steam, the most common heating medium, transfers its latent heat in condensing, causing heat flow to be proportional to steam flow. Thus, a measurement olsteam flow is essentially a measure of heat transfer. Consider raising a hquid from temperature Ti to To by condensing steam ... 

Coil-in-Tank or Jacketed Vessel Isothermal Heating Medium... 

Conauctive Heat Transfer Heat-transfer equipment in which heat is transferred by conduction is so constructed that the solids load (burden) is separated from the heating medium by a wall. 

Rehoiler Connected to the bottom of a fractionating tower, it provides the rehod heat necessary for distillation. The heating medium maybe either steam or a hot-process fluid. 

Waste-heat boiler Produces steam similar to steam generator, except that the heating medium is a hot gas or liquid produced in a chemical reaction. 

Heating medium separated from evaporating hquid by tubular heating surfaces. 

Heating medium confined by coils, jackets, double walls, flat plates, etc. 

Heating medium brought into direct contact with evaporating liquid. 

Indirect dryers using condensing fluids as the heating medium are generally economical from the standpoint of heat consumption, since they furnish heat only in accordance with the demand made by the material being dried. 

Control of trav and compartment equipment is usually maintained by control of the circulating-air temperature (and humidiy) and rarely by solids temperature. On vacuum units, control of the absolute pressure and heating-medium temperature is utihzed. In direct dryers, cycle controllers are frequently employed to vary the air temper-... 

Steam is the usual heating medium, and a standard heater arrangement consists of a main heater before the circulating fan. When steam is not available or the diying load is small, electricalheat can be used. For temperatures above 450 K, products of combustion can be used, or indirect-fired air heaters. 

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