30 Questions
What is the major difference in the primary sensing elements between steam boilers and hot water boilers?
Steam boilers require pressure-sensitive sensors, while hot water boilers require temperature-sensitive sensors.
In two-position firing rate control, what is the cut-in point for the burner in the case of steam boilers?
35 kPa
What is the primary manipulated variable in steam boiler control systems?
Firing rate
Why is two-position control only used in smaller capacity boilers?
Energy losses due to frequent cycling have less economic impact for small capacity boiler plants than for large capacity plants.
What happens when a hot water boiler continues to fire at an excessive rate?
The boiler will produce supersaturated hot water and over-pressurize due to liquid expansion.
What is the primary focus of this chapter?
Boiler-firing circuits
Which code most suitably addresses automatic firing rate controls for boilers?
ASME CSD-1 Controls and Safety Devices for Automatically Fired Boilers
What is the purpose of firing rate controls in boilers?
To increase boiler efficiency
In the event of an operating control failure, what must high limit controls be able to do?
Shut off the burner to prevent unsafe operating conditions
What must the energy output of a boiler always equal?
The energy demand
What is the purpose of high-low firing rate control in a boiler?
To maintain finer control of the boiler steam pressure or temperature
Why do high-low boilers require two operating pressure controls (pressuretrol) instead of one?
To maintain the high-fire and low-fire burner circuits separately
What happens when the low-fire burner circuitry is energized in a high-low boiler?
It remains energized until the boiler pressure reaches a certain level
In a multi-position firing rate control, what happens when the load demand increases?
The low-fire burner cannot satisfy the load demand and the boiler pressure continues to drop
What makes fully modulating firing rate control different from high-low and multi-position firing rate control?
It can operate at any one of an infinite number of firing rates between high-fire and low-fire
What causes mercury switches to occasionally fail?
Oxygen entering through small cracks and oxidizing the mercury
What type of switches are used instead of mercury switches?
Purely mechanical switches with a pressure capsule
How does the high-pressure cut-off (HPCO) differ from the operating pressure control?
It has a manual reset button
What is the purpose of the high-temperature limit control for hot water boilers?
To prevent the water temperature from exceeding the maximum allowable water temperature
How is the high-temperature cut-off (HTCO) different from the operating temperature control?
It has a higher setting than the operating temperature control
What is the purpose of limiting the boiler to low-fire when starting up a cold boiler with fully modulating firing rate control?
To prevent uneven thermal expansion and excessive stress on the boiler
What type of control devices do hot water boilers with fully modulating firing rate control use to control the firing rate?
Variable resistance temperature-actuated sensors
What type of switches are used as operating pressure controls, low fire controls, and high-pressure cut-offs in two-position (on-off) steam boilers?
Pressure-actuated two-position single pole, single throw switches
What is the purpose of the high-pressure cut-off control in hot water boilers with fully modulating firing rate control?
To prevent generation of steam pressure above the maximum allowable working pressure
What is the purpose of the pressuretrol in a steam boiler equipped for fully modulating firing rate control?
To position a modulating motor that drives the combustion air damper and the firing rate control valve
What is the purpose of the manual reset button on the high-temperature cut-off (HTCO)?
To lock the switch in the open position when activated
What is the purpose of a modulating pressure control in steam boilers with fully modulating burners?
To control how closely the boiler pressure tracks the set point
In hot water boilers with fully modulating burners, what is the purpose of a modulating temperature control?
To control how closely the boiler pressure tracks the set point
What is the function of a modulating motor in electrical control systems for boilers?
To position dampers and valves in electrical control systems
Why might some larger boilers use two modulating motors?
To control both air and fuel flow
Test your knowledge of boiler pressure control and its impact on plant efficiency. Learn about the operating principles and controls required for hot water boilers with fully modulating firing rate control.
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