Thermodynamics - Lecture 4

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Evaporation of sweat can be prevented if the existing VAPOUR PRESSURE of water in the atmosphere is too high.

Water Vapour in the air

-there is a fixed maximum amount of WATER VAPOUR that volume can hold at a particular temp.

-this corresponds to the saturated vapour Pressure

-in open environment water vapour isn't in equilibrium with any liquid water. Therefore any liquid water will EVAPORATE over time. as the air has less water vapour mixed with it than the maximum.

Relative Humidity - How much moisture is in the air

RH- (partial pressure of water vapour in air/saturated water vapour pressure) x100%

As dew-point temperature is less than temp of air because actual vapour press is less than the max achievable air temp.

This is determined through

Dew Point Temperature

-relates to daltons law (Ptotal= sum of all partial pressures)

-Normally partial pressure for water vapour is LESS than saturated vapour pressure

BUT NOW partial pressure = saturated vapour pressure at LOW TEMP.

-We can measure the temperature by cooling the surface UNTIL we get a dew/condensation on surface - DEW POINT TEMPERATURE

Moisture Content of Air - mass of moisture/kg of rest of air (DRY AIR) AKA Absolute humidity or humidity ratio

Mass of Water Vapour - use ideal gas law

mw = 18x10^-3 x Pw x V/RT

Mass of Rest of the Air

ma=28.97x10^-3 (Pt-Pw) x V / RT

then COMBINE to get MOISTURE CONTENT

Mw/ma = 0.621 X (Pw/ (Pt - Pw))

Psychometric Chart - assume Patm is 101.3 kPa

- shows lines of different RH using air temp and moisture content of air

Dry bulb temperature - normal air temperature

Wet bulb Temperature

- when air temperature passes over wet surface, moisture evaporates (as mechanism to cool down) when RH is LESS than 100%, thus the air GAINS water vapour its TEMP DROPS (heat provided from air for liquid to gas)

-Evaporation and cooling continues until RH reaches 100% = wet bulb temperature

ie Temp wet sleeve covering thermometer reaches when air is blown over it.

-the air can cool the air temp until its wet bulb temp, no further.

At 100% RH wet bulb = dry bulb