Operation of thermal networks. Temperature graph of a heat network - tips for drawing up Thermal regime of heat networks
Computers have long and successfully worked not only on tables office workers, but also in production and technological processes. Automation successfully manages the parameters of building heat supply systems, providing inside them ...
The set required air temperature (sometimes changing during the day to save money).
But the automation must be correctly configured, give it the initial data and algorithms for work! This article discusses the optimal temperature heating schedule - the dependence of the temperature of the coolant of the water heating system at various outdoor temperatures.
This topic has already been discussed in the article about. Here we will not calculate the heat losses of the object, but consider the situation when these heat losses are known from previous calculations or from the data of the actual operation of the operating object. If the object is active, then it is better to take the value of heat loss at design temperature outdoor air from statistical actual data previous years operation.
In the article mentioned above, to construct the dependences of the coolant temperature on the outdoor air temperature, a system of nonlinear equations is solved by a numerical method. This article will present "direct" formulas for calculating water temperatures on the "supply" and on the "return", which is an analytical solution to the problem.
You can read about the colors of Excel sheet cells that are used for formatting in articles on the page « ».
Calculation in Excel of the temperature graph of heating.
So, when setting up the boiler and / or thermal unit from the outside air temperature, the automation system must set a temperature graph.
Maybe, correct sensor place the air temperature inside the building and adjust the operation of the coolant temperature control system from the internal air temperature. But it is often difficult to choose the location of the sensor inside due to different temperatures in various premises object or due to the significant remoteness of this place from the thermal unit.
Consider an example. Let's say we have an object - a building or a group of buildings that receives thermal energy from one common closed source of heat supply - a boiler house and / or a thermal unit. A closed source is a source from which the selection of hot water for water supply is prohibited. In our example, we will assume that, in addition to the direct selection of hot water, there is no heat extraction for heating water for hot water supply.
To compare and verify the correctness of the calculations, we take the initial data from the above article "Calculation of water heating in 5 minutes!" and compose in Excel a small program for calculating the heating temperature graph.
Initial data:
1. Estimated (or actual) heat loss of an object (building) Q p in Gcal/h at design outdoor air temperature t nr write down
to cell D3: 0,004790
2. Estimated air temperature inside the object (building) t time in °C enter
to cell D4: 20
3. Estimated outdoor temperature t nr in °C we enter
to cell D5: -37
4. Estimated supply water temperature t pr enter in °C
to cell D6: 90
5. Estimated return water temperature t op in °C enter
to cell D7: 70
6. Indicator of non-linearity of heat transfer of applied heating devices n write down
to cell D8: 0,30
7. The current (of interest to us) outdoor temperature t n in °C we enter
to cell D9: -10
Values in cellsD3 – D8 for a specific object are written once and then do not change. Cell valueD8 can (and should) be changed by determining the coolant parameters for different weather.
Calculation results:
8. Estimated water flow in the system GR in t/h we calculate
in cell D11: =D3*1000/(D6-D7) =0,239
GR = QR *1000/(tetc — top )
9. Relative heat flux q determine
in cell D12: =(D4-D9)/(D4-D5) =0,53
q =(tvr — tn )/(tvr — tnr )
10. The temperature of the water at the "supply" tP in °C we calculate
in cell D13: =D4+0.5*(D6-D7)*D12+0.5*(D6+D7-2*D4)*D12^(1/(1+D8)) =61,9
tP = tvr +0,5*(tetc – top )* q +0,5*(tetc + top -2* tvr )* q (1/(1+ n ))
11. Return water temperature tabout in °C we calculate
in cell D14: =D4-0.5*(D6-D7)*D12+0.5*(D6+D7-2*D4)*D12^(1/(1+D8)) =51,4
tabout = tvr -0,5*(tetc – top )* q +0,5*(tetc + top -2* tvr )* q (1/(1+ n ))
Calculation in Excel of the water temperature at the "supply" tP and on the return tabout for selected outdoor temperature tn completed.
Let's make a similar calculation for several different outdoor temperatures and build a heating temperature graph. (You can read about how to build graphs in Excel.)
Let's reconcile the obtained values of the heating temperature graph with the results obtained in the article "Calculation of water heating in 5 minutes!" - the values match!
Results.
The practical value of the presented calculation of the heating temperature graph lies in the fact that it takes into account the type installed appliances and the direction of movement of the coolant in these devices. Heat transfer non-linearity coefficient n, which has a noticeable effect on the temperature graph of heating for different devices is different.
Economic consumption of energy resources in heating system, can be achieved if certain requirements are met. One of the options is the presence of a temperature chart, which reflects the ratio of the temperature emanating from the heating source to external environment. The value of the values makes it possible to optimally distribute heat and hot water to the consumer.
High-rise buildings are connected mainly to central heating. The sources that transmit thermal energy are boiler houses or CHPs. Water is used as a heat carrier. It is heated to a predetermined temperature.
Having passed full cycle through the system, the coolant, already cooled, returns to the source and reheating occurs. Sources are connected to the consumer by thermal networks. Since the environment changes the temperature regime, thermal energy should be regulated so that the consumer receives the required volume.
Heat regulation from central system can be produced in two ways:
- Quantitative. In this form, the flow rate of water changes, but the temperature is constant.
- Qualitative. The temperature of the liquid changes, but its flow rate does not change.
In our systems, the second variant of regulation is used, that is, qualitative. W Here there is a direct relationship between two temperatures: coolant and environment. And the calculation is carried out in such a way as to provide heat in the room of 18 degrees and above.
Hence, we can say that the temperature curve of the source is a broken curve. The change in its directions depends on the temperature difference (coolant and outside air).
Dependency graph may vary.
A particular chart has a dependency on:
- Technical and economic indicators.
- Equipment for a CHP or boiler room.
- climate.
High performance of the coolant provides the consumer with a large thermal energy.
An example of a circuit is shown below, where T1 is the temperature of the coolant, Tnv is the outdoor air:
It is also used, the diagram of the returned coolant. A boiler house or CHP according to such a scheme can evaluate the efficiency of the source. It is considered high when the returned liquid arrives cooled.
The stability of the scheme depends on the design values of the liquid flow of high-rise buildings. If the flow rate through the heating circuit increases, the water will return uncooled, as the flow rate will increase. And vice versa, when minimum flow, the return water will be sufficiently cooled.
The supplier's interest is, of course, in the receipt return water in a chilled state. But there are certain limits to reduce the flow, since a decrease leads to losses in the amount of heat. The consumer will begin to lower the internal degree in the apartment, which will lead to a violation building codes and the discomfort of the inhabitants.
What does it depend on?
The temperature curve depends on two quantities: outside air and coolant. Frosty weather leads to an increase in the degree of coolant. When designing a central source, the size of the equipment, the building and the section of pipes are taken into account.
The value of the temperature leaving the boiler room is 90 degrees, so that at minus 23°C, it would be warm in the apartments and have a value of 22°C. Then the return water returns to 70 degrees. Such norms correspond to normal and comfortable living in the house.
Analysis and adjustment of operating modes is carried out using a temperature scheme. For example, the return of a liquid with an elevated temperature will indicate high coolant costs. Underestimated data will be considered as a consumption deficit.
Previously, for 10-storey buildings, a scheme with calculated data of 95-70°C was introduced. The buildings above had their chart 105-70°C. Modern new buildings may have a different scheme, at the discretion of the designer. More often, there are diagrams of 90-70°C, and maybe 80-60°C.
Temperature chart 95-70:
temperature graph 95-70
How is it calculated?
The control method is selected, then the calculation is made. The calculation-winter and reverse order of water inflow, the amount of outside air, the order at the break point of the diagram are taken into account. There are two diagrams, where one of them considers only heating, the other one considers heating with hot water consumption.
For an example calculation, we will use methodological development Roskommunenergo.
The initial data for the heat generating station will be:
- Tnv- the amount of outside air.
- TVN- indoor air.
- T1- coolant from the source.
- T2- return flow of water.
- T3- the entrance to the building.
We will consider several options for supplying heat with a value of 150, 130 and 115 degrees.
At the same time, at the exit they will have 70 ° C.
The results obtained are brought into a single table for the subsequent construction of the curve:
So, we got three different schemes that can be taken as a basis. It would be more correct to calculate the diagram individually for each system. Here we have considered the recommended values, excluding climatic features region and building characteristics.
To reduce power consumption, it is enough to choose a low-temperature order of 70 degrees and uniform distribution of heat throughout the heating circuit will be ensured. The boiler should be taken with a power reserve so that the load of the system does not affect the quality operation of the unit.
Adjustment
Heating regulator Automatic control is provided by the heating controller.
It includes the following details:
- Computing and matching panel.
- Executive device at the water supply line.
- Executive device, which performs the function of mixing liquid from the returned liquid (return).
- boost pump and a sensor on the water supply line.
- Three sensors (on the return line, on the street, inside the building). There may be several in a room.
The regulator covers the liquid supply, thereby increasing the value between the return and supply to the value provided by the sensors.
To increase the flow, there is a booster pump, and the corresponding command from the regulator. The incoming flow is regulated by a "cold bypass". That is, the temperature drops. Some of the liquid that circulates along the circuit is sent to the supply.
Information is taken by sensors and transmitted to control units, as a result of which flows are redistributed, which provide a rigid temperature scheme for the heating system.
Sometimes, a computing device is used, where the DHW and heating regulators are combined.
The hot water regulator has more a simple circuit management. The hot water sensor regulates the flow of water with a stable value of 50°C.
Regulator benefits:
- The temperature regime is strictly maintained.
- Exclusion of liquid overheating.
- Fuel economy and energy.
- The consumer, regardless of distance, receives heat equally.
Table with temperature chart
The operating mode of the boilers depends on the weather of the environment.
If we take various objects, for example, a factory building, a multi-storey building and private house, all will have an individual heat chart.
In the table, we show the temperature diagram of the dependence of residential buildings on the outside air:
| Outside temperature | Temperature of network water in the supply pipeline | Temperature of network water in the return pipeline |
| +10 | 70 | 55 |
| +9 | 70 | 54 |
| +8 | 70 | 53 |
| +7 | 70 | 52 |
| +6 | 70 | 51 |
| +5 | 70 | 50 |
| +4 | 70 | 49 |
| +3 | 70 | 48 |
| +2 | 70 | 47 |
| +1 | 70 | 46 |
| 0 | 70 | 45 |
| -1 | 72 | 46 |
| -2 | 74 | 47 |
| -3 | 76 | 48 |
| -4 | 79 | 49 |
| -5 | 81 | 50 |
| -6 | 84 | 51 |
| -7 | 86 | 52 |
| -8 | 89 | 53 |
| -9 | 91 | 54 |
| -10 | 93 | 55 |
| -11 | 96 | 56 |
| -12 | 98 | 57 |
| -13 | 100 | 58 |
| -14 | 103 | 59 |
| -15 | 105 | 60 |
| -16 | 107 | 61 |
| -17 | 110 | 62 |
| -18 | 112 | 63 |
| -19 | 114 | 64 |
| -20 | 116 | 65 |
| -21 | 119 | 66 |
| -22 | 121 | 66 |
| -23 | 123 | 67 |
| -24 | 126 | 68 |
| -25 | 128 | 69 |
| -26 | 130 | 70 |
SNiP
There are certain rules that must be observed in the creation of projects on heating network and transporting hot water to the consumer, where the supply of water vapor must be carried out at 400°C, at a pressure of 6.3 bar. The supply of heat from the source is recommended to be released to the consumer with values of 90/70 °C or 115/70 °C.
Regulatory requirements should be followed for compliance with the approved documentation with the obligatory coordination with the Ministry of Construction of the country.
Considering thermal loads systems of communal heat supply (section Calculation of heating modes), their direct individual connection-dependence with the parameters of the environment has been established natural environment- temperature and humidity of the outside air, water temperature in water supply sources, wind speed and direction, radiation exposure - sunshine.
Any change in them necessitates adjustments. heat consumption both at the source of heat supply and directly at the consumer, by reducing or increasing the supply of heat, turning on or off certain types of equipment and appliances, establishing rational regime their work, taking into account heat losses during transportation. Thus, there is a need to control the processes of supply and consumption of thermal energy, i.e. thermal regulation by them.
The prevailing parameter for most heat loads is the outdoor temperature, which determines both the temperature of the water at the source of water supply and the temperature building materials and products, and parameters of the internal climate of residential and public buildings etc. The balance load equations include the temperature difference (t vn - t outside), showing their linear dependence on the current outdoor temperature (equations of straight lines).
If you build a graph of the heating heat load depending on the t of the outdoor environment, then it will look like a straight sloping line, graphs of ventilation loads and graphs of the dependence of the hot water supply load on the temperature of the source water will take similar forms (Fig. 1).
Figure 1. Graphs of changes in the heat loads of heating, ventilation and hot water supply of a residential building depending on t outdoor air.
AT practical work It is customary for designers and operators to build such graphs of the dependence of thermal loads Q (function) on the determining parameter t outdoor air (argument) in the coordinates “t outdoor air - Q”, where Q = ƒ(t outdoor air). At the same time, they are taken into account in a certain temperature range, for example, in the interval between the beginning of the heating period and the maximum heating load, called "calculated", t n.calc.
For the design temperature t n.o for the design of heating in each locality, the average outdoor temperature is taken equal to average temperature of the coldest five-day periods taken from the eight coldest winters over a 50-year observation period. Such values of tn.o have been determined for many cities of the country, they are given in the SNiP for building climatology, and maps of climatological zoning have been drawn up based on them.
Design temperatures were also determined and put into practice for the design of ventilation t n.v; duration of the heating period n, days; average outside temperature of the heating period; the average of the coldest month, as well as the average of the hottest month.
To establish the total loads, graphs of the total thermal loads are built (see Fig. 1), they are necessary to perform technological, technical and economic calculations and studies.
In the planning and economic work of enterprises (to determine fuel consumption, develop modes of equipment use, repair schedules, etc.), heat consumption graphs by months of the year (Fig. 2), seasonal load duration graphs (Fig. 3), and also integrated graphs of total loads (Fig. 4).
Figure 2.
Figure 3
Figure 4
With the help of duration graphs and integral graphs of the total load of the city/district, it is easy to establish economical modes of operation of heating equipment, determine the necessary parameters of the coolant at CHP and RTS, perform other technological and planning-economic calculations and studies. For example, the establishment of the operating mode and operational and dispatch planning of a specific DH system is based on three load schedules: daily, annual, and a schedule of changes in heat load by duration.
The regulation of thermal processes is carried out with the help of temperature graphs for the release of heat. These graphs (or tables) establish the relationship between the current water temperatures in heating systems t 1 and t 2 and in heating networks, depending on the outdoor temperature. Such a dependence is established from the heat balance equation of the heating device under design and any other temperature conditions:
where Q and G are the heat consumption, W h, and the coolant, kg / h, at the current and design outdoor air temperature; ∆t \u003d t 1 - t 2 - temperature difference in local heating devices at current and calculated (∆t p) outdoor temperature, to hail; t 1 and t 2 - the temperature of the supply and return water in local heating devices, deg; \u003d (t 1 + t 2) / 2 - T n - temperature difference of the heating device, deg; ∆T \u003d T in - T n - temperature difference in air inside (T in) and outside the room (T n) at the current and design temperature (∆T p), deg; k - heat transfer coefficient of the heating device, W / (m 2 · h · deg); F - surface of heating devices, m 2.
After a series of transformations of equation (1), we obtain the following expressions for t 1 and t 2:
Figure 5. Graph of water temperature in the supply and return mains of the heating network with high-quality regulation of the heating load at T p.r. = +18 °С
EXAMPLE 1. Initial conditions: Water heating system with design parameters T n.r = -25 °C, T p.r = +20 °C, t 1z = 95 °C, t 2p = 70 °C.
Required: Determine the supply and return water temperatures for the heating system at outdoor temperatures T n \u003d +8 ° C, -3.2 ° C and room temperature T p \u003d +20 ° C.
Solution: We find for T n \u003d +8 ° С:
According to formulas (2); (3) we get:
For T n \u003d -3.2 ° С similarly:
Based on the points obtained, we build a temperature graph (see lines 1 and τ "2 in Fig. 5).
Here are the values of water temperatures in the supply and return lines of the heating network τ 1 and τ 2 for different climatic regions with high-quality regulation of the heating load, for the estimated temperature difference in the local system ∆t p \u003d 95 - 70 \u003d 25 ° С, T p.r \u003d +18 °С; p \u003d (95 + 70) / 2 - 18 \u003d 64.5 ° С.
Due to the fact that heterogeneous heat consumers: heating and ventilation systems (seasonal, homogeneous loads), hot water supply systems (year-round loads), technological installations, temperature conditions heating networks must meet the needs and take into account the specifics of the heat consumption of each of them. Therefore, temperature graphs that are built according to the prevailing heat load (in cities - heating and ventilation) must take into account the requirements of hot water supply systems. The need for heating tap water to the level of 55-60 °С. Up to this level of heating of the secondary heat carrier, the primary network water must have its temperature not lower than 70 °C, therefore, the so-called spring-summer cutoff or “kink” in the flow line temperature at the level of 70 °C appears on the temperature heating curve.
In turn, maintaining such a temperature in the heating supply line during warm periods of the year leads to an undesirable phenomenon - overflowing of buildings, which causes discomfort for the population and, as a result, heat loss through open vents and window transoms. Overheating can be eliminated by adjusting the heat supply to the heating systems by passes (turning off the central heating systems for a while). So there is a combined load regulation (Fig. 6).
Figure 6
The duration of the heating system n, h, when regulated by gaps, is determined from the expression:
where Q - heat supply to the device, W, for time z, h; G - hot water supply to the device, kg / h; c is the heat capacity of water, W/(kg deg); t 1 and t 2 - the temperature of the supply and return water in heating device, hail; T p - ambient temperature of the heated medium, °C; F - heat sink heating surface, m 2 ; k - heat transfer coefficient of the heat sink W / (m 2 · h · deg); z - time, h
For the steam receiver we have:
Here, in addition to the notation adopted above:
D - steam consumption, kg/h; T - steam saturation temperature °С; ∆i - heat consumption of steam, kJ/kg.
In water DH systems, the amount of incoming heat Q can be influenced in different ways - by changing the temperature of incoming water t 1 (qualitative control), water flow G (quantitative control), heat supply time z (intermittent control), changing the heating surface of the heat exchanger F (rarely used ).
In domestic heat supply, the most widely used method is the central quality control of the heat load, in which the temperature of the incoming network water changes and its consumption remains unchanged. This method allows you to work with low steam pressure in the water heaters of CHP and provides significant fuel savings in cogeneration. It is easy to carry out and greatly simplifies group and individual adjustment. local systems.
Quantitative regulation has been widely used in foreign heat supply practice, in our country it has found partial use in group and local regulation of systems and individual devices. AT last years the combined method of qualitative-quantitative regulation has become widespread (see Fig. 6).
The regulation of the heating time (or as it is also called the regulation of gaps) has received limited use in the central regulation of water networks during the warm period heating season(when the mains pumps are stopped), as this will stop the hot water supply and the operation of the ventilation systems. With group and local regulation, this method allows to obtain significant savings in heat without the above restrictions.
AT steam systems intermittent group and local regulation are the main method of regulation steam plants heat supply.
Central and group regulation is carried out in accordance with regime schedules that establish the mode of temperature and water flow in heating networks and at subscriber inputs and allow you to control the correct operation and distribution of heat between consumers.
For correct regulation great importance has hydraulic stability of the local system. It is understood as the ability of individual heat receivers of the system to maintain the coolant flow rate set for them when the flow rate is changed by another heat exchanger of the system.
Hydraulic stability is determined by the ratio of the hydraulic resistance of the heat sink to the hydraulic resistance of the distribution network: the larger this ratio, the higher the hydraulic stability of the system.
To increase the hydraulic stability of the system, it is necessary to strive to increase the hydraulic resistance of heat sinks and reduce the resistance of heat networks.
Systems with low hydraulic stability cannot be accurately adjusted and are difficult to operate, therefore, it is often necessary to increase the hydraulic stability by installing artificial hydraulic resistances in front of the heat sinks (throttling-pucking systems), this is also facilitated by reducing the cross sections of the regulatory bodies, correct selection cones in elevators, serial, not parallel, inclusion of heat sinks of one unit (DHW heaters, etc.).
In centralized heat supply systems (especially in the heating networks of AO-energos), a certain system of division of labor and responsibility of personnel in the process of thermal regulation has developed. Thus, the station personnel is responsible for fulfilling the application daily schedule for the supply line temperature and for maintaining the specified pressures on the station collectors (in steam systems, for observing the schedule for steam pressure and temperature at the outlet of the station).
The personnel of the district of heating networks, in operational subordination of which is the duty personnel of subscribers, controls and is responsible for the parameters of the network economy - the heat carrier flow in the network, the temperature of the water in the return lines, the amount of make-up (in closed systems DH), condensate return to the station.
The temperature schedule determines the mode of operation of heat networks, providing central regulation of heat supply. According to the temperature graph, the temperature of the supply and return water in the heating networks, as well as in the subscriber input, is determined depending on the outdoor temperature.
The 150/70°C schedule used in Moscow (see columns 2 and 3 of the table) will allow heat to be transferred from a heat source with lower coolant consumption, however, coolant with a temperature above 105°C cannot be supplied to house heating systems. Therefore, it is produced according to reduced schedules.
For home heating systems of consumers, the Graph for the qualitative regulation of water temperature in heating systems is applied at various calculated and current outdoor temperatures with calculated differences in water temperature in the heating system of 95-70 and 105-70 ° C (see columns 5 and 6 of the table).
For networks operating according to temperature schedules of 95-70°С and 105-70°С (columns 5 and 6 of the table), the water temperature in the return pipeline of heating systems is determined by column 7 of the table.
For consumers connected according to an independent connection scheme, the water temperature in the direct pipeline is determined according to column 4 of the table, and in the return pipeline according to column 8 of the table.
The temperature schedule for regulating the heat load is developed from the conditions of the daily supply of heat energy for heating, which ensures the need for buildings in heat energy, depending on the outside temperature, in order to ensure that the temperature in the premises is constant at a level of at least 18 degrees, as well as covering the heat load of hot water supply with ensuring temperature of DHW in the places of water intake is not lower than + 60 ° C, in accordance with the requirements of SanPin 2.1.4.2496-09 " Drinking water. Hygiene requirements to the water quality of centralized systems drinking water supply. Quality control. Hygienic requirements for ensuring the safety of hot water supply systems. The temperature schedule for regulating the heat load is approved by the heat supply organization.
| T outdoor air | T1 | T "3 | T3 | T4 | T "4 | ||
| 150-70 with a surcharge | 150-70 cut at 130 | 120-70 | 105-70 | 95-70 | after the heating system | ||
| after the heating boiler | |||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| 10 | 80 | 70 | 43 | 38 | 37 | 33 | 34 |
| 9 | 80 | 71 | 45 | 41 | 39 | 34 | 35 |
| 8 | 80 | 74 | 47 | 43 | 41 | 35 | 36 |
| 7 | 80 | 75 | 49 | 45 | 42 | 36 | 37 |
| 6 | 80 | 77 | 51 | 47 | 44 | 38 | 39 |
| 5 | 80 | 78 | 53 | 49 | 46 | 39 | 40 |
| 4 | 80 | 79 | 56 | 51 | 48 | 40 | 42 |
| 3 | 80 | 81 | 58 | 53 | 49 | 41 | 43 |
| 2 | 81 | 82 | 60 | 55 | 52 | 42 | 44 |
| 1 | 83 | 84 | 62 | 57 | 53 | 43 | 45 |
| 0 | 85 | 85 | 64 | 59 | 55 | 45 | 47 |
| -1 | 88 | 86 | 67 | 61 | 57 | 46 | 48 |
| -2 | 91 | 88 | 69 | 63 | 58 | 47 | 49 |
| -3 | 93 | 89 | 71 | 65 | 60 | 48 | 50 |
| -4 | 96 | 90 | 73 | 66 | 62 | 49 | 52 |
| -5 | 98 | 92 | 75 | 68 | 64 | 50 | 54 |
| -6 | 101 | 93 | 78 | 70 | 65 | 51 | 54 |
| -7 | 103 | 95 | 80 | 72 | 67 | 52 | 56 |
| -8 | 106 | 96 | 82 | 74 | 68 | 53 | 57 |
| -9 | 108 | 97 | 84 | 76 | 70 | 54 | 58 |
| -10 | 110 | 99 | 87 | 77 | 71 | 55 | 59 |
| -11 | 113 | 100 | 89 | 79 | 73 | 56 | 60 |
| -12 | 116 | 102 | 91 | 81 | 74 | 57 | 61 |
| -13 | 118 | 103 | 93 | 83 | 76 | 58 | 62 |
| -14 | 121 | 105 | 96 | 84 | 78 | 59 | 63 |
| -15 | 123 | 107 | 98 | 86 | 79 | 60 | 64 |
| -16 | 126 | 108 | 100 | 88 | 81 | 61 | 65 |
| -17 | 128 | 112 | 102 | 90 | 82 | 62 | 67 |
| -18 | 130 | 114 | 104 | 91 | 84 | 63 | 69 |
| -19 | 132 | 116 | 107 | 93 | 85 | 64 | 70 |
| -20 | 135 | 118 | 109 | 95 | 87 | 65 | 70 |
| -21 | 137 | 121 | 111 | 96 | 88 | 66 | 72 |
| -22 | 140 | 123 | 113 | 98 | 90 | 67 | 73 |
| -23 | 142 | 125 | 115 | 100 | 91 | 68 | 74 |
| -24 | 144 | 128 | 117 | 102 | 93 | 69 | 74 |
| -25 | 146 | 130 | 119 | 103 | 94 | 69 | 75 |
| -26 | 148 | 130 | 120 | 105 | 95 | 70 | 76 |
| -28 | 150 | 130 | 120 | 105 | 95 | 70 | 76 |
Notation
T 1 (p. 2, 3) - water temperature in the main heating network from the source to the central heating station
T 3 (p. 5, 6) - water temperature in the heating distribution networks to the consumer after the central heating station
T "3 (p. 4) - water temperature in the heating distribution networks to the consumer with an independent connection scheme with an elevator at consumers
T 4 (p. 7) - the temperature of the water in the return pipeline of the heating network from the consumer for networks operating according to temperature charts p. 5, 6
T "4 (n 8) - water temperature after the heating heater in the central heating station with an independent connection scheme
Note:
1. All work schedules of sources and local systems may be different and are determined by the decision of the design and energy supply organization. The scheme for connecting the heating system is selected during design in accordance with the requirements of the rules.
The most important task in the design and operation of heat supply systems is the development of an effective hydraulic regime that ensures reliable operation of heat networks.
Under reliable performance means:
1) ensuring the required pressure in front of subscribers ();
2) exclusion of boiling of the coolant in the supply line;
3) elimination of emptying of heating systems in buildings, which means subsequent airing during restart;
4) exclusion of dangerous overpressure at consumers, causing the possibility of rupture of pipes and heating fittings.
Under hydraulic mode thermal networks understand the relationship between pressures (heads) and coolant flow rates at various points in the network at a given time.
The hydraulic regime of the heating network is studied by building pressure graph (piezometric graph).
The schedule is built after the hydraulic calculation of pipelines. It allows you to visually navigate in the hydraulic mode of operation of heat networks when different mode their work, taking into account the influence of the terrain, the height of buildings, pressure losses in heating networks. Using this graph, you can easily determine the pressure and available pressure at any point in the network and subscriber system, select the appropriate pump equipment pumping stations and a scheme for automatic control of the hydraulic mode of operation of the ITP.
Consider a piezometric graph for a heat network located on a terrain with a calm relief (Fig. 7.1). The plane with the zero mark is aligned with the location mark of the heat treatment plant. Main line profile 1 -2-3 -III aligned with the vertical plane in which the piezometric graph is drawn. At the point 2 branch connected to main 2 -I. This branch has its own profile in a plane perpendicular to the main line. To be able to display a branch profile 2 -I on the piezometric graph rotate it 90° counterclockwise around the point 2 and is compatible with the profile plane of the main line. After the planes are aligned, the branch profile will take the position shown by the line on the graph 2 - . Similarly, we build a profile for a branch 3 - .
Consider the operation of a two-pipe heat supply system, the schematic diagram of which is shown in fig. 7.1, in. From the heat treatment plant T, high-temperature water c enters the supply heat pipeline at the point P1 with full head in the supply manifold of the heat supply source 
(here is the initial total head after the network pumps (point K); - pressure loss of network water in the heat treatment plant). Since the geodetic mark of the installation of network pumps, the total pressure at the beginning of the network is equal to the piezometric pressure and corresponds to the overpressure in the collectors of the heat supply source. Hot water along the supply line 1-2-3-III and branches 2-I and 3-II enters the local systems of heat consumers I, II, III. The total pressures in the supply line and branches are shown in the head graphs P1-PIII,P2-PI,P3-PII. The cooled water is sent to the heat source through return pipelines. Graphs of total pressures in the return heat pipes are shown by lines OIII-O1, OII- O3, OI-O1.
The difference in pressure in the supply and return lines for any point in the network is called available pressure. Since the supply and return pipelines at any point have the same geodetic elevation, the available head is equal to the difference between the total or piezometric heads:
For subscribers, the available pressures are equal: ;
; 
. The total pressure at the end of the return line in front of the network pump on the return manifold of the heat supply source is . Therefore, available
pressure in the collectors of the heat treatment plant
Network pump increases the pressure of the water coming from the return line and directs it to the heat treatment plant, where it is heated to . The pump develops pressure.
Rice. 7.1. Piezometric graph (a), single line piping diagram (b) and a diagram of a two-pipe heating network (in)
I-III- subscribers; 1, 2, 3 - nodes; P- supply line; O - return line; H- pressure; T- heat treatment plant; SI- network pump; RD- pressure regulator; D- impulse selection point for RD; Mon- make-up pump; B - make-up water tank; DK - drain valve.
The pressure loss in the supply and return lines is equal to the difference in the total pressure at the beginning and end of the pipeline. For the supply line they are equal 
, and for the reverse 
.
The described hydrodynamic regime is observed during the operation of the network pump. Position of the piezometric return line at the point O1 maintained constant as a result of work make-up pump PN and pressure regulator RD. The pressure developed by the make-up pump at hydrodynamic mode, throttled by valve RD in such a way that at the point of selection of the pressure pulse D from the bypass line of the network pump, a head is maintained equal to the total head developed by the make-up pump.
On fig. 7.2 shows a graph of pressures in the make-up line and in the bypass line, as well as circuit diagram feeding device.
Rice. 7.2. Chart of pressure in the make-up line 1 -2 and in the bypass line of the network pump 2 -3(a) and power supply diagram (b):
H- piezometric heads; - pressure loss in the throttle bodies of the pressure regulator RD and in valves A and B; SN, MON- network and make-up pumps; DC- drain valve; B- make-up water tank
In front of the make-up pump, the total pressure is conditionally taken zero. make-up pump Mon develops pressure. This pressure will be in the pipeline to the pressure regulator RD. Loss of pressure due to friction in sections 1
-2
and 2
-3
neglected due to their smallness. In the bypass line, the coolant moves from the point 3
to the point 2.
In gate valves BUT and AT all the pressure developed by the network pump is used. The degree of closing of these valves is regulated in such a way that in the valve BUT the pressure was worked out and the total pressure after it was equal to 
.
in the valve AT pressure works out ,
and 
(here -
pressure after RD). The pressure regulator maintains a constant pressure at the point D between valves BUT and AT. At the same time, at the point 2
pressure will be maintained, and on the valve RD pressure will be generated.
With an increase in the leakage of the coolant from the network, the pressure at the point D starts to drop, valve RD opens a little, the supply of the heating network increases and the pressure is restored. When the leakage is reduced, the pressure at the point D starts to rise and the valve RD covered up. If the valve is closed RD the pressure will continue to increase, for example, as a result of an increase in the volume of water with an increase in its temperature, the drain valve will turn on DK, maintaining a constant pressure "to itself" at the point D, and dump excess water into the drain. This is how the make-up device works in hydrodynamic mode. When the network pumps stop, the circulation of the coolant in the network stops and the pressure in the entire system drops down to . pressure regulator RD opens and the feed pump Mon maintains a constant pressure throughout the system.
Thus, in the second characteristic hydraulic regime - static- at all points of the heat supply system, a full pressure is established, developed by the make-up pump. At the point D both in hydrodynamic and static modes, a constant pressure is maintained. Such a point is called neutral.
Due to the high hydrostatic pressure created by the water column, and high temperature transported water there are strict requirements for the allowable pressure range in both the supply and return pipelines. These requirements impose restrictions on the possible location of piezometric lines in both static and hydrodynamic modes.
To exclude the influence of local systems on the pressure regime in the network, we will assume that they are connected according to an independent scheme, in which the hydraulic regimes of the heating network and local systems are autonomous. Under such conditions, the following requirements are imposed on the pressure regime in the network.
During the operation of the heating network and when developing a schedule of piezometric pressures, the following conditions(both in dynamic and static modes), which are listed in the order in which they are checked when plotting.
1. The piezometric head in the network return must be higher than the static level of the connected systems (building heights N zd) by at least 5 m(reserve), otherwise return pressure H arr will be less than the static pressure of the building N zd and the water level in the buildings will be set at the height of the pressure of the reverse piezometer, and a vacuum will appear above it (the system will be exposed), which will cause air to leak into the system. On the graph, this condition will be expressed by the fact that the line of the inverse piezometer must pass 5 m above the building:
N arr N zd + 5 m; N st N zd + 5 m.
2. At any point in the return line, the piezometric pressure must be at least 5 m so that there is no vacuum and air leakage into the network (5 m- reserve). On the graph, this condition is expressed by the fact that the piezometric return line and the static pressure line at any point in the network must go at least 5 m above ground level:
N arr N s + 5 m; N st N s + 5 m.
3. The suction head of network pumps (feed pressure But) must be at least 5 m to ensure that the pumps are filled with water and that there is no cavitation:
But 5 m.
4. The water pressure in the heating system must be less than the maximum allowable, which can withstand heating appliances (6 kgf / cm 2). On the graph, this condition is expressed by the fact that at the inputs to the buildings, the piezometric pressures in the return line and the static level of the network should not be higher than N add \u003d 55 m(with a margin of 5 m):
N arr - N s 55 m; N st - N s 55 m.
5. In the supply pipeline to the elevator, where the water temperature is higher , pressure must be maintained not less than the boiling pressure of water at the temperature of the coolant - taken with a margin; (this is not necessary for the static level):
Hs=20 m at and Hs=40 m at .
On the graph, this condition will be expressed by the fact that the pressure line in the supply pipeline should be, respectively, by the value Hs above the highest point of superheated water in the heating system (for residential buildings this will be ground level, and for industrial buildings-highest point of superheated water in workshops):
H under H s + 5 m.
6. The static level of local systems (the level of the top of buildings) should not create in the systems of other buildings a pressure greater than the maximum allowable for them, otherwise, when the network pumps stop, the devices of these systems will be crushed due to the water pressure of high-lying buildings. On the graph, this condition will be expressed by the fact that the levels of high-lying buildings should not exceed more than 55 m ground levels of other buildings.
7. The pressure at any point in the system should not exceed the maximum allowable strength of equipment, parts and fittings. Usually take the maximum overpressure R additional=16…22 kgf / cm 2. This means that the piezometric head at any point of the supply pipeline (from ground level) must be at least N additional - 5 m(with margin5 m):
N under - N s N additional - 5 m.
8. The available pressure (the difference between the piezometric pressures in the supply and return pipelines) at the entrances to the buildings must be at least the pressure loss in the subscriber's system:
N r \u003d N under - N arr N zd.
Thus, the piezometric graph makes it possible to ensure an effective hydraulic regime of the heating network and to select pumping equipment.
1. State the main tasks of choosing the pressure regime of water heating networks from the condition of the reliability of the heat supply system.
2. What are the hydrodynamic and static modes of operation of the heating network? Justify the conditions for determining the position of the static level.
3. Present a technique for constructing a piezometric graph.
4. State the requirements for determining the position on the piezometric graph of the pressure lines in the supply and return lines of the heating network.
5. On the basis of what conditions are the levels of permissible maximum and minimum piezometric pressures for the supply and return lines of the heat supply system plotted on the piezometric graph?
6. What is the “neutral” point” on the piezometric graph and what device is used to regulate its position at the CHP or boiler house?
7. How is the operating pressure of network and make-up pumps determined?