In this article, we have provided you with detailed study notes on the Properties of Moist Air Psychrometric Chart for GATE, SSC JE, ESE & other Mechanical Engineering exams preparation.
It is the part of science which deals with the study of properties of moist air. In which simultaneous control of temperature, humidity (moisture in the air), air velocity and purity of the air are study.
Moist air = Dry air + water vapour
- It is a mixture of different gases such as oxygen (O2), nitrogen (N2), carbon dioxide (CO2), hydrogen (H2), argon (Ar), neon (Ne), krypton (Kr), helium (He), ozone (O3), and xenon (Xe). However, oxygen and nitrogen are the major part of the combination.
- Dry air is considered to consist of 21% oxygen and 79% nitrogen by volume, and 23% oxygen and 77% nitrogen by mass.
If the partial pressures of dry air and water vapour be pa and pv respectively, then according to Dalton's law of partial pressures:
Pa + Pv = PTotal
where PTotal is the atmospheric pressure. Since pv is very small, the saturation temperature of water vapour at pv is less than atmospheric temperature, Tatm. Since the water vapour exists in the superheated state in the air thus it is called unsaturated.
Specific Humidity (ω):
- It is defined as the ratio of mass of water vapour to the mass of dry air in a certain amount of moist air.
- It is also known as humidity ratio or percent humidity.
- Unit of specific humidity is kg of water vapour/ kg of dry air or gm of water vapour /gm of dry air.
Relative humidity (RH or ϕ):
- It is the ratio of mass of water vapour in given amount of air to the mass of water vapour in same amount of air under saturated conditions at the same temperature.
where mv and mvs : the mass of water vapour and mass of saturated air
pv and pvs : the partial pressure of water vapour and partial pressure of water vapour in saturated air at same temperature for saturated air.
- For saturated air: RH (ϕ) = 100%.
Degree of saturation(μ):
- It is the ratio of the actual specific humidity and the saturated specific humidity, both at the same temperature T.
- Therefore, μ varies between 0 and 1.
Dry bulb temperature (DBT): It is the temperature recorded by the thermometer with a dry bulb.
Wet bulb temperature (WBT): When the bulb of thermometer is enveloped by a cotton wick saturated with water, then the temperature recorded by thermometer is the WBT.
Dew Point Temperature (DPT): The temperature at which water vapour starts condensing is called the dew point temperature, TDTP, of the mixture.
- For unsaturated air, DBT > WBT > DPT
- For saturated air, DBT = WBT = DPT
Enthalpy of Air (h): The enthalpy of moist air is given by:
H = Ha + Hv
where T = Dry-bulb temperature of air-vapor mixture, °C
ω = Humidity ratio, kg of water vapor/kg of dry air
- It is the graphical representation of the thermodynamic properties of moist air.
- Standard psychrometric charts are bounded by the different parameters such as dry-bulb temperature (DBT) lines (on abscissa) and humidity ratio (ω) ( on ordinate) or vapour pressure. The Left-Hand Side of the psychrometric chart is bounded by curved line (known as the saturation line or saturation curve).
Case (i). Constant dry bulb temperature lines: These lines are drawn straight, not always parallel to each other slightly inclined from the vertical position. These are uniformly spaced.
Case (ii). Constant specific Humidity lines: These are the horizontal lines moving towards the saturation curve. These are uniformly spaced.
Case (iii). Constant Dew point Temperature line: These are horizontal line in psychrometric chart. These are Non-uniformly spaced.
Case (iv). Constant Relative Humidity curve: These hyperbolic lines are shown in interval of 10%. The saturation curve is at 100% RH, while dry air is at 0% RH.
Case (v). Constant WBT lines: They run diagonally downward from left to right across the chart. These lines are oblique lines and differ slightly from the enthalpy lines. WBT lines are Non-uniformly spaced.
Case (vi). Specific enthalpy: These are oblique lines which run diagonally downward from left to right across the chart. Enthalpy lines are uniformly spaced.
(vii). Specific volume: They run diagonally downward from left to right across the chart and are equally spaced straight lines.
Note: Enthalpy (h) and WBT have the same degree of inclination but are not coincident while specific volume (v) the highest degree of inclination.
Sensible Heating: It is a process of increasing the DBT (Dry bulb temperature) at constant humidity.
Sensible cooling: It is the process of decreasing the dry bulb temperature at constant specific humidity.
Humidification: It is a process of increasing the specific humidity at constant dry bulb temp.
Dehumidification: It is a process of decreasing the specific humidity at the constant dry bulb temperature.
Note: Pure Humidification & dehumidification are impossible to achieve practically. Therefore, these are combined either with sensible heating or sensible cooling.
STRAIGHT LINE LAW: It defines "When heat and mass (water) to or from a wetted surface is transferred to the air, the condition of air on psychrometric charts tends to shift towards the saturation line at the temperature of the wetted surface".
BYPASS FACTOR (X):
- It represents the loss or uncontacted air with the coil.
- It shows the fractional part of total inlet air which passes as it without coming in contact with the coil. Efficiency of the coil is given by:
η = 1 –BPF
Bypass factor in case of combined coil when there is more than one row of coils. BPF = XN
Where N = Total No. of coils, X= BPF of individual coil
SENSIBLE HEAT FACTOR (SHF): It is defined as the ratio of sensible heat to the total heat. Total heat is the summation of sensible heat & Latent heat.
EFFECTIVE TEMPERATURE: It is the temperature of saturated air at which a person or human being would feel same level of comfort as in actual environment. It includes comfort Temperature, Humidity, air velocity, and air motion. Effective temperature combines the effect of DBT & surrounding surface temperature into a single index.
Factors Affecting Temperature:
- Climate & seasonal difference
- Age & Gender
- Kind of activity
- Density of occupants
COMFORT CHART: This char is the result of research made by ASHRAE (American society of Heating, Refrigeration & Air conditioning Engineers) by conducting a survey & different kind of people subjected to wide range of environmental temperature condition humidity & purity. If the value of relative humidity is below 50% then the skin surface is too dry whereas if the RH is above 60% then there is a tendency of sticky sensation to develop.
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