An expansion tank is an essential component of any heating scheme. The expansion tank compensates for the thermal expansion of the coolant. It is necessary to qualitatively calculate the volume of the heating expansion tank, otherwise it will not perform its function. Incorrect selection of the volume of the expansion tank for the heating system will damage the heating devices, heat generator and communications. In the case of an open configuration of the circuit, an incorrect calculation may lead to a spill of the coolant.
Expansion tanks are used to eliminate thermal expansion, to accept excess coolant, to maintain stable hydraulic pressure in the equipment. In closed heating circuits, sealed tanks with a rubber membrane are installed, for open circuits - hollow vessels connected to the environment.
In open type heating systems, the excess volume of heated water is displaced into the open space of the expander. In case of overflow, an overflow from the expander to the sewer is organized. An open vessel is installed at the top of the system and at the same time performs the function of removing air pockets from the heating system. The size of the expansion tank for heating according to an open circuit when organizing the overflow of the coolant is chosen arbitrarily, but not less than 5% of the total volume of the coolant. In schemes with natural circulation (in the absence of plumbing), the tank is used to fill in water (coolant).
Membrane expander is a hermetic vessel divided by a membrane partition into two chambers. An outlet from the heating system is connected to one chamber, and during production, air is pumped into the other through a special valve with a pressure of 0.4 - 1.6 atmospheres. The volume of the tank depends on the total capacity of the equipment in terms of coolant. The coolant (water), warming up, expands and the resulting excess volume is squeezed out into the water chamber of the expander, creating pressure on the membrane partition. The membrane bends in the direction of the air chamber, the force of the coolant is compensated by the air pressure (the air is compressed). According to this principle, pressure compensation occurs in the heating system. The flexibility of the membrane and the air pressure of the tank of the expansion tank for closed-type heating maintains a constant pressure in the system.
Methods for calculating an expansion tank for heating
More often they use exact calculation according to formulas. It can be done by anyone with a calculator. The volume of the expansion tank for heating is calculated by the formula:
A \u003d VxS / K, where B is the volume of the coolant; C is an indicator of thermal expansion of the coolant; K is an indicator of the efficiency of the membrane tank.
Calculation of the volume of the coolant is carried out by three methods:
- Geometric - according to the internal volume of heating devices, boiler and pipelines;
- When filling the system - according to the metering device or addition when filling manually;
- The generalized method - 15 liters in the volume of the system is taken per 1 kW of boiler heat output.
The generalized method has a refined modification depending on the type of heating devices. When using radiators, the amount of water in them is on average 11 liters, in convectors - 7 liters, in the underfloor heating circuit - up to 18 liters. The volume of the heat exchanger is indicated in the equipment passport, the amount of water in the pipelines can be determined by counting their length and internal volume. These indicators are summarized (boiler, pipes, appliances) - the result is the total volume of the heating complex.
After calculating the volume of the system is made according to the following formula:
K = (DM - DB) / (DM + 1), where DB is the maximum coolant pressure, usually taken equal to the pressure of the safety valve on the safety group (3 atm.); DB - set air pressure in the air chamber of the expansion tank.
The thermal expansion of water is 4% when heated to 95 degrees Celsius. If there are non-freezing fractions in the coolant, the indicator increases depending on the percentage of additives. At 10% of the additive in the total volume, the water index of 4% is multiplied by a correction factor of 1.1, at 30% by 1.3, and so on.
Calculation of the expansion mat for a system with a 31 kW boiler
Before making calculations for the selection of an expansion tank, you should know that most wall-mounted boilers are equipped with built-in expansion tanks. The volume of the built-in tank is indicated in the technical documentation of the boiler. When recalculating the volume of the heating system by the boiler power (by multiplying 1 kW of heat power by 15 liters), the compliance of the tank with the volume of the system being constructed is checked. If there is a shortage, an additional tank is installed. Its volume is calculated minus the built-in expansion mat. Floor boilers, as a rule, do not have built-in equipment.
The calculation looks like this:
K \u003d (DM - DB) / (DM + 1) \u003d (3.0 - 1.5) / (3.0 - 1) \u003d 0.375
3.0 - pressure in the system, maximum, atm.;
1.5 – air pressure behind the membrane, atm.;
0.375 - tank efficiency indicator, K.
Coolant volume: B \u003d 31x15 \u003d 465 liters.
Then the volume of the tank will be:
A \u003d 465x0.04 / 0.375 \u003d 49.6 liters.
An expansion tank with a volume of at least 50 liters with an air pressure of 1.5 atm is selected. The general selection method (10% of A) shows the need to use a tank with a volume of at least 46.5 liters. In this case, the size of the expander is always rounded up to a larger volume - 50 liters.
The air pressure included in the calculation (1.5 atmospheres) can be changed. Expansion tanks have a built-in valve for filling with air. A hand pump can be connected to it and the pressure can be increased if the factory pressure is lower. In this case, care must be taken - with a significant increase in pressure, the membrane can be damaged, so the process must be controlled by a pressure gauge. The valve also performs the function of pressure relief when it rises to the limit values.
When calculating the expansion tank, it is better to increase the calculated volume by 5 - 10% - this measure eliminates calculation errors and will not affect the operation of the membrane vessel and the heating complex as a whole.
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