Contact of Hot and Cold Streams

Transfer of heat by direct contact is accomplished in spray towers, in towers with a multiplicity of segmented baffles or plates (called shower decks), and in a variety of packed towers. In some processes heat and mass transfer occur simultaneously between phases for example, in water cooling towers, in gas quenching with water, and in spray or rotary dryers. Quenching of pyrolysis gases in transfer lines or towers and contacting on some trays in fractionators may involve primarily heat transfer. One or the other, heat or mass transfer, may be the dominant process in particular cases. 

Data of direct contact heat transfer are not abundant. The literature has been reviewed by Fair (1972) from whom specific data will be cited. 

One rational measure of a heat exchange process is the number of transfer units. In terms of gas temperatures this is defined by 

The logarithmic meat temperature difference usually is applicable. For example, if the gas goes from 1200 to 150°F and the liquid countercurrently from 120 to 400°F, the mean temperature 

Heat transfer coefficients also have been measured on a volumetric or cross section basis. In heavy hydrocarbon fractionators, Neeld and O Bara (1970) found overall coefficients of 1360-3480 Btu/(hr)(°F)(sqft of tower cross section). Much higher values have been found in less viscous systems. 

---

Data of Heat Transfer Coefficients 182 Direct Contact of Hot and Cold Streams 185 Natural Convection 186... 

In direct contact heal exchange, there is no wall to separate hot and cold streams, and high rales of heal transfer are achieved. Applications include reactor off-gas quenching, vacuum condensers, desuperheating, and humidification. Water-cooling lowers are a particular example of a direct contact heal e.xchanger. In direct contact cooler-condensers, the condensed liquid is frequently used as the coolant. 

In direct-contact heat exchange the hot and cold streams are brought into contact without any separating wall, and high rates of heat transfer are achieved. 

As mentioned above, the main difference between microwave and conventional pyrolysis is the initial sonrce of thermal energy and the way this is transferred to the plastic. Nonetheless, there are other differences, particularly when microwave pyrolysis is compared with flnidized-bed pyrolysis equipment in the latter, the primary reaction prodncts are carried ont of the reactor by a hot gas stream which enables these products to take part in secondary and tertiary reactions. On the other hand, in microwave pyrolysis, once the pyrolytic prodncts leave the carbon bed, they stop receiving heat by conduction from the hot carbon and come in contact with a relatively cold carrier gas. This has an important effect in the nnmber of consecntive reactions occnrring and therefore, on the natnre of the prodncts, as is shown in Section 3.2.2. 

A problem of this type that has been subjected to careful analysis beginning with the early work of Marble and Adamson [62] is illustrated schematically in Figure 12.4. At the point x = 0, a stream of cold (temperature Tj) combustible gas traveling at the velocity Uj comes into contact with a stream of hot (temperature T2) inert gas traveling at the velocity U2. As... 

Analyser systems are expensive and should therefore be adequately protected from their environment and the process streams that they control. Every analyser is liable to malfunction upon contact with rain, snow, Ice, wind, sand, dust and so forth. After some time, alternate hot-cold or humid-dry periods cause expansions and compressions that results In erosion and corrosion of the analyser. On the other hand, industrial environments are particularly severe as the rain and atmospheric humidity react with traces of hydrocarbons, sulphurized products and nitrogen oxides to form acids which accelerate corrosion. All these reasons recommend protecting the analyser to an extent depending on the potential hazards of the area where the analyser is... 

---