Flue gas heat exchange
Thread Content
Dear experts: I know that the enthalpy value of flue gas is approximately 450 KJ/kg, while that of steam is about 2750 KJ/kg. If I use flue gas and steam, both at a constant temperature of 300 degrees, to heat 1 ton of water initially at 100 degrees to 200 degrees, which one will require more quantity? Is it calculated this way? Thank you
Is it to first use Q=CMT to calculate the heat required to raise the water temperature to 200 degrees, and then use Q=enthalpy x mass to determine the masses of the flue gas and steam respectively in order to compare them?
The latter should be Q = difference in enthalpy x amount of mass used, right? Because the flue gas still has an enthalpy value after heat exchange, just like steam. It’s an intuitive understanding; I’m not sure if it’s correct
This is definitely due to the large amount of flue gas used. Heat exchange with flue gas relies on temperature differences; in other words, the available heat from the flue gas comes from the enthalpy difference between the inlet and outlet flue gas. In contrast, heat exchange with steam utilizes the heat associated with phase changes, and this heat is significantly greater than the enthalpy difference. Generally, steam is used when a large amount of heat transfer is required, while flue gas is employed in situations where higher temperatures are needed for heat transfer, as high-temperature steam has very high pressure. As for the calculation, for flue gas heat exchange, an outlet flue gas temperature is first specified, then the enthalpy of the flue gas at that outlet temperature is determined, and finally the total heat transfer amount is divided by the difference between the enthalpies of the inlet and outlet flue gases. For steam heat exchange, it is simply possible to divide the total heat by the heat of vaporization of steam.
Is this poster trying to test everyone? They give the steam temperature but not the pressure ； The feed water is heated to 200°C without applying pressure, and there are also additional conditions such as the enthalpy values of flue gas and water vapor; let’s first discuss whether a phase change occurs during this process.