Use of the Psychrometric Chart
From two known properties of air, its condition can be located on the Psychrometric chart and all remaining properties can then be found by reading the appropriate scale.
Figure 1 Illustrates a condition plotted at the intersection of its dry bulb and wet bulb temperatures. The dry bulb temperature is represented on the chart by the vertical lines with its scale across the bottom. The wet bulb temperature is read along the saturation line and is represented on the chart by the solid diagonal lines. Enthalpy at a saturation, for a given wet bulb temperature is read from the diagonal scale at the left using the diagonal lines extending from the saturation line.
Figure 2 Illustrates a condition plotted at the intersection of its dry bulb temperature and relative humidity. Relative humidity is represented on the chart by the curved lines which are marked in percent relative humidity.
Figure 3 Illustrates a condition plotted at the intersection of its dry bulb and dew point temperatures. The dew point temperature is read along the saturation line at the intersection
of the Horizontal Humidity line. The value of the specific humidity is read from the scales at the right in either pounds or grains of moisture per pound of dry air by selecting the appropriate scale.
Figure 4 Illustrates the determination of specific volume from the chart. Specific volume is represented by the broken diagonal lines marked in cubic feet per pound of dry air. Intermediate points are read by interpolation between the lines.
Figure 5 Illustrates the use of sensible heat factor to determine the air conditions required to satisfy a specified space temperature and load conditions. The sensible heat factor is the ratio of internal sensible heat to internal total heat load of the space being conditioned. A straight line from the sensible heat factor scale through the circled point of the chart to the slope line from the space condition point to the saturation line. Air supplied to the space at any temperature condition located on the ratio line (and in the proper volume) will satisfy the room load.
Example — Using the point which is circled on the Psychrometric Chart, the following values are obtained:
| | | | | |
| Dry Bulb Temperature | | | | 80.0ºF. |
| Wet Bulb Temperature | | | | 67.0ºF. |
| Dew Point Temperature | | | | 60.3ºF. |
| Relative Humidity | | | | 51.1% |
| Specific Humidity | | | 78.1 | |
| A) 0.01115 lbs./lb. dry air = | SR/lb dry air |
| 7000 |
| B) 78.1 grains/lb. dry air | |
| | | | |
| Enthalpy at saturation | | 31.62 BTU/lb. dry air |
| Specific Volume | 13.83 Cu. Ft./lb. dry air |
Figure 6 … *Air Conditioned Process
1.Cooling and Dehumidification — A decrease in both dry bulb and specific humidity represented by a line sloping downward and to the left. Total heat content (both sensible and latent heat) is decreased.
2.Sensible Cooling — A decrease in dry bulb and sensible heat content represented by a horizontal line directed to the loft
along the constant specific humidity line. Specific humidity and dew point remain constant.
3.Evaporating Cooling — (Air passed through spray water or wetted surface at wet bulb temperature) – A decrease in dry bulb (reduced sensible heat content) and an increase in dew point and specific humidity (increased latent heat content) represented by a line sloping upward and to the left following a constant wet bulb line – no change in total heat content.
4.Humidification — An increase in the specific humidity as a result of moisture added, represented by a line directed upward.
5.Heating and Humidification — An increase in both sensible heat and specific humidity, represented by a line sloping upward and to the right.
6.Sensible Heating — An increase in dry bulb and sensible heat content, represented by a horizontal line directed to the right along the constant specific humidity line, Specific humidity and dew point remain constant.
7.Chemical Drying — (Air passed through a chemical drying agent) – A decrease in dew point and specific humidity, represented by a line sloping downward and to the right.
8.Dehumidification — a decrease in the specific humidity as a result of removing moisture, represented by a line directed downward.
Definitions
Dry Bulb Temperature — The temperature indicated by a thermometer, not affected by the water vapor content air.
Wet Bulb Temperature — The temperature of air indicated by a wet bulb thermometer; the temperature at which water, by evaporating into air, can bring the air to saturation adiabatically at the same temperature.
Dew Point Temperature — The temperature to which water vapor in air must be reduced to produce condensation of the moisture contained therein.
Relative Humidity — The ratio of actual vapor pressure in the air to the vapor pressure of saturated air at the same dry bulb temperature.
Specific Humidity (Moisture Content of Humidity Ratio) — The weight of water vapor per pound of dry air.
Sensible Heat — Heat which when added or subtracted from the air changes only its temperature with no effect on specific humidity.
Latent Heat — Heat which effects a change of state without affecting temperature, as in evaporating or condensing moisture.
Enthalpy (Total Heat) — The sum of sensible and latent heat. In the chart, enthalpy represents units of total heat content above an arbitrary base in terms of BTU per pound of dry air.
Specific Volume — Volume per unit of weight, the reciprocal of density, in terms of cubic feet per pound of dry air.
Sensible Heat Factor — The ratio of internal sensible heat to internal total heat load.
Ratio Line — The line extending from the space condition to the saturation line at a slope determined by the sensible heat factor.
