PSYCHROMETRIC CHART

The psychrometric chart shows graphically the parameters relating to water moisture in air. This application note describes the purpose and use of the psychrometric chart as it affects the HVAC {HVAC (heating, ventilating, and air conditioning; also heating, ventilation, and air conditioning) } engineer or technician.

Air comprises approximately 78% nitrogen, 21% oxygen, and 1% other gases. But air is never dry, even in a desert. Two-thirds of the earth’s surface is covered with water and this, along with other surface water and rain, maintain low pressure water vapor to be suspended in the air making up part of the 1% of other gases. The psychrometric chart indicates the properties of this water vapor through the following parameters, each of which is explained in more detail below:

•  dry bulb temperature

•  wet bulb temperature (also known as saturation temperature)

•  dew point temperature

•  relative humidity

•  moisture content (also known as humidity ratio)

•  enthalpy (also known as total heat)

•  specific volume (the inverse of density)

Any psychrometric chart is valid at a certain pressure of air. The pressure of air is related to the height above (or below) sea level. The chart provided by Power Knot is valid at sea level (760 mm of Hg). You must make corrections for different altitudes.

If you know any two of the parameters above, you can find the other five values from the chart.

The study of psychrometrics and therefore the usefulness of the chart are important in the HVAC industry because:

•  people feel comfortable over a narrow range of temperature and humidity

•  machines (especially electronic machines) operate over a specific range of temperature and humidity

•  to calculate the amount of heating or cooling required for a certain space requires knowledge of the moisture content of the air

For an example on how the psychrometric chart is used in practice, please read Power Knot’s application note on measuring enthalpy to evaluate the efficiency of an air conditioning system.

Properties on the Chart

1. Dry bulb (DB) temperatureDry bulb temperature graph

We measure the temperature of the air with a thermometer. Traditional thermometers have a bulb that contains a liquid that expands, and a tube indicating the temperature on a scale. As the liquid expands, it rises up the scale. In the HVAC business, we use a thermocouple and electronic meter or an infrared thermometer because these are faster and more rugged. Whichever method is used, this measurement is called the dry bulb temperature because the end of the thermometer that is making the measurement has no moisture on it.

The temperature of the air is measured in °F in the USA and in °C everywhere else.

This temperature is shown as the horizontal axis of the chart.

2. Wet bulb (WB) temperature

Wet bulb temperatureThe wet bulb temperature is measured by having the bulb of the thermometer moist. The moisture evaporates, lowering the temperature recorded by the thermometer. Less moisture in the air will result in a faster rate of evaporation and therefore a colder reading. In practice, we can use an electronic thermometer and wrap a paper tissue over the thermocouple. Make the paper tissue moist, but not too wet that water is dripping from it. Move air over the tissue (or move the thermocouple through the air) so the water evaporates.

When the air sample is saturated with water (that is, it has 100% relative humidity), no water can evaporate from the moist tissue so the WB temperature will read the same as the DB temperature. This temperature is therefore also referred to as the saturation temperature. This temperature is indicated by diagonal lines on the chart.

3. Relative humidity (RH)

Relative humidity graph

This is the ratio of the fraction of water vapor in the air to the fraction of saturated moist air at the same temperature and pressure. RH is dimensionless, and is usually expressed as a percentage. 100% RH indicates the air is saturated and cannot hold any more moisture. Preferred values of comfort for people are between 35% and 60%.

 

Lines of constant relative humidity are shown as exponential lines on the psychrometric chart. The line at 100% is referred to as the saturation line.

4. Dew point (DP) temperature

This is the temperature of the air at which a moist air sample reaches water vapor saturation. It is equivalent to a wet bulb temperature at 100% relative humidity. At this combination of temperature and humidity, further removal of heat results in water vapor condensing into liquid.

Dew point temperatureA practical view of the dew point is the temperature to which air must be cooled before condensation will begin.

An example is when you take a bottle of beer out of the refrigerator. Water condenses on the outside of the bottle only if the original temperature of the bottle was below the dew point. The instantaneous temperature of the water is the dew point temperature.

As a sample of air is cooled, its RH climbs until it reaches 100% RH (saturated air). This is the dew point temperature. At saturation, the dew point temperature equals the wet bulb temperature, which also equals the dry bulb temperature, and the RH is 100%. This temperature is shown as horizontal lines on the chart.

5. Moisture contentMoisture content graph

This is also known as the humidity ratio and is usually designated as W. It is the proportion of the mass of water vapor per unit mass of dry air. Humidity ratio is dimensionless, but in the US it is usually expressed as pounds of moisture per pound of dry air; elsewhere it may be expressed as grams of water per kilogram of dry air or as a percentage.1

The moisture content is the vertical axis of the chart.

6. Enthalpy (total heat)

Enthalpy is the heat content.Enthalpy (usually designated as h) is the total amount of heat energy of the moist air and therefore includes the amount of heat of the dry air and the water vapor in the air. In the approximation of ideal gases, lines of constant enthalpy are parallel to lines of constant WB temperature. Thus the enthalpy is indicated by diagonal lines.

In the US, enthalpy is measured in BTU per pound of dry air; elsewhere it is measured in Joules per kilogram of air.

7. Specific volume

This is the inverse of density. Specific volume is therefore the Specific Volume Graphvolume per unit mass of the air sample. This is shown as diagonal lines on the chart.

In the US, this is measured by cubic feet per pound of dry air; elsewhere it is measured by cubic meters per kilogram of dry air.

8. Comfort zones

People feel comfortable within a small range of temperatures and humidities. The ranges vary based on where you live and on the time of year. In the northern hemisphere, people typically wear more clothes in winter than in summer. Therefore, rooms are maintained at cooler temperatures in winter than in summer.

Comfort Zone graph with perceived tempurature and dry bulb temperature

symbolism of chart lines
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