Working principle analysis of centrifugal compressor
I. Explanation of common nouns:
(1) level: each level of impeller and its fixed components (such as diffuser, etc.) constitute a basic unit, called a level. For example: Hangzhou oxygen 2TYS100 2TYS76 oxygen turbine compressor high and low pressure cylinder has eight impellers, which is called eight levels.
(2) segment: the unit separated by intermediate cooler is called segment. In this way, the compressor can be divided into many sections according to the number of coolers. A section can include many levels, or only one level.
(3) standard state: 0℃, 1 standard atmospheric pressure.
(4) air intake status: generally refers to the temperature and pressure of the gas at the inlet.
(5) weight flow: the weight of gas flowing in one second.
Volume flow: the volume of gas flowing in one second.
Gauge pressure (G): The pressure measured based on the local atmosphere.
Absolute pressure (A): The pressure measured on the basis of complete vacuum.
Vacuum degree: negative difference with local atmosphere.
Pressure ratio: the ratio of outlet pressure to inlet pressure.
II. Gas flow in compressor stage
The impeller is dragged by the driver and rotates. After the gas enters the impeller, it works on the gas. Then the gas not only rotates with the impeller, but also flows in the impeller groove. It reflects that the pressure of gas rises, the temperature rises and the specific volume declines.
The speed of the impeller rotation (understanding "rotation") is the circular speed of the gas. There are different values in different radius, and the circular speed at the outlet of the impeller is the largest.
The relative speed of gas flow (understand "flow") in the impeller channel is relative speed. As the sectional area of the blade channel gradually increases from the inlet to the outlet, the relative speed gradually decreases.
The actual velocity of gas is the synthesis of circular velocity and relative velocity, which is also called absolute velocity.
III. Energy loss analysis of gas flow within the level
(1). Definition of energy:
The mass of a kind of substance that measures the movement of substance is generally interpreted as the ability of substance to work. The basic types of energy include potential energy, kinetic energy, thermal energy, electric energy, magnetic energy, luminous energy, chemical energy, atomic energy, etc. One energy can be converted into another. The unit of energy is the same as that of power. Energy is also called energy.
(II). Energy loss analysis of gas flow within the level
In the actual operation of the compressor unit, the energy is transmitted to the gas through the impeller, that is, besides the power and power consumed by the impeller to work on the gas through the blade, there are also wheel resistance loss caused by friction between wheel plate of impeller, outer side of wheel cap and wheel rim and surrounding gas, there are still rolling wheel exit gas pass shroud air seal leakage back to rolling wheel imported low-voltage low-voltage side leakage loss. All need to consume power. These losses are inevitable within the level. Only by carefully selecting the parameters in the design, processing according to the requirements in the remanufacturing, and carefully operating in the operation to make it reach the design working condition as far as possible, can these losses be reduced.
In addition, there are also flow loss, kinetic energy loss and impact loss generated in non-working condition inside the stage. The increase of impact loss will cause the compressor efficiency to reduce quickly. There is also the high-pressure shaft end. If the seal is not good, it will leak to the outside world, which will reduce the useful flow of pressing out.
Therefore, it is necessary for us to study the causes of these losses, so as to minimize the losses in the design, installation and operation, maintain the compressor operating in high-efficiency areas and save energy consumption.
1. Flow loss:
Definition: it is the energy loss when the airflow flows in the fixed components of the impeller and level.
Causes: mainly because the gas has viscosity, which causes friction loss in the flow, and these losses turn into heat which makes the temperature of the gas rise, and generate vortex in the flow, aggravating friction loss and Flow Energy loss will consume energy because of the generation of vortex; in the working wheel, there is also the second flow such as axial vortex, which causes flow loss. The tail loss is caused at the impeller outlet due to the influence of the thickness of the blade. Friction resistance and local resistance loss of Bend and recirculator, etc.
2. Impact loss:
Definition: it is a kind of flow loss generated under non-design working conditions.
The installation angle of impeller inlet blade β1a (actual) is generally decided by the inlet angle β1 (design) of design airflow. Generally, β1 = β1a, at this time, the air intake is non-impact air intake. However, when the working condition deviates from the design working condition, the airflow inlet angle β1 is larger than or smaller than β1a, which will cause the airflow impact on the vane.
We are used to calling the difference between the installation angle β1a (actual) of the impeller inlet blade and the inlet angle β1 (design) of the design airflow as the impact angle, which is called the impact angle for short. Expressed in I.
β1a <β1, I <0, is called negative impulse angle.
β1a> β1, I> 0, is called positive impact angle.
In the case of positive and negative impact angle, the separation of airflow from the blade surface will appear, forming a vortex area, resulting in energy loss. The increase of impact loss is proportional to the square of the absolute value of the flow deviation design flow.
3. Wheel resistance loss
The space between the non-working surface of the impeller and the chassis is full of gas. When the impeller rotates, because the gas has viscosity, friction loss will also occur. And because the rotating impeller generates centrifugal force, the gas on one side of the impeller flows upwards, and the gas on the other side of the chassis flows downwards, forming vortex and causing loss. There are experimental formulas for the calculation of wheel resistance loss. If you are interested, please refer to books.
4. Air leakage loss:
Including internal leakage and external leakage.
Internal Leakage means that the leaked gas leaks back into the compressed gas. It includes two situations: one is that the gas from the outlet of the impeller leaks back to the inlet from the space of the impeller and the chassis. The other is the single-axis centrifugal compressor. Because there is also a gap between the shaft and the chassis, the gas flows into the low-pressure side through the gap from the high-pressure side.
External leakage refers to the direct leakage of compressed gas into the atmosphere through the gap between the shaft and the enclosure seal or the gap between the body.
The loss of air leakage is a problem that can not be ignored, and we should pay special attention to it in maintenance and operation. The air leakage of some air compressor is caused by internal leakage and external leakage when it fails to reach the design value.
