Thermodynamic lecture slide Chapter 14 Final.pdf

Thermodynamic lecture slide Chapter 14 Final.pdf

1. THERMODYNAMICS -II
ME-234
LECTURE-18
Delivered By:
Completed in Week 12

2. 2
Summary
GasMixture
•CompositionofMixture
•P-V-TBehavior
•Dalton’sLaw
•Amagat’sLaw
•PropertiesofMixtures
Gas-VaporMixtures
•Twophasemixtures

3. 3
Gas-Vapor Mixture
•DryandAtmosphericAir
•CompositionofAir
•SpecificHumidity
•RelativeHumidity
•CharacteristicTemperaturesofAir
•DewpointTmperature
•Adiabaticsaturationtemperature
•Wet-bulbtemperatures
•Psychrometricchart
•ApplicationinAir-conditioning

4. Dry and Atmospheric Air
•Theairintheatmospherenormallycontainssome
watervaporandisreferredtoAtmosphericAir.
•AirthatcontainsnowateriscalledDryAir.
•Althoughtheamountofwatervaporintheairissmall,
itplaysamajorroleinhumancomfort.Thereforeitis
animportantconsiderationinairconditioning
applications
•Thedryairandvaporofatmosphericairinair
conditioningapplicationrangebetweentemperature
changesfrom-10℃to50℃

5. Total Atmospheric Pressure
•Whenthemixturetemperatureisbelow50
o
C,the
watervaporisanidealgaswithnegligibleerror
(<0.2%).Then,theatmosphericair(Dryair+Water
Vapor)canbetreatedasidealgasmixture.
•Thetotalatmosphericairpressure(P)issumof
partialpressureofdryair(P
a)andthatofwater
pressure(P
v):
P = P
a + P
v
•The partial pressure of water vapor is usually referred
to as the vapor pressure.

6. Enthalpy of Water Vapor
•Sincewatervaporisanidealgas,enthalpyofwater
vaporisafunctionoftemperatureonlyh=h(T)
h
v(T, low P) ~ h
g(T)

7. Composition of Atmospheric Air
Massofwatervaporpresentinunitmassofdryairis
calledspecificorabsolutehumidity
7

8. Saturated Air
8
•Bydefinition,dryaircontainsnowatervapor,
andthusitsspecifichumidityiszero
•Nowifweaddsomewatervaportothisdry
air,thespecifichumiditywillincrease
•Asmorevaporormoistureisadded,the
specifichumiditywillkeepincreasinguntilthe
aircanholdnomoremoisture.Atthispoint,
theairissaidtobesaturatedwithmoisture,
anditiscalledsaturatedair
•Anymoistureintroducedintosaturatedairwill
condense

9. Relative Humidity
Moistureintheairdeterminescomfortlevel
Comfortleveldependsmoreontheamount
ofmoisturetheairholds(m
v)relativetothe
maximumamountofmoisturetheaircanhold
atthesametemperature(m
g)
Theratioofthesetwoquantitiesiscalledthe
relativehumidity
9

10. Relative Humidity
Relativehumidityrangesfrom0fordryairto
1forsaturatedair.
Relativehumidityofairchangeswith
temperatureevenwhenitsspecifichumidity
remainsconstant.
10

11. Humidity
Combining both equations, we get
11

12. Enthalpy of Atmospheric Air
Amountofdryairintheair–water-vapormixture
remainsconstant,buttheamountofwatervapor
changes.Therefore,theenthalpyofatmosphericair
isexpressedperunitmassofdryairinsteadofper
unitmassoftheair–watervapormixture

13. 13
The enthalpy of water vapor in air can be taken to be
equal to the enthalpy of saturated vapor at the same temperature.
That is,

14. Example
14

15. Example
15

16. Example
16

17. Example
17

18. Temperatures of Atmospheric Air
Dry Bulb Temperature
Dew Point Temperature
Adiabatic Saturation Temperature
Wet Bulb Temperature
18

19. 1. Dry Bulb Temperature
Theordinarytemperatureofatmosphericair
isfrequentlyreferredtoasthedry-bulb
temperaturetodifferentiateitfromother
formsoftemperaturesthatshallbe
discussed.
19

20. 2. Dew Point Temperature
Thedew-pointtemperatureT
dpisthe
temperatureatwhichcondensationbegins
whentheairiscooledatconstantpressure.

21. 2. Dew Point Temperature

22. 3. Adiabatic Saturation Temperature
Tomeasurehumidityintheair
ω
1is unknown
Air exits as saturated air
T
2drops
No external work or heat
interaction

23. 3. Adiabatic Saturation Temperature

24. 3. Adiabatic Saturation Temperature

25. 3. Adiabatic Saturation Temperature
Relative and absolute humidity can be calculated by measuring
Pressure and Temperature at inlet and outlet

26. 4. Wet Bulb Temperature
Anothermethodtocalculate
humidityusingathermometer
whosebulbiscoveredwitha
cottonwicksaturatedwithwater
andbyblowingairoverthewick
Temperaturemeasuredinthis
manneriscalledthewet-bulb
temperatureT
wb
Aswaterevaporatesfromwick,its
temperaturedropsandreads
differentfromsurrounding
temperature

27. 4. Wet Bulb Temperature
Anotherarrangementiswhere
thermometerisrotated,called
“Slingpsychrometer”
Wet-bulbtemperatureT
wbcanbe
usedinplaceofT
2todetermine
thespecifichumidityofair.

28. Example
28

29. Example
29

30. Example
30

31. PSYCHROMETRIC CHART
31

32. Why Psychrometric Charts?
Stateofatmosphericairatanypressureis
completelyspecifiedby2independent
intensiveproperties
Psychrometricchartsallowthesecalculations
visuallyusingtwoproperties
Psychrometricchartsdevelopedfordifferent
pressures
32

33. Psychrometric Charts
DryBulbTemp
•OrdinaryTemp.
•Independentofmoisture
•X-axis
SpecificHumidity
•Y-Axis
•AbsoluteHumidity

34. Psychrometric Charts
SaturationPoints
•IncreaseswithTemp
•Ф=100%
Connectthesepointsto
getacurveforФ=100%

35. Psychrometric Charts
SaturationPoints
•IncreaseswithTemp
•Ф=100%
Connectthesepointsto
getacurveforФ=100%

36. Psychrometric Charts
RelativeHumidity
•LinesofconstantФ

37. Psychrometric Charts
RelativeHumidity
•LinesofconstantФ

38. Psychrometric Charts
WetBulbTemp
•T
wbisidenticaltoT
dbfor
saturatedair
T
wb= 15
T
wb= 20

39. Psychrometric Charts
WetBulbTemp
•T
wbisidenticaltoT
dbfor
saturatedair
T
wb= 15
T
wb= 20

40. Psychrometric Charts
Entalpy
•Constantenthalpylines
arealmostparallelto
constantwet-bulbtemp
lines

41. Psychrometric Charts
Entalpy
•Constantenthalpylines
arealmostparallelto
constantwet-bulbtemp
lines

42. Psychrometric Charts
Volume
•Constantvolumeliesare
also availableon
Psychrometriccharts

43. Psychrometric Charts
Volume
•Constantvolumeliesare
also availableon
Psychrometriccharts

44. Psychrometric Charts

45. Psychrometric Charts

46. Psychrometric Charts

47. Dew point Temperature
47

48. Human Comfort
Human wants:
Not hot, not cold, not humid, not dry -just
COMFORTABLE -Air-conditioning.
Typical comfort conditions:
22 –27
o
C and40 –60 %Relative Humidity
Air motion also effect human feeling (body heat loss).
48

49. Air-Conditioning
Maintainingspacedesiredtemperatureand
humidityrequires“air-conditioning
processes”includingsimpleheating,cooling,
humidifyinganddehumidifying
49

50. Air-Conditioning
Most Air-conditioning processes are steady
flow processes
Mass Balance for Air and water:
Energy Balance:
50

51. Air-Conditioning-No humidification
Ωremainssameforsimplecoolingand
heatingprocessprocess
51

52. APPLICATION IN AIR-
CONDITIONING
PSYCHROMETRIC CHARTS
52

56. Cooling with de-humidification
Specific humidity remains constant but relative
humidity increases until 100% (T
dp)
Further cooling results in condensation at ф= 100%
Air may be heated to desired ф
Condensate is assumed to leave cooling section at T
2
56

59. Adiabatic Mixing of Air Streams
Many air conditioning systems require mixing
of air with fresh air
No heat and work interaction during mixing
Mass and Energy balance:
Eliminate from these equations
59

60. Adiabatic Mixing of Air Streams
Thus we conclude that when two airstreams at two different
states (states 1 and 2) are mixed adiabatically, the state of the
mixture (state 3) lies on the straight line connecting states 1 and 2
on the Psychrometric chart, and the ratio of the distances 2-3 and
3-1 is equal to the ratio of mass flow rates

62. Other properties of mixture can be calculated
from Psychometric Chart at State 3