Relationship between available head and design circulation. Pressures in water supply systems

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 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 any 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 (a device for measuring pressure 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 - necessary two-stage hot water system;

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 hot water system is performed due to the circulation of hot water.

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.

1. 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.
2. 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:

1. 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.
2. 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:

1. Before the boiler installation (or boiler) and at the exit from it. At this point the pressure in the boiler is determined.
2. Before and after the circulation pump.
3. At the entrance of the heating main into a building or structure.
4. Before and after the pressure regulator.
5. At the inlet and outlet of the coarse filter (mud 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.

“Specification of indicators of the quantity and quality of communal resources in the modern realities of housing and communal services”

SPECIFICATION OF INDICATORS OF QUANTITY AND QUALITY OF COMMUNAL RESOURCES IN MODERN REALITIES OF HUSING AND UTILITIES

V.U. Kharitonsky, Head of Engineering Systems Department

A. M. Filippov, Deputy Head of the Engineering Systems Department,

State Housing Inspectorate of Moscow

Documents regulating the indicators of the quantity and quality of communal resources supplied to household consumers at the border of responsibility of the resource supply and housing organizations have not been developed to date. Specialists from the Moscow Housing Inspectorate, in addition to the existing requirements, propose to specify the values ​​of the parameters of heat and water supply systems at the entrance to the building, in order to maintain the quality of public services in residential apartment buildings.

A review of the current rules and regulations for the technical operation of the housing stock in the field of housing and communal services showed that currently construction, sanitary norms and regulations, GOST R 51617 -2000 * “Housing and communal services”, “Rules for the provision of utility services to citizens”, approved by Decree of the Government of the Russian Federation dated May 23, 2006 No. 307, and other current regulatory documents consider and establish parameters and modes only at the source (central heating station, boiler room, water pumping station) that produces communal resources (cold, hot water and thermal energy), and directly in the resident’s apartment, where utilities are provided. However, they do not take into account the modern realities of the division of housing and communal services into residential buildings and public utility facilities and the established boundaries of responsibility of the resource supply and housing organizations, which are the subject of endless disputes in determining the guilty party for the failure to provide services to the population or the provision of services of inadequate quality. Thus, today there is no document regulating the indicators of quantity and quality at the entrance to the house, at the border of responsibility of the resource supply and housing organizations.

However, an analysis of inspections of the quality of supplied utility resources and services carried out by the Moscow Housing Inspectorate showed that the provisions of federal regulatory legal acts in the field of housing and communal services can be detailed and specified in relation to apartment buildings, which will establish the mutual responsibility of resource supply and housing management organizations. It should be noted that the quality and quantity of communal resources supplied to the boundary of the operational responsibility of the resource supplying and managing housing organization, and public services to residents, is determined and assessed based on the readings, first of all, of common house metering devices installed at the inputs

heat and water supply systems to residential buildings, and an automated system for monitoring and accounting for energy consumption.

Thus, the Moscow Housing Inspectorate, based on the interests of residents and many years of practice, in addition to the requirements of regulatory documents and in development of the provisions of SNiP and SanPin in relation to operating conditions, as well as in order to maintain the quality of utility services provided to the population in residential apartment buildings, proposed regulating when introducing heat and water supply systems into the house (at the metering and control unit), the following standard values ​​of parameters and modes recorded by general house metering devices and an automated control and accounting system for energy consumption:

1) for a central heating system (CH):

The deviation of the average daily temperature of the network water entering the heating systems must be within ±3% of the established temperature schedule. The average daily temperature of the return network water should not exceed the temperature specified by the temperature schedule by more than 5%;

The network water pressure in the return pipeline of the central heating system must be no less than 0.05 MPa (0.5 kgf/cm2) higher than the static pressure (for the system), but not higher than permissible (for pipelines, heating devices, fittings and other equipment ). If necessary, it is allowed to install pressure regulators on the return pipelines in the ITP of heating systems of residential buildings directly connected to the main heating networks;

The network water pressure in the supply pipeline of central heating systems must be higher than the required water pressure in the return pipelines by the amount of available pressure (to ensure coolant circulation in the system);

The available pressure (pressure difference between the supply and return pipelines) of the coolant at the entrance of the central heating network into the building must be maintained by heat supply organizations within the limits:

a) with dependent connection (with elevator units) - in accordance with the design, but not less than 0.08 MPa (0.8 kgf/cm 2);

b) with independent connection - in accordance with the design, but not less than 0.03 MPa (0.3 kgf/cm2) more than the hydraulic resistance of the in-house central heating system.

2) For the hot water supply system (DHW):

The temperature of hot water in the DHW supply pipeline for closed systems is within 55-65 °C, for open heat supply systems within 60-75 °C;

Temperature in the DHW circulation pipeline (for closed and open systems) 46-55 °C;

The arithmetic mean value of the hot water temperature in the supply and circulation pipelines at the inlet of the DHW system in all cases must be at least 50 °C;

The available pressure (pressure difference between the supply and circulation pipelines) at the calculated circulation flow rate of the hot water supply system must be no lower than 0.03-0.06 MPa (0.3-0.6 kgf/cm2);

The water pressure in the supply pipeline of the hot water supply system must be higher than the water pressure in the circulation pipeline by the amount of available pressure (to ensure the circulation of hot water in the system);

The water pressure in the circulation pipeline of hot water supply systems must be no less than 0.05 MPa (0.5 kgf/cm2) higher than the static pressure (for the system), but not exceed the static pressure (for the highest located and high-rise building) more than by 0.20 MPa (2 kgf/cm2).

With these parameters in apartments, the sanitary fixtures of residential premises, in accordance with the regulatory legal acts of the Russian Federation, must have the following values:

Hot water temperature is not lower than 50 °C (optimal - 55 °C);

The minimum free pressure for sanitary fixtures in residential premises on the upper floors is 0.02-0.05 MPa (0.2-0.5 kgf/cm 2);

The maximum free pressure in hot water supply systems at sanitary fixtures on the upper floors should not exceed 0.20 MPa (2 kgf/cm2);

The maximum free pressure in water supply systems at sanitary fixtures on the lower floors should not exceed 0.45 MPa (4.5 kgf/cm2).

3) For a cold water supply system (CWS):

The water pressure in the supply pipeline of the cold water system must be at least 0.05 MPa (0.5 kgf/cm 2) higher than the static pressure (for the system), but not exceed the static pressure (for the highest located and high-rise building) by more than 0.20 MPa (2 kgf/cm2).

With this parameter in apartments, in accordance with regulatory legal acts of the Russian Federation, the following values ​​must be provided:

a) the minimum free pressure for sanitary fixtures in residential premises on the upper floors is 0.02-0.05 MPa (0.2-0.5 kgf/cm2);

b) the minimum pressure in front of the gas water heater on the upper floors is not less than 0.10 MPa (1 kgf/cm2);

c) the maximum free pressure in water supply systems at sanitary fixtures on the lower floors should not exceed 0.45 MPa (4.5 kgf/cm2).

4) For all systems:

The static pressure at the inlet to the heat and water supply systems must ensure that the pipelines of the central heating, cold water and hot water supply systems are filled with water, while the static water pressure should not be higher than permissible for this system.

The water pressure values ​​in the DHW and cold water systems at the entrance of pipelines into the house must be at the same level (achieved by setting automatic control devices for the heating point and/or pumping station), while the maximum permissible pressure difference must be no more than 0.10 MPa (1 kgf/cm 2).

These parameters at the entrance to buildings must be ensured by resource supplying organizations by implementing measures for automatic regulation, optimization, uniform distribution of thermal energy, cold and hot water between consumers, and for return pipelines of systems - also by housing management organizations through inspections, identification and elimination of violations or re-equipment and adjustment of building engineering systems. The specified measures should be carried out when preparing heating points, pumping stations and intra-block networks for seasonal operation, as well as in cases of violations of the specified parameters (indicators of the quantity and quality of utility resources supplied to the boundary of operational responsibility).

If the specified parameter values ​​and modes are not observed, the resource supplying organization is obliged to immediately take all necessary measures to restore them. In addition, in case of violation of the specified values ​​of the parameters of the supplied utility resources and the quality of the provided utility services, it is necessary to recalculate the payment for the provided utility services with a violation of their quality.

Thus, compliance with these indicators will ensure comfortable living for citizens, the effective functioning of engineering systems, networks, residential buildings and public utility facilities that provide heat and water supply to the housing stock, as well as the supply of utility resources in the required quantity and standard quality to the boundaries of the operational responsibility of the resource supply and managing housing organization (at the entrance of utilities into the house).

Literature

1. Rules for the technical operation of thermal power plants.

2. MDK 3-02.2001. Rules for the technical operation of public water supply and sewerage systems and structures.

3. MDK 4-02.2001. Standard instructions for the technical operation of municipal heating systems.

4. MDK 2-03.2003. Rules and regulations for the technical operation of housing stock.

5. Rules for the provision of public services to citizens.

6. ZhNM-2004/01. Regulations for the preparation for winter operation of heat and water supply systems of residential buildings, equipment, networks and structures of fuel, energy and public utilities in Moscow.

7. GOST R 51617 -2000*. Housing and communal services. General technical conditions.

8. SNiP 2.04.01 -85 (2000). Internal water supply and sewerage of buildings.

9. SNiP 2.04.05 -91 (2000). Heating, ventilation and air conditioning.

10. Methodology for checking violations of the quantity and quality of services provided to the population by accounting for heat energy consumption, cold and hot water consumption in Moscow.

(Energy Saving Magazine No. 4, 2007)

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 inlet pressure difference =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 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 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

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

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.

A more difficult task is the choice of a water heating network scheme due to its large length and large number of 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. When the diameters of the mains extending from the heat source are 700 mm or less, a radial heating network diagram is usually used with a gradual decrease in the pipe diameter as the distance from the source increases 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