The equipment for conveying and compressing gases is collectively known as gas pressurizing and conveying machinery, and its role is quite similar to that of liquid conveying equipment, both of which transfer energy to the fluid to make the fluid flow.
Gas pressure conveying machinery can be categorized according to its exit gas pressure or compression ratio. The pressure of the outlet gas of the press-feeding machinery is also known as the final pressure. The compression ratio is the ratio of the absolute pressure of the outlet and inlet gases of the pressurized conveying machinery. According to the final pressure roughly will be sent to the pressure machinery is divided into:
Ventilator final pressure is not greater than 15kPa (1500mm H20);
Blower final pressure of 0.015 ~ 0.3MPa (0.15 ~ 3kgf/cm2), the compression ratio of less than 4;
Compressor final pressure in the 0.3MPa (3kgf/cm2) more than the compression ratio is greater than 4;
Compressor final pressure in the 0.3MPa (3kgf/cm2), the compression ratio of greater than 4;
Vacuum pumps pump gases below atmospheric pressure from containers or equipment to the atmosphere.
In addition, according to its structure and working principle of the pressure and delivery machinery can be divided into centrifugal, reciprocating, rotary and fluid action type.
One, centrifugal ventilator, blower and centrifugal compressor
Centrifugal ventilator, blower and centrifugal compressor working principle and centrifugal pumps are similar to rely on the rotary movement of the impeller, so that the gas to obtain the energy to increase the pressure. Ventilators are usually single-stage, the resulting gauge pressure is less than 15kPa (1500mm H2O), the role of gas transportation. Blowers have single-stage and multi-stage, the generated gauge pressure is less than 3kgf/cm2, turbines are multi-stage, the generated gauge pressure is higher than 3kgf/cm2, the gas has a more significant compression effect.
(A) centrifugal ventilator
Centrifugal ventilator according to the wind pressure produced, can be divided into:
Low pressure centrifugal ventilator outlet wind pressure is lower than 1kPa (100mm H2O);
Medium pressure centrifugal ventilator outlet wind pressure of 1 to 3kPa (100-300mm H2O);
High pressure centrifugal ventilator The outlet wind pressure is 3～15kPa (300～1500mm H2O).
1. Centrifugal ventilator structure
Figure 2-21 shows the low-pressure centrifugal ventilator. Centrifugal fan structure and single-stage centrifugal pump similar. Its casing section has two kinds of square and round. Centrifugal ventilators have more blades than centrifugal pumps, and are not limited to backward curved blades, but also forward curved blades. In the medium and low pressure centrifugal ventilator, the main reason for the use of forward curved blades is due to the requirements of the pressure is not high. Before the curved blade is conducive to improving wind speed, thereby reducing the cross-sectional area of the ventilator, and thus the size of the equipment can be smaller than the back of the curved. However, when using forward curved blades, the efficiency of the fan is low and the energy loss is large.
Figure 2-21 Centrifugal ventilator
1-case; 2-impeller; 3-insertion; 4-exhaust outlet
2. Centrifugal ventilator performance parameters and characteristic curves
Centrifugal ventilator's main performance parameters are the volume of wind, air pressure, axial power and efficiency. Because of the gas through the fan pressure change is small, in the fan movement of the gas can be regarded as incompressible, so the centrifugal pump basic equation can also be used to analyze the performance of centrifugal ventilator.
(1) air volume air volume is the volume of gas discharged from the fan outlet in a unit of time, and in the state of the gas at the inlet of the fan, expressed in Q, the unit is m3/h.
(2) wind pressure wind pressure is a unit of volume of gas flow through the fan when the energy obtained, expressed in ht, the unit of J/m3 = N/m2. ht unit and the unit of the pressure is the same unit, so it is called wind pressure. Called wind pressure. Since it is the unit of pressure, usually with mmH2O to express.
The wind pressure of centrifugal fan depends on the structure of the fan, impeller size, speed and the density of the gas entering the fan.
Currently can not use theoretical methods to accurately calculate the wind pressure of centrifugal fan, but by experimental determination. Generally through the measurement of fan inlet and outlet gas flow rate and pressure data, according to Bernoulli equation to calculate the wind pressure.
Centrifugal fan on the gas to provide effective energy, often 1m3 gas as a benchmark. Set fan imports for the section 1-1′, exports for the section 2-2′, according to the unit volume of fluid as the basis for the Bernoulli equation can be obtained from the centrifugal fan wind pressure:
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In the formula ρ and (z2-z1) values are relatively small, (z2-z1) ρg can be ignored; fan inlet and outlet between the pipe is very short, ρ ∑hf1-2 can be ignored; and when the fan inlet and outlet of the pipe is very short, ρ ∑ hf1-2 can be ignored; and when the fan imports and the effective energy provided by 1m3 gas as a benchmark. Neglected; and when the fan inlet and the atmosphere directly connected, and section 1-1′ is located in the fan inlet outside, then v1 can be ignored, so the above formula can be simplified to:
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The above formula (p2-p1) is called static wind pressure, expressed as hpt. Known as dynamic wind pressure. Centrifugal ventilation and the exit of the gas flow rate is large, so the dynamic wind pressure can not be ignored, according to the above experimental device, the centrifugal ventilator wind pressure for the static wind pressure and dynamic wind pressure and the sum of the wind pressure, also known as the full wind pressure. Ventilator performance parameters listed on the wind pressure is the full wind pressure.
(3) shaft power and efficiency centrifugal ventilator shaft power is:
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Sequence N-axis power (kW);
Q-wind volume (m3/s);
Wind volume (m2) )
ht - wind pressure (Nm/m3);
η - efficiency, because of the full wind pressure is determined, it is also known as the full pressure efficiency.
The fan's shaft power is related to the density of the gas being transported, fan performance parameters listed on the table of the shaft power are experimental conditions, that is, the density of air is 1.2kg/m3 when the value, if the transported density of the gas is different, can be converted according to the following formula, that is:
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The formula N′ - shaft power (kW) when the gas density is ρ′;
N - shaft power (kW) when the gas density is 1.2kg/m3.
Centrifugal ventilator characteristic curve, shown in Figure 2-22. Indicates a certain type of ventilator at a certain speed, the air volume Q and wind pressure ht, static wind pressure hpt, shaft power, efficiency η relationship between the four.
Figure 2-22 Centrifugal ventilator characteristic curve diagram
3. Centrifugal ventilator selection
Centrifugal ventilator selection and centrifugal pumps are similar to the case of the selection of the steps are:
(1) according to Bernoulli's equation, calculated according to the transmission system of the wind pressure ht.
(2) according to the nature of the gas conveyed (eg. clean air, flammable, explosive or corrosive gases and dusty gases, etc.) and the range of wind pressure, determine the type of fan. If the conveyor is clean air, or air with similar properties of the gas, you can use the general type of centrifugal ventilator, commonly used 4-72 type, 8-18 type and 9-27 type. The former type belongs to low-pressure ventilator, the latter two types belong to high-pressure ventilator.
(3) according to the actual air volume Q (to the fan inlet state) and the experimental conditions of the wind pressure ht, from the fan samples or product catalogs in the characteristics of the curve or performance table to choose the right number, the same selection principles and centrifugal pumps, will not be described in detail.
Each type of centrifugal ventilator and a variety of different diameters of the impeller, so the centrifugal ventilator model is in the type and then add the machine number, such as 4-72No. 12. 4-72 indicates that the type, No. 12 represents the number of the machine, of which 12 indicates that the diameter of the impeller is 12cm.
(4) if the density of the gas conveyed is greater than 1.2kg/m, the need to calculate axial power in accordance with the formula ( 2-19) to calculate the shaft power.
Table 2-4 shows the performance and use of some domestic fans.
(B) centrifugal blower and centrifugal compressor
Centrifugal blower, also known as the turbine blower, the same principle of operation and centrifugal ventilator, can be a single-stage or multi-stage, multi-stage structure is similar to the multi-stage centrifugal pump. Figure 2-23 shows the schematic diagram of a five-stage centrifugal blower. The gas enters from the suction port, passes through the impeller and guide wheel of the first stage, and then turns into the entrance of the impeller of the second stage, and then passes through all the subsequent impellers and guide wheels in turn, and finally discharges from the discharge port.
The centrifugal blower delivers a large volume of gas, but the resulting wind pressure is still not high, and the outlet gauge pressure is generally no more than 0.3MPa (3kgf/cm3). Because the compression ratio of the gas in the centrifugal blower is not high, so there is no need for a cooling device, and the impeller diameters of all levels are roughly equal.
Centrifugal compressor is often called turbocompressor, the main structure, working principle are similar to centrifugal blower, but the centrifugal compressor impeller stage more, can be more than 10 levels, the speed is higher, so it can produce higher pressure. As the gas compression is higher, the volume change is larger, the temperature rise is also more significant. Therefore, the centrifugal compressor is often divided into several sections, the impeller diameter and width of the section by section narrowing, section and section between the intermediate cooler, so as not to gas temperature is too high.
Centrifugal compressor flow, gas supply uniformity, small size, less wear parts in the body, continuous operation and safe and reliable, easy maintenance, no lubricating oil in the body of the contaminated gas. Therefore, in recent years, in addition to the requirements of the pressure is very high, the centrifugal compressor application is becoming more and more widespread.
Table 2-4 common fan performance range and use of the table
Two, rotary blower
Currently the most widely used rotary blower is the Roots blower.
The working principle of the Roots blower is similar to the gear pump. As shown in Figure 2-24. Inside the casing there are two specially shaped rotors, often in the shape of a waist, between the two rotors, rotor and casing gap is very small, so that the rotor can rotate freely without excessive leakage. The two rotors rotate in opposite directions, which allows the gas to be drawn in from one side of the casing and discharged from the other. If the direction of rotation of the rotors is changed, the suction and discharge ports are interchanged.
Figure 2-23 Schematic diagram of a five-stage centrifugal blower
The air volume of a Roots blower is directly proportional to the rotational speed and is almost unaffected by changes in outlet strength. Roots blower speed is certain, the air volume can be kept largely unchanged, so it is called fixed-capacity blower. This type of blower air volume range is 2 ~ 500m3 / min, the export of table pressure within 80kPa (0.8kgf/cm2), but in the table pressure of 40kPa (0.4kgf/cm2) near the higher efficiency.
The outlet of the Roots blower should be installed with a gas stabilizer tank and a safety valve. Generally use the circuit branch to regulate the flow. The outlet valve cannot be completely closed. The operating temperature should not exceed 85℃, otherwise it will cause thermal expansion of the rotor and collision.
Figure 2-24 Roots blower
Three, reciprocating compressor
Reciprocating compressor structure, principle of operation and reciprocating pumps are relatively similar. The main components are cylinders, pistons, suction and discharge valves. Rely on the reciprocating motion of the piston and the gas will be inhaled and pressed out.
Figure 2-25 shows a vertical single-acting double-cylinder compressor, in the body is equipped with two parallel cylinders 1, known as double-cylinder, two pistons 2 connected to the same crankshaft 5. Suction valve 4 and exhaust valve 3 are in the upper part of the cylinder. Cylinders and pistons between the end of the closed volume is composed of the working volume of the compressor. Crank connecting rod mechanism to promote the piston constantly in the cylinder for reciprocating motion, so that the cylinder through the suction valve and exhaust valve control, cyclical suction-compression-exhaust-expansion process, in order to achieve the purpose of increasing the gas pressure. The cylinder wall is equipped with heat dissipation fins, so that the heat is easy to spread.
Figure 2-25 Vertical single-acting double-cylinder compressor
1-cylinder body; 2-piston; 3-exhaust valve; 4-suction valve; 5-crankshaft; 6-connecting rod
(a) the working process of reciprocating compressor
Reciprocating compressor structure and working principle and reciprocating pumps are close to each other, but because of reciprocating compressor deal with compressible gas, after compression the gas pressure increases. After compression of the gas pressure increases, the volume shrinks, the temperature rises, so the reciprocating compressor process and reciprocating pumps are different, Figure 2-26 for the single-acting reciprocating compressor work process. When the piston moves to the leftmost end of the cylinder (point A in the figure), the end of the press-out stroke. But because of the mechanical structure of the reasons, although the piston has reached the left end of the stroke, the left side of the cylinder there are a number of volume, called the gap volume. Due to the existence of the gap, the beginning of the inhalation stroke for the gap pressure of p2 high-pressure gas expansion process, until the gas pressure down to the inhalation of gas pressure p1 (Figure B point) inhalation valve is opened, the pressure of p1 of the gas is sucked into the cylinder. In the whole suction process, the pressure remains basically unchanged until the piston moves to the rightmost end (point C in the figure), the end of the inhalation stroke. When the piston moves to the left, the compression stroke begins, the suction valve is closed, the gas is compressed, when the pressure of the gas in the cylinder increases to slightly higher than p2 (point D in the figure), the discharge valve opens, the gas is discharged from the cylinder until the piston to the left end, the end of the discharge process.
This shows that a compressor's working cycle is composed of four stages: expansion - inhalation - compression - discharge. In Figure 2-26 p-V coordinates for a closed curve, BC for the inhalation stage, CD for the compression stage, DA for the discharge stage, and AB for the gap gas expansion stage. As a result of high-pressure gas in the cylinder gap, thus reducing the amount of inhaled gas, increasing power consumption. Therefore, the clearance should not be too large, the general clearance volume for the piston once swept through the volume of 3% to 8%, this percentage is also known as the clearance coefficient, indicated by the symbol ε.
Figure 2-26 reciprocating compressor process
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Va - clearance volume;
Vc-Va - piston swept volume.
When the gas is compressed the volume shrinks, the pressure increases and the temperature rises significantly. In order to improve the efficiency of the compressor, often used in the operation of the inter-segment cooling method, in order to reduce the rise in gas temperature, and at the same time in the cylinder structure set up air-cooled or water-cooled devices.
(B) the selection of reciprocating compressor
Reciprocating compressor selection is mainly based on the production capacity and exhaust pressure (or compression ratio) two indicators. Production capacity is usually used in the inlet state flow m3/min. Discharge pressure (or final pressure) is expressed in Mpa. In the actual selection, first of all should consider the special nature of the gas conveyed, the selected type of compressor and compression section. Then, according to the compressor according to the spatial location of the cylinder to divide the advantages and disadvantages of each type, select the type of compressor. Compressor type and model selected, according to the needs of production, according to the aforementioned production capacity and exhaust pressure of the two indicators, by the product samples, selected the required compressor.
Four, vacuum pump
From the vacuum container and pressurize the compressor discharged to the atmosphere is called a vacuum pump. There are many types of vacuum pumps, now the commonly used ones, briefly introduced as follows:
(A) reciprocating vacuum pump
Reciprocating vacuum pump basic structure and operating principle and reciprocating compressor is the same, but the vacuum pump operates at low pressure, the pressure difference between inside and outside the cylinder is very small, the valve must be more lightweight, easy to open and close. In addition, when the required vacuum degree is high, such as 95% of the vacuum degree, the compression ratio is about 20. such a high compression ratio, the residual gas in the gap on the vacuum pump pumping rate is bound to have a great impact. In order to reduce the impact of the gap, set up a balance between the two ends of the vacuum pump cylinder airway, in the end of the piston exhaust, so that the balance of the airway for a short period of time connected to the gap in the residual gas from one side to the other side of the flow, in order to reduce the pressure of the residual gas, reduce the impact of the gap.
(ii) water ring vacuum pump
Shown in Figure 2-27. The impeller is eccentrically installed in the casing 1, on which there are radial blades 2. the pump is filled with about half of the volume of water, and when rotating, it forms a water ring 3. the water ring has a liquid sealing effect, and the blades form a number of different sizes of sealing chambers between the gas is sucked in from the inlet 4 when the chambers increase gradually; when the chamber volume decreases gradually, the gas is discharged from the outlet 6. the water ring is used as a sealing device, and it is used as a sealing device to seal the gas.
The highest vacuum degree which can be caused by water ring vacuum pump is about 85kPa (0.85kgf/cm2), and it can also be used as a blower, but the generated gauge pressure is not more than 0.1MPa (1kgf/cm2). When the pumped gas should not be in contact with water, the pump can be filled with other liquids, so it is also called liquid ring vacuum pump.
Figure 2-27 Working diagram of water ring vacuum pump
1-pump body; 2-impeller; 3-water ring; 4-intake hole; 5-workshop; 6-exhaust hole; 7-exhaust pipe; 8-intake pipe; 9-discharge pipe; 10-water tank; 11-discharge pipeline; 12-control valve
This kind of pump has simple and compact structure, which is easy to manufacture and maintain, and has long service life because there is no mechanical friction in rotating part. There is no mechanical friction in the rotating part, long service life and reliable operation. It is suitable for pumping gases containing liquids, especially when pumping corrosive or explosive gases. But the efficiency is very low, about 30% to 50%, the vacuum degree can be caused by the liquid temperature limitations.