Expansion Tank Working Principle
When choosing an expansion tank, tank volume and pressure criteria should be taken into consideration. The basic working principle of expansion tanks, also known as hydro-pneumatic tanks, is based on these two criteria.
Today, at least one expansion tank can be found in every boiler room, as they are a standard part of both heating systems and hydro-pneumatic installations. Expansion tanks are directly integrated into small capacity devices such as combi boilers and radiators. The working principle of expansion tanks used in small capacity devices is no different from that of larger expansion tanks used in other systems. Expansion tanks, whether large or small, are used to absorb the pressure created by the expanding fluid in the system.
Open type expansion tanks balance pressure with atmospheric pressure. Closed expansion tanks are divided into two types: balloon diaphragm and diaphragm membrane:
Diaphragm Membrane Closed Expansion Tank
In diaphragm membrane tanks, water comes into contact with steel. Diaphragm membrane tanks can only be used in closed circuit heating systems. This is because only fresh water is introduced into the system during initial start-up. Afterwards, this water remains in the system continuously. The oxygen in the water causes minimal corrosion in the radiators, boiler, and tank walls. However, when oxygen is depleted, corrosion stops. If a diaphragm membrane expansion tank is connected to a hydro-pneumatic system, rapid corrosion occurs due to the continuous introduction of fresh water, causing the tank to puncture.
Balloon Diaphragm Closed Expansion Tank
In balloon diaphragm tanks, water does not come into contact with steel. It always remains inside the diaphragm. Therefore, balloon diaphragms can be used in both heating systems and hydro-pneumatic installations. Small capacity balloon diaphragms are without suspensions. However, in larger ones, the diaphragm is suspended from above.
Expansion Tanks in Natural Gas and Liquid Fuel Heating Systems
Expansion tanks are used in closed circuit natural gas or liquid fuel heating systems as well as hot water systems. This is because these systems have fully controlled fuel supply and operate entirely on electricity. If there is a power outage, everything stops and the boiler begins to cool immediately. Therefore, there is no danger of uncontrolled temperature increase. Ideally, nitrogen gas should be pressurized into expansion tanks. Nitrogen diffuses into water through the membrane at a slower rate compared to oxygen. Therefore, it can remain in the tank for a longer period of time. Additionally, nitrogen is non-corrosive like oxygen, so even if it mixes with water, it does not cause corrosion in the installation. However, due to the difficulty of finding compressed air, especially on construction sites or in buildings, and the fact that air is composed of 75-78% nitrogen, air is generally used instead of nitrogen.