what is the use of a psychrometric chart?

Answer 1

A psychrometric chart is a graph of the physical properties of moist air at a constant pressure. It graphically expresses how various properties relate to each other:

Dry-bulb temperature is that of an air sample, as determined by an ordinary thermometer, the thermometer's bulb being dry.
Wet-bulb temperature is that of an air sample after it has passed through a constant-pressure, ideal adiabatic saturation process, that is, after the air has passed over a large surface of liquid water in an insulated channel. In practice, this is the reading of a thermometer whose sensing bulb is covered with a wet sock evaporating into a rapid stream of the sample air.
Dew point temperature is that at which a moist air sample at the same pressure would reach water vapor saturation. At this saturation point, water vapor would begin to condense into liquid water fog or (if below freezing) solid hoarfrost, as heat is removed.
Relative Humidity is the ratio of the mole fraction of water vapor to the mole fraction of saturated moist air at the same temperature and pressure. The notion that air "holds" moisture, or that moisture dissolves in dry air and saturates the solution at some proportion, is an erroneous (although widespread) concept.
Humidity Ratio is the proportion of mass of moisture present in a unit mass of air at the given conditions (DBT, WBT, DPT, RH etc).
Enthalpy These values correspond to the saturated state and are to be read parallel to DPT values.
Inverse Density Mass per unit volume, inverted.
The versatility of the psychrometric chart lies in the fact that by knowing just two properties of moist air, the other properties can be determined.


The most common chart used by practitioners and students alike is the "w-t" (omega-t) chart in which the Dry Bulb Temperature (DBT) appears horizontally as the abscissa and the humidity ratios (w) appear as the ordinates.

In order to use the chart at least two of the 6 possible values must be known (DBT, DPT, WBT, RH, Humidity Ratio and Enthalpy content, Inverse Density). This gives rise to 6C2 or 15 possible combinations.

DBT : This can be read off from the abscissa

DPT : Follow the horizontal line from the point where the line from the horizontal axis hits the 100% saturation curve.

WBT : Line inclined to the horizontal and intersects saturation curve at DBT point.

RH : Hyperbolic lines drawn asymptotically with respect to the saturation curve which corresponds to 100% RH.

Humidity Ratio : Marked on Ordinate axis.

Inverse Density : Equally spaced parallel family of lines.

Answer 2

Amongst other things, usefull for designing air conditioning systems but not essential since all the parameters can be calculated from classical formulae. In the days before computers, charts of this type were very useful. These days charts have been taken over by specialist software or just spreadsheets. However a chart is still useful for one off calculations. I sometimes use it to design insulation systems on cryogenic plant (need to check that there is sufficient insulation to prevent condensation on the warm side ofthe insulation).

Answer 3

You need it for designing power stations. You can't do that without knowing how much water is in the air, and at what temperature it will condense out. This affects combustion, cooling towers and the like. It's affected by pressure, so a power station on a high plateau will be trickier to design than one by the seaside.