process and properties of dry air

Dry bulb, wet bulb, dew point temperature, and relative humidity, these terms are so closely related that if two properties are known, all others shown in the figure below may be read from the chart. When air is saturated, dry ­bulb, wet bulb, and dew point temperatures are identical. (See Example 2.)

Enthalpy of air for any given condition is the enthalpy at saturation corrected by the enthalpy deviation due to the air not being in a saturated state. The enthalpy (h) in Btu per pound of dry air is the enthalpy at the saturation (hwb) plus the enthalpy deviation (hd). (See Example 2.)

h = hwb + hd

If the air’s moisture content increases or decreases in a psychrometric process, the heat added (q) or removed (-q) is the difference between the enthalpy of the final or leaving air (hla) and the initial or entering air (hea) minus the enthalpy of the moisture (water in liquid or ice state) added (hw) or rejected hw.

q = hla + hea – hw

See Examples 4 and 5.

The enthalpy of added or rejected moisture is shown in the small graphs at the top of the chart.

Enthalpy of added or rejected moisture and enthalpy deviation are usually omitted in applications not requiring precise results – for example, comfort air conditioning.  Errors due to omissions for wet bulb temperatures below 32°F are much larger than for omissions above 32°F.

Sensible heat factor. This is part of certain calculations for installing air conditioning equipment. A scale along the right side of the figure in Example 4 below used with an origin at 80° dry bulb temperature and 50% RH provide a reasonable heat factor value. (See Example 4.)

Barometric pressures. In comfort air conditioning, a mercury reading of one inch or less either above or below the standard 29.92 inches of mercury is considered a standard reading.

When dry bulb and dew point temperatures are known for air at non-standard barometric pressures, values of percent RH and grains of moisture per cubic foot are correct on a standard chart, but for given dry bulb and wet ­bulb readings at non-standard barometric pressures, all properties must be corrected.

Interpreting the Air Conditioning Charts

Generally, in graphic presentations, humidifying is shown by an upward line and dehumidifying is shown by a downward line.

Heating and cooling air without changes in moisture content involve only a change in sensible heat and appear as a horizontal line, to right or left respectively. Changes occur in dry bulb, wet bulb, RH, and enthalpy. Specific humidity and dew point temperature remain constant.

In heating and humidifying, both sensible heat and specific humidity increase – shown as a line sloping upward and to the right. Changes occur in dry bulb, wet bulb, dew point temperatures, and enthalpy. A difference in RH depends on the slope of the line.

Air Conditioning Process
For cooling and dehumidifying, both sensible heat and specific humidity decrease, so the line slopes downward and to the left. Dry ­bulb, wet bulb, dew point temperatures, and enthalpy all change. Changes in RH are dependent on the slope of the line.

Evaporative cooling refers to air brought in contact with spray water at a temperature equal to the wet bulb temperature of the air. The process takes place upward along the wet bulb line. As sensible heat of the initial air vaporizes the water, the air’s dry bulb temperature falls. The sensible heat used to vaporize the water enters the air as latent heat in added vapor; thus no heat is added or removed. Wet bulb temperature remains constant. Dew point temperature, RH, specific humidity, and enthalpy increase. (In most evaporative cooling installations, heat may be added or removed during the process due to outside sources, although this amount is usually negligible.)

In chemical dehydration, the air that contacts the chemical either adsorbs or absorbs moisture from the air. Thus in this energy constant process, heat is liberated and added to the air and this amount is basically equally to the latent heat of vaporization of the moisture removed. Indicated by a downward sloping line approximating the wet bulb line, the slope of the chemical dehydration line may be either slightly greater or less than the wet bulb line, depending on if heat is stored, liberated, or absorbed.

AIR CONDITIONING PROCESSES such as heating, cooling, humidifying and dehumidifying may be shown graphically on the chart. (See Figure 1.)


Example 1. Reading Properties Of Air
Given:{ DB = 70°F
WB = 60°F Find:{ % RH
GR of moisture per lb dry air
GR of moisture per cu ft

Locate point of intersection on the chart of vertical line representing 700DB and oblique line representing 600WB. All values are read from this point of intersection.

Interpolate between relative humidity lines on 700DB line, read RH = 56%.
Follow horizontal line left to saturation curve, read DP = 53.6°F.
Interpolate between lines representing cubic feet per pound of dry air, read v = 13.53 cu ft.
Follow horizontal line to right, read grains of moisture per pound of dry air, W – 61.4 gr.
Grains of moisture per pound of dry air (61.4) divided by cubic feet per pound of dry air (13.53) = 4.54 gr per cu ft