行业应用

Industry Applications > Solutions > 正文

Powtech energy-saving retrofit of frequency converters in central air-conditioni

更新时间：2012-12-27点击次数：2651次字号：T|T

The principle of the fan and water pump were introduced in this paper, the principle of central air conditioning energy saving transformation and chilled water, cooling water control method.

1 Introduction

Since reform and opening-up, our country national economy rapid development, but the development of energy industry far cannot satisfy the need, and quite a period of time energy gap status will not change, so the state, and given priority to development and save energy policy. Especially is given priority to with valuable secondary energy - saving electric energy, electric energy is the largest user in the motor in our country, which accounts for about 50%, including fan, water pump power consumption accounted for 30% of the total electricity. Traditional fan pump is to rely on the regulation of the air flow, water inlet, the adjustment of the throttle valve. When the need to reduce the air volume, water, throttle, valve opening to reduce; When need to increase air volume, water volume, throttle, valve opening increased. The adjustment way is simple and feasible, has become a habit, but it is increase the network loss, consuming energy in the damper, the expense of the valve. And often in the design, the user fan pump design capacity is much higher than the actual need, so easy to form the people often say that the "big horse-drawn buggy" phenomenon, resulting in waste of electricity. If adopt the way of adjusting speed to adjust the flow rate, can fundamentally prevent the waste of electric power. In recent years, with the development of power electronic technology, frequency conversion speed regulating technology is more and more mature, so the promotion of frequency conversion speed regulation on fan and pump equipment applications, is of great significance to reduce energy waste.

2 The central air conditioning system energy saving transformation principle

Chilled water pump in central air conditioning system and the capacity of the cooling water pump is designed according to building the largest heat load is selected, and have margin, and operation is the long-term work under fixed maximum water flow all the year round, due to the changes of seasons, day and night, and the user load, the actual air conditioning thermal load in the most time is far less than the design load, the map of a building measured thermal load rate change, the chart below shows, with the pump flow and pressure of the maximum design load (load at a rate of 100%), compared to the year load rate below 50% the number of hours more than accounted for about 50% of the total running time. General design of cooling water temperature is 5 ~ 7 ℃, the design of the cooling water temperature difference is 4 ~ 5 ℃, under the condition of the system flow rate is fixed, most running time difference in temperature throughout the year is only 1.0 ~ 3.0 ℃, both work under low temperature difference and large flow conditions, thereby increasing the pipeline system energy loss, waste transport energy of the pump operation. According to statistical analysis, the power consumption of air conditioning pump accounts for about 20 to 30% of total system energy consumption, thus save energy water system of the conveyer during low load, has the very vital significance, therefore, change the variable flow water according to the thermal load of air conditioning water system shows its great superiority, and get more and more widely used. Adopting frequency converter to adjust pump speed can be conveniently regulating the flow of water, its energy-saving rate is usually 40% or more.

Heat load change rate graphs

Now the points from the following several aspects:

Fan, pump load speed regulation principle of energy conservation

Fan is used to transmit gas machinery and equipment, pump is used to transfer liquid of machinery and equipment, fan and pump are the motor shaft power into mechanical energy of a mechanical, both starting torque is small, load is light, with large energy saving potential; The structure and working principle of the basic which is similar.

To pump as an example under the analysis of its working principle and operation process:

(1) pump features and parameters

Purely used for pumping water power is effective power

Effective power = QH (1000)/(75 * 60/0.736) = QH / 6.11 (kW)

Type, Q as the flow rate (m3 / min); H as the total head (m).

Located within a head of 1 m3 of water weight is 1000 kg, so:

The pump shaft power = (effective power)/pump efficiency (kW)

Motor power output = (1.05 ~ 1.2) x shaft power (kW)

Because of pump lift and pump model, pump efficiency, instead, standard the general efficiency curve of the pump as shown.

Given the pump design and manufacturing will have error, so the output power of motor should be calculated value of the shaft power has 5 ~ 20% margin, and then find out the power of the motor according to the flow and head, flow and head characteristic curve of the figure below.

General standards of the pump efficiency Flow and head characteristic curve

(2) the pipe network of water resistance properties

When water pipe network resistance R remains unchanged, the relationship between the water quantity and water resistance are uncertain, namely water Q h according to the water resistance and the change law of resistance, its expression is: h = RQ2 (Pa)

Type of H - water resistance, R - water resistance coefficient.

H = f (Q) curve for water resistance characteristic curve, the parabola shape, as shown in figure A. The figure A shows that water resistance coefficient R is bigger, the curve is steep, namely water resistance.

3) fan and pump speed control principle of energy conservation

By fluid mechanics, the quantity Q and the speed of a direct ratio, pressure H is directly proportional to the square of the speed, power P is directly proportional to the vertical ejection of the rotational speed. Q = n/ne/Qe

H/He = (n/ne) 2

P = (n/ne/Pe) 2

Type: the Qe - blower, pump rated wind (flow);

He -- fan, pump pressure rating of the class;

Pe - blower, pump power rating;

Ne - rated speed of fan and pump.

By the above formula shows that if the efficiency of the pump must, when demand adjust water drop, is proportional to the speed is decreased, the shaft power of pump is established relationship.

Another according to the characteristic curve of pump products and water resistance characteristic curve of fans and pumps also can obviously see that energy saving effect. Figure B fans and pumps for speed regulation and energy saving principle diagram, diagram curve for constant speed under H - f H (Q) curve, its water resistance, wind resistance characteristic curve of R1 intersecting with A point. The corresponding airflow for Q1. At this time fan, water pump shaft power Q1AH1Q into rectangular area is proportional to the. When you would like to reduce airflow from Q1 to Q2 when using damper or valve, the new wind resistance, water resistance characteristic curve H intersect at point B, the fan shaft power is proportional to the Q2BH2Q round into the rectangular area. If the speed control method is used to reduce the speed of the fan and water pump to n2 make corresponding to the fan performance curve of H and wind resistance R2 characteristic curves intersect at point C. At this point is proportional to the fan shaft power Q2CH3Q round into the rectangular area is significantly reduced, shaft power down much, energy saving effect is obvious.

A water resistance characteristics Figure B fan , pump speed control , energy-saving diagram

Chilled water system control
For chilled water systems , the water temperature depends on the settings of the evaporator , the return water temperature depends on the heat received by the evaporator , central air conditioning chilled water temperature and frozen backwater temperature maximum design temperature difference of 5 ° C ( for example, water 7 ° C the backwater 12 ℃), now detect its temperature transmitter fitted in the outlet pipe of the evaporator and back to the water on the road the PID temperature difference between the regulator and the inverter to form a closed-loop control system , through the chilled water temperature difference ( △ T = 5 ° C ) control to make chilled water pump speed changes corresponding to the change of the thermal load , the specific control Figure shown in the following figure :

Chilled water pump control program

Chilled water system control mode

For cooling water systems , the cryogenic cooling water temperature depends on the condition of the cooling tower , we only need to control the temperature of the high - temperature cooling water ( condenser water ) , you can control the temperature difference is now using temperature transmitter , PID controller and inverter form a closed-loop control system , condenser water temperature control in T ( for example : 37 ° C) , corresponding to the change of the heat load of the cooling water pump speed , specific control method shown in the figure below .

Cooling water pump control program

3 conclusion

Adopts frequency closed loop control motor, according to the need to set temperature, make the equipment reserve capacity and seasonal variation of thermal load over time through the speed setting, maximum under meets the requirement of energy saving. In my company central air conditioning system, for example, part of our fan, water pump, according to the statistical analysis of electricity saving rate is an average of 20 ~ 50%. In addition, the use of frequency converter also has the following features:

(1) inverter for the protection of the motor is better, with over voltage, under voltage, over current, grounding protection;

(2) the inverter running stability, maintenance is simple and convenient;

(3) the starting current of frequency converter is small, less impact to power grid, the stable running of the power grid;

(4) the isolation effect of capacitor in the inverter input power factor is close to 1, the excitation reactive current of the motor is provided by capacitor and can save about 30% of the power grid capacity.

上一篇：Powtech energy saving analysis of frequency converter in cement plant
下一篇：已经没有了