Construction of an autonomous gravity heating network is selected if impractical and sometimes impossible to determine the circulating pump or to connect to a centralized power supply.
To heating system with natural circulation function smoothly, it is necessary to calculate its parameters correctly install the components and choose the reasonable water circuit diagram.
The content of the article:
- The principles of the process of natural circulation
- The maximum hydrostatic pressure difference
- Minimizing resistance to the motion of water
- Rules for the choice and installation of pipes
- Single-tube and double-tube heating circuit
- Circuit using one line
- Variant using feedback pipe
- Useful videos on the topic
The principles of the process of natural circulation
The process of movement of water in the heating circuit without using a circulation pump occurs due to natural physical laws.
Understanding the nature of these processes will allow well-designed heating system project for standard and non-standard cases.
The maximum hydrostatic pressure difference
The main physical feature of any coolant (water or antifreeze), which promotes its movement to the contour of natural circulation - decrease in density with increasing temperature. hot water density of less than cold and therefore there is a difference in the hydrostatic pressure of hot and cold liquid column. The cold water flowing to the heat exchanger, hot displaces up the pipe.
Home heating circuit can be divided into several fragments. By "hot" fragments water is directed upwards, and by "cold" - down. Border fragments are upper and lower heating point. The main task in the simulation of natural circulation of water is to achieve a maximum possible difference between the pressure of the liquid column in the "hot" and "cold" fragments.
A classic for natural circulation water circuit element is acceleration collector (main riser) - a vertical pipe directed upwards from the heat exchanger. Acceleration collector should have a maximum temperature, so it is insulated along the entire length. Although, if the height of the reservoir is not great (for both single-storey houses), you can not conduct heat insulation as well as water in it will not have time to cool down.
Usually design the system so that the upper point of dispersal collector coincide with the upper point of the entire circuit. There is set out in a tank-type or open-expander valve for discharging air when using a membrane tank. Then the length of the "hot" loop fragment is the minimum possible, which leads to a reduction of heat losses in this area.
It is also desirable to "hot" circuit moiety is not combined with a long portion, conveying the cooled heating medium. Ideally, the lowest point of the water circuit coincides with the lower point of the heat exchanger placed in the heating apparatus.
For the "cold" water loop segment, too, has its own rules, increasing the pressure of the liquid:
- more heat loss at the "cold" portion of the heating network, the lower the water temperature and the greater its density, so the operation of systems with natural circulation is only possible with considerable heat transfer;
- the greater the distance from the bottom point to the contour of the radiator connection points, the greater portion of the water column with a minimum temperature and maximum density.
To ensure that the latter rule is often a furnace or boiler installed in the lowest point of the house, for example, in the cellar. Thus placement of the boiler provides the greatest possible distance between the lower level and radiator water entry point to the heat exchanger.
However, the height between the upper and lower points of the water circuit in the natural circulation should not be too large (in practice not more than 10 meters). The furnace or boiler, heat exchanger, and only the lower part of the acceleration collector.
If the fragment is small relative to the entire height of the water circuit, the pressure drop in the "hot" circuit moiety will be negligible and circulation process is started.
Minimizing resistance to the motion of water
When designing a system with natural circulation is necessary to take into account the speed of the coolant circuit. Firstly, the faster the speed, the faster the heat transfer will take place according to the "boiler - heat - water circuit - radiators - room".
Second, the faster the velocity of the fluid through the heat exchanger, the lower the probability of boiling, which is particularly important in furnace heating.
In heating systems with forced circulation of water flow rate mainly depends on the circulation pump parameters. When water heating with natural circulation velocity depends on the following factors:
- pressure difference between the contour moieties in its lower point;
- hydrodynamic resistance heating system.
Methods to maximize pressure difference discussed above. Pressure drop of the real system does not lend itself to precise calculation because of complex mathematical models and a large number of incoming data, the accuracy of which is difficult to guarantee. However, there are general rules that will reduce the resistance of the heating circuit.
The main causes of reduced water velocity is resistance of the pipe wall and the presence of the restrictions as a result of the presence of pipe fittings or valves. At low flow rates walls practically no resistance, except long, thin tubes, for heating characteristic by means of floor heating. As a rule, it is isolated individual circuits with forced circulation.
When selecting the types of pipes for the loop with natural circulation would have to take into account the availability of technical restrictions when installing the system. Therefore, the metal pipes used in the natural circulation of water is undesirable because of their connection fittings, with much smaller internal diameter.
Rules for the choice and installation of pipes
Selection between steel or polypropylene tubes at any circulation occurs according to the criterion the possibility of their use for hot water, as well as the prices of products, ease of installation and life operation.
Riser feed mount of a metal pipe, as the water passes through the highest temperature, and in the case of oven heating or malfunction of the heat exchanger is possible embodiment the passage of steam.
When necessary to use a natural circulation pipe diameter somewhat larger than in the case of the circulation pump. Typically, for heating up to 200 square meters, and the diameter of the collector pipe to disperse the return inlet to the heat exchanger is two inches. This is due to lower velocity of water compared with forced circulation for this, which leads to the following problems:
- a smaller amount of heat transferred per unit time from the source to heat the room;
- a small head will not be able to push the blockage or air pockets.
Particular attention when using natural circulation circuit with the lower inlet flow problem must be given to removing air from the system. He can not completely discharged from the coolant through the expansion tank, because boil water flows first in line for equipment located below than themselves.
If forced circulation of the water pressure air herding fixed in the highest point of the air collector system - a device with automatic, manual or semi-automatic control. With the help of crane Majewski mainly produced heat adjustment.
The gravitational heating systems with supply, below the devices, cranes Majewski used directly for air bleed.
Air may also be discharged via air vent installed on each riser or air line laid parallel highways system. Because of the impressive number of devices for removing air from the bottom of gravity circuit wiring are used very rarely.
At low pressure a small air pocket is able to completely stop the heating system. Thus, according to snip 41-01-2003 not allowed without slope lay pipes of heating systems with water velocity less than 0.25 m / s.
In such natural circulation speed unattainable. Therefore, besides increasing the diameter of the pipes to be observed biases constant air output from the heating system. The slope of the design calculation 2- 3 mm per 1 meter, residential networks slope reaches 5 mm per meter horizontal line.
Bias supply make during the movement of water to the air moved to the tank or expander system, bleed air at the top point of the contour. Although it can be done and the counter-slope, but in this case it is necessary to install an additional valve for air removal.
The slope of the return line do, as a rule, the direction of travel of chilled water. Then the lower contour point coincides with a return pipe entering the heat source.
When installing floor heating a small area in the natural circulation circuit is necessary to prevent air from entering the narrow and horizontally disposed tube of the heating system. It is necessary to put the air removal device in front of a warm floor.
Single-tube and double-tube heating circuit
In designing heating circuit home with natural circulation of water may design as one or several separate circuits. They may differ significantly from each other. Regardless of the length, number of radiators and other parameters, they operate on a one-pipe or two-pipe circuit.
Circuit using one line
heating system using the same tube for sequentially supplying water to the radiators is called a single pipe. The easiest option is a one-pipe heating metal pipes without the use of heat sinks.
This is the cheapest and the least problematic way to solve heat your home when choosing in favor of the natural circulation of the coolant. The only significant negative - look bulky tubes.
When the economy circuit embodiment of a single pipe with radiators, hot water successively flows through each device. There must be a minimum number of pipes and valves. As the passing coolant cools, so subsequent radiators receive water cooler that should be considered when calculating the number of sections.
The most effective way to connect devices to the single-pipe heating network is considered to be the diagonal option. According to this scheme, the heating circuits with natural circulation type hot water enters the cooler from the top, after cooling is discharged through conduit located underneath. When passing the heated water in this way gives the maximum amount of heat.
At the bottom is connected to the battery like the inlet and outlet, the heat transfer is substantially reduced because the heated heat transfer medium have to go a long way as possible. Due to the significant cooling circuits in such batteries are not used with a large number of sections.
Heating circuits with radiators in this connection are called "Leningradka". Despite the marked loss of heat, they are given preference in the arrangement of apartment heating systems, due to the more aesthetic view of the pipeline is laid.
A major shortcoming of single-pipe networks is the inability to turn off one of the heating sections without stopping the circulation of water around the loop. So it is usually used the classical scheme of modernization with the installation of "bypass" to bypass the radiator via a branch with two ball valves or three-way stopcock. This allows to regulate the flow of water to the radiator, up to its complete disconnection.
For two or more storey buildings embodiments apply a single pipe circuit with vertical risers. In this case, the hot water distribution more uniform than in the horizontal struts. In addition, the vertical risers less extensive and better fit into the interior of the house.
Variant using feedback pipe
When a pipe is used for supplying hot water to the radiators, and the second - for discharging cooled to a boiler or furnace, a scheme called two-pipe heating. Such a system in the presence of radiators is used more often than single-pipe. It is more expensive, as it requires additional pipe installation, but has a number of significant advantages:
- occurs more uniform temperature distribution is supplied to the heat radiator;
- easier to perform the calculation of the parameters of the radiators on the area of heated space and the required temperature values;
- easier to perform heat flow adjustment to each radiator.
Depending on the direction of movement of chilled water relatively hot, the system is divided into two pipe passing and blind. The chilled water circuits free movement occurs in the same direction as the hot, so the cycle length for the entire contour point.
In dead-end circuits, chilled water moves toward the hot, however for different radiator coolant circulation cycle lengths are different. Since the rate of a small system, and the heating time can vary substantially. Those radiators, in which the water cycle is shorter than the cycle will be heated faster.
There are two types of liner positioned relative to the radiator, upper and lower. When the upper podvodke tube feed hot water is located above the radiator, and at the bottom podvodke - below.
At the bottom eyeliner is possible to remove the air through the radiators and there is no need for a pipe on top, which is good from the design position of the room. However, without collector acceleration pressure drop will be much less than when using the upper liner. Therefore, the bottom liner for heating the premises according to the principle of natural circulation is almost never used.
Useful videos on the topic
Single-pipe circuit on the basis of the electric boiler for a small house:
Two-pipe system for a single-storey wooden house on the basis of solid fuel boilers long burning:
Combined based on coal-fired boiler system to the presence of water tank:
The use of natural circulation of water during the motion in the heating circuit requires precise calculation and technically correct installation work. Under these conditions, the heating system will heat the quality of a private house premises and saves owners from the noise of the pump and dependence on electricity.
