Air inspection and elimination in wall-hung boiler pipes
1. Problems caused by trapped air
Although people can Recognize the presence of air in the system (e.g. the sound of water guzzling in pipes), but they are often unaware that trapped air can cause these hazards:
Causes chemical chemistry between metal parts such as steel in the system and the air Oxidation accelerates the corrosion of these components. Systems with long-term air entrapment corrode several times faster than normal systems.
The loss of heat load at the end of the heat sink or the entire system due to entrapped ‘air pockets’. This is often referred to as collection air. It comes in many ways. The most common is that the water in the screw compartment of the pump is replaced by it, preventing the pump from circulating the water in the system properly. Another possibility is that the pipe section at the top of the system is filled with an “air bag” and the pump cannot send water over the top across the pipe.
The pump head is reduced. Since the mixture of water and air can be compressed, the pump cannot transfer mechanical energy to the water efficiently. This will greatly reduce the heat output of the system.
The lubrication of the shield sleeve of the wet rotor pump is insufficient. Because the mixture of water and air causes the solution inside the pump to become foamy, the water film lubrication required by the shielding sleeve cannot be achieved. Wet rotor pumps rely entirely on this lubricating film, without which the pump will be damaged very quickly.
The heat transfer loss of the heat carrier. Air bubbles inside the heat carrier prevent thermal convection, which reduces the rate of thermal energy transfer. At the same time, the air in the heat exchange device inside the heat source may also cause the “superheated temperature point” to appear, which hinders the heat exchange and may eventually damage it.
2. Existence forms of trapped air
In a plumbing system, air exists in the following three forms:
Still Air pockets
Entrained air bubbles
Air dissolved in water
These three forms of air may exist in the system at the same time, especially at the initial stage of the system. Runtime. Each form of air behaves differently in the system.
2.1 Static pockets of air
Because air is lighter than water, it rises toward the high points of the system. These locations are not exactly the top of the system. Even at the cooling end of the lower layer, static pockets of air may still form in the upper part. A still air pocket is also formed in the horizontal section before the horizontal duct turns downward. A typical example is where a pipe rises across a building beam and then returns to the level. When the system begins to fill with water, these high points are dead spots for air to exist. Sometimes, because this trapped air displaces several liters of water, it will be necessary to re-inject water into the system in the future. Even after the system has first been purged, residual air bubbles can still build up and rise to a high point. This is especially true on low-flow equipment, where the flow is too low to push or carry the air away. Such as large radiators, large diameter pipes and water storage tanks.
2.2 Entrained air bubbles
When air exists in the form of bubbles, the water in the system can take away (entrain) these bubbles. Entrained air bubbles have both advantages and disadvantages. The advantage is that the air at the far end of the system can be delivered to the exhaust; the disadvantage is that the air may not be separated in the exhaust. The degree to which the water stream entrains air depends on its ability to carry the naturally rising air bubbles downward. Simply put, if the flow of water is going down faster than the bubbles are rising, it can carry the bubbles away in the direction of the flow. In a downward-flowing pipe, the flow velocity of the water is higher than the rising velocity of the bubbles, which is the key to entraining the bubbles.
The speed at which the bubble rises is determined by the diameter and density of the bubble, as well as the density and viscosity of the surrounding water. The larger the bubble diameter, the faster the rising speed. The higher the viscosity of the surrounding water, the lower the bubble density and the slower the bubbles rise. Among these factors, the diameter of the bubble plays a decisive role, and the rise rate of a bubble with a diameter smaller than half is only about 1/4 of the normal diameter. From a glass of carbonated beverages you can see the difference in how fast the bubbles rise.
A common type of air bubbles in plumbing systems is called microbubbles. Due to their small size, it is difficult to see. Dense clusters of microbubbles make clear water slightly cloudy. There are microbubbles in the upper part of the water put into the glass from the faucet with the filter.
The decomposed air also forms microbubbles when the water in the system heats; or microbubbles can also be created at an element that creates a vortex. Due to their low flow rate, they are easily carried away by the current. This feature makes it relatively difficult to isolate microbubbles in water. Separating microbubbles requires the flow of water to pass through an area of low velocity so that it has enough time to rise and form large bubbles.
Unfortunately, many air separation units in plumbing systems do not provide flow rates low enough to effectively isolate microbubbles. Bubbles with larger diameters rise faster and are eliminated, while relatively much smaller microbubbles are carried away by the water flow without being separated. Sometimes it’s because of the design of the air separator, sometimes it’s because the flow through the air separator is too fast.
2.3 Air dissolved in water
Perhaps the most difficult thing to understand is the presence of air in a plumbing system in the form of dissolved air. Molecules that make up air, including oxygen and nitrogen, can ‘coexist’ with water molecules. These molecules are invisible even under a microscope. Apparently clear, bubble-free water may also contain a large amount of dissolved air.
The amount of air dissolved in the water depends on the temperature and pressure of the water. As the water temperature increases, the amount of air dissolved in the water decreases. For example, when the absolute pressure is 2 kg, the water is heated from 20 degrees Celsius to 80 degrees Celsius, and the amount of air dissolved in the water drops from 35 liters to 17 liters. This explains why blisters collect on the surface of a hot pot of water, and it also explains the appearance of microbubbles on the outside walls of the boiler’s combustion chamber. Conversely, it redissolves the air when the water temperature cools.
Water pressure is also a factor in dissolved air. The water pressure decreases the amount of air it dissolves, and vice versa. For example, carbon dioxide bubbles appear immediately after opening the cap of a soda water bottle. This is because the water pressure decreases after the cap is opened, and the dissolved gas decreases rapidly and is released to make the bubbles appear immediately. The upper end of the multi-storey building has more air bubbles due to lower pressure, while the static pressure at the bottom of the system is relatively higher, which will absorb more air into the water.
3. Exhaust device
The exhaust device in the plumbing system can be divided into two types: high-level exhaust valve and centralized air separation device.
High-level vent valves are installed at multiple high points in the system to remove stationary air pockets, such as the top of the riser, the top of the radiator, the divider, or down bends across the pipe. The high-level vent valve is especially important for the initial water injection and venting of the system.
The centralized air separation device is used to remove the air mixed in the water and minimize the air content of the system water. It is usually installed at the water outlet of the heat source, and all system water flows through the air separation unit.
The following introduces several specific high-level exhaust valves and centralized air separation device products.
3.1. Manual exhaust valve
The simplest of the high-level exhaust valves is the manual exhaust valve. The manual exhaust valve is usually a small metal-metal sealed valve. The screw inside the valve is unscrewed by a handle, a flat screwdriver or a four-corner key, and the gas is discharged from a small hole on the side of the valve.
The manual exhaust valve is usually installed on the upper side of the radiator. Due to its small size and small air inlet, care should be taken not to block the air inlet with the sealing material during installation.
After the manual exhaust valve is exhausted, the system water will flow out from the exhaust port, so it is necessary to be monitored during exhaust, and a container for water should be prepared to prevent the discharged water from getting wet and soiling the floor . Since it is difficult to flush the direction of the exhaust port during installation, it is recommended to use a valve with an adjustable exhaust port direction.
3.2 Float type automatic exhaust valve
A high-level exhaust valve that automatically exhausts gas needs to be installed in places where the system is not easy to maintain and detect. Float type automatic exhaust valve is a good solution.
When the bubble accumulation in the valve body increases, the float ball drives the valve stem to open the exhaust piston to discharge the gas as the water level drops; after the gas is exhausted, the water level rises, and the float ball rises along with it to close the exhaust piston.
The exhaust cap of the exhaust port of the automatic exhaust valve prevents external impurities from blocking the exhaust port. After the automatic exhaust valve is installed, the exhaust cap needs to be slightly loosened to exhaust. In the event of water leakage from the exhaust port (such as the system water quality is too dirty), an optional moisture-absorbing exhaust cap can be used for safety protection.
There are many sizes, shapes and diameters of automatic exhaust valves. It is suitable for the top of the standpipe, the end of the water separator; the upper side of the radiator; for the parts with large gas volume, a large exhaust volume exhaust valve needs to be installed.
When there is a negative pressure at the location where the automatic exhaust valve is installed, such as when the expansion tank is installed at the pump output port and other design or installation errors, the air will enter the system through the exhaust port of the automatic exhaust valve . In this case, the error of the system should be corrected first, and an anti-suction exhaust cap should be installed on the exhaust port of the automatic exhaust valve.
4. How to Correct Persistent Venting Problems in Your System
A properly designed and installed system does not require frequent manual venting. Persistent airborne noise is a constant source of customer complaints and is rooted in one or more potential design and installation errors. These errors are often not directly related to the problem that manifests, so it can be difficult to pinpoint the source, especially for inexperienced engineers. The following is a description of some common problems and solutions. The actual running system may be caused by multiple symptoms concurrently.
Potential problem 1: The expansion tank is installed at the output port of the pump
Solution: The best solution is to reinstall the expansion tank at the suction port of the pump. This approach has been validated on many systems. Verifying the installation location of the expansion tank is the first step in verifying the system.
Potential problem 2: The operating pressure of the system is too low
Solution: The best solution is to use a visually adjustable automatic water replenishment valve, and the automatic replenishment valve is turned on. And ensure that the pressure set by the water supply valve is higher than the static pressure of the system + the maximum lift of the water pump by more than 0.5 meters.
Potential Problem 3: Water-immersed expansion tank or expansion tank volume is too small
Solution: If the airbag of the diaphragm expansion tank is soaked, open the air supply port of the airbag and there will be Water flowed out, indicating a perforated diaphragm of the expansion tank. The only solution is to replace the expansion tank. If the designed volume of the expansion tank is too small, it is necessary to recalculate the amount of expansion water and add or replace the expansion tank.
Potential problem 4: Air is continuously absorbed and released through water but cannot be eliminated
Solution: A special microbubble exhaust valve needs to be installed to collect and eliminate the dissolved air in the system.
Potential problem 5: No high-level exhaust valve is installed
Solution: It is necessary to install an automatic exhaust valve on these higher positions and the heat dissipation end. The vent cap must be loosened slightly for venting.
Potential Problem 6: Included Air Bubbles
Solution: The water flow velocity of all downward-flowing pipes must be maintained above 0.6 m/s to effectively remove air bubbles. If this flow rate cannot be achieved, an automatic vent valve will need to be installed at the highest point in the pipe.
Potential problem 7: The exhaust valve is installed at the top of the open system
Solution: The pipeline above the water level of the water storage tank in this type of system cannot be equipped with an exhaust valve, and the The flow rate must be set above 0.6 m/s to carry and transport the air bubbles back into the storage tank.
Potential problem 8: Improper operation when cleaning the system for the first time
Solution: In order to avoid the formation of static air pockets during cleaning, it is necessary to use a ‘pressurized’ cleaning method .
