PSYCHROMETRY AND HVAC

Psychrometry (hygrometry): science of moist airHVAC: 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=PL2E9_TlQKsGQ4PbJ0KJEvIFxjVUCw7s30Watch this tutorial on psychrometry

Plan

• Psychrometric properties • Measuring humidity • Psychrometric chart• HVAC

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Dry air

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name formula Molar mass M kg/kmol

number fraction y

nitrogen N2 28 0.78

oxygen O2 32 0.21

Moist air a mixture of many gases and water vapor, modeled as an ideal-gas mixture

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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

Humidity ratioabsolute humidity, specific humidity

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Relative humidity

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• 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%

Enthalpy

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Specific enthalpy of dry air:

Specific enthalpy of water vapor:

Enthalpy of moist air:

Specific enthalpy in terms of (P,T,):

Plan

• Psychrometric properties • Measuring humidity • Psychrometric chart• HVAC

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Dew-point

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liquid

gas

http://www.climate.be/textbook/glossary_s.xml

Foggy morning

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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

Frost point

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Adiabatic saturation temperature

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Conservation of dry air:

Conservation of water:

Conservation of energy:

saturated air at outlet:

Algebra:

P2

An experiment to measure humidity

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For air–water vapor mixtures at atmospheric pressure, Twb is approximately equal to the adiabatic saturation temperature.

Sling psychrometer

Wet-bulb temperature. Dry-bulb temperature

Hygrometerhumidity sensors

First inventor: Johann Heinrich Lambert (1755)Humidity affects everything. Everything is a hygrometer.

Today’s opportunity: The Internet of things.

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

• Bimaterial strip• Hair-tension hygrometer• Wet-bulb and dry-bulb• Dew-point hygrometer• Capacitor• Resistor• Thermal conductivity• Weight

Plan

• Psychrometric properties • Measuring humidity • Psychrometric chart• HVAC

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Psychrometric propertiesName all states of moist air by 3 variables: P,T,

1. P, (barometric, or total) pressure2. Pa, dry-air pressure

3. Pv, vapor pressure (partial pressure of water)

4. T, (dry-bulb) temperature5. Tdp, dew-point temperature

6. Twb, wet-bulb temperature

7. ma, mass of dry air

8. mv, mass of water vapor

9. , specific humidity (absolute humidity, humidity ratio)10. , relative humidity (RH)11. v, specific volume 12. h, specific enthalpy

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Name all states of moist air by 3 variables: P,T,

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Volume:

Specific volume in terms of (P,T,):

Specific humidity:

Partial pressure of water in terms of (P,T,):

Relative humidity:

Relative humidity in terms of (P,T,):

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

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Psychrometric chartHenry 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

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Psychrometric chartName all states of moist air using 3 variables: P,T,

P = 1 atm

Tdb

21https://www.ohio.edu/mechanical/thermo/Applied/Chapt.7_11/Chapter10b.html

Foggy morning

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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.

sunsetdew point

Plan

• Psychrometric properties • Measuring humidity • Psychrometric chart• HVAC

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Engineering a state of happiness

Lee Kuan YewThe founder of Singapore died on 23 March 2015, aged 91

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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.

http://www.economist.com/news/asia/leekuanyew

Singapore

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Boston

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Denver

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HVAC processesapproach the comfort zone by changing temperature and humidity

1. Hardware: duct, fan, heater, cooler, source and sink of water, flow of air

2. Psychrometric chart3. Steady-flow, control-volume analysis.

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Heating = constant, but drops

Conservation of dry air:

Conservation of water:

Conservation of energy:

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Heating and humidifying

Conservation of dry air:

Conservation of water:

Conservation of energy:

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Cooling and dehumidifying

dry air:

water:

energy:

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Evaporative Coolingh = constant

dry air:

water:

energy:

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Evaporative coolingDesert: T = 50 C, RH = 10%What is temperature of the cloth?

desert

cloth

Senator Rubio, There is no clash of civilization.Drink more water and study psychrometry!

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Adiabatic mixingRule of mixture

Conservation of dry air:

Conservation of water:

Conservation of energy:

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Coal power stationcoverts coal to electricity

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Cooling towerThe 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:

• Name all states of moist air by 3 psychrometric properties: (total) pressure P, (dry-bulb) temperature T, and humidity ratio .

• 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.

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Summary