Available pressure. Operation of thermal networks. Hydraulic calculation of water heating systems by the method of specific friction pressure losses

Q[KW] = Q[Gcal]*1160; Load conversion from Gcal to KW

G[m3/h] = Q[KW]*0.86/ ΔT; where ∆T- temperature difference between supply and return.

Example:

Supply temperature from heating networks T1 - 110˚ FROM

Supply temperature from heating networks T2 - 70˚ FROM

Heating circuit consumption G = (0.45 * 1160) * 0.86 / (110-70) = 11.22 m3 / h

But for a heated circuit with a temperature graph of 95/70, the flow rate will be completely different: \u003d (0.45 * 1160) * 0.86 / (95-70) \u003d 17.95 m3 / hour.

From this we can conclude: the lower the temperature difference (the temperature difference between the supply and return), the greater the required coolant flow.

Selection of circulation pumps.

When selecting circulation pumps for heating, hot water, ventilation systems, it is necessary to know the characteristics of the system: coolant flow rate,

which must be provided and the hydraulic resistance of the system.

Coolant consumption:

G[m3/h] = Q[KW]*0.86/ ΔT; where ∆T- temperature difference between supply and return;

hydraulic the resistance of the system must 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/h] =520*0.86/ 25 = 17.89 m3/h~ 18 m3/hour;

The resistance of the heating system wasξ = 5 meters ;

In the case of an independent heating system, it must be understood that the resistance of the heat exchanger will be added to this resistance of 5 meters. To do this, you need to look at his calculation. For example, let this value be 3 meters. So, the total resistance of the system is obtained: 5 + 3 \u003d 8 meters.

Now you can choose circulation pump with flow rate 18m3/h 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 provide a working pointflow / head at the first speed of its work. If, for any reason, this pressure is not enough, the pump can be “dispersed” up to 13 meters at the third speed. The best option is considered to be a pump option that maintains its operating point at the second speed.

It is also quite possible to put a pump with a built-in frequency converter instead of an ordinary pump with three or one speed, for example:

This version of the pump is, of course, the most preferable, since it allows the most flexible setting 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 without fail (main / backup), and for the circulation of the DHW line it is quite possible to supply one.

Drinking system. Selection of the feed system pump.

It is obvious that the boost pump is necessary only in the case of 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 a constant pressure in the secondary circuit in case of possible leaks.

in the heating system, as well as to fill the system itself. The recharge system itself consists of a pressure switch, a solenoid valve, and an expansion tank.

The make-up pump is installed only when the pressure of the coolant in the return is not enough to fill the system (the piezometer does not allow).

Example:

The pressure of the return heat carrier from the heating networks Р2 = 3 atm.

The height of the building, taking into account those. Underground = 40 meters.

3 atm. = 30 meters;

Required height = 40 meters + 5 meters (per spout) = 45 meters;

Pressure deficit = 45 meters - 30 meters = 15 meters = 1.5 atm.

The pressure of the feed pump is understandable, it should be 1.5 atmospheres.

How to determine the expense? The flow rate of the pump is assumed to be 20% of the volume of the heating system.

The principle of operation of the feeding system is as follows.

The pressure switch (pressure measuring device with relay output) measures the pressure of the return heat carrier in the heating system and has

presetting. For this particular example, this setting should be approximately 4.2 atmospheres with a hysteresis of 0.3.

When the pressure in the return of the heating system 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).

The make-up coolant is supplied until the pressure rises to a value of 4.2 atm + 0.3 = 4.5 atmospheres.

Calculation of the control valve for cavitation.

When distributing the available pressure between the elements of the heating point, it is necessary to take into account the possibility of cavitation processes inside the body

valves, which over time will destroy it.

The maximum allowable differential pressure across the valve can be determined from the formula:

∆Pmax= z*(P1 − Ps) ; bar

where: z is the coefficient of cavitation initiation, published in technical catalogs for the selection of equipment. 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 estimated differential pressure used to select the Kvs valve is not more than

∆Pmax, cavitation will not occur.

Example:

Pressure before valve P1 = 5 bar;

Coolant temperature Т1 = 140С;

Z valve catalog = 0.5

According to the table, for a coolant temperature of 140C, we determine Рs = 2.69

The maximum allowable differential pressure across the valve is:

∆Pmax= 0.5 * (5 - 2.69) = 1.155 bar

It is impossible to lose more than this difference on the valve - cavitation will begin.

But if the coolant temperature were lower, for example, 115C, which is closer to the real temperatures of the heating network, the maximum difference

pressure would be greater:ΔPmax\u003d 0.5 * (5 - 0.72) \u003d 2.14 bar.

From this we can draw a quite obvious conclusion: the higher the temperature of the coolant, the lower the pressure drop is 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/h

d – conditional diameter of the selected valve, mm

It is necessary to take into account the fact that the speed of the flow passing through the pipeline section should not exceed 1 m/s.

The most preferred flow velocity is in the range of 0.7 - 0.85 m/s.

The minimum speed should be 0.5 m/s.

The criterion for selecting a DHW system is usually determined from the technical specifications for the connection: the heat generation company very often prescribes

type of DHW system. In case the type of system is not prescribed, a simple rule should be followed: determination by the ratio of building loads

for hot water and heating.

If a 0.2 - necessary two-stage DHW system;

Respectively,

If a QDHW/Qheating< 0.2 or QDHW/Qheating>1; needed single stage hot water system.

The very principle of operation of a two-stage DHW system is based on heat recovery from the return of the heating circuit: the return heat carrier of the heating circuit

passes through the first stage of hot water supply and heats up cold water from 5C to 41...48C. At the same time, the return coolant of the heating circuit cools down to 40C

and already cold merges into the heating network.

The second stage of the hot water supply warms up cold water from 41 ... 48C after the first stage to the prescribed 60 ... 65C.

Advantages of a two-stage DHW system:

1) Due to the heat recovery of the heating circuit return, a cooled coolant enters the heating network, which dramatically 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 30 ° C, so that an even colder coolant merges into the return of the heating network.

2) The two-stage DHW system more accurately controls the temperature of hot water, which goes to the consumer for analysis and temperature fluctuations

at the exit from the system is much less. This is achieved due to the fact that the control valve of the second stage of domestic hot water, in the course of its operation, regulates

only a small part of the load, not the whole.

When distributing loads between the first and second stages of hot water supply, it is very convenient to proceed as follows:

70% load - 1 stage DHW;

30% load - 2nd stage DHW;

What does it give.

1) Since the second (adjustable) stage turns out to be small, then in the process of regulating the DHW temperature, temperature fluctuations at the outlet of

systems are small.

2) Due to this distribution of the DHW load, in the process of calculation we get the equality of costs and, as a result, the equality of diameters in the piping of the heat exchangers.

The consumption for DHW circulation must be at least 30% of the consumption of DHW analysis by the consumer. This is the minimum number. To increase reliability

system and stability of DHW temperature control, the flow rate for circulation can be increased to a value of 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 the DHW at the time of the peak analysis of the DHW, since the consumption

circulation will support the system at the moment the volume of the heat exchanger 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,

in the process of commissioning, the specialist is faced with extreme instability of the DHW system. It is appropriate here to even 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 setpoint. For example, when the setting

is 60C, then in the process of regulation, temperature fluctuations occur in the range from 40 to 80C. In this case, changing the settings

electronic controller (PID - components, stroke time, etc.) will not give a result, since the DHW hydraulics are fundamentally incorrectly calculated.

There is only one way out: to limit the flow of cold water and maximize the circulation component of hot water. In this case, at the mixing point

less cold water will mix with more hot (circulating) water 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.

Based on the results of the calculation of water supply networks for various modes of water consumption, the parameters of the water tower and pumping units are determined, ensuring the operability of the system, as well as free pressures in all network nodes.

To determine the pressure at the supply points (at the water tower, at the pumping station), it is necessary to know the required pressure of water consumers. As mentioned above, the minimum free pressure in the water supply network of a settlement with a maximum domestic and drinking water intake at the entrance to the building above the ground in a one-story building should be at least 10 m (0.1 MPa), with a larger number of storeys, 4 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 places, local pumping installations are provided. The free pressure at the standpipes must be at least 10 m (0.1 MPa),

In the external network of industrial water pipelines, free pressure is taken according to the technical characteristics of the equipment. The free pressure in the consumer's drinking water supply network should not exceed 60 m, otherwise, for certain areas or buildings, it is necessary to install pressure regulators or zoning the water supply system. During the operation of the water supply system at all points of the network, a free pressure of at least the normative one must be ensured.

Free heads at any point in the network are defined as the difference between the elevations of the piezometric lines and the ground surface. Piezometric marks for all design cases (during household 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 the minimum free pressure.

Typically, the dictate 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 head losses from the power source to the dictate point will be the largest). At the dictating point, they are set by a pressure equal to the standard one. If at any point in the network the pressure is less than the normative one, then the position of the dictating point is set incorrectly. In this case, they find the point that has the smallest free pressure, take it as the dictator, and repeat the calculation of the pressures 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 and most distant points of the territory served by the water supply from the power sources. According to the method of extinguishing a fire, water pipes are of high and low pressure.

As a rule, when designing water supply systems, low-pressure fire-fighting water supply should be taken, with the exception of small settlements (less than 5 thousand people). The installation of a high-pressure fire-fighting water supply system must be economically justified,

In low-pressure water pipes, the pressure increase is carried out only for the duration of the fire extinguishing. The necessary increase in pressure is created by mobile fire pumps, which are brought to the fire site and take water from the water supply network through street hydrants.

According to SNiP, the pressure at any point of the low-pressure fire water pipeline network at the ground level during fire fighting must be at least 10 m. network through leaky joints of soil water.

In addition, a certain supply of pressure in the network is required for the operation of fire pumps in order to overcome significant resistance in the suction lines.

The high-pressure fire extinguishing system (usually adopted at industrial facilities) provides for the supply of water at the fire rate established by the norms of fire and increasing the pressure in the water supply network to a value sufficient to create fire jets directly from hydrants. Free pressure in this case should provide a compact jet height of at least 10 m at full fire water flow and the location of the hose barrel at the level of the highest point of the tallest building and water supply through fire hoses 120 m long:

Nsv pzh \u003d N zd + 10 + ∑h ≈ N zd + 28 (m)

where N zd is the height of the building, m; h - pressure loss in the hose and barrel of the hose, m.

In the high-pressure water supply system, stationary fire pumps are equipped with automatic equipment that ensures that the pumps are started no later than 5 minutes after the fire signal is given. The pipes of the network must be selected taking into account the increase in pressure in the event of a fire. The maximum free pressure in the network of the integrated water supply should not exceed 60 m of the water column (0.6 MPa), and in the hour of a fire - 90 m (0.9 MPa).

With significant differences in the geodetic marks of the object supplied with water, a large length of water supply networks, as well as with a large difference in the values ​​\u200b\u200bof the free pressure required by individual consumers (for example, in microdistricts with different building heights), 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 on the basis of the following conditions: at the highest point of the network, the necessary free pressure must be provided, and at its lower (or initial) point, the pressure must not exceed 60 m (0.6 MPa).

According to the types of zoning, water pipelines come with parallel and sequential zoning. Parallel zoning of the water supply system is used for large ranges of geodetic marks within the city area. For this, lower (I) and upper (II) zones are formed, which are provided with water, respectively, by pumping stations of zones I and II with water supply at different pressures through separate conduits. 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 II (upper) zone; 3 - pumps of the I (lower) zone; 4 - pressure-regulating tanks

In water heat supply systems, consumers are provided with heat by appropriately distributing the estimated flow rates of network water between them. To implement such a distribution, it is necessary to develop the hydraulic regime of the heat supply system.

The purpose of developing the hydraulic regime of the heat supply system is to ensure optimally permissible pressures in all elements of the heat supply system and the necessary available pressures at the nodal points of the heating network, in group and local heating points, sufficient to supply consumers with estimated water consumption. Available pressure is the difference in water pressure in the supply and return pipelines.

For the reliability of the heat supply system, the following conditions are imposed:

Do not exceed the 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 units: 1.0 MPa - for sectional hot water heaters; 0.8-1.0 MPa - for steel convectors; 0.6 MPa - for cast iron radiators; 0.8 MPa - for heaters;

Providing excess pressure in all elements of the heat supply system to prevent cavitation of pumps and protect the heat supply system from air leakage. 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 at least 5 m of water above the point of the tallest building. Art.;

At all points in the heating system, pressure must be maintained in excess of the saturated water vapor pressure 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 design temperature of the network 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 head at the maximum coolant temperature.

Graphically, the hydraulic regime is conveniently depicted in the form of a piezometric graph. The piezometric graph is built for two hydraulic regimes: hydrostatic and hydrodynamic.

The purpose of developing a hydrostatic regime is to provide the necessary water pressure in the heat supply 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 heat supply system from air leakage. The hydrostatic mode is developed with the make-up pumps running and no circulation.

The hydrodynamic regime is developed on the basis of data from the hydraulic calculation of heat networks and is ensured by the simultaneous operation of make-up and network pumps.

The development of the hydraulic regime is reduced to the construction of a piezometric graph that meets all the requirements for the hydraulic regime. 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 pressure in the supply and return pipelines; available pressure at any point of the heating network, taking into account the terrain; according to the available pressure and height of buildings, choose consumer connection schemes; select automatic regulators, elevator nozzles, throttle devices for local systems of heat consumers; select mains and make-up pumps.

Building a piezometric graph(Fig. 7.1) is performed as follows:

a) scales are selected along the abscissa and ordinate axes and the terrain and the height of the building of the quarters are plotted. Piezometric graphs are built for main and distribution heating networks. For main heat networks, the scales can be taken: horizontal M g 1: 10000; vertical M at 1:1000; for distribution heating networks: M g 1:1000, M in 1:500; The zero mark of the y-axis (pressure axes) is usually taken as the mark of the lowest point of the heating main or the mark of network pumps.

b) the value of the static head is determined, which ensures the filling of consumer systems and the creation of a minimum excess head. This is the height of the highest building plus 3-5 meters of water.

After applying the terrain and the height of buildings, the static head of the system is determined

H c t \u003d [H zd + (3¸5)], m (7.1)

where N zd is the height of the tallest building, m.

The static head H st is drawn parallel to the abscissa axis, and it should not exceed the maximum operating head for local systems. The value of the maximum working pressure is: for heating systems with steel heaters and for heaters - 80 meters; for heating systems with cast-iron radiators - 60 meters; for independent connection schemes with surface heat exchangers - 100 meters;

c) Then a dynamic regime is built. The suction head of the network pumps Ns is arbitrarily chosen, which should not exceed the static head and provides the necessary head pressure at the inlet to prevent cavitation. The cavitation reserve, depending on the measurement of the pump, is 5-10 m.a.c.;

d) from the conditional pressure line at the suction of the network pumps, the pressure losses on the return pipeline DH arr of the main pipeline of the heating network (line A-B) are successively plotted using the results of hydraulic calculation. The magnitude of the pressure in the return line must meet the requirements specified above when constructing a static pressure line;

e) the required available pressure is postponed at the last subscriber DH ab, from the operating conditions of the elevator, heater, mixer and distribution heating networks (line B-C). The value of the available pressure at the point of connection of distribution networks is assumed to be at least 40 m;

e) starting from the last piping node, the pressure losses in the supply pipeline of the main line DH under (line C-D) are postponed. The pressure at all points of the supply pipeline, based on the condition of its mechanical strength, should not exceed 160 m;

g) the pressure loss in the heat source DH um (line D-E) is plotted and the pressure at the outlet of the network pumps is obtained. In the absence of data, the pressure loss in the communications of the CHP can be taken as 25 - 30 m, and for a district boiler house 8-16 m.

The pressure of network pumps is determined

The pressure of the make-up pumps is determined by the pressure of the static mode.

As a result of such a construction, the initial form of the piezometric graph is obtained, which allows you to evaluate the pressure 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 away from it, then the piezometric graph should be raised so that the pressure in the return pipeline ensures that the system is filled;

b) if the value of the maximum pressure in the return pipeline exceeds the allowable pressure in the heaters, 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 crosses the pressure line in the supply pipeline, then water may boil behind the intersection point. Therefore, the water pressure in this part of the heating network should be increased by moving the piezometric graph upwards, if possible, or 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. A piezometric graph is developed for two modes. Firstly, for a static mode, when there is no water circulation in the heat supply 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 the hydrodynamic regime - in the presence of coolant circulation in the system.

The development of the schedule begins with a static mode. The location of the full static pressure line on the graph should ensure that all subscribers are connected to the heating network according to a dependent scheme. To do this, the static pressure should not exceed the allowable one from the strength condition of subscriber installations and should ensure that local systems are filled with water. The presence of a common static zone for the entire heat supply system simplifies its operation and increases its reliability. If there is a significant difference in geodetic elevations of the earth, the establishment of 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 providing at the highest points of the systems of the tallest buildings located in the zone of the largest geodetic marks, an overpressure of at least 0.05 MPa. Such pressure turns out to be unacceptably high for buildings located in that part of the area that has the lowest geodetic marks. Under such conditions, it becomes necessary to divide the heat supply system into two static zones. One zone for a part of the area with low geodetic marks, the other - with high ones.

On fig. 7.2 shows a piezometric graph and a schematic diagram of the heat supply system for an area with a significant difference in geodetic elevations of the ground level (40m). The part of the area adjacent to the source of heat supply has zero geodetic marks, in the peripheral part of the area the marks are 40m. The height of the buildings is 30 and 45m. For the possibility of filling the heating systems of buildings with water III and IV located at the 40m mark and creating an excess head of 5m at the highest points of the systems, the level of the full static head should be located at the 75m mark (line 5 2 - S 2). In this case, the static head will be 35m. However, a head of 75m is unacceptable for buildings I and II located at zero. For them, the permissible highest position of the total static pressure level corresponds to 60m. 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 pressure - the lower one with a level of 50m (line S t-Si) and the upper one with a level of 75m (line S 2 -S2). 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 the water circulation in the system stops, the levels of static pressures are established in accordance with the accepted two zones, a separating device is located at the junction (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 stop, the pressure drop in the return pipeline of the upper zone is prevented by the pressure regulator “to itself” RDDS (10), which maintains a constant predetermined pressure HRDDS at the point of impulse selection. When the pressure drops, it closes. A pressure drop in the supply line is prevented by a non-return valve (11) installed on it, which also closes. Thus, RDDS and a check valve cut the heating network into two zones. To feed the upper zone, a booster pump (8) is installed, which takes water from the lower zone and delivers it to the upper one. The head developed by the pump is equal to the difference between the hydrostatic heads of the upper and lower zones. The bottom 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 - the line of the total static head of the lower zone;

S 2 - S 2, - line of the total static head of the upper zone;

N p.n1 - pressure developed by the make-up pump of the lower zone; N p.n2 - pressure developed by the make-up pump of the upper zone; N RDDS - head to which the RDDS (10) and RD2 (9) regulators are set; ΔN RDDS - pressure actuated on the valve of the RDDS regulator in hydrodynamic mode; I-IV- subscribers; 1-tank make-up water; 2.3 - make-up pump and bottom zone make-up regulator; 4 - upstream pump; 5 - main steam-water heaters; 6- network pump; 7 - peak hot water boiler; eight , 9 - make-up pump and make-up regulator for the upper zone; 10 - pressure regulator "to yourself" RDDS; 11- check valve

The RDDS regulator is set to the pressure Nrdds (Fig. 7.2a). The feed 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 a pressure H O1. The difference between these heads is used to overcome 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 released in the throttle substation at the RDDS valve. On fig. 8.9, and this part of the pressure is shown by the value of ΔН RDDS. The throttle substation in hydrodynamic mode allows maintaining 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 Figs. 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 head after the heat source is 160 m, which does not exceed the allowable pressure from the pipe strength condition. The 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 allowable pressure is 40 m.

The piezometric graph developed for static and hydrodynamic modes provides the possibility of connecting all subscribers according to a dependent scheme.

Another possible solution for the hydrostatic mode of the heat supply system shown in fig. 7.2 is the connection of a part of subscribers according to an independent scheme. There may be two options here. First option- set the total level of static pressure at 50m (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 head in the water-to-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 it will be determined by the height of the buildings. The second option is to set the total level of static pressure at around 75 m (line S 2 - S 2) with the buildings of the upper zone connected according to a dependent scheme, and the buildings of the lower zone - according to an independent one. In this case, the static head in water-to-water heaters on the side of the heating coolant will be 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;

DRe - natural circulation pressure in the settlement ring due to water cooling in pipes and heaters, Pa;

In pumping systems, it is allowed not to take into account DPe if it is less than 10% of DPn.

The available pressure drop at the entrance to the building DPr = 150 kPa.

Calculation of natural circulation pressure

The natural circulating pressure that occurs in the calculated ring of a vertical single-pipe system with lower wiring regulated with trailing sections, Pa, is determined by the formula

where is the average increase in water density with a decrease in its temperature by 1 °C, kg / (m3??C);

Vertical distance from heating center to cooling center

heater, m;

Water consumption 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 factor U according to the nomogram.

Available pressure difference at the inlet =150 kPa;

Heat carrier parameters:

In the heating network f1=150?С; f2=70?С;

In the heating system t1=95?C; t2=70?C;

We determine the mixing ratio by the formula

µ= f1 - t1 / t1 - t2 =150-95/95-70=2.2; (2.4)

Hydraulic calculation of water heating systems by the method of specific friction pressure losses

Calculation of the main circulation ring

1) Hydraulic calculation of the main circulation ring is performed 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 FCC into calculated sections.

3) For the preliminary selection of the pipe diameter, 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 proportion of local pressure losses in the system;

For a heating system with pump circulation, the share of losses due to local resistances is equal to b=0.35, to friction b=0.65.

4) We determine the flow rate of the coolant in each section, kg / h, according to the formula

Parameters of the heat carrier in the supply and return pipelines of the heating system, ?С;

Specific mass heat capacity of water, equal to 4.187 kJ / (kg?? С);

Coefficient for accounting for additional heat flow when rounding in excess of the calculated value;

Accounting coefficient for additional heat losses by heating devices near external fences;

6) We determine the coefficients of local resistance in the calculated sections (and write their sum in table 1) by .

Table 1

7) In each section of the main circulation ring, we determine the pressure loss due to local resistances Z, po, depending on the sum of the local resistance coefficients Uo and the water velocity in the section.

8) We check the reserve of the 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 scheme of coolant movement, the discrepancy between pressure losses in the circulation rings should not exceed 15%.

The hydraulic calculation of the main circulation ring is summarized 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 a secondary circulation ring through the riser 8 of a single-pipe water heating system

1) We calculate the natural circulation pressure due to the cooling of water in the heaters of the riser 8 according to the formula (2.2)

2) Determine the water flow in the riser 8 according to the 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 MCC sections, adjusted for the difference in natural circulation pressure in the secondary and main rings:

15128.7+(802-1068)=14862.7 Pa

4) We find the average value of the linear pressure loss according to the formula (2.5)

5) Based on the value, Pa/m, the flow rate of the coolant in the area, kg/h, and the maximum allowable speeds of the coolant, we determine the preliminary diameter of the pipes dу, mm; actual specific pressure loss R, Pa/m; actual coolant velocity V, m/s, according to .

6) We determine the coefficients of local resistance in the calculated sections (and write their sum in table 2) according to .

7) In the section of the small circulation ring, we determine the pressure loss due to local resistances Z, po, depending on the sum of the coefficients of local resistance Uo and the water velocity in the section.

8) The hydraulic calculation of the small circulation ring is summarized in Table 2 (Appendix B). We check the hydraulic balancing between the main and small hydraulic rings according to the formula

9) We determine the required pressure loss in the throttle washer according to the formula

10) Determine the diameter of the throttle washer by the formula

On the site it is required to install a throttle washer with a diameter of the internal passage DN = 5mm

Warning Insufficient pressure at the source Delta=X m. Where Delta is the required pressure.

MOST DIFFERENT CONSUMER: ID=XX.

Figure 283. Worst customer message




This message is displayed when there is not enough available pressure on the consumer, where DeltaH- the value of the pressure of which is not enough, m, and ID (XX)− individual number of the consumer for which the lack of pressure is maximum.



Figure 284. Insufficient pressure message




Double click the left mouse button on the message of the worst consumer: the corresponding consumer will flash on the screen.

This error can be caused by several reasons:

1. Incorrect data. If the value of the head shortage is beyond the real 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. Please check if the following information has been entered correctly:

   

   Hydraulic network mode.

   If there are no errors when entering the initial data, but there is a shortage of pressure and has a real value for this network, then in this situation, the cause of the shortage and the way to eliminate it are determined by the specialist working with this heating network.

ID=XX "Consumer name" Emptying the heating system (H, m)

This message is displayed when there is insufficient pressure in the return pipe to prevent the heating system from emptying the upper floors of the building, the total pressure in the return pipe must be at least the sum of the geodesic mark, the height of the building, plus 5 meters to fill the system. The pressure margin for filling the system can be changed in the calculation settings ().

XX− individual number of the consumer whose heating system is being emptied, H- head, in meters which is not enough;

ID=XX "Consumer name" Head in the return pipeline above the geodetic mark by N, m

This message is issued when the pressure in the return pipeline is higher than the permissible one according to the strength conditions of cast-iron radiators (more than 60 m of water column), where XX- individual consumer number and H- the value of the pressure in the return pipeline exceeding the geodetic mark.

The maximum pressure in the return line can be set independently in calculation settings. ;

ID=XX "Consumer name" Do not pick up the elevator nozzle. We set the maximum

This message may appear if there are large heating loads or if the connection scheme is incorrectly selected, which does not correspond to the calculated parameters. XX- individual number of the consumer, for which the elevator nozzle cannot be selected;

ID=XX "Consumer name" Do not pick up the elevator nozzle. We set the minimum

This message may appear if there are very low heating loads or if the connection scheme is incorrectly selected, which does not correspond to the calculated parameters. XX− individual number of the consumer, for which the elevator nozzle cannot be selected.

Warning Z618: ID=XX "XX" Number of washers on the CO supply pipe is greater 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 washer diameter is 3 mm (indicated in the calculation settings “Head loss calculation settings”), and the consumption for the consumer’s heating system ID=XX is very small, the result of the calculation is 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 in order to adjust this consumer, 16 washers with a diameter of 3 mm and 1 washer, the diameter of which is determined in the consumer database, should be installed.

Warning Z642: ID=XX The elevator at the central heating station is not working

This message is displayed as a result of the verification calculation and means that the elevator unit is not functioning.