The Psychrometric Chart

The psychrometric chart is a plot of the properties of atmospheric air. It graphically illustrates the relationship between db temperature, wb temperature, RH, humidity ratio, and enthalpy. It is essentially a graphic representation of the condition of air (air-water vapor mixture) at each point in the AC process. It relates db temperature to absolute moisture content of the air and includes all of the possible combinations of temperature, moisture content, density, and heat content properties that can occur in air. The chart can be used to make calculations to determine the sensible and latent loads associated with HVAC equipment processes. Figure 34-2 is a psychrometric chart in English units. The numbered lines or scales highlight the functionality of the chart. Figure 34-3 is a psychrometric chart in SI units.

Figure 34-4 is a basic sketch of a psychrometric chart; the scales and lines of the chart are highlighted as they relate to one specific set of air conditions. Included are db and wb temperatures, RH, specific humidity, vapor pressure, dewpoint temperature, and enthalpy. The darkened circle pinpoints the location of the specific condition and the intersecting horizontal and vertical lines show the scales from which the condition can be identified. If any two properties of an air mixture are known, the chart allows a quick determination of all of its other properties.

Following are a series of skeleton psychrometric charts. Each highlights particular scales and lines associated with each one of the seven conditions shown jointly in Figure 34-4.

Figure 34-5 shows db temperature lines, i.e., the temperature of air measured on a standard thermometer. These are shown as straight vertical lines. The scale called db temperature is laid out horizontally at the bottom of the chart in °F, with the incremental lines extending

Trane Psychrometric Chart Iso Standard
Fig. 34-2 Psychrometric Chart (English units). Source: The Trane Company
Psychrometrique Chart
Fig. 34-3 Psychrometric Chart (SI units). Source: Carrier Corp.
Pvs Psychrometric Chart
Fig. 34-4 Sketch of Psychrometric Chart Highlighting One Specific Set of Conditions.

70=F Dry Bulb

Fig. 34-7 Specific Humidity (Humidity Ratio) Lines.

Trane Psychrometric Chart Iso Standard

70°F Dry Bulb

Fig. 34-5 Dry-Bulb Temperature Lines.

70°F Dry Bulb

Fig. 34-5 Dry-Bulb Temperature Lines.

vertically. In SI unit charts, the scale is in °C.

Figure 34-6 shows RH lines. The RH lines are curved and the values appear in increments of 10%, representing the degree of saturation, or the ratio of the pressure of the vapor (pv) to the pressure of the vapor at saturation (pvs). At 100% RH, db temperature is equal to wb temperature. As RH decreases, the wb temperature becomes lower than the equivalent db temperature.

Figure 34-7 shows specific humidity, or humidity ratio (w or HR), lines. These are shown as straight horizontal lines that are perpendicular to db lines. The scale is in grains of moisture per lbm of dry air (grains/lbm) and typically ranges from 0 to about 200. In SI unit charts, the corresponding scale is kg/kg of dry air and typically ranges from 0 to 0.033. For example, to determine the grains/lbm of dry air removed in conditioning air from 90°F (32°C) and 70% RH to 80°F (27°C) and 40% RH, proceed vertically

70=FDry Bulb

Fig. 34-6 Relative Humidity Lines.

70=FDry Bulb

Fig. 34-6 Relative Humidity Lines.

70=F Dry Bulb

Fig. 34-7 Specific Humidity (Humidity Ratio) Lines.

70CF Dry Bulb

Fig. 34-8 Vapor Pressure Scale.

70CF Dry Bulb

Fig. 34-8 Vapor Pressure Scale.

from 90°F (32°C) db to the 70% RH curve. The intersection is at 150 grains (0.0214 kg/kg), while 80°F (27°C) and 40% RH intersect at 61 grains (0.0087 kg/kg). The difference is 90 grains (0.0128 kg/kg).

Figure 34-8 shows the vapor pressure (Pv or Pw) scale. This measures the pressure exerted by water vapor in the air. The scale, when included, is typically on the far right of the chart and the unit of measurement is in-Hg abs (cm-Hg abs or Pascal).

Figure 34-9 shows the dewpoint temperature scale. Dewpoint temperature lines run horizontally, like the grains of moisture lines, with a scale that typically ranges from 20 to 90°F (-7 to 32°C). To determine the dewpoint temperature of air at 80°F (27°C) and 50% RH, for example, start at the db temperature of 80°F (27°C) and proceed vertically to the 50% RH curve. From this intersection, proceed horizontally to the saturation curve (100% RH). This yields a dewpoint temperature of 60°F (16°C).

Figure 34-10 shows the enthalpy (h) scale. It is typi

70°F Dry Bulb

Fig. 34-11 Wet-Bulb Temperature Lines.

70°F Dry Bulb

Fig. 34-11 Wet-Bulb Temperature Lines.

Fig. 34-9 Dewpoint Temperature Scale.

Fig. 34-9 Dewpoint Temperature Scale.

cally on the left, but sometimes on both sides, of the chart. Typically, the scale ranges from about 5 to 60 Btu/lbm (12 to 140 kJ/kg). Commonly, the wb lines are used to represent enthalpy lines. While these are not exactly the same, they are generally considered to be close enough for practical purposes.

Figure 34-11 shows wb, or saturation, temperature lines. These indicate the temperature of air above 32°F (0°C). Below 32°F (0°C), temperatures are measured on a wb thermometer on which the water in the wick has frozen to ice. Note that the slope of the wb lines change below 32°F (0°C). The scale, in °F, is the curved line (with a slope of about 30 degrees) at the left edge of the chart.

Additional Features of Psychrometric Charts

Following are some additional lines and scales commonly found on psychrometric charts: • Specific volume (v) lines. Specific volume is the volume that a unit mass of air occupies. It is the

Fig. 34-10 Enthalpy Scale.

Fig. 34-10 Enthalpy Scale.

reciprocal of the density of air. Specific volume lines run almost diagonally from the upper left to the lower right of the chart, with values listed on each line. Values falling between each line can be found through interpolation by placing a straight edge over the point of intersection and paralleling the specific volume lines. The unit of measurement is ft3 (m3) of the air mixture per lbm (kg) of dry air, with typical values ranging from 12.5 to 14.5 (0.75 to 0.95) in standard charts.

• Sensible heat factor (SHF), or ratio, scale. This is useful when plotting some AC processes, such as cooling and dehumidification, and helps to determine the required supply air conditions. The scale typically ranges from 0.35 to 1.00, or 35 to 100%. This percentage represents the sensible work. The remaining percentage would be latent heat.

• Enthalpy deviation lines. These are used to correct enthalpy readings when extreme accuracy is required. Enthalpy is the total heat in the air at 100% saturation. If the air is not completely saturated, slight error is present in the enthalpy reading. The scale typically ranges from -0.02 to -0.30 Btu (-0.02 to -0.32 kJ). This is the amount of heat that would be subtracted from the enthalpy reading.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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