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Air compressor small class

Air compressor as an important form of energy generation
It is widely used in all aspects of production
Become an indispensable machine for social development
Faced with such commonly used equipment
What should we know?

Basic theoretical knowledge of air

What is air? What is normal air?

The atmosphere around the earth, we are used to calling it air. The air with a pressure of 0.1MPa, a temperature of 20 ° C, and a relative humidity of 36% is normal air. Normal air differs from standard air in that it is warm and contains moisture. When there is water in the air, once the water is separated, the volume of gas will be reduced.

What is the standard state definition of air?
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 processing capacity of air compressor displacement, dryer, filter and other post-processing equipment is marked by the flow rate under the air standard state, and the unit is written as Nm³/min).

What is saturated air and unsaturated air?
At a certain temperature and pressure, the content of water vapor in wet air (i.e. water vapor density) is limited; When the amount of water vapor contained at a certain temperature reaches the maximum possible content, then the wet air is called saturated air. Wet air when water vapor is less than the maximum possible content is called unsaturated air.

Under what conditions does unsaturated air become saturated air? What is “dew”?
The moment the unsaturated air becomes saturated, liquid water droplets in the wet air will condense out, a phenomenon called “condensation”. Condensation is a common phenomenon, such as the high humidity of the air in summer, which is easy to form water droplets on the surface of the water pipe, and water droplets will appear on the Windows of households in the winter morning, which are the results of dew condensation when the wet air is cooled to the dew point temperature at a constant pressure.

What is atmospheric pressure, absolute pressure, surface pressure? What are the common units of pressure?
The pressure exerted on the surface of the Earth or a surface object by a very thick layer of atmosphere enclosing the surface of the Earth is termed “atmospheric pressure”, symbol (RHO) b; The pressure acting directly on the surface of a vessel or object, whose absolute pressure value takes absolute vacuum as its starting point and symbol is Inner A. The pressure measured by a gauge, vacuum gauge, U-tube, etc., is called gauge pressure, starting from atmospheric pressure, symbol (s) g. The relationship among the three is as follows:
Ρ = Ρ b + Ρ g
Pressure is the force of a unit of area, N/ square, denoted by the unit of Inner Mongolia a, called PASCAL. MPa(MPA) commonly used in engineering
1MPa=10 ^ 6 Pa
1 Standard atmospheric pressure =0.1013MPa

What is temperature? What are the common temperature units?
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 at which the gas molecules stop moving, denoted 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”, the unit symbol is ℃.
In addition, British and American countries often use “Fahrenheit temperature”, the unit symbol is F.
The conversion relationship between the three temperature units is:
T (K) = T (℃) + 273.16

T (F) = 32 + 1.8 t (℃)

What is the partial pressure of water vapor in wet air?
Wet air is a mixture of water vapor and dry air. In a given volume of wet air, the amount of water vapor (in terms of 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 the wet air is the sum of the partial pressure of the constituent gases (i.e. dry air and water vapor). The pressure of water vapor in wet air is called partial pressure of water vapor, denoted as Inner so. Its value reflects the amount of water vapor in the wet air, the higher the water vapor content, the higher the partial pressure of water vapor. The partial pressure of water vapor in saturated air (RHO ab).

What is the humidity of the air? How many kinds of humidity?
The physical quantity that indicates the degree of dry and wet air is called humidity. Commonly used humidity expressions are: absolute humidity, relative humidity.
In the standard state, the mass of wet air containing water vapor in a volume of 1m³ is called the “absolute humidity” of wet air and is expressed in g/m³. Absolute humidity only indicates how much water vapor is contained in the unit volume of wet air, but it cannot indicate the ability of wet air to absorb water vapor, that is, it cannot indicate the humidity of wet air. Absolute humidity is the density of water vapor in wet air.
The ratio of the actual amount of water vapor contained in the wet air to the maximum amount of water vapor that can be contained at the same temperature is called “relative humidity” and is commonly expressed by φ. The relative humidity φ is between 0 and 100%. The smaller the φ value, the drier the air, the stronger the water absorption capacity. The hygroscopic capacity of wet air is also related to its temperature. As the temperature of wet air increases, the saturation pressure correspondingly increases, that is, the hygroscopic capacity of wet air increases. Therefore, when installing the air compressor room, pay attention to maintaining ventilation, reducing the temperature, and no indoor drainage and water to reduce the moisture in the air.

What is moisture content? How to calculate moisture content?
In wet air, the mass of 1kg dry air containing water vapor is called the “moisture content” of wet air, which is commonly used. To express, the moisture content ω is almost proportional to the partial pressure of water vapor Pso, and inversely proportional to the total pressure of air p. ω exactly reflects the amount of water vapor contained in the air, such as atmospheric pressure is generally unchanged, when the wet air temperature is certain, Pso is also certain, at this time the relative humidity increases, its moisture content increases, and the moisture absorption capacity decreases.

What does the density of water vapor in saturated air depend on?
There is a limit to the amount of water vapor in the air (water vapor density). In the range of pneumatic 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, and the higher the temperature, the greater the density of saturated water vapor. For example, at 40 ° C, 1 cubic meter of air, regardless of its pressure is 0.1MPa or 1.0MPa, its saturated water vapor density is the same.

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, and it has no special significance for the thermal performance of the whole wet air. Although the content of water vapor in wet air is not large, the change of content has a great effect on the physical properties of wet air. The amount of water vapor determines how dry and humid the air is. The working object of air compressor is wet air.

What is heat?
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 transferred spontaneously 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 transferred in reverse.

What is sensible heat? What is latent heat?
The heat absorbed or emitted by an object in the process of heating or cooling when the temperature rises or falls without changing its original phase state is called sensible heat. If the heat absorbed by the water is raised from 20 ° C to 80 ° C, it is called sensible heat.
In the process of absorbing or releasing heat, the phase state changes, such as: gas becomes liquid, but the temperature does not change, this absorption or release of heat 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 experimentally.
After the saturated air releases heat, a part of the water vapor phase will become liquid water, and the temperature of the saturated air does not drop at this time, and the heat released by this part is latent heat.

What is the enthalpy of air?
The enthalpy of air refers to the total heat contained in the air, usually based on the unit mass of dry air. Enthalpy is denoted by the symbol ι.

What is dew point? What does it relate to?
The temperature of the unsaturated air is called the dew point when the partial pressure of water vapor is kept constant (that is, the absolute water content is kept constant) and the temperature is lowered so that it reaches the saturated state. When the temperature drops to the dew point, condensation droplets are released 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 with high water content is high, and the dew point with low water content is low. At a certain wet air temperature, the higher the dew point temperature, the greater the partial pressure of water vapor in the wet air, and the greater the content of water vapor in the wet 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 and gas mixture will condensation in the oil and gas barrel 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.

What is the pressure dew point?
When wet air is compressed, the density of water vapor increases and the temperature rises. When the compressed air is cooled, the relative humidity increases, and when the temperature continues to drop to the relative humidity of 100%, there are water droplets precipitated from the compressed air, and the temperature is the “pressure dew point” of the compressed air.

What is the relationship between pressure dew point and atmospheric dew point?
The corresponding relationship between pressure dew point and normal pressure dew point is related to compression ratio. In the case of the same pressure dew point, the higher the compression ratio, the lower the corresponding normal pressure dew point. For example, when the pressure dew point of 0.7MPa compressed air is 2 ° C, it is equivalent to the normal pressure dew point of -23 ° C. When the pressure is increased to 1.0MPa, when the same pressure dew point is 2 ° C, the corresponding atmospheric pressure dew point is reduced to -28 ° C.

What instruments are used to measure the dew point of compressed air?
Although the unit of pressure dew point is Celsius (° C), its connotation is the water content of compressed air. So measuring the dew point is actually measuring the water content of the air. There are many instruments for measuring the dew point of compressed air, such as “mirror dew point meter” with nitrogen and ether as cold source, and “electrolytic hygrometer” with phosphorus pentoxide and lithium chloride as electrolytes. At present, the special gas dew point meter is widely used in industry to measure the dew point of compressed air.

What should I pay attention to when measuring the dew point of compressed air with a dew point meter?
When measuring the dew point of the air with a dew point meter, especially when the moisture content of the air being measured is very low, the operation should be very careful and patient. The gas sampling equipment and the connecting pipe must be dry (at least drier than the measured gas), the pipe connection should be completely sealed, the gas flow rate should be selected according to the regulations, and there is a long enough pretreatment time, a little careless, will bring a lot of error. It has been proved by practice that the error of measuring the pressure dew point of the air treated by the cold and dry machine is great when the “micro-moisture meter” using phosphorus pentoxide as electrolyte is used. This is because the compressed air will produce secondary electrolysis during the test, so that the reading value is higher than the actual value. Therefore, such instruments should not be used when measuring the dew point of the compressed air treated by the cold and dry machine.

Where should the pressure dew point of compressed air be measured in the dryer?
The dew-point of compressed air should be measured with a dew-point meter. The sampling point should be placed in the exhaust pipe of the dryer, and the sample gas should not contain liquid water droplets. There is an error in the dew point measured at any other sampling point.

Can the evaporation temperature be used instead of the pressure dew point?
The evaporation temperature (evaporation pressure) reading is not a substitute for the pressure dew point of compressed air in a cold dryer. 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 in the heat exchange process (sometimes up to 4~6℃); The temperature to which the compressed air can be cooled is always higher than the refrigerant evaporation temperature. The separation efficiency of the “gas-water separator” between the evaporator and the precooler can not be 100%, and there is always a part of the small water droplets that are not separated will enter the precooler with the airflow, and there will be “secondary evaporation” to reduce to water vapor, so that the water content of the compressed air increases and the dew point rises. Therefore, in this case, the measured refrigerant evaporation temperature is always lower than the actual pressure dew point of the compressed air.

Under what circumstances can temperature measurements be used instead of pressure dew points?
The theoretical basis for measuring the pressure dew point of compressed air with a thermometer is that if the compressed air is forced to cool by the evaporator and enters the precooler through the gas-water separator, the condensed water with it is completely separated in the gas-water separator, then the temperature of the compressed air measured at this time is its pressure dew point. Although in fact the separation efficiency of the gas-water separator can not reach 100%, in the case of good condensate discharge between the precooler and the evaporator, the condensate that enters the gas-water separator and needs to be removed by the gas-water separator only accounts for a small part of the total condensate water. Therefore, the error of measuring pressure dew point by 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 cold and dry machine or in the gas-water separator, because this point is the lowest temperature of compressed air.

What are the methods of compressed air drying?
Compressed air can be pressurized, cooled, adsorption and other methods to remove the water vapor, and can be heated, filtered, mechanical separation and other methods to remove liquid water.
The refrigerated dryer is a kind of equipment that cools the compressed air to remove the water vapor contained in it and obtain relatively dry compressed air. The rear cooler of the air compressor also uses cooling to remove the water vapor contained therein. Adsorption dryers use the principle of adsorption to remove water vapor contained in compressed air.

What is compressed air? What are the features?
Air has compressibility, and the air after the air compressor does mechanical work to reduce its volume and increase its pressure 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, easy to transport, no special harmful properties, and no pollution or low pollution, temperature is not high, no risk of fire, not afraid of overload, can work in many adverse environments, access to convenient, inexhaustible.

What impurities do compressed air contain?
The compressed air discharged by the air compressor contains many impurities: (1) water, including water mist, water vapor, condensate; ② oil, including oil, oil vapor; ③ All kinds of solid substances, such as rust mud, metal powder, rubber powder, tar particles and filter materials, sealing materials, etc., in addition to a variety of harmful chemical odor substances.

What is an air source system? What parts are they made of?
A system consisting of equipment that produces, processes and stores compressed air is called an air source system. A typical air source system is usually composed of the following parts: air compressor, rear cooler, filter (including pre-filter, oil-water separator, pipeline filter, oil filter, deodorization filter, sterilization filter, etc.), pressure storage tank, dryer (frozen or adsorption), automatic drainage sewerage, gas transmission pipeline, pipeline valves, instruments, etc. The above equipment is combined into a complete air source system according to the different needs of the process flow.

What are the hazards of impurities in compressed air?

The compressed air output from the air compressor contains a large number of harmful impurities, the main impurities are solid particles, water and oil in the air.
Vaporized lubricating oil will form an organic acid corrosion equipment, so that rubber, plastic, sealing material deterioration, blockage of small holes, resulting in valve action failure, pollution products.
The saturated water in the compressed air will condense into water under certain conditions and gather in some parts of the system. These moisture on the components and pipes have a rusting effect, causing the moving parts stuck or wear, so that the pneumatic components failure and leakage; In cold areas, water freezing can cause pipes to freeze or crack.
Impurities such as dust in the compressed air wear the relative moving surfaces in the cylinders, pneumatic motors and pneumatic directional valves, reducing the service life of the system.

Why purify compressed air?
As the hydraulic system has higher requirements for the cleanliness of hydraulic oil, the pneumatic system also has higher quality requirements for compressed air.
The air discharged by the air compressor cannot be used directly by the pneumatic device. The air compressor inhales air containing moisture and dust from the atmosphere, and the temperature of the compressed air is raised to more than 100 ° C, and the lubricating oil in the air compressor is also partly gaseous. In this way, the compressed air discharged by the air compressor is a high-temperature gas containing oil, water and dust. If the compressed air is sent directly to the pneumatic system, due to poor air quality, the reliability and service life of the pneumatic system are greatly reduced, and the resulting loss often greatly exceeds the cost and maintenance costs of the air source treatment device, so the correct selection of the air source treatment system is absolutely necessary.