Properties of Moist Air | Mechanical Engineering SSC JE (Technical) PDF Download

 PROPERTIES OF MOIST AIR

DALTON'S LAW

n = NO. of moles
AMAGAT'S LAW

γ = mole fraction of gas in mixture of gases

•  Molecular mass of mixture

•  Universal gas constant R = 8.314 kJ/kg mole K

Characteristic gas constant R_a =

•  Psychometric properties:

F + P = C + 2
F = Degree of freedom
P = Phases in mixture
C = Components of mixture
For a mixture of 2 gases we need 3 properties to fix its thermodynamic state whereas for pure substance we need only 2 properties.

•  Dry Bulb Temperature (DBT) is the actual temperature 't' of the normal by ordinary thermometer.

where,V =Volume
m =Mass
P = Pressure
T = Temperature
Specific humidity/Humidity ratio

P_v = Partial pressure of vapour
P = Total pressure of moist air

•  Specific humidity is a function of partial pressure.

Dew point Temperature

•  The temperature at which the super heated vapour present in a sample of unsaturated
•  At a given partial pressure of water vapour, the saturation temperature is DBT.

Degree of Saturation

•  The capacity of air to absorb moisture is called degree of saturation

 ω = Specific humidity at partial pressure P_v.
 ω = specific humidity at saturation pressure P_s

•  Relative humidity

ENTHALPY OF MOIST AIR

h = h_a + ωh_v
For 1 kg of dry air and w kg of water vapour.
h_a = enthalpy of dry air
h_v = enthalpy of water vapour

or 

h_fg = Latent heat of vapourization

WET BULB TEMPERATURE (WBT)

•  WBT is used to measure humidity by a psychrometer The temperature measured by thermometer whose bulb is covered with wet cloth is wet bulb temperature while the bare bulb measures dry bulb temperature.
  •  Wet Bulb Temperature (WBT) is always less than Dry Bulb Temperature (DBT) except when the air saturated. That time WBT is equal to DBT.
  •  Wet bulb depression = DBT – WBT
  •  WBT is an indirect measure of the dryness of the moist air.

Thermodynamic wet bulb temperature or temperature of adiabatic saturation

•  For any state of unsaturated moist air, there exist a temperature at which the air becomes adiabatically saturated by the evaporation of water into air. That temperature is temperature of adiabatic saturation.
  •  Only in case of saturated moist air, wet bulb temperature and thermodynamic wet bulb temperature and thermodynamic wet bulb temperature (temperature of adiabatic saturation) are equal.
    • For moist air having three degree of freedom, three of the four properties can be measured.
  • total pressure or barometer pressure.
    • dry bulb temperature
    • dew point temperature.
    • wet bulb temperature.

Thermodynamic WBT is given as

PSYCHROMETRIC CHARTS

Constant enthalpy lines are along the constant wet bulb temperature lines.
PSYCHROMETRY OF AIR CONDITIONING PROCESS

•  Mixing process
  •  Specific humidity of the mixture (w)
    
  •   Enthalpy of the mixture (h)
    
  •  Temperature of the mixture (t)
    

where for the two moist air stream
m_a1 , m_a2 – Mass of the dry air
W₁, W₂ – Specific humidity
h₁, h₂ – Specific enthalpy
t₁, t₂ – Temperature (in ºC)

•   Mixing with Condensation: When a cool current mixes with hot current with high relative humidity. the moisture may get condensed and relative humidity of mixed stream might be less that without condensation mass condensed per unit dry air

Basic Air Conditioning process

• If CMM be the cubic meter per minute supply of air and r_air = 1.2, C_pair = 1.02

Then Q_s = 0.02040 (CMM) Dt kW
Q_L = 50 (CMM) DwkW
Sensible Heating and Cooling
Q_s = m_a (1.005 + 1.88 w) (t₁ – t₂)
m_a – mass flow rate of dry air
ω – specific humidity of moist air
t₁, t₂ – initial and final temperature
Latent Heating and cooling

Q_L = m_a [h₁ – h₂]
m_a – mass flow rate of dry air
h₁, h₂ – initial and final specific enthalpy

Sensible Heat Factor (SHF)

SHF = 1
Þ SensibleHeatingorCooling

•   SHF = 0

Þ Humidification or Dehumidification
By Pass Factor (x)

•   By Pass Factor is defined as the fraction of air which doesn't come in contact with or which by passes the cooling surface.
•  Due to By Pass Factor, the exit state of air is a complex mixture of contacted and uncontacted air

• By pass factor represents how much effective the coil is high by pass factor coil implies less effective and vice-versa.

Apparatus Dew point Temperature

•  For dehumidification, the surface temperature of coil should be below the dew point temperature of the air. This surface temperature of coil is ADP. This point on the w – t graph, can be found by joining the initial and final condition of air and joining it to saturation curve which shows ADP. point S in the previous figure is ADP. Driving potential for sensible heat transfer is temperature difference whereas that for latent heat transfer is partial pressure or sp. Humidity difference i.e., (P_v – P_s) or (ω – ω_s)

Heating Load and cooling Load 

•  If a conditioned space gains heat from atmosphere it requires cooling of air to a lower temperature. This is called cooling load. If conditioned space losses heat to outside it is heating load.
•  Both these loads can be sensible or latent loads
•  Total heating or total cooling load is sum of sensible and latent loads.

Air Washer

• It is a device used in conditioning of air for large spaces. Air is passed through spray of water and following processes can take place depending upon the relative temperature of water droplets and air.

1. Heating and dehumidification (t_s > t₁)
2. Humidification (t_s = t₁)
3. Cooling and humidification (t₁' < t_s < t₁)
4. Adiabatic saturation (t₁' = t_s)
5. Cooling and humidification (t_d < t_s < t₁')
6. Cooling (t_s = t_d)
7. Cooling and dehumidification (t_s <t_d)

• Humidifying efficiency of air washer

By pass factor

Summer Air-Conditioning System

m₁ – Fresh air inducted in the room
m₂ – recirculated air
m₃ – air rejected from the room = m₁

Minimum quantity of air to be supplied is

• RSH = Room Sensible Heat 
  RLH = Room Latent Heat
  RTH = Room total heat
•  When ventilation air is used another load, the of bringing a ventilation air from outside condition to inside condition, comes on apparatus. This is called ventilation load.

OASH = Outside air sensible heat
OALH = outside air latent heat

•  Air conditioning equipment load

Total sensible TSH = RSH + OASH
Total latent TLH = RLH + OALH
Grand sensible heat factor