Abstract: There is significant potential for energy-saving retrofits of central air conditioning systems. This paper discusses the use of the logic control and communication functions of frequency converters and PLCs to analyze various important air conditioning systems. Based on this analysis, a system control scheme and implementation method are proposed, completing a comprehensive energy-saving retrofit plan. This is of great significance for reducing energy consumption and improving efficiency in central air conditioning systems.
Keywords : Central air conditioning inverter technology, PLC logic control, communication capability
Introduction of the Reformation of Central Air Conditioning System Energy-saving
WU Mu-Rong
Delta Greentech (China) Co., Ltd. (GuangZhou 510500 )
Abstract: The reformation of Central air conditioning energy-saving has great potential for reducing loss. This paper discusses the use of inverters and PLC logic control and communications functions, with the analysis of all important systems of air conditioning, proposes a system control scheme and its implementation. The full energy saving scheme is achieved. and has significant meaning for reducing the energy consumption of central air conditioning and improving the efficiency.
Keywords: central air conditioning, inverter technology, PLC, logic control, communication capability
1. Introduction
Central air conditioning is a major energy consumer in buildings, accounting for about 60% of annual electricity bills during normal heating or cooling seasons. Therefore, energy-saving retrofitting of central air conditioning systems is particularly important. Because central air conditioning systems are designed for maximum load with a 10-20% design margin, and in reality, the systems do not operate at full load most of the time, there is significant redundancy and considerable potential for energy savings. Furthermore, chilled water pumps and cooling water pumps cannot adjust their operating speed and number according to load changes; they can only regulate system flow and pressure differentials through valves and bypasses. This inevitably leads to significant flow losses and phenomena such as high flow, high pressure, and low temperature differences, resulting in substantial energy waste (increasing the extra load on chilled water pumps indirectly increases the load on chiller units) and situations where the lowest-level units of the central air conditioning system do not achieve optimal performance.
This article focuses on the specific characteristics of a hotel renovation project and utilizes a frequency converter and PLC control system to carry out energy-saving renovations on the original project's central air conditioning system, enabling it to make more rational use of energy. This is of great significance for reducing energy consumption and improving efficiency.
2. Project Introduction
The energy-saving renovation points of a hotel renovation project in Guangdong are as follows:
1. The chilled water pump control of the central air conditioning system in the East/West Building. Reason for modification: The cooling capacity was manually adjusted by adjusting the pipe resistance, which, although meeting the usage requirements, resulted in a huge waste of energy.
2. Cooling water pump control of the central air conditioning system in the East/West Building. Reason for modification: The cooling water flow (heat exchange capacity) was manually adjusted by adjusting the pipe resistance, which, although meeting the usage requirements, resulted in a huge waste of energy.
3. The cooling tower fan control of the central air conditioning system in the East/West Building was modified because: firstly, frequent starts resulted in large inrush currents, affecting the lifespan of contactors and motors; secondly, the air volume could not be automatically adjusted according to the supply and return water temperatures, leading to energy waste.
4. Fan coil unit cooling capacity exchange control, mainly distributed in East Building Conference Room 5, Tianbo Mansion and lobby, and West Building bowling alley, banquet hall, western restaurant, first floor lobby, Paradise Bar, Chaozhou City, second floor lobby, east lobby and conference rooms, etc. Reason for renovation: Currently, heat exchange and fresh air supply cannot be quickly adjusted according to the amount of people, and the temperature control is not accurate (the sampling point is at the return air vent; in winter, when heating is provided, hot air rises, and the temperature in the activity area is lower than the set temperature; conversely, in summer, when cooling is provided, cold air descends, and the temperature in the activity area is lower than the set temperature).
5. Water supply system of East Building/West Building. Reason for renovation: Currently, manual large capacity adjustment is used. Due to the large power of the water supply motors (55KW and 30KW respectively), large capacity adjustment will not only cause large power redundancy and energy waste, but also cause unstable water supply, water hammer and starting current impact, which will seriously affect the life of pipe fittings and water quality.
3. Control Scheme and Implementation Method
The hotel's central air conditioning system is divided into two parts: heating and cooling. The cooling part includes a cooling tower, chiller units, chilled water pumps, cooling water pumps, and terminals, while the heating part includes hot water pumps and heaters. The system structure of this central air conditioning system is shown in Figure 1.
Figure 1. System structure diagram of central air conditioning
The configuration diagrams for the west and east buildings of the central air conditioning system are shown in Figure 2 and Figure 3, respectively:
Figure 2 West Building Configuration Diagram
Figure 3 East Building Configuration Diagram
(1) Refrigeration unit
The typical design of a chiller unit control system involves installing a temperature sensor (T) on the supply and return water mains of the chiller unit, and a flow meter (F) on the chilled water return pipe. These three signals are simultaneously input to the controller. The controller calculates the building's cooling load Q = F * ΔT. Based on the chiller unit's efficiency curve, and after calculation and comparison, the combination with the lowest energy consumption is selected. Furthermore, based on the cumulative operating time of the equipment, the optimal chiller combination is automatically selected. This ensures that the system's total energy consumption remains at a minimum, achieving the best energy-saving effect.
Although large capacity regulation will result in large power redundancy and large energy waste, considering the operating characteristics of the chiller unit, it is not suitable to carry out frequency conversion modification without the cooperation of the manufacturer. Therefore, this solution will not be considered at this time.
(2) Refrigeration pump set/cooling pump set
Control method: Based on the temperature difference, flow rate and pressure difference between the supplied and returned water, the number of units to be started and the operating frequency of the variable frequency pump are calculated and determined, and the optimal heat exchange capacity is automatically adjusted.
Because the water pump uses a Y-Δ starting method, the starting current of the motor is 3 to 4 times its rated current. Under such a large current surge, the service life of the contactor and motor will be affected. The mechanical impact during startup and the water hammer phenomenon when the pump stops can easily damage mechanical parts, bearings, valves, and pipelines, increasing maintenance workload and spare parts costs. In addition, the need for startup alone will necessitate increasing the power distribution capacity of the entire building several times, increasing investment costs several times over. Variable frequency drives (VFDs) use a soft-start method. After using a VFD to control the motor, there is no inrush current during startup and operation. Inrush current is the most significant factor affecting the service life of contactors and motors. Furthermore, using a VFD to control the motor also avoids water hammer, thus greatly extending the service life of the motor, contactor, mechanical parts, bearings, valves, and pipelines.
Without the bypass valve, the frequency converter can adjust the speeds of the chilled water pump motor and the cooling water pump motor according to changes in the load of the chilled water pump and the cooling water pump, thus ensuring the normal operation of the central air conditioning system and achieving energy savings. As the pump motor speed decreases, the amount of electrical energy absorbed by the motor from the power grid is significantly reduced.
Reduced power consumption ΔP = PO(1 - (N1/N0)^3); Reduced flow rate ΔQ = Q0(1 - (N1/N0))
N1 is the changed rotational speed, N0 is the original motor rotational speed, P0 is the motor shaft power consumption at the original motor rotational speed, and Q0 is the water pump flow rate generated at the original motor rotational speed.
From the above formula, it can be seen that the flow rate Q is directly proportional to the first power of the rotational speed N, and the power consumption P is directly proportional to the cube of the rotational speed N. Assuming the original flow rate is 100 units and the energy consumption is also 100 units, if the rotational speed decreases by 10 units, we can deduce from ΔQ = Q0[1 - (N1/N0)] = 100 * [1 - (90/100)] = 10 that the flow rate has changed by 10 units. ΔP = P0[1 - (N1/N0)^3] = 100 × (1 - (90/100)^3) = 27.1, indicating that the power will decrease by 27.1 units, meaning that a 10% decrease in flow rate results in a 27.1% decrease in energy consumption.
When the opening degree of a butterfly valve is used to control the flow rate of chilled or cooling water, the butterfly valve resistance and power P change (as shown in Figure 3) from curve 1 to curve 2. The flow rate decreases, but the power does not decrease much. If the rotation speed is adjusted (as shown in Figure 4), the HQ curve changes from curve 1 to curve 2. When the butterfly valve opening is 100%, the butterfly valve resistance is zero, the pipeline resistance remains unchanged, and the power is saved significantly.
Figure 3. Curves showing the relationship between the resistance tube and the power P.
Figure 4. Curves showing the variation of resistance tube and power P at different rotational speeds.
The system modification method is as follows:
The control diagrams for the chilled water pumps in the west and east buildings are shown in Figures 5 and 6, respectively:
Figure 6. Control principle diagram of the chiller pump in the west building
(3) Cooling tower fan
Control method: The goal is to maintain a constant supply/return water temperature difference by adjusting the cooling tower fan airflow. However, changes in ambient temperature or heat exchange volume during operation generally do not require the cooling tower fan to operate at full speed.
n=60f(1-s)/p;-----p: Number of motor poles
According to fluid mechanics, the air pressure H is proportional to the rotational speed n; the power consumed P is equal to the product of the air volume Q and the air pressure H (i.e., the output power P is proportional to the rotational speed n), that is, Q=K1n; H=K2n2; P=Q*H=K1K2n3
A 20% reduction in airflow corresponds to a 20% reduction in rotational speed, resulting in a power saving of ΔP = K[n³ - (0.8n)³] = 0.438 K₁K₂n
A 50% reduction in airflow corresponds to a 50% reduction in rotational speed, resulting in a power saving of ΔP = K[n³ - (0.5n)³] = 0.875 K₁K₂n
It is evident that cooling tower fans are often performing unproductive work, wasting a significant amount of energy. Therefore, under the premise of ensuring normal system cooling airflow, even after accounting for the potential reduction in motor and fan efficiency due to the decrease in speed, the energy-saving effect of using frequency converter speed control is still very significant.
The modification method is shown in Figure 7:
Figure 7 Cooling tower fan system modification diagram
Control results: Significantly reduced energy consumption; no starting current surge.
(4) Fan coil unit heat/cooling exchange control
In central air conditioning systems, besides fresh air handling units and air conditioning units, fan coil units are widely used for heating and cooling in various rooms. A fan coil unit consists only of the coil, a three-speed fan, an electric regulating valve, a temperature sensing component, and a controller. Generally, the three-speed fan switch, temperature sensing component, and controller are integrated into a single unit. Currently, there are two types on the market: one type uses a DDC controller with communication capabilities with the main unit. This type of controller allows for centralized control via a calculation system, and is used in the west building. The other type lacks communication capabilities. It divides the fan coil units into several groups according to the water system connections. Each group's branch inlet is equipped with a flow meter, a supply and return water differential pressure transmitter, and supply and return water temperature sensors. This allows the system to calculate the opening degree of the fan coil unit's water valve and send a command to the electric regulating valve, adjusting it to the appropriate opening degree to ensure the required water flow through the coil. This type is used in the east building. To address the problem, the supply air temperature (setpoint adjustable) can be controlled by adjusting the opening of the two-way electric regulating valves of the heating coil and the cooling coil, as well as the supply air volume (fan speed), based on the sampled return air temperature and its carbon dioxide enthalpy. The method is shown in Figure 8.
Figure 8. Fan coil unit heat/cooling exchange control modification diagram
In addition to controlling room temperature, it is also necessary to monitor outdoor temperature and humidity to provide a comfortable environment. By detecting indoor temperature and humidity, the fresh air volume of air conditioners and fresh air units can be adjusted in real time, allowing for full fresh air during transitional seasons and reduced fresh air during peak air-conditioning seasons. The fresh/return air mixing ratio of the air conditioner can be automatically adjusted based on the monitored CO2 concentration to provide a comfortable environment for long-term activities while also achieving energy savings.
(5) Water supply
With the target water supply pressure as the goal, the VFD-F's automatic pump addition and subtraction function and frequency regulation function are used to make reasonable use of energy to maintain a constant water supply pressure based on the actual water consumption, while achieving shock-free start-up and avoiding water hammer effect.
The modification diagram is shown in Figure 9:
Figure 9. Water supply system renovation diagram
3. Conclusion
Based on the actual characteristics of the customer's equipment, this paper utilizes variable frequency technology and the powerful logic control and communication functions of PLC to provide the customer with a complete control system upgrade solution. This solution enables the central air conditioning system to utilize energy more rationally and avoid unnecessary energy waste. Operational practice shows that the system is stable, safe, reliable, and cost-effective, making it worthy of reference and promotion by industry peers.
References
[1] Programmable Logic Controller Application System Design and Communication Network Technology. Guo Zongren et al. People's Posts and Telecommunications Press, 2002.
[2] Selection and Application of Frequency Converters in Industry Liu Jidang et al. Science and Technology Information, No. 23, 2009
[3] Interference and its Suppression in Frequency Converter Applications Zhai Zhangzhi China Science & Technology Expo 2009, No. 02
About the Author
Wu Murong, male, is the Chief Automation Engineer of the South China Region of Delta Electronics' Electromechanical Business Unit. He has extensive application experience in printing, machine tools, textiles, HVAC and other fields. He is currently engaged in the integration, application and research of Delta's automation products and is a senior technical expert.
