Podcast
Questions and Answers
If the partial pressure of water vapor in the air remains constant, what happens to the dew-point temperature as the air temperature increases?
If the partial pressure of water vapor in the air remains constant, what happens to the dew-point temperature as the air temperature increases?
- The dew-point temperature remains constant. (correct)
- The dew-point temperature decreases.
- The dew-point temperature increases proportionally.
- The dew-point temperature approaches the air temperature.
A sealed container contains air and water vapor. If the temperature of the container is increased, what happens to the saturated vapor pressure inside the container?
A sealed container contains air and water vapor. If the temperature of the container is increased, what happens to the saturated vapor pressure inside the container?
- It remains the same because the amount of water vapor is fixed.
- It fluctuates randomly depending on air currents.
- It increases because higher temperature allows more water molecules to enter the vapor phase. (correct)
- It decreases because the air expands, lowering the water vapor concentration.
Which modification would directly reduce the amount of moisture condensation on walls in a flat?
Which modification would directly reduce the amount of moisture condensation on walls in a flat?
- Cool the walls to encourage condensation in a controlled area.
- Increase the surface temperature of the walls. (correct)
- Decrease the ventilation to minimize air exchange with the outside.
- Increase the relative humidity of the air in the flat.
How does the human body primarily utilize the latent heat of water for thermoregulation?
How does the human body primarily utilize the latent heat of water for thermoregulation?
Given that the saturated vapor pressure of water at 20°C is approximately 2.3 kPa and at 30°C is approximately 4.2 kPa, what can be inferred about the relationship between temperature and saturated vapor pressure?
Given that the saturated vapor pressure of water at 20°C is approximately 2.3 kPa and at 30°C is approximately 4.2 kPa, what can be inferred about the relationship between temperature and saturated vapor pressure?
If the air pressure in a closed room increases but the amount of water vapor remains constant, what will happen to the moisture content?
If the air pressure in a closed room increases but the amount of water vapor remains constant, what will happen to the moisture content?
In a scenario where the partial pressure of water vapor in the air is close to the saturated vapor pressure at the existing temperature, what physiological effect would be most likely?
In a scenario where the partial pressure of water vapor in the air is close to the saturated vapor pressure at the existing temperature, what physiological effect would be most likely?
Which strategy is most effective for preventing excessive moisture build-up in a poorly ventilated room during cold weather?
Which strategy is most effective for preventing excessive moisture build-up in a poorly ventilated room during cold weather?
Air at a certain temperature has a relative humidity of 50%. If the amount of water vapor in the air remains constant and the temperature decreases, what happens to the relative humidity?
Air at a certain temperature has a relative humidity of 50%. If the amount of water vapor in the air remains constant and the temperature decreases, what happens to the relative humidity?
Why does the evaporation of sweat provide a cooling effect for the human body?
Why does the evaporation of sweat provide a cooling effect for the human body?
How does increasing atmospheric vapor pressure affect the rate of sweat evaporation from the skin?
How does increasing atmospheric vapor pressure affect the rate of sweat evaporation from the skin?
What is the primary purpose of the psychrometric chart?
What is the primary purpose of the psychrometric chart?
Under what conditions will the dew-point temperature be equal to the air temperature?
Under what conditions will the dew-point temperature be equal to the air temperature?
If the temperature of a closed container of air increases, and no additional water vapor is added, what happens to the relative humidity inside the container?
If the temperature of a closed container of air increases, and no additional water vapor is added, what happens to the relative humidity inside the container?
For a closed system containing only air and water vapor, what is the relationship between total pressure ((P_t)), partial pressure of water vapor ((P_w)), and partial pressure of dry air ((P_a))?
For a closed system containing only air and water vapor, what is the relationship between total pressure ((P_t)), partial pressure of water vapor ((P_w)), and partial pressure of dry air ((P_a))?
Given two air masses at the same temperature, one with a high moisture content and one with a low moisture content, which air mass will have the higher dew-point temperature?
Given two air masses at the same temperature, one with a high moisture content and one with a low moisture content, which air mass will have the higher dew-point temperature?
What does the term 'dry-bulb temperature' typically refer to when used in the context of psychrometrics?
What does the term 'dry-bulb temperature' typically refer to when used in the context of psychrometrics?
In the context of moisture content of air, what do (M_w) and (M_a) represent, respectively, in the equation describing moisture content?
In the context of moisture content of air, what do (M_w) and (M_a) represent, respectively, in the equation describing moisture content?
According to the provided information, what value is used for the molar mass of water (M_w) in calculations related to moisture content of air?
According to the provided information, what value is used for the molar mass of water (M_w) in calculations related to moisture content of air?
What is the significance of the triple point on a pressure-temperature (P-T) diagram for water?
What is the significance of the triple point on a pressure-temperature (P-T) diagram for water?
Flashcards
Humidity
Humidity
The amount of water vapor present in the air.
Psychrometric chart
Psychrometric chart
A chart used to determine the properties of moist air.
Dew Point Temperature
Dew Point Temperature
The temperature to which air must be cooled to become saturated with water vapor.
Moisture content
Moisture content
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Dry-bulb temperature
Dry-bulb temperature
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Relative humidity
Relative humidity
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Study Notes
- Water vapor in the air, humidity, and the psychrometric chart will be discussed
Lecture Objectives
- Quantities to describe water vapor-dry air mixtures, including dew-point temperature and moisture content, will be covered
- These quantities will be related to the concept of vapor pressure
- The psychrometric chart will be used to measure water vapor-dry air mixtures
Water Vapor in the Air
- The human body contains millions of sweat glands
- The body can produce up to 3 liters of sweat per hour
- Evaporation of sweat, consisting mostly of water, regulates body temperature
- This makes use of the latent heat of water
- High atmospheric vapor pressure can impede sweat evaporation
Dew Point Temperature
- The partial pressure of water vapor in the air is generally less than the saturated vapor pressure of water at the air temperature
- The partial pressure of water vapor in the atmosphere equals the saturated vapor pressure of water at a lower temperature
- Condensation or dew forms on the surface when cooling it
- The creation of dew happens at the dew-point temperature
Vapor Pressure of Water
- The vapor pressure of water doubles approximately every 10°C for temperatures between 0°C and 30°C
Moisture Content of Air
- Moisture content of air measures the mass of moisture per kg of dry air
- It is also known as absolute humidity or the humidity ratio
- We can find this from the ideal gas law PV=nRT using n=m/M
- The mass of dry air is ma = (MaPaV) / (RT)
Moist Air Continued
- mw / ma = (Mw Pw) / (Ma Pa)
- Mw is molar mass of water [18 g/mol]
- Ma is molar mass of air [28.97 g/mol]
- Pw is partial pressure of water
- Pa is partial pressure of air
- The sum of partial pressures equals total pressure: Pw + Pa = Pt, so that Pa = Pt - Pw
- mw / ma = 0.621 Pw / (Pt - Pw)
- Pw is the partial pressure of water
- Pt is the total pressure
- Typically Pw << Pt, mw / ma ≈ 0.621 Pw / Pt, where moisture content is directly proportional to the partial pressure of water vapor
Example
- Saturated vapor pressure for water is 4.24 kPa at 30 °C
- Question for the viewer: What is the moisture content for air when the dew-point temperature is 30 °C and the air pressure is 101.3 kPa?
Psychrometric Chart
- Displays the properties of moist air in a composite graph
- Normal air temperature to distinguish it from dew-point temperature is called dry-bulb temperature
- Measuring dew-point temperature requires cooling a surface until dew condenses
- Assuming normal atmospheric pressure of 101.3 kPa, air serves its purpose
Absolute Humidity
- Absolute humidity is (18 x 10^-3 Pw)/ (28.97 x 10^-3 (Pt - Pw)) = 0.621 x (Pw / (Pt - Pw))
- If dew point temperature is known, Pw moisture content, absolute humidity, and humidity ratio can be determined
Relative Humidity
- Temperature of dew-point is less than the air temperature
- Actual vapor pressure is less than the maximum achievable at the air temperature
- RH = (Actual partial vapor pressure of water) / (Saturated vapor pressure at the same temperature)
- Psychrometric chart displays relative humidity using air temperature and the moisture content
Example #2
- Dew-point temperature is 20°C, dry-bulb temperature is 35°C
- Question for the viewer: Find the realtive humidity
Example #3
- The temperature in a student flat is 15 °C with a relative humidity of 80%
- Mould growths are likely to occur if the walls regularly get wet through moisture condensation
- Question for the viewer: How cold can the walls get to avoid condensation and associated health risks?
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