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Guide to Inspections of Low Acid Canned Food 13

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some times 4 or 5 stories high. The fire-tube boiler is operated at pressures normally under 150 psi. One advantage of the fire-tube boiler is the large water storage capacity. Because of this feature, wide and sudden changes in steam demand are met with little change in pressure. Watertube units on the other hand are capable of considerably higher overloads. Boilers may be fired using a variety of fuels including, natural gas, liquid propane gas, fuel oil, coal and wood. The method of firing the boiler may have an impact on the steam pressure supply and recovery time.

Boiler or steam generator capacities are normally listed in terms of horsepower or lbs of steam per hour. The term horsepower is defined as the ability of the unit to change approximately 34 lbs of water at 212° F to 34 lbs of steam at 212° F. If boiler capacity is given in pounds of steam per hour it can be converted to equivalent number of horse power by dividing by 34.5. The rated capacity of a boiler is not an absolute measure of the amount of steam that can be generated. Fire tube boilers may be operated at 135 to 150% of the rated capacity and water tube boilers may be operated at 150 to 200% of capacity. Operation of the boiler depends upon proper maintenance of the boiler. A boiler with heavily encrusted scale in the tubes or one which is not properly fired may not be able to even meet the rated capacity of the boiler. This is usually beyond the ability of the investigator to determine.

Steam pressures in the main steam line (header) may vary depending upon the operation of the boiler(s) and the demand upon the steam supply. Boilers are normally operated by either of two methods, use of modulating burners, or on-off operation of the boiler. Modulating control is normally used in larger installations. Small plants usually operate by on-off operation of one boiler; if additional boilers are used they are normally fired at a fixed rate. The pressure gauge on the main steam line should be watched over a period of time to determine if there are large fluctuations in steam pressure. Modulated systems will normally have only small variations in the steam supply; however the variation in on-off systems can be quite large. A reserve steam capacity is sometimes obtained by operating the boilers at a high pressure (150 psi) and using a reducing valve to reduce the steam pressure to (100 psi) at the retort steam supply header.

Steam header pressure at the retorts of 100 to 125 psi is recommended for best performance. Lower steam pressures can provide for adequate operation of retort systems if the steam supply valve and plumbing of the retorts is carefully selected. Temperature distribution testing in the retort system should be performed on these systems to document adequate operation under the highest plant steam demand. The greatest steam demand for retort systems normally occurs when the maximum number of steam retorts are being vented or during the heating or come-up period of other retort systems. A standard three or four crate vertical still steam retort can require from 2,500 to 6000 lbs of steam per hour (depending upon the size of the steam inlet) during venting. One-fourth to one-half of the total steam demand of the retort is required for venting. The steam demand of the retort rapidly drops to 100 to 150 lbs per hour after the vent is closed and the retort reaches processing temperature. Other retort systems have similar peak demands during the initial heating phases of the retort and product. The steam supply to the retorts must take into consideration the maximum number of retorts to be vented or brought to processing temperature at one time and any other equipment, such as blanchers, steam kettles, steam peelers, and steam exhaust boxes, which may have a steam demand at the same time. The steam demand of the entire plant should be taken into consideration during the establishment of venting and come-up schedules for retort systems. Fluctuations in the header steam pressure should be correlated with fluctuations in retort temperatures, long vent periods and long come-up times in the retort systems.

Most boiler systems require the addition of water treatment chemicals to prevent the buildup of scale on the boiler tubes and to prevent corrosion in the boiler system. When the steam comes into contact with the lacf, through direct injection of the steam into the food; during the exhausting of containers in a steam exhaust box; through injection of steam into the headspace of containers to form a vacuum, or through any other means, the boiler additives must be approved for use as a food additive and labeled for that use as per Title 21 CFR Part 173.310, Boiler Water Additives.


Although retort systems vary from still batch type retorts to more complex continuous retorts, employ different heating mediums during processing, and may use control systems ranging from operator to computer controls; there are still basic equipment requirements specified in the lacf regulations 21 CFR Part 113 that are applicable to all retort systems.

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