Boiler Fundamentals: Examining All Sides of the Combustion Triangle
The first step toward the safe and productive operation of boilers is a good working knowledge of boiler fundamentals.
Industrial plants need steam for heating and other processes. Boilers are commonly used to provide this steam, and you may be called on to operate the boilers in your plant. The first step toward the safe and productive operation of boilers is a good working knowledge of boiler fundamentals.
The function of boilers is to produce steam. Steam is produced by heating water to its boiling point. When water reaches its boiling point, it changes from a liquid phase to a vapor phase. It’s this vapor that we call steam. To produce steam, you essentially need two things — water and heat. To generate the heat needed for steam production, boilers rely on the process of combustion, which is the burn of fuel to provide the required heat.
In simple terms, there are three requirements for combustion, which may be illustrated on a combustion triangle. One requirement is fuel. Most boilers burn oil, natural gas, or coal. The second requirement is air. More specifically, air that contains the oxygen needed for combustion. The third requirement is heat. Heat is required to raise the temperature of the fuel air mixture to a point where a chemical reaction or ignition takes place. If any one of the requirements is missing, combustion will not occur. Using natural gas as the fuel, the process of combustion occurs following the below chemical equation. This equation states when natural gas and combustion air are mixed, heat will be generated and will result in increasing water temperature.
CH4 + 2 O2 → CO2 + 2 H2O
In this process, NO, NO2, and NO3 (nitrous oxides, NOx), as well as carbon dioxide, are produced. The NO family is formed as nitrogen in the air is mixed with excess air. This process is common for all types of boilers.
The two requirements for steam production are water and heat. The heat is provided by combustion. Boilers are designed to allow the requirements for steam production and combustion to come together. The function of a boiler is to convert energy contained in the fuel into high temperature steam. Power plants, depending on the size, use different types of boilers. Water-tubed boilers, where water flows through the tube and fire surrounds it, are among the most common types of boilers. These units are ideal for high-pressure applications.
Fire tube boilers also can be used for small applications. Burners in the boiler convert chemical energy into thermal energy. The hot gas that is produced travels all over the boiler. The first heat absorption unit of the boiler is the economizer. Highly pressurized water supplied by the feed water pump enters the economizer. The economizer absorbs energy from the flue gas, which raises the temperature of the liquid. From there, the water flows into a steam drum to the down comer. It then passes through a narrow tube section called a water wall. The phase change of the water happens in the water wall. The steam that is produced, in addition to some water droplets, enters the steam drum again. The real function of the steam drum is to separate the water droplets from the steam. As a result, the steam leaving the steam drum is saturated and pure.
According to the second law of thermodynamics, the greater the temperature of the heat source, the more efficient the cycle is. Therefore, making the steam temperature very high will result in a highly efficient power plant, but this temperature increase has certain limitations. The steam turbine blade material cannot withstand temperatures greater than 600ºC. So one more heat exchange is added after the steam drum to superheat the steam and make sure it is heated to the maximum allowable limit. Superheating also reduces the moisture content in the steam. The steam is then supplied to the turbine inlet. After the first turbine stage, the temperature drops. One of the best solutions to increase power plant efficiency is to bypass the steam completely after the first stage and add more heat. This process is known as reheating. A heat exchanger known as a reheater is used for this purpose. Reheating and superheating also increase the power plant capacity along with the efficiency.
When the temperature increases, will the pressure increase too? Water inside the boiler is an open flowing system. The water is free to expand when the liquid is converted into steam, which means the pressure should remain the same throughout an ideal boiler. In reality, the pressure doesn’t remain constant, and usually there is a small drop across the boiler due to friction and other irregularities.
A condensing boiler is a boiler with a heat exchanger that is made of materials that can withstand acidic condensate. Typical materials for condensing boilers in the market are aluminum alloy, 316L, cast iron, SA 240 316Ti, and copper tube without a secondary heat exchanger. Condensing boilers achieve high efficiencies of 99 percent-plus by condensing water vapor in the exhaust gas and recovering the latent heat of vaporization, which otherwise would’ve been wasted.
The differences between condensing boilers and noncondensing boilers are explained in Table 1.
Most everyone in the boiler industry knows that condensing boilers are generally more expensive than noncondensing boilers, but the increase in initial cost varies based on the type of heat exchanger, configurations, and manufacturers. Based on different analysis, it was determined the maintenance costs for a full condensing boiler and a partial condensing boiler are almost the same, while a full condensing boiler is approximately 25-30 percent more expensive than other boilers of the same size.
The initial higher cost for a condensing hot water boiler system doesn’t necessarily end at the boiler, as equipment selection would be different depending on the temperatures of hot water supply and return. The most common change that makes a difference is the physical size of the hot water coils in the system and, hence, the corresponding cost of them. Hot water coils will require deeper coils, which act as a fin with large surface area for the heat transfer. These deep coils allow a lower hot water supply temperature by increasing the heat transfer area. The deeper the coil is, the higher the initial cost. This isn’t limited to just the initial cost. The operating cost will increase, too. As the hot water supply temperature decreases, the deeper the coils are needed, hence water pressure drops. Designers need to take into account the initial cost of the boilers, maintenance costs, designed hot water supply temperature, pumps, and pressure drops in the fittings.
The typical heating systems in industrial/commercial facilities include boilers, hot water pumps, VFDs to adjust speed, pipes, discharge pressure sensor, AHU heating coils, and VAV reheat coils. Figure 4 shows a heating system serving a five-story hospital. The system includes three gas-fired boilers piped in parallel with three non-dedicated hot water pumps. The pumps are equipped with VFDs, which allows them to modulate based on the discharge pressure sensor. How well the condensing boilers fit within the boiler/heating system’s design and operate in condensing mode depends on the return water temperature from the building.
The rule of thumb in designing heating systems is the greater the temperature differential between the supply hot water temperature and return hot water temperature, the more efficient the system will be. Usually, HVAC designers find a balance between the design gpm and the delta T to minimize the energy consumption and maximize system efficiency. Typical measures in order to reduce energy usage include the following:
Boiler management system — Install a BAS system or master controller to stage the boilers and modulate the firing rates to achieve the highest boiler efficiency given different load requirements. This controller will also rotate the lead/lag/standby/backup boilers based on a given schedule to smoothly distribute the run-time hours smoothly and avoid any excessive overtime cycle.
Reset the hot water supply temperature (HWST) set point with the outside air temperature (OAT) — By adjusting the HWST based on OAT, hot water return temperature will be reduced as well.
Add/optimize the boiler lockout — Add or optimize control of a heating water system to disable the boiler when the outside air dry-bulb temperature exceeds a specified temperature set point. Generally, a 60ºF lock out temperature is considered an acceptable temperature.
- Reduce water loss/leakage in the system;
- Proper insulation of heating pipes and traps;
- Add boiler stack energy recovery units to preheat the hot water return temperature;
- Run the steam system at low pressure; and
- Perform regular and timely maintenance on all steam condensate traps.