(dry-bulb) temperature, relative humidity

57 Pages·2009·2.81 MB·English

by  Dr Ali Jawarneh

CChhaptter 1144
GGAASS–VVAAPPOORR MMIIXXTTUURREESS
AND AIR-CONDITIONING
Dr Ali Jawarneh
Department of Mechanical Engineering
Hashemite University
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Objectives
• Differentiate between dry air and atmospheric air.
• Define and calculate the specific and relative
humidity of atmospheric air.
• Calculate the dew-point temperature of
aattmmoosspphheerriicc aaiirr.
• Relate the adiabatic saturation temperature and
wweett-bbuullbb tteemmppeerraattuurreess ooff aattmmoosspphheerriicc aaiirr.
• Use the psychrometric chart as a tool to
determine the ppropperties of atmosppheric air.
• Apply the principles of the conservation of mass
and energy to various air-conditioning processes.
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14-1 DRY AND ATMOSPHERIC AIR
AAttmosphheriic aiir: AAiir iin tthhe attmosphhere conttaiiniing
some water vapor (or moisture).
Dry air: Air that contains no water vapor.
WWaatteerr vvaappoorr iinn tthhee aaiirr ppllaayyss aa mmaajjoorr rroollee iinn hhuummaann
comfort. Therefore, it is an important consideration
in air-conditioning applications.
The c of air can be
p
aassssuummeedd ttoo bbee ccoonnssttaanntt
Water vapor in air behaves as if it existed alone at 1.005 kJ/kg · °C in the
and obeys the ideal-gas relation Pv = RT. Then the temperature range −10
atmospheric air can be treated as an ideal-gas to 50°C with an error
miixtture: under 0.2%.
the subscript adenotes dry air
and the subscript vdenotes
The temperature of air in air-conditioning
water vapor
applications ranges from about
PP PPaarrttiiaall pprreessssuurree ooff ddrryy aaiirr -1100 ttoo aabboouutt 5500°CC. IInn tthhiiss rraannggee, ddrryy aaiirr
a
P Partial pressure of vapor (vapor pressure) can be treated as an ideal gas with
v
a constant c value of 1.005 kJ/kg · K
p
(It is the pressure water vapor would exert if it existed
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alone at the temperature and volume of atmospheric air)
For water h =h(T ) since water
h = 2500.9 kJ/kg at 0°C vapor is an ideal gas
g
c = 11.8822 kkJJ//kkg · °°CC att −1100 tto 5500°°CC range
p,avg
Therefore, the enthalpy of
water vapor in air can be taken
to be eqqual to the enthalppyy of
saturated vapor at the same
temperature
In the temperature range −10 to You can use
50°C, the h of water can be
g Table A-4
determined from this eqquation
with negligible error.
Below 50°C, the h =const. lines
At 50°C, the saturation pressure of water is 12.3 kPa. At
coincide with the T =const. lines in the pressures below this value, water vapor can be treated
as an ideal gas with negligible error (under 0.2 %)
superheated vapor region of water.
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Is Water Vapor an Ideal
GGaass??
- At P<10 kPa, water vapor can
bbee ttrreeaatteedd aass aann iiddeeaall ggaass
-
In air-conditioning applications, the water
vvaappoorr iinn tthhee aaiirr ccaann bbee ttrreeaatteedd aass aann iiddeeaall
gas with essentially no error since the
pressure of the water vapor is very low. In
steam power plant applications, however,
tthhe pressures iinvollvedd are usualllly very
high; therefore, ideal-gas relations should
not be used.
•Gases follow the ideal-gas equation
closely at low pressures and high
temperatures
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14-2 SPECIFIC AND RELATIVE HUMIDITY OF AIR
AAbbssoolluuttee oorr ssppeecciiffiicc hhuummiiddiittyy
(humidity ratio): The mass of water
vapor present in a unit mass of dry air.
Fig. 14-4 For saturated air, the
vapor pressure is equal to the
ssaattuurraattiioonn pprreessssuurree ooff wwaatteerr..
R =0.287, R =0.4615 kJ/kg.K
a v
Specific humidity is the amount of water vapor present in a unit mass of dry air.
SSaattuurraatteedd aaiirr:: TThhee aaiirr ssaattuurraatteedd wwiitthh mmooiissttuurree.
Consider 1 kg of dry air. By definition, dry air contains no water vapor, and thus its specific humidity is
zero. Now let us add some water vapor to this dry air. The specific humidity will increase. As more vapor or
moisture is added, the specific humidity will keep increasing until the air can hold no more moisture. At this
ppooiinntt,, tthhee aaiirr iiss ssaaiidd ttoo bbee ssaattuurraatteedd wwiitthh mmooiissttuurree,, aanndd iitt iiss ccaalllleedd ssaattuurraatteedd aaiirr.. AAnnyy mmooiissttuurree iinnttrroodduucceedd
into saturated air will condense. The amount of water vapor in saturated air at a specified temperature and
pressure can be determined from Eq. 14–8 by replacing P by P , the saturation pressure of water at that
v g
temperature (Fig. 14–4).
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Relative humidity: The ratio of the
amount of moisture the air holds (m) to the
vv
maximum amount of moisture the air can
hold at the same temperature (m).
g
Relative humidity is the ratio of the actual amount of
vvaappoorr iinn tthhee aaiirr aatt aa ggiivveenn tteemmppeerraattuurree ttoo tthhee mmaaxxiimmuumm
amount of vapor air can hold at that temperature.
The difference between specific
Combining Eqs. 14–8 and 14–9, we can and relative humidities.
also express the relative humidity as
What is the relative humidity
of dryy air and saturated air?
The relative humidity ranges from 0 for dry air to 1 for saturated air. Note
that the amount of moisture air can hold depends on its temperature.
TThheerreeffoorree, tthhee rreellaattiivvee hhuummiiddiittyy ooff aaiirr cchhaannggeess wwiitthh tteemmppeerraattuurree eevveenn wwhheenn iittss
specific humidity remains constant.
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In most practical applications, the
amount of dry air in the air–
water-vapor mixture remains
constant, but the amount of water
vvaappoorr cchhaannggeess..
Therefore, the enthalpy of
atmospheric air is expressed per
unit mass of dry air.
Dry-bulb temperature: The enthalpy of moist (atmospheric) air is
TThhee oorrddiinnaarryy tteemmppeerraattuurree eexxpprreesssseedd ppeerr uunniitt mmaassss ooff ddrryy aaiirr, nnoott ppeerr
of atmospheric air. unit mass of moist air.
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EXAMPLE: A tank contains 21 kg of dry air and 0.3 kg of
water vappor at 30°C and 100 kPa total ppressure. Determine
(a) the specific humidity, (b) the relative humidity, and (c)
the volume of the tank.
SSOOLLUUTTIIOONN
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