air_conditioning_psychrometry.pdf
TRANSCRIPT
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
Comfort
Air conditioning
1
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
2Introduction
Air Conditioning => process of conditioning air to suit specific needs e.g. human comfort, process air
Human comfort => influenced by:
Air Temperature Air Humidity Air velocity
Air consists of dry air and water vapour mixture => humidity
Air conditioning process controls temperature, humidity and velocity of air to specified state
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
3Introduction
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
4
Atmospheric Air
Composition of dry air remains relatively constant
Composition of water vapour changes due to:Condensation Evaporation
Within typical range of air-conditioning applications (-10 to 50 °C), dry air and water vapour can be treated as ideal gas
va PPP +=
Partial pressure of water vapour
Partial pressure of dry air
Total pressure of atmospheric air
Dalton’s Law
∑=
=k
immim VTPP
1),(
Air ConditioningProcesses
Atmospheric air consists of:Dry air ( N2, O2, CO2 )Water vapour ( H2O )
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Air Humidity
Air Temperatures
PsychrometricChart
5
Atmospheric Air
Atmospheric Air
Within -10 to 50 °C, specific heat of dry air ≅ constant
TTCTh apa
005.1)( ,
==
Tha ∆=∆ 005.1
Within -10 to 50 °C:
Specific heat of water vapour ≅ constant
Enthalpy of water vapour ≅ enthalpy of saturated vapour at same temperature
TTCh
ThThvpCg
gv
82.13.2501
)()(,0,
+≅+=
=°
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Temperatures
PsychrometricChart
6
Air Humidity
Air Humidity
Humidity => measure amount of water vapour in air
2 ways to specify humidity:Absolute or specific humidity ω:- mass of water vapour present in 1 kg of dry air- represented by
v
v
a
v
a
v
PPP
PP
mm
−===
622.0622.0ω
Unit: kg H2O/kg dry air
Relative humidity φ:- ratio of actual mass of water vapour in air (mv) to maximumamount of water vapour air can hold at that temperature (mg)
- represented by
Tsatgg
v
g
v PPPP
mm
@; ===φ
Unit: %
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Temperatures
PsychrometricChart
7
Air Humidity
Air Humidity
Specific humidity ω and relative humidity φ are related:
g
g
g PPP
PP
φφ
ωω
ωφ−
=+
=622.0
;)622.0(
Relative humidity:Has more effect on human comfort level
Relative humidity changes with temperature because amount of vapour air can hold depends on temperature
Enthalpy of air:Sum of enthalpies of dry air and water vapour
Expressed per unit mass of dry air
air)dry (kJ/kgga hhh ω+=
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
PsychrometricChart
8
Air Temperatures
Air Temperatures
Dew-point temperature: temperature at which condensation begins when air is cooled at constant pressure
vPsatdp TT @=Air ConditioningProcesses
Air temperatures:Dew-point temperature Dry-bulb temperatureAdiabatic saturation temperature Wet-bulb temperature
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
PsychrometricChart
9
Air Temperatures
Air TemperaturesLiquid water
2
2
21
2
22
2121
622.0where
)(
g
g
fg
fgp
PPP
hhhTTC
−=
−
+−=
ω
ωω
Air saturation temperature:Temperature air attains after flowing over water in long channel until it is saturated
Used to measure humidity
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
PsychrometricChart
10
Air Temperatures
Air Temperatures
Dry-bulb temperature: ordinary atmospheric air temperature
Wet-bulb temperature:
Air ConditioningProcesses
* thermometer bulb covered with wet cotton wick
* approximates adiabatic saturationtemperature
* provide more practical way of measuring air humidity
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
11
Psychrometric chart
PsychrometricChart
Psychrometric chart => chart that present properties ofatmospheric air at specified total atmospheric pressure
Air ConditioningProcesses
ωφ
φ
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
Air ConditioningProcesses
12
Psychrometric chart
PsychrometricChart
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
13
Air conditioning processes
Air ConditioningProcesses
Comfort of human body mainly depends on:(dry-bulb) temperaturerelative humidity => affect body heat dissipation thru evaporationair motion => removes warm and moist air
Factor Comfort zone Temperature 22 - 27°C Relative humidity 40 - 60% Air motion ~ 15 m/min
Thermal comfort as defined by American Society of Heating, RefrigeratingAnd Air Conditioning Engineers (ASHRAE)
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
14
Air conditioning processes
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
15
Air conditioning processes
Air ConditioningProcesses
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
16
Air conditioning processes
Air ConditioningProcesses
Cooling Heating
Humidifying Dehumidifying
Cooling & Dehumidifying Heating & humidifying
Air-conditioning processes:
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
17
Air conditioning processes
Air ConditioningProcesses
Air conditioning analysis:
∑ ∑∑ ∑∑ ∑
∑ ∑
−=−==>=
=
iieeeeaiiaewiw
eaia
hmhmWQmmmm
mm
:Energy:massWater
:massair Dry
,,,,,,
ωω
Dry air
Vapour(Humidity)
Dry air
Vapour(Humidity)
Heat Transfer
Mass (water) transfer
Conditioning process
Steady-flowKE & PE negligible
Utilise mass & energy balance
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
18
Air conditioning processes
Air ConditioningProcesses
Simple heating:* (dry-bulb) temperature increase* specific humidity constant* relative humidity decrease
ωφ
φ
1 2
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
19
Air conditioning processes
Air ConditioningProcesses
Simple cooling:* (dry-bulb) temperature decrease* specific humidity constant* relative humidity increase
ωφ
φ
12
Dry air
Vapour(Humidity)
Dry air
Vapour(Humidity)
Heat Transfer
Conditioning process
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
20
Air conditioning processes
Air ConditioningProcesses
Heating with humidifying:* 1 - 2: Simple heating* 2 - 3: humidifying
(Adding of moisture)* Increase in specific humidity
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
21
Air conditioning processes
Air ConditioningProcesses
Cooling with dehumidifying:* Air need to be cooled tosaturation temperature (φ = 100%)for vapour to condense & able to be removed
* May need simple heating toget to required temperature
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
22
Air conditioning processes
Air ConditioningProcesses
Evaporative cooling:* principle: as water evaporates,
latent heat of vaporisation is absorbed from water and surrounding air
* Follow approximately line of constant wet-bulb temperatureand enthalpy
constant
constant
≅≅
hTwb
Sychrometry & Air Conditioning
Adv. Thermo-fluids
Atmospheric Air
Air Humidity
Air Temperatures
PsychrometricChart
23
Air conditioning processes
Air ConditioningProcesses
Adiabatic mixing of airstreams:* State of air mixture lies on lines
connecting states 1 & 2 on chart
* Ratio of distances 2-3 & 3-1 equals to ratio of mass flow ma1 and ma2
321
321
21
321
321
:Energy
: vapourof Mass
:airdry of Mass
hmhmhm
mmm
mmm
maa
aaa
aaa
=+
=+
=+
ωωω
13
32
13
32
2
1
hhhh
mm
a
a
−−
=−−
=ωωωω