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In water heat supply systems, the provision of heat to consumers is carried out by appropriately distributing the estimated costs of network water between them. To implement such distribution, it is necessary to develop a hydraulic mode of the heat supply system.
The purpose of developing the hydraulic mode of the heating supply system is to ensure optimal permissible pressures in all elements of the heating supply system and the necessary available pressures at the nodes of the heating network, at group and local heating points, sufficient to supply consumers with the calculated water flows. The available pressure is the difference in water pressure in the supply and return pipelines.
To ensure reliable operation of the heat supply system, the following conditions apply:
Not exceeding permissible pressures: in heat supply sources and heating networks: 1.6-2.5 mPa - for steam-water network heaters of the PSV type, for steel hot water boilers, steel pipes and fittings; in subscriber installations: 1.0 mPa - for sectional water-water heaters; 0.8-1.0 mPa - for steel convectors; 0.6 mPa - for cast iron radiators; 0.8 mPa - for air heaters;
Security overpressure in all elements of the heat supply system to prevent pump cavitation and protect the heat supply system from air leaks. The minimum value of excess pressure is assumed to be 0.05 MPa. For this reason, the piezometric line of the return pipeline in all modes must be located above the point of the tallest building by at least 5 m of water. Art.;
At all points of the heating system, a pressure must be maintained that exceeds the pressure of saturated water vapor at maximum temperature water, ensuring that the water does not boil. As a rule, the danger of water boiling most often occurs in the supply pipelines of the heating network. Minimum pressure in the supply pipelines is taken according to the design temperature of the supply water, table 7.1.
Table 7.1
The non-boiling line must be drawn on the graph parallel to the terrain at a height corresponding to the excess pressure at the maximum temperature of the coolant.
It is convenient to depict the hydraulic mode graphically in the form of a piezometric graph. The piezometric graph is plotted for two hydraulic modes: hydrostatic and hydrodynamic.
The purpose of developing a hydrostatic mode is to ensure the necessary water pressure in the heating system, within acceptable limits. The lower pressure limit should ensure that consumer systems are filled with water and create the necessary minimum pressure to protect the heating system from air leaks. The hydrostatic mode is developed with charging pumps running and no circulation.
The hydrodynamic regime is developed based on the data hydraulic calculation heating networks and is provided simultaneous work make-up and network pumps.
The development of a hydraulic mode comes down to constructing a piezometric graph that meets all the requirements for the hydraulic mode. Hydraulic modes of water heating networks (piezometric graphs) should be developed for heating and non-heating periods. The piezometric graph allows you to: determine the pressures in the supply and return pipelines; available pressure at any point in the heating network, taking into account the terrain; select consumer connection schemes based on available pressure and building heights; select auto regulators, elevator nozzles, throttle devices for local systems heat consumers; select network and make-up pumps.
Construction of a piezometric graph(Fig. 7.1) is done as follows:
a) scales are selected along the abscissa and ordinate axes and the terrain and the height of the building blocks are plotted. Piezometric graphs are constructed for main and distribution heating networks. For main heating networks the following scales can be adopted: horizontal M g 1:10000; vertical M in 1:1000; for distribution heating networks: M g 1:1000, M v 1:500; The zero mark of the ordinate axis (pressure axis) is usually taken to be the mark of the lowest point of the heating main or the mark of the network pumps.
b) the value of the static pressure is determined to ensure the filling of consumer systems and the creation of minimal excess pressure. This is the height of the highest building plus 3-5 m.water column.
After plotting the terrain and building heights, the static head of the system is determined
H c t = [N building + (3¸5)], m (7.1)
Where N rear- height of the highest building, m.
The static head H st is parallel to the x-axis, and it should not exceed the maximum operating pressure for local systems. The maximum operating pressure is: for heating systems with steel heating devices and for air heaters - 80 meters; for heating systems with cast iron radiators- 60 meters; for independent connection schemes with surface heat exchangers - 100 meters;
c) Then the dynamic mode is constructed. The suction pressure of network pumps H sun is arbitrarily selected, which should not exceed the static pressure and provides the necessary supply pressure at the inlet to prevent cavitation. The cavitation reserve, depending on the size of the pump, is 5-10 m.water column;
d) from the conditional pressure line at the suction of network pumps, pressure losses are successively postponed to return pipeline DN of the main line of the heating network ( line A-B) using the results of hydraulic calculations. The amount of pressure in the return line must meet the requirements specified above when constructing the static pressure line;
e) the required available pressure is set aside at the last subscriber DN ab, based on the operating conditions of the elevator, heater, mixer and distribution heating networks (line B-C). The amount of available pressure at the connection point of distribution networks is assumed to be at least 40 m;
e) starting from the last pipeline node, pressure losses are deposited in the supply pipeline of the main line DN under ( line C-D). Pressure at all points of the supply pipeline based on its conditions mechanical strength should not exceed 160 m;
g) pressure losses are delayed in the heat source DН it ( line D-E) and the pressure at the outlet of the network pumps is obtained. In the absence of data, the pressure loss in the communications of a thermal power plant can be assumed to be 25 - 30 m, and for a district boiler house 8-16 m.
The pressure of the network pumps is determined
The pressure of the charging pumps is determined by the pressure of the static mode.
As a result of this construction, the initial form of a piezometric graph is obtained, which allows one to estimate pressures at all points of the heat supply system (Fig. 7.1).
If they do not meet the requirements, change the position and shape of the piezometric graph:
a) if the pressure line of the return pipeline crosses the height of the building or is less than 3¸5 m from it, then the piezometric graph should be raised so that the pressure in the return pipeline ensures filling of the system;
b) if the maximum pressure in the return pipeline exceeds the permissible pressure in heating devices, and it cannot be reduced by shifting the piezometric graph down, it should be reduced by installing booster pumps in the return pipeline;
c) if the non-boiling line intersects the pressure line in the supply pipeline, then boiling of water is possible beyond the intersection point. Therefore, the water pressure in this part of the heating network should be increased by moving the piezometric graph upward, if possible, or by installing a booster pump on the supply pipeline;
d) if the maximum pressure in the equipment of the heat treatment plant of the heat source exceeds the permissible value, then booster pumps are installed on the supply pipeline.
Division of the heating network into static zones. The piezometric graph is developed for two modes. Firstly, for static mode, when there is no water circulation in the heating system. It is assumed that the system is filled with water at a temperature of 100°C, thereby eliminating the need to maintain excess pressure in the heat pipes to avoid boiling of the coolant. Secondly, for hydrodynamic mode - in the presence of coolant circulation in the system.
The development of the schedule begins with the static mode. The location of the full static pressure line on the graph should ensure the connection of all subscribers to the heating network according to a dependent scheme. To do this, the static pressure should not exceed what is permissible based on the strength of subscriber installations and should ensure that local systems are filled with water. The presence of a common static zone for the entire heating system simplifies its operation and increases its reliability. If there is a significant difference in geodetic elevations of the earth, establishing a common static zone is impossible for the following reasons.
The lowest position of the static pressure level is determined from the conditions of filling local systems with water and ensuring high points systems of the tallest buildings located in the area of the highest geodetic elevations, excess pressure of at least 0.05 MPa. This pressure turns out to be unacceptably high for buildings located in that part of the area that has the lowest geodetic elevations. Under such conditions, it becomes necessary to divide the heat supply system into two static zones. One zone is for part of the area with low geodetic marks, the other - with high ones.
In Fig. 7.2 shows the piezometric graph and circuit diagram heat supply systems for an area with a significant difference in geodetic ground level marks (40m). The part of the area adjacent to the heat supply source has zero geodetic marks; in the peripheral part of the area the marks are 40 m. The height of the buildings is 30 and 45 m. To be able to fill building heating systems with water III and IV, located at the 40 m mark and creating an excess pressure of 5 m at the upper points of the systems, the level of the total static pressure should be located at the 75 m mark (line 5 2 - S 2). In this case, the static head will be equal to 35m. However, a head of 75m is unacceptable for buildings I And II, located at the zero mark. For them, the permissible highest position of the level of total static pressure corresponds to 60 m. Thus, under the conditions under consideration, it is impossible to establish a common static zone for the entire heat supply system.
A possible solution is to divide the heat supply system into two zones with different levels full static pressure - to the lower one with a level of 50m (line S t-Si) and the upper one with a level of 75m (line S 2 -S 2). With this solution, all consumers can be connected to the heat supply system according to a dependent scheme, since the static pressures in the lower and upper zones are within acceptable limits.
So that when water circulation in the system stops, the static pressure levels are established in accordance with the accepted two zones, a separating device is placed at the point of their connection (Fig. 7.2 6 ). This device protects the heating network from high blood pressure when the circulation pumps stop, automatically cutting it into two hydraulically independent zones: upper and lower.
When the circulation pumps are stopped, the pressure drop in the return pipeline of the upper zone is prevented by the pressure regulator “towards itself” RDDS (10), which maintains a constant set pressure RDDS at the point where the pulse is taken. When the pressure drops, it closes. A pressure drop in the supply line is prevented by a check valve(11), which also closes. Thus, the RDDS and the check valve cut the heating network into two zones. To feed the upper zone, a feed pump (8) is installed, which takes water from the lower zone and supplies it to the upper one. The pressure developed by the pump is equal to the difference between the hydrostatic heads of the upper and lower zones. The lower zone is fed by the make-up pump 2 and the make-up regulator 3.
Figure 7.2. Heating system divided into two static zones
a - piezometric graph;
b - schematic diagram of the heat supply system; S 1 - S 1, - line of total static pressure of the lower zone;
S 2 – S 2, - line of total static pressure of the upper zone;
N p.n1 - pressure developed by the feed pump of the lower zone; N p.n2 - pressure developed by the top zone make-up pump; N RDDS - pressure to which the RDDS (10) and RD2 (9) regulators are set; ΔН RDDS - pressure activated on the RDDS regulator valve in hydrodynamic mode; I-IV- subscribers; 1-make-up water tank; 2.3 - make-up pump and lower zone make-up regulator; 4 - pre-switched pump; 5 - main steam-water heaters; 6- network pump; 7 - peak hot water boiler; 8 , 9 - make-up pump and top zone make-up regulator; 10 - pressure regulator “towards you” RDDS; 11- check valve
The RDDS regulator is set to the pressure Nrdds (Fig. 7.2a). The make-up regulator RD2 is set to the same pressure.
In hydrodynamic mode, the RDDS regulator maintains the pressure at the same level. At the beginning of the network, a make-up pump with a regulator maintains the pressure of H O1. The difference in these pressures is spent on overcoming the hydraulic resistance in the return pipeline between the separating device and the circulation pump of the heat source, the rest of the pressure is activated in the throttle substation on the RDDS valve. In Fig. 8.9, and this part of the pressure is shown by the value ΔН RDDS. The throttle substation in hydrodynamic mode makes it possible to maintain the pressure in the return line of the upper zone not lower than the accepted level of static pressure S 2 - S 2.
Piezometric lines corresponding to the hydrodynamic regime are shown in Fig. 7.2a. The highest pressure in the return pipeline at consumer IV is 90-40 = 50m, which is acceptable. The pressure in the return line of the lower zone is also within acceptable limits.
In the supply pipeline, the maximum pressure after the heat source is 160 m, which does not exceed what is permissible based on the strength of the pipes. Minimum piezometric head in the supply pipeline is 110 m, which ensures that the coolant does not boil over, since at a design temperature of 150 ° C the minimum permissible pressure is 40 m.
The piezometric graph developed for static and hydrodynamic modes provides the ability to connect all subscribers according to a dependent circuit.
To others possible solution hydrostatic mode of the heating system shown in Fig. 7.2 is the connection of some subscribers according to an independent scheme. There may be two options here. First option- set the general level of static pressure at 50 m (line S 1 - S 1), and connect the buildings located at the upper geodetic marks according to an independent scheme. In this case, the static pressure in water-water heating heaters of buildings in the upper zone on the side of the heating coolant will be 50-40 = 10 m, and on the side of the heated coolant will be determined by the height of the buildings. The second option is to set the general level of static pressure at 75 m (line S 2 - S 2) with the connection of the buildings of the upper zone according to a dependent scheme, and the buildings of the lower zone - according to an independent one. In this case, the static pressure in water-water heaters on the side of the heating coolant will be equal to 75 m, i.e. less than the permissible value (100 m).
Main 1, 2; 3;
add. 4, 7, 8.
The task of hydraulic calculation includes:
Determination of pipeline diameter;
Determination of pressure drop (pressure);
Determination of pressures (pressures) at various points in the network;
Linking all network points in static and dynamic modes in order to ensure permissible pressures and required pressures in the network and subscriber systems.
Based on the results of hydraulic calculations, the following problems can be solved.
1. Determination of capital costs, metal (pipes) consumption and the main volume of work on laying a heating network.
2. Determination of the characteristics of circulation and make-up pumps.
3. Determination of operating conditions of the heating network and selection of subscriber connection schemes.
4. Selection of automation for the heating network and subscribers.
5. Development of operating modes.
a. Schemes and configurations of heating networks.
The layout of the heating network is determined by the location of heat sources in relation to the area of consumption, the nature of the heat load and the type of coolant.
The specific length of steam networks per unit of design heat load is small, since steam consumers - usually industrial consumers - are located at a short distance from the heat source.
More challenging task is the choice of the scheme of water heating networks due to the large length, large quantity subscribers. Water vehicles are less durable than steam vehicles due to greater corrosion, and are more sensitive to accidents due to the high density of water.
Fig.6.1. Single-line communication network of a two-pipe heating network
Water networks are divided into main and distribution networks. The coolant is supplied through main networks from heat sources to areas of consumption. Through distribution networks, water is supplied to GTP and MTP and to subscribers. Subscribers very rarely connect directly to backbone networks. At the points where distribution networks are connected to the main ones, sectioning chambers with valves are installed. Sectional valves on main networks are usually installed every 2-3 km. Thanks to the installation of sectional valves, water losses during vehicle accidents are reduced. Distribution and main vehicles with a diameter of less than 700 mm are usually made dead-end. In the event of an emergency, a break in the heat supply to buildings for up to 24 hours is acceptable for most of the country. If a break in heat supply is unacceptable, it is necessary to provide for duplication or loopback of the heating system.
Fig.6.2. Ring heating network from three thermal power plants Fig.6.3. Radial heat network
When supplying heat to large cities from several thermal power plants, it is advisable to provide for mutual interlocking of thermal power plants by connecting their mains with interlocking connections. In this case, a ring heat network with several power sources is obtained. Such a scheme has higher reliability and ensures the transmission of redundant water flows in the event of an accident on any part of the network. For pipeline diameters emanating from a heat source of 700 mm or less, it is usually used radial scheme heating network with a gradual decrease in pipe diameter as it moves away from the source and the connected load decreases. This network is the cheapest, but in the event of an accident, the heat supply to subscribers is stopped.
b. Basic calculation dependencies
Based on the results of calculating water supply networks for various water consumption modes, the parameters of the water tower and pumping units are determined to ensure the operability of the system, as well as free pressures in all network nodes.
To determine the pressure at supply points (at the water tower, at the pumping station), it is necessary to know the required pressures of water consumers. As mentioned above, the minimum free pressure in the water supply network of a settlement with maximum domestic and drinking water supply at the entrance to the building above the ground surface in a one-story building should be at least 10 m (0.1 MPa), with a higher number of storeys it is necessary to add 4 to each floor m.
During the hours of lowest water consumption, the pressure for each floor, starting from the second, is allowed to be 3 m. For individual multi-storey buildings, as well as groups of buildings located in elevated areas, local pumping installations are provided. The free pressure at the water dispensers must be at least 10 m (0.1 MPa),
IN external network industrial water pipelines free pressure is taken according to technical specifications equipment. The free pressure in the consumer's drinking water supply network should not exceed 60 m, otherwise for individual areas or buildings it is necessary to install pressure regulators or zoning the water supply system. When operating a water supply system, a free pressure of no less than the standard must be ensured at all points in the network.
Free heads at any point in the network are determined as the difference between the elevations of the piezometric lines and the ground surface. Piezometric marks for all design cases (for domestic and drinking water consumption, in case of fire, etc.) are calculated based on the provision of standard free pressure at the dictating point. When determining piezometric marks, they are set by the position of the dictating point, i.e., the point with a minimum free pressure.
Typically, the dictating point is located in the most unfavorable conditions both in terms of geodetic elevations (high geodetic elevations) and in terms of distance from the power source (i.e., the sum of the pressure losses from the power source to the dictating point will be the greatest). At the dictating point they are set by a pressure equal to the normative one. If at any point in the network the pressure is less than the standard one, then the position of the dictating point is set incorrectly. In this case, they find the point with the lowest free pressure, take it as the dictating one, and repeat the calculation of the pressure in the network.
The calculation of the water supply system for operation during a fire is carried out on the assumption that it occurs at the highest points and remotest from power sources in the territory served by the water supply. According to the method of fire extinguishing, water pipelines are of high and low pressure.
As a rule, when designing water supply systems, low pressure fire water supply should be adopted, with the exception of small settlements(less than 5 thousand people). Fire-fighting water supply system high pressure must be economically justified,
In low-pressure water supply systems, the pressure is increased only while the fire is being extinguished. The necessary increase in pressure is created by mobile fire pumps, which are transported to the site of the fire and take water from the water supply network through street hydrants.
According to SNiP, the pressure at any point in the low-pressure fire-fighting water supply network at ground level during fire fighting must be at least 10 m. Such pressure is necessary to prevent the possibility of vacuum formation in the network when water is drawn from fire pumps, which, in turn, can cause penetration into network through leaky soil water joints.
In addition, a certain supply of pressure in the network is required for the operation of fire truck pumps in order to overcome significant resistance in the suction lines.
A high-pressure fire extinguishing system (usually adopted at industrial facilities) provides for the supply of water to the fire site as required by fire regulations and increasing the pressure in the water supply network to a value sufficient to create fire jets directly from the hydrants. The free pressure in this case should ensure a compact jet height of at least 10 m at full fire water flow and the location of the fire nozzle barrel at the level of the highest point of the tallest building and water supply through fire hoses 120 m long:
Nsv = N building + 10 + ∑h ≈ N building + 28 (m)
where H building is the height of the building, m; h - pressure loss in the hose and barrel of the fire nozzle, m.
In high-pressure water supply systems, stationary fire pumps are equipped with automatic equipment that ensures that the pumps start no later than 5 minutes after a signal about a fire is given. The network pipes must be selected taking into account the increase in pressure during a fire. The maximum free pressure in the combined water supply network should not exceed 60 m of water column (0.6 MPa), and during the hour of a fire - 90 m (0.9 MPa).
When there are significant differences in the geodetic elevations of the object supplied with water, a large length of water supply networks, as well as when there is a large difference in the values of free pressure required by individual consumers (for example, in microdistricts with different number of storeys), zoning of the water supply network is arranged. It may be due to both technical and economic considerations.
The division into zones is carried out based on the following conditions: at the highest point of the network the necessary free pressure must be provided, and at its lowest (or initial) point the pressure must not exceed 60 m (0.6 MPa).
According to the types of zoning, water supply systems come with parallel and sequential zoning. Parallel zoning water supply systems are used for large ranges of geodetic elevations within the city area. To do this, lower (I) and upper (II) zones are formed, which are supplied with water by pumping stations of zones I and II, respectively, with water supplied at different pressures through separate water pipelines. Zoning is carried out in such a way that at the lower boundary of each zone the pressure does not exceed the permissible limit.
Water supply scheme with parallel zoning
1 — pumping station II lift with two groups of pumps; 2—pumps of the II (upper) zone; 3 — pumps of the I (lower) zone; 4 - pressure-regulating tanks
Q[KW] = Q[Gcal]*1160;Converting load from Gcal to kW
G[m3/hour] = Q[KW]*0.86/ ΔT; where ΔT– temperature difference between supply and return.
Example:
Supply temperature from heating networks T1 – 110˚ WITH
Supply temperature from heating networks T2 – 70˚ WITH
Heating circuit flow G = (0.45*1160)*0.86/(110-70) = 11.22 m3/hour
But for a heated circuit with temperature chart 95/70, the flow rate will be completely different: = (0.45*1160)*0.86/(95-70) = 17.95 m3/hour.
From this we can conclude: the lower the temperature difference (temperature difference between supply and return), the greater the coolant flow required.
Selection of circulation pumps.
When selecting circulation pumps for heating, hot water, ventilation systems, you need to know the characteristics of the system: coolant flow,
which must be ensured and the hydraulic resistance of the system.
Coolant flow:
G[m3/hour] = Q[KW]*0.86/ ΔT; where ΔT– temperature difference between supply and return;
Hydraulic The system resistance should be provided by specialists who calculated the system itself.
For example:
We consider the heating system with a temperature graph of 95˚ C /70˚ With and load 520 kW
G[m3/hour] =520*0.86/25 = 17.89 m3/hour~ 18 m3/hour;
The heating system resistance wasξ = 5 meters ;
In the case of an independent heating system, you need to understand that the resistance of the heat exchanger will be added to this resistance of 5 meters. To do this, you need to look at its calculation. For example, let this value be 3 meters. So, the total resistance of the system is: 5+3 = 8 meters.
Now it’s quite possible to choose circulation pump with flow rate 18m3/hour and a head of 8 meters.
For example this one:
In this case, the pump is selected with a large margin, it allows you to ensure the operating pointflow/pressure at the first speed of its operation. If for some reason this pressure is not enough, the pump can be “accelerated” to 13 meters at third speed. The best option A pump version is considered that maintains its operating point at the second speed.
It is also quite possible, instead of an ordinary pump with three or one operating speed, to install a pump with a built-in frequency converter, for example this:
This pump version is, of course, the most preferable, since it allows the most flexible adjustment of the operating point. The only downside is the cost.
It is also necessary to remember that for the circulation of heating systems it is necessary to provide two pumps (main/backup), and for the circulation of the DHW line it is quite possible to install one.
Recharge system. Selection of the charging system pump.
Obviously, a make-up pump is necessary only in the case of using independent systems, in particular heating, where the heating and heated circuit
separated by a heat exchanger. The make-up system itself is necessary to maintain constant pressure in the secondary circuit in case of possible leaks
in the heating system, as well as for filling the system itself. The make-up system itself consists of a pressure switch, a solenoid valve, and an expansion tank.
A make-up pump is installed only when the coolant pressure in the return is not enough to fill the system (the piezometer does not allow it).
Example:
Return coolant pressure from heating networks P2 = 3 atm.
The height of the building taking into account technical requirements. Underground = 40 meters.
3atm. = 30 meters;
Required height = 40 meters + 5 meters (at spout) = 45 meters;
Pressure deficit = 45 meters – 30 meters = 15 meters = 1.5 atm.
The pressure of the feed pump is clear; it should be 1.5 atmospheres.
How to determine consumption? The pump flow rate is assumed to be 20% of the volume of the heating system.
The operating principle of the recharge system is as follows.
A pressure switch (pressure measuring device with a relay output) measures the pressure of the return coolant in the heating system and has
pre-setting. For this concrete example this setting should be approximately 4.2 atmospheres with a hysteresis of 0.3.
When the pressure in the heating system return drops to 4.2 atm, the pressure switch closes its group of contacts. This supplies voltage to the solenoid
valve (opening) and make-up pump (switching on).
Make-up coolant is supplied until the pressure rises to a value of 4.2 atm + 0.3 = 4.5 atmospheres.
Calculation of a control valve for cavitation.
When distributing the available pressure between the elements of a heating point, it is necessary to take into account the possibility of cavitation processes inside the body
valves that will destroy it over time.
The maximum permissible pressure drop across the valve can be determined by the formula:
ΔPmax= z*(P1 − Ps) ; bar
where: z is the cavitation onset coefficient, published in technical catalogs for equipment selection. Each equipment manufacturer has its own, but the average value is usually in the range of 0.45-06.
P1 – pressure in front of the valve, bar
Рs – saturation pressure of water vapor at a given coolant temperature, bar,
Towhichdetermined by the table:
If the calculated pressure difference used to select the valve Kvs is no more
ΔPmax, cavitation will not occur.
Example:
Pressure before valve P1 = 5 bar;
Coolant temperature T1 = 140C;
Valve Z according to catalog = 0.5
According to the table, for a coolant temperature of 140C we determine Рs = 2.69
The maximum permissible pressure drop across the valve will be:
ΔPmax= 0.5*(5 - 2.69) = 1.155 bar
You cannot lose more than this difference on the valve - cavitation will begin.
But if the coolant temperature was lower, for example 115C, which is closer to the actual temperatures of the heating network, the maximum difference
pressure would be greater: ΔPmax= 0.5*(5 – 0.72) = 2.14 bar.
From here we can draw a quite obvious conclusion: the higher the temperature of the coolant, the lower the pressure drop possible across the control valve.
To determine the flow rate. Passing through the pipeline, it is enough to use the formula:
;m/s
G – coolant flow through the valve, m3/hour
d – nominal diameter selected valve, mm
It is necessary to take into account the fact that the flow velocity of the pipeline passing through the section should not exceed 1 m/sec.
The most preferable flow speed is in the range of 0.7 - 0.85 m/s.
The minimum speed should be 0.5 m/s.
Selection criterion DHW systems, as a rule, is determined from technical specifications for connection: the heat generating company very often prescribes
type of DHW system. If the type of system is not specified, a simple rule should be followed: determination by the ratio of building loads
for hot water supply and heating.
If 0.2
Respectively,
If QDHW/Qheating< 0.2 or QDHW/Qheating>1; necessary single-stage DHW system.
The very principle of operation of a two-stage hot water system is based on heat recovery from the return of the heating circuit: return coolant of the heating circuit
passes through the first stage of the hot water supply and heats up cold water from 5C to 41...48C. At the same time, the return coolant of the heating circuit itself cools down to 40C
and already cold it merges into the heating network.
The second stage of the hot water supply heats up the cold water from 41...48C after the first stage to the required 60...65C.
Advantages of a two-stage DHW system:
1) Due to heat recovery from the heating circuit return, cooled coolant enters the heating network, which sharply reduces the likelihood of overheating
return lines This point is extremely important for heat generating companies, in particular heating networks. Now it is becoming common to carry out calculations of heat exchangers of the first stage of hot water supply at a minimum temperature of 30C, so that even colder coolant is drained into the return of the heating network.
2) The two-stage hot water system allows for more precise control of the temperature of hot water, which is used for analysis by the consumer and temperature fluctuations
at the exit from the system is significantly less. This is achieved due to the fact that the control valve of the second stage of DHW, during its operation, regulates
only a small part of the load, and not the whole thing.
When distributing loads between the first and second stages of DHW, it is very convenient to do the following:
70% load – 1st DHW stage;
30% load – DHW stage 2;
What does it give?
1) Since the second (adjustable) stage is small, in the process of regulating the DHW temperature, temperature fluctuations at the outlet
systems turn out to be insignificant.
2) Thanks to this distribution of the DHW load, in the calculation process we obtain equality of costs and, as a consequence, equality of diameters in the heat exchanger piping.
The consumption for DHW circulation must be at least 30% of the consumption for DHW disassembly by the consumer. This is the minimum number. To increase reliability
system and stability of DHW temperature control, circulation flow can be increased to 40-45%. This is done not only to maintain
hot water temperature, when there is no analysis by the consumer. This is done to compensate for the “drawdown” of DHW at the time of peak DHW withdrawal, since the consumption
circulation will support the system while the heat exchanger volume is filled with cold water for heating.
There are cases of incorrect calculation of the DHW system, when instead of a two-stage system, a single-stage one is designed. After installing such a system,
During the commissioning process, the specialist is faced with extreme instability of the hot water supply system. Here it is even appropriate to talk about inoperability,
which is expressed by large temperature fluctuations at the outlet of the DHW system with an amplitude of 15-20C from the set setpoint. For example, when the setting
is 60C, then during the regulation process, temperature fluctuations occur in the range from 40 to 80C. In this case, changing the settings
an electronic regulator (PID - components, rod stroke time, etc.) will not give a result, since the DHW hydraulics are fundamentally incorrectly calculated.
There is only one way out: limit the consumption of cold water and maximize the circulation component of the hot water supply. In this case, at the mixing point
a smaller amount of cold water will be mixed with a larger amount of hot (circulation) and the system will work more stable.
Thus, some kind of imitation of a two-stage DHW system is performed due to the circulation of DHW.
The operating pressure in the heating system is the most important parameter on which the functioning of the entire network depends. Deviations in one direction or another from the values specified in the design not only reduce the efficiency of the heating circuit, but also significantly affect the operation of the equipment, and in special cases can even cause it to fail.
Of course, a certain pressure drop in the heating system is determined by the principle of its design, namely the difference in pressure in the supply and return pipelines. But if there are larger spikes, immediate action should be taken.
- Static pressure. This component depends on the height of the column of water or other coolant in the pipe or container. Static pressure exists even if the working medium is at rest.
- Dynamic pressure. It is a force that acts on the internal surfaces of the system when water or other medium moves.
The concept of maximum operating pressure is distinguished. This is the maximum permissible value, exceeding which can lead to the destruction of individual network elements.
What pressure in the system should be considered optimal?
Table of maximum pressure in the heating system.
When designing heating, the coolant pressure in the system is calculated based on the number of floors of the building, the total length of the pipelines and the number of radiators. As a rule, for private houses and cottages, the optimal values of medium pressure in the heating circuit are in the range from 1.5 to 2 atm.
For apartment buildings up to five floors high, connected to a central heating system, the pressure in the network is maintained at 2-4 atm. For nine- and ten-story buildings, a pressure of 5-7 atm is considered normal, and in taller buildings - 7-10 atm. The maximum pressure is recorded in the heating mains through which the coolant is transported from boiler houses to consumers. Here it reaches 12 atm.
For consumers located at different heights and at different distances from the boiler room, the pressure in the network must be adjusted. Pressure regulators are used to reduce it, and pumping stations are used to increase it. However, it should be taken into account that a faulty regulator can cause an increase in pressure in certain areas of the system. In some cases, when the temperature drops, these devices can completely shut off the shut-off valves on the supply pipeline coming from the boiler plant.
To avoid such situations, the regulator settings are adjusted so that complete shutoff of the valves is impossible.
Autonomous heating systems
Expansion tank in an autonomous heating system.
In the absence of centralized heating, houses are equipped with autonomous heating systems, in which the coolant is heated by an individual low-power boiler. If the system communicates with the atmosphere through an expansion tank and the coolant circulates in it due to natural convection, it is called open. If there is no communication with the atmosphere, and the working medium circulates thanks to the pump, the system is called closed. As already mentioned, for the normal functioning of such systems, the water pressure in them should be approximately 1.5-2 atm. This low figure is due to the relatively short length of pipelines, as well as a small number of instruments and fittings, which results in relatively low hydraulic resistance. In addition, due to the low height of such houses, the static pressure in the lower sections of the circuit rarely exceeds 0.5 atm.
At the stage of launching the autonomous system, it is filled with cold coolant, maintaining a minimum pressure in closed heating systems of 1.5 atm. There is no need to sound the alarm if, some time after filling, the pressure in the circuit drops. Pressure losses in this case are caused by the release of air from the water, which dissolved in it when the pipelines were filled. The circuit should be de-aired and completely filled with coolant, bringing its pressure to 1.5 atm.
After heating the coolant in the heating system, its pressure will increase slightly, reaching the calculated operating values.
Precautionary measures
A device for measuring pressure.
Since when designing autonomous heating systems, in order to save money, a small safety margin is included, even a low pressure surge of up to 3 atm can cause depressurization of individual elements or their connections. In order to smooth out pressure drops due to unstable pump operation or changes in coolant temperature, an expansion tank is installed in a closed heating system. Unlike a similar device in an open type system, it does not communicate with the atmosphere. One or more of its walls are made of elastic material, due to which the tank acts as a damper during pressure surges or water hammer.
The presence of an expansion tank does not always guarantee that pressure is maintained within optimal limits. In some cases it may exceed the maximum permissible values:
- if the expansion tank capacity is incorrectly selected;
- in case of malfunction of the circulation pump;
- when the coolant overheats, which is a consequence of malfunctions in the boiler automation;
- due to incomplete opening of shut-off valves after repairs or maintenance work;
- due to the appearance of an air lock (this phenomenon can provoke both an increase in pressure and a drop);
- when the throughput of the dirt filter decreases due to its excessive clogging.
Therefore, in order to avoid emergency situations when installing closed-type heating systems, it is mandatory to install a safety valve that will release excess coolant if the permissible pressure is exceeded.
What to do if the pressure in the heating system drops
Pressure in the expansion tank.
When operating autonomous heating systems, the most common emergency situations are those in which the pressure gradually or sharply decreases. They can be caused by two reasons:
- depressurization of system elements or their connections;
- problems with the boiler.
In the first case, the location of the leak should be located and its tightness restored. You can do this in two ways:
- Visual inspection. This method is used in cases where the heating circuit is laid in an open manner (not to be confused with an open-type system), that is, all its pipelines, fittings and devices are visible. First of all, carefully inspect the floor under the pipes and radiators, trying to detect puddles of water or traces of them. In addition, the location of the leak can be identified by traces of corrosion: characteristic rusty streaks form on radiators or at the joints of system elements when the seal is broken.
- Using special equipment. If a visual inspection of the radiators does not yield anything, and the pipes are laid in a hidden way and cannot be inspected, you should seek the help of specialists. They have special equipment that will help detect leaks and fix them if the home owner is unable to do this themselves. Localizing the depressurization point is quite simple: water is drained from the heating circuit (for such cases, a drain valve is installed at the lowest point of the circuit during the installation stage), then air is pumped into it using a compressor. The location of the leak is determined by the characteristic sound that leaking air makes. Before starting the compressor, the boiler and radiators should be insulated using shut-off valves.
If the problem area is one of the joints, it is additionally sealed with tow or FUM tape and then tightened. The burst pipeline is cut out and a new one is welded in its place. Units that cannot be repaired are simply replaced.
If the tightness of pipelines and other elements is beyond doubt, and the pressure in a closed heating system still drops, you should look for the reasons for this phenomenon in the boiler. You should not carry out diagnostics yourself; this is a job for a specialist with the appropriate education. Most often the following defects are found in the boiler:
Installation of a heating system with a pressure gauge.
- the appearance of microcracks in the heat exchanger due to water hammer;
- manufacturing defects;
- failure of the make-up valve.
A very common reason why the pressure in the system drops is the incorrect selection of the expansion tank capacity.
Although the previous section stated that this may cause increased pressure, there is no contradiction here. When the pressure in the heating system increases, the safety valve is activated. In this case, the coolant is discharged and its volume in the circuit decreases. As a result, the pressure will decrease over time.
Pressure control
For visual monitoring of pressure in the heating network, dial pressure gauges with a Bredan tube are most often used. Unlike digital instruments, such pressure gauges do not require electrical power. Automated systems use electrical contact sensors. A three-way valve must be installed at the outlet to the control and measuring device. It allows you to isolate the pressure gauge from the network during maintenance or repair, and is also used to remove an air lock or reset the device to zero.
Instructions and rules governing the operation of heating systems, both autonomous and centralized, recommend installing pressure gauges at the following points:
- Before the boiler installation (or boiler) and at the exit from it. At this point the pressure in the boiler is determined.
- Before and after the circulation pump.
- At the entrance of the heating main into a building or structure.
- Before and after the pressure regulator.
- At the inlet and outlet of the coarse filter (sludge filter) to control its level of contamination.
All control and measuring instruments must undergo regular verification to confirm the accuracy of the measurements they perform.
