Application of frequency conversion speed regulating pump in HVAC system
Pumps, as we all know, are power components used to transport liquids and are used in many sectors of the national economy. The variety of specifications. Its classification methods are also different, according to their working principle can be divided into three categories: vane pumps, positive displacement pumps, other types of pumps. Grundfos production of the main products for the centrifugal pump is a vane pump, but also the most widely used pump. The working principle of this kind of pump is that when the impeller rotates at a high speed, the blade pushes the liquid to rotate so that the liquid gets the centrifugal force to complete the water delivery process of the water pump. Therefore, the pump is called a centrifugal pump. Its application areas include domestic hot water supply, sewage drainage, industrial applications, commercial building HVAC cycle, cooling water transport and other aspects. As the centrifugal pump is an important equipment, it needs a lot of power to operate! According to statistics, 20% of the world's electricity is consumed in the pump system. In fact, by taking the necessary technical measures and control measures, 30% -50% of the energy consumption can be saved.
A: constant speed pump and variable speed pump:
The traditional heating and air-conditioning system is based on a separate qualitative adjustment of the operation mode to select the circulating pump, pump principle is the pump flow can not be less than the required flow outside the network, generally in accordance with the theory of external network traffic 1.1 to 1.2 times the head According to the pipeline and the user's total resistance of 1.05 ~ 1.10 times to choose, then the corresponding shaft power is greater than 100%. Can be seen by the constant flow of operation, water pump power consumption is great. And bring the adjustment effect is not very satisfactory.
Pump running at a constant flow rate, as shown in Figure 1: When part of the load state, the system reduces the flow required, in order to adapt to its flow changes, by reducing the valve opening adjustment to change the system characteristic curve, which consumes excess pressure Head, wasting a lot of power!
In addition to changing the valve opening to complete the adjustment of the pump operating point, we can also use to change the pump speed:
It can be seen that when the speed of the pump is changed, the performance curve of the pump will change at the same time, and the speed will change as the frequency [Hz] changes. Analysis of the performance of the circulating pump shows that: the pump flow, lift and shaft power and pump impeller speed there is a certain proportion between:
As can be seen from this, the pump head and the square of the motor speed is proportional to the pump shaft power and the motor speed is proportional to the cube. That is, when the flow rate of the pump is reduced by 20%, the motor speed should be reduced by 20% and the pump power consumption will be reduced by 50%. When the flow rate of the pump is reduced by 50%, the motor speed will be reduced by 50% A reduction of 87.5%. When the system needs to reduce the flow rate, by reducing the speed, the corresponding pump to reduce the flow, the pump shaft power reduction, saving energy significantly. And, due to the use of variable speed regulation, but also to avoid the use of valve adjustment unnecessary head pressure loss.
Second: the principle of speed control:
When the flow rate decreases, the controller will detect the pressure signal (through the sensor or motor current or speed state). At this point, the controller sends a signal to the drive to lower the output (lower frequency) until the pressure returns to the desired level (set point). Conversely, when the flow rises again, the controller will detect that the pressure is decreasing. The controller signals the frequency converter to raise the output (higher frequency) until the pressure returns to the desired level (set point).
Three: variable speed pump overall efficiency
Usually people refer to the pump efficiency is only the pump efficiency [hp], and strictly speaking, we should measure the total efficiency:
The overall efficiency [ht] is evaluated by considering both the motor efficiency [hm], the pump efficiency [hp], and the conversion efficiency [hd]. The overall efficiency refers to the P1 to P4 changes. Motor efficiency varies depending on the motor efficiency level (EFF1 or EFF2) and the motor load.
What is the motor efficiency rating?
The U.S. Congress enacted and implemented the Energy Policy Act (EPACT) in October 1997, which defines efficient electrical machines. A similar definition is also found in the European Union. The European Association of Electrical and Electronics Manufacturers (CEMEP) also defines the minimum requirements for high-efficiency motors in the 1.1-90KW, 2-pole and 4-pole ranges. Now CEMEP and EPACT are the world recognized 50Hz and 60HZ efficient motor standards. According to the CEMEP standard, motor efficiency is divided into three levels: EFF1 - the highest efficiency level; EFF2 - the medium efficiency level; and the EFF3 - the low efficiency level.
Now Grundfos pumps can be equipped with EFF1 (CEMEP standard motor).
What are the advantages of Eff1 motor?
*higher efficiency
Compared to a typical motor, colleges and universities mean that for the same amount of energy, the motor itself has less energy loss and more output. Also, because the motor itself has less energy loss, the energy input to the motor is reduced at the same energy requirements for the same workload, saving on energy consumption and operating costs. Figure 7, when the load is very low (less than 25%), means that the efficiency will be dramatically reduced. But Eff1 motor efficiency is obviously higher than Eff2 motor at this moment.
* Partial load more efficient
Most motors rarely work at 100% full load. It is therefore important that the nominal efficiency of the motor be less than full load. For normal motors, this data is not normally provided, but the efficiency of the EFF1 motor must be at 75% load and 100% load efficiency. For EFF1 motors, these two efficiency values ​​are usually the same.
* Motor operating temperature is low
Because Grundfos EFF1 motor high efficiency, energy loss generated by the heat is very small, so the work of the motor temperature is lower than the average motor temperature. As a result, high-efficiency motors are able to operate at higher ambient temperatures and require less ambient cooling than the standard motors typically used.
Similarly, the resistance of the motor stator windings affected by temperature can be kept within the normal range, and the Grundfos EFF1 motor is less prone to overheating of the stator windings of the motor. In this way, the motor stator winding insulation and motor bearings will increase the service life.
* Low noise
Under normal circumstances, the motor noise is mainly generated by the cooling fan. For the EFF1 motor, less energy is lost due to higher efficiency and less cooling air is needed to reduce motor temperature rise. So most of the EFF1 motors have a small cooling fan and less noise.
* Lower operating costs
The EFF1 motor saves energy, reduces maintenance costs and other associated operating costs, and greatly reduces owner costs. In this regard, EFF1 motors are very competitively priced. In fact, in the case of normal work, the initial cost of procurement increased in less than three years can be fully recovered.
Conversion efficiency
Factors to be considered when calculating conversion efficiency are the motor load and the frequency output. However, when the load is reduced to 30%, the efficiency will vary greatly.
Pump efficiency
Pump efficiency varies depending on the flow rate.
At the same time, the maximum efficiency of the pump varies according to the size of the pump. In general, a large pump will be more efficient than a small pump. When the pump speed changes, the efficiency of the pump will change at the same time. If the speed is reduced by less than 20%, the change in efficiency is small.
Although the overall efficiency is important for energy consumption, the control model has a greater impact on energy consumption. The figure below shows the energy savings in different control modes:
Four: Conclusion:
As technology advances, more and more frequency conversion technology will be recognized and applied by the people, with the correct system design (such as the three pump system) and the pump with the election of the pump, allowing users to feel more comfortable at the same time , Get more obvious energy-saving effect.
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