Psychrometry
1) Difference between Moist Air and Dry Air
| Dry Air | Moist Air |
|---|---|
| Does not contain water vapour. | Contains water vapour. |
| More density | Less density |
| Heavy | Lighter |
| Exerts more pressure in atmosphere | Exerts less pressure in atmosphere |
| Less specific heat capacity | More specific heat capacity |
| Takes less energy to be heated | Takes more energy to be heated |
2) Difference between Dew Point Temperature and Wet Bulb Temperature
| Dew Point Temperature (DPT) | Wet Bulb Temperature (WBT) |
|---|---|
| It is the temperature at which the air must be cooled to become saturated with water vapor. | It is the temperature that we get when exposing a thermometer having a moistened bulb to the air flow. |
| At high relative humidity, dew point temperature is close to dry air temperature. | At 100% relative humidity, the wet bulb temperature equals the dry air temperature. |
| Indicates the formination of dew or frost from water vapor. | Can measure wet bulb temperature using evaporation rate of water from water soaked cloth covering thermometer. |
3) What is Dry Bulb Temperature?
Definition: It is the temperature measured by the thermometer exposed to the air.
Dry Bulb Temperature refers to the ambient air temperature. Generally when the term temperature is used, it refers to the dry bulb temperature.
Designated by: Tdb
4) What is Wet Bulb Temperature?
Definition: It is the temperature measured by the thermometer bulb covered with a wet cloth and exposed to air.
The wet bulb temperature is always between the dry bulb temperature and the dew point.
Designated by: Twb
5) What is Dew Point Temperature?
Definition: It is the temperature at which the water vapor starts to condense out of the air.
When the Dew Point Temperature is close to the dry air temperature, relative humidity is high.
When the Dew Point Temperature is well below the dry air temperature, relative humidity is low.
Designated by: Tdp
6) What is Relative Humidity?
Definition: The mole fraction of water vapor in a sample divided by the mole fraction in saturated air at the same temperature and pressure.
Relative humidity tells us how much water vapor is in the air, compared to how much it could hold at that temperature.
Designated by: RH
7) What is Humidity Ratio?
Definition: The humidity ratio is the mass of water vapor divided by the mass of dry air in the sample.
The information about the relative humidity helps us to control the moisture around the desired product.
8) What is Degree of Saturation?
Definition: The humidity ratio of the water vapor in a sample divided by the humidity ratio of the water vapor in saturated air at the same temperature and pressure is known as Degree of Saturation.
The degree of saturation denotes the actual relationship between the weight of moisture existing in a space and the weight that would exist if the space were saturated.
9) What is Vapor Density?
Definition: Density of a particular gas relative to that of the hydrogen at the same temperature and pressure.
If a gas has a vapor density of less than one it will generally rise in air. If the vapor density is greater than one the gas will generally sink in air.
By vapor density, we can estimate position of the air.
9) What is Specific Humidity?
Definition: It is the mass of water vapor in a unit mass of moist air.
It is the most reliable unit of measurement of humidity.
This measures weight of water vapor per unit weight of air. It is express as gram of water vapor per kilogram of air.
Unit: g.kg-1
10) What is Sensible Heat?
Definition: Thermal energy whose transfer to or from a substance which results in change in temperature.
Phase does not change here.
11) What is Latent Heat?
Definition: Energy absorbed or released by a substance during a change in its physical state (Phase) that occurs without changing its temperature.
Temperature remains constant here.
12) What is the use of psychrometric chart?
A psychrometric chart presents physical and thermal properties of moist air in a graphical form. It can be very helpful in troubleshooting and finding solutions to greenhouse or livestock building environmental problems. Understanding psychrometric charts can help you visualize environmental control concepts, such as why heated air can hold more moisture or, conversely, how allowing moist air to cool will result in condensation.
No responses yet