Psychrometrics
Elements of Psychrometric Chart
Learning Objectives:
 Be able to understand the basic terminology and definitions related to air properties.
 Be able to read psychrometric chart.
 Be able to apply the psychrometric charts on airconditioning or drying applications.
 Identify the four basic parts of the Psychrometric chart.
 Plot a point on the psychrometric chart using two measured conditions of the air.
 Recognize the relationship of the conditions of the air and the operation of the heating and cooling equipment.

Dry Bulb Temp.: The temp.of air readon a standard thermometer. Lines of constant DBT are straight, vertical lines on the chart. The DBT scale is at the bottom of the chart. Units are Deg. C.

Wet Bulb Temp.: It is indicated by a thermometer whose bulb is covered by a wet wick and exposed to a stream of air moving at a velocity of 5m/s. The WBT scale is on the curved line at the left of the chart. Units are Deg. C.

Humidity Ratio: It is the ratio the mass of water vapor to the mass of dry air. Lines of constant humidity ratio are straight, horizontal lines at right angle to the DBT lines. The humidity ratio scale is at the right of the chart. Units are grains of moisture per kg of dry air.

Enthalpy: It is a measure of heat energy in the air. Units are KJ/kg of dry air.

Specific Volume: It is the volume per unit mass. Units are m\(^3\)/kg. Lines of constant specific volume indicated along the bottom axis of the psychrometric chart with the constant volume slanting upward to the left.

Dew Point Temp.: Units are Deg. C. It is the temp. at which water vapor has reached the saturation point (100% Relative Humidity)

Relative Humidity: Lines of constant relative humidity curve upward from left to right. It is the ratio of mole fraction of water vapor to the mole fraction of water vapor saturated at the same temp. and barometric pressure.
Source: https://tex.stackexchange.com/questions/59051/tikzhowtoconsideracurveasanaxisinachart
Solved Example: 77101
The temperature of air recorded by a thermometer, when it is not effected by the moisture present in the air, is called:
A. Wet bulb temperature
B. Dry bulb temperature
C. Dew point temperature
D. None of these.
In a sling thermometer, there are two thermometers.
 One of these thermometers records regular temperature which is referred as dry bulb temperature.
 Other thermometer has its bulb convered with wet cloth, which records wet bulb temperature. The moisure in the cloth tries to evaporate, and while doing so it lowers the temp. in the second thermometer. Hence, the temperature in the second thermometer (only) gets affected by the dryness (or the moisture level) of the surrounding air.
Correct Answer: B
Solved Example: 77102
The relative humidity is defined as:
A. The mass of water vapour present in 1 kg of dry air
B. The ratio of the actual mass of water vapour in a unit mass of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature and pressure.
C. The ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure
D. The mass of water vapour present in 1 $m^3$ of dry air
Relative humidity: It is defined as the ratio of the mass of water vapour ($m_v$) in a certain volume of moist air to the mass of water vapour ($m_{vs}$) in the same volume of saturated air (air having the maximum amount of water vapor without condensing) at the same temperature.
Correct Answer: C
Solved Example: 77103
The humidity ratio or specific humidity is the mass of water vapour present in:
A. 1 kg of dry air
B. 1 kg of wet air
C. 1 $m^3$ of wet air
D. 1 $m^3$ of dry air
Please refer the psychrometric chart in the SI units on page 175 of FEReference handbook. The yaxis on the RIGHT side refers specific humidity or humidity ratio. The units are kg of vapor per kg of DRY air.
Correct Answer: A
Solved Example: 77104
One kg of steam sample contains 0.8 kg dry steam; its dryness fraction is:
A. 0.2
B. 0.8
C. 1.0
D. 0.6
Dryness fraction is defined as: \[\dfrac{\mathrm{mass\ of\ vapor}}{\mathrm{mass\ of\ vapor} + \mathrm{mass\ of\ liquid}}\] \[= \dfrac{0.8}{1} = 0.8\]
Correct Answer: B
Solved Example: 77105
Absolute humidity is defined as:
A. Mass of water vapour present in unit volume of air
B. Mass of water vapour present in 1 kg of air
C. Mass of moist air per m$^3$ of volume
D. None of the above
Correct Answer: A
Reading a Psychrometric Chart
Learning Objectives:
 Be able to understand the basic terminology and definitions related to air properties;
 Be able to read psychrometric chart.
 Be able to apply the psychrometric charts on airconditioning or drying applications.
Relative Humidity can be calculated with a wet bulb thermometer and a dry bulb thermometer, using a psychrometric chart as below. Wetbulb temperature is measured using a thermometer, with the glass bulb or sensor tip wrapped in a wick, which is kept wet. The evaporation of water from the wet wick has a cooling effect on the thermometer and the rate of evaporation from the wetbulb thermometer depends on the humidity of the air. Therefore, less humidity means more cooling, which means a lower wet bulb temperature, and a bigger difference between wet and dry bulb temperatures.
The difference in the temperatures (wet bulb depression) indicated by the two thermometers gives a measure of atmospheric humidity. This chart allows humidity to be calculated from wet and dry bulb readings. (The dry bulb temperature is another name for air temperature). It also explains common atmospheric behaviour that we see daily. We will break the chart down for ease of explanation:

The vertical lines represent dry bulb temperatures.

The diagonal lines represent wet bulb temperatures.

Lines of constant humidity are also shown on the chart.

Read the relative humidity by finding where the wet bulb temperature and the dry bulb temperature readings intersect. The nearest constant humidity line is the relative humidity for the given wet and dry bulb temperatures.

You can also read the wet bulb temperature from the chart, when you know the air temperature and relative humidity, using the same principle of finding the intersection of the relevant lines.
Solved Example: 77201
On Mollier chart, flow through turbine is represented by:
A. Horizontal straight line
B. Vertical straight line
C. Straight inclined line
D. Curved line
Correct Answer: B
Solved Example: 77202
On Mollier chart, the constant pressure lines:
A. Diverge from left to right
B. Diverge from right to left
C. Are equally spaced throughout
D. First rise up and then fall
Correct Answer: A
Solved Example: 77203
On Mollier chart, free expansion, or throttling process from high pressure to atmosphere is represented by:
A. Horizontal straight line
B. Vertical straight line
C. Straight inclined line
D. Curved line
Correct Answer: A
Solved Example: 77204
In a psychrometric chart, specific humidity (moisture content) lines are:
A. Vertical and uniformly spaced
B. Horizontal and uniformly spaced
C. Horizontal and nonuniformly spaced
D. Curved lines
Correct Answer: B
Solved Example: 77205
A room contains 35 kg of dry air and 0.5 kg of water vapor. The total pressure and temperature of air in the room are 100 kPa and 25$^{\circ}\mathrm{C}$ respectively. Given that the saturation pressure for water at 25$^{\circ}\mathrm{C}$ is 3.17 kPa, the relative humidity of the air in the room is:
A. 83%
B. 67%
C. 55%
D. 71%
\begin{align*} P_{total} &=100\ kPa\\ P_{saturated} &=3.17\ kPa\\ \end{align*} \[RH=\dfrac {P_{water}}{P_{saturated}} \times 100\] \[P_{water}=x_{water}\times P_{total}\] where, x$_{water}$ = mole fraction of water \[x_{water} =\dfrac {n_{H_{2}O}}{n_{H_{2}O} + n_{air}} =\dfrac {\dfrac{0.5}{18}}{\dfrac{0.5}{18}+\dfrac{30}{29}} =0.0265\] Therefore, \[P_{water} = 0.265\times 100 = 2.65\ kPa\] \[RH =\dfrac {2,65}{3.17}\times 100 =83.5\% \]
Correct Answer: A
Solved Example: 77206
The pressure, dry bulb temperature and relative humidity of air in a room are 1 bar, 30$^{\circ}\mathrm{C}$ and 70%, respectively. If the saturated steam pressure at 30$^{\circ}\mathrm{C}$ is 4.25 kPa, the specific humidity of the room air in kg water vapor/kg dry air is:
A. 0.0191
B. 0.0083
C. 0.0101
D. 0.0232
Specific humidity is given by,
\[w = 0.622 \times \dfrac{p_v}{p_a  p_v}\]
where, p$_v$ = Relative humidity $\times$ Saturated steam pressure
\[p_v = \phi \times p_s = 0.7 \times 0.0425 = 0.02975\ \mathrm{bar}\]
p$_a$ =1 bar
So that from the above equation, we have
\begin{align*}
w &= 0.622 \times \dfrac{0.02975}{1  0.02975}\\
&= 0.0191\ \mathrm{kg/kg\ of\ dry\ air}
\end{align*}
Correct Answer: A
Solved Example: 77207
If a mass of moist air contained in a closed metallic vessel is heated, then its:
A. Relative humidity decreases
B. Relative humidity increases
C. Specific humidity increases
D. Specific humidity decreases
Given, that moist air is contained in a closed vessel, so we can say that mass of moist air is constant.
$\omega$ = specific humidity = $\dfrac{m_v}{m_{dry\ air}}$ = constant
When the container is heated, temperature increases due to which saturation pressure increases, hence mass at saturation pressure increases.
So, $\phi$ = $\dfrac{m_v}{m_{saturated\ vapor}}$ = decreases
Correct Answer: A
Solved Example: 77209
The specific heat of superheated steam in kcal/kg is generally of the order of:
A. 0.1
B. 0.3
C. 0.5
D. 0.8
Correct Answer: C
Solved Example: 77210
The dry saturated steam at very low pressure, (510 kg/cm$^2$) when throttled to atmosphere will become:
A. Wet
B. Superheated
C. Remain dry saturated
D. Dry
Correct Answer: B
Solved Example: 77211
Water at pressure of 4 kg/cm$^2$ and 160$^\circ$C temperature when exposed to atmosphere will:
A. Boil
B. Flash i.e. get converted into steam
C. Remain as it was
D. Cool down
Correct Answer: B
Solved Example: 77212
The dry saturated steam at very high pressure (150200 kg/cm$^2$) when throttled to atmosphere will become:
A. Wet
B. Superheated
C. Remain dry saturated
D. Dry
Correct Answer: A
Solved Example: 77213
In a throttling process:
A. Steam temperature remains constant
B. Steam pressure remains constant
C. Steam enthalpy remains constant
D. Steam entropy remains constant
Correct Answer: C
Solved Example: 77214
In an experiment to determine dryness fraction of steam, the mass of water separated was 1.2 kg in 15 minutes and the mass of steam passed out in same time was 4.8 kg. Dryness fraction is:
A. 40%
B. 25%
C. 50%
D. 80%
Correct Answer: D