Psychrometry Questions and Answers

Q: When a fixed mass of air-water vapour mixture is heated at constant pressure,

On heating at constant pressure, relative humidity decreases, but specific humidity remains unchanged.Specific humidity is the mass of water vapour per unit mass of dry air. Heating a fixed mass of mixture adds no water and removes none, so it stays the same, and at constant total pressure the vapour partial pressure stays the same too.Relative humidity is the vapour partial pressure divided by the saturation pressure at the mixture's temperature. The saturation pressure rises steeply with temperature, so the relative humidity falls. That is why heated air feels drier. Any option in which specific humidity changes is wrong, since no moisture is added or condensed.

Q: The relative humidity of air (in percentage) is

The relative humidity is about 81.80%.Saturation pressure at 30°C, then the actual vapour pressure from the humidity, using 18/29 = 0.6207:\[ \begin{aligned} \ln p_{w}^{0} &= 18.6556 - \frac{5217.635}{303} = 1.4357 \\ p_{w}^{0} &= 4.203\ \text{kPa} \\ p_{w} &= \frac{0.0218(101.325)}{0.6207 + 0.0218} = 3.438\ \text{kPa} \\ RH &= \frac{3.438}{4.203} = 81.8\% \end{aligned} \]The other options do not follow from the given humidity and vapour pressure equation.

Q: The enthalpy (in kJ/kg) of moist air is

The enthalpy of the moist air is about 85.93 kJ/kg of dry air.Taking 0°C liquid water as the reference, the enthalpy is the sensible heat of the dry air plus that of the vapour, including its latent heat:\[ \begin{aligned} h &= c_{a}T + H(\lambda_{0} + c_{v}T) \\ &= 1.005(30) + 0.0218(2502.3 + 1.884 \times 30) \\ &= 30.15 + 55.78 = 85.93\ \text{kJ/kg} \end{aligned} \]Most of it is the latent heat carried by the water vapour. 31.38 kJ/kg is roughly the dry air term with the vapour's sensible heat only, which forgets the latent heat. The other values do not follow from the data.

Q: 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°C respectively. Given that the saturation pressure for water at 25°C is 3.17 kPa, the relative humidity of the air in the room is

The relative humidity is about 71 %.The specific humidity is the mass of vapour per unit mass of dry air:\[ \begin{aligned} \omega &= \frac{0.5}{35} = 0.01429 \\ &= \frac{0.622\,p_{v}}{p-p_{v}} \end{aligned} \]Solving for the vapour pressure with p = 100 kPa:\[ \begin{aligned} p_{v} &= \frac{\omega p}{0.622+\omega} = \frac{1.429}{0.6363} \\ &= 2.245\ \text{kPa} \\ \phi &= \frac{2.245}{3.17} = 0.708 \approx 71\ \% \end{aligned} \]The relative humidity compares the actual vapour pressure with the saturation pressure at the same temperature, so the 3.17 kPa enters only in the last step. 67 %, 55 % and 83 % do not follow from the specific humidity of 0.0143.

Q: In a psychrometric chart, one axis represents dry-bulb temperature. The axis, that is perpendicular to the dry-bulb temperature axis, represents

The perpendicular axis represents specific humidity.A psychrometric chart plots dry-bulb temperature on the horizontal axis and specific humidity (humidity ratio, kg of vapour per kg of dry air) on the vertical axis.Wet-bulb temperature, relative humidity and enthalpy are shown as families of lines or curves drawn across the chart, not as the axis perpendicular to dry-bulb temperature.

Q: An air-conditioning system consists of an insulated rigid mixing chamber designed to supply air at 24 °C to a building. The mixing chamber mixes two air streams: (i) a cold air stream at 10 °C and mass flow rate \(\dot{m}_{c}\) (kg/s), and (ii) a stream of fresh ambient air at 30 °C and mass flow rate \(\dot{m}_{a}\) (kg/s). Assume air to be an ideal gas with constant specific heat (\(c_{p} = 1.005\) kJ/(kg K), \(\gamma = c_{p}/c_{v} = 1.4\)). Neglect change in kinetic and potential energies as compared to change in enthalpy. Under the steady state condition, the ratio of the mass flow rates of the two streams (\(\dot{m}_{c}/\dot{m}_{a}\)) is

The ratio is \(\frac{3}{7}\).For an insulated steady mixing chamber with constant \(c_{p}\), the enthalpy given up by the warm stream equals that gained by the cold stream:\[ \begin{aligned} \dot{m}_{c}c_{p}(24 - 10) &= \dot{m}_{a}c_{p}(30 - 24) \\ \frac{\dot{m}_{c}}{\dot{m}_{a}} &= \frac{6}{14} = \frac{3}{7} \end{aligned} \]\(7/3\) is the inverse ratio, \(\dot{m}_{a}/\dot{m}_{c}\). \(2/7\) and \(4/7\) do not satisfy the energy balance. The values of \(c_{p}\) and \(\gamma\) cancel and are not needed.

Q: With reference to 'Psychometric Chart', which of the following statement is correct? Conventionally-

Conventionally, the moisture content is taken as the ordinate and the temperature as the abscissa.On the standard psychrometric chart (the paper spells it 'psychometric'), dry-bulb temperature is plotted along the horizontal axis and humidity ratio (moisture content, kg of water per kg of dry air) along the vertical axis, usually on the right-hand side. Lines of relative humidity, wet-bulb temperature, enthalpy and specific volume are drawn across this grid.Option A reverses the axes. Wind speed and time do not appear on a psychrometric chart at all.

Q: The Psychometric chart is used to-

The chart is used to represent the state of moist air.A psychrometric chart shows the thermodynamic properties of air-water vapour mixtures at a given total pressure. Any two known properties, such as dry-bulb and wet-bulb temperature, fix a point on the chart, from which the others (humidity ratio, relative humidity, dew point, enthalpy, specific volume) can be read. It is used to design and analyse drying, cooling and air conditioning.It does not show air speed, and atmospheric pressure is held constant for a given chart, not represented on it. 'Potential energy in the air' is not one of its properties.

Direction for [ Question No: 1 To 1 ] :

Notation used:
P : pressure, V : volume, T : temperature, s : specific entropy, h : specific enthalpy, C_p : molar heat capacity at constant pressure, C_v : molar heat capacity at constant volume, R : universal gas constant

When a fixed mass of air-water vapour mixture is heated at constant pressure,

A.both relative and specific humidity decrease.
B.relative humidity decreases, but specific humidity remains unchanged.
C.specific humidity decreases, but relative humidity remains unchanged.
D.both relative and specific humidity increase.

Exam Year(s)

GATE XE - 2018,

Answer: B

On heating at constant pressure, relative humidity decreases, but specific humidity remains unchanged.

Specific humidity is the mass of water vapour per unit mass of dry air. Heating a fixed mass of mixture adds no water and removes none, so it stays the same, and at constant total pressure the vapour partial pressure stays the same too.

Relative humidity is the vapour partial pressure divided by the saturation pressure at the mixture's temperature. The saturation pressure rises steeply with temperature, so the relative humidity falls. That is why heated air feels drier. Any option in which specific humidity changes is wrong, since no moisture is added or condensed.

Discuss in Forum

Direction for [ Question No: 2 To 3 ] :

Air at 1 atmospheric pressure (101.325 kPa) and 30 °C with absolute humidity of 0.0218 kg/kg of dry air is flowing in a drying chamber. The saturated vapor pressure of water (\(p_{w}^{0}\), in kPa) is related to temperature (T, in °C) as given below

\[ \ln p_{w}^{0} = 18.6556 - \frac{5217.635}{T + 273} \]

Heat capacities of dry air (average molecular weight 29) and that of water vapor (molecular weight 18) are 1.005 and 1.884 kJ/kg.K, respectively. Latent heat of vaporization of water at reference temperature (0 °C) is 2502.3 kJ/kg.

The relative humidity of air (in percentage) is

A.62.82
B.68.22
C.86.62
D.81.80

Exam Year(s)

GATE XE - 2012,

Answer: D

The relative humidity is about 81.80%.

Saturation pressure at 30°C, then the actual vapour pressure from the humidity, using 18/29 = 0.6207:

\[ \begin{aligned} \ln p_{w}^{0} &= 18.6556 - \frac{5217.635}{303} = 1.4357 \\ p_{w}^{0} &= 4.203\ \text{kPa} \\ p_{w} &= \frac{0.0218(101.325)}{0.6207 + 0.0218} = 3.438\ \text{kPa} \\ RH &= \frac{3.438}{4.203} = 81.8\% \end{aligned} \]

The other options do not follow from the given humidity and vapour pressure equation.

Discuss in Forum

The enthalpy (in kJ/kg) of moist air is

A.85.93
B.54.55
C.31.38
D.99.38

Exam Year(s)

GATE XE - 2012,

Answer: A

The enthalpy of the moist air is about 85.93 kJ/kg of dry air.

Taking 0°C liquid water as the reference, the enthalpy is the sensible heat of the dry air plus that of the vapour, including its latent heat:

\[ \begin{aligned} h &= c_{a}T + H(\lambda_{0} + c_{v}T) \\ &= 1.005(30) + 0.0218(2502.3 + 1.884 \times 30) \\ &= 30.15 + 55.78 = 85.93\ \text{kJ/kg} \end{aligned} \]

Most of it is the latent heat carried by the water vapour. 31.38 kJ/kg is roughly the dry air term with the vapour's sensible heat only, which forgets the latent heat. The other values do not follow from the data.

Discuss in Forum

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°C respectively. Given that the saturation pressure for water at 25°C is 3.17 kPa, the relative humidity of the air in the room is

A.67%
B.55%
C.83%
D.71%

Exam Year(s)

GATE ME - 2012,

Answer: D

The relative humidity is about 71 %.

The specific humidity is the mass of vapour per unit mass of dry air:

\[ \begin{aligned} \omega &= \frac{0.5}{35} = 0.01429 \\ &= \frac{0.622\,p_{v}}{p-p_{v}} \end{aligned} \]

Solving for the vapour pressure with p = 100 kPa:

\[ \begin{aligned} p_{v} &= \frac{\omega p}{0.622+\omega} = \frac{1.429}{0.6363} \\ &= 2.245\ \text{kPa} \\ \phi &= \frac{2.245}{3.17} = 0.708 \approx 71\ \% \end{aligned} \]

The relative humidity compares the actual vapour pressure with the saturation pressure at the same temperature, so the 3.17 kPa enters only in the last step. 67 %, 55 % and 83 % do not follow from the specific humidity of 0.0143.

Discuss in Forum

In a psychrometric chart, one axis represents dry-bulb temperature. The axis, that is perpendicular to the dry-bulb temperature axis, represents

A.wet-bulb temperature
B.specific humidity
C.relative humidity
D.enthalpy

Exam Year(s)

GATE ME - 2025,

Answer: B

The perpendicular axis represents specific humidity.

A psychrometric chart plots dry-bulb temperature on the horizontal axis and specific humidity (humidity ratio, kg of vapour per kg of dry air) on the vertical axis.

Wet-bulb temperature, relative humidity and enthalpy are shown as families of lines or curves drawn across the chart, not as the axis perpendicular to dry-bulb temperature.

Discuss in Forum

An air-conditioning system consists of an insulated rigid mixing chamber designed to supply air at 24 °C to a building. The mixing chamber mixes two air streams: (i) a cold air stream at 10 °C and mass flow rate \(\dot{m}_{c}\) (kg/s), and (ii) a stream of fresh ambient air at 30 °C and mass flow rate \(\dot{m}_{a}\) (kg/s). Assume air to be an ideal gas with constant specific heat (\(c_{p} = 1.005\) kJ/(kg K), \(\gamma = c_{p}/c_{v} = 1.4\)). Neglect change in kinetic and potential energies as compared to change in enthalpy. Under the steady state condition, the ratio of the mass flow rates of the two streams (\(\dot{m}_{c}/\dot{m}_{a}\)) is

A.\(\frac{7}{3}\)
B.\(\frac{3}{7}\)
C.\(\frac{2}{7}\)
D.\(\frac{4}{7}\)

Exam Year(s)

GATE XE - 2022,

Answer: B

The ratio is \(\frac{3}{7}\).

For an insulated steady mixing chamber with constant \(c_{p}\), the enthalpy given up by the warm stream equals that gained by the cold stream:

\[ \begin{aligned} \dot{m}_{c}c_{p}(24 - 10) &= \dot{m}_{a}c_{p}(30 - 24) \\ \frac{\dot{m}_{c}}{\dot{m}_{a}} &= \frac{6}{14} = \frac{3}{7} \end{aligned} \]

\(7/3\) is the inverse ratio, \(\dot{m}_{a}/\dot{m}_{c}\). \(2/7\) and \(4/7\) do not satisfy the energy balance. The values of \(c_{p}\) and \(\gamma\) cancel and are not needed.

Discuss in Forum

With reference to 'Psychometric Chart', which of the following statement is correct? Conventionally-

A.The temperature is taken as the ordinate and the humidity as the abscissa.
B.The moisture content is taken as the ordinate and the temperature as the abscissa.
C.The speed of wind is taken as the ordinate and the time is taken as the abscissa.
D.The time is taken as the ordinate and the speed of wind is taken as the abscissa.

Exam Year(s)

RSMSSB JE - 2025, RSMSSB JE Agriculture - 2025, RSMSSB - 2025,

Answer: B

Conventionally, the moisture content is taken as the ordinate and the temperature as the abscissa.

On the standard psychrometric chart (the paper spells it 'psychometric'), dry-bulb temperature is plotted along the horizontal axis and humidity ratio (moisture content, kg of water per kg of dry air) along the vertical axis, usually on the right-hand side. Lines of relative humidity, wet-bulb temperature, enthalpy and specific volume are drawn across this grid.

Option A reverses the axes. Wind speed and time do not appear on a psychrometric chart at all.

Discuss in Forum

The Psychometric chart is used to-

A.Represent the state of moist air.
B.Represent the speed of air.
C.Represent the atmospheric pressure.
D.Represent the value potential energy in the air.

Exam Year(s)

RSMSSB JE - 2025, RSMSSB JE Agriculture - 2025, RSMSSB - 2025,

Answer: A

The chart is used to represent the state of moist air.

A psychrometric chart shows the thermodynamic properties of air-water vapour mixtures at a given total pressure. Any two known properties, such as dry-bulb and wet-bulb temperature, fix a point on the chart, from which the others (humidity ratio, relative humidity, dew point, enthalpy, specific volume) can be read. It is used to design and analyse drying, cooling and air conditioning.

It does not show air speed, and atmospheric pressure is held constant for a given chart, not represented on it. 'Potential energy in the air' is not one of its properties.

Discuss in Forum

Loading…

Topics

Other Subjects

Tags: Psychrometry Questions and Answers || Psychrometry MCQ Questions and Answers || Psychrometry GK Questions and Answers || Psychrometry GK MCQ Questions || Psychrometry Multiple Choice Questions and Answers || Psychrometry GK || GK on Psychrometry || Thermodynamics Questions and Answers || Thermodynamics MCQ Questions and Answers || Thermodynamics GK Questions and Answers || GK on Thermodynamics

Psychrometry Questions and Answers

Direction for [ Question No: 1 To 1 ] :

Direction for [ Question No: 2 To 3 ] :

Topics

Other Subjects

About Us

Stay informed about all important Dates & Notifications