Industrial food processes applying heat at one or more stages are the vast majority: heat is still one of the most widespread means to reduce or virtually eliminate microbiological risk in food. Furthermore, heat is needed to cook food and thus to render it softer, smoother, more digestible and in general more appetizing. Heat is also needed for many other operations, as sanitation of packaging at pre-filling stage, pasteurization or sterilization of packaged food, cleaning and sanitation procedures of equipments, surfaces and food plant in general. Heat is often delivered via hot water, usually produced by boilers. Boilers are two-phase-flow heat exchange equipments: on one side is a boiling fluid, on the other side is either a single-phase or two-phase flow. If the equipment is such to provoke water phase change from liquid to vapor in a controlled and continuous way, it is also defined as “steam generator”. Hot water/steam produced by boilers is usually forced into heat exchangers, devices in which heat is transferred from a hot fluid to a cold fluid/mass commonly through a thin separating wall that can take on a wide variety of geometries: metal structures as pipes (shell and tube, multitube, double pipe or pipe-in-pipe exchangers, etc.) or plates (gasketed plates or matrix and plate fin-tube exchangers, etc.), in order to indirectly contact the food/beverage and transfer heat to it. Less commonly the heat exchange is not through a metal wall and there is direct contact between hot water/steam and food/liquid, i.e. uperization processes for milk treatment. In this particular example, heated water quality must be food grade; in all other cases (no direct food/beverage contact), feed-water microbiological quality is not paramount but other parameters are, as mineral content, in order to prevent fouling. Fouling is the accumulation of undesirable substances on a surface, and can rapidly diminish heat exchange efficiency and increased pressure drop in boiler equipments, especially in presence of an high surface area to water quantity ratio. Fouling of heat transfer surfaces introduces one of the major uncertainty factors into the design and operation of boilers.
Repair and maintenance of coal-fired steam boilers
1 During the operation of the boiler, the following points should be noted:
(1) Check the operation of the fan every shift, if the fan vibrates violently, it should be stopped and checked;
(2) No water can accumulate on the ground at the bottom of the boiler to prevent moisture from corroding the base.
2 The boiler shall be operated for 1-2 weeks, and an inspection shall be conducted to see if the valves, instruments and pipelines are damaged by leakage, and if necessary, they shall be repaired.
3 Every 6 months of boiler operation, the boiler shall be shut down for comprehensive inspection and maintenance. In addition to the above work, the following shall also be carried out:
(1) Remove the scale and mud on the bottom of the pot shell, and rinse with clean water;
(2) Carry out internal and external inspections on the boiler, such as whether there is corrosion inside and outside the welded steel plate of the pressure part, if there is, it should be repaired in time;
(3) After the inspection is completed, the fire surface can be painted with boiler paint;
(4) Check whether the fan is normal;
(5) If necessary, the insulation layer can be removed for a thorough inspection;
(6) The boiler base, insulation cover and flue duct should be painted at least once a year.