Analysis of Causes of Hydraulic Imbalance and Design Preventive Measures
The causes of hydraulic imbalance in air-conditioning water systems are complex and varied, and adopting preventive measures at the design stage is the fundamental approach to solving hydraulic balance problems. According to Section 8.5 of GB 50736 "Code for Design of Heating Ventilation and Air Conditioning of Civil Buildings," the piping layout of air-conditioning water systems should follow the principle of symmetrical loop arrangement, so that the path lengths of each terminal loop are as close as possible. A five-star hotel typically contains different functional zones such as guest room areas, lobby areas, dining areas, conference areas, and fitness/SPA areas; the air-conditioning terminal arrangements in each zone differ greatly, and the pipeline resistance characteristics differ, making hydraulic imbalance highly likely. From the perspective of fluid mechanics, the root cause of hydraulic imbalance is the non-uniform distribution among pressure energy, kinetic energy, and potential energy in the Bernoulli equation across each loop. When the resistance losses of parallel loops differ, the loop with lower resistance receives more flow, resulting in over-flow; the loop with higher resistance receives insufficient flow, resulting in insufficient cooling supply. Technical measures to prevent hydraulic imbalance at the design stage include: (1) The piping system should adopt a reverse-return arrangement, i.e., the water flow direction of the supply and return mains between each riser or terminal is the same, so that the resistance of each loop is similar. Article 8.5.3 of GB 50736 recommends that air-conditioning water systems adopt a reverse-return arrangement; when the system complexity is high and the transmission distance differences are large, balancing valves should be installed for adjustment. (2) Reasonably determine pipe diameters so that the specific frictional resistance is controlled within the range of 100-300Pa/m, and the specific frictional resistance of terminal branch pipes should not exceed 150Pa/m. (3) Preset balancing valve interfaces at key nodes of the piping system, including: on the return pipe of each main riser, on the supply and return branch pipes of each zone, on the supply and return pipes of each air handling unit or fresh air handling unit, and on the return pipe of each fan coil unit. According to the Marriott MIDS standard, all terminal equipment of the air-conditioning water system in a five-star hotel should be equipped with balancing valves, with a minimum size of DN15. (4) For vertical systems with a height difference exceeding 50m, the influence of gravitational head on hydraulic balance should be considered; static balancing valves should be installed on the supply and return mains of vertical zones, and the influence of hydrostatic pressure on the pressure-bearing capacity of terminal equipment on each floor should be verified. Article 8.5.6 of GB 50736 stipulates that the difference in pressure loss between parallel loops of an air-conditioning water system should not exceed 15%. When calculation shows that the imbalance rate exceeds 15%, balancing valves or orifice plates should be installed in the low-resistance loop. For a project such as a five-star hotel with a large number of terminal devices and a large system scale, it is recommended to use fluid simulation software (such as PipeFlow, AFT Fathom, etc.) at the design stage to conduct full-system hydraulic analysis, simulate the flow distribution under various operating conditions, and optimize the piping routing and equipment selection. Practice shows that investing 1-2 weeks in hydraulic simulation analysis at the design stage can reduce more than 80% of the hydraulic balance commissioning problems at the construction stage.
Static Balancing Valve Selection and Flow Pre-setting Technology
Static balancing valves are the core components for achieving precise hydraulic balance in air-conditioning water systems, and their selection, installation, and setting directly affect the balancing effect. According to Article 11.2.3 of GB 50243 "Code for Acceptance of Construction Quality of Ventilation and Air Conditioning Works," balancing valves installed in air-conditioning water systems should have linear flow characteristics, and the adjustment accuracy should not be lower than ±2% of the dial scale. The working principle of a static balancing valve is to increase local resistance by changing the opening of the valve core, simulating the design resistance of the loop, so that each parallel loop achieves the design flow distribution. When selecting a static balancing valve, the following technical parameters should be considered with emphasis: (1) Nominal diameter (DN), which should be consistent with the nominal diameter of the pipe; reduced-diameter valves should not be used, otherwise water resistance and noise will increase; (2) Nominal pressure (PN); the operating pressure of chilled water pipelines in hotels is usually between 0.6-1.0MPa, and PN16 or PN25 is recommended; (3) Kv value (flow coefficient), Kv=Q/√ΔP, where Q is the design flow (m³/h) and ΔP is the pressure drop when the valve is fully open (bar). Generally, the pressure drop when the balancing valve is fully open is controlled between 3-8kPa, and the Kv value is calculated and determined according to the design flow and the set pressure drop. (4) Adjustment characteristics; linear or equal-percentage adjustment characteristics should be selected to ensure uniform flow change throughout the entire stroke range from fully open to fully closed. (5) Measurement accuracy; a high-quality static balancing valve should be equipped with high-precision measuring connectors, which can be connected to a differential pressure gauge to directly measure the pressure difference before and after the valve, with an accuracy of ±1%. The implementation steps for flow pre-setting are as follows: Step 1, according to the design flow of each terminal and the valve Kv value, calculate the preset opening value (number of turns or scale) of each balancing valve. Step 2, after the system is filled with water and air is removed, start the water pump to operate under design conditions. Step 3, commission according to the steps of the Proportional Method: first commission the terminal valve of the most unfavorable loop to be fully open, then sequentially commission the balancing valves of each branch so that the deviation between the measured flow and the design flow is within ±5%. The core principle of the Proportional Method is that when each terminal valve is adjusted proportionally at the same time, the hydraulic conditions of the entire system remain similar, avoiding repeated commissioning. Article 8.5.8 of GB 50736 clarifies the application requirements of static balancing valves in air-conditioning water systems: when the calculated pressure loss difference between parallel loops of the system is greater than 15%, static balancing valves should be installed in the low-resistance loop. For five-star hotels, it is recommended to install static balancing valves at the following locations: the return pipe of the horizontal branch pipe on each floor, the return pipe of each air handling unit, the return pipe of each fresh air handling unit, and the return pipe of the main riser. Taking a five-star hotel as an example, a total of about 180 static balancing valves were installed (of which about 120 of DN15-DN25 were used for fan coil units, about 50 of DN32-DN80 were used for air handling units and fresh air handling units, and about 10 of DN100-DN150 were used for riser main valves).
Combined Control of Dynamic Differential Pressure Control Valves and Variable-Frequency Water Pumps
In the air-conditioning water system of a five-star hotel, the combined control of dynamic pressure-independent control valves (PICV) and variable-frequency pumps is a key technology for achieving efficient operation of a variable-flow system. Compared with a pure static balancing valve solution, adding dynamic differential pressure control can significantly improve the system's dynamic response characteristics and part-load operating efficiency. According to Clause 8.5.5 of GB 50736, a variable-flow air-conditioning water system shall adopt differential pressure control or temperature control to regulate pump speed, and electric two-way valves or regulating valves shall be installed at terminal equipment. The dynamic pressure-independent control valve (PICV) integrates the functions of a static balancing valve and a dynamic differential pressure regulator, and can automatically adjust the valve plug position when the differential pressure across the valve fluctuates (e.g., caused by the action of valves in other loops), maintaining a constant differential pressure across the terminal equipment and thereby ensuring stable flow. The technical parameter requirements of the PICV include: differential pressure control range (typically 20-200 kPa), maximum shut-off differential pressure (must meet the maximum pump head requirement), leakage rate (≤0.05% Kvs) and response time (≤2 s). When PICVs are adopted, static balancing valves only need to be installed at main branches such as the distribution/collection headers or risers, and no separate static balancing valves are required at terminal equipment. For the control strategy of variable-frequency pumps, the most unfavorable loop differential pressure control method is recommended: a differential pressure sensor is installed in the terminal loop farthest from the pump, and the differential pressure signal is transmitted to the variable-frequency controller, compared with the set value, and then the pump speed is adjusted. Clause 8.5.9 of GB 50736 stipulates that the minimum frequency of variable-frequency pumps should not be lower than 25 Hz, and the maximum frequency should not exceed 50 Hz, so as to avoid cavitation of the pump under low-flow conditions or overload under high-flow conditions. The combined control system also needs to be provided with a minimum differential pressure bypass valve: when the variable-frequency pump frequency drops to the lower limit while the system load continues to decrease, the bypass valve opens automatically to maintain the minimum flow required by the chillers. The bypass valve capacity shall be selected at 120% of the minimum allowable flow of a single chiller. According to the recommendation of ASHRAE Guideline 12-2020, the combined PICV + variable-frequency pump control scheme in a variable-flow system can save 40%-60% of pump energy consumption, and when all terminals are equipped with electric regulating valves, the annual hydraulic balance rate of the system remains above 95%. The Marriott MIDS standard requires that the hotel air-conditioning water system adopt a variable-flow design, all terminal equipment be equipped with electric regulating valves (or PICVs), and the supply/return water differential pressure sensor be located at 2/3 of the distance to all terminal equipment. In actual projects, five-star hotels usually adopt the scheme of electric two-way valve + static balancing valve for fan coil units in guest room areas, which has lower cost; for air handling units and fresh air handling units, PICVs are configured to obtain better dynamic regulation performance.
Commissioning process and acceptance criteria
Hydraulic balancing commissioning of the air-conditioning water system is a key step in verifying design and construction quality, and must be carried out strictly in accordance with the specified process and with complete commissioning records. According to the requirements of Section 11.2 of GB 50243 "Code for Acceptance of Construction Quality of Ventilation and Air Conditioning Works", water system commissioning shall be carried out in the following process: system flushing → individual equipment trial run → water system balancing commissioning → system joint trial run. System flushing is a prerequisite for commissioning and shall adopt the sectional cleaning method, with drain valves installed at the lowest points of the pipe network, and the flushing water velocity shall not be lower than 1.5 m/s. Clause 11.2.2 of GB 50243 stipulates that pipeline flushing shall remove welding slag, rust, sediment and other debris from the piping system, and the flushing water shall be discharged to a safe outdoor location. The flushing acceptance criteria are: the color and transparency of the water at the discharge outlet are visually consistent with those at the inlet, and the impurity content in the water is ≤20 mg/L. The specific steps of hydraulic balancing commissioning are as follows: (1) Preparation: confirm that all balancing valves have been installed in place and numbered, and prepare differential pressure gauges (accuracy ±1%), flow meters (ultrasonic flow meters, accuracy ±2%), thermometers and other relevant instruments. (2) Confirmation of pump operating conditions: check that the pump rotation direction is correct, the operating current does not exceed the rated value, and the inlet/outlet differential pressure is within the normal range. (3) Identification of the most unfavorable loop: determine the theoretical most unfavorable loop location according to the hydraulic calculation report, such as the highest floor of the building or the farthest terminal equipment. (4) Proportional balancing method commissioning: first fully open the balancing valve at the terminal of the most unfavorable loop, connect the differential pressure gauge to measure the differential pressure, and calculate the actual flow; then sequentially open the other balancing valves on the same branch, adjust the opening of each valve in turn so that the ratio of the flow at each terminal to the design flow is consistent, and finally adjust the main pipe valve uniformly so that the total system flow reaches the design value. (5) Review and spot check: conduct flow review for no less than 20% of the terminals to ensure the deviation is within ±5%. According to Clause 11.2.6 of GB 50243, the acceptance criteria for air-conditioning water system commissioning are: the deviation between the actual water flow and the design water flow of each air handling unit, fresh air handling unit and fan coil unit shall not be greater than ±10%, and when fine adjustment with balancing valves is adopted, the deviation shall not be greater than ±5%. The acceptance standards of five-star hotels are usually stricter, and the Marriott MIDS requires the deviation to be controlled within ±5%. After commissioning is completed, a complete commissioning report shall be submitted, including: commissioning record sheets (including the preset values and actual openings of each valve), flow review records, temperature review records and the signature of commissioning personnel. Clause 11.2.8 of GB 50243 emphasizes that commissioning records shall be included in the completion acceptance documents for archiving.