What are the selection steps and variable-frequency control strategy for air-conditioning chilled water pumps and cooling water pumps?
I am an HVAC expert with many years of experience in HVAC system design and optimization, and I know well that pump selection and variable-frequency control strategy are the core of the energy efficiency and stability of central air-conditioning systems. Improper pump selection will lead to a surge in system energy consumption, hydraulic imbalance, and even equipment damage; if the variable-frequency control strategy is unreasonable, it cannot realize energy-saving potential and may even cause oscillation or inefficient operation. The following is a detailed analysis based on standards and engineering experience.
I. Selection steps for chilled water pumps and cooling water pumps
1. Clarify system design parameters
According to Article 8.2.1 of GB 50736-2012 "Design Code for Heating Ventilation and Air Conditioning of Civil Buildings," the design flow of pumps shall be calculated based on cooling load and supply-return water temperature difference:
- Chilled water pumps: flow \( Q_{ch} = \frac{0.86 \times Q_c}{\Delta t} \) (\( Q_c \) is cooling load, kW; \( \Delta t \) is supply-return water temperature difference, usually taken as 5℃).
- Cooling water pumps: flow \( Q_{cw} = \frac{0.86 \times (Q_c + P_{chiller})}{\Delta t} \) (\( P_{chiller} \) is the compressor power of the chiller, kW; the cooling water temperature difference is usually taken as 5℃).
2. Calculate system resistance
According to Article 9.6.3 of GB 50019-2015 "Design Code for Heating Ventilation and Air Conditioning of Industrial Buildings," the pump head shall overcome the total resistance of the most unfavorable loop, including:
- Chilled water side: chiller evaporator resistance (about 5~10mH₂O), terminal equipment (fan coil unit/air handling unit) resistance (about 3~8mH₂O), and pipeline frictional and local resistance (calculated according to pipe length, pipe diameter, valves, etc.).
- Cooling water side: chiller condenser resistance (about 5~8mH₂O), cooling tower resistance (about 3~5mH₂O), and pipeline and valve resistance.
3. Determine pump model and number
- Number configuration: According to Article 8.2.2 of GB 50736, the number of pumps should preferably correspond one-to-one with the number of chillers, and standby pumps should be provided (usually 1 standby pump).
- Selection principle: Adopt the method of matching the pump performance curve with the system characteristic curve to ensure that the operating point is in the high-efficiency zone (efficiency ≥70%). Give priority to low specific speed, high-efficiency pumps (such as vertical centrifugal pumps).
- Engineering experience: For large systems, it is recommended to use a combination of large and small pumps (such as 3 large pumps + 1 small pump) to adapt to load changes and avoid frequent start-stop.
II. Variable-frequency control strategy
1. Control objectives and principles
The core of variable-frequency control is to adjust pump speed to match real-time load, following the affinity laws: flow is proportional to speed, head is proportional to the square of speed, and power is proportional to the cube of speed. Article 8.5.9 of GB 50736 explicitly requires that air-conditioning water systems should adopt variable-frequency speed control.
2. Chilled water pump variable-frequency strategy
- Differential pressure control: Install a differential pressure sensor at the end of the most unfavorable loop of the system to maintain a constant supply-return water differential pressure (usually set to 80%~100% of the design differential pressure). When the terminal valve opening decreases (load decreases), the differential pressure rises, and the frequency converter reduces the pump speed.
- Temperature control: Use the supply-return water temperature difference as the feedback signal and adjust the pump speed to keep the temperature difference constant (such as 5℃). However, note that this method responds slowly and is easily affected by system thermal inertia.
- Comprehensive strategy: A compound control of "differential pressure as the primary control, temperature difference as correction" is recommended. For example, in a commercial complex project, the differential pressure setpoint was dynamically adjusted by season (0.15MPa in summer, 0.12MPa in transition seasons), combined with PID regulation, and the system energy-saving rate reached 25%.
- Temperature difference control: Take the cooling water supply-return temperature difference as the target (usually set to 5℃), and adjust the speed according to changes in the chiller condenser heat load. Note that the condensing pressure must be avoided from being too low (below the minimum allowable value of the unit).
- Optimization strategy: Combined with outdoor wet-bulb temperature, adopt dual-parameter control of "cooling water temperature difference + condensing pressure." For example, in a data center project, when the outdoor wet-bulb temperature was below 18℃, the temperature difference was relaxed to 7℃ to reduce pump energy consumption while ensuring unit safety.
- Minimum speed limit: The speed of variable-frequency pumps should not be lower than 30%~50% of the design speed; otherwise, problems such as insufficient motor heat dissipation and shaft seal leakage may occur.
- Multiple pumps in parallel: When multiple variable-frequency pumps are operated in parallel, the logic of "first started, first stopped" or "cyclic rotation" should be adopted to avoid water contention between pumps or inefficient operation.
- Interlock with chillers: Variable-frequency control needs to be coordinated with the chiller loading/unloading logic to avoid frequent start-stop. For example, in a hospital project, because the cooling water pump variable-frequency response was too fast, the unit condensing pressure fluctuated; the problem was solved after setting a 100-second delay.
III. Actual engineering case
Case: A super high-rise office building (building area 120,000 m²)
- Selection: Cooling load 8000kW, chilled water temperature difference 5℃, calculated flow 1376m³/h, system resistance 32m. Four vertical centrifugal pumps were selected (3 duty + 1 standby), each with a flow of 460m³/h, head of 35m (including 5% margin), and efficiency of 82%.
- Variable-frequency control: Chilled water pumps adopted differential pressure control (differential pressure at the end of the most unfavorable loop 0.12MPa), and cooling water pumps adopted temperature difference control (set at 5℃).
- Effect: After one year of operation, pump energy consumption accounted for 12% of total system energy consumption (conventional fixed-frequency systems are usually 18%), with annual power savings of about 150,000 kWh.
Summary
The selection of chilled water pumps and cooling water pumps must strictly follow GB 50736 and GB 50019, determine flow and head in combination with hydraulic calculation, and reasonably configure the number of pumps; variable-frequency control strategies should give priority to differential pressure or temperature difference control, and attention must be paid to safety limits and multi-pump coordination. Through optimized selection and control, system energy efficiency can be significantly improved and equipment life extended.
The above are reference opinions. The actual situation of the project still needs to be comprehensively evaluated and implemented in combination with specific operating conditions, site conditions, and other factors.