Cooling tower is widely used in air conditioning cooling system and aluminum profile processing, steam turbine, tanning, power generation, injection molding, foaming, air compressor and other industrial water cooling fields.
As a leading enterprise in the field of industrial control, STEP has mastered the core technology of frequency conversion drive and accumulated rich industrial application experience. It has developed the driving layer solution of AS170Plus frequency converter + permanent magnet direct drive motor for cooling tower, which has the characteristics of low noise, large torque, good control performance, high protection level and convenient maintenance. To help customers produce higher value-added and competitive products, but also with practical actions to help achieve the "dual carbon" goal.
#01 Introduction to the working principle of cooling tower
Cooling tower is a kind of water cooling device, generally composed of catchment pool, ventilation tower, water distribution pipe, fan and water remover. The water pump will press the hot water to the water distribution pipe, with the help of the nozzle on the pipe, the hot water will be evenly sprayed on the filler, and form a water film, at the same time with the dry air flowing into the bottom of the ventilation tower for heat exchange to evaporate to the air to take away the heat, and the cooling water drops into the bottom of the catchment pool, continue to recycle.
The evaporation of water into the air is not endless. With the increase of air humidity, when the air on the water and air contact surface reaches saturation, the water molecules will not evaporate out. At this time, the fan needs to play a role in sending hot and humid air to the upper air to prevent reflux, and constantly sucking dry air from the bottom of the cooling tower, so that there is airflow inside the cooling tower. The role of the fan in the whole process is to increase the air supply and speed up the air flow to facilitate heat exchange.
#02 Characteristics of cooling tower fans
√ Large moment of inertia;
√ There may be initial velocity before starting;
√ Commonly used PID control with temperature as the control object;
√ Work in a humid environment for a long time;
√ High noise, easy to cause resonance;
√ Direct drive, transmission, belt and other driving modes can be adopted.
#03 Cooling tower fan control scheme
Scheme 1: frequency converter + asynchronous motor + gearbox/belt + fan
Scheme 2: frequency converter + permanent magnet synchronous motor + gearbox/belt + fan
Scheme 3: frequency converter + permanent magnet direct drive motor + fan
Plan 1
Advantages: low initial investment cost, simple debugging.
Disadvantages: high power consumption, poor control performance, poor system stability, high maintenance cost, high noise.
Plan 2
Advantages: Compared with scheme one, it has the following advantages
① Energy saving;
② Large torque, high efficiency, strong overload capacity;
③ Good control performance, high control precision, fast response speed;
④ motor volume is smaller, weight is easy to install and maintain;
⑤ High protection level, long service life.
Disadvantages:
The upfront cost is high, because the fan is driven through the gearbox, the system stability is poor, the maintenance cost is high, and the noise is high.
Plan 3
Advantages: Compared with scheme two, the motor directly drives the fan, removing the middle drag structure, stable structure, easy installation, good control performance, high precision, high system stability, reduce wear, almost no maintenance in the later period.
Cons: High upfront costs.
#04 Energy saving of cooling tower
In the fan control system, energy saving has become the main concern of many customers. The following figure shows the efficiency comparison between asynchronous motor and permanent magnet synchronous motor.
At 50% speed, the efficiency of permanent magnet motor reaches 90%, while the efficiency of asynchronous motor is only about 76%, and the efficiency of permanent magnet motor is about 14% higher than that of asynchronous motor. When the speed is about 100%, the efficiency of permanent magnet motor can reach more than 96%, while the efficiency of asynchronous motor almost reaches the maximum value, about 94%; The efficiency of permanent magnet synchronization is about 2%~14% higher than that of asynchronous motor. Taking 55kw as an example, assuming that the average operation is 20 hours a day, 330 days a year, and the electricity cost is 0.7 yuan/KWH, then:
Power saving: 55kw×(2%~14%)=1.1kw~7.7kw
Cost saved: (1.1kw~7.7kw)×20h×330 days ×0.7 yuan/degree =(5082-35,574) yuan
Compared with asynchronous motor, permanent magnet synchronous motor has good energy saving effect, and can save a considerable amount of electricity. STEP frequency converter can meet the requirements of asynchronous motor, permanent magnet synchronous, permanent magnet direct drive control has the corresponding solution, especially "STEP frequency converter + permanent magnet direct drive" scheme not only retains the good energy saving effect of permanent magnet synchronous motor, but also improves the control accuracy of the system, reduce noise, reduce maintenance costs and so on.
#05 STEP Inverter features
A variety of installation methods: according to different schemes, different working conditions, optional cabinet machine, all-in-one machine (frequency converter + motor).
Match a variety of motor types: such as asynchronous motor, permanent magnet synchronous motor, permanent magnet direct drive motor, reluctance motor, etc.
High protection level: AS170Plus protection level is IP55, can be combined with permanent magnet direct drive motor.
Intelligent function design:
√ Solve the pain points of cooling tower fans working in cold, wet and other harsh environments;
√ Solve problems such as resonance and communication failure and shutdown;
√ Make the cooling tower fan system have good self-diagnosis and self-protection ability.
#06 Cooling tower application case
Case 1: Frequency converter + asynchronous motor
Key Features:
By setting the built-in PID function of the frequency converter, the temperature is taken as the control object to complete the closed-loop control;
Through the frequency detection upper limit and lower limit control motor frequency conversion, power frequency switching;
Set acceleration and deceleration rounded corners to make the fan start and stop more smoothly;
With flying start, when the fan is reversed by the external wind or has an initial speed, flying start enables the fan to start smoothly and without impact.
Case 2: Frequency converter (cabinet) + permanent magnet direct drive
The frequency converter used for this application is AS500, which is installed in the control cabinet:
Key Features:
Using permanent magnet direct drive mode, compared with the use of gearbox/belt and other driving modes, it has the advantages of high efficiency and energy saving, high control accuracy, good control performance, large starting torque, etc. When used with AS500 frequency converter, it can bring customers more intelligent use experience, improve convenience, reduce maintenance costs.
Case 3: Frequency converter (all-in-one) + permanent magnet direct drive
As shown in the figure, the frequency converter has a high level of protection, directly installed on the motor, simple structure, convenient installation, high efficiency, large torque, good control performance, low noise, almost no maintenance.