Hot-water systems must have an expansion tank in the fluid circulation loop. Water expands as it’s heated and, since it’s in a closed fluid loop, the hydronic system must be able to accommodate the increased volume. At night the water cools and contracts, and during the day it expands as it’s heated, into an expansion tank in the loop.
In a multistory building, the pumps and expansion tank are usually located in a basement central plant. The expansion tank is filled with air, under pressure. This pressure comes from the pump head as well as from static pressure from the loop since the tank is at the low point in the system. This type of pressurized system, with water and air in direct contact, causes the air to go into solution. The greater the pressure and temperature, the higher the solubility of air in water. When the water in the loop passes through the low-pressure upper part of the fluid circuit, the air comes out of solution and becomes bubbles in the water system. The presence of this air in the water can cause control problems, pump cavitation and wear, vibration, and diminished heat transfer at the coils and boiler tubes.
Since the major source of air is unavoidable, as no hot-water hydronic system can operate without an expansion tank, an air separator is needed to extract air from the circulating fluid. Air is taken from the water and returned to the top of the expansion tank, to maintain the air cushion there; or the air is vented, and the air cushion is replenished from the pneumatic control air system or by a small dedicated compressor. Either way, the pressured air-water interface remains in the expansion tank, a constant source of air to dissolve into the circulating fluid.
This inevitable air problem in hot-water circulating systems has resulted in the standard solution: Increase the water flow (to a level far above that required for efficient heat-transfer processes) so that the air bubbles do not cause as much trouble. This is a practical solution that has stood the test of time in countless installations. However, it is also a costly solution. Larger pumps, piping, valves, and hot-water coils are needed, and for the life of the facility the pump works harder and uses more energy than is necessary to do the job.
One solution to the problem is to simply relocate the expansion tank to the lowest-pressure point in the fluid circuit, usually at the top of the loop in a multi-storied building. Air still dissolves into solution, but since the static pressure is higher throughout the rest of the fluid circuit, perhaps enough to overcome any dips in dynamic pressure at valves or fittings, the air doesn’t have a chance to come out of solution. Another approach is to replace the expansion tank with a bladder-type tank. The air’s trapped in a rubber bladder within the tank, permitting expansion and contraction but eliminating the air-to-water interface. Consequently, far less air enters solution than with an open system. Some small amount of air does get into the circulating fluid, from valves or loose fittings. Therefore, an air separator is still necessary for the system. Retrofitting to a bladder-type expansion tank can generate significant savings. Often the water flow can be reduced by installing a smaller, high-efficiency pump and rebalancing the hydronic loop to the new flow regime. This small pump can handle the demand at great savings in electricity use, with little effect on the degree of environmental control in the building.
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Excerpt from Energy Conservation Projects.