Scubbing Bubbles--Microbubble Air Separation in Hydronic Systems
Air control in hydronic systems has been a concern ever since closed-loop hydronic systems came into existence during the 1930s. At that time, the idea was to purge the “bulk air” out of the system as it was filled and then separate any remaining air from the water and move it to the expansion tank. Designers understood the basics, namely that air bubbles rise, and that air is easier to separate from hot water than from cold water. This led to devices such as the boiler air collection fitting shown in Figure 1.
Some boiler manufacturers even designed their castings with integral air separation chambers. The idea was to capture bubbles forming on the vertical walls of the cast-iron sections and rising to the top where they would collect and eventually be ejected through a float-type air vent.
Although systems of this type have been functioning for years, they do not represent the best currently available technology for air elimination. As one who’s been in this industry almost 30 years, I can bear witness to a transformation in which air control/elimination has moved from a “keep your fingers crossed” proposition in some situations to a relatively simple, consistent, and reliable process in well-designed modern systems. This article explores the current state-of-the-art systems in air elimination.
It Has Got to GoWhen a closed-loop hydronic system is not properly deareated, the fluid within it is partially compressible and also contains free oxygen molecules. This can lead to several problems, such as:
• Flow noises as the mixture of air and water cascades through piping and heat emitters.
• Poor circulator performance along with accelerated wear on bushings in wet rotor circulators. Air accumulating in vertically oriented circulators with integral check valves at top flange can eventually cause complete loss of flow and permanent damage to the circulator. Under the right conditions, air within the water also can trigger gaseous cavitation within the circulator.
• Poor heat transfer at the heat emitters, especially if large amounts of air accumulate at high points within the emitters.
• Accelerated corrosion due to the presence of free oxygen in systems containing ferrous metals such as cast iron or steel.
• Unstable operation of balancing valves leading to poor capacity control.
Although early air separation systems did work, they focused on handling bubbles large enough to have significant buoyancy, and thus, capable of rising upward through fluid and eventually collecting at high points in the system. The larger the bubble, the faster it rises. More specifically, the terminal (maximum) rise velocity of small bubbles through water is proportional to the square of its diameter. This affect can be seen with bubbles rising in a pool or even a glass of soda. The large bubbles easily out sprint the smaller ones in their race to the surface.
If all air bubbles in a hydronic system were, say, 1/4” in diameter, air separation would be a matter of collecting them at a high point and ejecting them through a simple float-type vent. However, all bubbles are not the same size. Big bubbles come from small bubbles, small bubbles come from microbubbles, and microbubbles come from molecules of oxygen, nitrogen, hydrogen, carbon dioxide, and other gases mingled among the water molecules. Let’s start at the molecular level and work up.
Suck It UpWater has the ability to contain the gases that constitute air. These gases exist as molecules interspersed with water molecules and, thus, are said to be dissolved into the water. The maximum amount of dissolved gas molecules contained in the water is limited by the pressure and temperature of the water. A graph showing the maximum amount of dissolved air gases that water can contain in solution based on the water’s temperature and pressure is shown in Figure 2.
This graph shows that the ability of water to retain dissolved gases decreases with increasing temperature and vice versa. For example, at 30 psi absolute pressure (about 15.3 psi gauge pressure) and a corresponding temperature of 65°F, up to 3.6% of the molecules in a container of water can be dissolved gases. However, if the water’s temperature is raised to 170°F while maintaining the same pressure, its ability to contain dissolved gases is reduced to 1.8% of its volume, half the previous level. Such a temperature change would be typical of cold water heated within a boiler for the first time.
The decreasing ability of the water to retain gas molecules in solution as the water is heated forces the “excess” molecules to come out of solution in the form of microbubbles. The latter are so small they cannot be seen as individual bubbles. Instead, they typically appear as a cloud, as if a small of amount of milk had been stirred into the otherwise clear water.
Microbubbles often can be seen when a clear glass is filled with cold tap water as shown in Figure 3.
Put the glass with this mixture in it on the countertop for a few minutes and watch what happens. The microbubble cloud slowly rises to the top of the water, like fog slowly rising from a valley. Eventually the microbubbles reach the surface and disappear, making the water appear bubble free, but don’t assume that bubble free implies air free.
Catch Me If You CanThe small size of microbubbles makes their rise velocity very low, and, hence, they are easily entrained with water flowing through a hydronic system at normal flow velocities. This makes it very difficult to collect them based on the premise they will simply gather at high points where vents can do the rest.
Efficient collection of microbubbles requires a coalescing media in combination with a low velocity chamber. The coalescing media provides a high amount of surface area with solid edges that create very localized low pressure areas due to vortex formation. Microbubbles flowing past the coalescing media tend to cling to these edges. From there the coalescing media provides pathways along which the microbubbles can slowly rise through the active flow zone of the separator without being torn away from the media.
When the flow velocity into a microbubble air separator is relatively low, its bubble capture efficiency is high. However, if the flow velocity across the coalescing media is too high, some of the bubbles are re-entrained into the flow stream and are carried out of the separator. In the latter scenario, more passes of the system fluid through the separator will be needed to achieve a high degree of air removal.
Most microbubble air separators use a vertically oriented cylindrical chamber to house the coalescing media and create acceptably low flow velocities so that bubbles can rise above the active flow zone of the separator. Once above this zone, the bubbles collect at the top of the air separator, and eventually leave through a float-operated vent. They are driven out of the vent by positive system pressure.
Designers should always verify the maximum flow velocity at which the microbubble air separator is rated to operate and be sure pipe sizes are selected accordingly. A typical maximum entering flow velocity specification is four feet per second or lower, although some commercial size microbubble separators are rated for operation up to 10 feet per second.
Microbubble air separators are best placed where the water is hottest and where the pressure is reasonably low. In heating systems, the preferred placement is near the outlet of the boiler and on the inlet side of the system circulator as shown in Figure 4. In chilled-water cooling systems, the best location is within the warm water return piping to the chiller.
In some cases, the make-up water line for the system also connects to the bottom of the air separator. Although this is fine from the standpoint of air separation, be sure to verify that the float venting device in the separator will not be adversely effected by highly turbulent water as the system is filled.
If there is any concern on this issue, connect the make-up water line to the system near the inlet side of the separator. In such cases I also suggest a boiler drain valve on the bottom of the air separator. This valve can occasionally be opened to flush out minor amounts of sediment that can accumulate at the bottom of the separator due to the low flow velocity within it.
Go Get It!The ability of a microbubble air separator to lower the air content of water as it passes through at a higher temperature allows the water to be in an “unsaturated” state as it cools while passing through the hydronic distribution system. Think of water in this state as a “sponge for air.” When unsaturated water passes into a component containing air molecules, it has the ability to absorb some of those air molecules and carry them back to the boiler.
As the water is reheated in the boiler, the “sponge for air” gets squeezed, and gives up the captured air molecules as microbubbles.
The air separator then captures the microbubbles and ejects them from the system. This process of capture – return – saturate - coalesce - eject takes place continually as the water temperature cycles up and down during normal system operation as depicted in Figure 5.
Eventually, a microbubble air separator can lower the air content of the water to no more than 0.4%. At that point, the system is, for all intents and purposes, fully deareated. Circulators will operate quietly and efficiently, heat emitters will perform to their best ability, and occupants will not hear gurgling or hissing from trapped air in radiators or other components. It’s Nirvana as far as the hydraulics of the system is concerned.
The coalescing media need for efficient microbubble air separation is also being built into some currently available hydraulic separators. This combines the functions of air separation and dirt separation to hydraulic separation, thus creating a 3-in-1 device. It eliminates the need for separate air- and dirt-separating devices and provides good value engineering. Such devices are ideal in new systems, as well as boiler retrofit applications where older black iron piping often suggests sediment is likely to be present.