I. Overview:

In the central air conditioning system, the capacity of the chilled water pump and the cooling water pump is selected according to the maximum design heat load of the building with a certain design margin is left. In a system that does not use speed regulation, the pump runs at full speed throughout the year, and has to use check valve or return valve to regulate the flow, resulting in a large amount of loss. And for the pump motor, It runs at full speed all the time, thus causing a huge waste of energy.       

Due to changes in the four seasons, the temperature of the outside air is different in cloudy and rainy weather, in day and night, making the thermal load of the central air conditioner much lower than the design load for most of the time. That is to say, the central air conditioner actually runs at low load condition most of the time. According to statistics, 67% of the engineering design heat load value is 94-165W/m2, while in fact 83% of the engineering heat load is only 58-93 W/m2, and the full load operation time does not exceed 10-20 hours per year. It is proved that adding variable frequency system to the circulating system of the central air conditioner (cooling pump and refrigerating pump), and using variable frequency technology to control motor speed and adjust the flow and pressure can achieve significant energy saving effect.

ii the principle of energy saving:

The working principle of the water pump and the fan shows that the flow rate (air volume) is proportional to the rotation speed; while the pressure (pumping head) is proportional to the squared value of rotation speed. And the shaft power of the water pump and the fan is equal to the flow rate multiply the pressure, so the shaft power is proportional to the cubic value of the speed (ie, proportional to the cubric of the power frequency). Tell from the above principle, the power of the pump and the fan can be reduced more by reducing its speed.

For example, if the frequency is reduced from 50Hz to 45Hz, then P45/P50=(45/50)3=0.729, ie P45=0.729P50 (P is the motor shaft power); if the power supply frequency is reduced from 50Hz to 40Hz, then P40/ P50=(40/50)3=0.512, ie P40=0.512P50 (P is the motor shaft power).

It can be seen from the above theory that when the flow rate (air volume) is controlled by the frequency converter, a large amount of electric energy can be saved. The central air-conditioning system is designed according to the maximum cold air demand in the site. And the cooling pump and refrigeration pump power is chosen according to the maximum cool air demand. But in actual use, more than 90% of the time the cooling pump and chiller pump runs lower than full-load. The use of check valve and return valves not only increases the system loss, but also the A/C system fluctuates because the adjusting is staged. The problem can be solved completely by installing frequency converters on the cooling pump and chilling pump. Automation is realized and investment can be recouped by saving energy cost. Further more, the VFD features soft starting and smooth regulating. It ensures the system works stably and extends the service life of A/C system and pipe network.

Therefore, The VFD can regulate the pump speed according to the flow demand required by the actual cooling load. The load change feedback to inverter PID function and form the closed-loop control, and so the pump runs along with the changing load. The energy saving is normally higher than 20%. The retrofit plan’s energy saving efficiency is related to the application site conditions. Generally speaking, the energy saving in Spring and Autumn is higher which can be up to 40%; while in summer the cooling demand is quite large, and so the energy saving capacity is limited to around 20%.

iii Application case:

1. The existing central air conditioner of Renda Building is a set of units consists of 2 sets of 15KW cooling pumps (one for use, one for backup) and 2 sets of 11W refrigerating pumps (one for use, one for backup). Since the main compressor retrofit space is very limited, we only carry out the energy-saving retrofits for cooling and refrigeration systems.

When the central air conditioner is running, the cooling system and the refrigeration system temperature difference between the inlet and outlet (△T) is about 2 °C, according to:

Heat taken away by chilled water and cooling water (r)_= Flow rate (Q) × temperature difference (△T)

We can increase the temperature difference (△T) and reduce the flow rate (Q), that is, reduce the speed, to achieve energy saving.

The principle diagram of the frequency inverter transformation of the central air conditioning system is as follows:

2. chilled water system

For the chilled water system, the temperature of the chilled water depends on the operating parameters of the evaporator, it is only necessary to control the temperature of the higher temperature chilled water (return water) to control the temperature difference. The temperature sensor, the PID regulator and the frequency inverter are now used to form a closed loop control. The system, the chilled water return water temperature is controlled at T1, so that the chilled water pump speed changes according to the change of the heat load.

3. Cooling water system

For the cooling water system, the temperature of the cooling water on both sides of the condenser is taken as the controlled targets. the temperature sensor, the PID temperature difference regulator, the frequency converter and the cooling water pump form a closed-loop control system. and the temperature difference of the cooling water is controlled at ΔT2 to make the speed of the cooling water pump changes to the heat load. The temperature difference of the cooling water is kept at the set value, so that the cooling pump system can save the maximum amount of energy under the same application conditions.

4, Control method

This retrofit program is installed on the basis of retaining the original power frequency system. It adopts PLC to automatically switch the pumps, monitor electricity consumption time and condition. With the installation of the meter, the power saving situation can be clearly known, and the contract energy management (EMC) can be easily carried out.

iv Central A/C system performance after the transformation

(1) Using inverter to form the closed-loop control. The software can be configured as needed and the temperature can be set for PID adjustment, so that the output power of the motor changes with the change of the heat load, and the maximum energy saving can be achieved under the premise of meeting the use requirements.

(2) Due to the lower speed operation and soft start, vibration, noise and wear are reduced; the equipment maintenance period and service life are prolonged; the MTBF (mean time between failures) value of the equipment is increased; and the impact on the power grid is reduced; system reliability is enhanced.

(3) The system has various protection measures to greatly improve the operating rate and safety reliability of the system.

(4) The variable frequency closed-loop control system and the original power frequency control system are interlocked with each other. When the variable frequency speed control closed-loop control system is in maintenance or faulty, the original power frequency control system can still operate normally.