1. PSYCHROMETRY AND HVAC Psychrometry (hygrometry): science of moist air
HVAC: Heating, ventilating, and air-conditioning
I am teaching Engineering Thermodynamics to a class of 75 undergraduate students.
I plan to go through these slides in one 90-minute lecture.
Zhigang Suo, Harvard University
Watch this tutorial on psychrometry

2. Plan Psychrometric properties Measuring humidity Psychrometric chart
HVAC

3. Dry air name formula Molar mass M kg/kmol number fraction y nitrogen28
0.78
oxygen O2 32
0.21

4. Moist air a mixture of many gases and water vapor, modeled as an ideal-gas mixture
Dry air is far from the dome
Water vapor is near the dome
Partial pressure of dry air, Pa
Partial pressure of water vapor, Pv
Total (barometric) pressure

5. Humidity ratio absolute humidity, specific humidity

6. Relative humidity
Obtain the saturated pressure Pg(T) from the steam table.
Dry air. Relative humidity = 0%
Air saturated with water. Relative humidity = 100%
Human comfort. Relative humidity = 40-60%

7. Enthalpy Specific enthalpy of dry air:
Specific enthalpy of water vapor:
Enthalpy of moist air:
Specific enthalpy in terms of (P,T,w):

8. Plan Psychrometric properties Measuring humidity Psychrometric chart
HVAC

9. Dew-point
liquid
gas

10. Foggy morning
At sunset, the temperature is 20 C and the relative humidity is 50%. In the morning, how much must the temperature have dropped for fog to form? Assume that the partial pressure of water remains fixed.
sunset
dew point

11. Frost point

12. Adiabatic saturation temperature
An experiment to measure humidity
Conservation of dry air:
Conservation of water:
Conservation of energy:
saturated air at outlet:
Algebra:

13. Wet-bulb temperature. Dry-bulb temperature
Sling psychrometer
For air–water vapor mixtures at atmospheric pressure, Twb is approximately equal to the adiabatic saturation temperature.

14. Hygrometer humidity sensors First inventor: Johann Heinrich Lambert (1755) Humidity affects everything. Everything is a hygrometer. Today’s opportunity: The Internet of things.
Bimaterial strip
Hair-tension hygrometer
Wet-bulb and dry-bulb
Dew-point hygrometer
Capacitor
Resistor
Thermal conductivity
Weight

15. Plan Psychrometric properties Measuring humidity Psychrometric chart
HVAC

16. Psychrometric properties Name all states of moist air by 3 variables: P,T,w
P, (barometric, or total) pressure
Pa, dry-air pressure
Pv, vapor pressure (partial pressure of water)
T, (dry-bulb) temperature
Tdp, dew-point temperature
Twb, wet-bulb temperature
ma, mass of dry air
mv, mass of water vapor
w, specific humidity (absolute humidity, humidity ratio)
f, relative humidity (RH)
v, specific volume
h, specific enthalpy

17. Name all states of moist air by 3 variables: P,T,w
Volume:
Specific volume in terms of (P,T,w):
Specific humidity:
Partial pressure of water in terms of (P,T,w):
Relative humidity:
Relative humidity in terms of (P,T,w):

18. Since Chinese invented paper, humans have spent disproportional amount of time on one particular number: 2

19. Psychrometric chart Fix P = 1 atm
Henry Ford: “You can have Model T in any color, so long as it’s black.”
You can draw figure in any dimensions, so long as it’s two-dimensional.
Fix P = 1 atm

20. Name all states of moist air using 3 variables: P,T,w
Psychrometric chart
Name all states of moist air using 3 variables: P,T,w
P = 1 atm w
Tdb

22. Foggy morning
At sunset, the temperature is 20 C and the relative humidity is 50%. In the morning, how much must the temperature have dropped for fog to form? Assume that the partial pressure of water remains fixed.
dew point
sunset

23. Plan Psychrometric properties Measuring humidity Psychrometric chart
HVAC

24. Engineering a state of happiness

25. Lee Kuan Yew The founder of Singapore died on 23 March 2015, aged 91
Among a number of 20th-century luminaries asked by the Wall Street Journal in 1999 to pick the most influential invention of the millennium, he alone shunned the printing press, electricity, the internal combustion engine and the internet and chose the air-conditioner. He explained that, before air-con, people living in the tropics were at a disadvantage because the heat and humidity damaged the quality of their work.

26. Singapore

27. Boston

28. Denver

29. HVAC processes approach the comfort zone by changing temperature and humidity
Hardware: duct, fan, heater, cooler, source and sink of water, flow of air
Psychrometric chart
Steady-flow, control-volume analysis.

30. Heating  = constant, but f drops Conservation of dry air:
Conservation of water:
Conservation of energy:

31. Heating and humidifying Conservation of dry air:
Conservation of water:
Conservation of energy:

32. Cooling and
dehumidifying
dry air:
water:
energy:

33. Evaporative Cooling
h = constant
dry air:
water:
energy:

34. Senator Rubio, There is no clash of civilization.
Drink more water and study psychrometry!
Evaporative cooling
Desert: T = 50 C, RH = 10%
What is temperature of the cloth?
cloth
desert

35. Adiabatic mixing Rule of mixture Conservation of dry air:
Conservation of water:
Conservation of energy:

36. Coal power station coverts coal to electricity

37. Cooling tower
The moist air in the tower has a high water-vapor content, and is lighter than the outside air. The light air in the tower rises, and the heavier outside air fills the vacant space, creating an airflow from the bottom of the tower to the top.
Conservation of dry air:
Conservation of water:
Conservation of energy:

38. Summary
Name all states of moist air by 3 psychrometric properties: (total) pressure P, (dry-bulb) temperature T, and humidity ratio w.
Hygrometer
Psychrometric chart: Fix P, and represent all other psychrometric properties on a plane by lines of constant values.
Comfort zone (T = C, RH = 40%-60%).
Reach comfort zone by HVAC processes.