The central air-conditioning is the indispensable facility in modern buildings, hotels and shopping malls. It can make people feel like spring all the year round and comfortable every day.Due to its great power and energy consumption, moreover, the phenomenon of “strong horse drawing small cart” in design, all the electric charges from the central air-conditioning become a large sum of expenses. Because of changes from seasons, days and nights, check in rates of guests in hotels and open time of entertainment places, these will change the indoor heat source absorbance of the central air-conditioning system. In addition, motor power has considerable margin in technological design. So electricity saving space exists obviously. Introducing frequency conversion technology into the central air-conditioning can keep indoor temperature constant. The energy saving improvement is a shortcut to reduce costs and enhance efficiency.
Central Air-conditioning System
Figure 1 shows a typical system of the central air-conditioning unit mainly consisting of the frozen water cycle system, cooling water circulation system and master.
Figure1 Central air-conditioning system schematic diagram
lFrozen Water Cycle System
This part is made up of the frozen pump, indoor fan and frozen pipe. The low temperature frozen water flows from the master evaporator. After pressurized by the frozen pump, water runs into the frozen water pipe (outlet water), enters indoors for heat exchange, takes away the heat and finally comes back to the master evaporator (inlet water). The indoor fan is used to blow air through the frozen pipe, reduce air temperature and accelerate indoor heat exchange.
lCooling Water Circulation System
The system consists of the cooling pump, cooling water pipe, cooling tower and condenser. While exchanging indoor heat, the system will take away lots of heat. The heat taken away can be transferred to the cooling water by the refrigerant in the master, raising the temperature of the cooling water. The water will be pressurized into the cooling tower by the cooling pump (outlet water), exchange heat with air and then go back to the master condenser after lowering its temperature (inlet water).
It consists of the compressor, evaporator, condenser and refrigerant (cold-producing medium).The circulation process goes as follows: At first the low pressure gas refrigerant is pressurized into the condenser by the compressor and is gradually condensed into the high pressure liquid. In the process of condensation, the refrigerant releases a lot of heat which is absorbed by cooling water in condenser, then is sent to the outdoor cooling tower, and finally releases in air. When the following high pressure liquid refrigerant in the condenser flows through the throttle depressurizing installation in front of the evaporator, it will gasify due to pressure mutation and then form a gas-liquid mixture into the evaporator. The refrigerant gasifies constantly in the evaporator and meanwhile absorbs the heat in the frozen water to reach a lower temperature. Finally the gasifying refrigerant is transferred into low pressure gas again back into the compressor and the cycle repeats.
lEnergy Saving Theory
Operating Condition Before Energy Saving Improvement for Central Air-conditioning. When the central air-conditioning system is designed, motor capacity selection of frozen and cooling pumps has a certain design margin according to the maximum design heat load of buildings. Due to season climate, day and night temperature changes, the heat load for the central air-conditioning is changing constantly all the time. The following figure 2 is about the average heat load from a residential building.
Figure 2 Heat load rate graphs
According to the above figure, a year heat load rate of the central air-conditioning below 50% time exceeds 50% of the total operating time. The design temperature difference of the cooling pipe usually ranges 5～6℃ while practice indicates the temperature difference is only 2～4℃ most of the time. This illustrates the flow of the frozen and cooling water for refrigeration usually below the design flow, forming the operating condition of the central air-conditioning with low temperature difference, low load and large flow.
Before using the energy saving system, the water pump runs at full speed all along under frequency power supply and the water supply flow in pipes can be regulated only by the valve or reflux, which will cause a large quantity of throttle and reflux loss, simultaneously increase the motor load and consume a lot of power.
The power consumption of the water pump motor in the central air-conditioning accounts for 30-40% of the total power consumption, so energy saving improvement for the central air-conditioning will have an obvious effect.
lBasis of Energy Saving Theory
According to the fluid mechanics theory, the output flow Q for the transmission equipment (such as centrifugal pumps, fans) of the centrifugal fluid is proportional to the rotating speed n; the output pressure P (head) is proportional to the square of its rotating speed n; the output power N is proportional to the dimension of its rotating speed n. Mathematical formulas show as follows:
Q ＝ K1 × n
P ＝ K2 × n2
N ＝ Q × P ＝ K3 × n3 (K1, K2, K3 are proportional constants)
From the above theory, slow down the rotating speed of the water pump and its output power will be reduced more. If the power supply frequency of the motor is reduced from 50 Hz to 40 Hz ,theoretically, the output power ratio of low frequency 40 Hz and high frequency 50 Hz is (40/50)3=0.512.
It has been found that the way of connecting the frequency energy saving system to the central air-conditioning, regulating the flow in pipes by frequency technology of changing the rotating speed of water pump instead of the valve regulation and reflux can have an obvious effect on energy saving. The general saving rate is above 30%. Simultaneously, soft start function of the inverter and smooth speed regulation performance can realize the stable regulation of the central air-conditioning and prolong the life of the unit and pipes.
Energy Saving Scheme Analysis
The temperature difference between the outlet and inlet water of each circulating water system in the central air-conditioning reflects heat exchange for the whole system. Therefore, controlling the flow of the circulating water according to above difference and thus controlling the heat exchange speed are the preferred energy saving scheme.
lFrozen Water Cycle System
The outlet temperature of the frozen water is determined by the refrigeration effect from the master and is always relatively stable, so the inlet water temperature can reflect indoor heat load accurately. On basis of this condition, the energy saving improvement for the frozen water cycle system can take the inlet water temperature as its control target, and realize the indoor temperature control by the automatic regulation of the inverter for the frozen pump flow.
lCooling Water Circulation System
The system is simultaneously affected by the outdoor environment temperature and indoor heat load. The one-sided water temperature of circulating pipes can’t reflect heat exchange amount accurately, so the reasonable energy saving way is to take the temperature difference between the outlet and inlet water as the basis of the temperature in the control room. Under the condition of the constant temperature of external environment with large temperature difference, it indicates the indoor heat load is relatively great. Increase the rotating speed of the cooling pump and the circulating speed of the cooling water. On the contrary, under a small temperature difference, slow down the rotating speed of the cooling pump.
lScheme Structure Diagram
On basis of above analysis, the total engineering structure for energy saving is shown in Figure 3.
Figure 3 Structure for energy saving improvement of central air-conditioning system
According to above diagram, the basic idea for the energy saving scheme is as follows:
Install temperature sensors respectively at the inlet water pipe of the master evaporator, the outlet and inlet water pipes of the master condenser to carry out the real-time detection of the network temperature and feed back to the inverter as analog signals (0~10V or 4~20mA). After corresponding frequency instructions are calculated and output by the built-in PID of the inverter, regulate the rotating speed of pumps automatically and thus regulate the heat exchange speed of the circulating water. Finally realize the control over the indoor constant temperature. In particular, the feedback handling function of the temperature difference is integrated in the internal design of the inverter, so it’s unnecessary to configure another special control module.
lCircuit Control Scheme
The following chart shows the data of the central air-conditioning unit from a certain company.
Among the three pumps, only use one in spring and autumn and two standby pumps; usually use two during summer peak and one standby.
Requirements: One is operated with frequency conversion and it can be transferred manually.
Other can be started directly with power frequency by manual operation.
Design: Three pump motors with one optional inverter. Select an arbitrary pump working as the master and directly driven with frequency conversion by the inverter (controlled by built-in PID ); other two auxiliary pumps are under manual on-off control in terms of refrigeration features and motors are operated with power frequency. See the following diagram.
Use EV100 Series general inverter in the scheme and equip other control cabinet and electric accessories for AC and the power saving bypass.
lFeatures of Energy Saving Conversion System
1. LED display for the inverter interface, rich monitoring parameters; concise layout for keyboard, easy operation;
2. With the digital double-screen LED display for temperature/ temperature difference sensors, it is easy to set temperature parameters and monitor;
3. With a variety of electric protection installations against overcurrent, overload, overvoltage and overheat, and rich fault alarm output functions, the inverter can effectively keep the water supply system operate properly;
4. After additionally installed the inverter, motors have soft-start and stepless speed regulation functions to reduce the machinery wear of pumps and motors and prolong the life of pipes;
5. The inverter is internally installed with the filter capacitor of large capacity which will effectively raise the power factor of electric equipment;
6. Realize PID closed loop regulation for temperature, balance indoor temperature changes, and make people feel comfortable.
Applying frequency technology to the central air-conditioning plays an important part in raising the automation level for the central air-conditioning, reducing the energy consumption, limiting the impact on power grid, and prolonging the life of machinery and pipes.