HVAC Expansion Tanks
Expansion tanks are required in a closed loop heating or chilled water HVAC system to absorb the expanding fluid and limit the pressure within a heating or cooling system. A properly sized expansion, or compression tank, will accommodate the expansion of the system fluid during the heating or cooling cycle without allowing the system to exceed the critical pressure limits of the system. The expansion tank uses compressed air to maintain system pressures by accepting and expelling the changing volume of water as it heats and cools. Some tank designs incorporate a diaphragm or bladder to isolate the expanded water from the pressure controlling air cushion. As water is expanded, it is contained in the bladder preventing tank corrosion and water logging potentials. The pressure controlling air cushion is pre-charged at the factory and can be adjusted in the field to meet critical system requirements. This design and operation of this style of expansion tank allows the designer/specifying engineer to reduce tank sizes up to 80%.
There are several different functions that a hydropneumatic tank can perform. In a booster pump application, it can provide water to the system during periods of a no flow shutdown of the booster pump or it can provide water to replace leak loads. In a well water application, it can provide the desired volume of water required between the pump shut down pressure and the pump turn on pressure. In a sprinkler or irrigation pump application, the tank may provide a cushion to maintain necessary pressure so the jockey pump will not short cycle. In any case, the amount of water that the tank will be required to supply to the system during any given cycle is called the drawdown. Drawdown mustfiret be determined to properly size the hydropneumatic tank.
Thermal Expansion Tanks
Thermal expansion tanks are designed for pressure control in a potable water system. The building's water heater or water heating system typically creates the thermally expanded water. A properly sized thermal expansion tank will accommodate this additional volume of water created through expansion during the heating cycle, and control the system pressure increases, keeping pressures from reaching critical limits. The tank uses compressed air to maintain system pressures by accepting and expelling the changing volume of water as it heats and cools. These tanks are designed utilizing a flexing butyl barrier that separates the stored water from the captured pressure control air cushion. This barrier, a replaceable bladder or fixed diaphragm, allows the water to be contained within the bladder preventing corrosion and potential water logging. The thermal expansion tanks are engineered to meet ASME standards, and are also available in non-code designs. Wessels' thermal expansion tanks are offered in industry's broadest range from 2 to 4,000 gallons and up to 250 psi.
Pressurized thermal expansion tanks differ from plain steel expansion tanks in that a flexible bladder or diaphragm separates the air cushion from the system fluid. The air side pre- charge of the tank must be field adjusted to equal the system supply pressure. As the system water expands, the bladder expands open, pushing against the air cushion to accept the expanded water.
Stock & Surge Tanks
Shock and Surge Tanks are ASME replaceable bladder type precharged hydropneumatic tanks for commercial, industrial, municipal and well water systems, to effectively control water hammer shock, and pump start-up/shot-down water surge. The vessels are fabricated to 250 PSI with large water system connection designed to accept potentially harmful water surge pressures quickly, with minimal pressure drop. The water is contained in a butyl bladder.
The compression tank has been used for many years, and in some systems has worked very well. Using a plain steel expansion tank makes the systems an air control system. One must control the air volume or air cushion above the water level in the tank. The common face between this air cushion and the water in the tank allows the air to be absorbed by the water. If the air is not removed properly from the water and placed back into the air cushion, the compression tank will become waterlogged.