Discussion on Several Questions of Conversion and Speed Gove

Discussion on Several Questions of Conversion and Speed
Governing Constant Pressure Supply of Water

Yao Zhiqiang

The Architectural Design Institute of Hunan Province,
Add:No.140,Renming Rd, changsha City Zip code:410011
Tel:0731-5166214 Fax:5163176

Abstract: Based on similarity law on impeller of centrifugal pumps, the conditions which assure the performance of conversion and speed governing pumps working in high are analyzed. Both the energy saving conditions of frequency controlled water supplying and the matter of attention in selecting the complete performance parameter of speed governing are proposed.
Key words: conversion, speed governing, water supply, energy saving

Conversion and speed governing constant pressure supply of water, using alternating conversion, speed governing and SCM technique is an effective way of energy saving. It has the advantage of achieving constant pressure supply of water and avoiding the energy waste caused by surplus pressure water supply. Thus, preferable economical profit and social benefit can be attained by considering the fact that energy shortage in China at present is evident. However, the adjustment of the flow of speed governing pumps is limited in a certaing range, if the pump and its performance parameter are chosen incorrectly, or water supplying in a normal way which features as one pump working in frequency conversion with the others are power frequency, energy waste is presented. This article involved the discussion of the questions mentioned above.
The equation H=KQ² can be attained according to the rule of pumps, from which we can conclude that each operating point which accords with the rule is distributed on a curve, which is a conic with the coordinate origin as its vertex, named as equivalent curve.
1 The Energy Saving Conditions of Conversion and Speed Governing Supply of Water
At present the way of water supply which one pump is conversion and the others are power frequency is often adopted in the market. Whether this way is reasonable could be explained by analyzing the process of pumps in paralleled connection. Now given that three pumps with the same type are working in paralleled connection, both the characteristic curves of the pumps and the pipes are shown in figure 1. In figure 1, the high-efficiency zone of the constant speeding pump is from A to B, the rating

　

The1、2 curves are the respective equivalent curves which are through A、B The Ⅰ curve is the characteristic curve that one pump works at ratin revolution(n₀); the I‘ curve is the characteristic curve that two pumps in paralleled connection work at rating revolution (n_min); the Ⅱ curve is the characteristic curve that two pumps in paralleled connection work at rating revolution(n₀); the Ⅲ curve is the characteristic that three pumps in paralleled connection all work at the minimum revolution (n_min); the V curve is the characteristic curve that two pumps work at n_min, one works at n0, and the three pumps are paralleled connection; the Ⅵ curve is the characteristic curve that two pumps work at n0, one works at n_min, and the three pumps are paralleled connection ;the 3、4 curves are the characteristic curves of the pipes.

revolution of the pump is n0, and the minimum revolution of the speed governing pump in the high-efficiency zone is n_min. According to the rule mentioned above, when the revolution of pump decrease from n0 to n_min, the high-efficiency zone is located in the range between parabola 1 and 2. If the constant pressure value is set as Hb, according to figure 1, when two pumps are power frequency, the flow is Qc. While if two pumps are power frequency and one is conversion, the minimum flow of the paralleled pumps in the high-efficiency zone is Qd. Obviously, Qd＞Qc, for the operating point of the network is random. If the flow (Q) vary from Qc to Qd, the operating point of speed governing pump will be out of the high-efficiency zone. That is to say ,the variety of flow in high-efficiency zone is not continuous, this will bring about energy wastage. Therefore, only one pump working as frequency conversion is not reasonable, it must be adopted that two pumps are conversion and one is constant. Of course, it is feasible that the three pumps are all conversion ,while the price of speed governing pump is still a problem. It is out of the reality that the three pumps are all conversion. However, the manner that two pumps are conversion and one is power frequency is just one of the conditions of energy saving. In order to settle the problem, the performance parameters of paralleled connection pumps must also accord with the condition:

Qb/Qa≥ （Hb/Ha）^1/2 （1）

In the formula:

Hb----designed constant pressure value;
Ha----the maximum value of head in the high-efficiency segment of the

pump claracteristic curve;

Qb----the flow corresponding to Hb;
Qa----the flow corresponding to Ha;

As shown in figure 1, assuming that the constant pressure is set as Hb, in order to ass ure the continuous flow when all the three pumps are working parallelly in high-efficiency zone, at the same time, the operating point of the speed conversion pump isn‘t allowed to appear out of the high-efficiency zone, the curve Ⅱ must be passed through B or presented under it, thus, accordingly, curve V can be passed through C or under it. From this, the following formula is availble:

Qb≥ 2Qe

According to the rate of pumps, the relation between the similitude operationg point A and E is:

Qa/Qe=(Ha/He)^1/2

According to figure 1,to ensure that the operating point of the speed-constant pump and speed controlled pump are included in their corresponding high-efficiency zone, the characteristic curve of the speed controlled pump after its revolution decreased must not present under E, in other words, the characteristic curve of the speed controlled pump which work in its high-efficiency zone with the minimum revolution, i.e. n_min is surely passed through E, thus, He=Hb

Therefore, Qa/Qe≥（Ha/He)^1/2 Qe=Qa (Hb/Ha)^1/2 (3)

By combining formula (2) and equation(3), the following one can be attained:

Qb/Qa≥2 （Hb/Ha)^1/2 (4)

Given that: Qb/Qa=α2（Hb/Ha)^1/2=β

It can be induced that α＞β （5）

The result above is achieved from three paralleled connection pumps. It is the same with the condition that when more than three pumps paralleled.

2 The Selection of Constant pressure Value of Conversion and Speed Governing Water Supply pump

Now, the exit pressure of pumps is adopted by conversion and speed governing water supply equipment in market to achieve constant pressure governing automatically. Yet it come to us that how to determine the constant pressure value? In some product sample, the head provided is the middle value be used as the constant pressure value? This will be discussed next.

The characteristic curve(Q-H) of one pump is shown in figure 2, thereinto the segment from A to B are high-efficiency segment of the pump, and the three points of A、B、C are three high-efficiency points provided in pump sample. In can be known from figure 2 that the energy saving amount at different constant pressure value. It can be known from figure 2 that the energy saving amount when Hc is the setting constant pressure value of speed governing pump exit is obviously greater than the that when the setting value is Hb.
It can be known from accessorial able that the parameters of all pumps can‘t accord with the energy saving conditions mentioned above, i.e. α＞β when the middle value of high-efficiency zone of pump head is selected as the constant pressure value of the pump exit. When the minimum of high-efficiency zone of pump head is selected as the constant pressure value. Most TSWA pumps　can‘be looked the constant pressure value. As a result, under the condition of fulfilling the

Accessorial Table
The α、β value of several pump in common use at different
constant pressure value

pump type High-efficiency Zone H(m) High-effciency Zone Q(mg/h) Constant Pressure value α β compare Betweenα、β constant Pressure value α β compare Between α、β 50DL×2 26.6～24.4～21.2 9～12.6～16.2 24.4 1.4 1.92 α＜β 21.2 1.8 1.78 α＞β 65DL×2 37～82～2.9 18～30～35 82 1.67 1.86 α＜β 29 1.94 1.77 α＞β 80DL×2 43.2～40～34.2 32.4～50.4～65.2 40 1.56 1.92 α＜β 34.2 2.01 1.78 α＞β 100DL×2 43.4～4.～34 72～100～126 40 1.39 1.92 α＜β 34 1.75 1.77 α＞β IS50-32-125 22～20～18.5 7.5～12.5～15 20 1.67 1.91 α＜β 18.5 2 1.83 α＞β IS65-50-125 21.8～20～18.5 15～25～30 20 1.67 1.92 α＜β 18.5 2 1.84 α＞β IS80-65-125 22.5～20～18 30～50～60 20 1.67 1.89 α＜β 18 2 1.79 α＞β IS100-80-125 24～20～16.5 60～100～120 20 1.67 1.83 α＜β 16.5 2 1.65 α＞β IS125-100-200 57.5～50～44.5 120～200～240 20 1.67 1.89 α＜β 17.5 2 1.76 α＞β IS150-125-250 22.5～20～17.5 120～200～240 20 1.67 1.89 α＜β 17.5 2 1.76 α＞β 80LG50-20×2 43.6～40～31.7 40～50～68.4 40 1.25 1.92 α＜β 31.7 1.71 1.71 α＞β 65LG36-20×2 45.5～40～34 27～36～43.2 40 1.33 1.88 α＜β 34 1.6 1.73 α＞β 50LG-24-20×2 44～40～33 18～24～30 40 1.33 1.91 α＜β 33 1.67 1.73 α＞β 40LG12-15×2 32～30～24 9～12～18 30 1.33 1.94 α＜β 24 2 1.73 α＞β 32LG6.5-15×2 32.6～60～23 4.5～6.5～9.5 30 1.44 1.92 α＜β 23 2.11 1.68 α＞β 150TSWA×3 97.4～90～82.3 119～155～191 90 1.30 1.92 α＜β 32.3 1.61 1.84 α＞β 125TSWA×2 46～43.2～40 72～90～1108 43.2 1.25 1.94 α＜β 40 1.5 1.87 α＞β 100TSWA×2 32.4～31.2～28 62～68～80 31.2 1.11 1.96 α＜β 28 1.29 1.86 α＞β 75TSWA×2 25～23～20 30～36～42 23 1.20 1.92 α＜β 20 1.4 1.79 α＞β 50TSWA×3 20～18.4～16.8 15～18～22 18.4 1.20 1.92 α＜β 16.8 1.47 1.83 α＞β

demand pressure of water supply system, the constant pressure value should be close to the minimum head in high-efficiency zone instead of using the middle value as constant pressure value(as nearly as possible).
3 Conclusions
(1)To assure the conversion and speed governing always working in high-efficiency zone, and achieve the aim of energy saving, the optimal condition is that two speed governing pumps are working, and the performance parameter of pumps must fulfill the condition:α＞β
(2)The value that equal to the middle of high-efficiency segment and greater than it should not be chosen as the constant pressure value of pump exit. As long as the pressure of water supply system is fulfilled, the constant value should be close to the minimum head in high-efficiency segment as nearly as possible.
(3)The IS DL TG should be used as paralleled connections pumps for conversion and speed governing water supply.

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