Boilers are part of a hydronic heating system. Hydronic systems use water to transfer heat to a distribution source, like a radiator, to heat a home. Hydronic systems can heat via hot water or steam, depending on the type of boiler used. The boiler is the part of the system that heats the water to be distributed. The key elements of a boiler include the burner, combustion chamber, heat exchanger, exhaust stack, and controls. Boiler accessories including the flue gas economizer are also commonly used as an effective method to recover heat from a boiler.
Key Components of Boilers are:-
- Burner – The burner is the component of boiler that provides the heat that heats the water of system. The fuels used can be natural gas or oil.
- Heat exchanger – The heat exchanger of boiler allows the heat from the burner to heat the water in system. The job of the heat exchanger is to carry the heat from the burner to the water without having direct contact with the water. It’s a similar idea to boiling water in a pot.
- Supply lines – Hydronic heating systems use piping to deliver the heated water or steam to the distribution points, and the supply lines are the pipes that distribute the hot water or steam to distributor.
- Return lines – When the water cools, or the steam cools and changes states back to water, the return lines bring this water back to the boiler for re-heating.
- Firebox – The firebox is where the fuel of system meets the air, creating a flame.
- Refractory – Refractory actually refers to refractory materials that are used for filling any gaps and/or openings that may be around the fire box – this helps ensure the fire stays in the fire box.
- Circulator pumps – circulator pumps push the hot water or steam from system to the heat distributors in our homes.
- Deaerators/Condenser – Deaerator and condenser tanks are only used in steam boiler systems and not in hot water and hot oil boil because here the fluid always is on liquid form. The construction of these two types of tanks is almost identical, but as their names suggest, they are used for different purposes.
Two primary principles are used with this form of tank design: thermal and vacuum. This depends on which type of boiler being used. Each principle also has different pump construction requirements.
A tank using the thermal principle is connected to the atmosphere. This design is normally used in smaller plants. Here, steam is used to maintain tank water temperature at around 105°C, which removes air from the water.
Here, an ejector pump is used to create a vacuum in the tank. This causes the tank water to start boiling because of the low temperature, which in turn removes air from the water. This principle is normally used for steam turbine applications.
Historically, economizers have only been used in large-scale power plants. However, the demand for more efficient boilers within industry and marine means that economizers are now far more commonplace. An economizer is a heat exchanger that is placed in the exhaust from a boiler or in the exhaust funnel of the main engine of a ship. Pump requirements differ greatly, depending on where the economizer is installed.
It is integral part of boiler and is placed in the path of hot flue gases from the furnace. The heat recovered from the flue gases is used in superheating the steam before entering into the turbine (i.e. prime mover).Its main purpose is to increase the temperature of saturated steam without raising its pressure.
Natural gas boilers employ one of two types of burners, atmospheric burners, also called natural draft burners and forced draft burners, also called power burners. Due to more stringent air quality regulations, low NOx burners and pre-mix burners are becoming more commonly used and even required in some areas. By ensuring efficient mixing of air and fuel as it enters the burner, these types of burners can ensure that NOx emissions are reduced.
The combustion chamber, usually made of cast iron or steel, houses the burners and combustion process. Temperatures inside the combustion chamber can reach several hundred degrees very quickly.
Heat exchangers may be made from cast iron, steel tube bundles, or, in the case of some smaller boilers, copper or copper-clad steel.
The exhaust stack or flue is the piping that conveys the hot combustion gasses away from the boiler to the outside. Typically this piping is made of steel, but in the case of condensing boilers it needs to be constructed of stainless steel to handle the corrosive condensate. Another consideration is whether the exhaust stack will be under a positive or negative pressure. This can determine how the joints of the exhaust stack must be sealed.
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, if 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 providing 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.