Chuyên ngành ĐHKK

1. Các tên gọi theo thuật ngữ tiếng Anh của các Đại lượng Độ ẩm không khí at
http://www.engineeringtoolbox.com/absolute-humidity-air-d_681.html
Humidity - or moist - is the water vapor present in air where (commonly used
terms) are Absolute Humidity, Specific Humidity and Relative Humidity
Humidity is the quantity of water vapor present in air. It can be expressed as absolute, specific or a
relative values.
1-AbsoluteHumidity of Air
Absolute humidity is the actual mass of water vapor - mh20 - present in the air water vapor mixture
( in a volume unit of Air-Vapor mixture)
 In SI-unitsthe absolutehumiditymaybe expressedinkilogramsof watervapor (kg/m3
,g/m3
)
 In imperial unitsthe absolute humiditymaybe expressedinlbsof watervapor (lbs/ft3
,gr/ft3
)
specific humidity SH is: độ ẩm riêng
1- the ratio of (the mass of water vapor) in air to (the total mass) of (the mixture of air and water vapor).
2- 1) the mass of water vapour in (a sample of moist air) divided by 2) the mass of the sample
SH= mH2O / ( mH2O + mDRY-AIR)
Compare dewpoint,mixingratio,relativehumidity,specifichumidity.
2- Humidity Ratio HR of Air : tỷ số độ ẩm hay độ chứa ẩm
The ratio between actual mass of water vapor present in moist air - to the mass of dry air only
HR= mH2O / mDRY-AIR
Humidity ratio can be expressed by mass of water - or by the vapor partial pressure in the moist air
HumidityRatio by Mass
Humidity ratio can be expressed as the ratio between the actual mass of water vapor present in moist air
- to the mass of the dry air. Humidity ratio is normally expressed in kilogram or pounds of water vapor
per kilogram or pounds of dry air.
Humidity ratio expressed by mass:
x = mw / ma (1)

2. where
x = humidity ratio (kgwater/kgair, lbwater/lbdry_air)
mw = mass of water vapor (kg, lb)
ma = mass of dry air (kg, lb)
HumidityRatio by VaporPartial Pressure
Humidity ratio can also be expressed with the partial pressure of water vapor:
x = 0.62198 pw / (pa - pw) (2)
where
pw = partial pressure of water vapor in moist air (Pa, psi)
pa = atmospheric pressure of moist air (Pa, psi)
The maximum amount of water vapor in the air is achieved when pw = pws the saturation pressure of
water vapor at the actual temperature. (2) can be modified to:
xs = 0.62198 pws / (pa - pws) (3) where
xs = specific humidity at saturation (kgwater/kgair, lbwater/lbdry_air)
pws = saturation pressure of water vapor
The water vapor pressure is small regarding to the atmospheric pressure and the relation between the
humidity ratio and the saturation pressure is almost linear.
Note! - be careful with these equations at higher temperatures - as indicated in Temperature and
Moisture Holding Capacity of Air.
Maximum specific humidity at some common temperatures:
Temperature (o
C)
SaturationPressure of WaterVapor
(Pa)
MaximumHumidityRatio(kgw /kga)
0 609.9 0.003767
5 870 0.005387
10 1225 0.007612

3. Temperature (o
C)
SaturationPressure of WaterVapor
(Pa)
MaximumHumidityRatio(kgw /kga)
15 1701 0.01062
20 2333 0.014659
25 3130 0.019826
30 4234 0.027125

4. Note that the saturation pressure of water vapor, - and the maximum humidity ratio, increases
dramatically with air temperature. This important for the capacity of drying processes.
Example- HumidityRatio ofMoistAir
The specific humidity for saturated humid air at 20oC with water vapor partial pressure 2333 Pa at
atmospheric pressure of 101325 Pa (1013 mbar, 760 mmHg) can be calculated as:
x = 0.62198 (2333 Pa) / ((101325 Pa) - (2333 Pa))
= 0.0147 (kg/kg)
= 14.7 (g/kg)
3- Moisture Holding Capacity of Air
Moisture holding capacity of air varies with temperature
The moisture holding capacity of air increases dramatically with increasing air temperature.
AirMoistureHoldingCapacity -SI Units
Moisture holding capacity of air - kg water per kg dry air:

5. AirMoistureHoldingCapacity -Imperial Units
Moisture holding capacity of air - lb water per lb dry air:

6. Moisture holding capacity of air - lb water per 1000 cubic feet dry air:

7. Note - the moisture holding capacity of air at 100oF (38 oC) is 10 times the moisture holding capacity of
air at 30oF (~ 0 oC). This is a very important observation - especially when working with drying
processes where air is used to transport moisture away from the processes and the products.
In general
 it isgoodenergyeconomytoincrease the temperaturesindryersasmuchas possible! - the increased
moisture transportcapacitiesof airat highertemperaturesnormallyout-weightsthe increasedenergy-
consumptionforheatingdryairto highertemperatures!

8. 4- Relative Humidity in Air
The relative humidity in air can be expressedby partial vapor and air pressure - density of the
vapor and air - or by the actual mass of the vapor and air
Humidity is the quantity of water vapor present in air. It can be expressed as an absolute, specific or
relative value.
Relative humidity is expressed by
 partial vaporand air pressure,
 densityof the vaporand air,or
 by the actual mass of the vaporand air
Relative humidity is usually expressed in per cent and abbreviated by φ or RH.
RelativeHumidityandVaporPartial Pressure
Relative humidity as the ratio of vapor partial pressure in the air - to the saturation vapor partial pressure
if the air at the actual dry bulb temperature.
φ = pw / pws 100% (1)
where
φ = relative humidity (%)
pw = vapor partial pressure (mbar)
pws = saturation vapor partial pressure at the actual dry bulb temperature (mbar)
Vapor saturation pressures at different temperatures:
Temperature
SaturationVaporPressure
(mbar,millibar,mb)
(o
C) (o
F)
-18 0 1.5
-15 5 1.9
-12 10 2.4
-9 15 3.0

9. Temperature
SaturationVaporPressure
(mbar,millibar,mb)
(o
C) (o
F)
-7 20 3.7
-4 25 4.6
-1 30 5.6
2 35 6.9
4 40 8.4
7 45 10.3
10 50 12.3
13 55 14.8
16 60 17.7
18 65 21.0
21 70 25.0
24 75 29.6
27 80 35.0
29 85 41.0
32 90 48.1
35 95 56.2
38 100 65.6
41 105 76.2
43 110 87.8
46 115 101.4

10. Temperature
SaturationVaporPressure
(mbar,millibar,mb)
(o
C) (o
F)
49 120 116.8
52 125 134.2
 1 bar= 1000 mbar= 105
Pa (N/m2
) =0.1 N/mm2
= 10,197 kp/m2
= 10.20 m H2O = 0.9869 atm= 14.50 psi
(lbf/in2
) = 106
dyn/cm2
= 750 mmHg
 Air- SaturationVaporPressure - (Celciusandmilibar) (pdf)
If the water vapor pressure in the air is 10.3 mbar the vapor saturates on a surface with 45oF (7oC).
Note! The atmospheric pressure of air is 1013 mbar (101.325 kPa, 760 mmHg). As we can see the
maximum water vapor pressure - the saturation pressure - is relatively small.

11. Example - Relative Humidity and VaporPressure
From the table above the saturation pressure at 70oF (21oC) is 25.0 mbar. If the vapor pressure in the
actual air is 10.3 mbar the relative humidity can be calculated as:
φ = (10.2 mbar) / (25.0 mbar) 100% = 41%
RelativeHumidityandVaporDensity
Relative humidity can also be expressed as the ratio of the vapor density of the air - to the saturation
vapor density at the the actual dry bulb temperature.
Relative humidity by density:
φ = ρw / ρws 100% (2b) where
φ = relative humidity (%)
ρw = vapor density (kg/m3)
ρws = vapor density at saturation at actual dry bulb temperature (kg/m3)
A common unit for vapor density is g/m3. If the actual vapor density at 20oC (68oC) is 10 g/m3 and the
the saturation vapor density at this temperature is 17.3 g/m3, the relative humidity can be calculated as
φ = (10 g/m3) / (17.3 g/m3) 100%
= 57.8%
RelativeHumidityandVaporMass
Relative humidity can also be expressed as the ratio at actual mass of water vapor in a given air volume -
to the mass of water vapor required to saturate at this volume.
Relative humidity can be expressed as:
φ = mw / mws 100% (2c)
where
φ = relative humidity (%)
mw = mass of water vapor in the given air volume (kg)
mws = mass of water vapor required to saturate at this volume (kg)

12.  Weightof Water VaporinAir
RelativeHumidityChart- degreesFahrenheit
 Relative humidityfromdryandwetbulbtemperatures- degreesFahrenheit(pdf)

13. RelativeHumidityChart- degreesCelsius
 Relative humidityfromdryandwetbulbtemperatures- degreesCelsius (pdf)

14. Altitudeand CorrectionFactors
BarometricAltitude (mbar)
Abs.Altitude
CorrectionFactorfor
φ
(m) (ft)
1013 0 0 1.000
1000 108 354 0.987
989 200 656 0.976
966 400 1312 0.953
943 600 1996 0.931
921 800 2624 0.909
899 1000 3281 0.887
842 1500 4922 0.731
795 2000 6562 0.785