1. Psychrometric Chart Basics
This presentation in on the basics of psychrometrics and the primary purpose is to gain and understanding of its importance in residential housing.

2. Basic Concepts
All changes in the properties of air can be described graphically on a psychrometric chart. Psychrometric charts can also be used to determine the value for various air properties but it is more important to understand what must be done to more from one point on the graph to another. This figure will appear in following examples because it can help explain what must be occurring when air changes conditions.
The following descriptions include some common examples. While some are uncommon all can be achieved with the proper HVAC equipment and controls.
Sensible Cooling occurs when the air temperature is lowered without any change in it’s moisture content The most common example is outside air as it cools in the evening.
Sensible Cooling & Dehumidification occurs with both the air temperature and moisture content are lower. This is what is desired when an air conditioner is operating in a warm humid climate. This is also what occurs with the outside air temperature falls below the dew point and you see dew on the grass.
Dehumidification alone is uncommon but can be achieved with a balance between cooling the air to remove moisture and then reheating it. It could also be done by a material that absorbs moisture from the air.
Heating and Dehumidification is also not common because it requires the air temperature to increase and the moisture content to decrease. Because the most common way to decrease moisture content is by cooling the air below the dew point, this would require cooling and then reheating the air. A dehumidifier running in a small room would achieve this result because the heat given off by the motors is more than the cooling. This causes the air temperature to rise while the moisture content was reduced.
Sensible Heating occurs when the air temperature increases with any change in it’s moisture content. A furnace or heat pump would provide this type of change.
Heating & Humidification occurs when both the air temperature and moisture content increase. This would occur in a furnace with a humidifier or a unvented combustion appliance, such as an unvented gas fireplace.
Humidification is not common. It occurs when moisture is added to the air without any change in air temperature.
Humidification & Cooling occurs when the air temperature decreases while the moisture content increases. This is what happens during evaporative cooling. In dry hot climates evaporative cooling has been used as a means of cooling, however, it does increase the air’s moisture content.

3. Saturation Line
The only value that can not be calculated from air and water vapor equations is the saturation line. The saturation line is point at which the air can hold no additional water vapor. This line will represent several air values: Dew point, Web Bulb Temperature. With the saturation line, the bottom axis is the air temperature and the right side axis is the quantity of moisture in the air.

4. Constant Dry Bulb Temperature
Lines of constant dry bulb temperature run vertically up the psychrometric chart. The dry bulb temperature is what if measured by a normal air thermometer.
At the point where a dry bulb temperature line reaches the saturation line the following conditions exist:
Relative Humidity = 100%
Wet Bulb Temperature = Dry Bulb Temperature
Dew Point Temperature = Dry Bulb Temperature

5. Constant Humidity Ratio
The vertical axis represents the water vapor in the air and is commonly referred to as the Humidity Ratio. A horizontal line represents a constant amount of water vapor in the air. It no water is added to the air, humidification, or removed from the air, dehumidification, then any change in air properties will follow this line.

6. Constant Humidity Ratio Saturation Temperature – Dew Point
At a constant humidity ratio if the air temperature drop it will eventually reach the point where it can no longer hold the water vapor, the saturation line. That point is called the Dew Point. Later examples will show the importance of the dew point to residential housing. One simple example is that the dew point of the air in the afternoon is a simple predictor of the overnight low temperature.

7. Constant Relative Humidity
The chart becomes more complex as more lines are added. Another essential line is constant relative humidity. The primary use of the constant relative humidity line is to identify a point on the chart. A simple example is that if you raise the air temperature and do not add water vapor, constant humidity ratio, the relative humidity decreases. So while the humidity is lower in the hottest part of the day, the moisture in the air has not changed.

8. Constant Specific Volume
Specific volume is a measure of the weight of an air and water vapor mixture. When calculating changes in air properties caused by heating and cooling systems it is necessary to know the mass of air flowing through them.

9. Constant Wet Bulb Temperature
The constant wet bulb lines run diagonally across the chart. A constant web bulb means the energy used to evaporate water to raise the water vapor in the air comes from the air in the form of a lowered air temperature. It is important in residential housing because a simple, accurate measurement is a wet bulb thermometer. Knowing the wet bulb temperature and dry bulb temperature a specific point on the chart can be found.

10. Constant Enthalpy
If calculations are to be performed on changes in air properties then it is important to know the actual energy in the air vapor mixture, the enthalpy. The enthalpy lines are close to the web bulb lines and can create some confusion.

11. Constant Enthalpy and Web Bulb
This figure illustrates how close the two lines are.

12. Typical Chart With Enthalpy Lines
The resulting chart showing all the lines is complex, however, remember what is important is finding a point on the chart.

13. Typical Chart Without Enthalpy Lines
This is a more common example of a chart where the enthalpy lines have been removed for the chart. On this type of chart a straight edge would be used to connect the red enthalpy lines on the outside of the chart to read a red lne

14. State Point State Point
A specific point on a psychrometric chart is called a state point.

15. Reading a Psychrometric Chart Practice
The following examples show how to find a state point and determine another property of the air vapor mixture.

16. Dry-bulb temperature = 70 F
State Point
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%

17. State Point 70 F Dry Bulb Constant Dry Bulb Temperature5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95
100
105
110
115
120
DRY BULB TEMPERATURE - °F
130
140
150
160
170
180
190
200
210
HUMIDITY RATIO - GRAINS OF MOISTURE PER POUND OF DRY AIR
Linric Company Psychrometric Chart,
Constant Dry Bulb Temperature
Dry Bulb
The first step is to locate the air dry bulb temperature on the bottom axis. The state point will be somewhere on the vertical line above that point.

18. State Point 60%
The relative humidity lines slope upward to the right. In this case the relative humidity is one of the lines. This state point is where the 70 F line crosses the 60% RH line. If the relative humidity is a number like 62%, the location is a estimated between 60% and 70%.

19. Dry-bulb temperature = 70 F Wet-bulb temperature = ? F
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Wet-bulb temperature = ? F

20. Wet-bulb
Using the state point as the starting point the web bulb line is followed to the saturation line. The web bulb temperature is read using the scale on the saturation line.

21. Dry-bulb temperature = 70 F Wet-bulb temperature = 61 F
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Wet-bulb temperature = 61 F

22. Dry-bulb temperature = 70 F
Dew Point
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Dew point = ?

23. Dew Point
Dew Point
Using the state point as the starting point the constant humidity ratio line is followed to the saturation line. The dew point temperature is read using the scale on the saturation line.

24. Dry-bulb temperature = 70 F
Dew Point
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Dew point = 55.5 F

25. Dry-bulb temperature = 70 F
Specific Volume
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Specific volume = ?

26. Specific Volume
The specific volume lines are typically widely spaced and their value must be estimated be the distance between two lines.

27. Dry-bulb temperature = 70 F Specific volume = 13.6 ft3 / lb dry air
Sea Level Chart
Dry-bulb temperature = 70 F
Relative Humidity = 60%
Specific volume = 13.6 ft3 / lb dry air

28. Dry-bulb temperature = 70 F
Humidity Ratio
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Humidity ratio = ?

29. Humidity Ratio
Using the state point as the starting point the constant humidity ratio line is followed to the scale on the right.

30. Humidity Ratio Sea Level Chart Dry-bulb temperature = 70 F
Relative humidity = 60%
Humidity ratio = lb water / lb dry air or
7000 grains = 1 lb water
7000 x = 65.8
The humidity ration scale uses units are either “pounds of water per pound of dry air” or “grains of water per pound of dry air”.

31. Dry-bulb temperature = 70 F
Enthalpy
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Enthalpy = ?

32. Enthalpy
Determining the enthalpy depends on the psychrometric chart. This example is a chart were the enthalpy lines are not drawn through the chart. A straight edge is useful when determining the enthalpy. For an estimate the wet bulb lines could be used.

33. Dry-bulb temperature = 70 F Enthalpy = 27.0 Btu / lb dry air
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Enthalpy = 27.0 Btu / lb dry air

34. Calculating Air Property Values
If exact values of air vapor mixture properties are needed for calculations the use of a psychrometric calculator allows for the values to be determined. The calculator shown is available online.

35. Now we will look at examples of the use of a psychrometric chart and this figure.

36. Sensible Heating
If the state points for the supply and return air temperatures are located on a chart they should be on a constant humidity ratio line unless a humification system is operating.

37. Raise Room Temperature 1 degree F Constant Humidity Ratio
27.31 Btu/lb
27.06 Btu/lb
00.25 Btu/lb
Using a calculator the energy needed to raise the air temperature, at a constant humidity ratio, of one pound of air one degree F is 0.25 btu. The 0.25 is close the specific heat of air, 0.24, and the difference is most likely just the rounding error in the calculation method.

38. Raise Room Temperature 1 degree F Constant Humidity Ratio
Length (ft) x Width (ft) x Height (ft) = Volume (ft3)
Volume (ft3) / Specific Volume (ft3/lb) = Mass of Air (lb)
Mass of Air (lb) x Enthalpy Change (Btu/lb) = Heat Input (Btu)
Bedroom
12 x 12 x 8 = 1152 ft3
1152 / = lb
82.17 x 0.25 = Btu
Using that value one can illustrate on little heat is actually required to just raise the air temperature in a room. This does not account for any energy gain or loss from the room over time but just the energy needed to quickly raise a room’s air temperature one degree. One match provides approximately 1 Btu.

39. Heating System Example supply air from furnace1 2
Furnace
Expected Result
1) Return Air 65 F and 60%
2) Supply Air 95 F and 22%
supply air from furnace
return air to furnace
The is what one would expect to occur if the return and supply air conditions were plotted on a chart.

40. Heating System Example supply air from furnace
Return 65 F and 60%
Supply 95 F and 18% 1 2
Measured Values
How is this possible?
supply air from furnace
return air to furnace
How could this condition occur? How can dehumification occur in a heating system.?

41. Mixing Air Streams ma1 h1 w1 Energy Balance ma1 h1 + ma2 h2 = ma3 h3
Mass Balance
ma1 w1 + ma2 w2 = ma3 w3
By Algebra
h2 – h3 = w2 – w3 = ma1
h3 – h1 w3 – w1 ma2
ma3 h3 w3
The most likely answer is that another source of air has been mixed with the return air. The slide shows the actual equations that control what happens but the simple answer is that if you mix two air streams the result is a simple combination of the two.
ma2 h2 w2

42. Mixing Air Streams
Resulting mixture lies on the line between the two state points.
Mixing Two Equal Air Streams = the center of the line
The location of the state point of the mixture on a psychrometric chart is on the line connecting the two points. Its location is a simple ratio of the two streams. If both streams are equal the point is in the center.

43. Heating System Example supply air from furnace
Return 65 F and 60%
Supply 95 F and 18% 1 2
Measured Values
How is this possible?
supply air from furnace
return air to furnace
How is this possible? Sensible heating of a furnace would result in a horizontal line between the return and supply air.

44. Heating System Example
The most like cause of this is a second air source is mixing with the return air before it gets to the furnace. If one knows the conditions of the other air source then it is possible to estimate the amount of air being added by that source.

45. Heating System Example
In this case assume the outside air is at 35F, then we can find the approximate amount of duct leakage. A straight line between point 1, the return air state point measured in the house, and the second possible air source represents the line where the mixed air stream state point must be. Given that the heating system does not add moisture the supply air humidity ratio must represent the humidity ratio of the mixed air stream. Using that line it is possible to locate the state point of the mixed air streams. The ratio of the total length of the line to the length of line to the combined point represents the approximate percentage duct leakage.

46. Cooling
If cooling occurs with now change in the moisture in the air the two state points would be on the same humidity ratio line.

47. Cooling Systems
In order to achieve dehumidification air must be cooled beyond the dew point of the initial state point 1. Two forms of energy are required to achieve this change, one to reduce the air temperature and the other to remove moisture. Looking at the figure it can be seen that even on the saturation line energy is required to both reduce the air temperature and energy to remove moisture.

48. Cooling Systems
The energy to remove moisture is called latent heat and the energy to remove temperature is called sensible heat. On the chart one can determine the amount of each required by going vertically down from state point 1 to the final humidity ration of state point 2 to determine the latent heat. From that point going horizontally to state point 2 provides the sensible heat requirement.

49. Solving the General Equations for Air Conditioning
Total Heat
q = (cfm) (4.5) (∆h)
q = Btuh
h = enthalpy, Btu per lb dry air
Sensible Heat
q = (cfm) (1.08) (∆t)
t = temperature F
Latent Heat
q = (cfm) (4840) (∆w)
w = humidity ratio, lb water per lb dry air
This can be applied to an actual AC system but using the standard equations that calculate total heat energy change, sensible heat change, and latent heat change. The equations use cfm under standard conditions to simplify the calculation.

50. Solving the General Equations for Air Conditioning - Sources of Error
Sensible Heat
q (Btuh) = (cfm) (1.08) (∆t)
(∆t) = q (Btuh)
(cfm) (1.08)
cfm lower - (∆t) higher
cfm higher - (∆t) lower
q lower - (∆t) lower
q higher - (∆t) higher
If one were to use this process to an actual AC system to determine if the unit was operating properly one must be aware of several possible errors. Air flow rates are difficult to measure accurately and depending on the type of AC system, the cooling, Btuh, is sensitive to the refrigerant charge.

51. Standard Operation of an Air Conditioner Sensible heat fraction = 0.70
12,000 Btuh / ton
400 cfm /ton
Sensible heat fraction = 0.70
Latent heat fraction = 0.30
Looking at a standard AC system one can use to psychrometric get a general idea of what should be expected.

52. Standard Operation of an Air Conditioner
Total Heat
q = (cfm) (4.5) (∆h)
12,000 = (400) (4.5) (∆h)
(∆h) = 27.3 Btu / lb dry air
Sensible Heat
q = (cfm) (1.08) (∆t)
12,000 (0.7) = (400) (1.08) (∆t)
(∆t) = 19 F
Latent Heat
q = (cfm) (4840) (∆w)
12,000 (.3) = (400) (4840) (∆w)
(∆w) = lb water / lb dry air
Knowing the total cooling energy and what part is for sensible heat removal and latent heat removal it is possible to calculate the change one would apply to the state point of return air to an AC to find the state point of the supply air. Notice that if one increases the total cooling energy in this example to 2 tons, Btuh, the air flow would be increased to 800 cfm so the results calculated do not change.

53. Standard Operation of an Air Conditioner
When the second state point is found on the chart using the results from the previous side, one can see what the supply air conditions should be. This would be when an AC was operating at standard conditions. If one were to measure the return and supply air conditions the AC would have to had run long enough to be operating this way.

54. Operation of an Air Conditioner
What if humidity ratio of return air equals the humidity ratio of the supply air?
How could this occur? This assumes the AC is operating at standard conditions. One answer that the example does not address is if the evaporator is larger than necessary and there is little or no latent heat removal. In climates with a high average humidity in the summer this provides cooling but no dehumidification. In that case the answer is that the air temperature change would be greater than 19 degrees.

55. Air Conditioner Example
If the supply air has the same humidity ratio and the air conditioner is removing moisture then the return air must have more moisture then the original return air. This is most likely caused by a leaky return duct.

56. Air Conditioner Example
If the second air source state point is located on the chart, a mixture of the two air sources would be located on a straight line between the two points.

57. Air Conditioner Example
0.002
0.002
If one assumes that the moisture removal by the AC is the same than one can use that value to determine where on the connecting line the new return air state point is.

58. Air Conditioner Example
0.002
19 F
The same total heat is removed, with the same latent and sensible heat removal. The new supply air temperature is higher than the original supply air temperature.

59. Evaporative Cooling
When evaporative cooling occurs the new state point would be on the wet bulb line. This condition could be measure in a house when the cooling fan is running without the compressor. Right after the compressor stop the evaporator coil is wet and the system is now getting a little sensible heat out of the metal in the evaporator coil but mainly it is acting as an evaporative cooler. The result is that moisture is being added back in the house air.

60. Annual Hourly Outside Air Conditions – Lexington KY
This chart shows the actual outside air conditions over the course of one year.

61. Human Comfort Zones
This is the same chart where the normal comfort range for heating, in red, and cooling, in blue is shown. The chart demonstrates how much of the time the air must be conditioned for proper comfort. One can also determine the type of conditioning that must be done to achieve the proper comfort.

62. Annual Hourly Outside Air Conditions – Phoenix, AZ
Notice the difference between Phoenix and Lexington. The air is much dryer and hotter.

63. Crawl Space Conditions Example
Crawl spaces create two possible problems at can be illustrated on a psychrometric chart.

64. Annual Hourly Outside Air Conditions – Lexington KY
The red line would represent the typical supply air temperature in a house duct system, about 55 degrees. If the surface of the duct were uninsulated and exposed to the outside air, in this case either in a attic or crawl system, any time the outside air was above the horizontal red line condensation would occur on the surface.
The green represents a crawl space without sufficient air vents or the AC supply ducts leak and is cooler than the outside air in the summer. In this case the assumption is that it stays around 65 degrees. One can observe how much the humidity will increase and create conditions that could lead to mold, above 60% humidity.