Vapor Pressure
3
Vapor Pressure The vapor pressure of a liquid, at a designated temperature, is the absolute pressure at which the liquid would boil if its pressure were lowered to that point. The vapor pressure of water at 212°F is 14.7 psia. That is why water boils away into steam at 212°F when the atmospheric pressure is 14.7 psia. High on a mountain top water might boil at 200°F. The vapor pressure of water at 200°F is 11.5 psia, and, therefore, the barometric pressure up on that mountain must have been 11.5 psia. Vapor pressure can be very important in a flowmetering application if the liquid enters the meter at a pressure near its vapor pressure. Any pressure reduction in the meter could cause the liquid to flash partly to a gas. This can completely destroy the performance of many types of flowmeters. If the gas pressure rises above the vapor pressure, the gas bubbles will implode, causing cavitation to occur. Cavitation can destroy many types of flowmeters. A common problem arises when trying to measure steam condensate if it is not subcooled or, if not, pumped to increase its pressure. Vapor pressure does not vary in proportion to temperature. Instead, it typically changes only slightly at lower temperatures and then increases much more drastically at relatively higher temperatures.
-
Upload
carmen-sandoval -
Category
Documents
-
view
4 -
download
1
description
Presion de vapor
Transcript of Vapor Pressure
Vapor Pressure
The vapor pressure of a liquid, at a designated temperature, is the absolute pressureat which the liquid would boil if its pressure were lowered to that point. Thevapor pressure of water at 212°F is 14.7 psia. That is why water boils away intosteam at 212°F when the atmospheric pressure is 14.7 psia. High on a mountaintop water might boil at 200°F. The vapor pressure of water at 200°F is 11.5 psia,and, therefore, the barometric pressure up on that mountain must have been 11.5psia.
Vapor pressure can be very important in a flowmetering application if the liquidenters the meter at a pressure near its vapor pressure. Any pressure reduction inthe meter could cause the liquid to flash partly to a gas. This can completelydestroy the performance of many types of flowmeters. If the gas pressure risesabove the vapor pressure, the gas bubbles will implode, causing cavitation tooccur. Cavitation can destroy many types of flowmeters.
A common problem arises when trying to measure steam condensate if it is notsubcooled or, if not, pumped to increase its pressure. Vapor pressure does not varyin proportion to temperature. Instead, it typically changes only slightly at lowertemperatures and then increases much more drastically at relatively higher temperatures.
Boiling Point
The boiling point of a liquid is a property directly related to vapor pressure. It issimply the temperature at which a liquid would boil at a designated pressure. Tabulationsof boiling point for various liquids at atmospheric pressure are commonlypublished. They are of little help in flowmeter design because they apply only atone pressure.
