Psychrometrics

Outline

• What is psychrometrics?
• Psychrometrics in daily life and food industry
• Psychrometric chart
– Dry bulb temperature, wet bulb temperature, absolute humidity, relative humidity, specific volume, enthalpy
– Dew point temperature
• Mixing two streams of air
• Heating of air and using it to dry a product
• Psychrometrics is the study of properties of mixtures of air and water vapor
• Water vapor
– Superheated steam (unsaturated steam) at low pressure
– Superheated steam tables are on page 817 of textbook
– Properties of dry air are on page 818 of textbook
– Psychrometric charts are on page 819 & 820 of textbook
• What are these properties of interest and why do we need to know these properties?

Psychrometrics in Daily Life

• Sea breeze and land breeze
– When and why do we get them?
• How do thunderstorms, hurricanes, and tornadoes form?
• What are dew, fog, mist, and frost and when do they form?
• When and why does the windshield of a car fog up?
– How do you de-fog it? Is it better to blow hot air or cold air? Why?
• Why do you feel dry in a heated room?
– Is the moisture content of hot air lower than that of cold air?
• How does a fan provide relief from sweating?
• How does an air conditioner provide relief from sweating?
• When does a soda can “sweat”?
• When and why do we “see” our breath?
• Do sailboats perform better at high or low relative humidity?

Key factors: Temperature, Pressure, and Moisture Content of Air

Do Sailboats Perform Better at low or High RH?
• Does dry air or moist air provide more thrust against the sail?
• Which is denser – humid air or dry air?
– Avogadro’s law: At the same temperature and pressure, the no. of molecules in a given volume is the same, no matter what the gas is
– Air has ~80% N

2 and ~20% O
2 (Ratio of N
2:O
2 = 4:1)
– If 10 molecules of water are added to air, 8 molecules of N

2 and 2
molecules of O
2 are displaced

– Atomic weight gained by air = 10 x 18 = 180
– Atomic weight lost by air = 8 x 28 + 2 x 32 = 288
– Thus, there is a net decrease in weight and hence density
– So, humid air is less dense than dry air

Thus, there is more thrust for the sail with dry air and hence sailboats perform better at low RH

Psychrometrics in the Food Industry

• Heating or cooling of air
– To heat or cool a product
• Mixing different streams of air
• Drying a food product using hot and dry air
– Drier the air, better the drying efficiency

Psychrometric Chart: 6 Quantities

• Dry bulb temperature, Tdb (°C)
– Temperature recorded by a regular thermometer
• Wet bulb temperature, Twb (°C)
– Temp. of a thermometer with air blowing over a moist wick on its bulb
• Moisture content or specific humidity, W (kg water / kg dry air)
– Amount of moisture in air (also called, absolute humidity)
• Relative humidity (RH),

 (%)

– Ratio of amount of moisture in air to max. amt. of moisture air can hold
• Specific volume, V’ (m

3/kg dry air)

– Volume of moist air per unit mass of dry air (specific volume = 1/density)
• Enthalpy, H (kJ/kg dry air)
– Energy content of air

…

Source: https://projects.ncsu.edu/project/foodengineer/231/notes/psychrometrics/Psychro-ppt.pdf