How to increase the speed of the coolant in the pipeline. Hydraulic calculation of the heating system, taking into account pipelines
Heat Supply News magazine No. 1, 2005, www.ntsn.ru
Ph.D. O.D. Samarin, Associate Professor, Moscow State University of Civil Engineering
The currently existing proposals regarding the optimal speed of water movement in pipelines of heat supply systems (up to 3 m/s) and permissible specific pressure losses R (up to 80 Pa/m) are based mainly on technical and economic calculations. They take into account that with increasing speed, the sections of pipelines decrease and the volume of thermal insulation decreases, i.e. capital investments in the network device are reduced, but at the same time, operating costs for pumping water increase due to an increase in hydraulic resistance, and vice versa. Then the optimal rate corresponds to the minimum of the reduced costs for the estimated depreciation period of the system.
However, in a market economy, one should definitely take into account the discounting of operating costs E (rubles/year) and capital costs K (rubles). In this case, the formula for calculating the total discounted costs (SDZ), when using borrowed funds, takes the following form:
In this case, the discount coefficients for capital and operating costs, calculated depending on the estimated depreciation period T (years), and the discount rate p. The latter takes into account the level of inflation and investment risks, i.e., ultimately, the degree of instability of the economy and the nature of changes in current tariffs, and is usually determined by the method of expert assessments. As a first approximation, the value of p corresponds to the annual interest for a bank loan. In practice, it can be taken in the amount of the refinancing rate of the Central Bank of the Russian Federation. Starting from January 15, 2004, it is equal to 14% per annum.
Moreover, it is not known in advance that the minimum SDZ, taking into account discounting, will correspond to the same level of water velocity and specific losses that are recommended in the literature. Therefore, it is advisable to carry out new calculations using the current range of prices for pipelines, thermal insulation and electricity. In this case, if we assume that the pipelines operate under the conditions of a quadratic resistance mode, and calculate the specific pressure losses using the formulas given in the literature, the following formula can be obtained for the optimal water flow rate:
Here K ti is the coefficient of rise in the cost of pipelines due to the presence of thermal insulation. When using domestic materials such as mineral wool mats, K ti = 1.3 can be taken. Parameter C D is the unit cost of one meter of pipeline (rubles/m2) divided by inner diameter D (m). Since the price lists usually indicate the price in rubles per ton of metal C m, the recalculation must be carried out according to the obvious ratio, where is the thickness of the pipeline wall (mm), \u003d 7.8 t / m 3 - the density of the pipeline material. The value of C el corresponds to the tariff for electricity. According to OAO Mosenergo, for the first half of 2004 for communal consumers C el = 1.1723 rubles/kWh.
Formula (2) is obtained from the condition d(SDZ)/dv=0. The determination of operating costs was carried out taking into account the fact that the equivalent roughness of the walls of pipelines is 0.5 mm, and the efficiency of network pumps is about 0.8. Water density p w was considered equal to 920 kg/m 3 for the typical temperature range in the heating network. In addition, it was assumed that circulation in the network is carried out all year round, which is quite justified, based on the needs of hot water supply.
An analysis of formula (1) shows that for long depreciation periods T (10 years and more), typical for heating networks, the ratio of discount coefficients is practically equal to its limiting minimum value p/100. In this case, expression (2) gives the lowest economically feasible water velocity corresponding to the condition when the annual interest for a loan taken for construction is equal to the annual profit from reducing operating costs, i.e. with an infinite payback period. At the end time, the optimal speed will be higher. But in any case, this rate will exceed the calculated one without discounting, since then, as it is easy to see, , but in modern conditions, it turns out 1/T< р/100.
The values of the optimal water velocity and the appropriate specific pressure losses corresponding to them, calculated by expression (2) at an average level of C D and a limiting ratio, are shown in Fig.1. It should be borne in mind that formula (2) includes the value D, which is not known in advance, therefore, it is first advisable to set the average velocity value (of the order of 1.5 m/s), determine the diameter from the given water flow rate G (kg/h), and then calculate the actual speed and the optimal speed from (2) and check if v f is greater than v opt. Otherwise, reduce the diameter and repeat the calculation. It is also possible to obtain the relation directly between G and D. For the average level C D, it is shown in fig. 2.
Thus, the economically optimal water velocity in heating networks, calculated for the conditions of a modern market economy, in principle does not go beyond the limits recommended in the literature. However, this rate is less dependent on the diameter than when the condition for allowable specific losses is met, and for small and medium diameters, higher values of R up to 300 - 400 Pa/m turn out to be appropriate. Therefore, it is preferable to further reduce capital investments (in
in this case - to reduce the cross section and increase the speed), and thus more the higher the discount rate. Therefore, the desire to reduce one-time costs in the construction of engineering systems, which is present in a number of cases in practice, receives a theoretical justification.
Literature
1. A.A. Ionin et al. Heat supply. Textbook for high schools. - M.: Stroyizdat, 1982, 336 p.
2. V. G. Gagarin. Cost recovery criterion for increasing the thermal protection of building envelopes in various countries. Sat. report conf. NIISF, 2001, p. 43 - 63.
Individual hydronic heating systems
In order to correctly carry out the hydraulic calculation of the heating system, it is necessary to take into account some operational parameters of the system itself. This includes the speed of the coolant, its flow rate, hydraulic resistance stop valves and pipeline, inertia and so on.
It may seem that these parameters have nothing to do with each other. But this is a mistake. The connection between them is direct, so it is necessary to rely on them in the analysis.
Let's take an example of this relationship. If you increase the speed of the coolant, then the resistance of the pipeline will immediately increase. If you increase the flow, then the speed of hot water in the system increases, and, accordingly, the resistance. If you increase the diameter of the pipes, then the speed of the coolant decreases, which means that the resistance of the pipeline decreases.
The heating system includes 4 main components:
- Boiler.
- Pipes.
- Heating appliances.
- Shut-off and control valves.
Each of these components has its own resistance parameters. Leading manufacturers must indicate them, because the hydraulic characteristics may vary. They largely depend on the shape, design, and even on the material from which the components of the heating system are made. And it is these characteristics that are the most important when conducting a hydraulic analysis of heating.
What is hydraulic performance? These are specific pressure losses. That is, in every form heating element, whether it is a pipe, a valve, a boiler or a radiator, there is always resistance from the side of the structure of the device or from the side of the walls. Therefore, passing through them, the coolant loses its pressure, and, accordingly, its speed.
Coolant consumption
Coolant consumption
To show how the hydraulic calculation of heating is carried out, let's take for example a simple heating scheme, which includes a heating boiler and heating radiators with a kilowatt heat consumption. And there are 10 such radiators in the system.
Here it is important to correctly divide the entire scheme into sections, and at the same time strictly adhere to one rule - in each section, the diameter of the pipes should not change.
So, the first section is a pipeline from the boiler to the first heater. The second section is a pipeline between the first and second radiator. And so on.
How does heat transfer occur, and how does the temperature of the coolant decrease? Getting into the first radiator, the coolant gives off part of the heat, which is reduced by 1 kilowatt. It is in the first section that the hydraulic calculation is made under 10 kilowatts. But in the second section it is already under 9. And so on with a decrease.
Please note that this analysis is carried out separately for the supply and return circuits.
There is a formula by which you can calculate the flow rate of the coolant:
G \u003d (3.6 x Qch) / (with x (tr-to))
Qch is the calculated thermal load site. In our example, for the first section it is 10 kW, for the second 9.
c - specific heat capacity of water, the indicator is constant and equal to 4.2 kJ / kg x C;
tr - coolant temperature at the entrance to the site;
to - coolant temperature at the exit from the site.
Coolant speed
Schematic calculation
There is a minimum speed of hot water inside the heating system, at which the heating itself works optimally. This is 0.2-0.25 m / s. If it decreases, then air begins to be released from the water, which leads to the formation air locks. Consequences - heating will not work, and the boiler will boil.
This is the lower threshold, and as for the upper level, it should not exceed 1.5 m / s. Exceeding threatens the appearance of noise inside the pipeline. The most acceptable indicator is 0.3-0.7 m / s.
If you need to accurately calculate the speed of water movement, you will have to take into account the parameters of the material from which the pipes are made. Especially in this case, the roughness of the inner surfaces of the pipes is taken into account. For example, by steel pipes hot water moves at a speed of 0.25-0.5 m/s, on copper 0.25-0.7 m/s, on plastic 0.3-0.7 m/s.
Selecting the main contour
Hydraulic switch separates boiler and heating circuits
Here it is necessary to consider separately two schemes - one-pipe and two-pipe. In the first case, the calculation must be carried out through the most loaded riser, where a large number of heating devices and valves are installed.
In the second case, the busiest circuit is selected. It is on its basis that you need to do the calculation. All other circuits will have much lower hydraulic resistance.
In the event that a horizontal junction of pipes is considered, then the busiest ring of the lower floor is selected. Load is understood as thermal load.
Conclusion
Heating in the house
So let's sum it up. As you can see, in order to make a hydraulic analysis of the heating system at home, a lot needs to be taken into account. The example was deliberately simple, since it is very difficult to figure out, say, a two-pipe heating system for a house with three or more floors. To conduct such an analysis, you will have to contact a specialized bureau, where professionals will sort everything “by the bones”.
It will be necessary to take into account not only the above indicators. This will have to include pressure loss, temperature drop, circulation pump power, system operation mode, and so on. There are many indicators, but all of them are present in GOSTs, and the specialist will quickly figure out what's what.
The only thing that needs to be provided for calculation is the power of the heating boiler, the diameter of the pipes, the presence and number of valves and the power of the pump.
If you are starting to install a heating system, you must make all the necessary calculations before starting work. Particular attention should be paid to the calculation of the diameter of the heating pipeline. If it is made incorrectly, then the hydrodynamics of the heating system will suffer first of all. And also we will get poor performance systems at high energy costs. Including the wrong choice of pipe diameter can lead to more significant problems, such as system failures, breaks or leaks. In order to prevent this from happening, you need to competently approach the issue of installing the heating pipeline.
As a rule, the main characteristics of pipes for heating include inner and outer diameters, as well as nominal diameter- the rounded total value of the diameter, which is determined in inches or in fractions of an inch.
The difference between the outer and inner diameter of the pipe differs by the thickness of the pipe. Depending on the material from which the pipe is made, this value varies.
The outer diameter of the pipe must be taken into account during installation, since it requires the attachment of all kinds of fasteners. The inner diameter is the main criterion for choosing a pipe for the heating system. Thanks to him, it is determined throughput systems. And this, in turn, significantly affects the possibility of the length of the pipeline and how many radiators it will be possible to connect to the heating system.
Additionally, taking into account the diameter of the pipe, it will be possible to predict the heat loss of the heating system.
First of all, it must be borne in mind that the rules for choosing pipes for various heating schemes differ significantly.
If the heating system is connected to the central heating main, then the pipe diameter is calculated similar to apartment heating systems.
If it is planned heating system, then the diameter here can be different depending on whether the system will work with a circulation pump, or by natural circulation.
The choice is influenced by:
- Material pipe manufacturing
- Type of coolant
- Wiring specifics heating system
- Alleged water pressure
- Current speed water in the system
When calculating the diameter of the pipeline, you should initially take into account what type of pipes the installation will be carried out from. This is necessary because the system for measuring and marking pipes differs based on the material from which it is made. As a rule, pipes made of steel and cast iron are marked based on the internal diameter, and plastic and copper pipes according to the outer section. This is a particularly important factor if it is planned to install a pipeline in a combination of several materials.
Ideally, you should entrust the calculation procedure to a specialist, however, if you do not have such an opportunity or just have a desire, then you can completely cope on your own.
Calculation of the diameter of the pipes of the heating system
This calculation is based on a number of parameters. First you need to define thermal power heating systems, then calculate at what speed the coolant - hot water or another type of coolant - will move through the pipes. This will help to make calculations as accurately as possible and avoid inaccuracies.
Calculation of the power of the heating system
The calculation is made according to the formula. To calculate the power of the heating system, you need to multiply the volume of the heated room by the heat loss coefficient and the difference between the winter temperature inside and outside the room, and then divide the resulting value by 860.
You can determine the heat loss coefficient based on the material of the building, as well as the availability of insulation methods and its types.
If the building has standard parameters, then the calculation can be made in the average order.
To determine the resulting temperature, the average outside temperature in winter time year and internal no less than it is regulated by sanitary requirements.
Coolant velocity in the system
According to the standards, the speed of movement of the coolant through the heating pipes should exceed 0.2 meters per second. This requirement is due to the fact that at a lower speed of movement air is released from the liquid, which leads to air locks that can disrupt the operation of the entire heating system.
The upper speed level should not exceed 1.5 meters per second, as this may cause noise in the system.
In general, it is desirable to maintain a medium velocity barrier in order to increase circulation and thereby increase the productivity of the system. Most often, special pumps are used to achieve this.
Calculation of the pipe diameter of the heating system
The correct determination of the pipe diameter is very important point, since it is responsible for the high-quality operation of the entire system, and if you make an incorrect calculation and mount the system on it, then it will be impossible to correct something partially. Will be needed replacement of the entire piping system. And these are significant costs. In order to prevent this, you need to approach the calculation with all responsibility.
The pipe diameter is calculated using special formula. It includes:
- desired diameter
- thermal power of the system
- coolant speed
- the difference between the supply and return temperatures of the heating system.
This temperature difference must be chosen based on entry requirements(not less than 95 degrees) and on the return line (as a rule, it is 65-70 degrees). Based on this, the temperature difference is usually taken as 20 degrees.
Hydraulic calculation of pipes
The complexity of the work depends on the calculation of the diameter of the pipes, the thickness of their walls and other parameters.
The diameter of the pipes depends on the length and type of the heating network. The coolant during its passage through various sections of the pipeline loses part of the energy. Reducing the diameter of the pipe contributes increase in the speed of passage of the coolant and thus increase heat transfer.
In addition, the coefficient of resistance to the flow of the coolant is determined by the roughness of the inner surface of the pipeline. In this regard, it may significantly different pressure on the different areas heating systems.
The use of hydraulic calculations is necessary to accurately determine the pressure parameters. Otherwise, this may lead to a decrease in the efficiency of the heating system due to the fact that the pressure that drives the coolant did not exceed the total losses.
It is also necessary to take into account the fact that the thickness of the pipe matters no less than its diameter.
If the pipe diameter is chosen incorrectly, this threatens with serious complications during the operation of the heating system or even its premature failure:
- Excessive heating pipe diameter. This will lead to insufficient pressure in the heating system and thus to a violation of circulation. Because of this, it will break temperature regime in the room, in other words, it will not be warm enough.
- Too small diameter of the heating pipe. Due to the increase in pressure inside the small diameter pipe, the heating system will work too noisily.
During the design and installation of the heating system, it is necessary to carefully comply with all parameters and rules. Errors made at the system design stage most often simply cannot be corrected selectively, and a complete dismantling of the heating system pipeline and its new laying is necessary. This leads to tangible financial costs and, as a result, dissatisfaction with the operation of the system. To prevent this from happening, it is enough to carefully consider all stages of the process, including calculations of the diameter of the pipe of the heating system.
Hydraulic calculation heating systems including pipelines.
In carrying out further calculations, we will use all the main hydraulic parameters, including the flow rate of the coolant, the hydraulic resistance of fittings and pipelines, the speed of the coolant, etc. There is a complete relationship between these parameters, which must be relied upon in the calculations.
For example, if you increase the speed of the coolant, at the same time the hydraulic resistance of the pipeline will increase. If you increase the flow rate of the coolant, taking into account the pipeline of a given diameter, the speed of the coolant will simultaneously increase, as well as the hydraulic resistance. And the larger the pipeline diameter, the lower the coolant velocity and hydraulic resistance. Based on the analysis of these relationships, it is possible to turn the hydraulic (the calculation program is available on the network) into an analysis of the parameters of the efficiency and reliability of the entire system, which, in turn, will help reduce the cost of the materials used.
The heating system includes four basic components: a heat generator, heaters, pipelines, shut-off and control valves. These elements have individual hydraulic resistance parameters that must be taken into account when performing the calculation. Recall that the hydraulic characteristics are not constant. Leading manufacturers of materials and heating equipment must indicate information on specific pressure losses (hydraulic characteristics) for the equipment or materials produced.
For example, the calculation for FIRAT polypropylene pipelines is greatly facilitated by the given nomogram, which indicates the specific pressure or head losses in the pipeline for 1 meter running pipe. Analysis of the nomogram makes it possible to clearly trace the above-mentioned relationships between individual characteristics. This is the main essence of hydraulic calculations.
Hydraulic calculation of water heating systems: coolant flow
We think you have already drawn an analogy between the term "coolant flow rate" and the term "coolant quantity". So, the flow rate of the coolant will directly depend on what kind of heat load falls on the coolant in the process of transferring heat to it. heater from the heat generator.
Hydraulic calculation involves determining the level of coolant flow in relation to a given area. The calculated section is a section with a stable coolant flow rate and a constant diameter.
Hydraulic calculation of heating systems: an example
If the branch includes ten kilowatt radiators, and the coolant flow rate was calculated for the transfer of heat energy at the level of 10 kilowatts, then the calculated section will be a cut from the heat generator to the radiator, which is the first in the branch. But only on condition that this section is characterized by a constant diameter. The second section is located between the first radiator and the second radiator. At the same time, if in the first case the transfer rate of 10 kilowatts of thermal energy was calculated, then in the second section the estimated amount of energy will be already 9 kilowatts, with a gradual decrease as the calculations are carried out. The hydraulic resistance must be calculated simultaneously for the supply and return pipelines.
Hydraulic calculation of a single-pipe heating system involves calculating the flow rate of the coolant
for the design area according to the following formula:
Guch \u003d (3.6 * Quch) / (s * (tg-to))
Qch is the thermal load of the calculated area in watts. For example, for our example, the heat load on the first section will be 10,000 watts or 10 kilowatts.
s (specific heat capacity for water) - a constant equal to 4.2 kJ / (kg ° С)
tg is the temperature of the hot coolant in the heating system.
tо is the temperature of the cold coolant in the heating system.
Hydraulic calculation of the heating system: coolant flow rate
The minimum coolant velocity should take a threshold value of 0.2 - 0.25 m/s. If the speed is lower, excess air will be released from the coolant. This will lead to the appearance of air pockets in the system, which, in turn, may cause a partial or complete failure of the heating system. As for the upper threshold, the coolant velocity should reach 0.6 - 1.5 m/s. If the speed does not rise above this indicator, then hydraulic noise will not form in the pipeline. Practice shows that the optimal speed range for heating systems is 0.3 - 0.7 m / s.
If there is a need to calculate the coolant velocity range more accurately, then the parameters of the pipeline material in the heating system will have to be taken into account. More precisely, you will need a roughness factor for the inner pipe surface. For example, if we are talking about pipelines made of steel, then the coolant speed at the level of 0.25 - 0.5 m / s is considered optimal. If the pipeline is polymer or copper, then the speed can be increased to 0.25 - 0.7 m / s. If you want to play it safe, carefully read what speed is recommended by manufacturers of equipment for heating systems. A more accurate range of the recommended coolant velocity depends on the material of the pipelines used in the heating system, and more precisely, on the roughness coefficient of the inner surface of the pipelines. For example, for steel pipelines, it is better to adhere to a coolant velocity from 0.25 to 0.5 m / s for copper and polymer (polypropylene, polyethylene, metal-plastic pipelines) from 0.25 to 0.7 m / s, or use the manufacturer's recommendations if available.
Calculation of the hydraulic resistance of the heating system: pressure loss
The pressure loss in a certain section of the system, which is also called the term "hydraulic resistance", is the sum of all losses in hydraulic friction and local resistances. This indicator, measured in Pa, is calculated by the formula:
ΔPuch=R* l + ((ρ * ν2) / 2) * Σζ
where
ν is the speed of the coolant used, measured in m/s.
ρ is the heat carrier density, measured in kg/m3.
R - pressure loss in the pipeline, measured in Pa / m.
l is the estimated length of the pipeline in the section, measured in m.
Σζ - the sum of the coefficients of local resistance in the area of equipment and valves.
As for the total hydraulic resistance, it is the sum of all hydraulic resistance settlement areas.
Hydraulic calculation two-pipe system heating: selection of the main branch of the system
If the system is characterized by a passing movement of the coolant, then for a two-pipe system, the ring of the most loaded riser is selected through the lower heating device. For a one-pipe system - a ring through the busiest riser.
If the system is characterized by a dead-end movement of the coolant, then for a two-pipe system, the ring of the lower heating device is selected for the busiest of the most remote risers. Accordingly, for a one-pipe heating system, a ring is selected through the most loaded of the remote risers.
If we are talking about a horizontal heating system, then the ring is selected through the most loaded branch related to the lower floor. When we talk about loading, we mean the "heat load" indicator, which was described above.
In order for the water heating system to function correctly, it is necessary to ensure the desired coolant velocity in the system. If the speed is low, the heating of the room will be very slow and the distant radiators will be much colder than the near ones. On the contrary, if the speed of the coolant is too high, then the coolant itself will not have time to heat up in the boiler, the temperature of the entire heating system will be lower. Added to the noise level. As you can see, the speed of the coolant in the heating system is a very important parameter. Let's take a closer look at what should be the most optimal speed.
Heating systems where natural circulation occurs, as a rule, have a relatively low coolant velocity. The pressure drop in the pipes is achieved by the correct location of the boiler, the expansion tank and the pipes themselves - straight and return. Only the correct calculation before installation allows you to achieve the correct, uniform movement of the coolant. But still, the inertia of heating systems with natural circulation liquid is very large. The result is slow heating of the premises, low efficiency. The main advantage of such a system is the maximum independence from electricity, there are no electric pumps.
Most often, houses use a heating system with forced circulation of the coolant. The main element of such a system is a circulation pump. It is he who accelerates the movement of the coolant, the speed of the liquid in the heating system depends on its characteristics.
What affects the speed of the coolant in the heating system:
- scheme of the heating system;
- type of coolant;
- power, performance of the circulation pump;
- what materials the pipes are made of and their diameter;
— absence of air jams and blockages in pipes and radiators.
For a private house, the most optimal would be the coolant speed in the range of 0.5 - 1.5 m / s.
For administrative buildings - no more than 2 m / s.
For industrial premises - no more than 3 m / s.
The upper limit of the coolant velocity is chosen mainly due to the noise level in the pipes.
Many circulation pumps have a fluid flow rate regulator, so it is possible to choose the most optimal one for your system. The pump itself must be chosen correctly. It is not necessary to take with a large power reserve, as there will be more electricity consumption. With a large length of the heating system, a large number of circuits, number of storeys, and so on, it is better to install several pumps of lower capacity. For example, put the pump separately on the warm floor, on the second floor.