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渣浆泵内的损失与效率计算方式
添加时间:2019.11.17

 渣浆泵内的损失与效率计算方式

在本章第四节中讲过,的有效功率P(输出功率)总是小泵的轴功率P (菜输功率),因为泵中有各种损失,泵的效率总是小1。在我国,泵耗的电能是很大的,约占总发电量的20% - 30%。所以提高的效率,降低泵内的各种损失是很重要的。泵内的各种损失当然与泵本身的设计有关,但同时也与使用有关。迄今为止还不能精确进行计算泵内的各种损失,只能借助经验公式和经验数据来粗略计算。但重要的是通过本节的讨论,分析泵内的各种损失,可以知道如何避免或减小这些损失,提高泵的效率。
    泵内的损失可分为三大类:机械损失、容积损失和水力损失。

一、 机械损失与机械效率

机械损失可分为两部分,是泵的轴承和轴封的机械摩擦损失:二是液体与叶轮盖板之间的机械摩擦损失,即圆盘摩擦损失。
    (1) 轴承和轴封的摩擦损失,正常情况下是不大的,AP ~(0.0 ~ 0)P”。大采取小值,小泵取大值。

但当轴承中缺油或油质不好,如使用油脂时,时间过长,油干后都会增加摩擦损失。当轴承磨损后,也会增加摩擦损失。
    当使用机械密封时,轴封的摩擦损失是很小的,所以从节能的角度,应当尽量采用机械密封。在采用填料密封时,如果压盖压得太紧,轴封的摩擦损失就会增加很多,甚至烧毁填料。
    (2) 圆盘摩擦损失是比较大的,是机械损失的主要部分,尤其对于低比转数的离心泵,圆盘摩擦损失更大,是泵内的最主要损失。圆盘摩擦损失与比转数的关系如图2 -32所示。当比转数n=30时,圆盘摩擦损失接近于有效功率的30%
    kf是实验系数,与泵体形状、叶轮盖板粗糙度有关,对于一般整体铸造叶轮,可用下式近似计算:

从式(2-21)可看出,圆盘摩擦损失与叶轮外径5次方成正比,在泵转速和流量不变的情况下,用增大叶轮外径的办法来提高单级扬程,伴随而来的是圆盘摩擦损失急速增大。这就是为什么低比转数的泵效率低的原因。
    从式(2-21) 还可看出,圆盘摩擦损失与转速3次方成正比,与叶轮直径5次方成正比,而增加转速可以减小叶轮直径,所以提高泵的转速可以有效地减小圆盘摩擦损失。
    圆盘摩擦损失还与叶轮盖板表面粗糙度有关,减小表面粗糙度可以减少叶轮圆盘摩擦损失,所以叶轮的盖板应尽量光滑,如果表面比较粗糙可在表面涂漆改善。当盖板锈蚀严重时,要重新清理,打磨涂漆。
    叶轮圆盘摩擦损失还与叶轮和泵体间的侧隙大小有关,如图2-33所示。在B/D2 =2%- 5%范围内较好,并采用开式泵腔能回收一部分能量。 渣浆泵厂家
总的机械损失为:
                       P = P+ Pdf

则机械效率为:

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: