Psychrometric Chart

Psychrometric chart is one of the chart that is commonly used in solving HVAC problems. The properties of air such as wet bulb temperature, dry bulb temperature, dew point temperature, relative humidity, humidity ratio, specific enthalphy and specific volume are shown on this chart.

It is used to determine how these properties vary as the amount of moisture in the air changes. The chart is based on the standard atmospheric pressure of 76mm Hg or 1.01325 bar.

Basic Properties of Air

Dry Bulb Temperature is the actual temperature of air measured by an ordinary thermometer. SI unit is °Celcius. Other unit used is °Fahrenheit. This temperature is located at the horizontal scale of the chart or abscissa.

Wet Bulb Temperature is the temperature of evaporation of which the air would become saturated if moisture were added to it without the addition or substraction of heat. These lines are oblique lines on the chart. SI unit is °Celcius. Other unit used is °Fahrenheit.

Dew Point Temperature is the temperature of saturation of a given atmospheric pressure. Here, the moisture began to condense in the form of dew. SI unit is °Celcius. Other unit used is °Fahrenheit.

Relative Humidity is the ratio of the absolute humidity to the maximum possible density of water vapor in the air at the same pressure and temperature. It has no unit and is the hyperbolic lines that are shown in intervals of 10%. The saturation curve is at 100% RH, while dry air is at 0% RH.

Humidity Ratio is the weight of water in a sample of air compared to the weight of the same sample of air without any moisture in it. It is measured in grams of water/kg of dry air, or grains of water/pound of air. 1 pound is equal to 7,000 grains.

Specific Enthalphy is the total energy of the sample of dry air and water vapor. These are oblique diagonal lines that are drawn from left to right and are parallel to one other. Take note that these lines are not parallel to the wet bulb temperature lines though they may look so. The SI unit is joules/kg of air or BTU/pound of dry air.

Specific Volume is the volume of dry air and water vapor occupied by one unit of mass of dry air. The SI units are cubic meters per kilogram of dry air; other units are cubic feet per pound of dry air

Examples of using the Psychrometric Chart

Question 1: If the DB (dry bulb temperature) is 80°F and WB (wet bulb temperature) is 60°F, find the relative humidity. 

Follow the vertical line of the DB temperature of the chart at 80°F. Similarly, follow the line of the WB temperature at 60°F. Find out the point where these two lines intersect. Read the humidity line where this point is. It is about 30.5% RH(relative humidity).

Question 2: The DB temperature of a sample of air is 85°F and relative humidity is 70% RH. Find the dew point. 

Follow the vertical line of the DB temperature of the chart at 85°F. Find out where the intersection point is with RH of 70%. The dew point of about 73°F can be read out using the chart. 

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