Gas convection oven

Temperature distribution in a typical gas convection oven. Each gray tone represents 1°C. From Mora and Macosko (1991). 

Gas convection ovens can increase temperature very rapidly as illustrated by curves 2a and 2b in Figure 8.2.13. The sample... 

Cardboard baking dishes coated with crystalline PET or crystalline PBT can be used in convection ovens up to temperatures of 200-220 °C. Single portion dishes made from heat formed films find wide applications in microwave ovens. Biaxially stretched PET covers an important application area of bottles, wide mouth jars and cans. These containers are particularly well suited for carbonated beverages, edible oils and spirits. The gas barrier properties can be improved by coextrusion with a barrier layer such as polyamide. With improved barrier properties it can also be used for beer and wine. 

Conventional industrial coatings materials of the thermoset type are usually acrylic, polyester, epoxy, polyurethane or silicone resins dispersed or dissolved in organic or water/ether-alcohol coupling solvents. They are cured with gas convection or electric IR ovens. The raw materials for the polymers come from petroleum feedstocks which are processed or manufactured into a finished coating system. 

In a conventional oven, the air in the oven is first heated to the desired temperature by the electric or gas heating element. This preheating may take several minutes. The heat is then transferred from the air to the skin of the chicken by natural convection in older ovens or by forced convection in the nevrer convection ovens that utilize a fan. The air motion in convection ovens increases the convection heat transfer coefficient and thus decreases the cooking time. Heat is then conducted toward the inner parts of the chicken from its outer parts as in microwave ovens. 

Table II. Alternative Energy Sources to Gas Fuel Convection Ovens for Cure of Coatings...

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. 

Jacketed molds heated with hot oil or salts were used in the past. The heat transfer efficiency of these systems was good but the maintenance problems have made these systems uncommon today. Modem machines employ forced air convection ovens to heat the molds. In the majority of industrial machines the ovens are heated by the combustion of a gas, while in smaller machines electricity is normally used. Ovens are provided with blowers and baffles to improve heat transfer. 

FIGURE 15.3 (A) Vitamin E bath placed inside a convection oven is used to dope irradiated UHMWPE under inert gas purge (B) Doping creates a... 

Early convection ovens consumed over 50m /hr for an ROL of 10-20 ppm. In modern systems, this consumption level has been reduced without impacting the heat transfer performance or ROL. However, as has already been mentioned, the ROL required depends on the inherent wettability of the parts to be assembled. Because the cost of inerting gas increases with purity and the need for higher flow rates to achieve a lower ROL, only a purity level necessary to achieve acceptable wetting should be used. These measures can result in substantial savings. A common recipe is to set the ROL level in the peak zone at 100 ppm, and this can usually be achieved in optimized systems with a flow rate in the range of 20-30 N m /hr. 

The heating of rotational moulds may be achieved using infra-red, hot liquid, open gas flame or hot-air convection. However, the latter method is the most common. The oven temperature is usually in the range 250-450°C and since the mould is cool when it enters the oven it takes a certain time to get up to a temperature which will melt the plastic. This time may be estimated as follows. 

Preheating methods are convection, infrared, radio frequency, and steam. Thermostatically controlled gas or electrically heated ovens are inexpensive methods of heating. The quickest, and possibly the most efficient, method is radio-frequency heating, but it is also the most expensive. Preheaters are located adjacent to the molding press and are manually operated for each cycle. 

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