Air Conditioning MCQ Quiz - Objective Question with Answer for Air Conditioning - 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:

Low-Velocity Duct

• A low-velocity duct is a type of air duct system where the mean velocity of air flow is kept intentionally low to minimize noise, reduce energy consumption, and maintain a more stable and comfortable distribution of airflow. These ducts are typically utilized in systems where efficient energy usage and comfort are prioritized over compactness or high-flow rates.

Criteria for Low-Velocity Ducts:

• In HVAC (Heating, Ventilation, and Air Conditioning) systems, ducts are classified based on the mean velocity of the air that flows through them. The classification helps in determining the design and operational parameters of the duct system. A duct is considered a low-velocity duct when the mean velocity of air is less than 10 m/s. This classification ensures that the air movement remains quiet, and the system operates efficiently without excessive energy losses.

Advantages of Low-Velocity Ducts:

• Noise Reduction: Lower air velocities result in significantly reduced noise levels, making the system suitable for residential, office, and other noise-sensitive environments.
• Energy Efficiency: By minimizing turbulence and frictional losses, low-velocity systems consume less energy compared to high-velocity systems.
• Improved Air Quality: Reduced air speed minimizes particle disturbance and ensures better air filtration, leading to improved indoor air quality.
• Comfort: Low-velocity airflow promotes better thermal comfort by avoiding drafts and ensuring uniform temperature distribution.

Explanation:

Optimum Effective Temperature for Human Comfort

• The optimum effective temperature refers to the specific range of temperature and humidity conditions that provide maximum comfort to the majority of people under normal circumstances. Human comfort is influenced by several factors, including air temperature, humidity, air movement, and individual clothing and activity levels. The concept of effective temperature combines these factors into a single index to predict comfort levels.
• The optimum effective temperature for human comfort is lower in winter than in summer. This is because human comfort is not only influenced by the surrounding temperature but also by psychological and physiological factors. During winter, individuals tend to wear heavier clothing and are accustomed to cooler surroundings, making them comfortable at lower effective temperatures. In contrast, during summer, lighter clothing and higher ambient temperatures lead to a higher optimum effective temperature for comfort.

Let us delve deeper into the reasoning behind this:

• Clothing and Insulation: In winter, people typically wear insulating clothing to retain body heat. These layers of clothing reduce the need for higher ambient temperatures to feel comfortable. Conversely, in summer, lighter clothing allows for better heat dissipation, making slightly higher temperatures more tolerable and comfortable.
• Metabolic Rate: During winter, the human body generates more heat to maintain its core temperature. This natural adaptation reduces the reliance on external heating and allows for comfort at lower effective temperatures. In summer, the metabolic rate may decrease slightly as the body attempts to cool itself, leading to a preference for higher effective temperatures.
• Psychological Factors: People psychologically associate winter with cooler conditions and summer with warmer conditions. This association impacts their perception of comfort, influencing the optimum effective temperature for each season.
• Environmental Adaptation: Over time, people adapt to seasonal changes in their environment. This adaptation affects their comfort levels and the corresponding effective temperature ranges.

Explanation:

Interaction of Unsaturated Air with Water Droplets in Air Washer

• The interaction of unsaturated air with water droplets in an air washer refers to the process where the air comes into contact with water droplets, leading to changes in its humidity and temperature properties. This process is widely used in air-conditioning systems for cooling, humidifying, or dehumidifying air to achieve desired indoor environmental conditions.

When unsaturated air interacts with water droplets inside an air washer, several phenomena can occur depending on the initial condition of the air and the water temperature. These phenomena include dehumidification, humidification, and operation along a constant dew point temperature (DPT) line. Let’s analyze each aspect:

1) Dehumidification:

• If the water temperature inside the air washer is lower than the dew point temperature of the air, the water droplets cause condensation of moisture from the air. This results in a reduction of the air’s humidity, referred to as dehumidification. The condensed moisture is removed, and the air becomes drier. This principle is commonly used in cooling and dehumidifying systems where maintaining low humidity levels is necessary.

2) Humidification:

• If the water temperature is higher than the air’s dew point temperature, evaporation of water occurs, adding moisture to the air. This process increases the air’s humidity, known as humidification. Humidification is crucial in scenarios where the air is excessively dry, such as in winter conditions or specific industrial applications, to maintain optimal humidity levels for comfort or manufacturing processes.

3) Constant Dew Point Temperature Line:

• In certain conditions, the process may occur along a constant dew point temperature (DPT) line. This happens when the water temperature is equal to the dew point temperature of the air. In such a scenario, there is no net addition or removal of moisture in the air, and the interaction mainly results in a change in air temperature without altering its humidity ratio. This constant DPT process is significant in maintaining stable environmental conditions.

Explanation:

Air-Cooling Systems:

• Air-cooling systems are widely used in various applications, including automotive engines, electronics cooling, and small to medium-sized internal combustion engines.
• These systems rely on the natural or forced circulation of air over the surfaces of components to dissipate heat.
• Here, we will break down the fundamental aspects of air-cooling systems, their advantages, disadvantages, and why uniform cooling is a characteristic rather than a limitation.

Principle of Operation:

• Air-cooling systems operate by transferring heat from the surface of a component to the surrounding air.
• This can be achieved through natural convection, where air flows over the hot surface due to temperature differences, or forced convection, where fans or blowers actively push air over the component to enhance cooling efficiency.
• The heat is then carried away by the air, preventing the component from overheating.

Advantages of Air-Cooling Systems:

• Simplicity: Air-cooling systems are simple in design and construction, making them relatively easy to manufacture and maintain.
• Cost-Effective: These systems do not require a liquid coolant, reducing costs associated with coolant production, storage, and disposal.
• Weight Reduction: Air-cooled systems eliminate the need for heavy radiators, pumps, and coolant, resulting in a lighter overall system.
• Environmental Impact: Air-cooling systems have a lower environmental impact since they do not involve liquid coolants that may require special handling or disposal.

Disadvantages of Air-Cooling Systems:

• Limited Cooling Capacity: Air has a lower heat capacity compared to liquids, limiting the amount of heat that can be dissipated.
• Dependence on Ambient Temperature: The efficiency of air-cooling systems decreases with higher ambient temperatures, as the temperature gradient between the component and the air decreases.
• Noise: Forced air-cooling systems can produce significant noise due to the operation of fans or blowers.

Concept:

Relative humidity is given by

\(ϕ = \frac{{{P_v}}}{{{P_{vs}}}}\)

where P _v is pressure of water vapour and P_vs is pressure of water vapour at saturation point.

For isothermal process:

PV = constant

Calculation:

Given:

ϕ₁ = 50%, P_v1

Since volume is reduced to half so the pressure has become twice.

P_v2 = 2P_v1

\(\frac{{{ϕ _2}}}{{{ϕ _1}}} = \frac{{{P_{{v_2}}}}}{{{P_{v1\;}}}}\)

\(\frac{{{ϕ _2}}}{{{ϕ _1}}} = 2\)

ϕ₂ = 2ϕ₁

ϕ₂ = 2 × 50 ⇒ 100%.

Additional InformationHere, the initial condition is given as Moist air, but we cannot apply ideal gas to it. Technically the wording is incorrect but here we have to assume it as ideal and solve accordingly.

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:

By pass factor for N no. of coils is given by:

BPF = XN

Calculation:

Given:

By Pass Factor = 0.8, N = 3

Now,

∴ BPF = XN 

∴ BPF = 0.83 

∴ BPF = 0.512

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:

Carnot Cycle:

Reversed Carnot Cycle:

\({η _{carnot}} = \frac{{{T_H} - {T_L}}}{{{T_H}}}\)

By reversing the Carnot cycle we can have either heat pump or refrigerator.

COP of Reversed Carnot cycle (H.P):

\(CO{P_{HP}} = \frac{{{T_H}}}{{{T_H} - {T_L}}} = \frac{1}{η }\)

COP of Refrigerator:

\(CO{P_{Ref}} = \frac{{{T_L}}}{{{T_H} - {T_L}}} = CO{P_{HP}} - 1\)

Calculation:

Given: η = 0.8

Using the relation \(CO{P_{HP}} = 1 + CO{P_R} = \frac{1}{{{η _{HE}}}}\)

\( \Rightarrow CO{P_R} = \frac{1}{{0.8}} - 1 = 0.25\)

Concept:

This will be the case of sublimation. As the weather is sub-zero, so the water present in clothes will freeze and will sublimate directly and we feel the clothes as dry.

Sublimation

• Sublimation is a process in which solid changes into the gaseous state without passing through the liquid state.
• Sublimation is endothermic in nature and occurs at temperature and pressure below the one at which all three states of the substance exist simultaneously.
• The energy required for sublimation is known as the enthalpy of sublimation.
• The sublimation process can be seen in the case of camphor, naphthalene balls, wet cloths turn into dry clothes, etc.

​Vaporization

• Vaporization can be defined as the process in which liquid state changes into the vapor state.

Melting

• ​Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid.
• This occurs when the internal energy of the solid increases, typically by the application of heat or pressure, which increases the substance's temperature to the melting point.

Condensation

• Condensation is the deposition of a liquid or a solid from its vapor, generally upon a surface that is cooler than the adjacent gas.

Concept:

A psychrometric chart is represented as shown in the figure.

Calculation:

Given, that moist air is contained in CLOSED VESSEL, so we can say that mass of moist air is constant.

\(w = specific\;humidity = \frac{{{m_v}}}{{{m_{dry\;air}}}} = constant\)

So specific humidity is also same.

Now as the container is heated, so temperature increases. On psychometry chart, it can be represented by process 1 - 2.

Along the saturation line, RH is 100%. So in this case, RH is decreasing.