By Dan Whitezell
Air is present in system water in a variety of forms. The first, free air bubbles, float around looking for a place to leave the system (float vents), or collect and create problems. Second, entrained air is present as tiny microbubbles that travel at the same speed as the water. They either maintain that form or combine together or become larger. Third is dissolved air, which is always present to some degree. This is because water is part oxygen. How much oxygen is dependent upon pressures and temperatures?
Releasing air best occurs where the temperature is the highest and pressure is the lowest. For example, think about opening a bottle of soda pop. When you open it, you will most often get a hissing sound. With a glass bottle you can see the bubbles form and cling to the side. This is because the pressure has been released from the container and the gases have been allowed to leave the fluid. The same is true when boiling water. The bubbles, which cling to the side of a pot as it heats and the boiling action itself, is the air being driven out by the temperature change.
Those conditions also occur in a hydronic system. The water is either heated or gains heat. The temperature increases and air is released. In addition, any place in the system where the pressure decreases (coils, valves, or at the end of a loop), air will be released and seek a collection point. The problems are obvious by the need for purging high points, radiators, coils, etc. Noise may result from the water sloshing around air pockets and creating gurgling sounds or a waterfall effect. Circulation can and does occur around these pockets by flowing along the walls of the pipe with air acting as an insulator in the center. Any place an air pocket is restricting or preventing circulation, there will be a heat transfer problem. Other system problems attributed to air can be pump cavitation (by attempting to pump an air water mixture) and the obvious effects of corrosion caused by air and water together.
Most piping systems are designed for 6 to 10 feet per second, which is too fast for effective air separation. Many devices currently in place have been installed at "line size," which results in high velocities and pressure drops, thus reducing their effectiveness. An air separation device should be selected with nozzle velocities at 4 feet per second or less and head losses at less than 1 foot unless specifically designed for higher velocities. There is a new product solution, which combines best practice selection criteria with new and unique features. Superior to air scoops and centrifugal air separators, which have been the standard for decades, it not only removes all of the large bubbles, it eliminates 100 percent of the entrained air and 99.6 percent of the dissolved air.
Using a patented coalescing medium (scrubber), it forces the system water to release air to the point that it goes back into the system in an absorptive state. The water then acts like a sponge and absorbs air pockets lodged in various locations and brings them back to the eliminator. Constant and continuous operation keeps the system virtually air-free, even after the feed valve has opened. Without the air, system purging will not be required every time the system cycles or is serviced. The pump will quiet down, heat transfer will improve, and most of the corrosion problems will be eliminated. Multiple old-style float vents may be removed with only manual vents to be used during system fill. Air problems, previously thought of as inherent in hydronic systems, can be eliminated with a product that is cost effective, easily installed, and available throughout the world.
Dan Whitezell is a 27-year veteran in the hydronic industry, having worked for many leading manufacturers. Presently, he is vice president of marketing and sales at Spirotherm Inc. (www.spirotherm.com), Glendale Heights, IL.