Expansion tank, also known as pressure expansion tank or volume compensation tank, is a core auxiliary equipment used in water systems and refrigeration systems to balance system pressure and compensate for changes in medium volume. It is widely used in central air conditioning water systems, boiler systems, underfloor heating systems, refrigeration unit cooling water circuits, and other scenarios. Its core function is to absorb the volume expansion of system media (water, refrigerant) caused by temperature changes, buffer system pressure fluctuations, avoid system overpressure, protect pipelines and equipment, and extend system service life.
The core working principle of the expansion tank is based on “gas compression and expansion”, utilizing the compressibility of pre filled nitrogen gas inside the tank to achieve volume compensation and pressure stability: the expansion tank is equipped with an airbag, which is pre filled with nitrogen gas (usually at a pre filling pressure of 0.6MPa-1.0MPa), and the medium chamber is located between the outside of the airbag and the inner wall of the tank body, connecting the system pipeline. When the temperature of the system medium increases and the volume expands, the medium enters the medium chamber, squeezes the airbag, compresses the nitrogen inside the airbag, and increases the pressure, thereby absorbing the expansion volume of the medium and buffering the system pressure; When the temperature of the system medium decreases and the volume shrinks, the nitrogen gas inside the airbag expands, pressing the medium in the medium chamber back into the system to fill the volume gap and maintain stable system pressure.
According to their structural form and working pressure, expansion tanks are mainly divided into two types, which are suitable for different system scenarios:
1. Diaphragm type expansion tank: The tank is equipped with an elastic diaphragm (airbag) to completely separate nitrogen from the system medium, avoiding nitrogen dissolution in the medium and causing pressure instability; Compact structure, good sealing, easy maintenance, pre filled nitrogen can be replenished at any time, suitable for small and medium-sized water systems, household HVAC systems, and small refrigeration units, with a working pressure range of 0.6MPa-1.6MPa.
2. Vertical/horizontal expansion tank: designed without diaphragm, with direct contact between nitrogen and system medium, simple structure, low cost, large volume, suitable for cooling water circuits of large water systems, boiler systems, and industrial refrigeration units. The working pressure range is 1.0MPa-2.5MPa, and nitrogen gas needs to be regularly replenished to avoid pressure drop.
The core pressure control technique of the expansion tank directly affects its working effect and system stability:
1. Pre charge pressure setting: The pre charge pressure should be determined based on the system working pressure. Under normal circumstances, the pre charge pressure is 0.8-0.9 times the static working pressure of the system to avoid excessive or insufficient pre charge pressure, which may cause pressure buffering failure.
2. Pressure upper limit control: Through the pressure linkage between the system safety valve and the expansion tank, the pressure upper limit is set (not exceeding the system design pressure). When the system pressure exceeds the upper limit, the safety valve automatically releases pressure, and the expansion tank absorbs excess volume, providing dual protection for the system.
3. Regular pressure check: Check the pressure gauge of the expansion tank every week to confirm that the pre charge pressure and system pressure are stable. If the pressure drops, nitrogen should be replenished in a timely manner; Check the sealing of airbags or diaphragms every quarter to avoid medium leakage and pressure instability.