An expansion tank is required in a closed loop heating or chilled water HVAC System for two very important reasons. 1.
To control or set the system s operating range.
2.
To give the expanded water in the system a place to go as the water is heated. In a heating system this occurs when the system is heated form its coldest fill temperature to operation temperature. In a chilled water system this expansion occurs when the system is shut down and the system temperature rises from operating to ambient.
The goal in sizing any expansion or compression tank is to make the system able to accommodate the expansion of the system water throughout the heating or cooling cycles without allowing the system to exceed the pressure limits of the lowest pressure-rated component in that system. The lowest rated component in most systems is, by design, the pressure relief valve. The maximum system pressure is normally set at 90% of the pressure relief valve rating at its point of installation.
B.
DIAPHRAGM OR BLADDER TANKS COMPARED TO PLAIN STEEL TANKS
The Plain Steel Expansion Tank has been used for many years, and in some systems has worked very well. Using a plain steel expansion tank makes the system 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 expansion tank will become waterlogged. A waterlogged tank is an expansion tank that no longer has an air cushion large enough to allow all the expanded water form the system to enter the tank without exceeding the maximum system pressure. When this occurs the safety relief valve will open and heated system water will be discharged to the drain. CAUTION:
An expansion tank does not need to be 100% full to be waterlogged. The same symptoms will also exist if the expansion tank is sized too small!
The advantage of a plain steel tank is that the initial purchase cost is less than a diaphragm/bladder tank, but in many cases the installed cost will offset this advantage.
The Diaphragm/Bladder Expansion Tank has been developed to allow the systems air cushion to be separated form the systems water. No waterlogging of the tank can occur as the air is held between the tank wall and the outside of a bladder placed inside the tank, while the system water is contained inside the bladder. This changes the system to an air elimination system, as any air extracted from the system water is passed out of the system into the atmosphere. The bladder tank is much smaller, usually around 50%, than a plain steel tank for the same application as they are precharged with air to the system operating pressure before the system is filled with water. The only water that needs to be accommodated by a diaphragm/bladder tank is the expanded water. The air elimination system allows the air vent and air separator to be placed at the most advantageous point in the system for air removal, usually at the system s high point where the pressure is the lowest or at the boiler outlet where the water temperature is the highest. The expansion tank can now be placed at floor level, since air no longer needs to be returned to the tank. The diaphragm/bladder tank can also be placed at the most advantageous point in the system. C. TYPICAL PALCEMENT OF AN EXPANSION TANK The system connection of an expansion is known as The Point of No Pressure Change . This means that wherever the expansion tank is connected to the system the pressure will always be the same as the pressure inside the tank. This is true if the tank is a plain steel or diaphragm/bladder style. This is also true whether the system pump is on or off. This pressure is only changed as water or air are added to or removed from the tank. To better understand this The Point of No Pressure Change , an in-depth study of Boyl s Law (The principle that the volume of a gas times its pressure is constant at a fixed temperature.) is necessary. Because of this The Point of No Pressure Change the system sees a pump additive pressure from the pump discharge to the expansion tank connection. From the expansion tank connection back to the pump suction, the system sees a negative pressure change from the tank pressure, due to the friction loss when there is flow. With this loss of pump addictive pressure and the loss due to flow, it is usually best to place the The Point of No Pressure Change or expansion tank system connection as close to the pump suction as possible.
SUBMITTAL TYPE: NLA ASME PRESSURIZED EXPANSION TANKS FOR HEATING & COOLING SYSTEMS MODELS: NLA 35 TO NLA 800L Submittal Sheet No. N-3350A
Patterson Type NLA Tanks are ASME removable bladder type pre-charged expansion tanks. They are designed to absorb the expansion forces and control the pressure in heating/cooling systems. The system’s expanded water (fully compatible with water/glycol mixtures) is contained in a heavy-duty bladder preventing tank corrosion and waterlogging problems. NLA expansion tanks reduce tank sizes up to 80%.
Shell: Carbon Steel Bladder: Heavy duty butyl
PERFORMANCE LIMITATIONS Maximum Design Temperature: 240ºF Maximum Design Pressure: 125 PSIG* *200 & 250 PSIG available
TYPICAL SPECIFICATION Furnish and install as shown on plans a ______________ gallon ___________” diameter X __________” (high) pre-charged steel expansion tank with heavy-duty butyl rubber bladder. The tank shall have NPT system connections and a .302”-32 charging valve connection (standard tire valve) to facilitate the on-site charging of the tank to meet system requirements. The tank must be constructed in accordance with most recent addition of Section VIII of the ASME Boiler and Pressure Vessel Code. Each tank shall be Patterson model number NLA______________ or approved equal.
101Ayersville TANK STRd● ● GREENWOOD, IN 46143 ● (317) 888-9800 ● (317) 888-9988 ● www.westank.com 9201 Toccoa, GA 30577 ● (706) 886-2101 ● (706) 886-0023 FAX FAX ● www.pattersonpump.com
SUBMITTAL TYPE: NLA-SERIES ASME PRESSURIZED EXPANSION TANKS FOR HEATING & COOLING SYSTEMS MODELS: NLA-2500 TO NLA-15000 Submittal Sheet No. PAT-3350 Date: 11-03
Patterson Type L-Series Tanks are ASME removable bladder type pre-charged expansion tanks. They are designed to absorb the expansion forces and control the pressure in heating/cooling systems. The system’s expanded water (fully compatible with water/glycol mixtures) is contained in a heavy-duty bladder preventing tank corrosion and waterlogging problems. AV L expansion tanks reduce tank sizes up to 80%.
Shell: Carbon Steel Bladder: Heavy duty butyl
Model NLA-2500 NLA-3000L NLA-3000S NLA-4000 NLA-5000 NLA-7500 NLA-10000 NLA-15000 Notes:
PERFORMANCE LIMITATIONS Maximum Design Temperature: 240ºF Maximum Design Pressure: 125 PSIG 200 PSIG 250 PSIG
TYPICAL SPECIFICATION Furnish and install as shown on plans a ______________ gallon ___________” diameter X __________” (high) precharged steel expansion tank with heavy-duty butyl rubber bladder. The tank shall have NPT system connections and a .302”-32 charging valve connection (standard tire valve) to facilitate the on-site charging of the tank to meet system requirements. The tank must be constructed in accordance with most recent addition of Section VIII of the ASME Boiler and Pressure Vessel Code. Each tank shall be Patterson model numberNLA-______________ or approved equal.
A. WHY AN EXPANSION TANK? An expansion tank is required in a ...
A.
WHY AN EXPANSION TANK?
An expansion tank is required in a closed loop heating or chilled water HVAC System for two very important reasons. 1.
To...
