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.
Read also:
|
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;
Ensuring excess pressure 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 the maximum water temperature, 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. The minimum pressure in the supply pipelines is taken according to the calculated 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 mode is developed on the basis of hydraulic calculation data for heating networks and is ensured by the simultaneous operation of 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, throttling devices for local heat consumer systems; 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 the network pumps, the pressure losses in the return pipeline DН return of the main heating network (line A-B) are successively plotted 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;
f) starting from the last pipeline node, pressure losses are deposited in the supply pipeline of the main line DH under (line C-D). The pressure at all points of the supply pipeline, based on the condition of its mechanical strength, should not exceed 160 m;
g) pressure losses in the heat source DН it are postponed (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, then 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 that at the highest points of the systems of the tallest buildings located in the area of the highest geodetic marks, an 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. Figure 7.2 shows a piezometric graph and a schematic diagram of the heat supply system for an area that has 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 of total static heads - the lower one with a level of 50 m (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 increased 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. The pressure drop in the supply line is prevented by the non-return valve (11) installed on it, 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. The minimum piezometric pressure 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.
Another possible solution to the 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 available pressure drop to create water circulation, Pa, is determined by the formula
where DPn is the pressure created by the circulation pump or elevator, Pa;
ДПе - natural circulation pressure in the calculation ring due to cooling of water in pipes and heating devices, Pa;
In pumping systems, it is allowed not to take into account DP if it is less than 10% of DP.
Available pressure drop at the entrance to the building DPr = 150 kPa.
Calculation of natural circulation pressure
The natural circulation pressure that arises in the design ring of a vertical single-pipe system with bottom distribution, adjustable with closing sections, Pa, is determined by the formula
where is the average increase in water density when its temperature decreases by 1? C, kg/(m3?? C);
Vertical distance from heating center to cooling center
heating device, m;
Water flow in the riser, kg/h, is determined by the formula
Calculation of pump circulation pressure
The value, Pa, is selected in accordance with the available pressure difference at the inlet and the mixing coefficient U according to the nomogram.
Available pressure difference at the inlet =150 kPa;
Coolant parameters:
In the heating network f1=150?C; f2=70?C;
In the heating system t1=95?C; t2=70?C;
We determine the mixing coefficient using the formula
µ= f1 - t1 / t1 - t2 =150-95/95-70=2.2; (2.4)
Hydraulic calculation of water heating systems using the method of specific pressure loss due to friction
Calculation of the main circulation ring
1) Hydraulic calculation of the main circulation ring is carried out through riser 15 of a vertical single-pipe water heating system with bottom wiring and dead-end movement of the coolant.
2) We divide the main central circulation system into calculation sections.
3) To pre-select the diameter of the pipes, an auxiliary value is determined - the average value of the specific pressure loss from friction, Pa, per 1 meter of pipe according to the formula
where is the available pressure in the adopted heating system, Pa;
Total length of the main circulation ring, m;
Correction factor taking into account the share of local pressure losses in the system;
For a heating system with pump circulation, the share of loss due to local resistance is b=0.35, and due to friction b=0.65.
4) Determine the coolant flow rate in each section, kg/h, using the formula
Parameters of the coolant in the supply and return pipelines of the heating system, ?C;
Specific mass heat capacity of water equal to 4.187 kJ/(kg??С);
Coefficient for taking into account additional heat flow when rounding above the calculated value;
Coefficient of accounting for additional heat losses by heating devices near external fences;
6) We determine the coefficients of local resistance in the design areas (and write their sum in Table 1) by .
Table 1
|
1 plot Gate valve d=25 1 piece Bend 90° d=25 1 piece |
|
|
2nd section Tee for passage d=25 1 piece |
|
|
Section 3 Tee for passage d=25 1 piece Bend 90° d=25 4pcs |
|
|
Section 4 Tee for passage d=20 1 piece |
|
|
5th section Tee for passage d=20 1 piece Bend 90° d=20 1 piece |
|
|
6th section Tee for passage d=20 1 piece Bend 90° d=20 4pcs |
|
|
Section 7 Tee for passage d=15 1 piece Bend 90° d=15 4pcs |
|
|
8th section Tee for passage d=15 1 piece |
|
|
Section 9 Tee for passage d=10 1 piece Bend 90° d=10 1 piece |
|
|
10th section Tee for passage d=10 4pcs Bend 90° d=10 11pcs Crane KTR d=10 3 pcs Radiator RSV 3 pcs |
|
|
11th section Tee for passage d=10 1 piece Bend 90° d=10 1 piece |
|
|
Section 12 Tee for passage d=15 1 piece |
|
|
Section 13 Tee for passage d=15 1 piece Bend 90° d=15 4pcs |
|
|
Section 14 Tee for passage d=20 1 piece Bend 90° d=20 4pcs |
|
|
15th section Tee for passage d=20 1 piece Bend 90° d=20 1 piece |
|
|
16th section Tee for passage d=20 1 piece |
|
|
17th section Tee for passage d=25 1 piece Bend 90° d=25 4pcs |
|
|
Section 18 Tee for passage d=25 1 piece |
|
|
19th section Gate valve d=25 1 piece Bend 90° d=25 1 piece |
7) At each section of the main circulation ring, we determine the pressure loss due to local resistance Z, depending on the sum of the local resistance coefficients Uo and the water speed in the section.
8) We check the reserve of available pressure drop in the main circulation ring according to the formula
where is the total pressure loss in the main circulation ring, Pa;
With a dead-end coolant flow pattern, the discrepancy between pressure losses in the circulation rings should not exceed 15%.
We summarize the hydraulic calculation of the main circulation ring in Table 1 (Appendix A). As a result, we obtain the pressure loss discrepancy
Calculation of a small circulation ring
We perform a hydraulic calculation of the secondary circulation ring through riser 8 of a single-pipe water heating system
1) We calculate the natural circulation pressure due to the cooling of water in the heating devices of riser 8 using formula (2.2)
2) Determine the water flow in riser 8 using formula (2.3)
3) We determine the available pressure drop for the circulation ring through the secondary riser, which should be equal to the known pressure losses in the main circulation circuit sections, adjusted for the difference in natural circulation pressure in the secondary and main rings:
15128.7+(802-1068)=14862.7 Pa
4) Find the average value of linear pressure loss using formula (2.5)
5) Based on the value, Pa/m, of the coolant flow rate in the area, kg/h, and based on the maximum permissible speeds of coolant movement, we determine the preliminary diameter of the pipes dу, mm; actual specific pressure loss R, Pa/m; actual coolant speed V, m/s, according to .
6) We determine the coefficients of local resistance in the design areas (and write their sum in Table 2) by .
7) In the section of the small circulation ring, we determine the pressure loss due to local resistance Z, depending on the sum of the local resistance coefficients Uo and the water speed in the section.
8) We summarize the hydraulic calculation of the small circulation ring in Table 2 (Appendix B). We check the hydraulic connection between the main and small hydraulic rings according to the formula
9) Determine the required pressure loss in the throttle washer using the formula
10) Determine the diameter of the throttle washer using the formula
At the site it is required to install a throttle washer with an internal passage diameter of DN=5mm
General principles of hydraulic calculation of pipelines for water heating systems are described in detail in the section Water heating systems. They are also applicable for calculating heat pipelines of heating networks, but taking into account some of their features. Thus, in the calculations of heat pipelines, the turbulent movement of water is taken (water speed is more than 0.5 m/s, steam - more than 20-30 m/s, i.e. quadratic calculation area), values of the equivalent roughness of the inner surface of large-diameter steel pipes, mm, accepted for: steam pipelines - k = 0.2; water network - k = 0.5; condensate pipelines - k = 0.5-1.0.
The estimated coolant costs for individual sections of the heating network are determined as the sum of the costs of individual subscribers, taking into account the connection diagram of the DHW heaters. In addition, it is necessary to know the optimal specific pressure drops in pipelines, which are previously determined by technical and economic calculations. They are usually taken equal to 0.3-0.6 kPa (3-6 kgf/m2) for main heating networks and up to 2 kPa (20 kgf/m2) for branches.
When performing hydraulic calculations, the following tasks are solved: 1) determining the diameters of pipelines; 2) determination of pressure-pressure drop; 3) determination of current pressures at various points in the network; 4) determination of permissible pressures in pipelines under various operating modes and conditions of the heating network.
When carrying out hydraulic calculations, diagrams and a geodetic profile of the heating main are used, indicating the location of heat supply sources, heat consumers and design loads. To speed up and simplify calculations, instead of tables, logarithmic nomograms of hydraulic calculations are used (Fig. 1), and in recent years, computer calculation and graphic programs are used.
Picture 1.
PIEZOMETRIC GRAPH
When designing and in operational practice, piezometric graphs are widely used to take into account the mutual influence of the geodetic profile of the area, the height of subscriber systems, and operating pressures in the heating network. From them it is easy to determine the pressure (pressure) and available pressure at any point in the network and in the subscriber system for the dynamic and static state of the system. Let's consider the construction of a piezometric graph, and we will assume that pressure and pressure, pressure drop and pressure loss are related by the following dependencies: H = p/γ, m (Pa/m); ∆Н = ∆р/ γ, m (Pa/m); and h = R/ γ (Pa), where Н and ∆Н - pressure and pressure loss, m (Pa/m); р and ∆р - pressure and pressure drop, kgf/m 2 (Pa); γ - mass density of the coolant, kg/m3; h and R - specific pressure loss (dimensionless value) and specific pressure drop, kgf/m 2 (Pa/m).
When constructing a piezometric graph in dynamic mode, the axis of the network pumps is taken as the origin of coordinates; taking this point as a conditional zero, they build a terrain profile along the route of the main highway and along characteristic branches (the elevations of which differ from the elevations of the main highway). The heights of the connected buildings are drawn on the profile on a scale, then, having previously assumed a pressure on the suction side of the network pumps collector H sun = 10-15 m, the horizontal line A 2 B 4 is drawn (Fig. 2, a). From point A 2, the lengths of the calculated sections of heat pipelines are plotted along the abscissa axis (with a cumulative total), and along the ordinate axis from the end points of the calculated sections - the pressure loss Σ∆H in these sections. By connecting the upper points of these segments, we obtain a broken line A 2 B 2, which will be the piezometric line of the return line. Each vertical segment from the conventional level A 2 B 4 to the piezometric line A 2 B 2 indicates the pressure loss in the return line from the corresponding point to the circulation pump at the thermal power plant. From point B 2 on a scale, the required available pressure for the subscriber at the end of the line ∆H ab is plotted upward, which is taken to be 15-20 m or more. The resulting segment B 1 B 2 characterizes the pressure at the end of the supply line. From point B 1, the pressure loss in the supply pipeline ∆Н p is postponed upward and a horizontal line B 3 A 1 is drawn.
Figure 2.a - construction of a piezometric graph; b - piezometric graph of a two-pipe heating network
From line A 1 B 3 downward, pressure losses are deposited in the section of the supply line from the heat source to the end of the individual calculated sections, and the piezometric line A 1 B 1 of the supply line is constructed similarly to the previous one.
With closed PZT systems and equal pipe diameters of the supply and return lines, the piezometric line A 1 B 1 is a mirror image of line A 2 B 2. From point A, the pressure loss in the boiler room of the thermal power plant or in the boiler room circuit ∆Н b (10-20 m) is postponed upward. The pressure in the supply manifold will be N n, in the return manifold - N sun, and the pressure of the network pumps will be N s.n.
It is important to note that when connecting local systems directly, the return pipeline of the heating network is hydraulically connected to the local system, and the pressure in the return pipeline is entirely transferred to the local system and vice versa.
During the initial construction of the piezometric graph, the pressure at the suction manifold of the network pumps N vs was taken arbitrarily. Moving the piezometric graph parallel to itself up or down allows you to accept any pressure on the suction side of network pumps and, accordingly, in local systems.
When choosing the position of the piezometric graph, it is necessary to proceed from the following conditions:
1. The pressure (pressure) at any point in the return line should not be higher than the permissible operating pressure in local systems, for new heating systems (with convectors) the operating pressure is 0.1 MPa (10 m of water column), for systems with cast iron radiators 0.5-0.6 MPa (50-60 m water column).
2. The pressure in the return pipeline must ensure that the upper lines and devices of local heating systems are filled with water.
3. The pressure in the return line, in order to avoid the formation of a vacuum, should not be lower than 0.05-0.1 MPa (5-10 m of water column).
4. The pressure on the suction side of the network pump should not be lower than 0.05 MPa (5 m water column).
5. The pressure at any point in the supply pipeline must be higher than the boiling pressure at the maximum (design) temperature of the coolant.
6. The available pressure at the end point of the network must be equal to or greater than the calculated pressure loss at the subscriber input for the calculated coolant flow.
7. In summer, the pressure in the supply and return lines takes on more than the static pressure in the DHW system.
Static state of the central heating system. When the network pumps stop and water circulation in the central heating system stops, it goes from a dynamic state to a static one. In this case, the pressures in the supply and return lines of the heating network will be equalized, the piezometric lines will merge into one - the static pressure line, and on the graph it will take an intermediate position, determined by the pressure of the make-up device of the MDH source.
The pressure of the make-up device is set by the station personnel either by the highest point of the pipeline of the local system directly connected to the heating network, or by the vapor pressure of superheated water at the highest point of the pipeline. So, for example, at the design temperature of the coolant T 1 = 150 °C, the pressure at the highest point of the pipeline with superheated water will be equal to 0.38 MPa (38 m of water column), and at T 1 = 130 °C - 0.18 MPa (18 m water column).
However, in all cases, the static pressure in low-lying subscriber systems should not exceed the permissible operating pressure of 0.5-0.6 MPa (5-6 atm). If it is exceeded, these systems should be transferred to an independent connection scheme. Reducing the static pressure in heating networks can be achieved by automatically disconnecting high buildings from the network.
In emergency cases, in the event of a complete loss of power supply to the station (stopping the network and make-up pumps), circulation and make-up will stop, while the pressures in both lines of the heating network will be equalized along the line of static pressure, which will begin to slowly, gradually decrease due to the leakage of network water through leaks and cooling it in pipelines. In this case, boiling of superheated water in pipelines is possible with the formation of vapor locks. Resuming water circulation in such cases can lead to severe water hammer in the pipelines with possible damage to fittings, heating devices, etc. To avoid this phenomenon, water circulation in the central heating system should begin only after the pressure in the pipelines has been restored by replenishing the heating network at a level not lower than the static one.
To ensure reliable operation of heating networks and local systems, it is necessary to limit possible pressure fluctuations in the heating network to acceptable limits. To maintain the required level of pressure in the heating network and local systems, at one point of the heating network (and in difficult terrain conditions - at several points), a constant pressure is artificially maintained under all operating modes of the network and during static conditions using a make-up device.
The points at which the pressure is maintained constant are called the neutral points of the system. As a rule, pressure is secured on the return line. In this case, the neutral point is located at the intersection of the reverse piezometer with the static pressure line (point NT in Fig. 2, b), maintaining constant pressure at the neutral point and replenishing coolant leakage is carried out by make-up pumps of the thermal power plant or RTS, KTS through an automated make-up device. Automatic regulators are installed on the make-up line, operating on the principle of “after” and “before” regulators (Fig. 3).
Figure 3. 1 - network pump; 2 - make-up pump; 3 - heating water; 4 - make-up regulator valve
The pressures of the network pumps N s.n are taken equal to the sum of the hydraulic pressure losses (at the maximum - design water flow): in the supply and return pipelines of the heating network, in the subscriber's system (including inputs to the building), in the boiler installation of the thermal power plant, its peak boilers or in boiler room Heat sources must have at least two network and two make-up pumps, of which one is a reserve pump.
The amount of recharge for closed heat supply systems is assumed to be 0.25% of the volume of water in the pipelines of heating networks and in subscriber systems connected to the heating network, h.
In schemes with direct water withdrawal, the amount of recharge is taken to be equal to the sum of the calculated water consumption for hot water supply and the amount of leakage in the amount of 0.25% of the system capacity. The capacity of heating systems is determined by the actual diameters and lengths of pipelines or by aggregated standards, m 3 / MW:
The disunity that has developed on the basis of ownership in the organization of operation and management of urban heat supply systems has the most negative impact on both the technical level of their functioning and their economic efficiency. It was noted above that the operation of each specific heat supply system is carried out by several organizations (sometimes “subsidiaries” of the main one). However, the specificity of district heating systems, primarily heating networks, is determined by the tight connection of the technological processes of their functioning, and uniform hydraulic and thermal regimes. The hydraulic mode of the heat supply system, which is the determining factor in the functioning of the system, is extremely unstable by its nature, which makes heat supply systems difficult to control compared to other urban engineering systems (electricity, gas, water supply).
None of the links in the district heating systems (heat source, main and distribution networks, heating points) can independently provide the required technological modes of operation of the system as a whole, and, consequently, the end result - reliable and high-quality heat supply to consumers. Ideal in this sense is an organizational structure in which heat supply sources and heating networks are under the jurisdiction of one enterprise structure.
Incorrect data. If the amount of pressure shortage goes beyond the actual values for a given network, then there is an error when entering the initial data or an error when plotting the network diagram on the map. You should check whether the following data has been entered correctly:
Hydraulic network mode.
If there are no errors when entering the initial data, but a lack of pressure exists and is of real significance for a given network, then in this situation the determination of the cause of the shortage and the method for eliminating it is carried out by the specialist working with this heating network.
Warning There is not enough pressure at the source Delta=X m. Where Delta is the required pressure.
WORST CONSUMER: ID=XX.
Figure 283. Message about the worst consumer
This message is displayed when there is a lack of available pressure at the consumer, where DeltaH− the value of the pressure that is not enough, m, a ID (XX)− individual number of the consumer for whom the pressure shortage is maximum.
Figure 284. Message about insufficient pressure
Double-click the left mouse button on the message about the worst consumer: the corresponding consumer will blink on the screen.
This error can be caused by several reasons:
ID=ХХ "Name of consumer" Emptying the heating system (H, m)
This message is displayed when there is insufficient pressure in the return pipeline to prevent emptying of the heating system of the upper floors of the building; the total pressure in the return pipeline must be at least the sum of the geodetic mark, the height of the building plus 5 meters to fill the system. The head reserve for filling the system can be changed in the calculation settings ().
XX− individual number of the consumer whose heating system is being emptied, N- pressure, in meters of which is not enough;
ID=ХХ "Name of consumer" Pressure in the return pipeline is higher than the geodetic mark by N, m
This message is issued when the pressure in the return pipeline is higher than permissible according to the strength conditions of cast iron radiators (more than 60 m. water column), where XX- individual consumer number and N- pressure value in the return pipeline exceeding the geodetic mark.
The maximum pressure in the return pipeline can be set independently in calculation settings. ;
ID=XX "Name of consumer" Elevator nozzle cannot be selected. Set the maximum
This message may appear when there is a large heating load or when an incorrect connection diagram is selected that does not correspond to the design parameters. XX- individual number of the consumer for whom the elevator nozzle cannot be selected;
ID=XX "Name of consumer" Elevator nozzle cannot be selected. Set the minimum
This message may appear when there are very small heating loads or when an incorrect connection diagram is selected that does not correspond to the design parameters. XX− individual number of the consumer for whom the elevator nozzle cannot be selected.
Warning Z618: ID=XX "XX" The number of washers on the supply pipe to CO is more than 3 (YY)
This message means that, as a result of the calculation, the number of washers required to adjust the system is more than 3 pieces.
Since the default minimum diameter of the washer is 3 mm (indicated in the calculation settings “Setting up the calculation of pressure losses”), and the consumption of the consumer’s heating system ID=XX is very small, the calculation results in determining the total number of washers and the diameter of the last washer (in consumer database).
That is, a message like: The number of washers on the supply pipeline for CO is more than 3 (17) warns that to set up this consumer, you should install 16 washers with a diameter of 3 mm and 1 washer, the diameter of which is determined in the consumer database.
Warning Z642: ID=XX The elevator at the central heating station is not working
This message is displayed as a result of a verification calculation and means that the elevator unit is not functioning.
