Foundation for Sustainable Facility and Process Heat
Typically fueled by natural gas, the steam boiler is the workhorse of a food facility. This pressure vessel produces the heat necessary for ovens, kettles, vessels, vats, reactors, water heating, and building heating. Boilers must be chosen with care: the food production process, fluctuating operational loads (now and projected), efficiency, maintenance cost and effort, and the availability of operational resources affect the type of boiler system available to you.
The key to designing energy efficient systems is to not just consider the peak efficiency of energy consuming equipment, but also the efficiency for the conditions during actual equipment operations. For example, a boiler heater might be rated highly efficient at a certain operating point temperature, but much less efficient at the actual operating point for which it will be used.
A food facility’s steam system consists of a boiler and its components, including condensate pumps, pressure and temperature valves, a deaerator, steam trap, and a blowdown separator. (A water treatment system for makeup water is also necessary.)
There are many types of boilers, yet two basic types are typically used in the food industry: fire-tube and water-tube boilers.
A fire-tube boiler uses hot gases in tubes to transfer the heat to water contained in a pressure vessel. Most fire-tube boilers use a horizontal arrangement of tubes with a burner located at one end of the boiler. Fire-tube boilers are designed for long-term industrial use. The shell of the boiler stores the water, and many sizes are available to support large-scale, high-demand critical operations that experience swings in steam requirements. The water stored in the shell of fire-tube boilers is reserved to buffer a changing steam demand.
Fire-tube boilers are also distinguished by wetback design or dryback design. The difference is the end of the boiler that is opposite to the burner. This space is used to return (or turn) the hot combustion gases back to the front of the boiler. The hot gases typically flow through the bottom tubes, turn back at the end of the boiler, and then flow through the top tubes and out the stack.
If the space where the gases turn back is lined with refractory material, the boiler has a dryback design. These boilers cost less upfront but are usually less efficient.
If the space where the gases turn back has water outside the gas chamber, the boiler is a wet-back design. These boilers cost more upfront but are typically more efficient.
Each of these boiler types can also be constructed with more than two passes of the combustion gases in the boiler; some fire tube boilers include three-pass or four-pass designs. In general, the greater number of passes in the design, the higher the efficiency for extracting heat.
A water-tube boiler circulates water inside tubes that have combustion gases flowing along their exterior. Very large boilers (boilers that are built at the site), such as those in power generating plants, have this type of design. This design, however, has also been adapted for smaller, modular factory-built boilers.
Factory-build modular water tube boilers require less space that a fire-tube boiler. Rather than invest in a large fire-tube boiler that may use more energy than needed at a given time, some facilities opt for vertical, modular water-tube boilers. These can be located side-by-side and controlled as a group to produce steam for varying demands. The downside is that these systems are very sensitive to water treatment, and proper water treatment (specific to the incoming conditions of the supply water and boiler needs) is critical in modular systems for consistent operation and longevity of the system.
Steam generators are water-tube boilers as well. They utilize a single pass coil through which water is pumped and converted to steam. These types of units are often smaller and quicker to start up but less able to provide steam during periods of variable demand. They are designed to produce a fixed number of pounds per hour of steam.
Open vs Closed Loop Systems
Both closed and open steam systems are commonly used in food processing facilities. A closed system involves recapturing the steam, turning it into condensate water, and returning it to the boiler. This steam does not come into contact with food or surfaces that will touch food.
An open system produces steam that is not returned to the boiler. These systems can be used to humidify a space, such as ovens and proofers. The steam needs to be culinary quality; it can be directly injected into product, used to rehydrate or blanch food, humidify or heat product by direct contact, or it may be used to clean/sterilize surfaces. Therefore, any water treatment needs to be food grade and approved for its intended use. The use of strainers and filters may be needed at the point of steam use.