air_conditioning_psychrometry.pdf

Sychrometry & Air Conditioning

Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

Comfort

Air conditioning

1

Air ConditioningProcesses


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



Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

3Introduction



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),(


Atmospheric air consists of:Dry air ( N2, O2, CO2 )Water vapour ( H2O )


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,

+≅+=

=°



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: %



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+=



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


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



Adv. Thermo-fluids

Atmospheric Air

Air Humidity

PsychrometricChart

10

Air Temperatures

Air Temperatures

Dry-bulb temperature: ordinary atmospheric air temperature

Wet-bulb temperature:


* thermometer bulb covered with wet cotton wick

* approximates adiabatic saturationtemperature

* provide more practical way of measuring air humidity


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


ωφ

φ


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures


12

Psychrometric chart

PsychrometricChart


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

13

Air conditioning processes


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)


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

14




Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

15




Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

16



Cooling Heating

Humidifying Dehumidifying

Cooling & Dehumidifying Heating & humidifying

Air-conditioning processes:


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

17



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


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

18



Simple heating:* (dry-bulb) temperature increase* specific humidity constant* relative humidity decrease

ωφ

φ

1 2


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

19



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


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

20



Heating with humidifying:* 1 - 2: Simple heating* 2 - 3: humidifying

(Adding of moisture)* Increase in specific humidity


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

21



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


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

22



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


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

23



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


Adv. Thermo-fluids

Atmospheric Air

Air Humidity

Air Temperatures

PsychrometricChart

24



Wet cooling towers:

Induced DraftNatural Draft

air_conditioning_psychrometry.pdf

Documents

air mv

velocity of air

temperatureenthalpy

dry air n2

process of conditioning

water vapour mixture

mass of water vapour

water vapourexpressed