The Application Of The VVVF Technique In The Fans And Pumps

The Application of the VVVF Technique in the fans and pumps

I Foreword:

With the world facing a growing energy crisis because of limited oil reserves and carbon emissions causing global warming, China is committed to wage a long-lasting campaign for energy saving around the whole country.

Statistics show that the total capacity of China’s high-, medium- and low voltage motors hits 35000MW, most of which are fan and pump units. However, the fans and pumps, covering the power, oil, metallurgy, chemical, manufacturing industries, always word with low efficiency, but consume a large quantity of energy.

China is aiming a 20% decrease in the unit GDP energy consumption in the "11th Five-Year Plan" period through three methods -to adjust the industrial structure, to promote energy saving technology and to enhance energy management level.

II Energy saving principle of fans:

As the fan’s static pressure Pst = 0, its energy saving principle can be represented by the diagram 1.


Diagram 1 Energy saving elementary diagram of fan

In the diagram 1, the curve (1) represents the wind pressure and wind quantity characteristics curve (P-Q) at rated speed N1. The curve (2) is the pipe network resistance characteristics curve when the air valve is widely open. The intersection A of the curve (1) and (2) indicates the point at which the motor runs with the highest efficiency. At this time, the wind pressure is assumed as P2, the wind quantity is Q1, and the axis power N1=k*Q1*P2, which can be represented by the area of AP2OQ1.

On condition that the wind quantity is to be decreased from Q1 to Q2 according to production requirements:

If adjusting the wind quantity by the valve, it will increase resistance of the pipe network, resulting in the pipe network resistance curve to change to the curve (3), and the intersection A change to B. At this time, the wind press is assumed as P1 and wind quantity is Q2, so the axis power N2 = k*Q2*P1, which can be represented by the area of BP1OQ2. Compared with AP2OQ1, it is easy to find out that the axis power decreases just a bit.

When adopting the inverter to conduct speed adjustment, the fan speed will reduce from N1 to N2. As the fan’s static pressure Pst=0, conforming to the similarity theorem, the wind pressure and wind quantity characteristics curve(P-Q) at the speed N2 can be represented by the curve (4), which converges the curve (2) at point C. At this time, the wind pressure is assumed as P3 and wind quantity is Q2, so the axis power N3=K*Q2*P3, which can be represented by the area of CP3OQ2. It is easy to draw a conclusion that N3 decreases steeply due to hefty decline of wind pressure.

Compared with the valve control mode, the inverter can help save huge energy. The saved axis power N=K*(P1-P3)*Q2, which can be represented by the area of P1BCP3.

III Energy saving principle of pumps:

Different from fans, the static pumping head of pumps Hst≠0. The outlet pressure (pumping head) of pumps is required to maintain constant in most occasions. The pump’s energy saving principle can be represented by the diagram 2.


Diagram 2 Energy saving elementary diagram of pump

When decreasing the flow quantity from Q1 to Q2 by the valve, the pump speed maintains its rated speed N1, but the pipe network resistance curve (1) will change to the curve (2), leading the working intersection A to change to B. At this time, the consumed axis power can be represented by the area of H1BOQ2, almost the same to that of H2AOQ1, resulting in huge energy to be wasted.

When the inverter is installed, it can decrease the pump speed from N1 to N2 with the pipe network resistance curve (1) being unchanged, so the working intersection A transfers to C. At this time, the consumed axis power can be represented by the area of H3COQ2. Compared with H1BOQ2, it is easy to find out that the inverter has prominent energy-saving capacity.

IV Typical application case:

One power plant owns a 200 MW-annual-production-capacity generator, which has two 1250kW/6kV induced draft fans.

The following diagram shows energy-saving efficiency after WIN-HV medium-voltage inverter, manufactured by Shenzhen Winner S&T Co., Ltd., is installed to operate the fan units.

Machine load
(MW)

         Before Reformation

After Reformation

Total power of the induced draft fans(kW)

Motor load ratio of the induced draft fans (%)

Total power of the induced draft fans(kW)

Energy saving ratio
%

100

960

38.40

297.6

69.00

120

1020

40.88

427.2

58.12

140

1628

65.12

712.0

56.27

160

1656

66.24

988.8

40.29

180

1754.4

70.18

1247.2

28.91

200

1790.4

71.62

1288.0

28.06

In most China’s power plants, the induced draft fan of boiler furnace commonly runs at a constant speed with power frequency. A massive of energy is being wasted in the process of adjusting the wind quantity via the valve. The 200WM generator usually runs with peak shaving, with huge valley-to-peak difference. When WIN-HV inverter is installed, the induced draft fans get access to realize speed adjustment, as well as wind pressure and wind quantity control no matter whether the valve is open or not. Accordingly, WIN-HV inverter is able to help the power producer save energy and cut cost. Field operation shows that energy-saving rate of 20%~70%.

Moreover, WIN-HV inverter will also bring many indirect economic benefits based on its upgraded functions:

  • strengthen reliability of the equipments
    Realize soft starting when WIN-HV inverter is installed, eliminating huge surge current and mechanical wear abrasion, so as to prolong service life of the equipments. Relieve vibration of the fan and prevent axletree from wear abrasion.
  • Reduce maintenance and repair cost
    WIN-HV inverter can help the user reduce maintenance and repair cost as it eliminates huge surge current and mechanical wear abrasion.
  • Enhance automatization level
    WIN-HV inverter is equipped with several external signal and computer interfaces, which enable the inverter convenient to connect with the control system PLC, DCS &FCS, communication system, energy management system and other management system. It greatly enhances the machines’ roboticized control and management level.
  • Relieve labor intensity
    On condition of the load changed, it is very inconvenient to adjust the wind quantity by changing the valve opening. And this control mode has poor linearity and high fault rate. In contrast, when adopting WIN-HV inverter, it can adjust the speed of the fan by regulating the fan’s motor frequency. It is much easier to operate, with perfect linearity and control sensitivity.

V Conclusion:

WIN-HV series inverters are suitable in the fields of power, iron and steel, HVAC, oil, mine, construction, etc for the high-voltage motor’s speed adjustment, energy saving, soft starting and intelligent control. The performance characteristics and effect of energy saving technology, which is on the basis of converting frequency to change the speed of fan and pump units, are pretty satisfying.