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 air. At a certain altitude, the composition and proportion of dry air are basically stable. It has no special significance for the heat supply performance of the whole wet air. Although the content of water vapor in the wet air is small, the change of the content has a great impact on the physical properties of the 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 unit: KJ/(kg. ℃)  cal/(kg. ℃), etc. 1kcal=4.186kj，1kj=0.24kcal。
According to the laws of thermodynamics, heat can transfer 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 reverse direction.
13. What is sensible heat? What is latent heat?
In the process of heating or cooling, the heat needed to be absorbed or released by an object when its temperature rises or decreases without changing its original phase state is called sensible heat. It can make people feel obvious cold and hot changes, which can usually be measured by thermometer. If water is raised from 20 ℃ to 80 ℃ to absorb heat, it is called sensible heat.
In the process of absorbing or releasing heat, the phase state of the object changes (such as gas changing into liquid), but the temperature does not change. The heat absorbed or released is called latent heat. Latent heat cannot be measured by thermometer, nor can human body feel it, but it can be calculated by experiment.
After the saturated air releases heat, part of the water vapor will change into liquid water, and the temperature of the saturated air does not drop at this time. The heat released from this part is latent heat.
14. What is the enthalpy of air?
The enthalpy value of air refers to the total heat contained in air, which is usually based on the unit mass of dry air. Enthalpy is expressed by the symbol l.
15. What is dew point? What is it related to?
The temperature of unsaturated air is called dew point when the temperature is reduced to reach saturation state under the condition that the partial pressure of water vapor is kept constant (that is, the absolute water content is kept constant). When the temperature drops to the dew point, there will be condensation droplets in the wet air.
The dew point of wet air is not only related to temperature, but also to the moisture content in the wet air. The dew point is high with high water content and low with low water content. At a certain wet air temperature, the higher the dew point temperature, the higher the partial pressure of water vapor in the wet air, and the greater the content of water vapor in the wet air. Dew point temperature has important applications in compressor engineering. For example, when designing the outlet temperature of air compressor, it must be ensured that the dew point temperature is not lower than the corresponding partial pressure.
16. What is the pressure dew point?
After the wet air is compressed, the water vapor density 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 precipitate from the compressed air. At this time, the temperature is the pressure dew point of the compressed air.
17. What is the relationship between pressure dew point and atmospheric 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 pressure ratio, the lower the corresponding atmospheric dew point. For example, when the pressure dew point of 0.7Mpa compressed air is 2 ℃, it is equivalent to the atmospheric dew point of - 23 ℃. When the pressure is increased to 1.0Mpa, the same pressure dew point is 2 ℃, and the corresponding atmospheric dew point is - 28 ℃.
18. What instrument is used to measure the dew point of compressed air?
Use the dew point meter to measure the air dew point, especially when the water content of the measured air is very low, the operation should be very careful and patient. The gas equipment and the connecting pipeline must be dry (at least 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 bring great errors. Practice has proved that there is a big error when measuring the pressure dew point of the compressed air processed by the cold dryer with a micro-moisture tester with phosphorus pentoxide as electrolyte. This is because compressed air will produce secondary electrolysis during the test, which makes the reading value higher than the actual value. Therefore, such instruments 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. There are errors in the dew points measured at other sampling points.
21. Can the evaporation temperature be used to replace the 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 degrees Celsius); The temperature that compressed air can be cooled to is always higher than the evaporation temperature of refrigerant. The separation efficiency of the steam-water separator between the evaporator and the precooler cannot be 100%. There are always some small water droplets that cannot be separated completely into the precooling with the air flow. The measured refrigerant evaporation temperature is always lower than the actual pressure dew point of the compressed air.
22.。 Under what circumstances can temperature measurement be used instead of pressure dew point?
The procedure of measuring the air pressure dew point with the gap sampling of the SHAW dew point meter in the industrial field is quite troublesome, and the test results are often affected by the incomplete test conditions. Therefore, in places where the requirements are not very strict, a thermometer is often used to approximate the pressure dew point of compressed air.
The theoretical basis for measuring the pressure dew point of compressed air with a thermometer is: if the compressed air enters the precooler through the steam-water separator after being forced cooled by the evaporator, the condensed water contained therein is completely separated in the gas-water separator, and wonder that the measured compressed air is its pressure dew point at this time. Although the separation efficiency of the gas-water separator cannot reach 100% in fact, under the condition that the condensate from the precooler and evaporator is discharged well, the condensate that enters the gas-water separator and needs to be discharged through the gas-water separator only accounts for a small part of the total condensate. 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 measuring point should be selected at the end of the evaporator of the cold dryer or in the gas-water separator, because this kind of compressed air has the lowest temperature.
23. What are the drying methods of compressed air?
Compressed air can remove the water vapor through pressurization, cooling, adsorption and other methods, and remove the liquid moisture through heating, filtration, mechanical separation and other methods.
Freeze-type dryer is a device that cools the compressed air to remove the water vapor in it and obtain relatively dry compressed air. The rear cooler of the air compressor also uses cooling to remove the water vapor in it. The adsorption dryer uses the adsorption principle to remove the water vapor in the compressed air.
24. What is compressed air? What are the characteristics?
The air is compressible, and the air after its volume is reduced and the pressure is increased 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 transportation, no special harmful performance, and no pollution or low pollution, low temperature, no fire hazard, not afraid of overload, can work in many adverse environments, easy access, and inexhaustible.
25. What impurities are contained in the compressed air?
The compressed air discharged by the air compressor contains many impurities: 1. Water, including water mist, water vapor and condensate; 2. Oil, including oil dirt and oil vapor; 3. Various solid substances, such as rust mud, metal powder, rubber fines, tar particles and fine powder of sealing materials, and other harmful chemical odor substances.
26. What is the air source system? What are the components?
The system composed of equipment for producing, processing and storing compressed air is called air source system. A typical air source system is usually composed of the following parts: air compressor, rear cooler, filter (including prefilter, oil-water separator, pipeline filter, oil removal filter, deodorization filter, sterilization filter, etc.), stabilized pressure air storage tank, dryer (refrigeration or adsorption) automatic drainage and blowdown device, gas transmission pipeline, pipeline valve, instrument, etc. The above equipment is combined into a complete air source system according to the different needs of the process flow.
27. What are the hazards of impurities in compressed air?
The compressed air from the air compressor contains a large number of harmful impurities, the main impurities are solid particles, water and oil in the air.
The vaporized lubricating oil will form a kind of organic acid corrosion equipment, which is caused by the deterioration of rubber, plastic and sealing materials, blocking the small hole, causing the valve action failure and polluting the product.
The saturated water in the compressed air will condense into water under certain conditions and gather in some parts of the system. These moisture will corrode the components and pipes, cause the moving parts to be stuck or worn, and make the pneumatic components fail or leak; In cold areas. The freezing of water will cause the pipeline to freeze or crack.
The dust and other impurities in the compressed air will wear the relative moving surface in the cylinder, pneumatic motor and pneumatic reversing valve, and reduce the service life of the system.
28. Why should compressed air be purified?
As the hydraulic system has higher requirements for the cleanliness of hydraulic oil, the pneumatic system also has higher requirements for the quality of compressed air.
The air discharged by the air compressor cannot be directly used by the pneumatic device. The air compressor inhales the air containing water and dust from the atmosphere, and the temperature of the compressed air increases to more than 100 ℃, at this time, the lubricating oil in the air compressor also partially becomes gaseous. In this way, the compressed air discharged by the air compressor is a high-temperature gas containing oil, water and dust. If this compressed gas is sent directly to the pneumatic system, the reliability and service life of the pneumatic system will be greatly reduced due to poor air quality, and the resulting losses often exceed the cost of gas source disposal and maintenance costs, so it is absolutely necessary to select the correct gas source treatment system.
29. What are the standards for air source quality?
Different users have different requirements for air source quality. GB/T13277-1991 (Quality grade of compressed air for general use) is formulated for this purpose. This standard specifies the amount of solid particles, water and oil in compressed air.
In a broader sense, a good gas source should also have stable pressure in the process of use and minimal pollution to the surrounding environment.
The expression method of compressed air quality grade. The quality grade of compressed air is expressed in three Arabic numerals. If there is no requirement for one of the pollution levels, use one instead.