lecture 11-12 water vapour as a major energy carrier
DESCRIPTION
Terrestrial Energy Systems LectureTRANSCRIPT
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CIV300 Terrestrial Energy Systems
Water Vapour as a Major Energy Carrier Lecture 11 - 12
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l Humidity Ra+o = Mass of vapour / mass of dry air (g/kg or no dims)
l This is the absolute amount of water vapour in the airand we represent this as a percentage of the total possible with the relaNve humidity (RH as a %).
l Atmospheric pressure varies with atmospheric humidity (Daltons Law of ParNal Pressures)
l E = Actual Vapour Pressure
l E* = Saturated Vapour Pressure
How to Dene Humidity? 2
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E* is the saturated vapour pressure
SaturaNon Vapour Pressure Curve 3
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Psychrometric Chart 4
Dry Bulb Temperature
Humidity RaNo (g/kg)
Enthalpy (kJ/kg of dry air)
SaturaNon temperature
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l Chart has a bewildering range of potenNal measurements
l Focus on dry bulb, wet bulb, relaNve humidity, dew point, humidity raNo, enthalpy
l Psychrometric Chart provides a graphic relaNonship of the condiNon of air
Key Items 5
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l Wet Bulb temperature relates relaNve humidity to the dry air temperature (Dry Bulb) via the energy, or Enthalpy (kJ/kg) - a measure of how it feels
l A direct measure of the ability to evaporate and therefore cool a thermometer. The higher the RH, the less the ability to evaporate and cool
l The drier the air the more rapid evaporaNon on a thermometer the lower the wet bulb. OR the moister the air, the closer dry bulb to wet bulb.
l Dry Bulb Temperature (deg C) = Web Bulb temperature at 100% RH follow the enthalpy lines to the curve to read it.
Wet Bulb 6
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Wet Bulb Temperature Measurement 7
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l ASHRAE states we need to meet 27oC dry bulb / 21oC wet bulb
l Equates to 27oC / 58% RH
Toronto Design Day 8
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l ASHRAE states we need to meet 30oC dry bulb / 25oC wet bulb
Equates to 30oC / 67% RH
Melbourne, FL Design Day 9
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How Air CondiNoning Works. 10
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Reading from the Chart.
Air at 30oC/50% RH? 22oC wet bulb 65 kJ/kg enthalpy 0.013 kg/kg dry air moisture content
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l Dead of winter
l Tropical summer
l Desert heat
l Humidity in the home
Real Life 14
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l Can now see why we get dew and frost
l AdiabaNc cooling forces RH to 100% condensaNon
l Find the dry bulb and RH, follow the line horizontally as it cools, unNl it meets the curve
l If ambient temperature is
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Dew point
Frost point
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Reading from the Chart.
0.013 humidity raNo 18oC dew point
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l Which window got caulked?
! !!
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Light Relief
l What is the Humidex? 19
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So the humidex IS related to the wet bulb in laymans terms
Humidex 20
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The humidex 21
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1. Radia+on fog
Ground cools o at night. Reduces temperature of moist air which then reaches the dew point. Fog.
l Why does fog burn o in sunlight?
Types of Fog 22
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2. Advec+on fog
Moist air blows across cold surface.
Welcome to Newfoundland.
Types of Fog 23
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Catch the Fog 24
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l Warm moist air rises up e.g. summer over Lake Ontario
l What happens on the chart?
Low Level Cloud FormaNon 25
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Air Mass and Mountains
l The moist wind approaches from the westor east in Nanaimo
l Recall psychrometrics
l What happens when it hits the dew point?
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How Air CondiNoning Weather Works. 29