Heating by direct steam – how to calculate the heating time
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Heat 1 ton of water by direct contact with steam, raising its temperature from 20°C to 100°C; the steam pressure is 0.3 MPa, resulting in a temperature of 135°C. It is possible to calculate that less than 300 kg of steam is required, but how is the heating time determined? Derivation of the direct heat transfer formula for gas-liquid phases.
In \"Principles of Chemical Engineering,\" there is an introduction to mass and heat transfer due to direct gas-liquid contact. Inside, gas and liquid circulate around each other in the cooling tower to cool the liquid; then, the height of the tower that meets the requirements is determined through sectional calculations of the tower. The situation is as follows: the liquid is in the container, and steam is introduced into the container through a pipe at the bottom of the tank to provide direct heating; we want to know the heating time required for the material inside the tank to reach the desired temperature. Can the time parameter be used directly in place of the tower height parameter when inserting it into the heat and mass transfer equation set for calculations?
If stirring is involved, theoretically, this time depends on the amount of time it takes to introduce the required amount of heating steam; if 300 kg of steam can be introduced in 1 second, then the heating time will be 1 second.
The mass and heat transfer rates of the liquid and vapor inside the container are related to the solution temperature. The greater the difference between the saturated steam supplied and the vapor pressure at the liquid temperature, the faster the heat transfer. Just as you can eat 2 jin of food, you can’t say that if you’re given 2 jin of food in one second, you’ll be able to finish it in that same second; it depends on your eating speed. The more you eat, the slower your speed will become. When steam is introduced into the water, it absorbs a certain amount of heat over time, and the excess heat is discharged outside the container along with the gas.