Steam Boiler Types
The generation part of a steam system uses a boiler to add energy to a feedwater supply to generate steam. The boiler provides a heat transfer surface (generally a set of tubes) between the combustion products and the water. There are several general types of boilers with varying qualities and differing boiler designs to serve different purposes. Some boilers run on natural gas, while others use heating oil or propane. There are both high pressure and low-pressure boilers, and there are some that can do either. Steam boilers are of different types but there are two basic ones and others are somewhat an extension of these two: fire tube and water tube.
A fire-tube boiler is a type of boiler in which hot gases from a fire pass through one or more tubes running through a sealed container of water (Figure 23.1). The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam. The tank is usually cylindrical as it is the strongest practical shape for a pressurized container. This cylindrical tank may be either horizontal or vertical.
Water-tube boilers consist of many tubes that travel through the combustion chamber, heating the water inside the tubes to the point of producing steam. The tubes themselves can vary in size depending on capacity and style. Water-tube boilers are typically constructed with a lower and upper header with many tubes between the two headers. Water is always in the lower drum and steam is always in the upper drum. Water tube boilers are suitable for low and high pressure needs as well but can achieve pressures significantly higher than fire-tube boilers.
Some manufactures offer modular steam boilers to address load swings, high efficiency in low demand periods, and floor space constraints. The main components of a modular system are a feedwater inlet, lower header, series of tubes or coils, upper header, and steam separator (Figure 23.2). This design may seem similar to the water-tube boiler, but its application is what sets it apart from the traditional water tube design. A modular boiler converts water to steam in only one pass. Flames engulf the tubes, quickly transferring heat to the water and turning the water to steam faster than traditional designs.
High-Pressure or Low-Pressure Steam
A steam boiler is used to heat water into steam, and the steam is circulated through a closed-loop piping system to transfer heat and/or humidity to a process. If there are processes that require different steam pressures and temperatures within the system, pressure reducers can be used to modify the pressure of the steam for specific pieces of equipment. Alternatively, the brewer can select separate steam boilers with different pressure ratings for each process with a different pressure.
Burner design for packaged boilers varies with boiler output. Packaged boilers can be fueled by a range of fuels, including natural gas, #2 fuel oil, propane, syngas, biogas, #6 fuel oil, and electric. Burners can be designed to be dual-fuel units with a primary fuel and secondary or backup fuel. Natural gas is by far the most common fuel source for both fire tube and water-tube boilers. If not available, then propane and diesel (#2 oil) would be the other two options. In parts of the country where fuel oil is common, this can be used as the fuel source. Propane is also an option where natural gas or fuel oil is not available.
Efficiency. Since electric boilers do not require vent stacks, they are very efficient; with outputs between 95 and 99 percent. Natural gas boilers are typically between 80 and 85 percent efficient which means that 15 to 20 percent of the heat goes up the stack.
Electric Steam Boilers. An electric steam boiler is a type of boiler where the steam is generated using electricity, rather than through the combustion of a fuel source. Electric boilers are known for being clean, quiet, and easy to install, and compact. The lack of combustion results in reduced complexity in design and operation and less maintenance.
Sizing a Boiler for a Brewery
A steam boiler should be sized according to the connected load. The connected load includes all the vessels and tanks as well as the piping. The manufacturer of the brewing equipment should provide the Btu consumption of their equipment. Another sizing consideration is the planned expansion of the brewery. An oversized boiler can result in frequent cycling while an undersized boiler may never be able to meet the temperature requirements of the brewery.
Distribution System. From the boiler house the steam is carried to the various processes within the brewery by the distribution system, where it should be available at the correct pressure, in sufficient quantity and in the best possible condition. It is important that pipework carrying steam from the boiler house to the equipment is sized correctly. If the piping is too large, it could inflate the installation cost. If the piping is too small, the boiler will cycle continuously affecting the ability to brew.
Steam boilers are often required to be installed in a separate “boiler room.” Ideally, the location of a boiler room in the brewery should be as close to the equipment that it is serving as possible. In general, this means as close to the kettle and fermenters as possible. This is meant to minimize the distance that steam, cold water, etc. needs to travel to get to and from your equipment, which reduces piping costs and heating/cooling loss. There isn’t a well-defined rule about proximity to equipment, but it is an important design consideration.
The unique nature of the brewing industry applications referred to earlier is that brewing is primarily a batch process whereby large amounts of steam are required for the mashing process for a limited period, and the same condition occurs for the wort boiling process. Other steam uses such as hot water production, clean-in-place, pasteurization, and steam heating, plus other non-production uses, take away from boiler capacity, possibly when that capacity is needed most.
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