Air Separators

Mon, May 05, 2016

Hydronic systems, by their very definition, heat or cool using water, or some other heat transfer fluid like glycol. Many of the potential problems in these systems occur when something other than fluids are in the system. One of the most common contaminates is air. This is particularly because water contains air when it comes from the city for fill. The air can be in the form of bubbles, which we can see, and/or entrained air, which cannot always be seen. Another source of air in a system are pockets of air that can't removed from the system when it is filled. Not using an air separator can cause big problems. They include, but are not limited to: noise, corrosion, "air lock", and even fluctuating pressure changes caused by having more than one "point of no pressure change". Bell & Gossett and other manufacturers make specialized equipment that is designed to remove both visible and entrained air from the heat transfer fluid. These devices perform with varying degrees of effectiveness, depending upon their design and system conditions, to collect the air then, in combination with various vents and tanks, expel the air from the system or use it as a cushion for the expansion forces in the system. The first air separators, like the Bell & Gossett IAS (Inline Air Scoop) were little more than "bumps in the pipe". The larger diameter of the fitting slows down the water enough that the air comes out of entrainment and bubbles to the top of the fitting, where it would either leave through an automatic air vent, or enter the compression tank. Those of you who regularly read this column know what style of tank to use depending upon whether the air is vented, or collected. With this style of separator, it is imperative the the 'bump' in the fitting is installed on the top! I have seen several instances in the field when the fitting was installed upside down and the effectiveness of the device was severely compromised. Since these devices work based on their ability to slow the water down, it is important that they are not over-pumped. If the flow is too high, the air will simply be carried through the device as if the separator wasn't there. Since then, the major innovation in residential air separators has been the addition some type of coalescing element in the fluid stream reduction in size, and integration of features. The coalescing element (which often looks like a brush) acts to improve the efficiency of the device in two ways: 1.) the filaments of the brush give the 'micro-bubbles' something to cling to until enough of then glob together to form a big enough bubble to release and be removed; and 2.) they create a more uniform velocity across the entire cross-section of the fitting, which allows the separator to be smaller than before. A further refinement is the integration of the automatic air vent into the separator. This last one, however, makes the vent more difficult to be replaced if it fails, and it is the most common component to fail. Large, commercial air separators work on the same premise as their smaller residential bretheren, and devices like Bell & Gossett's 'Rolairtrol' have tangential connections which swirl the water through them, creating a low pressure area in the center, where the air bubbles separate more readily. Commercial air separators also offer options like removable strainer screens, lifting lug attachments, legs for floor mounting, and internal modifications that use the swirling action to centrifugally separate heavier than water contaminates like sand and collect them in the bowl at the bottom of the vessel where it can easily be blown-down and removed. In addition to being more readily removable at low velocities and pressures, air is also driven out of entrainment when the water is warmer. Therefore, an air separator located just downstream of the boiler will be more efficient than one located on the cooler return. Used with philosophy of air control, when the separated air is sent to the compression tank, the air separator should be on the suction side of the pump. When used with the philosophy of air removal, the air separator location is not tied to the location of the expansion tank, and it can be located wherever convenient. There is an air separator in the size and configuration fro your system. Air separators are low maintenance items with no wearing parts unless the vent is integrated. Bell & Gossett design schools at 'The Little Red Schoolhouse' discuss the proper application of air separators and other hydronic specialties. Feel free to sign-up for classes at Bornquist, or our stocking wholesalers.