Psychrometric Chart and Processes MCQ Quiz - Objective Question with Answer for Psychrometric Chart and Processes - Download Free PDF

Concept:

Relative humidity is given by:

\( \phi = \frac{P_v}{P_{sat}} \times 100 \)

Where partial vapor pressure is calculated using:

\( P_v = \frac{\omega \times P}{0.622~ + ~\omega} \)

Given:

• \( \omega = 0.018 \)
• \( P = 760 \, \text{mm Hg} \)
• \( P_{sat} = 28.34 \, \text{mm Hg} \)

Calculation:

\( P_v = \frac{0.018 \times 760}{0.622 + 0.018} \approx 21.375 \, \text{mm Hg} \)

\( \phi = \frac{21.375}{28.34} \times 100 \approx {75.42\%} \)

Explanation:

Comfort Air-Conditioning Assumptions

• Comfort air-conditioning refers to the process of treating air to control its temperature, humidity, cleanliness, and distribution to maintain the comfort of occupants in a specific space.
• When designing an air-conditioning system, it is crucial to have accurate data regarding the required conditions.
• However, in cases where sufficient data is not available, standard assumptions must be made.
• In the absence of specific data, the commonly assumed standard conditions for comfort air-conditioning are a dry-bulb temperature of 21°C (70°F) and a relative humidity of 50%.
• These conditions are generally considered to provide a comfortable environment for most people.
• Comfort air-conditioning aims to create an indoor environment that is comfortable for the majority of occupants.
• Several factors are considered to achieve this, including temperature, humidity, air quality, and air distribution.
• The following sections explain why 21°C and 50% relative humidity are considered optimal for comfort air-conditioning.

Temperature:

• The dry-bulb temperature is a measure of the air temperature without considering humidity. For comfort air-conditioning, a temperature range of 20°C to 22°C (68°F to 72°F) is generally recommended. This range is deemed comfortable for most people, as it aligns with the body's natural temperature regulation mechanisms. Within this range, 21°C is often chosen as a standard assumption for design purposes.

Relative Humidity:

• Relative humidity is the percentage of moisture in the air relative to the maximum amount of moisture the air can hold at a given temperature. A relative humidity of 50% is considered ideal for comfort air-conditioning because it balances moisture levels, reducing the risk of respiratory discomfort and mold growth. Humidity levels that are too high or too low can lead to discomfort and health issues.

Combined Effect of Temperature and Humidity:

• The combination of 21°C and 50% relative humidity creates an indoor environment that feels neither too dry nor too humid. This balance is crucial for maintaining comfort and preventing issues such as dry skin, respiratory problems, and discomfort due to excessive sweating or shivering.

Explanation:

The temperature at which liquid droplets just appear when moist air is cooled continuously is called the dew point temperature.

The concept of dew point temperature is fundamental in understanding the behavior of moist air. When moist air is cooled, it reaches a temperature at which it can no longer hold all the water vapor it contains. This temperature is known as the dew point. At this point, the air is saturated with water vapor, and any further cooling results in the condensation of water vapor into liquid droplets. This process is crucial in various fields, including meteorology, HVAC (Heating, Ventilation, and Air Conditioning), and environmental science.

Detailed Explanation:

Moist air is a mixture of dry air and water vapor. The amount of water vapor that air can hold depends on its temperature. Warm air can hold more water vapor than cool air. When air is cooled, its capacity to hold water vapor decreases. The dew point temperature is the temperature at which air becomes saturated with water vapor and any further cooling causes condensation.

To understand this better, let's consider a practical example. Imagine a warm, humid day. The air is filled with water vapor. As the evening approaches, the temperature begins to drop. The air cools down and its capacity to hold water vapor decreases. When the temperature drops to a certain point, known as the dew point, the air becomes saturated with water vapor. At this point, if the temperature continues to drop, the excess water vapor starts to condense into tiny liquid droplets, forming dew on surfaces like grass, car windows, and other objects.

The dew point temperature is a critical parameter in weather forecasting and climate studies. It provides valuable information about the humidity and moisture content of the air. A high dew point indicates high humidity, which can make the air feel warmer and more uncomfortable. Conversely, a low dew point indicates low humidity, which can make the air feel cooler and more pleasant.

In HVAC systems, understanding the dew point is essential for controlling indoor air quality and preventing issues like mold growth and condensation on windows and walls. By maintaining the indoor air temperature above the dew point, HVAC systems can prevent the formation of condensation and ensure a comfortable and healthy indoor environment.

Advantages:

• Provides valuable information about the humidity and moisture content of the air.
• Helps in weather forecasting and climate studies.
• Essential for controlling indoor air quality in HVAC systems.
• Prevents issues like mold growth and condensation on surfaces.

Disadvantages:

• High dew point can make the air feel warmer and more uncomfortable.
• Low dew point can make the air feel cooler and drier, which might not be desirable in certain conditions.

Applications:

The concept of dew point temperature is widely used in various applications, including:

• Weather forecasting and climate studies to predict humidity and moisture levels.
• Design and operation of HVAC systems to control indoor air quality.
• Environmental monitoring to assess the potential for dew or frost formation.
• Agriculture to determine the best conditions for planting and harvesting crops.
• Industrial processes that require precise control of humidity levels.

Correct Option Analysis:

The correct option is:

Option 4: Dew point temperature

This option correctly identifies the temperature at which liquid droplets just appear when moist air is cooled continuously. The dew point temperature is the point at which air becomes saturated with water vapor, and any further cooling causes condensation.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Flash point temperature

The flash point temperature is the lowest temperature at which a liquid can form an ignitable mixture in air. This is not related to the condensation of water vapor or the formation of liquid droplets in the air.

Option 2: Fire temperature

The fire temperature refers to the temperature at which a material can ignite and sustain combustion. This is also unrelated to the condensation of water vapor or the formation of liquid droplets in the air.

Option 3: Pour point temperature

The pour point temperature is the lowest temperature at which a liquid (usually a lubricant or oil) remains pourable. This is not related to the condensation of water vapor or the formation of liquid droplets in the air.

Conclusion:

Understanding the concept of dew point temperature is essential for various applications, including weather forecasting, HVAC systems, and environmental monitoring. The dew point temperature is the temperature at which air becomes saturated with water vapor, and any further cooling causes condensation. This knowledge helps in predicting humidity levels, preventing mold growth, and ensuring a comfortable indoor environment. The correct option, dew point temperature, accurately describes this phenomenon, while the other options refer to different concepts that are not related to the condensation of water vapor.

Concept:

• The process in which the moisture or water vapour or humidity is added to the air without changing its dry bulb (DB) temperature is called a humidification process.
• This process is represented by a straight vertical line on the psychometric chart starting from the initial value of specific humidity, extending upwards and ending at the final value of the specific humidity.
• The various types of processes on the psychometric chart are:

In the above process, we can say that whenever the humidification is involved in any process the humidity ratio increases or we can say that specific humidity increases.

Concept:

A psychrometric chart is represented as shown in the figure.

Dry Bulb Temperature: Actual temperature of gas or mixture of gases

Wet bulb temperature: Temperature obtained by an accurate thermometer having a wick moistened with distilled water

Dew point temperature: Temperature at which the liquid droplets just appear when the moist air is cooled continuously.

Relative humidity along saturation line is 100%.

Basic Processes in Conditioning of Air:

Sensible heating: Moisture content of air remains constant so specific humidity is constant, temperature increases as it flows over a heating coil

Sensible cooling: Moisture content of air remains constant so specific humidity is constant, but its temperature decreases as it flows over a cooling coil

Cooling and dehumidification: When moist air is cooled below its dew-point by bringing it in contact with a cold surface, some of the water vapour in the air condenses and leaves the air stream as a liquid, as a result, both the temperature and humidity ratio of air decreases.

Heating and Humidification: During winter it is essential to heat and humidify the room air for comfort. It is done by first sensibly heating the air and then adding water vapour to the air stream through steam nozzles, as a result, both the temperature and humidity ratio of air increases.

Cooling & humidification: Air temperature drops and its humidity increases. 

Heating and de-humidification: This process can be achieved by using a hygroscopic material, which absorbs the water vapour from the moisture. If this process is thermally isolated, then the enthalpy of air remains constant, as a result, the temperature of air increases.

Calculation:

Inlet (State 1): 35°C and 100% RH

Outlet (State 2): 25°C and 100% RH

From the above figure, relative humidity is same but as the temperature is decreasing and specific humidity is also decreasing. So the process 1-2 is cooling and dehumidification process.

The name of the device is dehumidifier.

Explanation:

Basic Processes in Conditioning of Air:

Sensible heating: 

The moisture content of air remains constant so specific humidity is constant, temperature increases as it flows over a heating coil.

Sensible cooling: 

The moisture content of air remains constant so specific humidity is constant, but its temperature decreases as it flows over a cooling coil.

Dehumidification:

• When the temperature remains constant but specific humidity decreases.
• It is represented by a vertical line.

Humidification:

• When in a process, the temperature remains constant but specific humidity increases.
• It is represented by a vertical line.

Cooling and dehumidification: 

• This process involves lowering both the air temperature and specific humidity. 
• This process is commonly used in summer air conditioning in which air passes over a cooling coil. 
• When moist air is cooled below its dew point, the vapor is condensed from the air resulting in simultaneous cooling and dehumidification.

Heating and Humidification: 

• During winter it is essential to heat and humidify the room air for comfort.
• It is done by first sensibly heating the air and then adding water vapor to the air stream through steam nozzles, as a result, both the temperature and humidity ratio of air increases.

Cooling & humidification: 

Air temperature drops and its humidity increases. 

Heating and de-humidification: 

This process can be achieved by using a hygroscopic material, which absorbs the water vapor from the moisture. If this process is thermally isolated, then the enthalpy of air remains constant, as a result, the temperature of air increases.

Additional Information

Dry Bulb Temperature: Actual temperature of gas or mixture of gases.

Wet Bulb temperature: Temperature obtained by an accurate thermometer having a wick moistened with distilled water.

Dew point temperature: Temperature at which the liquid droplets just appear when the moist air is cooled continuously.

Relative humidity along the saturation line is 100%.

Concept:

Psychrometry is the study of the properties of mixtures of air and water vapor. 

Every psychometric chart includes

• Vertical lines that represent the dry bulb temperatures,
• The diagonal line represents the wet-bulb temperature
• Curved lines represent relative humidity.

Dry Bulb Temperature: Actual temperature of gas or mixture of gases

Wet Bulb temperature: Temperature obtained by an accurate thermometer having a wick moistened with distilled water

Dew point temperature: Temperature at which the liquid droplets just appear when the moist air is cooled continuously.

• Relative humidity along the saturation line is 100%.
• From the Psychrometric Chart, we can conclude that at constant specific humidity, DPT remains constant and is independent of DBT.

Explanation:

If the dry bulb temperature is increased, then it's compulsory that the relative humidity will decrease, since the saturation pressure increase.

When the temperature is increased, the specific humidity (w) is constant.

Concept:

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 vapour without condensing) at the same temperature.
• It is denoted by ϕ.

\(RH\;\left( \phi \right) = \frac{{{m_v}}}{{{m_{vs}}}}\)

Concept:

Every psychometric chart includes vertical lines that represent the dry bulb temperatures, diagonals line represents wet bulb temperature and curved lines represent relative humidity.

Dry Bulb Temperature: Actual temperature of gas or mixture of gases

Wet Bulb temperature: Temperature obtained by an accurate thermometer having a wick moistened with distilled water

Dew point temperature: Temperature at which the liquid droplets just appear when the moist air is cooled continuously.

Relative humidity along the saturation line is 100%.

Explanation:

Wet-bulb temperature:

• It is the temperature obtained by an accurate thermometer having a wick moistened with distilled water
• Wet-bulb temperature is the measure of relative humidity. 
• If air is saturated then its relative humidity is 100% that means it can't absorb more moisture, therefore if it is being made to pass over the water it will not absorb the moisture consequently and its temperature will not change.

​

Additional Information

Every psychometric chart includes vertical lines that represent the dry bulb temperatures, the diagonals line represents wet bulb temperature and the curved lines represent relative humidity.

• Dry Bulb Temperature: Actual temperature of gas or mixture of gases.
• Dew point temperature: Temperature at which the liquid droplets just appear when the moist air is cooled continuously.

When the relative humidity of the air is 100%, i.e the air is saturated, the dew point temperature (DPT) equals the wet-bulb temperature (WBT), which is also equal to the dry-bulb temperature.

So DBT = WBT = DPT

For unsaturated air:

DBT > WBT > DPT

Psychrometry chart:

Concept:

• The process in which the moisture or water vapor or the humidity is removed from the air keeping its dry bulb (DB) temperature constant is called the dehumidification process. 
• This process is represented by a straight vertical line on the psychrometric chart starting from the initial value of relative humidity, extending downwards and ending at the final value of the relative humidity.
• The process in which the moisture or water vapor or humidity is added to the air without changing its dry bulb (DB) temperature is called a humidification process.

Explanation:

Humidity:

• Humidity is the amount of water vapor in the air.
• If there is a lot of water vapor in the air, the humidity will be high. The higher the humidity, the wetter it feels outside.
• When the humidity is high it feels oppressive outside because sweat doesn't evaporate and provide cooling.
• When the humidity is low, you feel cooler but your skin dries out and you get dehydrated more easily because more moisture is being evaporated from your body. 
• For example, a completely saturated parcel of air at constant pressure cannot hold any more water molecules, giving it a relative humidity of 100 percent. As air temperature increases, air can hold more water molecules, and its relative humidity decreases. When temperatures drop, relative humidity increases. Temperature therefore directly relates to the amount of moisture the atmosphere can hold. Hence, we can conclude from the above points that humidity decreases air temperature.

There are three main measurements of humidity: relative, absolute, and specific. 

Absolute humidity:

• Its unit is grams of water vapor per cubic meter volume of air.
• It is a measure of the actual amount of water vapor in the air, regardless of the air's temperature.
• The higher the amount of water vapor, the higher the absolute humidity.
• For example, a maximum of about 30 grams of water vapor can exist in a cubic meter volume of air with a temperature in the middle 80s. 

Relative humidity:

• It is expressed as a percent, is a measure of the amount of water vapor that air is holding compared the amount it can hold at a specific temperature.
• Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have higher relative humidity.
• Relative humidity of 50% means the air holds on that day (specific temperature) holds 50% of the water needed for the air to be saturated.
• Saturated air has a relative humidity of 100%. 
• The relative humidity of an air-water mixture is also defined as the ratio of the partial pressure of water vapor in the mixture to the saturated vapor pressure of water at a given temperature.
• Thus the relative humidity of air is a function of both water content and temperature.

Specific humidity:

• Specific humidity refers to the weight of water vapor contained in a unit weight (amount) of air (expressed as grams of water vapor per kilogram of dry air).
• Absolute and specific humidity are quite similar in concept.

Explanation:

Boiling:

• The boiling point is the temperature at which a liquid boils and turns to vapour. In other words, the liquid state turns into a gaseous state. In a more scientific language, it is the temperature at which the pressure exerted by the surroundings upon a liquid is equalled by the pressure exerted by the vapour of the liquid or the vapour pressure drops below the pressure above the liquid surface.
• When a liquid is boiling in an airtight vessel, It implies that the vapour pressure is equal to or below the liquid pressure. Using the exhaust tube the vapour is pumped out, Hence the vapour pressure further decreases and boiling continue.
• As high-temperature vapour being pumped out the temperature of the vessel decreases.
• The boiling point of the water at 1 bar pressure is 100°C.
• If the pressure is decreased, the boiling point of the substance also decreases.
• At high altitudes, water boils at low temperature because atmospheric pressure decreases with an increase in altitude.