Oven heating temperature distribution

Oven heating is the most common source of heat for curing epoxy adhesives that require an elevated-temperature cure. Good air circulation within the oven is mandatory for uniform heating. Temperature distribution within an oven should always be checked before items are placed in the oven. Many ovens will have significant temperature distribution variation and dead spaces, especially in corners where air circulation is not uniform. The number of items and their arrangement in the oven also affect the time for the joint to reach the proper temperature. The geometry and size of the part may affect air circulation and cause variations in temperature distribution. 

A piece of beef steak is cooked either in a microwave oven or a radiant heating oven. Sketch temperature distributions at specific times during the heating and the cooling processes in each oven. 

Dry-heat sterilization is generally conducted at 160—170°C for >2 h. Specific exposures are dictated by the bioburden concentration and the temperature tolerance of the products under sterilization. At considerably higher temperatures, the required exposure times are much shorter. The effectiveness of any cycle type must be tested. For dry-heat sterilization, forced-air-type ovens are usually specified for better temperature distribution. Temperature-recording devices are recommended. 

The inner chamber of the oven has curved walls for smooth circulation of air the radiant heat from the sample injection port units and the detector oven is completely isolated. These factors combine to provide demonstrably uniform temperature distribution. (The temperature variance in a column coiled in a diameter of 20cm is less than 0.75°K at a column temperature of 250°C). 

Heating should be carried out in an oven or other equipment which will achieve sterilising conditions throughout the load. The method of loading used should not be such as to lead to an uneven temperature distribution. 

Catalytic or flameless combustion of hydn en exhibits many advantages in comparison with flame combustion. It occurs at low temperatures (ambient - 800 K), is safe and leads to a very high conversion of the burning gas (99.9 %). The NOX formation which usually occurs in conventional combustion at temperatures of about 1700 K, is here almost completely suppressed. The catalytic combustion in diffusion burners (Fig. 7-2) occurs in the presence of small amounts of Pt or Pd catalysts. It is adequate, e.g., for kitchen appliances such as cooker, oven, water heater, space heater. Drawbacks are the possible non-uniformity of the temperature distribution at die catalyst surface, rapid changes in the operational state, and relatively small heat flux densities [47]. 

Temperature Distribution. Temperature distribution across the width of the tunnel determines the quality and uniformity of heating. Top-quality ovens have rail-to-raU thermal nniformity of about 2°C, even for large board widths. Such tight thermal distributions are necessary for controllable and consistent soldering. 

FIGURE 47.9 Schematic plan view and detailed side view of an adjustable oven characterization fixture (oven diagnostic tool) composed of pairs of thermocouples, shielded from one another by a rigid, heat-resistant insulator. The thermocouple positions are adjustable in width and length for investigating temperature distribution along the tunnel width. 

The distribution of molecular speeds in gas can be determined experimentally. To du so, the gas is heated to the required temperature in an oven. The molecules then stream out of the oven through a small hole into an evacuated region. To ensure that the molecules form a narrow Iteam, they may also pass through a senes of slits, and the pressure must Ik kept very low so that collisions within the beam do not cause spreading. 

Other microwave-assisted parallel processes, for example those involving solid-phase organic synthesis, are discussed in Section 7.1. In the majority of the cases described so far, domestic multimode microwave ovens were used as heating devices, without utilizing specialized reactor equipment. Since reactions in household multimode ovens are notoriously difficult to reproduce due to the lack of temperature and pressure control, pulsed irradiation, uneven electromagnetic field distribution, and the unpredictable formation of hotspots (Section 3.2), in most contemporary published methods dedicated commercially available multimode reactor systems for parallel processing are used. These multivessel rotor systems are described in detail in Section 3.4. 

When coke-oven or petroleum-tar pitches were heated to an appropriate temperature (about 420°C.) and quenched, the product contained spheres of various sizes mostly in the range 0-10 microns. The spheres were normally, fairly regularly distributed through the pitch matrix, with small and larger spheres intermixed. When the toluene-soluble extract of the pitch was carbonized to the same temperature, fewer spheres were present in the product, but their diameters were considerably greater. Under these conditions many spheres were more than 10 microns across, and some exceeded 50 microns. This result suggested that nucleation occurred less commonly or less easily in the toluene-soluble pitch fraction and was associated with the presence of C1 -insoluble material. 

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