Compression and delivery of air compressor

Compressor

The compressor can convert the mechanical energy of the motor or internal combustion engine into the pressure of compressed air. Air compressors are divided into two categories: reciprocating and rotary. The following figure shows the basic types of compressors:

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Reciprocating compressor

Single-stage piston compressor

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Compress the inhaled atmospheric pressure air to the required pressure in only one process.

The piston moves down, the volume increases, the pressure in the cylinder is lower than the atmospheric pressure, and the air enters the cylinder from the intake valve.

At the end of the stroke, the piston moves upward, the intake valve is opened, and the output air enters the air storage tank.

This type of compressor is usually used in systems that require a gas range of 3-7 bar.

Two-stage piston compressor

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In a single-stage compressor, if the air pressure exceeds 6 bar, the resulting superheat will greatly reduce the efficiency of the compressor. Therefore, piston compressors used in industry are usually two-stage.

The inhaled atmospheric pressure air is compressed to the final pressure by two-sided three-sword stages.

If the final pressure is 7 bar, the first stage usually compresses it to 3 bar, then it is cooled, and then sent to the second stage cylinder to be compressed to 7 bar.

After the compressed air passes through the intercooler, the temperature drops greatly, and then enters the second-stage cylinder. Therefore, the efficiency is improved compared to a single-stage compressor. The final output temperature may be around 120°C.

Diaphragm compressor

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Diaphragm compression can provide 5 bar compressed air. Because it has no oil at all, it is widely used in instrument medicine and similar industries.

The diaphragm changes the volume of the air chamber, sucking in air during the down stroke and compressing air during the up stroke.

Rotary compressor

Rotary vane compressor

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When the rotor rotates, the centrifugal force causes the blades to contact the inner wall of the stator. From the inlet to the outlet, the space between two adjacent blades is gradually reduced, so the air can be compressed.

The purpose of spraying oil into the airflow near the inlet is for lubrication and sealing. The oil can also take away part of the heat generated by the compressed air and limit the output temperature to about 190°.

Screw compressor

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GA full performance model at full load

Pneumatic system:

A Intake filter

B Air intake valve

C compressor host

D Check valve

E air/oil separator

F small pressure valve

G after cooler

H Water separator with automatic steam trap

GA full performance model at full load

Oil system:

J fuel tank

K thermostatic bypass valve

L oil cooler

M oil filter

N oil return valve

O oil cut valve refrigeration system:

P refrigeration compressor

Q Condenser

R heat exchanger

S Bypass system

T air outlet filter

The two meshed spiral rotors move in opposite directions, and the volume of the free space among them decreases in the axial direction, thereby compressing the air between the two rotors.

The oil is sprayed to lubricate and seal the two rotating screws, and the oil separator separates the oil from the output air.

This type of compressor can continuously output more than 400M³/MIN and pressure up to 10 bar.

Compared with vane compressors, this type of compressor can deliver continuous compressed air without pulsation.

Although screw-type and vane-type compressors have become more and more popular, reciprocating compressors are still the most commonly used in industry.

Scroll compressor

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The scroll compressor is a high-tech compressor developed and introduced in the late 1990s. Because of its simple structure, few parts, high efficiency and good reliability, it is much better than other types in terms of low noise and long life. The compressor has been paid attention and recognized by the compressor industry. Known as the "environmentally friendly compressor".

Due to the unique design of the scroll compressor, it has become the most energy-efficient compressor in the world today.

Because the main moving parts of the scroll compressor are scroll pairs, only running-in and no wear, so the service life is longer, and it is known as a maintenance-free compressor.

Because the scroll compressor runs smoothly, has low vibration and quiet working environment, it is also known as the "super static compressor".

The scroll compressor has few parts and only four moving parts. The compressor working cavity is formed by the phase-moving scroll pair to form multiple mutually closed sickle-shaped working chambers. When the moving scroll moves in translation, the sickle-shaped working chamber is made From large to small, the purpose of compressing and discharging compressed air is achieved.

Characteristic parameters of compressor

The capacity or output of the compressor is expressed by the standard volume flow, the unit is m³/S, Mn³/min, DMn³/S, or L/min. Capacity can also be expressed by displacement or "theoretical input". For piston compressors:

Q (L/MIN) = Piston area (dm²) X stroke (dm) X number of first stage air negative cylinder X speed (rpm)

For a two-stage compressor, only its first-stage cylinder is considered.

Due to volume and heat loss, the output is usually less than the input.

At the end of the compression process, it is impossible to exhaust all the compressed air, so volume loss is inevitable. There is still a certain amount of space after compression, which is called "dead volume".

The heat loss is due to the high temperature during compression, so the volume increases, and when it is cooled to room temperature, its volume decreases. (See Charlie's Law in Chapter 3).

Volumetric efficiency

Ratio Free air output/discharge volume expressed as a percentage is called volumetric efficiency, which varies with the size, model and processing of the compressor, the number of stages and the final pressure. The volumetric efficiency of the two-stage compressor is less than that of the first stage because there is a "dead volume" between the first and second cylinders.

Thermal efficiency and total efficiency

In addition to the aforementioned losses, the effect of heat also reduces the efficiency of compressed air. These losses further reduce the overall efficiency, the extent of which depends on the compression ratio and load. A compressor operating at full capacity accumulates a lot of heat and reduces efficiency. In a two-sided three-knife stage compressor, the compression ratio gradually decreases, and part of the air compressed in the first stage is cooled by an intercooler before the second stage cylinder is pressed to the final pressure.

For example, if the air pocket sucked by the first-stage cylinder is compressed to one-third of its volume, its absolute pressure at the output will reach 3 bar. Relatively speaking, the heat generated due to the small compression ratio is correspondingly low. , The compressed air enters the second-stage cylinder after passing through the intercooler, and then compressed to one-third of its volume, so the final pressure is 9 bar (ABS).

The heat generated by directly compressing air from atmospheric pressure to 9 bar (ABS) in the first-stage compressor is much more than that of the second-stage compressor, and the overall efficiency will also be greatly reduced.

For single-stage compression with lower final pressure, its pure volumetric efficiency is higher. However, as the final pressure gradually increases, heat loss becomes more and more important, and the superiority of the two-stage compressor with higher thermal efficiency is reflected.

"Unit energy consumption" is a measure of total efficiency and can be used to estimate the cost of manufacturing compressed air. On average, 1Kw electric energy produces 120-150l/min (=0.12-0.15M²n/min/kw) and the working pressure is 7 bar of compressed air.

Compressor accessories

Gas tank

The air storage tank is a pressure vessel made of welded steel plates. It is installed horizontally or vertically directly behind the aftercooler to store compressed air. Therefore, the pulsation of the air flow can be reduced.

Its important function is to reserve enough air to meet the requirements beyond the compressor capacity, and to minimize the "full load" and "no load" phenomena that often occur in the compressor. It replenishes and condenses from the aftercooler before further distributing air Therefore, it is best to put the gas tank in a cool place. Such containers should be equipped with safety valves, pressure gauges, drain valves, and manhole covers for easy inspection and cleaning.

The size of the gas storage tank is based on the output of the compressor, the size of the system is based on the output of the compressor, and the size of the system is determined by whether the demand is fixed or variable.

In industry, the compressor is driven by electricity supplied to a network, usually switching between the minimum pressure and the maximum pressure. This kind of control is called "automatic control". This requires an equivalent minimum gas storage tank volume to avoid such frequent switching.

The flow compressor driven by the internal combustion engine does not stop after the air is pressed to the maximum pressure, but the intake air rises so that the air enters the cylinder freely without being compressed. The pressure difference between compression and no-load movement is very small. Smaller gas storage tank.

For factories, the principles for calculating the size of gas storage tanks are:

The capacity of the air storage tank = the output of compressed air per minute by the compressor (not F.A.D)! (FREE. AIR DELIVER).

For example, the compressor outputs a flow rate of 18 mn³/min (free air), and the average pressure is 7 bar. Therefore, the output of compressed air per minute is 18000/7 which is approximately equal to 2500l, that is, an air tank with a volume of 2750l is suitable.

Inlet filter

Typical urban air contains 40 million units/M³ of solid particles, namely dust, sludge, pollen, etc. If this air is compressed to 7 bar, the concentration will reach 320 million units/m³. An important condition for reliable compressor operation is to provide suitable and effective filters to avoid excessive loss of cylinders and piston rings. This loss is mainly caused by the friction of such impurities.

The filter does not need to be too fine, because the efficiency of the compressor decreases as the air resistance increases. Therefore, fine particles (2-5µ) cannot be filtered out.

The intake port should be set as far as possible, clean and dry air flows upward, and the diameter of the intake pipe is large enough to avoid excessive pressure. When applying a muffler, the filter should be placed on its upper end to minimize the pulsation of the air flow.