Hot spot heating

Hot-spot heating. In this mechanism, a hot spot is formed whose temperature rises to a value at which it cannot be rewetted, thus initiating the CHF transition. 

Hot-Spot Heating. This mechanism has been investigated by Unal et al. [169], The stages envisaged are that, on departure of the vapor mushroom in the fully developed... 

Since aluminium has a good thermal conductivity, heat is dissipated more rapidly than with other metals. This avoids hot spots. Heat diffusion is accompanied by deformation, because of aluminium s high linear expansion coefficient. 

The temperature in stirred tank reactors may be influenced by chemical or physical reactions within the tank. Cooling or heating devices might be required to control the process temperature. In many endothermic processes heat has to be added to raise and maintain the temperature of the bulk. In other exothermic processes heat is removed to avoid hot spots. Heating and cooling of the process fluid are accomplished by heat transfer between the process fluid and a heating or cooling media that is circulated within a closed heat transfer surface. Different types of heat transfer equipment are used in industrial processes such as jackets, external or internal helical coils, as sketched in Fig. 7.10. Heat transfer from the bulk of the tank to the heat transfer medium can be calculated by the standard heat transfer model ... 

Generally speaking, temperature control in fixed beds is difficult because heat loads vary through the bed. Also, in exothermic reactors, the temperature in the catalyst can become locally excessive. Such hot spots can cause the onset of undesired reactions or catalyst degradation. In tubular devices such as shown in Fig. 2.6a and b, the smaller the diameter of tube, the better is the temperature control. Temperature-control problems also can be overcome by using a mixture of catalyst and inert solid to effectively dilute the catalyst. Varying this mixture allows the rate of reaction in different parts of the bed to be controlled more easily. 

Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

In the refinery the salts deposit in the tubes of exchangers and reduce heat transfer, while in heater tubes, hot spots are created favoring coke formation. 

In addition to the circuit breaker, there have been a number of other SMA appHcations for various functions in electric power generation (qv), distribution, and transmission systems. One such device is a thermal indicator that provides a signal visible from the ground of a hot junction or connector in a distribution yard. Such hot spots occur as a result of the loosening of bus bar connectors owing to cycHc temperature as the electric load varies. In addition to the use of SMA flags as a hot-spot indicators, actuators that automatically maintain the contact force in a bus bar connection have been demonstrated. Based on a BeUeviHe washer fabricated from a Cu—Al—Ni SMA trained to exhibit two-way memory, these washers, when heated by a hot joint, increase their force output and correct the condition. A 30 mm diameter washer 3 mm thick can produce a force of over 4000 N. Similar in purpose... 

The hydrocarbon gas feedstock and Hquid sulfur are separately preheated in an externally fired tubular heater. When the gas reaches 480—650°C, it joins the vaporized sulfur. A special venturi nozzle can be used for mixing the two streams (81). The mixed stream flows through a radiantly-heated pipe cod, where some reaction takes place, before entering an adiabatic catalytic reactor. In the adiabatic reactor, the reaction goes to over 90% completion at a temperature of 580—635°C and a pressure of approximately 250—500 kPa (2.5—5.0 atm). Heater tubes are constmcted from high alloy stainless steel and reportedly must be replaced every 2—3 years (79,82—84). Furnaces are generally fired with natural gas or refinery gas, and heat transfer to the tube coil occurs primarily by radiation with no direct contact of the flames on the tubes. Design of the furnace is critical to achieve uniform heat around the tubes to avoid rapid corrosion at "hot spots."... 

The surface of the coal particles oxidizes or weathers resulting in cracks, finer particles, and reduced agglomeration all of which may destroy coking properties. If spontaneous heating takes place, the calorific value of the coal is reduced. Hot spots must be carefully dug out and used as quickly as possible. Without spontaneous heating and with good compaction, calorific value losses below 1%/yr have been recorded. 

Catalysts such as iron oxides cause isomeriza tion of the ethylene oxide to acetaldehyde with the evolution of heat. The acetaldehyde has a much lower autoignition temperature in air than does ethylene oxide, and the two effects may lead to hot-spot ignition (190,191). 

External Coils and Tracers Tanks, vessels, and pipe hnes can be equipped for heating or cooling purposes with external coils. These are generally 9.8 to 19 mm (% to V4 in) so as to provide good distribution over the surface and are often of soft copper or aluminum, which can be bent by hand to the contour of the tank or hne. When necessary to avoid hot spots, the tracer is so mounted that it does not touch the tank. 

In many cases, cold spots on the reactor shell will result in condensation and high corrosion rates. Sufficient insulation to maintain the shell and appurtenances above the dew point of the reaction gases is necessary. Hot spots can occur where refractory cracks allow heat to permeate to the shell. These can sometimes be repaired by pumping castable refractoiy into the hot area from the outside. 

During filling, the catalyst is distributed uniformly to avoid the possibility of channeling that coiild lead to poor heat transfer, poor conversion, and harm to the catalyst because of hot spots. During startup, sudden surges of flow may disturb the bed and are to be avoided. For instance, in a study of a hydrodesulfuiizer by Murphree et al. (Ind. Eng. Chem. Proc. Des. size unit varied between 47 and 80 percent with different modes of loading and startup. 

Premature shutdown of fans/venti-lation system immediately following shutdown of heat input (prior to sufficient cooling) resulting in hot spots and flammable pockets (dryers, carbon beds, and thermal oxidizers). Possibility of subsequent ignition resulting in fire or explosion. 

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