Hydronic Systems: Are Air and Noise Problems Necessary?

March Deep Links: Many engineers have come to believe in the need for constant venting and are aware of the noise problems quite often associated with hydronic systems. This has resulted in purging high points and fan coil units, repairing corrosion damage and fixing circulators just to try to keep things quiet. Fortunately, there is now a solution to these problems.

04/06/2001


Many maintenance and plant engineers for senior-housing facilities have come to believe in the need for constant venting and are aware of the noise problems quite often associated with hydronic systems. In fact, it has been said that "it's just part of a hydronic system". This has resulted in purging high points and fan coil units, repairing corrosion damage and fixing circulators just to try to keep things quiet. Fortunately, there is now a solution to these problems.

First, one must define the cause of most of the problems. Air is present in system water in a variety of forms. The first are actual bubbles which float around in the water looking for a place to either leave the system (vents) or collect and create problems. Second is entrained air, which is present as tiny microbubbles that move along at the same speed as the water. Sometimes they maintain that form and at other times they combine together and become larger bubbles.

The third is dissolved air, which is always present to some degree. This is because water is obviously part oxygen. How much oxygen is dependent upon pressures and temperatures (Henry's Law).

The amount of air in the water is greatest at low temperatures and high pressures. For example, cold water wants to retain air and higher pressures force the air bubbles to dissolve and stay in solution. For example, think about opening a bottle of beer or soda pop. When you open it, you will most often get a hissing sound as the gas escapes. This is because the pressure has been released from the container and the gases have been allowed to leave the fluid. In glass bottles you will actually see the tiny bubbles being released. The same is true when boiling water. The bubbles, which are part of the boiling activity, are the air being driven out by the heat.

These conditions occur in a hydronic system as well. The water is heated by the boiler or by gaining heat as it returns to the chiller. These temperature increases release air. Along with that, any place in the system where the pressure decreases, such as a coil or valve; air will come out of the water and look for a location to collect. These problems are obvious by the need for purging high points, radiators, coils, etc.

Other problems can also be attributed to air in the system. Noise, for example, is often caused by water sloshing around air pockets and making anything from a gurgling sound to something that sounds like a waterfall. Circulation can and does occur around these pockets; therefore, air problems do not have to be a complete binding of the system. The water actually moves along the walls of the pipe with the air staying in the center. In severe cases, circulation may actually stop. Any place an air pocket is restricting or preventing circulation, there will be a heat transfer problem. Air is a great insulator, and it will prevent the occupied space from achieving the right comfort levels as well as good system balance.

Last, but not least, are circulating pump and component problems. A pump that is cavitating (shaking in the pipe or on its base) could be the result of a poor selection, too large an impeller or air. Yes, air can cause a pump to shake, gurgle and run hot. When the water enters the pump suction, it sees an immediate pressure decrease because it has entered a large chamber. When pressure drops, bubbles result.

The air/water mixture then enters the impeller, which creates a pressure increase as it develops discharge head. The increase in pressure forces the bubbles to collapse but before they dissolve completely, the pump is trying to pump this mixture of air and water, which causes the vibrations. In severe cases, the impeller will actually start to spin free (in the air) and when fluid takes up the space; it slams against the water and can snap a pump shaft. Air can also cause corrosion and wreak havoc upon boiler sections, radiation, valves and many other components.

There is a solution to the problems, which cause resident complaints. It is far superior to air scoops and standard centrifugal-air separators because not only does it remove the large bubbles, it eliminates the microbubbles and dissolved air which are the cause of recurring system difficulties. By means of a patented coalescing medium (scrubber), it keeps forcing the system water to give up its air to the point that it actually goes back into the system and reabsorbs pockets. This is because the water is capable of holding more air than it has in it after passing through the device and goes back to the system looking for more. This is a constant and continuous activity that will keep the system virtually air free.

Very few systems are completely tight and every time the feed valve opens, more air arrives. This is why continuous operation is so important. Without the air, system purging and bleeding will not be required every time the system cycles, the pump will quiet down, heat transfer will improve and most of the corrosion problems will be eliminated. The venting mechanism on this product is not your standard float type air vent and in fact the manufacturer recommends that you take out or close off all of the old float vents which may or may not still be working. These, quite often, become a source of air as they wear out and leak. Only one manual vent at the high point is required for system fill.

When properly selected, these units have a pressure drop of only 1/2 feet of water and control the water going through at approximately 4 feet per second. This allows for the continuous operation and results in elimination of 100 percent of the free air and 99.6 percent of the dissolved air based upon the temperatures and pressures where it is installed. The air problems sometimes categorized as inherent in hydronic systems can be eliminated by looking beyond the methods employed for many years and reaching forward to a new product that is cost effective, easily installed and available throughout North America.

Written by Dan Whitezell, Spirotherm, Inc., Glendale Heights, IL.





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