Smart Boiler Specifications
Engineers responsible for selecting and designing base building heating systems usually have many questions from owners regarding boiler selections. Other than cooling-water systems, boiler plants are typically the most costly—and thoroughly debated.
Methods of controlling boiler and burner should be discussed so the engineer can specify a “smart boiler.” The key is a smart specification—a guide for obtaining the most reliable, cost effective and efficient system possible.
Boiler plant controls integrate boilers, burners, pumps and other components to create an efficient, safe and reliable source of heat. The system oversees all components to ensure that flame and feedwater rates are balanced, and steps in to restore safe conditions if balance is upset. The goal of control system design is to regulate evaporation of boiler water to match the condensing rate at use points.
The heart of the control system is measurement of steam pressure and boiler-water level. These readings are fed to burners and feedwater pumps, where the flame rate and feedwater flow rate are controlled. The cycle of pressure/water-level measurement and flame/feedwater-rate control forms the feedback loop.
Another set of redundant devices monitors the steam pressure, boiler water level and flame to ensure that these variables stay within safe ranges. Generally, these devices have two set points: pre-alarm and safety cut-off. Pre-alarm setpoints are outside normal operating range and alert operating personnel to system failures, but these setpoints are within safe operating level and allow warnings to correct the failure. The safety cut-off setpoint is at a level where damage is imminent and initiates flame shutdown. As a last resort, boilers are equipped with safety-relief valves for pressure in excess of designed vessel limits.
A higher level of control concerns communication of elementary pressure/water level control to a central system for boiler room and, ultimately, building management.
• Water level controls. Boiler-water level depends upon synchronization of evaporation and condensate return. Condensate return rates can be unpredictable, due to waste at use points or variable usage. This may result in variable water levels, which can lead to the risk of thermal shock. There are two main methods to control the water level of a boiler:
Intermittent control. This method employs a set of low water-level switches to turn on feedwater pumps and another set of high water level switches to turn pumps off. This results in a continuously varying water level, acceptable for boilers with large water volumes, such as the firetube (see discussion of boiler types below).
Continuous control. This approach brings a more precise level control, as is needed for watertube boilers and in cases of frequent load swings. Feedwater pumps run continuously, constantly refreshing the feedwater supply, and a modulating control valve adjusts feedwater rates to match rates of water-level change.
Integration with building-wide systems
Integration with a building automation system (BAS) can enhance boiler efficiencies and maximize operational advantages. Some key considerations:
• Boiler management panels. These panels (see Figure 2) are microprocessor-based controllers that serve as “the brains of the boiler.” The panels read real-time data such as firing rate, flame safeguard and operating parameters such as water level, pressure, etc., via solid-state sensors. They take this information and process it to maintain efficient firing of the boiler. Complete, self-diagnostic checks and digital displays are provided to enhance operation and troubleshooting.
• Boiler management communication bus. A number of boiler management panels can be linked together via a communication bus (see Figure 3), which allows an exchange of information among individual panels, a direct digital control (DDC) system and an operator via a personal computer. The communication network can be extended to remote locations via modem.
• DDC strategies and capabilities. The flexibility, reliability and energy efficiency of the boiler plant is enhanced by its ability to gather, analyze and communicate information to the DDC system and remote operator stations. Some of the software strategies available with a fully integrated boiler-management system include:
Historical data logging. Useful for “seeing” load profiles and dynamic relationships between operating variables, this is available in some software packages with graphing capabilities or exporting of data to spreadsheets for analysis.
Alarm annunciation. Communication of impending dangerous conditions can be carried through the communication bus, which can be configured for remote paging of operating personnel. Although most states and insurance authorities require a watch engineer, this feature can free up operators to perform other duties.
Time-of-day scheduling. Operating setpoints can be reset based on day of week, time of day, etc., to adjust firing rates and pressure settings—coordinated with the overall building control system—for unoccupied time setbacks. Optimum start/stop is a further development of time scheduling, an adaptive software strategy that adjusts start, stop and setback times to minimize energy usage.
Lead/lag: This software compares the operating hours logged by units in multiple boiler plants to vary sequencing and ensure even wear.
Firing level control. Remote adjustments to firing are possible, but boiler management panels automatically determine safe, efficient firing rates that should only rarely be overruled.
Operating variable status. This allows anyone with access to the communication network the convenience of checking the value of any real time operating variable.
Combustion optimization: The amount of air permitted into the combustion chamber is usually regulated by installing oxygen sensors in the stack that measure oxygen in the flue gas and modulate combustion fans and dampers to optimize air entry and, thus, combustion.
The smartest specs
Specifying the correct boiler and controls can be a monumental proposition. With so many manufacturers and options available, the key is to determine which actually add value to building operations, yield cost savings and ease of maintenance.
Rather than simply specifying expensive options that are hard to use, understand and repair, engineers should carefully weigh advantages of each boiler types, their operating characteristics—and suitable system control and integration schemes—in order to specify a truly “smart boiler.”
Boilers at a Glance
Control strategies for the boiler plant depend greatly on system features. An overview helps place these specification decisions in perspective.
• Firetube boilers. Two basic designs are offered: Firebox boilers, in which the boiler shell sits on top of the combustion chamber, in sizes ranging from 12 to 300 boiler horse-power (bhp); and scotch marine boilers, designed with multiple tube passes in which hot flue gases heat water to desired temperature (hot water) or saturation (steam) in capacities to 1,500 bhp.
Firetube boilers are available in dryback designs (with refractory-lined chambers outside the vessel to direct combustion gases from furnace to tube bank) and wetback designs (with water-cooled chambers directing flue gases from furnace to chambers.)
• Watertube boilers. For industrial high pressure steam or hot water, these offer fast generating capacities. Since they hold less water than firetube models, they permit faster response and can generate steam at higher pressures. Some packaged designs can generate steam at up to 3000 psig and 450°F, thanks to strong, thin boiler tubes.
• Cast-iron boilers. Limited to producing low pressure steam or hot water, these are usually used in residential or light commercial applications, in sizes ranging from 25 to 200 bhp. In spite of advantageous modular designs, they have lower efficiencies, smaller capacities, and higher installed costs and maintenance needs.
Common fuels for industrial and commercial heating plants include lightweight oil (no. 2), natural gas and “dual fuel” combinations. Two burners are available for these fuels:
• Power burners. Associated with gas, power burners must be specified with attention to operating altitude or the heat content of the gas being used, or both. Also critical is the pressure of gas provided by local utilities, since it must satisfy required levels at gas train assemblies. Methods for controlling airflow:
Natural draft. The chimney draws products of combustion through the boiler, and the atmospheric burner relies on a natural draft to deliver air for combustion. Here, chimney design is critical, since air flows at all times.
Forced draft. A fan pushes air through the burner and boiler, so that combustion occurs under positive pressure. Gas and combustion air can be modulated effectively, although the possibility of positive pressure in the stack may require leak-tight construction to prevent the migration of deadly flue gases.
Gas modulation is accomplished through a gas train assembly.
• Pressure-atomizing burners. These are associated with oil, but—like the power gas burner—combustion air is introduced either as natural or forced draft. In pressure-atomizing burners, however, the fuel is introduced directly into the combustion chamber.
Fuel oil is pumped at pressures of 100—300 psig through a nozzle orifice creating a fine mist, and fan air is forced across the mist. A spark-ignited gas or oil igniter creates combustion.
• Dual-fuel (oil/gas) burners. Typically forced-draft burners, these units offer a delay for pre- and post-purge cycles to commence before changeover to the “new” fuel. Automatic changeover (based on outside air temperatures) is available on larger boilers.
Fuel delivery to burners is regulated in response to a steam-pressure sensor, reverse-acting type, which increases firing rate as steam pressure is reduced. There are three methods for controlling output:
• On/off cycling control. Typical for small boilers (up to 1,000,000 btu/hr), the burner is cycled on or off to maintain required pressure (steam) or temperature (hot water). This method may not be favorable due to inefficient cooling of heat-transfer surfaces within the boiler.
• High/low fire. The oil or gas burner has two preset firing rates: high and low. Efficiency is increased because burner fire rate of fuel input more closely matches load profile. This method reduces the extreme thermal cycling of boiler heat-transfer surfaces.
• Modulating control. Used in most large applications, boiler output is adjusted to match load when above the low-fire limit (usually about 15 percent of capacity). Pressure or temperature is measured to closely match amount of entering fuel. This method is highly energy efficient, since fuel input exactly matches load profile and eliminates thermal cycling.
For fuel modulation, gas shut-off valves can be specified with position motors to operate on/off or high/low/off. When high/low/off is specified, the air-intake damper (or valve) is controlled by the valve’s modulating motor. Valve position governs the amount of combustion air required for the gas-input rate.
Boiler feedwater systems
Steam condenses and returns to boilers at unpredictable rates, potentially compromising the need for steady water supplies. Also, method of return can directly impact efficiency.
Boiler feed systems provide a reservoir with sufficient surge capacity to satisfy large, intermittent condensate flow rates and provide stable suction conditions for boiler feedwater pumps. Therefore, accurate calculation of peak condensate return is vital for proper selection of boiler feed tanks. Common methods and equipment:
• Deaerators. When makeup water exceeds 25 percent of total supply, a deaerator is needed to remove oxygen and carbon dioxide—which can cause corrosion—from makeup water and returning condensate. Another function of deaerators is to preheat makeup water with exhaust steam and/or return condensate, improving boiler efficiency and reducing the risk of thermal shock.
• Surge tanks. Used for intermittent peak return-condensate loads exceeding deaerator storage capacity, surge tanks help reduce the amount of makeup water needed.
• Boiler feed. These systems combine boiler-feed pumps with a tank to store condensate and makeup water. When makeup water exceeds 50 percent, preheating is needed. Typically, feedwater is heated to about 210°F, increasing boiler efficiency and helping release oxygen and carbon dioxide.
A Sample Boiler Specification
Provide boilers rated, tested and built as per AMBA, UL and ASME, and install in accordance with NFPA 31, NFPA 54 and all insurance companies. Provide factory-assembled and tested packaged firetube boiler, 3- or 4-pass design with horizontal tubes and minimum 5 sq. ft of heating surface per rated BHP. Mount on steel base with forced-draft burner and packaged burner controls.
Provide complete boiler trim (water column, feedwater pump control, low-water cutoff and auxiliary, steam pressure gauges, relief valves, and controls mounted near water column) to regulate burner operation, as well as front and rear doors, hinged, sealed with heat-resistant gaskets, and fastened with lugs. Include doors and observation port for inspection and cleaning, and a manhole for boilers 48 inches and above. Additional items: Refractory and insulation in all doors, exhaust vent and thermometer, 2-inch thick fiberglass insulation jacket on boiler shell, and hard enamel finish.
Fuel and piping specifications: Burner shall be combination low-pressure, air atomizing type (for No.2 oil) and multiport type (for gas). Gas pilot shall be premix type with automatic electric ignition and electronic monitor for pilot; pilot train shall include two shut-off valves, solenoid valve, pressure regulator and gauge. Provide oil pump with capacity of two times maximum burning rate.
Fuel-oil piping includes oil-pressure regulating devices, oil-metering controls, solenoid shut-off valves, pressure gauges, and low oil-pressure switch. A fuel-oil controller shall be provided to combine all fuel-oil controls.
Mount a separate air-compressor module on boiler with low atomizing air-pressure switch. Gas burner piping shall include a motor-operated primary gas shut-off valve with proof-of-closure switch and plugged leakage-test connection, and a lubricated butterfly valve at the front of gas train as a means for testing. Provide high and low gas-pressure switches; turndown ratio of the boiler shall be 10:1 for gas and 8:1 for oil.
Control panel and flame-safeguard control: Controls system shall consist of computer-based boiler controller and solid-state sensors integrated for automatic burner sequencing, flame supervision, and status indication, with full modulating control of fuel, combustion air and firing rate as per system demand.
Status indicators shall be Ready, Demand, Main fuel valve open, Low water cutoff, and Alarm. System shall have pre-purge cycle and allow selection of rate-control programs for precise load tracking to reduce on-off cycling due to load swings. A low-fire cutoff shall be included. System monitors inputs from sensors for gas pressure, oil pressure, and temperature, with high/low limits programmable from keyboard display.
Control panel shall come with a modem to allow for remote monitoring and inputs/outputs for such options as lead/lag and oxygen trim adjustments.