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
When the free air output/discharge volume is expressed as a percentage, it 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 above losses, the influence 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. Compressors working at full capacity accumulate a lot of heat and reduce 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 gap 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).
Compressing air directly from atmospheric pressure to 9 bar (ABS) in the one-stage compressor generates much more heat than the two-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 storage tank
The air 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, it can reduce the pulsation of the air flow.
Its important function is to reserve enough air to meet the requirements of exceeding the capacity of the compressor, 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 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 suction 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 tank = the output of compressed air per minute by the compressor (not F.A.D)!
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 the reliable operation of the compressor is to provide a suitable and effective filter to avoid excessive loss of the cylinder and piston ring, which 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 suction 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.