Three aspects of energy loss in centrifugal pump
The centrifugal pump is a kind of energy conversion machine, the energy conversion process must be accompanied by the loss of energy, and efficiency is the measure of this conversion. How to improve the efficiency of the pump and reduce the loss of energy? It is necessary to find out the loss of energy in the pump. The energy loss in the pump mainly includes the following three aspects:
1. Mechanical loss
It is mainly the friction loss (also known as disc loss) between the liquid and the outer surface of the front and rear cover plates of the impeller and the centrifugal pump cavity.
Disk loss accounts for a large proportion, even up to 30% of the effective power. The test shows that the disc loss is proportional to the third power of rotating speed and the fifth power of impeller outer diameter of the centrifugal pump. Therefore, the larger the outer diameter of impeller, the greater the disc loss. Although the disc loss is proportional to the third power of rotation speed, under a given head, with the increase of rotation speed, the outer diameter of impeller of centrifugal pump decreases correspondingly (it can be considered that the increase of pump speed doubles, the outer diameter of impeller decreases by half), and the disc loss decreases in the fifth power proportion. Therefore, with the increase of rotation speed, the disc loss does not increase, but decreases, which is the reason for the development of high-speed pump One.
2. Volume loss
A part of the liquid in the impeller of centrifugal pump leaks back to the impeller inlet through the clearance of the impeller sealing ring, which can not be effectively used, resulting in loss. Therefore, the smaller the clearance of the sealing ring is, the better. However, due to processing and assembly reasons, the smaller the clearance may form eccentric wear or jamming. The national standard has made special provisions for the clearance of various types of centrifugal pumps.
3. Hydraulic loss
The fluid in the overflow part of the centrifugal pump (from the inlet to the outlet) must have the loss caused by the change of speed and direction, which are the hydraulic loss. In order to reduce this part of the loss, in addition to improving the smoothness of the flow passage parts, try to select excellent hydraulic model
3、 Analysis of pump efficiency formula
What is the efficiency of a centrifugal pump? How about the formula?
Answer: it refers to the ratio of effective power and shaft power of the centrifugal pump. η = power of PE / P pump generally refers to the input power, that is, the power transmitted from the prime mover to the pump shaft, so it is also called shaft power of centrifugal pump, which is represented by P.
The effective power is the product of the lift, mass flow and acceleration of gravity.
PE = ρ g QH (W) or PE = γ QH / 1000 (kw)
ρ: Density of liquid delivered by pump (kg / m3)
γ: Gravity of liquid delivered by pump: γ = ρ g (n / m3)
g: Acceleration of gravity (M / s)
Mass flow QM = ρ Q (T / h or kg / s)
4、 General methods to improve centrifugal pump efficiency
Improve pipeline system and reduce resistance. The length of the pipeline shall be shortened and kept as straight as possible, and the flow rate shall be reduced to reduce the head loss along the pipeline; the number of gate valve, bottom valve, elbow, orifice plate and other components shall be reduced to reduce the local head loss. Reduce the surplus of water centrifugal pump outlet pressure to meet the requirements of pipeline system for outlet pressure.
Centrifugal Pump is the most widely used general-purpose machinery, but also a large consumer of electricity and energy, so improving the efficiency of the pump itself and the use efficiency of the pump plays an important role in energy conservation.
Improve the efficiency of the centrifugal pump itself
1. The blade extends to the suction port and thins, so that the liquid is affected by the blade in advance, which can reduce the outer diameter of the impeller, also can increase the length of the streamline in the blade passage, and reduce the relative diffusion; but the extension should be appropriate, too forward extension will make the area of the inlet et too small, so that the wall angle between the blade inlet and the blade cover plate will be smaller, but it will increase the hydraulic friction loss, squeeze the inlet flow channel, and affect the cavitation and efficiency All are unfavorable.
2. In order to reduce the diffusion loss, the ratio of the outlet area to the inlet area of the runner between the adjacent blades should be controlled between 1.0 and 1.3. If the ratio is greater than L.3, the flow channel will diffuse seriously and the efficiency will decrease.
3. The larger the hydraulic radius of the runner is, the better. Try to make the blade inlet section close to the square as much as possible to reduce the friction loss. From hydraulics, we know that the ratio of the cross-section area of the flow passage of centrifugal pump to the wet circumference is called the hydraulic radius, that is, the hydraulic radius - the cross-section area of the flow passage / the wet circumference. Large wetted perimeter, in fact, is the contact area between the liquid and the wall. When the cross-section of the flow passage is changed from an approximate square to a long and narrow rectangle, it is essentially to let the liquid flow through the gap of the long and narrow cross-section, so the resistance must be large.
4. Because of the large hydraulic loss of the curved diffusion pipe, the diffusion section with a slight bend close to a straight line is mostly used now. For the anti guide vane, its inlet angle and position in the circumferential direction shall be determined in combination with the outflow of liquid flow in the diffusion section. The principle is to form a continuous flow channel, so as to avoid the narrow inlet section of the anti guide vane, otherwise, eddy current and impact loss of centrifugal pump will be caused at the inlet of the anti guide vane.
5. For the multistage centrifugal pump, the impeller inlet is pre rotated (the outlet angle of the anti guide vane is less than 90). ）At the same time, the relative velocity diffusion is reduced when the outlet angle of the guide vane is less than 90. When the water flows into the impeller of centrifugal pump, there is a pre rotation, that is to say. 1≠0。
6. In order to make the "1vul" term in the theoretical head of centrifugal pump formula ht-u2vu2-lvul zero in the design, the outlet angle of the anti guide vane should be 90. For the last stage guide vane, the rotating component can be eliminated. However, experiments show that this is not good for efficiency and stable performance curve, especially for some low specific speed centrifugal pumps, in order to obtain the declining performance curve, the outlet angle of the anti guide vane should be less than 90. Usually at 60. ～80。。 Both ends of the blade should be thinner to avoid collision and eddy current loss.
Section of Blade Runner
7. Increase the width of impeller outlet of centrifugal pump, reduce the absolute speed of impeller outlet, and reduce the hydraulic loss in the water chamber.
8. Bevel the impeller outlet of centrifugal pump to reduce the length difference of front and rear streamline