Heat release schedule. Temperature schedules of central quality regulation regimes for heat supply to consumers and their use in heat supply. Regulation of heat supply

Graphs of temperature and water flow in the heat network and the local heating system with qualitative and quantitative regulation of heat supply for the heating complex with elevator node are shown in Fig. 5.3.

With a surface heat exchanger and a pump unit, the types of heat management in the local heating system and the parameters of the network water entering the heat exchanger can be the same or different. So, in a local heating system, qualitative regulation can be carried out with quantitative regulation of the flow of network water. With such heat exchangers at the input, interruptions in the supply of network water to the user's heat exchanger do not stop the circulation of water in the local heating system, whose appliances continue to give the rooms some time to heat accumulated in the water and pipelines of the local system.

With elevator fills with a constant mixing ratio, qualitative regulation of the network water parameters leads to a qualitative regulation of the local water parameters, and a purely quantitative regulation of the network water entering the elevator leads not only to a proportional change in the flow of water in the local system, but also to a change in temperature] local water, i.e., leads to a quantitative and qualitative change in the water parameters of the local heating system. Stopping the supply of network water to the elevator causes an immediate cessation of water circulation in the local heating system and, accordingly, a rapid cessation of heat supply to the heated rooms.

Fig. 5.3. Graphs of the temperatures (a) and relative costs (b) of water in the heat network and the local heating system with qualitative and quantitative regulation of heat supply

1, 1 '- water temperature in the supply pipeline of the heat network, respectively, with qualitative and quantitative regulation; 2, 2'- water temperature in the local heating system, respectively, with qualitative and quantitative regulation; 3, 3'- return water temperature, respectively, with qualitative and quantitative regulation; 4.4 "- relative water flow, respectively, with qualitative and quantitative regulation

Let's consider some features of regulation of heat supply for heating. The main feature is that in a heat-supply area there can be buildings with different values ​​of relative internal heat release relative to heat losses through external fences. Consequently, at the same outdoor temperature, different networked buildings have to supply network water at different temperatures, which is almost impossible. In these conditions, the most rational is the assignment of water temperatures in the network for the consumption of heat for heating residential buildings. This is explained by the following reasons: first, for residential buildings account for up to 75% of the total heat consumption for heating residential and public buildings of urban development, and secondly, accounting for internal heat in residential buildings can reduce the annual heat consumption by their heating by 10% . For those public buildings, the relative internal heat dissipation in which, while people are in them, is less than in residential buildings, the insufficient temperature of the water in the heat network must be compensated by an increase in the flow of the network water.

Active regulation of heat supply (subscriber, instrument, etc.) should only reduce the heat transfer of heating dribors in comparison with its normalized value, but in no case should it exceed this value. This is due to the fact that at present centralized heat supply is calculated for a limited heat output for heating (in the amount necessary to maintain the standard value of air temperature in the heated rooms). With this restriction, any over-consumption of heat by one of the subscribers of the heat supply system- or one of the devices of the local heating system - entails a shortage of heat by another subscriber or other device.

Theoretical substantiation of the method of hydraulic calculation of pipelines of water heating networks (application of the Darcy equation, limiting Reynolds number, practical coolant velocities, hydraulic operation mode).

As a result of the hydraulic calculation of the heat network, the diameters of all sections of heat pipes, equipment and shut-off valves are determined, as well as the loss of pressure of the coolant on all elements of the network. From the obtained values ​​of pressure losses, the heads are calculated, which the pumps of the system must develop. The pipe diameters and frictional pressure losses (linear losses) are determined by the Darcy formula

(7.1)

where - pressure loss for friction (linear), Pa; - coefficient of friction; l, d- length and diameter of the pipeline section, m; w is the flow rate, .m / s; - heat carrier density, kg / m 3.

If the energy of the flow, J, is attributed to the unit of force, H, we obtain a formula for calculating the head loss, m. For this, all terms of equation (7.1) should be divided by the specific weight, N / m 3:

(7.2)

The coefficient of friction depends on the fluid motion regime, the roughness of the inner surface of the pipe and the height of the roughness ridges k.

The motion of the coolant in water and steam networks is characterized by turbulent regime. At relatively low values ​​of the Reynolds number (2300

(7.3)

With the development of flow turbulence, the thickness of the laminar layer decreases, the roughness protrusions begin to rise above it and resist flow movement. With this in the flow, both viscous and inertial hydraulic resistance are observed. The latter is due to the failure of turbulent eddies with ridges of roughness. Turbulent vortices have inertial resistance to acceleration, which arises due to their movement to a zone of high velocities to the axis of the flow.

The considered modes of motion refer to the transient turbulent regime. The established turbulent regime is characterized by a quadratic law of resistance when the resistance is due to the presence of inertial forces and does not depend on the viscosity of the fluid. The coefficient of friction for this regime is calculated by the formula of BL Shifrinson:

(7.4)

where k e is the absolute equivalent uniformly-grained roughness, which creates a hydraulic resistance equal to the actual resistance of the pipeline; k e / d is the relative roughness.

  The limiting Reynolds number, which separates the transient and steady turbulent regimes, is

At Re\u003e Re np, the quadratic law of resistance is observed. Let us determine the limiting velocity of water motion, which corresponds to the quadratic law of resistance. The maximum water flow in the heat networks corresponds to the break point of the temperature profile, so the limit regime is calculated for the water temperature t-70 ° C, at which v = 0.415-10 -6 m 2 / s. Equivalent roughness for water networks k e = 0.0005 m. Then:

The speed of water movement in heat pipes usually exceeds 0.5 m / s, therefore, in most cases they operate in the region of quadratic mode.

The limiting velocity of the medium-pressure steam, corresponding to the boundary of the region of the quadratic law of resistance, is determined at a pressure p = 1.28 MPa (absolute). At this pressure, the saturation temperature t = 190 ° C, and the kinematic viscosity = 2.44-10 -6 m 3 / s. The limiting speed for k e = 0.0002 m will be:

In steam pipelines, the speed is usually greater than 7 m / s, hence they also operate in the quadratic mode.

For saturated low-pressure steam at t = 115 ° C, p = 0.17 MPa (absolute) and = 13.27-10 -6 m 2 / s, the limiting speed is, respectively:

This speed is close to the maximum in the steam pipelines, therefore low-pressure steam pipes work mainly in the field of hydraulically smooth pipes.

Calculation of the hydraulic resistance for transients and steady-state turbulent regimes can be carried out using the universal formula of AD Al'tshul:

(7.5)

At Re k e / d<<68 эта формула совпадает с формулой Блазиуса (7.3), а при Re k э /d>\u003e 68 it coincides with the formula of BL Shifrinson (7.4).

In hydraulic calculations, the following values ​​of the absolute equivalent roughness of the inner surface of the pipes are assumed:

Heating network  Steam Water Hot Water and Condensate Pipes

k e, m. 0,0002 0,0005 0,001

20 Tasks and general provisions of the methodology of engineering hydraulic calculation of pipelines of heating networks. Determination of design coolant flow and head losses in branched water heating networks in accordance with the requirements of SNiP 2.04.07-86 *.

Calculated water costs for all sections of the branched network are determined uniquely, depending on the estimated flow rates of the coolant in consumers. Possible pressure losses in the heat networks depend on the pressure developed by the circulating pumps adopted for installation, and can be very different. Thus, in the formulation of the problem of hydraulic calculation there is uncertainty, for the elimination of which it is necessary to add additional conditions. Such conditions are formulated from the requirements of the maximum economic efficiency of the heat supply system, which determine the tasks of the technical and economic calculation of heat pipes. Consequently, the techno-economic calculation is organically linked to hydraulic calculation and allows one to uniquely calculate the diameters of all elements of the heat network using hydraulics formulas.

  The main point of the technical and economic calculation of heat pipes is as follows. From the adopted diameters of the elements of the heat network, the hydraulic losses in them depend. The smaller the diameters, the greater the loss. With the reduction of diameters, the cost of the system decreases, which increases its economic efficiency. But with the growth of losses, the pressure that the pumps have to develop increases, and with increasing pressure, their cost and energy spent on pumping the coolant increase. Under such conditions, when with a change in diameters one group of cost indicators decreases and the other increases, there always exist optimal diameters for which the total cost of the network will be minimal.

In this paragraph, the hydraulic calculation of the heat network is considered according to an approximate method, when the values ​​of the specific pressure loss for friction, recommended SNiP, are used to select the diameters of heat pipes.

Fig. 7.4. Heat network layout

1,2, ... .., 7 - number of parcels

The calculation is carried out in the following order:

1) first calculate the main line. Diameters are selected according to the average hydraulic gradient, taking specific frictional pressure losses to 80 Pa / m, which gives a solution close to economically optimal. When determining pipe diameters, the value of k e is equal to 0.0005 m and the velocity of the coolant is not more than 3.5 m / s;

2) after determining the diameters of the sections of the heat main, the sum of the coefficients of local resistances is calculated for each section using the heat network scheme, the data on the location of the valves, compensators and other resistances, and the values ​​of the local resistance coefficients. For each site, the length equivalent to local resistance is found at = 1 and the equivalent length k e for this section is calculated. After the determination of l er, the calculation of the heat main is completed and the head losses in it are determined. On the basis of the head loss in the supply and return lines and the necessary head pressure at the end of the line, which is assigned taking into account the hydraulic stability of the system, the necessary pressure is determined on the output manifolds of the heat source;

3) the branches are calculated using the remaining head, provided that at the end of each branch the necessary available head is maintained and the specific pressure loss for friction does not exceed 300 Pa / m. Equivalent lengths and head losses in the sections are determined in the same way as for the main line.

Method of hydraulic calculation of steam pipelines of heating networks: determination of pipeline diameters, calculation of head losses, recommended speeds, consideration of the influence of steam density on hydraulic losses, the structure of tables and nomograms.

Energy losses during fluid flow through pipes are determined by the motion regime and the nature of the internal surface of the pipes. The properties of a liquid or gas are taken into account in calculation using their parameters: density and kinematic viscosity. The formulas used to determine the hydraulic losses, for both liquid and vapor, are the same.

A distinctive feature of the hydraulic calculation of the steam pipeline is the need to take into account the changes in the vapor density when determining the hydraulic losses. When calculating gas pipelines, the gas density is determined depending on the pressure according to the equation of state written for ideal gases, and only at a high pressure (greater than about 1.5 MPa) is introduced into the equation a correction factor that takes into account the deviation of the behavior of real gases from the behavior of ideal gases.

When using the ideal gas laws to calculate the pipelines along which saturated steam moves, significant errors are obtained. The laws of ideal gases can only be used for highly superheated steam. When calculating steam lines, the vapor density is determined depending on the pressure from the tables. Since the vapor pressure in turn depends on the hydraulic losses, the steam pipelines are calculated by the method of successive approximations. First, the pressure losses in the section are set, the average vapor pressure is determined from the average pressure and then the actual pressure loss is calculated. If the error is not valid, recalculate.

When calculating steam networks, the steam flow, its initial pressure and the required pressure before the installations using steam are set. The method of calculating steam pipelines is given in the example.

Example 7.2. Calculate the steam line (Figure 7.5) with the following initial data: initial steam pressure when exiting the heat source Р н = 1.3 MPa (redundant); saturated steam; the final vapor pressure of consumers p k = 0,7 MPa; steam consumption by consumers, t / h: D 1 = 25; D II = 10 ;, D III = 20; D IV = 15; length of sections, m: l 1-2 = 500; l 2-3 == 500; l 3-4 = 450; l 4- IV = 400; l 2- I = 100; l 3- II = 200; l 4- III = 100.

  1. We determine the approximate value of the specific friction losses on the sections from the heat source to the most remote consumer IV:

Here is the total length of sections 1-2-3-4-IV; a is the fraction of pressure losses in local resistances, taken equal to 0.7 for both a line with U-shaped compensators with welded outlets and assumed diameters of 200-350 mm.

2.Calculate the plot 1-2. The initial pressure in the section p 1 = 1.4 MPa (absolute). The density of saturated vapor at this pressure is determined. according to the tables of water vapor, = 7, l kg / m 3. We are given the final pressure in the region p 2 = 1.2 MPa (absolute). At this pressure = 6.12 kg / m 3. Average density of steam in the area:

Spray consumption in the section 1-2: D l -2 = 70 t / h = 19.4 kg / s. According to the assumed specific pressure loss of 190 Pa / m and a flow rate of 19.4 kg / s according to the nomogram in Fig. 7.1 we find the diameter of the steam pipe. Since the nomogram is composed for a vapor with a density p n-1 = 2.45 kg / m 3, we first recalculate the specific pressure drop by tabular density:

  For the values ​​(= 513 Pa / m and D 1-2 = 19.4 kg / s, we find the diameter of the steam pipe d 1-2 = 325x8 mm () = 790 Pa / m. The speed of the steam w m = 107 m / s. actual pressure loss and steam velocity:
Speed ​​is recalculated similarly:

We determine the sum of the coefficients of local resistances in Section 1-2 (see Table 7.1):

Gate valve .......... 0.5

Compensator U-shaped with welded taps (3 pieces) ............. 2.8-3 = 8.4

Tee at flow separation (passage). . .1

The value of the equivalent length at = l for k e = 0.0002 m for a pipe with a diameter of 325x8 mm according to Table. 7.2 l e = 17.6 m, therefore, the total equivalent length for the section 1- 2: 1 e = 9.9 * 17.6 = 174 m.

The reduced length of section 1-2: l Ex.1-2 = 500 + 174 = 674 m.

The loss of pressure on friction and in local resistance in the section 1-2:

The vapor pressure at the end of section 1-2:

which is practically equal to the previously accepted value of 1.2 MPa. The average density of the para is also equal to 6.61 kg / m 3. In this regard, we do not make a recount. If there is a significant deviation of the obtained value of the average vapor density from the previously adopted value, we recalculate.

The remaining sections of the steam pipeline are calculated in the same way as section 1-2. The results of all calculations are summarized in Table. 7.7. We calculate the equivalent lengths of local resistances in the same way as in Example 7.1.

Hydraulic mode and reliable operation of heat networks. Theoretical justification and methodology for constructing a piezometric graph, calculation of the required heads of network and make-up pumps.

Because of the high density, water exerts considerable hydrostatic pressure on the pipes and equipment, so the hydraulic calculation of the water heating systems includes two parts: the first is the actual hydraulic calculation, in which the diameters of the heat pipes are determined, and the second is checking the compliance of the hydraulic regime with the requirements.

Check mode in the static state of the system (hydrostatic mode), when the circulating pumps do not work, and with the dynamic state of the system (hydrodynamic mode), taking into account the geodetic height of the pipeline. As a result, the maximum pressure lines in the supply and return heat pipes are determined from the condition of mechanical strength of the system elements and the minimum pressure line from the condition of preventing effervescence of the high-temperature coolant and the formation of a vacuum in the elements of the system. The piezometric lines of the projected object should not extend beyond these extreme limits. When developing the hydrodynamic mode of the heat network, the parameters for selecting circulating pumps are revealed, and in the development of the hydrostatic regime, for selecting the make-up pump.

With the hydraulic calculation of steam networks, due to the low vapor density, the difference in elevation marks of individual points of the steam pipe is neglected.

Piezometric graphs are widely used to study the pressure regime in heat networks and local building systems. On the charts at a certain scale, the relief of the terrain is plotted along the thermal traces, indicates the height of the attached buildings, shows the head in the feed and return lines of the heat pipes and in the equipment of the heat-preparation plant. The role of the piezometric graph in the development of hydraulic regimes of heat supply systems is very high, since it allows to visually show the permissible pressure limits and their actual values ​​in all elements of the system.

Consider the graph of heads in the heat pipeline laid underground (Figure 8.1). In settlements, the heating networks are buried about 1 m. In view of the small depth at drawing out the profile of the heat pipe route, its axis is conventionally combined with the ground surface.

A horizontal OO plane passing through the zero mark is taken as the horizontal reference plane. All geodetic marks of the profile of the route correspond to the scale indicated on the scale on the left. Thus, the value of z i indicates the geodesic height of the pipeline axis at point i above the reference plane.

The notion of reliability  reflects two main approaches to assessing the performance of a device or system. The first is a probabilistic evaluation of the system's operability. The need for probabilistic estimation is due to the fact that the duration of the operation of the elements of the system is determined by a number of random factors, it is impossible to foresee the effect of which on the operation of the element. Therefore, the deterministic estimation of the operating time of an element is replaced by a probabilistic estimate, i.e., the law of distribution of work time. Accounting for work time is the second main approach to assessing the health of the system. Reliability is the preservation of qualities by an element or system in time. In accordance with these basic properties of the concept of reliability, its main criterion is the probability of failure-free operation of the system (element) P during a given period t.

Fig. 8.1. Graph of pressure in the heat pipe

1 - full pressure line without taking into account friction losses; 2-line of full pressure head without loss of friction and high-speed head; 3 - full pressure line with allowance for friction losses; 4-line full pressure taking into account friction losses and without taking into account the high-speed head; 5 - axis of the heat-conductor.

According to GOST, reliability is defined as a property of the system to perform specified functions while maintaining the specified performance indicators during the accepted operating time. For heat supply, a given function is to supply consumers with a certain amount of water with a specified temperature and pressure and a certain degree of purification.

There are two ways to create reliable systems. The first way is to improve the quality of the elements that make up the system; the second is the redundancy of the elements. Increase reliability, realizing first of all the first way. But, when the technical possibilities for improving the quality of the elements are exhausted or when further quality improvement turns out to be economically unprofitable, they follow the second path. The second way is necessary when the reliability of the system must be higher than the reliability of the elements of which it consists. Increase of reliability is achieved by reservation. For heat supply systems, duplication is used, and for thermal networks, duplication, ringing and sectioning are used.

Reliability is characterized by longevity - the property to remain operative up to the limit state with permissible interruptions or without them during maintenance and repairs. Heat supply systems are durable systems.

Heat supply systems are repairable systems, therefore they are characterized by maintainability, which is the ability of the system to prevent, detect and eliminate failures and failures through maintenance and repairs. The main indicator of maintainability of heat supply systems is the time of restoration of the failed element trem. The recovery time is of great importance in justifying the need for redundancy of the system. It mainly depends on the diameters of pipelines and network equipment. With small diameters, the repair time may be less than the permissible interruption of heat supply. In this case, there is no need for a reservation.

In order to assess the reliability of a system, it is first of all necessary to formulate precisely the concept of failure of an element and a system. When formulating the concept of failure of an element of a heat network, they start with the suddenness and duration of a break in the heat supply of consumers. A sudden failure of an element is a violation of its operability, when the failing element must immediately be shut down from work. With a gradual refusal at first, it is possible to carry out a preliminary repair of the element without disturbance or with a permissible violation of heat supply, transferring the full repair repair for a while, when its shutdown does not lead to a failure of the system.

When calculating system reliability and determining the degree of redundancy, only sudden failures should be considered.

Thus, the failure of the element, taken into account in calculating the reliability of heat supply systems, is a sudden failure, provided that t rep\u003e t d o n. Such a failure in non-redundant systems results in system failure, and for redundant systems - to change the hydraulic mode of operation.

The reasons for the failures associated with breaking the strength of the elements are random coincidences of overloads in the weakened places of the elements. Both the overloading of elements and their weakening are determined by the values ​​of a number of independent random variables. For example, a decrease in the strength of a welded seam may be due to lack of penetration, the presence of slag inclusions and other causes, which in turn depend on the qualification of the welder, the quality of the electrodes used, the welding conditions, etc. Thus, failures are of a random nature.

The study of failures associated with the corrosion of pipelines, a violation of the operability of equipment, also leads to the conclusion that their nature is random. At the same time, the coincidence of a number of random factors that can lead to rejection is a rare event, therefore refusals are classified as rare events.

Thus, the main properties of failures considered in the calculation of reliability are that they are random and rare events. If the failure of the performance of the element is not an accidental event, it can be provided for in the calculations.

The task of heat supply systems is to provide the required levels of parameters for consumers, under which comfortable living conditions of people are achieved. Emergency failures violate the heat supply of residential and public buildings, as a result of which the working and leisure conditions of the population are unacceptably worsening, which causes social consequences. To these consequences, first of all, the very fact of violation of normal working conditions and people's lives, which leads to an increase in the number of people's diseases, to a decline in their performance. Social consequences can not be estimated economically. At the same time, their importance is very high, therefore, in the methodology for assessing the reliability of heat supply systems, the social consequences of interruptions in the supply of heat must be taken into account.

Considering the foregoing, in assessing the reliability of heat supply, one should proceed from the principle inadmissibility of failures, considering that the failure of the system leads to irreparable consequences for the fulfillment of the task.

As noted above, the damage to the sections of heat pipes or network equipment that lead to the need for their immediate disconnection are regarded as failures. The following damage to the elements of heat networks leads to failures:

1) pipelines: through corrosion damage pipes; breaks in welds;

2) gate valves: corrosion of the valve body or bypass; bending or falling of discs; loose flange connections; clogging leading to leakage of sections;

3) gland compensators: corrosion of the glass; failure of the packing plates.

All the above-mentioned damages occur during operation as a result of exposure to a number of unfavorable factors. The causes of some damage are construction defects.

The most common cause of damage to heat pipes is external corrosion. The amount of damage associated with the rupture of longitudinal and transverse welds of pipes is much less than that of corrosion. The main reasons for the rupture of welded seams are factory defects in the manufacture of pipes and defects in pipe welding during construction.

The reasons for the damages of the valves are very diverse: these are external corrosion, and various problems that arise during operation (clogging, jamming and falling of disks, flange connections).

All the above causes, causing damage to network elements, are a consequence of the impact on them of various random factors. If there is a damage to the pipeline section, it is switched off, repaired and re-commissioned. Over time, it may appear a new damage, which will also be repaired. The sequence of the resulting faults (failures) on the elements of the heat network is the flow of random events - the flow of failures.

THEM. Saprykin, the chief technologist,
LLC PTC "Energy Technologies", Nizhny Novgorod

Introduction

In heat supply systems, there are very significant reserves of saving heat energy resources, in particular thermal and electric energy.

Recently, a lot of new high-efficiency equipment and technologies have appeared on the market aimed at improving the comfort and efficiency of heat supply systems. Correct application of innovations makes high demands on the engineering corps. Unfortunately, the reverse is happening with the engineering staff: a decrease in the number of qualified specialists in the field of heat supply.

For the identification and best use of savings reserves, including knowledge of the laws governing the regulation of heat supply. In the technical literature, the practical application of regimes for regulating heat release has not been given due attention. In this article, an attempt is made to fill this gap, while a slightly different approach is proposed to the formation of basic equations describing regimes for regulating heat release than those set forth in the technical literature, for example.

Description of the proposed methodologies

It is known that the laws governing the heating loads of buildings can be obtained from a system of three equations describing the heat loss of the building through the enclosing structures, the heat transfer of heating appliances in the building and the supply of heat through the heating networks. In a dimensionless form, this system of equations looks as follows)