PSYCHROMETRICS

 PSYCHROMETRICS

PSYCHROMETRIC:-

                            Psychometric is the science of studying the thermodynamic properties of moist air. The amount of moisture vapor in the air varies quite significantly under different conditions. When the air is hot, it can contain a large amount of moisture vapor sometimes as much as 5% by volume. When it is cold, its capacity to hold the moisture is reduced. When the temperature of warm air begins to fall, the vapor also cools and, if cooling continues, it will condense into tiny moisture droplets. In the atmosphere this results in the formation of clouds and eventually rain.

DEFINITION OF AIR:-

    Three basic definitions are used to describe air under various conditions:

1.ATMOSPHERIC AIR:-
                                            It contains nitrogen, oxygen, carbon dioxide, water vapor, other gases, and miscellaneous contaminants such as dust, pollen, and smoke. This is the air we breathe and use for ventilation.

2. DRY AIR:-

                           It exists when all of the contaminants and water vapor have been removed from atmospheric air. By volume, dry air contains about 78 percent nitrogen, 21 percent oxygen, and 1 percent other gases. Dry air is used as the reference in psychrometrics.

3. MOIST AIR:-

                             It is a mixture of dry air and water vapor.

For practical purposes, moist air and atmospheric air can be considered equal under

the range of conditions normally encountered.

PSYCHROMETRIC CHART :-

                                                                The Psychrometric Chart provides a graphic relationship of the state or condition of the air at any particular time. It displays the properties of air: dry bulb temperature (vertical lines), wet bulb temperature (lines sloping gently downward to the right), dew point temperature (horizontal lines), and relative humidity (the curves on the chart). Given any two of these properties, the other two can be determined using the chart.

READING PSYCHROMETRIC CHART:-

                                                                        The psychrometric chart seems a dizzying maze of lines and curves going every which way, but once a few fundamental things are understood about the psych chart, it is not really that difficult to understand.

DRY BULB TEMPERATURE:-

                                                      Dry Bulb Temperature (DBT) is the temperature that we measure with a standard thermometer that has no water on its surface. When people refer to the temperature of the air, they are commonly referring to its dry bulb temperature. Several temperature scales commonly are used in measuring the temperature. In the inch-pound (I-P) system of units, at standard atmosphere, the Fahrenheit scale has a water freezing point of 32°F and a boiling point of 212°F. In the International System (SI) of units, the Celsius scale has a water freezing point of 0°C and a boiling point of 100°C. On theKelvin scale, 0ºK equals -273°C.

Dry-bulb temperature is located on the X-axis, of the psychrometric chart and lines of constant temperature are represented by vertical chart lines.

WET BULB TEMPERATURE:-

                                                                The wet bulb temperature (WBT) is a temperature associated with the moisture content of the air. Wet bulb temperature is taken by surrounding the thermometer with a wet wick and measuring the reading as the water evaporates. Because of the evaporative cooling effect, Wet bulb temperatures are always lower than dry bulb temperatures and the only time that they will be the same is at saturation (i.e. 100% relative humidity).

                                                        

Wet bulb temperature on psychrometric chart is represented by lines that slant diagonally from the upper right of the chart (along the line of saturation) down to the lower left of the chart. These follows lines of constant enthalpy but values are read off at the upper, curved, saturation temperature boundary. The unit of measure used for wet bulb temperature is degrees Celsius (°C) or degrees Farenheight (°F)

ENTHALPY:-

                            Enthalpy is the measure of heat energy in the air due to sensible heat or latent heat.Sensible heat is the heat (energy) in the air due to the temperature of the air and the latent heat is the heat (energy) in the air due to the moisture of the air. The sum of the latent energy and the sensible energy is called the air enthalpy. Enthalpy is expressed in Btu per pound of dry air (Btu/lb of dry air) or kilojoules per kilogram (kJ/kg).

The enthalpy scale is located above the saturation, upper boundary of the chart. Lines of constant enthalpy run diagonally downward from left to right across the chart; follow almost exactly the line of constant wet bulb temperature.

RELATIVE HUMIDITY (RH):-

                                                    Relative humidity (RH) is a measure of the amount of water air can hold at a certain temperature. Air temperature (dry-bulb) is important because warmer air can hold more moisture than cold air. As a rule of thumb, the maximum amount of water that the air can hold doubles for every 20°F increase in temperature.

Lines of constant relative humidity are represented by the curved lines running from the bottom left and sweeping up through to the top right of the chart. The line for 100 percent relative humidity, or saturation, is the upper, left boundary of the chart.

ABSOLUTE HUMIDITY or HUMIDITY RATIO:-

                                                                                        Amount of grains of moisture present in sample air called humidity ratio.

Humidity ratio is represented on the chart by lines that run horizontally and the values are on the right hand side (Y-axis) of the chart increasing from bottom to top.

DEW POINT :-
                            Dew point temperature indicates the temperature at which water will begin to condense out of moist air. When air is cooled, the relative humidity increases until saturation is

reached and condensation occurs. Condensation occurs on surfaces which are at or below the dew point temperature.

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