Uniform heating

Induction heating using low frequency and low power density when apphed to a stationary or moving bar can produce a uniformly heated part suitable for introduction to a rolling mill (4). A cod line capable of producing 32 t/h of 17.8 cm (7 in.) diameter alloy steel bars heated to 1177°C is shown in Figure 5. 

A special coil configuration is used to heat thin strips of metal that caimot be heated efficiently with a coil that encircles the load, as the strip thickness is small compared to the depth of penetration. The transverse flux induction coil is positioned on either side of a strip to produce a uniformly heated strip with good efficiency in a much smaller space than conventional radiant or convective strip heating furnaces (6). 

NUh2 is the Nusselt number for uniform heat flux boundary condition along the flow direction and periphery. 

The stainless steel plates are separated (ca 3 pm between) by nonabsorbent vulcanized gaskets. Various profiles and configurations, including raised knobs, crescents, channels, or diamonds, provide a rapid, uniform heat-transfer plate surface. During operation the plates must be pressed together to provide a seal, and mounted and coimected in such a manner that air is eliminated and that the product drains from the plates without opening. 

To speed up the molding process, the required amount of molding powder or granules is often pressed iato a block and prewarmed before placing it ia the mold. Rapid and uniform heating is accompHshed ia a high frequency preheater essentially an iadustrial microwave oven. The prewarmed block is then transferred to the hot mold, pressed iato shape, and cured. 

The usual practice is to normalize at 50—80°C above the upper critical temperature. For some alloy steels, however, considerably higher temperatures may be used. Heating may be carried out in any type of furnace that permits uniform heating and good temperature control. 

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."... 

Cracking reactions are endothermic, 1.6—2.8 MJ/kg (700—1200 BTU/lb) of hydrocarbon converted, with heat supplied by firing fuel gas and/or fuel oil in side-wall or floor burners. Side-wall burners usually give uniform heat distribution, but the capacity of each burner is limited (0.1—1 MW) and hence 40 to 200 burners are required in a single furnace. With modem floor burners, also called hearth burners, uniform heat flux distribution can be obtained for coils as high as 10 m, and these are extensively used in newer designs. The capacity of these burners vary considerably (1—10 MW), and hence only a few burners are required. The selection of burners depends on the type of fuel (gas and/or liquid), source of combustion air (ambient, preheated, or gas turbine exhaust), and required NO levels. 

For parallel plates and annuli with Do/Di < 1.4 and uniform heat... 

Cylinder Heat-Transfer Units Sometimes called can dryers or drying rolls, these devices are differentiated from drum diyers in that they are used for solids in flexible continuous-sheet form, whereas drum dryers are used for liquid or paste forms. The construction of the individual cyhnders, or drums, is similar in most respec ts to that of drum diyers. Special designs are used to obtain uniform distribution of steam within large drums when uniform heating across the drum surface is critical. 

In the vei tical-tube single-row double-fired heater, a single row of vertical tubes is arrayed along the center plane of the radiant section that is fired from both sides. Usually this type of heater has an overhead horizontal convec tion bank. Although it is the most expensive of the fired heater designs, it provides the most uniform heat transfer to the tubes. Duties are 21 to 132 GJ/h (20 to 125 10 Btu/h) per cell (twin-cell designs are not unusual). 

In static systems, the cooling jackets are normally filled with a glycol and water mixture to provide for uniform heat distribution within the cylinder. This system may be used where the AT of the gas is less than 150°F and discharge gas temperature is less than 190°F. 

Before entering the pipeline, the gas is also adjusted in the field to achieve a uniform heating value of 1,00(1 Btu (British thermal units) per cubic foot. And for safety, because natural gas is odorless and colorless, an odorant is added to provide a distinctive and disagreeable smell that is easy to recognize. 

But problems persisted. The catalyst, moving at rapid rates, tended to disintegrate as it impacted the inside surface of equipment. Dust particles formed, clogging pipes and transfer lines and disrupting the smooth flow of operation. This difficulty in turn prevented uniform heat distribution through the system and affected the rate and extent of both cracking and regeneration. Both the volume and C uality of fuel produced suffered. 

The volumetric capacity is adjustable to provide uniform heat transfer to the oil passing through. Heaters are flexible in operation down to about 50 per cent of their designed temperature rise, when the quantity of oil passing through is approximately double. Where the temperature level required with a reduced throughput is outside this range, the heater manufacturer should be consulted. 

Products are affected dimensionally by the difference between their forming temperature and their product-use temperature. Thus, a plastic s coefficient of thermal expansion and contraction has a significant effect on service conditions. The thermoforming pressure, time, and temperature variations that can exist will affect the final dimensions. Of these factors, evenness in heating throughout the sheet thickness before forming is usually the most important control. Type of heater has a direct effect on obtaining uniform heat... 

The next step involves heating the mold while it is rotating. Molds can be heated by a heated oven, a direct flame, a heat-transfer liquid (either in a jacket around the mold or sprayed over the mold), or electric-resistance heaters placed around the mold. With uniform heat transfer through the mold, the plastic melts to build up a layer of molten plastic on the molds inside surface. 

---