Expansion tank

Expansion vessels (abbreviation ADG, colloquially bomb), also the expansion vessel or surge tank are components in hydraulic systems, which receive the changes in volume of hydraulic fluid between the minimum and maximum temperature and so the pressure largely kept constant ( pressure maintenance ). They are often designed as diaphragm expansion vessel (abbreviation MAG).

Technically analogous work the diaphragm accumulators, however, the application here is the storage of energy and the damping pressure surges.

Purpose

The volume of liquids increases with increase in temperature and decrease in temperature from as far as the density anomaly of water is left at 4 ° C in mind. This results due to the low strain capacity of the pipe materials to the fact that the pressure of a confined fluid may increase greatly, even at low temperature increase. Without additional measures, such as expansion tanks this increase in pressure can lead to the destruction of pipelines and pressure vessels.

The change in volume of liquid per Kelvin temperature change is described by the media- dependent coefficient of cubic expansion:

The higher the value, the higher the pressure increase.

Rule of thumb: 1 liter expansion per kW ⇒ ⇒ 12 liter 12 kW

Is the expansion tank designed incorrect or defective, the pressure difference can significantly affect the efficiency of the heating system, even if it still works. An efficiency loss of up to 10% is quite possible here.

Areas of application

Most common application areas of expansion vessels are heating systems, domestic hot water, cold water, solar (with water -glycol mixture ) and hydraulic oil circuits.

Pressure maintenance with conservator

Closed expansion vessel

Maintaining pressure on prestressed gas cushion

Diaphragm expansion vessel

Membrane expansion vessels are equipped with a flexible rubber membrane, which separates the liquid and gas cushion, and avoid as far as possible the gas transition in the liquid. An otherwise regularly become necessary system maintenance is required, which is why only this construction is used in modern heating and solar panel installations.

When heated, the nearly incompressible liquid expands and compresses the gas cushion on the other side of the membrane. Due to the flexible diaphragm, there is a pressure balance between the liquid and the gas cushion, as far as the system pressure is at the lowest temperature to the biasing pressure of the membrane. The gas cushion can be considered as an ideal gas. The relative change in pressure in the system when the temperature change is then proportional to the relative change in volume of the liquid ( hydraulic capacity).

When dimensioning a membrane expansion vessel must be considered:

• Liquid volume
• Lowest and highest temperature of the heat carrier
• Cubic expansion coefficient of the fluid and
• The maximum system pressure.

The expansion vessel must be dimensioned in terms of volume, so that the pressure at the highest and lowest temperature in the system is not impermissibly exceeded or fallen short. The use of other liquids than water ( for example, ethylene glycol / water mixture ) is to be noted that the coefficient of expansion is significantly above the water and the volume of the expansion vessel, has to be correspondingly larger.

Basically four states of membrane expansion vessels can be distinguished:

Expansion vessel without membrane

Closed expansion tank without membrane are running with a direct pressurized nitrogen blanketing. They are usually provided with a nitrogen feeding module, so you can find only in special cases use.

Open expansion vessel

Open expansion tanks are mounted at the highest point of the cycle ( pressure maintenance over head) and are connected to the hydraulic system unabsperrbar a safety line. Since atmospheric oxygen can dissolve in water and cause corrosion, one finds this form only for old heating systems. An open expansion vessel is also the elevated water tank or water tower in drinking water supply systems. Through this an equalization of pressure is achieved among consumers, while provision of a larger amount of water.

Pump pressure maintenance

For systems with very large volumes of liquid, the pump pressure maintenance is used: when the pressure drops promotes Diktierpumpe water into the system, due to pressure of water over relief valves or pressure-dependent controlled solenoid valves is conducted in an unpressurized container. It has a rubber diaphragm to prevent the diffusion of atmospheric oxygen into the liquid as a rule.

Special Requirements

• The ADG is part of the safety equipment in hot water heating systems and must be serviced annually in accordance with DIN EN 12828.

• Closed hydraulic circuits with expansion vessel must be equipped with a safety valve, as in case of damage of the membrane and loss of gas overpressure may occur in the system.

• Since the operating temperature of the membrane is limited (usually to about 80 ° C ), a series container must be installed before the diaphragm expansion vessel for plants with high heat transfer temperatures, where the temperature coated and cooling takes place.

• A diaphragm expansion vessel in the drinking water network must have a forced circulation to prevent legionella.