Flash evaporation

Flash evaporation is the generation of steam by decreasing the pressure in a liquid filled, under pressure ( to ambient) related closed container.

Classic examples are the reduction in pressure in the drum of a steam generator (forced circulation boiler or natural circulation boiler ) or the bubbling when you open a bottle of mineral water.

In the drum, it is used in the flash evaporation of the liquid to overheat and thus the generation of steam. In the mineral water bottle is reduced by opening the lid of the pressure in the bottle, so that dissolved in the water portions of carbon dioxide " gassing ". This is not, however, the transition from a liquid into the gaseous state, but to the emergence of a dissolved gas from a liquid, which, however, expresses similar ( formation of gas bubbles, etc.).

Physical viewing

For flash vaporization occurs in a closed container which is filled with saturated boiling liquid and the associated vapor phase when the pressure by releasing an outlet cross-section is lowered (for example, a valve ).

In a closed container, the liquid has a pressure at a given temperature pstart T1 - corresponding to the saturation pressure of the temperature, which is determined by the state 1 in the TS diagram of the boiling. If the valve is opened, the pressure drops, the new pressure level p * (p * < pstart ) propagates at the speed of wave propagation in a from container. The temperature, however, can not adapt at the same rate, as they at the phase boundary water / steam is subject to heat and mass transfer mechanisms, which run slower. This time delay can be described as Siedeverzugszeit.

In this state, the liquid is overheated, it and its vapor are not in thermodynamic equilibrium. This condition means that the liquid has to give off heat - in this case this is done by an energy input to boiling nuclei and already existing steam bubbles in the container. This leads to bubble growth, and the rise of the liquid, and finally to the surge and destaging of mass and energy in the form of steam - the process which is generally referred to as "cooking". Thus the steam energy is transported from the container.

The mixture passes through assuming isentropic processes in this case the vertical Isentropic "s" from the initial state T1 ( pstart ) to the new final state pende. Considering a snapshot of the state 2, which is achieved by opening the valve and is located between initial and final states, the flash evaporation proceeds in such a way that the liquid from the point 2 of the point 2a, and thereby the state of " saturated boiling point ", the vapor state 2b, " saturated steam " is achieved.

Is the thermodynamic equilibrium between the phases is reached again, the flash evaporation stops. The composition of liquid and vapor phase of a mixture of chemical substances can be determined by a calculation Flash.

The process of Massenausspeicherung in the form of steam is utilized in steam generators with drum, for example, Natural circulation or forced circulation boiler, since after a pressure reduction to increase performance immediately - depending on type and size - about 200 to 600 kg of steam per bar pressure drop are available. In power plants are common at operating pressures of 70-200 bar pressure relief by 5-10% of the nominal pressure with load change. This corresponds to a power change rate of about 5-10% per minute.

Add keywords

• Pressure reduction leads to overheating of the liquid
• New print spreads with wave propagation velocity in a container
• Temperature change is slowed down by the heat and mass transfer pending at the phase boundaries
• Thermodynamic equilibrium is no longer present
• Overheating is degraded by energy transfer to boiling nuclei and existing vapor bubbles
• Energy input into boiling nuclei and bubbles leads to the bubble growth and rise of bubbles
• Reclaiming ground in the form of steam escapes the system, the temperature
• New thermodynamic equilibrium is achieved