As a closed water circulation device for the terminal of central air conditioning, the water hammer effect in the fan coil unit (FCU) system belongs to a low-pressure, small-amplitude water hammer. Due to the system’s low operating pressure (generally 0.3~1.0MPa) and small pipe diameter (branch pipes mostly DN15~DN25) compared with industrial large-diameter water systems, the intensity of water hammer is low but easy to be ignored. Long-term or sudden water hammer impact will directly damage terminal equipment and pipeline accessories, which is a common problem in the operation and maintenance of air conditioning water systems.
Core Definition: In the closed air conditioning water system where FCUs are applied, the pressure shock wave generated by the sudden step change of water flow velocity/system pressure causes the rapid conversion of fluid kinetic energy into pressure energy, resulting in the phenomenon of reciprocating propagation and impact in pipelines and equipment. Essentially, it is a pressure mutation induced by the inertial force of the fluid.
I. Core Causes of Water Hammer in FCU Systems
The water hammer in FCU systems is not caused by a single factor, but is mostly related to terminal valve operation, water pump start/stop, and system hydraulic imbalance. Moreover, due to the thin branch pipes of the terminal and fast flow velocity response, water hammer is more likely to occur locally at the inlet and outlet of FCUs. The main causes are divided into four categories:
- Rapid opening and closing of electric two-way valves (the primary cause)
The electric two-way valves (solenoid valves/electric ball valves/butterfly valves) standard equipped with FCUs are the core inducement: when an FCU starts or stops, if the valve is of a quick-opening and quick-closing type (closing time < 1s), the water flow in the branch pipe will be cut off instantly. The high-speed flowing water body suddenly stops due to inertia, and the kinetic energy is converted into pressure energy in an instant, forming a local pressure impact in the valve rear and the FCU heat exchanger; if a normally closed valve is opened suddenly, high-pressure water from the main pipe rushes into the empty pipe/low-velocity branch pipe rapidly, and the sudden acceleration of water flow will also produce negative pressure water hammer (cavitation type).
- Abrupt start and stop of water pumps
When the air conditioning chilled/hot water pumps are started or stopped directly (without soft start/stop devices), the flow velocity of the main pipeline water will rise from 0 to the design value (0.8~1.5m/s) or drop from the design value to 0 in a short time. The pressure shock wave will transmit to each terminal FCU along the main pipe, especially the terminals close to the water pump, where the impact is more obvious; the flow mutation during the parallel switching of multiple water pumps will also cause system pressure fluctuations and indirectly induce terminal water hammer.
- Rough operation of manual valves/pipeline accessories
If the manual globe valves and the valves before and after strainers on FCU branch pipes are opened/closed rapidly during operation and maintenance, the local water flow will be blocked directly; the sudden adjustment of regulating valves and butterfly valves on the system main pipe will also lead to the redistribution of main pipe pressure, causing the flow velocity mutation of terminal branch pipes and generating water hammer.
- Improper water make-up/air exhaust + hydraulic imbalance
Rapid water make-up during the make-up process of the closed system will cause a sudden pressure rise and trigger pressure impact; incomplete air exhaust at the high points of the pipeline will lead to air blockage, and the water flow will impact the air mass to produce local pressure fluctuations. The compression/expansion process of the air mass will repeatedly impact the copper tubes of the FCU heat exchanger; in addition, the system hydraulic imbalance (excessively high flow velocity >2.0m/s in some FCU branch pipes) will amplify the inertial effect of flow velocity mutation and aggravate the water hammer intensity.
- Secondary inducements
Scaling of the FCU heat exchanger and blockage of branch pipe strainers will reduce the local pipeline flow cross-section and cause a sudden rise in local water flow velocity. If the scaling part is unblocked suddenly, the flow velocity mutation will also cause micro water hammer; if the check valve of the system (e.g., at the water pump outlet) is of a quick-closing type, the sudden closing of the valve clack will also transmit pressure shock waves to the terminal.
II. Main Hazards of Water Hammer in FCU Systems
Due to the low system pressure, the water hammer of FCUs will not cause extreme situations such as “pipeline burst” in industrial systems, but the local impact + repeated fatigue damage is the core hazard, which is mainly concentrated on terminal equipment and small-diameter accessories, directly affecting the service life and operational stability of the system:
- Damage to core components of FCUs
The heat exchanger is made of thin-walled red copper tubes (wall thickness 0.3~0.5mm). Repeated pressure impacts will cause cracking at the welding joints of copper tubes and fins and fatigue perforation of copper tube walls, leading to water leakage of the heat exchanger; the impact force of water hammer will also cause loosening of the joints of water distributors/collectors inside the FCU, resulting in internal leakage.
- Valve failure and pipeline leakage
The valve core and valve seat of the electric two-way valve are deformed by pressure impact, leading to poor sealing (constant leakage) or jamming during opening and closing of the valve; the copper joints, union joints and rubber gaskets at the inlet and outlet of the FCU are loosened after impact, causing external leakage of branch pipes; the joints of small-diameter metal corrugated pipes and hoses are easy to disengage due to impact.
- Generation of noise and vibration
When the pressure shock wave impacts the pipeline and equipment, it will cause pipeline vibration + impact noise (mostly “bang” and “click” sounds), which is more obvious during low-load operation at night and affects indoor use; vibration will also cause loosening of pipeline fixed supports, further aggravating hydraulic imbalance.
- Damage to instruments and accessories
Precision accessories such as pressure gauges, temperature sensors and flow switches on FCU branch pipes will suffer damage to diaphragms and sensing elements due to pressure mutation, resulting in distortion of monitoring data; the valve cores of the system’s air vents and water make-up valves will also lose their sealing performance due to impact, leading to water leakage or air blockage.
- Long-term hydraulic imbalance
Repeated water hammer will cause scale and impurities in the pipeline to fall off due to impact, blocking the FCU strainer or heat exchanger, further aggravating the system hydraulic imbalance, resulting in poor heating/refrigeration effect of some FCUs, and forming a vicious circle of “water hammer – blockage – flow velocity mutation – more severe water hammer”.
III. Targeted Prevention and Control Measures for Water Hammer in FCU Systems
The core of water hammer prevention and control for FCUs is “suppressing flow velocity mutation + absorbing pressure impact + optimizing valves and operation”. Combined with the characteristics of the closed air conditioning water system, measures are taken from three dimensions of design, installation, and operation and maintenance, considering both operability and economy. No complex industrial water hammer protection equipment is needed, and the focus is on the optimization of terminals and small pipelines:
(I) Design Stage: Avoid Water Hammer from the Source
- Select slow-closing electric two-way valves (core design requirement)
The FCU terminal must be equipped with slow-opening and slow-closing electric two-way valves, requiring a closing time ≥3~5s and opening time ≥2~3s. By extending the valve opening and closing time, the acceleration/deceleration of water flow is reduced, and pressure mutation induced by inertial force is reduced from the source; electric ball valves/butterfly valves (with good slow-closing characteristics) are preferred to replace quick-closing solenoid valves (only applicable to extremely small diameter DN15).
- Strictly control pipeline flow velocity to avoid hydraulic imbalance
The design flow velocity of FCU branch pipes (DN15~DN25) is ≤1.0m/s, and the main pipe flow velocity is controlled at 0.8~1.5m/s. The water flow inertia is reduced through reasonable pipe diameter ratio; hydraulic balance calculation is carried out during design, and static balance valves are configured for each branch to avoid excessively high flow velocity in local branch pipes from the source.
- Arrange simple water hammer protection accessories locally
For the air conditioning water system of multi-storey/high-rise buildings, install miniature diaphragm-type water hammer arresters or surge tanks at the floor main pipes where FCUs are concentrated, water pump outlets and main risers to absorb pressure shock waves by the elasticity of the diaphragm and prevent the impact from transmitting to the terminal; flexible rubber joints can be installed on the terminal branch pipes (one at each inlet and outlet of the FCU), which not only absorb vibration but also buffer local pressure mutation.
- Select water pumps with soft start/stop functions
Configure variable frequency soft starters, star-delta starters for chilled/hot water pumps, or use variable frequency water pumps to make the water flow velocity rise and fall linearly (instead of step change) when the water pump starts and stops, reducing the intensity of pressure shock waves from the system source; install slow-closing check valves (closing time 3~5s) at the water pump outlet to replace quick-closing swing check valves.
(II) Installation Stage: Reduce Local Pipeline Resistance and Avoid Impact Amplification
- Optimize pipeline layout to reduce local mutations
Avoid 90° sharp bends, sudden diameter changes and multi-valve series connection for FCU branch pipes; use bends with R≥1.5D for elbows and eccentric reducers for diameter changes to reduce the local head loss of water flow and avoid flow velocity mutation caused by sudden changes in pipeline cross-section; the branch pipe direction should be as straight as possible to shorten the distance between the FCU and the main pipe.
- Ensure proper pipeline fixation and vibration reduction
Fix FCU branch pipes with pipe clamps with vibration-damping pads to avoid pipeline suspension; set vibration-damping supports at the connection between the main pipe and branch pipes to prevent the vibration of water hammer from transmitting and amplifying in the pipeline; add vibration-damping rubber pads between the FCU body and the wall/ceiling to isolate the vibration between the equipment and the building structure.
- Standardize the installation of air vents and water make-up valves
Install automatic air vents at the highest points of the system and the high points of FCU water distributors to ensure complete air exhaust of the system and avoid air mass impact caused by air blockage; select pressurized water make-up valves (with pressure reducing and stabilizing functions) for water make-up valves to avoid pressure sudden rise caused by rapid water make-up, and make up water slowly through the bypass valve during the make-up process.
- Arrange strainers in front to reduce local blockage
Install Y-type strainers (40~60 mesh filter screen) at both the inlet and outlet of the FCU, and arrange the strainers in front of the electric two-way valve to filter scale and impurities in water in advance and prevent local flow velocity mutation caused by blockage of heat exchangers and valves.
(III) Operation and Maintenance Stage: Standardize Operation and Eliminate Late Inducements
- Standardized operation of valves and water pumps
Follow the principle of “slow opening and slow closing” when manually operating FCU branch valves and main pipe valves to avoid one-time shut-off/opening; start and stop water pumps in strict accordance with the operating procedures, open the bypass valve first, and then start and stop the main pump step by step to avoid system pressure mutation; regularly inspect the slow-closing mechanism of electric two-way valves, and repair or replace them in time if jamming or quick closing occurs.
- Regular cleaning and descaling
Carry out chemical descaling and physical flushing on the FCU heat exchanger and pipeline before the start and stop of the air conditioning season every year to remove scale on the inner wall of copper tubes and in the pipeline, ensure uniform flow cross-section and avoid excessively high local flow velocity; clean Y-type strainers regularly and remove impurities in time to prevent blockage.
- Daily inspection of system pressure stabilization and air exhaust
Inspect the pressure of the pressurized expansion tank of the closed water system daily to ensure the pressure is stable within the design range (0.3~0.5MPa), avoid cavitation caused by excessively low pressure and impact caused by excessively high pressure; inspect automatic air vents weekly to ensure unobstructed air exhaust, and dredge them in time if blockage occurs to eliminate system air blockage.
- Timely treatment of leakage and vibration
If leakage at FCU joints and pipeline gaskets, or obvious pipeline vibration and water hammer noise are found, stop the machine for inspection immediately, investigate problems such as the slow-closing function of electric two-way valves, pipeline fixation and strainer blockage, and restart the operation after eliminating the inducements to avoid long-term fatigue damage caused by minor problems.
IV. Key Differences Between FCU System Water Hammer and Industrial Large-System Water Hammer
The water hammer in FCU systems is a low-pressure, local, low-frequency (or high-frequency micro-impact) water hammer, which is essentially different from the water hammer in industrial water supply and drainage and heating large-diameter water systems. There is no need to adopt industrial-grade water hammer protection measures (such as pressure relief valves, quick-closing valves, and air tanks). The core differences are as follows:
| Comparison Item | FCU System Water Hammer | Industrial Large-Diameter Water System Water Hammer |
| System Pressure | 0.3~1.0MPa (low pressure) | Above 1.0MPa (medium and high pressure) |
| Pipeline Diameter | Branch pipes DN15~DN25, main pipes ≤DN100 | Mostly above DN100 (large diameter) |
| Water Hammer Intensity | Pressure mutation amplitude ≤0.5MPa | Pressure mutation amplitude ≥2MPa |
| Influence Range | Local (FCU inlet/outlet/branch pipe) | Whole system (main pipe + equipment + pump station) |
| Protection Core | Valve slow closing + local vibration reduction and buffering | Pump station speed regulation + pipeline pressure relief + valve interlock control |
Summary
Although the intensity of water hammer in FCU systems is low, the hazard of “high-frequency micro-impact” is far greater than that of a sudden single impact. The key to its prevention and control is not to install complex protection equipment, but to select slow-closing electric two-way valves and control pipeline flow velocity from the design source, and then eliminate late inducements through installation optimization and standardized operation and maintenance.
In daily operation and maintenance, if an FCU has phenomena such as “bang” impact noise, joint leakage, and poor sealing of electric two-way valves, it is mostly caused by water hammer. Priority should be given to investigating the slow-closing function of electric two-way valves and strainer blockage problems, which is the most direct and efficient means to solve terminal water hammer.
