PSYCHROMETRY AND HVAC Psychrometry hygrometry science of moist

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 https: //www. youtube. com/playlist? list=PL 2 E 9_Tl. QKs. GQ 4 Pb. J 0 KJEv. IFxj. VUCw 7 s 30 Watch this tutorial on psychrometry

Plan • • Psychrometric properties Measuring humidity Psychrometric chart HVAC 2

Dry air name formula Molar mass M kg/kmol number fraction y nitrogen N 2 28 0. 78 oxygen O 2 32 0. 21 3

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 4

Humidity ratio absolute humidity, specific humidity 5

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% 6

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

Plan • • Psychrometric properties Measuring humidity Psychrometric chart HVAC 8

Dew-point liquid gas 9 http: //www. climate. be/textbook/glossary_s. xml

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 10

Frost point 11

Adiabatic saturation temperature P 2 An experiment to measure humidity Conservation of dry air: Conservation of water: Conservation of energy: saturated air at outlet: Algebra: 12

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. 13

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 https: //en. wikipedia. org/wiki/Hygrometer 14

Plan • • Psychrometric properties Measuring humidity Psychrometric chart HVAC 15

Psychrometric properties Name all states of moist air by 3 variables: P, T, w 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 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 16

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 in terms of (P, T, w): 17

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

Psychrometric chart 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 19

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

https: //www. ohio. edu/mechanical/thermo/Applied/Chapt. 7_11/Chapter 10 b. html 21

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 22

Plan • • Psychrometric properties Measuring humidity Psychrometric chart HVAC 23

Engineering a state of happiness 24

Lee Kuan Yew The founder of Singapore died on 23 March 2015, aged 91 Among a number of 20 th-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 airconditioner. 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. http: //www. economist. com/news/asia/leekuanyew 25

Singapore 26

Boston 27

Denver 28

HVAC processes approach the comfort zone by changing temperature and humidity 1. Hardware: duct, fan, heater, cooler, source and sink of water, flow of air 2. Psychrometric chart 3. Steady-flow, control-volume analysis. 29

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

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

Cooling and dehumidifying dry air: water: energy: 32

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

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

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

Coal power station coverts coal to electricity 36

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: 37

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 = 22 -27 C, RH = 40%-60%). • Reach comfort zone by HVAC processes. 38