Boilers are pressure vessels designed to heat water or produce steam, which can then be used to provide space heating and/or service water heating to a building. In most commercial building heating applications, the heating source in the boiler is a natural gas fired burner. Boilers have several strengths that have made them a common feature of buildings. They have a long life, can achieve efficiencies up to 95% or greater, provide an effective method of heating a building, and in the case of steam systems, require little or no pumping energy.
However, fuel costs can be considerable, regular maintenance is required, and if maintenance is delayed, repair can be costly. Guidance for the construction, operation, and maintenance of boilers is provided primarily by the ASME (American Society of Mechanical Engineers), which produces the following resources:
• Rules for construction of heating boilers, Boiler and Pressure Vessel Code, Section IV-2007
• Recommended rules for the care and operation of heating boilers, Boiler and Pressure Vessel Code, Section VII-2007
Boilers are often one of the largest energy users in a building. For every year a boiler system goes unattended, boiler costs can increase approximately 10%. Boiler operation and maintenance is therefore a good place to start when looking for ways to reduce energy use and save money.
How Boilers Work
Both gas and oil fired boilers use controlled combustion of the fuel to heat water. The key boiler components involved in this process are the burner, combustion chamber, heat exchanger, and controls.
Boiler controls help produce hot water or steam in a regulated, efficient, and safe manner. Combustion and operating controls regulate the rate of fuel use to meet the demand. The main operating control monitors hot water temperature or steam pressure and sends a signal to control the firing rate, the rate at which fuel and air enters the burner. Common burner firing sequences include on/off, high/low/off and modulating.
Boiler safety controls include high pressure and temperature, high and low gas/oil pressure, and high and low water level and flame safeguard controls. These controls are considered safeties or limits that break the electrical circuit to prevent firing of the boiler. For example, in the event pressure in the boiler exceeds the pressure limit setting, the fuel valve is closed to prevent an unsafe, high pressure condition. The safety circuit of a flame safeguard control system typically includes switch contacts for low water cutoff, high limits, air proving switches, redundant safety and operating controls, and flame detectors. Flame detectors often consist of flame rods, and ultraviolet or infrared scanners to monitor the flame condition and deactivate the burner in the event of a non ignition or other unsafe condition. Flame safeguard controls are programmed to operate the burner and cycle it through the stages of operation.
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