Within the food industry, steam is used for processing, drying, and heating, and for general use such as in sanitizing. Some type of steam generation system is included in most food processing plants. The demand for steam varies in different operations; and in some plants or parts of the plant, steam is used continuously, while it is used intermittently in others. The steam generation system must be adapted to the operations that will be performed within a plant. Since the boiler room is the energy source for most processes, the system needs to be designed with possible expansion and maintenance in mind.
Steam is produced in a boiler or steam generator. The boiler is housed in a building that is frequently separate from the main processing area. It is always surprising to step into a boiler room to get the cleanest and best maintained piece of equipment on the premises. There are many types and sizes of boilers that can be found in industry. In simple terms, a boiler can be seen as a closed container with water under pressure in it. In a furnace, the chemical energy of a fuel is converted to heat energy that must be transferred to the water in such a way that as little as possible of the heat is lost. The heat of combustion is transferred to the water with conduction, convection, and radiation. The contribution of each of these modes of heat transfer depends upon the type and design of the boiler. The two principal types of boilers used for industrial applications are fire-tube and water-tube boilers.
In this boiler, hot combustion gases run through the boiler tubes. The tubes are surrounded with the water that will be heated. This system is contained in a large pressure vessel that will also contain the water and steam. In some applications, the furnace is also housed within the same vessel. Fire-tube boilers are available with pressure outputs of 103.4 to 2413 kPa. When higher pressures are required, the shell of the boiler needs to be made of thicker material.
Fire-tube boilers are designed to work with large water content and require a large space for the water pressure vessel. It requires a large amount of space. The boilers take a long time to produce steam from a cold start and are, therefore, not suitable for intermittent use. However, they are reliable and capable of producing high-quality steam. They have a large water capacity and can handle variable loads with almost no variation in steam pressure. It can be envisaged as a shell and tube heat exchanger. The large number of tubes causes a very large surface area resulting in a large heat transfer area. This increases the overall boiler effectiveness.
In water-tube boilers, the combustion gases pass around tubes that are filled with water. The tubes are interconnected on the feed side and the steam side. This boiler is often called a safety boiler because there is less chance of a serious accident when one of the water filled pipes inside the boiler bursts. Water-tube boilers can normally operate at higher pressures than former boilers. In power plant applications, pressures of 10,342 to 13,789 kPa are often used. The water tubes contain steam and steam and water mixtures, while the hot combustion products pass around them. They can have more than one drum. Tubes, some of which can be in the form of waterfall tubes lining the furnace area, connect the steam drum and mud drum. This boiler is faster than former boiler to come up to full pressure because they contain a relatively small amount of water. The design of this type allows for the production of superheated steam. In the case of load variation, and where high pressure or superheated steam is required in the process, the use of a water-tube boiler should be considered.