Term Pressure

Atmospheric Pressure

Fig-1 Atmospheric pressure balances a column of mercury(Hg) to the height of 760 mm at sea level or water column of 10.33 m. The atmospheric pressure at sea level is 1.033 kg/cm2 at normal temperature (15.550C or 600F).

Figure shows a vessel filled with mercury exposed to atmospheric pressure. A capillary is placed in the center of the vessel. Due to the atm pr, the liquid, i.e. Mercury, rises up to 760mm.

The atm. pr. drops at the rate of about 1 meter for every 1000 meters increase in altitude, above mean sea level.

The Physical Atmosphere (Atm.)

It measures 1.033kg/cm2 = 760 mm of Hg (at sea level). This should not be confused with the Technical Atmosphere of 1kg/cm2. The standard atmospheric is 1.013 bar or 760 mm of Hg column.

The Technical Atmosphere (Atm.)

The unit of 1 atm = 1kg/cm2 = 735.5 mm of Hg is called the technical atmosphere.

The Concept of Vacuum

Refer Fig-2 (click here) Theoretically zero atm. pr. would be attained in a perfect vacuum. This is known as Absolute ZERO pr. All practical pr. are then made either relative to this datum or to the atm. pr. Pl refer the figure above

Vapor Pressure (VP)

Ref Fig-3 (click here) Substance/ Matter exists in three phases: 1) Solid, 2) Liquid and 3) Gas.

The term vapor pressure is frequently used in connection with centrifugal pumps, Particularly in Thermal plants and Process Industries.

All liquids have a tendency to evaporate when exposed to atm. The rate at which this evaporation occurs depends on the molecular energy of the liquid.(i.e. type of liquid and its temp.) and the condition of pr. adjoining the liquid surface.

Ref. fig above Consider a liquid contained in a sealed in beaker A. Assuming the constant temp is maintained, some of liquid molecules have sufficient energy to break away from the liquid and enter the air in the vapor form as in A. As the time passes, the air will contain vapor molecules to the extent that some of them will be forced to re-enter the liquid by the partial air pressure as shown in B. Eventually equilibrium will be attained when the rate at which molecules break away from liquid, same will re-enter back in to the liquid. As shown in C. Under such condition, the air above the liquid is saturated with liquid molecule and the pr. on the liquid surface is called its vapor pressure at prevailing temp.

Under vacuum, the pressure of vaporisation is accelerated. The boiling point of liquid has direct relation with the pressure above its surface. At higher pr. the boiling point increases and vice versa.

When a liquid has a very high vapor pr. is evaporates readily even at room temp. It is called a "Volatile Liquid" (like Petrol).

V.P. has great effects on working of centrifugal pumps. The energy available at the eye of the impeller gets reduced as liquid vapor pr. increases. In section of NPSH, will study further.