The basic theoretical knowledge of air that everyone who uses an air compressor should understand

The basic theoretical knowledge of air that everyone who uses an air compressor should understand

1 What is air? What is normal air?

　　 Answer: The atmosphere around the earth, we are accustomed 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 is different from standard air in temperature and contains moisture. When there is moisture in the air, once the moisture is separated, the air volume will be reduced.

　　2 What is the standard definition of air?

　　Answer: The definition of the standard state is: when the air suction pressure is 0.1MPa and the temperature is 15.6℃ (the domestic industry defines it as 0℃), the air state is called the standard state of air.

In the standard state, the air density is 1.185kg/m3 (the processing capacity of post-processing equipment such as air compressors, dryers, filters, etc. are all marked with the flow rate under 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, there is a certain limit to the water vapor content (ie, water vapor density) in the moist air; when the amount of water vapor at a certain temperature reaches z*maximum possible content, then The humid air is called saturated air. Moist air when the water vapor does not reach the maximum possible content of z* 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 drops of water will condense in the moist air. This phenomenon is called "condensation". Condensation is common. For example, the air humidity in summer is very high, and it is easy to form water drops on the surface of the water pipe. In winter morning, water droplets will appear on the glass windows of residents. These are all humid air cooled under constant pressure to reach the dew point. The result of temperature and condensation.

　　5 What is atmospheric pressure, absolute pressure, gauge pressure? What are the common pressure units?

Answer: The pressure caused by the thick atmosphere surrounding the surface of the earth on the surface of the earth or objects on the surface is called "atmospheric pressure", and the symbol is PB; the pressure directly acting on the surface of the container or object is called "absolute pressure". The pressure value takes absolute vacuum as the starting point and the symbol is Pa; the pressure measured by pressure gauges, vacuum gauges, U-shaped tubes and other instruments is called "gauge pressure", and the "gauge pressure" starts from atmospheric pressure and the symbol is Pg. The relationship between the three is

　　 Ρa=Ρb+Ρg

　　 Pressure refers to the force per unit area, the unit of pressure is N/square, denoted as Pa, called Pascal. Commonly used in engineering MPa (Mega Pa)

　　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 unit system, the pressure is usually kgf/cm2 (kilogram force/square centimeter) as the unit.

　　6 What is temperature? What are the commonly used temperature units?

　　Answer: Temperature is the statistical average of the thermal motion of material molecules.

　　Absolute temperature: The temperature starting from the z*lower limit temperature when the 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(℃)+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, water vapor takes up much less weight (by mass) than dry air, but it occupies the same volume as dry air. , Also has the same temperature. The pressure of wet air is the sum of the partial pressures of the constituent gases (ie, dry air and water vapor). The pressure of water vapor in the humid air is called the partial pressure of water vapor and is 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, and is recorded as Pab.

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

　　Answer: The physical quantity indicating the degree of air dryness and humidity is called humidity. Commonly used humidity expressions include absolute humidity and relative humidity.

　　 In the standard state, the mass of water vapor contained in the humid air in a volume of 1m3 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 cannot indicate the ability of the humid air to absorb water vapor, that is, it cannot indicate the degree of humidity of the humid air. Absolute humidity is the density of water vapor in humid air.

　　The ratio of the amount of water vapor actually contained in the humid air to the amount of water vapor likely to be contained at the same temperature z* is called "relative humidity" and 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 more humid the air and the weaker the water absorption capacity. The moisture absorption capacity of moist air is also related to its temperature. As the temperature of moist air rises, the saturation pressure increases accordingly. If the content of water vapor does not change, the relative humidity φ of moist air will decrease, that is, the moisture absorption capacity of moist air. increase. Therefore, during the installation of the air compressor room, attention should be paid to maintaining ventilation, lowering the temperature, and no drainage or accumulation of water in the room to reduce the moisture in the air.

　　9What is moisture content? How to calculate the moisture content?

　　 Answer: In moist air, the mass of water vapor contained in 1kg of dry air is called the "moisture content" of moist air, and it is commonly used. To show that the moisture content ω is almost proportional to the partial pressure of water vapor Pso, and inversely proportional to the total air pressure p. ω accurately 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, the 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: There is a limit to the amount of water vapor in the air (water vapor density). 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 will have the same saturated water vapor density regardless of whether its pressure is 0.1 MPa or 1.0 MPa.

　　11 What is humid air?

　　 Answer: Air containing a certain amount of water vapor is called moist air, and air without water vapor is called dry air. The air around us is humid air. At a certain altitude, the composition and proportion of dry air remain basically stable, and it has no special significance to the thermal performance of the entire wet air. Although the water vapor content in humid air is not large, the change of content has a great influence on the physical properties of humid air. The water vapor content determines the dryness and humidity of the air. The working object of an air compressor is humid air.

　　12 What is calories?

　　 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 law 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: The amount of heat that an object needs to absorb or release when its temperature increases or decreases without changing its original phase state during heating or cooling is called sensible heat. It can make people feel 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.

　　In the process of absorbing or releasing heat, the phase state of the object changes (such as gas becomes liquid...), but the temperature does not change. This kind of 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 through experiments.

　　 After the saturated air emits heat, part of the water vapor will phase into liquid water, and the temperature of the saturated air does not decrease at this time. The heat released by this part is latent heat.

　　14 What is the enthalpy of air?

　　 Answer: The enthalpy value 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: Under the condition of keeping the partial pressure of water vapor constant (that is, keeping the absolute water content constant), the temperature of unsaturated air is reduced, and the temperature when it reaches the saturated state is called the dew point. When the temperature drops to the dew point, condensed water droplets will precipitate in the humid air.

　　 The dew point of humid air is not only related to temperature, but also to the amount of moisture in the humid air. The dew point with high water content is high, and the dew point with low water content is low. 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 content of water vapor in the humid air. Dew point temperature has important uses in compressor engineering. For example, when the outlet temperature of the air compressor is too low, the oil and gas barrel will cause condensation of the oil and gas mixture due to the low temperature, 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.

　　16 What is the pressure dew point?

　　 Answer: After the humid air is compressed, the water vapor density increases and the temperature also rises. When the compressed air is cooled, the relative humidity increases. When the temperature continues to drop to a relative humidity of 100%, water droplets are precipitated 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 atmospheric dew point?

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

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

　　 Answer: Although the unit of pressure dew point is degrees 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 and ether as the cold source, and the "electrolytic hygrometer" using phosphorus pentoxide and lithium chloride as the electrolyte. At present, the industry generally uses a dedicated gas dew point meter to measure the dew point of compressed air, such as the British SHAW dew point meter, the measuring range of the instrument can reach -80 ℃.

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

　　 Answer: Use a dew point meter to measure the air dew point, especially when the measured air moisture content is extremely low, the operation must be very careful and patient. The gas sampling equipment and the connecting pipeline must be dry (at least drier than the measured gas), the pipeline connection should be completely sealed, the gas flow rate should be selected according to regulations, and a sufficient pretreatment time is required. Being cautious will bring great errors. Practice has proved that when using the "moisture analyzer" with phosphorus pentoxide as the electrolyte to measure the pressure dew point of the compressed air processed by the cold dryer, the error is very large. This is because the compressed air will produce secondary electrolysis during the test, making the reading higher than the actual value. Therefore, this type of instrument should not be used when measuring the dew point of the compressed air processed by the refrigeration dryer.

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

　　 Answer: 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 must not contain liquid water droplets. The dew points measured at other sampling points have errors.

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

　　 Answer: In the refrigeration dryer, the reading of the evaporation temperature (evaporation pressure) cannot be used to replace the pressure dew point of the compressed air. This is because in the evaporator with limited heat exchange area, there is a non-negligible temperature difference between the compressed air and the refrigerant evaporation temperature during the heat exchange process (sometimes up to 4~6℃); the compressed air can be cooled to a higher temperature than the refrigerant The evaporation temperature is high. The separation efficiency of the "gas-water separator" between the evaporator and the pre-cooler cannot be 100%. There will always be a part of the infinitely separated small water droplets that enter the pre-cooler with the airflow and "secondary evaporation" there. It is reduced to water vapor to increase the water content of the compressed air and the dew point. Therefore, in this case, the measured evaporation temperature of the refrigerant is always lower than the actual pressure dew point of the compressed air.

　　22 Under what circumstances can the method of measuring temperature be used instead of pressure dew point?

　　Answer: The procedure of intermittent sampling and measurement of air pressure dew point with SHAW dew point meter in industrial field is quite troublesome, and the test results are often affected by incomplete test conditions. Therefore, in occasions where the requirements are not very strict, a thermometer is often used to approximate the pressure dew point of the compressed air.

The theoretical basis for measuring the pressure dew point of compressed air with a thermometer is: if the compressed air that enters the pre-cooler through the gas-water separator after being forcedly cooled by the evaporator, the condensate contained therein is completely separated in the gas-water separator, then at this time The measured compressed air temperature is its pressure dew point. Although in fact the separation efficiency of the gas-water separator cannot reach 100%, the condensate that enters the gas-water separator and needs to be removed through the gas-water separator only accounts for when the condensate from the precooler and evaporator is discharged well. A small part of the total amount of condensate. 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 measurement point should be selected at the end of the evaporator of the refrigeration dryer or in the air-water separator, because the compressed air temperature z* is low at this point.

　　23 What are the compressed air drying methods?

　　Answer: Compressed air can remove water vapor from it by pressurization, cooling, adsorption and other methods, and can remove liquid water by heating, filtering, mechanical separation and other methods.

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

　　24 What is compressed air? What are the characteristics?

　　Answer: Air is compressible. The air compressor performs mechanical work to reduce its volume and increase its pressure.