E. Polyakova
The task of any membrane tank - be it an expansion tank, a hydraulic shock absorber or a hydraulic accumulator - is to ensure reliable, safe and long-term operation of the elements of the engineering system. The completeness of its solution depends on how correctly the "membrane" is selected and mounted.
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The place of the tank in the heating system
The function of an expansion tank (expansomat) in a heating system is to compensate for an increase in the volume of water due to its thermal expansion.
The pressure at the point of connection of the device to the system is always equal to the static pressure at this point at the existing temperature. It is very easy to prove this: if we assume that the pressure at the tank connection point changes, then we will have to admit that the volume of the coolant in the tank has also changed. And this cannot be, because. There is no place to take an extra coolant in a closed system, and it also cannot disappear without a trace. However, this rule only applies to systems with one expansion tank.
Thus, the operation parameters of all other elements of the heating system, the required initial pressure in the tank and its volume depend on the location of the expansion tank.
When choosing a place to connect an expansion tank, remember that the higher the pressure in the heating system, the less likely it is to air.
On rice. 1 there are several options for connecting a membrane tank to a heating system with the following height parameters:
. excess of the upper point of the system over the lower ( H) - 10 m;
. the heat generator and the safety valve are located 2 m above the lowest point of the system ( h 1);
. the expansion tank is placed 1 m above the point of its connection to the system ( h 2);
. static pressure at the level of the lowest point of the system - 15 m of water. Art.
If the membrane tank is connected to the system directly after the circulation pump (diagram b), it should be checked that an anti-cavitation pressure reserve is maintained in front of the pump.
At remote flags on rice. 1 the design values of the working pressure at the characteristic points of each system (in m of water column) are indicated.
The setting value of the safety valve is assumed to be 33 m of water. Art., pump pressure - 6 m of water. Art., system capacity - 200 l. The difference between the maximum and minimum coolant temperatures is 80 °C.
IN tab. 1 the calculated characteristics of membrane tanks for circuits with their different connections are given.
The coolant must enter the membrane tank from above. In this case, there is no chance of air entering the liquid compartment of the tank. If this requirement cannot be met, the following rules are recommended:
. the make-up point should be as close as possible to the tank connection point;
. when filling the system with coolant, it is not allowed to use automatic air vents to release air (they must be closed).
. air must be removed from the system through the fittings with taps provided for this (Fig. 2a) or combination faucets with drain and manual air vent (fig. 2b);
. if possible, membrane tanks should be used, having an upper branch pipe for connecting the air vent to the liquid cavity.
Tank selection
A sufficient volume of the membrane expansion tank is recommended to be determined by the formula:
Includes the volume of water in pipes, boiler, radiators and other elements of the system. This indicator is calculated by the actual capacity of each element of the system; P a min - initial (setting) absolute pressure in the expansion tank, bar; P a max - maximum absolute pressure possible in the expansion tank, bar.
With a certain error, the value of the coolant volume in the system can be selected from tab. 2. When calculating at the stage of the feasibility study, it is allowed to take the specific capacity of the heating system as 15 l/kW.
The values of the coefficient of thermal expansion of the coolant βt, corresponding to the maximum difference in water temperatures in a non-working and working system, it is recommended to take according to tab. 3.
The set absolute pressure is calculated using the formula:
Absolute maximum pressure possible in the expansion vessel:
As the analysis shows formula 1, the optimal choice of the volume of the expansion membrane tank is directly related to the correct setting of the safety valve (according to SP 41-101-95 "Design of heat points" this is a mandatory element for the expansion meter). It is usually set to a pressure that exceeds the allowable pressure for the most vulnerable element of the system by 10% (taking into account the difference in the heights of the valve and the protected element). Therefore, for heating systems, it is recommended to use valves with the ability to adjust the setting pressure. In addition, the valve must necessarily have a device for forced opening ("undermining") to periodically check its performance and to avoid sticking of the spool. An example of such a valve is shown in rice. 3.
Installing an expansion tank of insufficient volume or incorrect installation can cause the heating system to malfunction and even fail.
The set pressure of the tank must not be lower than the hydrostatic pressure at the level of the center of the tank by more than 1 m of water. Art. (0.1 bar). Otherwise, already in the process of filling the system, the useful volume of the tank will be filled with coolant, and during subsequent heating and expansion of the liquid, a smaller volume will be provided than necessary. In other words, if the setting (factory) pressure in the tank is 1.5 bar, then the system must be filled to a pressure at the level of the center of the tank that does not exceed 1.6 bar. If, according to the project, it is necessary to establish a higher hydrostatic pressure in the system, then for this, before installing the tank, it is necessary to increase the pressure in it using an air pump.
A certain amount of coolant in the tank, provided by its "under-pumping" to hydrostatic per 1 m of water. Art., is necessary in the event that the coolant is cooled down. For example, if the system was filled during the day at a water temperature of 20 °C, and the boiler was not started for some reason, when the coolant cools down at night, its volume will decrease, which can lead to rarefaction at the upper points of the system and intensive air suction through the air vents.
In two identical systems, differing only in the type of coolant, a larger expansion tank will be required in the system that uses an antifreeze coolant based on glycol (ethylene or propylene glycol). After all, the expansion coefficient of glycol solutions is slightly higher than that of water.
Thus, when switching from a water system to a glycol system, it may be necessary to replace the tank with a larger one in size or install an additional "membrane".
A signal that the system needs a larger tank is the frequent operation of the safety valve.
The diameter of the supply line to the diaphragm expansion tank must be at least as calculated according to the following formula:
Strapping examples
On rice. 4 the option of installing an expansion tank in a system with one boiler is shown. In this case, the expander is located on the return pipeline of the system, which makes it possible to operate it at a lower coolant temperature than if it were installed on the supply line. This solution allows you to extend the life of the device. Connecting a tank to the suction port of the pump prevents the pump from cavitation.
On rice. 5- installation diagram of expansion tanks in a system with several boilers and automatic limitation of the minimum water temperature in the return pipeline. Here, one expander per boiler is provided. The capacity of each of them must be no less than the calculated capacity for the entire system, i.e. if, according to the calculation, it needs a tank with a capacity of 80 liters, then the capacity of each of the installed devices should be the same. This is due to the fact that during operation at reduced power, when the burner of one of the boilers is turned off, the corresponding circulation pump is also turned off and the three-way valve is closed. In this case, there is no water circulation through the switched off boiler, and the expansion tank installed on this boiler is isolated from the rest of the system. The expansomat remaining in operation must provide compensation for the expansion of the coolant in the entire volume of the system. This situation is also valid when using two-way valves that perform the function of blocking the boilers.
A system with several boilers and automatic limitation of the minimum return water temperature can also have one expansion tank. In this case, it must be mounted as shown in rice. 6.
Membrane tanks for domestic hot water
The main difference between membrane tanks for water supply is that the water in them should not come into contact with the walls of the case, as is allowed in heating systems. Therefore, they always use a chamber-type membrane (in the form of a bag). In addition, increased requirements are imposed on the material of the membrane of tanks for water supply in terms of the admissibility of contact with food liquids.
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