Author Topic: Adiabatic Flash  (Read 6805 times)

mbeychok

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Adiabatic Flash
« on: December 27, 2007, 05:18:41 AM »
When the pressure of a saturated liquid is decreased by the liquid flowing across a throttling valve or other device to a lower pressure, a part of the liquid immediately "flashes" into vapor. Both the flashed vapor and the residual liquid are cooled to the saturation temperature of the liquid at the lower pressure. The flashing is an isenthalpic (i.e., constant enthalpy) process and is often referred to as an "adiabatic flash". It is also sometimes referred to as "auto-refrigeration" and is the basis of most conventional vapor compression refrigeration systems.

If the liquid is a single-component liquid (i.e., liquid propane or liquid ammonia), the following equation derived from a simple heat balance around the throttling valve or device is used to predict how much of a single-component  liquid is vaporized.



( Note: The words "upstream" and "downstream" refer to before and after the liquid passes through the throttling valve or device.)

The adiabatic flashing of a multi-component liquid is much more complex and requires an iterative, trial and error solution involving the Rachford Rice equation.

An adiabatic flash of a liquid is entirely different from the Joule-Thomson effect of a gas.

Multi-stage flashing is used in the desalination of ocean water. The water is heated and then routed into a reduced pressure flash evaporation "stage" where some of the water flashes into steam which is subsequently condensed into salt-free water. The residual salty liquid from that first stage is introduced into a second flash evaporation stage at a pressure lower than the first stage pressure where more water is flashed into steam which is also subsequently condensed into more salt-free water.  This sequential use of multiple flash evaporation stages is continued until the design objectives of the system are met. A large part of the installed desalination capacity worldwide uses multi-stage flashing and typically such plants may have up to 24 or more stages of flash evaporation.

Engineer Forum

Adiabatic Flash
« on: December 27, 2007, 05:18:41 AM »