Psychrometry

Usual applications

The principles of the psychrometry can apply to any system bringing into play gas-steamer mixtures. Most current being the Ventilation, the Air conditioning and the Meteorology.

The psychrometric report/ratio

The psychrometric report/ratio connects the absolute humidity and maximum moisture with the difference between the wet temperature of bulb and that in adiabatic saturation. In the case of the mixture vapor the water-air one, the report/ratio is about the unit what simplifies calculations for the problems of drying and cooling.

Psychrometric diagrams

A psychrometric diagram gathers the main features of the humid air for a given atmospheric pressure (in general that with the sea level, i.e. 1013hPa):

Température of dry bulb noted $\ theta$ is the temperature given by a dry thermometer, placed in a draft wet and protected from the parasitic radiations (coming from cold or hot objects like the sun).
wet Température of bulb noted $\ theta_ {H}$ is the temperature of the air circulating with the top of a liquid large surface of water in an insulated system. It by expemple the temperature is indicated by a thermometer placed in a wet linen subjected to draft.
Température of dew noted $\ theta_ {R}$ is the temperature to which a humid air is with the saturating steam pressure. At this temperature the liquid cannot store steam any more without this one not condensing.
relative Humidité noted HR is the relationship between the quantity of water present in the air and the maximum quantity of water which the air could contain at the same temperature. It is also the relationship between the steam pressure and the saturating steam pressure.

HR= \ frac {P_ {v}} {P_ {v sat}}

absolute Humidité noted Y is the quantity of water contained in the air, it is expressed in gram (S) per kilogram of dry air.

Y= \ frac {m_ {v}} {m_ {dry air}} = \ frac {M_ {dry air}. P_ {v}} {M_ {v}. P_ {dry air}}

Y=0,622. \ frac {P_ {v}} {P-P_ {v}}

Enthalpy specific symbolized by the letter h* . That corresponds to the energy contained in the fluid in a state given (temperature, pressure,… fixed)

the specific enthalpy of the dry air is given by:

h_ {has} ^ {*} (\ theta) =c_ {has}. \ theta

with

c_ {has} \ approx 1,006kJ/(kg. K)

heat-storage capacity of the air for temperatures of air lain between -20°C and 50°C the specific enthalpy of the steam is given by:

h_ {L} ^ {*} (\ theta) =L_ {v} (0) +c_ {v}. \ theta

with

L_ {v} (0) \ approx 2500.8kJ/kg

latent heat of vaporization and

c_ {v} \ approx 1,8266 kJ/(kg. K)

heat-storage capacity of the vapor the specific enthalpy of liquid water is given by:

h_ {L} ^ {*} (\ theta) =c_ {L}. \ theta

with

c_ {L} \ approx 4,194kJ/(kg. K)

heat-storage capacity of liquid water for temperatures of air lain between 0°C and 50°C the specific enthalpy of the humid air is given by:

h_ {\ theta, Y} =h_ {has} ^ {*} +Y.h_ {v} ^ {*}

Y < Y_ {sat}

h_ {\ theta, Y} =h_ {has} ^ {*} +Y_ {sat} .h_ {v} ^ {*} + (Y there _ {sat} .h_ {L} ^ {*}

Y > Y_ {specific saturation}

Volume , noted $\ nu$ is the volume of humid air per unit of mass of dry air.

\nu=\frac{461,51.(0,622+Y). T} {(1+Y). P}

By knowing only two characteristics of the air one can find the whole of the properties of the humid air thanks to the diagram.

How to read the diagrams

There exist two type of diagrams:

the diagram of Carrier: $Y = F (\ theta)$

The curve of dew is the higher curve of the diagram (HR=100%), the other relative curves of moisture have the same pace. The values of the enthali are located on oblique lines.

the diagram of Mollier: $h = F (Y)$

The isenthalpes are on obliques although the title of the diagram lets think that the isenthalpes are horizontal lines. The temperatures are read on quasi horizontal lines. Finally the curve of dew is the lower curve of the diagram.

See too

Air
Air conditioning
Law of Dalton on the pressures partial
Dewpoint
Evaporation
Moisture
relative Humidity
psychrometric Table
Enthalpy

Bonds

Psychrometric Chart - detailed psychrometric Diagram with the curves of enthalpy, mass of air, water mass.
American company of the engineers in heating, refrigeration and air-conditioned
Universal Industrial Gases, Inc. webpage - Bonds towards the tables of the physical properties, the diagrams…
Corwin' S Calculators Calculator of the water content and the temperature of dew.

Random links: