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渣漿泵內(nèi)的損失與效率計(jì)算方式
在本章第四節(jié)中講過(guò)，泵的有效功率P。(泵輸出功率)總是小于泵的軸功率P (菜輸入功率)，因?yàn)楸弥杏懈鞣N損失，泵的效率總是小于1。在我國(guó)，泵耗耗的電能是很大的，約占總發(fā)電量的20% - 30%。所以提高泵的效率，降低泵內(nèi)的各種損失是很重要的。泵內(nèi)的各種損失當(dāng)然與泵本身的設(shè)計(jì)有關(guān)，但同時(shí)也與使用有關(guān)。迄今為止還不能精確進(jìn)行計(jì)算泵內(nèi)的各種損失，只能借助經(jīng)驗(yàn)公式和經(jīng)驗(yàn)數(shù)據(jù)來(lái)粗略計(jì)算。但重要的是通過(guò)本節(jié)的討論，分析泵內(nèi)的各種損失，可以知道如何避免或減小這些損失，提高泵的效率。
泵內(nèi)的損失可分為三大類:機(jī)械損失、容積損失和水力損失。
一、 機(jī)械損失與機(jī)械效率
機(jī)械損失可分為兩部分，一是泵的軸承和軸封的機(jī)械摩擦損失:二是液體與葉輪蓋板之間的機(jī)械摩擦損失，即圓盤摩擦損失。
(1) 軸承和軸封的摩擦損失，正常情況下是不大的，AP ~(0.0 ~ 0)P”。大采取小值，小泵取大值。
但當(dāng)軸承中缺油或油質(zhì)不好，如使用油脂時(shí)，時(shí)間過(guò)長(zhǎng)，油干后都會(huì)增加摩擦損失。當(dāng)軸承磨損后，也會(huì)增加摩擦損失。
當(dāng)使用機(jī)械密封時(shí)，軸封的摩擦損失是很小的，所以從節(jié)能的角度，應(yīng)當(dāng)盡量采用機(jī)械密封。在采用填料密封時(shí)，如果壓蓋壓得太緊，軸封的摩擦損失就會(huì)增加很多，甚至燒毀填料。
(2) 圓盤摩擦損失是比較大的，是機(jī)械損失的主要部分，尤其對(duì)于低比轉(zhuǎn)數(shù)的離心泵，圓盤摩擦損失更大，是泵內(nèi)的最主要損失。圓盤摩擦損失與比轉(zhuǎn)數(shù)的關(guān)系如圖2 -32所示。當(dāng)比轉(zhuǎn)數(shù)n。=30時(shí)，圓盤摩擦損失接近于有效功率的30%。
kf是實(shí)驗(yàn)系數(shù)，與泵體形狀、葉輪蓋板粗糙度有關(guān)，對(duì)于一般整體鑄造葉輪，可用下式近似計(jì)算:
從式(2-21)可看出，圓盤摩擦損失與葉輪外徑5次方成正比，在泵轉(zhuǎn)速和流量不變的情況下，用增大葉輪外徑的辦法來(lái)提高單級(jí)揚(yáng)程，伴隨而來(lái)的是圓盤摩擦損失急速增大。這就是為什么低比轉(zhuǎn)數(shù)的泵效率低的原因。
從式(2-21) 還可看出，圓盤摩擦損失與轉(zhuǎn)速3次方成正比，與葉輪直徑5次方成正比，而增加轉(zhuǎn)速可以減小葉輪直徑，所以提高泵的轉(zhuǎn)速可以有效地減小圓盤摩擦損失。
圓盤摩擦損失還與葉輪蓋板表面粗糙度有關(guān)，減小表面粗糙度可以減少葉輪圓盤摩擦損失，所以葉輪的蓋板應(yīng)盡量光滑，如果表面比較粗糙可在表面涂漆改善。當(dāng)蓋板銹蝕嚴(yán)重時(shí)，要重新清理，打磨涂漆。
葉輪圓盤摩擦損失還與葉輪和泵體間的側(cè)隙大小有關(guān)，如圖2-33所示。在B/D2 =2%- 5%范圍內(nèi)較好，并采用開(kāi)式泵腔能回收一部分能量。 渣漿泵廠家
總的機(jī)械損失為:
△P = △P+ △Pdf
則機(jī)械效率為:
Calculation method of loss and efficiency in slurry pump
As mentioned in the fourth section of this chapter, the effective power P of the pump. (pump output power) is always less than the pump shaft power P (vegetable input power), because there are various losses in the pump, the efficiency of the pump is always less than 1. In China, the power consumption of pumps is very large, accounting for about 20% - 30% of the total power generation. Therefore, it is very important to improve the efficiency of the pump and reduce various losses in the pump. The various losses in the pump are certainly related to the design of the pump itself, but also to the use. Up to now, it is not able to calculate all kinds of losses in the pump accurately, only with the help of empirical formula and empirical data. But it is important to analyze all kinds of losses in the pump through the discussion in this section, and know how to avoid or reduce these losses and improve the efficiency of the pump.
The loss in the pump can be divided into three categories: mechanical loss, volume loss and hydraulic loss.
I. mechanical loss and mechanical efficiency
The mechanical loss can be divided into two parts: one is the mechanical friction loss of the pump bearing and shaft seal; the other is the mechanical friction loss between the liquid and the impeller cover plate, that is, the disc friction loss.
(1) the friction loss of bearing and shaft seal is not large under normal condition, AP ~ (0.0 ~ 0) P ". Small value is adopted for large pump and large value is adopted for small pump.
But when the bearing is short of oil or the oil quality is not good, such as using grease, if the time is too long, the friction loss will increase after the oil is dry. When the bearing is worn, it will also increase the friction loss.
When using mechanical seal, the friction loss of shaft seal is very small, so from the point of view of energy saving, mechanical seal should be used as far as possible. When using packing seal, if the gland is pressed too tightly, the friction loss of the shaft seal will increase a lot, or even burn the packing.
(2) the disc friction loss is relatively large, which is the main part of the mechanical loss, especially for the centrifugal pump with low specific speed, the disc friction loss is greater, which is the most important loss in the pump. The relationship between disc friction loss and specific revolution is shown in figure 2-32. When the specific revolution is n. =At 30 ℃, the friction loss of the disc is close to 30% of the effective power.
KF is the experimental coefficient, which is related to the shape of the pump body and the roughness of the impeller cover plate. For general integrally cast impeller, the following formula can be used for approximate calculation:
From equation (2-21), it can be seen that the disc friction loss is directly proportional to the 5th power of the outer diameter of the impeller. When the pump speed and flow rate are constant, the method of increasing the outer diameter of the impeller is used to increase the single stage lift, accompanied by the rapid increase of the disc friction loss. This is why pumps with low specific speed are inefficient.
It can also be seen from equation (2-21) that the disc friction loss is directly proportional to the third power of rotating speed and the fifth power of impeller diameter, while increasing rotating speed can reduce the impeller diameter, so increasing the rotating speed of pump can effectively reduce the disc friction loss.
The disc friction loss is also related to the surface roughness of the impeller cover plate. Reducing the surface roughness can reduce the disc friction loss of the impeller, so the cover plate of the impeller should be as smooth as possible. If the surface is relatively rough, it can be improved by painting on the surface. When the cover plate is seriously rusted, it shall be cleaned again, polished and painted.
The friction loss of impeller disc is also related to the size of side clearance between impeller and pump body, as shown in Figure 2-33. It is better in the range of B / D2 = 2% - 5%, and part of the energy can be recovered by using the open pump chamber.
The total mechanical loss is:
△P = △P+ △Pdf
Then the mechanical efficiency is:
