Cooling damage zone

Equipment for food freezing is designed to maximize the rate at which foods are cooled to —18° C to ensure as brief a time as possible in the temperature zone of maximum ice crystal formation (12,13). This rapid cooling favors the formation of small ice crystals which minimize the dismption of ceUs and may reduce the effects of solute concentration damage. Rapid freezing requires equipment that can deHver large temperature differences and/or high heat-transfer rates. 

Analysis of the ventilation system indicates that combustion gases will not damage downstream components of the system (Mitchell and Naegeli 1999). Cooling of combustion gases in the approximately 20 feet of ducting from an SCB to the Zone 1 exhaust plenum will not appreciably decrease the gas temperature. However, the exhaust from each SCB is diluted by a factor of 10 1 in the Zone 1 exhaust plenum. Thus, the combustion gas exhaust from Zone 1 would have a temperature of no more than approximately 250 F and damage to the ventilation system due to the fire is an improbable event. 

Whether batch or inline, furnace selection is based not only on throughput rates, but is also determined by the type of product being assembled. A greater complexity to the circuit board requires more control of the time-temperature profile to ensure that all of the solder joints are completed at minimum defect levels. In some applications, the extra furnace zones are used to control the cooling rate of the soldered circuit board to prevent thermal shock damage to sensitive components or substrates. 

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