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A boiler is comprised of two basic systems. (See Figure 1-1.) One system is the steam water system also called the waterside of the boiler. In the waterside, water is introduced and heated by transference through the water tubes, converted to steam, and leaves the system as steam. Boilers must maintain a chemical balance. The manner in which this is done can interact with the feedwater control system. The amount of blowdown must be considered in the feedwater control scheme, especially if the blowdown is continuous. Often, the blowdown flow is divided by the concentration ratio times the feedwater flow. Continuous blowdown is the common method for controlling the chemical concentration. On large boilers this may be done auto- matically by measuring the boiler water conductivity to control the blowdown rate.
The blowdown rate may also be achieved by combining the conductivity with ratio control of blowdown, ratioing blowdown to feedwater flow. In utility plants, conductivity is usually meas ured and blowdown is achieved manually. This is required on a periodic basis or when the conductivity gets too high. Conductivity is measured in micro mhos which is equal to the reciprocal of 1 mechanical ohm (resistance). The other boiler system is the fuel air-flue gas system, also referred to as the fireside of the boiler. This system provides the heat that is transferred to the water. The inputs to this system are the fuel and air required to burn the fuel. The fuel and air chamber is also referred to as the windbox.The outputs are the flue gas and ash.
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