The most complete version｜Basic theoretical knowledge of compressed air (2)

The most complete version｜Basic theoretical knowledge of compressed air (2)

Six, compression and compression ratio

　　1, compression

　　 Adiabatic compression is a compression process in which gas heat is not significantly transferred or transferred during the compression process. The above process can become a reality in a fully insulated cylinder. Isothermal compression is a compression process in which the gas maintains a constant temperature during the compression process.

　　2, compression ratio: (R)

　　Compression ratio refers to the absolute pressure ratio of the compressor discharge and intake. Example: At sea level, the intake absolute pressure is 0.1 MPa, and the exhaust pressure is 0.8 MPa absolute. The compression ratio:

　　The most complete version｜Basic theoretical knowledge of compressed air (2)

　　The advantages of multi-stage compression:

　　 (1), save compression work;

　　 (2), reduce the exhaust temperature;

　　(3), increase the volume coefficient;

　　(4) For the piston compressor, reduce the thrust of the gas on the piston.

　　 The most complete version｜Basic theoretical knowledge of compressed air (2)

　　 Seven, compressed media

　　 Why use air as the compression medium?

　　Because the air is compressible, clear and transparent, and easy to transport (non-condensing), harmless, safe, and inexhaustible.

　　 Inert gas is a gas that has no chemical effect on the environment. Standard compressors can compress inert gas as well. Both dry nitrogen and carbon dioxide are inert gases.

　　 Nature of air:

　　 Dry air composition:

　　Nitrogen (N2) 78.03%

　　 Oxygen (O2) 20.93%

　　Carbon dioxide (CO2) 0.03%

　　 Molecular weight: 28.96

　　Specific gravity: when 0℃, 760mmHg column, r0=1.2931kg/m3

　　 Specific heat: At 25℃ and 1 atmosphere, Cp=0.241 kcal/kg－℃

　　 At t℃, the pressure is H (mmhg), the specific gravity of air:

　　The specific gravity of humid air should also consider the saturated water vapor partial pressure (0.378ψ, Pb).

　　 pressure

　　1, pressure

　　 This is only the force of a certain unit area, such as the pressure of 1 Newton on a square meter and the unit is 1 Pascal:

　　 means: 1Pa = 1N/㎡

　　1Kpa = 1,000 Pa = 0.01 kg/c㎡

　　1Mpa = 106Pa = 10 kg/ c㎡

　　2, absolute pressure

　　Absolute pressure considers that the ambient atmosphere we live in has an absolute pressure of 0.1Mpa compared to a complete vacuum or absolute zero. At sea level, the absolute pressure can be obtained by adding the gauge pressure to the atmospheric pressure of 0.1 MPa. The higher the altitude, the lower the atmospheric pressure.

　　3, atmospheric pressure

　　 Barometer is used to measure atmospheric pressure. When added to the instrument pressure, the absolute pressure can be obtained.

　　Absolute pressure = pressure gauge pressure + atmospheric pressure

　　Atmospheric pressure is usually in mercury mm, but any pressure unit can be explained equally well:

　　1 physical atmospheric pressure = 760 mmHg = 10.33 m water column = 1.033kgf/c㎡≌0.1MPa.

　　 The relationship between atmospheric pressure and altitude:

　　H=altitude,

　　P0=Atmospheric pressure (0℃, 760mmHg)

　　4, pressure unit conversion:

　　Unit: MPa, Psi (bf/in2)

　　1Psi=0.006895MPa,

　　1bar=0.1MPa,

　　1kgf/c㎡=98.066KPa=0.098066MPa≌0.1Mpa

　　 temperature

　　1, temperature

　　Temperature refers to a method of measuring the energy level of a substance at a certain time. (Or more simply, how hot or cold a certain thing is).

　　The temperature range is based on the freezing point and boiling point of water. On a Celsius thermometer, the freezing point of water is zero degrees and the boiling point is 100 degrees. On a Fahrenheit thermometer, the freezing point of water is 32 degrees and the boiling point is 212 degrees.

　　 Convert from Fahrenheit to Celsius: Fahrenheit = 1.8 Celsius + 32, Celsius = 5/9 (Fahrenheit -32)

　　2, absolute temperature

　　 This is the temperature explained by using absolute zero as the base point.

　　The base point is minus 459.67 degrees Fahrenheit or minus 273.15 degrees Celsius

"Absolute zero" refers to the temperature that exists when all the heat is removed from a substance or the temperature that exists when a certain volume of gas is theoretically reduced to zero.

　　3, cooling temperature difference

　　 Cooling temperature difference is a term that determines the efficiency of the cooler. Because the cooler cannot achieve 100% efficiency, we can only use the cooling temperature difference to measure the efficiency of the cooler.

　　 The cooling temperature difference is the difference between the temperature of the cold water or cold air entering the cooler and the temperature of the compressed air after cooling.

　　4, intercooler

　　 Intercooler is a device used to cool down the temperature of compressed air or gas between stages in a multi-stage compressor. The intercooler reduces the compression power by reducing the temperature of the compressed air entering the next stage to help increase efficiency.

　　 dew point and relative humidity

　　 1. Dew point and relative humidity

　　 Just as the temperature drop at night will produce dew, the temperature drop in the compressed air system will also produce moisture. The dew point is the temperature at which moist air is cooled to saturation while the partial pressure of water vapor remains unchanged.

Why is this? Air containing moisture can only hold a certain amount of moisture. If the volume is reduced by pressure or cooling, there is not enough air to hold all the water, so the excess water will be analyzed as condensed water.

　　The air leaving the aftercooler is usually fully saturated. The condensed water in the separator shows this, so any drop in air temperature will produce condensed water.

　　The set humidity can be considered as the weight of water vapor in the humid air, that is, the ratio of the weight of water vapor to the weight of dry air.

　　Relative humidityψ

　　The most complete version｜Basic theoretical knowledge of compressed air (2)

　　 When Ps=0, ψ=0, it is called dry air;

　　 Ps=Pb, ψ=1, it is called saturated air.

　　Absolute humidity-the weight of water vapor in 1m3 of humid air.

　　The most complete version｜Basic theoretical knowledge of compressed air (2)

　　2, saturated air

　　 When no more water vapor can be contained in the air, the air is saturated, and any pressurization or temperature drop will cause the precipitation of condensed water.

　　3, water vapor separator

　　 water-gas separator is a device used to collect and remove water condensed from air or gas during cooling.

　　Air storage cylinder is a container used to store compressed air and gas discharged from the compressor. The air storage cylinder is beneficial to eliminate the pulse in the exhaust pipe, and can store and supplement the compressed air when the demand is greater than the capacity of the compressor.

　　 4. Dryer

　　Dryer is a device for drying air. In our terminology, it is to use its dry compressed air. The air leaving the aftercooler is usually fully saturated, which means that any temperature drop will produce condensed water. The refrigerated dryer reduces the temperature of the compressed air to remove moisture, and then reheats the air to a temperature close to the original temperature.

　　The regenerative dryer passes air through a filter containing chemical substances to extract moisture. This kind of device can absorb water vapor better than the refrigerated device.

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　　 state and capacity

　　1, standard state

　　The definition of the standard state is: the volume of air provided to the user's system when the air suction pressure is 0.1MPa and the temperature is 15.6°C (the domestic industry defines it as 0°C). If you need to use the standard state to reflect the actual operating conditions, such as altitude, temperature and relative humidity, convert the actual inhalation state to the standard state.

　　2, normal 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.

　　3, inhalation state

　　 The air at the inlet of the compressor.

　　 4. Altitude

　　 Measured vertically upwards at sea level, altitude simply refers to the height above sea level. Altitude occupies an important factor in compressor engineering, because the higher the altitude, the thinner the air becomes and the lower the absolute pressure becomes. Since the air at altitude is relatively thin, the cooling effect of the motor is relatively poor, which makes the standard motor can only operate within a certain altitude. The maximum allowable operating altitude of the EP200 standard unit is 2286 meters.

　　 5. Factors that affect the displacement

　　 Pj, Tj, altitude, n, V, leakage, etc.

　　 6. The influence of altitude on compressor

　　The higher the altitude, the thinner the air, the lower the absolute pressure, the higher the pressure ratio, the greater the Nd;

　　The higher the altitude, the worse the cooling effect, and the greater the motor temperature rise;

　　The higher the altitude, the thinner the air, the greater the fuel-air ratio of the diesel engine, and the smaller the N.

　　7, volume flow

　　Volume flow rate refers to the flow rate of air sucked by the compressor in a standard state per unit time. Use the unit: m3/min (cubic meters per minute). The standard is expressed in Nm3/min.

　　1CFM=0.02832m3/min or 1m3/min=35.311CFM　S--standard state, A--actual state

　　 8. Clearance volume

　　 Clearance volume refers to the volume left at the end of the stroke of a positive displacement volumetric (reciprocating or screw) compressor. The compressed air of this volume returns to the suction port after expansion and has a huge impact on the volume coefficient.

　　9, load factor

　　Load factor refers to the ratio of the average output of the compressor to the maximum rated output of the compressor in a certain period of time. It is not wise to sell the compressor to the user to meet the user's greatest demand. Adding one or more tools or leaks will cause the pressure of the factory to drop. In order to avoid this situation, Ingersoll Rand has been recommending a load factor for many years: take the maximum value of the gas required by the user's system and divide it by a load factor of 0.9 or 0.8. (Or any user thinks it is a safety factor)

　　This kind of comprehensive air volume selection can take into account the unexpected increase in air demand. Some small expansions can be made without additional capital investment.

　　 10. Air volume test

　　 (1), reciprocating compressor cylinder volume

　　The volume of the compressor cylinder refers to the volume of piston movement minus the volume occupied by the piston rod. It is usually expressed in cubic meters per minute. The volume of the multi-stage compressor is only the volume compressed by the first stage, because the gas that passes through all stages one by one comes from the first stage.

　　 (2), test

　　 Low pressure nozzle test is a method to accurately measure the air supplied by the compressor. This method is recognized by the Compressed Air and Gas Society and also accepted by the ASME Energy Test Code Committee.

　　 ASME PTC-9 describes the use of low pressure nozzles to test reciprocating compressors. ASME PTC-10 has a description of using low pressure nozzles to test power compressors.

　　The most complete version｜Basic theoretical knowledge of compressed air (2)

　　 power and specific power (energy consumption ratio, volume specific energy)

　　 1. Compressor efficiency

　　The volumetric efficiency is the ratio of the actual volume of the compressor to the theoretical volume, expressed as a percentage.

　　Compression efficiency is the ratio of the actual power required to compress a given amount of gas to the theoretical power. The theoretical power can be calculated according to isothermal conditions or adiabatic conditions. The corresponding compression efficiency can be determined and expressed as a percentage. For steam-driven or internal combustion engine-driven compressors, mechanical efficiency refers to the ratio of the indicated power component horsepower of the compressor to the braking component horsepower on the shaft. For motor-driven compressors, mechanical efficiency refers to the ratio of the indicated power in the compression cylinder to the shaft power of the compressor. Expressed as a percentage.

　　2, overall efficiency

　　 The overall efficiency is the sum of the compressor's compression efficiency and mechanical efficiency.

　　 Compressor shaft power (brake power) includes: gas compression work-indicating work, friction work

　　Rough calculation: Nad=1.634PjVm(k/k-1)[ε(k-1/k)-1] Kw

　　N motor=N axis/η transmission, η transmission (belt: 0.92～0.98, gear: 0.97～0.99)

In the screw compressor, the shaft power of the air-cooled compressor must be added to the power of the fan motor.

　　3, volume specific energy

　　 Volume specific energy refers to the power consumed by the compressor in a unit time of sucking a unit air volume, usually expressed in Kw/m3/min, the smaller the volume specific energy under the same discharge pressure. That is, less power consumption. The compressor efficiency is a measure of the true efficiency of the compressor.

　　 Specific power: Specified working conditions:

　　Pj=1bar(A), tj=20℃, ψ=0, twater=15℃

　　Pc=7bar (meter), water volume≤2.5L/m3

　　 Power is the work done per unit time, such as horsepower (kilowatt) is set at 76Kg-m/hour

　　 Power is an index measured in the conversion of energy.

　　 In order to get the cost of power, we must also include time. For example, the cost of money is not kilowatts but kilowatt hours.

　　 To take horsepower and convert it into user-consuming costs, we need to use the following formula:

　　 Noise and sound evaluation

　　 Noise is considered annoying or disturbing.

　　 The user is completely willing to sit in the discotheque all night, smoking and enjoying disco music up to 95 decibels, but it is incredible that he can not tolerate the 65 decibels of the copier noise the next morning. Users like the noise of disco but not the noise of copier. A typical cocktail party has a noise level of 90 decibels, and a rock band’s noise is between 100 and 138 decibels.

So what is decibel?

The definition of 　　 decibel can be interpreted as the logarithm (base 10) of the ratio of the two energies and multiply by 10.

　　 An increase of 10 decibels means an energy increase of 10:1, an increase of 20 decibels means an energy increase of 100:1, and an increase of 30 decibels means an increase of 1000:1.

　　 For our application, we are discussing the sound power level-set the reference value of W1 to 10-12, the formula becomes:

　　 For example, if we have a sound source that emits a power level of 10-5 watts, the sound power is:

　　 When the ears face the noise, people find that the ears automatically "cannot hear" low-frequency noise, which is very similar to the "A" class network below.

　　 For this reason, the standard for the measurement of industrial noise is the "A" noise level, and the term dBA is used.

　　 Since the reflected noise can be easily captured by the test probe, another standard is set. The standard requires all noise measurements to be made under "open field conditions".

　　 The ANSIS 51 rule for measuring the sound of gas equipment states that the noise should be measured at a height of 1.5 meters and one meter away from the machine.

　　 Therefore, here we determined the location of the test probe and the measurement location and measured the noise with an "A" network.

　　 All manufacturers use these same basic rules to measure noise.

　　 If two machines with the same noise level are operated side by side, the result of the noise level will increase by 3dBA (twice)

　　 For example: in our original formula:

　　If we double our sound power level to 2×10-5

　　 A compressor manufacturer’s statement: The noise level guarantee is +3dBA means that its noise level will be twice or one half of the declared noise level.

　　 Two units operating at different speeds may have the same noise level, but they sound completely different. One may be harsher than the other. This is because the noise is based on the sum of all the frequencies in the spectrum to form a number to form dBA.

　　 is the measurement of the noise level, the noise of each scale band is measured, and the number of answers (dBA) is obtained by inverse evaluation with "A" and comparison and addition.

　　 What does all this mean:

　　 1. This means that we cannot place a compressor in a room due to reflections and expect to have the same noise level as in the open field conditions.

　　2. We cannot make a clever guess about the noise level with two different units (with different speeds, different drives, different components and different enclosures). The only way to measure noise is to use a sound measuring device.

　　The most complete version｜Basic theoretical knowledge of compressed air (2)

　　 8. How do we overcome obvious differences in noise levels?

　　 1. Through accurate measurement of noise level

　　2. By knowing how the noise level is composed, sensibly specify the frequency difference and harshness factors.

　　3. Knowing that two units have the same noise level, but the noise of the unit with different frequency characteristics causes the same damage to the ears, even if one of them does "sound" lighter.

　　 Nine, how can we further reduce noise?

　　1. Ensure that all joints in the machine body are safe, the forklift holes are closed, and the base body of the machine body on the ground is solidly sealed.

　　2. The intake and exhaust are delivered through pipes.

　　 3. Reduce reflected noise.

　　 Sound and noise measurement is at best a very imprecise science. The discussion on this topic hopes to avoid field problems, the large cost of field corrections and user dissatisfaction.

　　 1. All noise level measurement uses ANSLS51 standard. This is an industry standard. We should use this standard to refer to the noise level used for reference. In short, the standard requires open field measurements (without reflecting walls and roofs), multi-point measurements around the unit, and an average of the measured values. It should be measured one meter away from the unit, 1.5 meters above the ground and foundation level. Any single-point measurement can reach the quoted A noise level. As long as the average reading can meet or fall below the reference level. In addition, the measurement used is the average value of the measured noise that should be broadband. When frequency band analysis is required or given, some mid-band readings can, and usually do, are higher than the noise measurement average. Again, this is what the standard accepts.

　　2. There is no tolerance given and applicable in the standard structure

　　3. Units that have not been placed in the field should be placed in the open field. In fact, the noise level in regular locations is always higher because of reflections from nearby walls and/or roofs and nearby equipment. 　　4. The noise level data that may be submitted is obtained by using actual measurements when the measurement should be made for a specific compressor, and repeated operation of a compressor of the same type under the same conditions can be interpreted as a typical noise level.

　　 Note:

　　 For any multi-point measurement or repeated compressor measurement, there is a certain error link. These errors indicate that in order to ensure that the noise level is a problem for a particular compressor, 3 decibels should be added to the total dBA measurement. When guaranteeing a user's noise level, it is absolutely necessary to comply with the above requirements.