Convection ovens limitations

The only disadvantage to the use of hydrogen as a carrier gas is the real or perceived explosion hazard from leaks within the column oven. Experience has shown that the conditions required for a catastrophic explosion may never be achieved in practice with forced air convection ovens. However, commercially available gas sensors will automatically switch off the column oven and carrier gas flow at air-hydrogen mixtures well below the explosion threshold limit. A considerable difference in the relative cost of helium in the USA and Europe has resulted in different preferences on the two continents. For open tubular columns helium is widely used in the USA for safety rather than theoretical considerations while hydrogen is commonly used in Europe. 

Thermal dryers are sometimes used for predrying, but are almost always used for final drying because of the limitations of mechanical dryers. After mechanical predrying, much of the remaining water is chemically bonded to the fiber and must be evaporated. This is accomplished using several types of thermal system such as heated cans, convection ovens, and radio frequency (RF) dryers. Recently, microwave dryers have been designed for drying textiles and carpet tiles [5]. 

IR ovens are typically used to predry materials that are finally dried in tenter-frame convection ovens. They are also used to augment the existing ovens where additional production is needed and space is limited. IR ovens are not normally used to accomplish final drying because IR energy is absorbed by dry textiles and there is a danger of scorching or burning the product. 

Infrared cure is gaining increased attention from coaters as a result of shorter cure cycles and the possibihty of smaller flcK)r space requirements when compared to convection oven curing. Figure 14.13 shows an example of an IR oven set up for composite curing. IR curing is limited in its use for composite curing as detailed in Table 14.10 however, it has potential for fast and efficient heating for various applications such as preform binder activation. 

HIGH-SPEED TEMPERATURE PROGRAMMING 5.11.1 Limitations of Convection Ovens... 

We first recall the physical situation to facilitate this, we draw a sketch (see Fig. 1). At high oven temperatures, the heat is transferred from the heating elements to the meat surface by both radiation and heat convection. From there, it is transferred solely by the unsteady-state heat conduction that surely represents the rate-limiting step of the whole heating process (Fig. 1). 

Preparation of Active Mn02 by Pyrolysis of MnCOs. Powdered MnCOs was spread in a one-inch thick layer in a Pyrex glass and heated at 220-280 °C for about 18 h in an oven in which air circulated by convection. The initially tan powder turned darker at about 180 °C, and black when maintained at over 220 °C. No attempt was made to determine lower temperature or time limits, nor the upper limit of temperature. The Mn02 prepared as above was stirred with about 1 L of a solution made up of 15% HNO3 in H2O. The slurry was filtered with suction, the solid was washed on the Buchner funnel with distilled water until the washes were about pH 5, and finally was dried at 220-250 °C. The caked, black solid was readily crushed to a powder which retained its oxidizing ability even after having been stored for several months in a loosely stoppered container. 

---