1. What is air? What is normal air?
Answer: The atmosphere around the earth, we are used to call it air.
The air under the specified pressure of 0.1MPa, temperature of 20°C, and relative humidity of 36% is normal air. Normal air differs from standard air in temperature and contains moisture. When there is water vapor in the air, once the water vapor is separated, the air volume will be reduced.
2. What is the standard state definition of air?
Answer: The definition of the standard state is: the air state when the air suction pressure is 0.1MPa and the temperature is 15.6°C (the domestic industry definition is 0°C) is called the standard state of the air.
In the standard state, the air density is 1.185kg/m3 (the capacity of air compressor exhaust, dryer, filter and other post-processing equipment is marked by the flow rate in the air standard state, and the unit is written as Nm3/min ).
3. What is saturated air and unsaturated air?
Answer: At a certain temperature and pressure, the content of water vapor in humid air (that is, the density of water vapor) has a certain limit; when the amount of water vapor contained in a certain temperature reaches the maximum possible content, the humidity at this time Air is called saturated air. The moist air without the maximum possible content of water vapor is called unsaturated air.
4. Under what conditions does unsaturated air become saturated air? What is “condensation”?
At the moment when unsaturated air becomes saturated air, liquid water droplets will condense in the humid air, which is called “condensation”. Condensation is common. For example, the humidity of the air in summer is very high, and it is easy to form water droplets on the surface of the water pipe. In the winter morning, water droplets will appear on the glass windows of the residents. These are the humid air cooled under constant pressure to reach the dew point. The result of condensation due to temperature.
5. What are atmospheric pressure, absolute pressure and gauge pressure? What are the common units of pressure?
Answer: The pressure caused by a very thick layer of atmosphere surrounding the earth’s surface on the earth’s surface or surface objects is called “atmospheric pressure”, and the symbol is Ρb; the pressure directly acting on the surface of the container or object is called “absolute pressure”. The pressure value starts from absolute vacuum, and the symbol is Pa; the pressure measured by pressure gauges, vacuum gauges, U-shaped tubes and other instruments is called “gauge pressure”, and “gauge pressure” starts from atmospheric pressure, and the symbol is Ρg. The relationship between the three is
Pa=Pb+Pg
Pressure refers to the force per unit area, and the pressure unit is N/square, denoted as Pa, called Pascal. MPa (MPa) commonly used in engineering
1MPa=10 sixth power Pa
1 standard atmospheric pressure = 0.1013MPa
1kPa=1000Pa=0.01kgf/square
1MPa=10 sixth power Pa=10.2kgf/square
In the old system of units, pressure is usually expressed in kgf/cm2 (kilogram force/square centimeter).
6. What is temperature? What are the commonly used temperature units?
A: Temperature is the statistical average of the thermal motion of the molecules of a substance.
Absolute temperature: The temperature starting from the lowest limit temperature when gas molecules stop moving, denoted as T. The unit is “Kelvin” and the unit symbol is K.
Celsius temperature: The temperature starting from the melting point of ice, the unit is “Celsius”, and the unit symbol is ℃. In addition, British and American countries often use “Fahrenheit temperature”, and the unit symbol is F.
The conversion relationship between the three temperature units is
T (K) = t (°C) + 273.16
t(F)=32+1.8t(℃)
7. What is the partial pressure of water vapor in humid air?
Answer: Humid air is a mixture of water vapor and dry air. In a certain volume of humid air, the amount of water vapor (by mass) is usually much less than that of dry air, but it occupies the same volume as dry air. , also have the same temperature. The pressure of moist air is the sum of the partial pressures of the constituent gases (ie, dry air and water vapor). The pressure of water vapor in humid air is called the partial pressure of water vapor, denoted as Pso. Its value reflects the amount of water vapor in the humid air, the higher the water vapor content, the higher the water vapor partial pressure. The partial pressure of water vapor in saturated air is called the saturated partial pressure of water vapor, denoted as Pab.
8. What is the humidity of the air? How much humidity?
Answer: The physical quantity that expresses the dryness and humidity of the air is called humidity. Commonly used humidity expressions are: absolute humidity and relative humidity.
Under standard conditions, the mass of water vapor contained in humid air in a volume of 1 m3 is called the “absolute humidity” of the humid air, and the unit is g/m3. Absolute humidity only indicates how much water vapor is contained in a unit volume of humid air, but does not indicate the ability of the humid air to absorb water vapor, that is, the degree of humidity of the humid air. Absolute humidity is the density of water vapor in moist air.
The ratio of the actual amount of water vapor contained in humid air to the maximum possible amount of water vapor at the same temperature is called “relative humidity”, which is often expressed by φ. The relative humidity φ is between 0 and 100%. The smaller the φ value, the drier the air and the stronger the water absorption capacity; the larger the φ value, the humid the air and the weaker the water absorption capacity. The moisture absorption capacity of humid air is also related to its temperature. As the temperature of the humid air rises, the saturation pressure increases accordingly. If the content of water vapor remains unchanged at this time, the relative humidity φ of the humid air will decrease, that is to say, the moisture absorption capacity of the humid air Increase. Therefore, during the installation of the air compressor room, attention should be paid to maintaining ventilation, lowering the temperature, no drainage, and water accumulation in the room to reduce the moisture in the air.
9. What is moisture content? How to calculate the moisture content?
Answer: In humid air, the mass of water vapor contained in 1kg of dry air is called the “moisture content” of humid air, which is commonly used. To show that the moisture content ω is almost proportional to the water vapor partial pressure Pso, and inversely proportional to the total air pressure p. ω exactly reflects the amount of water vapor contained in the air. If the atmospheric pressure is generally constant, when the temperature of the humid air is constant, Pso is also constant. At this time, the relative humidity increases, the moisture content increases, and the moisture absorption capacity decreases.
10. What does the density of water vapor in saturated air depend on?
Answer: The content of water vapor (water vapor density) in the air is limited. In the range of aerodynamic pressure (2MPa), it can be considered that the density of water vapor in saturated air depends only on the temperature and has nothing to do with the air pressure. The higher the temperature, the greater the density of saturated water vapor. For example, at 40°C, 1 cubic meter of air has the same saturated water vapor density no matter its pressure is 0.1MPa or 1.0MPa.
11. What is humid air?
Answer: The air containing a certain amount of water vapor is called humid air, and the air without water vapor is called dry air. The air around us is moist air. At a certain altitude, the composition and proportion of dry air are basically stable, and it has no special significance for the thermal performance of the whole humid air. Although the water vapor content in the humid air is not large, the change of the content has a great influence on the physical properties of the humid air. The amount of water vapor determines the degree of dryness and humidity of the air. The working object of the air compressor is moist air.
12. What is heat?
Answer: Heat is a form of energy. Commonly used units: KJ/(kg·℃), cal/(kg·℃), kcal/(kg·℃), etc. 1kcal=4.186kJ, 1kJ=0.24kcal.
According to the laws of thermodynamics, heat can be spontaneously transferred from the high temperature end to the low temperature end through convection, conduction, radiation and other forms. In the absence of external power consumption, heat can never be reversed.
13. What is sensible heat? What is latent heat?
Answer: In the process of heating or cooling, the heat absorbed or released by an object when its temperature rises or falls without changing its original phase state is called sensible heat. It can make people have obvious changes in cold and heat, which can usually be measured with a thermometer. For example, the heat absorbed by raising water from 20°C to 80°C is called sensible heat.
When an object absorbs or releases heat, its phase state changes (such as gas becomes liquid…), but the temperature does not change. This absorbed or released heat is called latent heat. Latent heat cannot be measured with a thermometer, nor can the human body feel it, but it can be calculated experimentally.
After the saturated air releases heat, part of the water vapor will phase into liquid water, and the temperature of the saturated air does not drop at this time, and this part of the released heat is latent heat.
14. What is the enthalpy of air?
Answer: The enthalpy of air refers to the total heat contained in the air, usually based on the unit mass of dry air. Enthalpy is represented by the symbol ι.
15. What is dew point? What is it related to?
Answer: The dew point is the temperature at which the unsaturated air lowers its temperature while keeping the partial pressure of water vapor constant (that is, keeping the absolute water content constant) so that it reaches saturation. When the temperature drops to the dew point, condensed water droplets will be precipitated in the humid air. The dew point of humid air is not only related to temperature, but also related to the amount of moisture in the humid air. The dew point is high with high water content, and the dew point is low with low water content. At a certain humid air temperature, the higher the dew point temperature, the greater the partial pressure of water vapor in the humid air, and the greater the water vapor content in the humid air. The dew point temperature has an important use in compressor engineering. For example, when the outlet temperature of the air compressor is too low, the oil-gas mixture will condense due to the low temperature in the oil-gas barrel, which will make the lubricating oil contain water and affect the lubrication effect. Therefore, the outlet temperature of the air compressor must be designed to ensure that it is not lower than the dew point temperature under the corresponding partial pressure.
Post time: Jul-17-2023
