Most boiler thermodynamic calculations involve heat transfer between fuel, air, flue gas, water, and steam. To perform boiler heat transfer calculations, it's necessary to pre-calculate the relevant thermophysical properties.
Before introducing thermophysical properties, let's first understand some relevant common sense:
◆ The specific heat of a gas is measured per cubic meter at 0°C and 1 atmosphere.
◆ The specific heat of moist air is calculated as 10g of water per 1kg of dry air, measured per cubic meter of dry air.
◆ Average-composition flue gas, also known as standard flue gas, refers to flue gas at 1 atmosphere, with the volume fractions of CO2, H2O, and N2 being rCO2 = 0.13, rH2O = 0.11, and rN2 = 0.76, respectively. The flue gas temperature should be ≤1600°C.
Summary of Thermophysical Properties
There are four main thermophysical properties we commonly use:
More specifically, they are:
◆ Specific heat of fuel, including gaseous fuels, solid fuels, and heavy oil
◆ Specific heat of dry and moist air
◆ Specific heat of flue gas
◆ Kinematic viscosity, thermal conductivity, and Prandtl number of flue gas
Main Applications of Thermophysical Properties
The specific heat of fuel is relatively simple to use, primarily for calculating fuel consumption in heat balance calculations.
The specific heat of air is also used in heat balance calculations. Furthermore, due to the presence of excess air coefficient, flue gas after complete combustion contains a certain amount of air, so it is also used in the specific heat of flue gas.
The specific heat of flue gas refers to the specific heats of various components in the flue gas and is generally listed in a table. What does it look like? Please refer to Screenshot 1 below. The formula for calculating the specific heats of air and flue gas components in Screenshot 1 is relatively complex. To simplify the calculation, we often approximate the formula to a linear relationship between specific heat and temperature. What does this simplified formula look like? Refer to Screenshot 2 below. Using the formula in the table, we can calculate the enthalpy of flue gas at different temperatures and compositions, thereby generating an enthalpy-temperature table.
In flue gas convective heat transfer calculations, a key task is calculating the heat transfer coefficient, which requires the flue gas's kinematic viscosity, thermal conductivity, and Prandtl number. Screenshot 3 lists the kinematic viscosity, thermal conductivity, and Prandtl number for air and average-composition flue gas from 0°C to 2200°C.
Additional Note
When performing engineering calculations, the actual flue gas composition after complete combustion differs from that of the average-composition flue gas, resulting in errors in the calculated kinematic viscosity, thermal conductivity, and Prandtl number. If the actual flue gas composition is similar to that of the standard flue gas, the actual flue gas can be approximated as the standard flue gas.
If the deviation is significant, a coefficient can be introduced before the kinematic viscosity, thermal conductivity, and Prandtl number of the standard flue gas. Because the use of coefficients is rare, we will not explain this in detail. If necessary, you can check the relevant chapters in the "Standard Method for Thermal Calculation of Boiler Units" yourself.
Most boiler thermodynamic calculations involve heat transfer between fuel, air, flue gas, water, and steam. To perform boiler heat transfer calculations, it's necessary to pre-calculate the relevant thermophysical properties.
Before introducing thermophysical properties, let's first understand some relevant common sense:
◆ The specific heat of a gas is measured per cubic meter at 0°C and 1 atmosphere.
◆ The specific heat of moist air is calculated as 10g of water per 1kg of dry air, measured per cubic meter of dry air.
◆ Average-composition flue gas, also known as standard flue gas, refers to flue gas at 1 atmosphere, with the volume fractions of CO2, H2O, and N2 being rCO2 = 0.13, rH2O = 0.11, and rN2 = 0.76, respectively. The flue gas temperature should be ≤1600°C.
Summary of Thermophysical Properties
There are four main thermophysical properties we commonly use:
More specifically, they are:
◆ Specific heat of fuel, including gaseous fuels, solid fuels, and heavy oil
◆ Specific heat of dry and moist air
◆ Specific heat of flue gas
◆ Kinematic viscosity, thermal conductivity, and Prandtl number of flue gas
Main Applications of Thermophysical Properties
The specific heat of fuel is relatively simple to use, primarily for calculating fuel consumption in heat balance calculations.
The specific heat of air is also used in heat balance calculations. Furthermore, due to the presence of excess air coefficient, flue gas after complete combustion contains a certain amount of air, so it is also used in the specific heat of flue gas.
The specific heat of flue gas refers to the specific heats of various components in the flue gas and is generally listed in a table. What does it look like? Please refer to Screenshot 1 below. The formula for calculating the specific heats of air and flue gas components in Screenshot 1 is relatively complex. To simplify the calculation, we often approximate the formula to a linear relationship between specific heat and temperature. What does this simplified formula look like? Refer to Screenshot 2 below. Using the formula in the table, we can calculate the enthalpy of flue gas at different temperatures and compositions, thereby generating an enthalpy-temperature table.
In flue gas convective heat transfer calculations, a key task is calculating the heat transfer coefficient, which requires the flue gas's kinematic viscosity, thermal conductivity, and Prandtl number. Screenshot 3 lists the kinematic viscosity, thermal conductivity, and Prandtl number for air and average-composition flue gas from 0°C to 2200°C.
Additional Note
When performing engineering calculations, the actual flue gas composition after complete combustion differs from that of the average-composition flue gas, resulting in errors in the calculated kinematic viscosity, thermal conductivity, and Prandtl number. If the actual flue gas composition is similar to that of the standard flue gas, the actual flue gas can be approximated as the standard flue gas.
If the deviation is significant, a coefficient can be introduced before the kinematic viscosity, thermal conductivity, and Prandtl number of the standard flue gas. Because the use of coefficients is rare, we will not explain this in detail. If necessary, you can check the relevant chapters in the "Standard Method for Thermal Calculation of Boiler Units" yourself.