Basic theoretical knowledge of air about air compressor that should understand.

1. What is air? What is normal air?

The atmosphere around the earth is customarily called air.

The air under the specified pressure of 0.1MPa, temperature of 20 ℃ and relative humidity of 36% is normal air. Normal air differs from standard air in temperature and moisture content. When there is moisture in the air, once the moisture is separated, the gas volume will be reduced.

2. What is the definition of the standard state of air?

The standard state is defined as the air state when the air suction pressure is 0.1MPa and the temperature is 15.6 ℃ (0 ℃ as defined by the domestic industry).

In the standard state, the air density is 1.185kg/m3 (the processing capacity of post-processing equipment such as air compressor exhaust, dryer, filter, etc. is indicated by the flow rate under the standard state of air, and the unit is Nm3/min).

3. What are saturated air and unsaturated air?

Under a certain temperature and pressure, the content of water vapor (i.e., the density of water vapor) in wet air has a certain limit; When the amount of water vapor at a certain temperature reaches the maximum possible content, the wet air at this time is called saturated air. The wet air when the water vapor does not reach the maximum possible content is called unsaturated air.

4. Under what conditions does unsaturated air become saturated? What is "dewing"?

When unsaturated air becomes saturated, liquid water droplets will condense out of wet air, which is called "condensation". Dew condensation is common. For example, in summer, the air humidity is very high, and it is easy to form water droplets on the surface of the water pipe. In winter, in the morning, water droplets will appear on the windows of households. These are the results of dew condensation when the moist air is cooled to the dew point temperature under constant pressure.

5. What are atmospheric pressure, absolute pressure and gauge pressure? What are the common units of pressure?

The pressure on the earth's surface or surface objects caused by a very thick atmosphere surrounding the earth's surface is called "atmospheric pressure", and the symbol is Ρ b； The pressure directly acting on the surface of the container or object is called "absolute pressure". The absolute pressure value takes the absolute vacuum as the starting point, and the symbol is Ρ a； The pressure measured by pressure gauge, vacuum gauge, U-tube and other instruments is called "gauge pressure". The "gauge pressure" starts from atmospheric pressure, and the symbol is Ρ g。 The relationship between the three is Ρ a= Ρ b+ Ρ g

6. What is temperature? What are the common temperature units?

Temperature is the statistical average value of the thermal motion of material molecules.

Absolute temperature: the temperature starting from the lowest limit temperature when the gas molecules stop moving, recorded 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, the British and American countries often use "Fahrenheit temperature", the unit symbol is F.

7. What is the partial pressure of water vapor in wet air?

Wet air is a mixture of water vapor and dry air. In a certain volume of wet air, the component of water vapor (by mass) is usually much less than that of dry air, but it occupies the same volume as dry air and has the same temperature. The pressure of wet air is the sum of the partial pressures of the constituent gases (i.e. dry air and water vapor). The pressure of water vapor in wet air is called the partial pressure of water vapor, which is recorded as Ρ. The value reflects the amount of water vapor in the wet air. The higher the water vapor content, the higher the water vapor pressure. The partial pressure of water vapor in saturated air is called the saturated partial pressure of water vapor, which is recorded as Ρ ab.

8.What is air humidity? How many kinds of humidity are there?

The physical quantity representing the degree of dryness and wetness of air is called humidity. Commonly used humidity expressions include absolute humidity and relative humidity.

In the standard state, the mass of water vapor contained in 1m3 of wet air is called the "absolute humidity" of wet air, and the unit is g/m3. The absolute humidity only indicates how much water vapor is contained in the unit volume of wet air, but cannot indicate the ability of wet air to absorb water vapor, that is, it cannot indicate the humidity degree of wet air. Absolute humidity is the density of water vapor in wet air.

The ratio between the actual amount of water vapor contained in the wet air and the maximum amount of water vapor likely to be contained at the same temperature is called "relative humidity" φ To express. relative humidity φ Between 0 and 100%. φ The smaller the value is, the drier the air is, and the stronger the water absorption capacity is; φ The higher the value, the wetter the air and the weaker the water absorption capacity. The moisture absorption capacity of wet air is also related to its temperature. When the temperature of wet air rises, the saturation pressure increases accordingly. In this case, the content of water vapor remains unchanged, then the relative humidity of wet air φ It will decrease, that is, the moisture absorption capacity of wet air will increase. Therefore, during the installation of the air compressor room, attention should be paid to maintaining ventilation, lowering temperature, and no drainage or ponding in the room to reduce the moisture in the air.

9. What is the moisture content? How is the moisture content calculated?

In wet air, the mass of water vapor contained in 1kg dry air is called the "moisture content" of wet air, commonly used. To express the moisture content ω It is almost proportional to the partial pressure of water vapor Pso and inversely proportional to the total air pressure p. ω It accurately reflects the amount of water vapor contained in the air. If the atmospheric pressure is generally unchanged, when the wet air temperature is constant, Pso is also constant. At this time, the relative humidity increases, its moisture content increases, and its moisture absorption capacity decreases.

10. What does the density of water vapor in saturated air depend on?

There is a limit to the amount (density) of water vapor in the air. Within the range of pneumatic pressure (2MPa), it can be considered that the density of water vapor in saturated air only depends 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, the saturated water vapor density of 1 cubic meter of air at 40 ℃ is the same regardless of its pressure of 0.1MPa or 1.0MPa.

11. What is wet air?

Air containing a certain amount of water vapor is called wet air, and air without water vapor is called dry air. The air around us is wet. At a certain altitude, the composition and proportion of dry air are basically stable, which has no special significance for the thermal performance of the whole wet air. Although the content of water vapor in wet air is small, the change of the content has a great impact on the physical properties of wet air. The amount of water vapor determines the dryness and humidity of the air. The working object of the air compressor is wet air.

12. What is heat?

Heat is a form of energy. Common units: KJ/(kg ·℃), cal/(kg ·℃), kcal/(kg ·℃), etc. 1kcal=4.186kJ，1kJ=0.24kcal。

According to the laws of thermodynamics, heat can be transferred spontaneously from the high temperature end to the low temperature end through convection, conduction, radiation and other forms. Without external power consumption, heat can never be transferred in the opposite direction.

13. What is sensible heat? What is latent heat?

Sensible heat refers to the heat that an object needs to absorb or release when its temperature rises or decreases without changing its original phase state during heating or cooling. It can make people have an obvious feeling of cold and hot changes, which can usually be measured with a thermometer. For example, the heat absorbed by raising water temperature from 20 ℃ to 80 ℃ is called sensible heat.

When an object absorbs or emits heat, its phase state changes (e.g., a gas becomes a liquid...), but the temperature does not change. The heat absorbed or emitted is called latent heat. Latent heat cannot be measured with a thermometer, nor can it be felt by the human body, but it can be calculated through experiments.

After the saturated air emits heat, part of the water vapor will change into liquid water. At this time, the temperature of the saturated air does not drop. This part of the heat is latent heat.

14. What is the enthalpy of air?

The enthalpy of air refers to the total heat contained in air, usually based on the unit mass of dry air. Symbol for enthalpy ι express.

15. What is dew point? What is it about?

The temperature of unsaturated air is reduced under the condition of keeping the partial pressure of water vapor unchanged (that is, keeping the absolute water content unchanged), and the temperature when it reaches the saturation state is called the dew point. When the temperature drops to the dew point, condensate droplets will be separated from the wet air.

The dew point of wet air is not only related to temperature, but also to the amount of moisture in the wet air. The dew point is high for high water content, and low for low water content. At a certain wet air temperature, the higher the dew point temperature, the greater the partial pressure of water vapor in the wet air, the greater the content of water vapor in the wet air. Dew point temperature plays an important role in compressor engineering. For example, when the outlet temperature of the air compressor is too low, the oil and gas mixture will be dewed in the oil and gas barrel due to the low temperature, which will make the lubricating oil containing water affect the lubricating effect. Therefore, the outlet temperature of the air compressor must be designed to be no lower than the dew point temperature under the corresponding partial pressure.

16. What is the pressure dew point?

After the wet air is compressed, the density of water vapor increases, and the temperature also increases. When the compressed air is cooled, the relative humidity will increase. When the temperature continues to drop to 100% of the relative humidity, water droplets will be separated from the compressed air. The temperature at this time is the "pressure dew point" of the compressed air.

17. What is the relationship between pressure dew point and normal pressure dew point?

The corresponding relationship between pressure dew point and atmospheric dew point is related to the compression ratio. Under the same pressure dew point, the higher the compression ratio, the lower the corresponding atmospheric dew point. For example, when the dew point of 0.7MPa compressed air pressure is 2 ℃, it is equivalent to the dew point of normal pressure is - 23 ℃. When the pressure is increased to 1.0MPa and the dew point of the same pressure is 2 ℃, the corresponding normal pressure dew point drops to - 28 ℃.

18. What instrument is used to measure the dew point of compressed air?

Although the unit of pressure dew point is Celsius (℃), its connotation is the water content of compressed air. Therefore, measuring the dew point is actually measuring the moisture content of the air. There are many instruments for measuring the dew point of compressed air, such as the "mirror dew point meter" using nitrogen, ether, etc. as the cold source, and the "electrolytic hygrometer" using phosphorus pentoxide, lithium chloride, etc. as the electrolyte. At present, a special gas dew point meter is widely used in industry to measure the dew point of compressed air, such as the SHAW dew point meter in Britain, whose measuring range can reach - 80 ℃.

19. What should be paid attention to when measuring the dew point of compressed air with a dew point meter?

Use a dew point meter to measure the air dew point, especially when the moisture content of the air being measured is extremely low, the operation should be very careful and patient. The gas sampling equipment and connecting pipeline must be dry (at least more dry than the measured gas), the pipeline connection should be completely sealed, the gas flow rate should be selected according to the regulations, and the pretreatment time should be long enough. A little carelessness will lead to large errors. Practice has proved that the "micro moisture tester" using phosphorus pentoxide as electrolyte has a large error when measuring the pressure dew point of compressed air treated by the cold dryer. This is because the compressed air will produce secondary electrolysis during the test, which makes the reading higher than the actual value. Therefore, this kind of instrument should not be used when measuring the dew point of compressed air processed by the cold dryer.

20. Where should the pressure dew point of compressed air be measured in the dryer?

Use a dew point meter to measure the pressure dew point of compressed air. The sampling point should be placed in the exhaust pipe of the dryer, and the sample gas should not contain liquid water droplets. Dew points measured at other sampling points have errors.

21. Can evaporation temperature be used instead of pressure dew point?

In the cold dryer, the reading of evaporation temperature (evaporation pressure) cannot be used to replace the pressure dew point of compressed air. This is because in the evaporator with limited heat exchange area, the evaporation temperature of compressed air and refrigerant can not be ignored during the heat exchange process (sometimes up to 4~6 ℃); The temperature that the compressed air can be cooled to is always higher than the refrigerant evaporation temperature. The separation efficiency of the "gas water separator" between the evaporator and the precooler cannot be 100%. There is always a part of small water droplets that cannot be separated completely will enter the precooler with the airflow, where the "secondary evaporation" will be reduced to water vapor, which will increase the water content of the compressed air and the dew point. Therefore, in this case, the measured refrigerant evaporation temperature is always lower than the actual pressure dew point of compressed air.

22. Under what circumstances can temperature measurement be used to replace pressure dew point?

The procedure of intermittent sampling with SHAW dew point meter to measure air pressure dew point in industrial field is rather troublesome, and the test results are often affected due to incomplete test conditions. Therefore, thermometers are often used to approximately measure the pressure dew point of compressed air when the requirements are not very strict.

The theoretical basis for measuring the pressure dew point of compressed air with a thermometer is: if the compressed air entering the precooler through the air-water separator after forced cooling by the evaporator is completely separated from the condensate in the air-water separator, then the measured temperature of the compressed air is its pressure dew point. Although the separation efficiency of the gas water separator is unlikely to reach 100%, the condensate entering the gas water separator and to be discharged through the gas water separator only accounts for a small part of the total condensate when the condensate from the precooler and evaporator is discharged well. Therefore, the error of measuring pressure dew point with this method is not very large.

When using this method to measure the pressure dew point of compressed air, the temperature measuring point should be selected at the end of the evaporator of the dryer or in the air-water separator, because the compressed air temperature is the lowest at this point.

23. What are the drying methods of compressed air?

Compressed air can remove water vapor by pressurizing, cooling, adsorption and other methods, and remove liquid water by heating, filtering, mechanical separation and other methods.

Refrigeration dryer is a kind of equipment that cools the compressed air to remove the water vapor and obtain relatively dry compressed air. The rear cooler of the air compressor also uses cooling to remove the steam contained therein. The adsorption dryer uses the adsorption principle to remove the water vapor contained in the compressed air.

24. What is compressed air? What are the characteristics?

Air is compressible. The air that is reduced in volume and increased in pressure by the mechanical work of the air compressor is called compressed air.

Compressed air is an important power source. Compared with other energy sources, it has the following obvious characteristics: clear and transparent, convenient transmission, no special harmful performance, no pollution or low pollution, low temperature, no fire risk, and no fear of overload. It can work in many adverse environments, and is convenient to obtain.

25. What impurities are contained in the compressed air?

The compressed air discharged from the air compressor contains many impurities: ① water, including water mist, steam and condensate; ② Oil, including oil stain and oil vapor; ③ Various solid substances, such as rust mud, metal powder, rubber fines, tar particles, filter materials, fine fines of sealing materials, and other harmful chemical odor substances.