Systems of internal cold and hot water supply
As already noted in the classification of HW systems, circulation serves to prevent cooling of water in the distribution pipelines of systems in the absence of water intake (for example, at night in residential buildings). According to the level of coverage of circulation systems, there are:
systems without circulation;
systems with circulation only in main pipelines;
systems with circulation both in distribution lines and
and in stands.
It is allowed not to provide circulation if the water temperature at the points of water intake during the time-regulated water intake will not decrease below the minimum allowable.
Required to compensate for the total heat loss of the system Q h t , (see § 1.5) the circulation flow will be
where b- coefficient of circulation misalignment;
Q ht i - heat loss of individual parts of the HW system, kW;
D t h - drop in water temperature in the system;
with p - heat capacity of water, kJ/(kg × K).
In systems with circulation only through distribution pipelines Q h t is determined only by distributing pipelines and the calculation is performed at D t h = 10 °С; b=1.
In systems with circulation in distribution pipelines and in risers with unequal resistance Q h t is determined by distributing pipelines and risers and the calculation is performed at D t h =10 °С; b=1.
In systems with circulation in distribution pipelines and in risers with the same resistance Q h t is determined only by risers and the calculation is performed at D t h \u003d 8.5 ° С; b=1,3.
The circulation flow determined in this way in the head section of the system is distributed among the sections in proportion to their heat losses, determined in accordance with the provisions of § 1.5.
The principle of such a distribution can be demonstrated by the following conditional example. Imagine that the next node No. 6 is subject to the distribution of the circulation flow (the distribution is made in the direction of water movement: from the heating center to the most remote riser). The node is formed by connecting to the main circulation half-ring of the next riser with conditional No. 3 (the half-ring is the riser itself and its circulation part).
If the circulation flow in section 6-7, determined when linking the previous node 7, is, then the desired flow rate directed further along section 5-6 will be:
and the circulation flow directed to riser No. 3:
In these formulas - the sum of the heat losses of the system in from the most remote point to node 6, and - the heat loss of the riser. For the convenience of using this method, the heat loss calculation itself is recommended to be carried out with consistent accounting for the losses of each riser at the point of its connection.
1.7-1. Hydraulic calculation of SGW in circulation mode
After the distribution of circulation flow rates over the calculated sections, the hydraulic balancing of the system is carried out in the "clean" circulation mode. The sequence of calculation is as follows.
Pre appointed the diameters of the circulation pipelines (by 1-3 standard sizes) are smaller than the corresponding supply pipeline. The determination of pressure losses in sections is carried out according to the same formulas and nomograms as in the hydraulic calculation of supply pipelines, but with circulation flow rates. The calculation is carried out in parallel along the supply and circulation pipelines with summation to the next branching node. Similarly, pressure losses are determined in a semi-ring connected to the same node, formed by a riser, sectional node or branch of the system.
The resulting pressure losses in the half-rings joining in a given node should not differ by more than 10%. If this condition is not met, the node is linked in the following order (each next method is applied if the previous one does not give the proper result).
1) Piping diameters vary.
2) A diaphragm is installed on the circulation pipeline of a half ring with less pressure loss. The aperture diameter is determined by the expression
where q h m - water flow in a semicircle with less head loss;
D H - the difference in pressure losses in the half-rings, which should be "switched off" in the diaphragm, daPa.
The diaphragm cannot be less than 10 mm (due to gradual overgrowth and the possibility of its off-design operation).
3) The circulation flow changes, but not more than 30%. The change in flow must be taken into account in all subsequent (to the heating center) sections.
4) An additional crane is installed to adjust the system during commissioning. A valve is installed on the circulation pipe in addition to the normal shut-off valve.
The hydraulic calculation of the circulation rings is carried out for the circulation mode, i.e. at the highest circulation rate. Each circulation ring consists of heat pipelines: supply pipes, the diameters of which are selected in the maximum drawdown mode, and circulation heat pipes. The hydraulic calculation of circulation rings includes the calculation of pressure losses in the supply heat pipelines in the absence of water intake and the passage of only circulation water flows and the calculation of pressure losses in the circulation heat pipelines when the circulation water flows are passed. The calculation is carried out similarly to the calculation of supply heat pipes. The diameters of the prefabricated circulation heat pipe and the most remote riser should be taken based on the allowable water flow rates. In this case, the diameters of the circulation heat pipelines should be 1-2 calibers less than the diameters of the corresponding sections of the supply heat pipelines.
For hot water supply systems with sectional water-folding-circulation units with the same diameters of the risers, it is recommended to calculate the circulation as follows. First, the circulation flow is determined for the most remote sectional node, taking the value åQht equal to the heat loss by the supply heat pipelines of the entire unit, and the temperature difference Dt due to the cooling of water in the water-folding-circulation unit, it is 2-3 ° C less than the temperature drop in the entire system. In this case, the circulation flow rates for the remaining nodes of the system will always be greater than for the most remote one, since the pressure difference at the points of connection of the sectional nodes to the supply and circulation heat pipelines will increase as the sectional nodes approach circulation pump. In order to increase the hydraulic stability of the system, it is advisable to take the pressure losses in the circulation risers of the units to be sufficiently large compared to the pressure losses in the prefabricated circulating heat pipelines. It is recommended that at the circulation flow, the pressure loss in the water-folding-circulation unit be in the range of 0.03-0.06 MPa.
The hydraulic calculation of the circulating heat pipelines is recorded in table 4. The section numbers have their own numbering, different from the distribution line, and are supplied with the index "C".
Table 4.1. Calculation of the most remote section node.
Lot number | Section length L, m | Compass. consumption, l/s | Diameter D, mm | Speed, m/s | ||||
15-14 | 5.1 | 2.1 | 80 | 0.42 | 60 | 0.2 | 367 | 0.37 |
14-13 | 28.1 | 1.74 | 65 | 0.5 | 100 | 0.2 | 3372 | 3.74 |
13-12 | 61.23 | 0.7 | 50 | 0.42 | 100 | 0.2 | 7348 | 11.09 |
12-11 | 17.7 | 0.33 | 40 | 0.3 | 60 | 0.2 | 1274 | 12.36 |
11-10 | 6.35 | 0.31 | 40 | 0.29 | 58 | 0.2 | 442 | 12.8 |
10-9 | 14.2 | 0.16 | 32 | 0.2 | 40 | 0.2 | 681 | 13.48 |
9-8 | 6.8 | 0.1 | 32 | 0.12 | 15 | 0.2 | 122 | 13.6 |
8-7 | 4 | 0.1 | 32 | 0.12 | 15 | 0.2 | 72 | 14.32 |
7-6 | 4 | 0.05 | 25 | 0.12 | 25 | 0.2 | 120 | 14.44 |
6-5 | 4.5 | 0.05 | 25 | 0.12 | 25 | 0.5 | 169 | 14.61 |
5-4 | 4.5 | 0.05 | 25 | 0.12 | 25 | 0.5 | 169 | 14.78 |
4-3 | 4.5 | 0.05 | 25 | 0.12 | 25 | 0.5 | 169 | 14.95 |
3-2 | 4.5 | 0.05 | 20 | 0.2 | 100 | 0.5 | 675 | 15.62 |
2-1 | 7.7 | 0.05 | 20 | 0.2 | 100 | 0.5 | 1155 | 16.78 |
1-1ts | 4.9 | 0.05 | 15 | 0.4 | 700 | 0.5 | 4725 | 21.5 |
1c-2c | 3.6 | 0.05 | 15 | 0.4 | 700 | 0.2 | 3360 | 24.86 |
2c-3c | 1.9 | 0.1 | 15 | 0.85 | 3500 | 0.2 | 16800 | 41.66 |
3ts-4ts | 20.5 | 0.16 | 20 | 0.6 | 900 | 0.2 | 22140 | 63.8 |
4c-5c | 18.5 | 0.16 | 25 | 0.35 | 200 | 0.2 | 4440 | 68.24 |
5c-11c | 10.3 | 0.31 | 32 | 0.4 | 160 | 0.2 | 1978 | 70.22 |
11c-12c | 17.7 | 0.32 | 32 | 0.4 | 160 | 0.2 | 3398 | 73.62 |
12c-13c | 61.23 | 0.7 | 40 | 0.75 | 400 | 0.2 | 29390 | 103.01 |
13ts-14ts | 28.1 | 1.74 | 50 | 0.8 | 400 | 0.2 | 13488 | 116.49 |
14c-15c | 5.1 | 2.1 | 50 | 1 | 600 | 0.2 | 3672 | 120.17 |
Table 4.2 Calculation of the most approximateJ sectional node.
Lot number | Section length L, m | Compass. consumption, l/s | Diameter D, mm | Speed, m/s | Specific pressure loss , Pa/m | Pressure loss at the account, Pa | Total pressure loss, kPa | |
15-14 | 5.1 | 2.1 | 80 | 0.42 | 60 | 0.2 | 367 | 0.37 |
14-16 | 23.1 | 0.33 | 40 | 0.32 | 80 | 0.2 | 2218 | 2.59 |
16-17 | 9.7 | 0.16 | 32 | 0.17 | 25 | 0.2 | 291 | 2.88 |
+55.44 | ||||||||
17ts-16ts | 9.7 | 0.16 | 25 | 0.35 | 200 | 0.2 | 2328 | 60.65 |
16ts-14ts | 23.1 | 0.33 | 25 | 0.8 | 1100 | 0.2 | 3049 | 91.14 |
14c-15c | 5.1 | 2.1 | 50 | 1 | 600 | 0.2 | 3672 | 94.81 |
Circulation risers rely on the pressure difference at their junctions with the supply risers and the circulation line. The difference in pressure losses in different circulation rings is allowed no more than 10%.
If it is impossible to link pressure losses by changing the diameters of pipelines in sections of the circulation network, diaphragms are installed at the base of the circulation risers. Diaphragm hole diameter d d is determined by the formula:
(7)
where q- water flow through the diaphragm, m 3 / h;
H out- excess head, which must be extinguished by the diaphragm, m.
If, during the calculation, the aperture diameter of the diaphragm is less than 10 mm, it is allowed to install a valve instead of it to repay the excess pressure. However, when installing diaphragms, an increase in sludge and scale formation is observed in these places, therefore, it is allowed to link pressure losses in the circulation rings by increasing hydraulic resistance risers, introducing inserts from pipes of smaller diameters into their lower part.
Description:
The technique for determining the circulation flow in water supply systems is given. hot water multi-storey residential buildings. The calculation is integral part to ensure the water supply of the network. The purpose of the calculation is to determine cost-effective pipe diameters for passing the estimated water flow rates and pressure losses from the dictating water-folding device in the building to the point of connection of the input to the external water supply network.
Determination of circulation flow in hot water plumbing systems
Physical and mechanical parameters of produced autoclaved aerated concrete
The calculation algorithm, adapted to the use of Excel spreadsheets, corresponds to that given in.
The total friction pressure loss is 3.88 m, and taking into account the local resistance losses accepted for systems with heated towel rails on water risers in the amount of 50% of the friction loss, the total pressure loss is equal to: ∆ H\u003d 1.5 3.88 \u003d 5.83 m.
It should be borne in mind, however, that in the head section (up to the first node), i.e. in this case, in the 4th section of the hot water plumbing system, it is necessary to check whether it is necessary to take into account the effect of circulation on the water flow. To do this, we need to calculate the ratio q 4 / q cir, where the circulation flow q cir is determined based on the heat loss of the risers and the allowable decrease in water temperature to the upper draw-off point, which in buildings over 4 floors is 8.5 ° C. With an average floor height of 3.3 m, as was adopted in the example under consideration, the average heat loss on the ground floor for water folding open non-
isolated risers with heated towel rails according to the table. 10.4 will be approximately 186 watts for D y25 and 232 W for D y32. Heat loss in
branches to the risers from the main, as a first approximation, can be ignored due to their insignificant length and the alleged presence of thermal insulation in the basement. From here we get:
q cir= q circle 0 N this N st,
where N et - the number of floors, N st - the number of risers in the water assembly, and the specific circulation flow per floor q circle 0 at the specified heat losses is about 0.0052 l / s at D y25 and 0.0065 l/s for D y32. In our case, the stand D y25, N floor = 8, N st = 4, whence:
q cir\u003d 0.0052 8 4 \u003d 0.167 l / s,
and relation:
q 4 / q cir = 1,138 / 0,167 = 6,8 > 2,1,
therefore, when calculating the flow rate in the 4th section, the presence of circulation does not need to be taken into account.
For comparison, let us now repeat the calculation for a 16-storey building with a similar layout.
In this case: N= 2 96 = 192, U= 2 108 = 216 people; Q h rh \u003d 10.9 l / h (according to the data, since the building has more than 12 floors), then:
P = (10.9 216) / (3600 0.18 192) = 0.0189,
those. somewhat larger than in the first case, but still significantly less than 0.1. The results of the calculations are summarized in Table. 2.
Consequently, here the diameter of the riser naturally turns out to be one step larger.
At the same time, the value q cir for D y32, N floor = 16 and N st \u003d 4 is equal to: 0.0065 16 4 \u003d 0.416 l / s,
then the relation q 4 / q cir= 1.832 / 0.416 = 4.4, which is also greater than 2.1, so adjust the value here too q 4 is not needed.
However, since the building already has 16 floors, we check the pressure at the taps on the first floor. It consists of the geometric height:
H g \u003d 3.3 15 + 1.5 \u003d 51 m,
where 1.5 is the difference between the marks of the dictating and the lowest located device within the apartment, m; 3.3 - floor height from floor to floor, m; quantities ∆ H and free pressure at the dictating device H St.
For bath with shower H sv \u003d 3 m according to the table. 2.1 or appendix 2 (for most other taps in residential buildings H sv = 2 m). As ∆ H in this case, it is necessary to take into account losses only in section 1, i.e., above the connection of the apartment wiring of the first floor. According to the table 2, taking into account local resistances, we obtain:
∆H\u003d 1.659 1.5 \u003d 2.49 m.
∑H = 51 + 2,49 + 3 = 56,49 < 60 м.
Thus, we have obtained an approximate method for determining the circulation flow in hot water plumbing systems of multi-storey residential buildings. The considered technique has a simple and engineering form and is available for use in the practice of mass design. At the same time, the calculations showed that for the most typical cases, the influence of the circulation flow on the drawdown mode can be neglected.
Literature
- Samarin O.D. Hydraulic calculations engineering systems. M.: ASV, 2014.
- SNiP 2.04.01–85* “Internal water supply and sewerage of buildings”. M.: GUP TsPP, 2000.
- SP 30.13330.2012 "Updated edition of SNiP 2.04.01-85* Internal water supply and sewerage of buildings". Moscow: Ministry of Regional Development of Russia, 2012.
- Samarin O. D. Calculation of pressure losses in polymer pipes // Sanitary engineering. - 2014. - No. 1. - S. 22–23.
- Designer's Handbook. Internal sanitary devices. Part 2. Water supply and sewerage / Pod. ed. I.G. Staroverov and Yu.I. Schiller. Moscow: Stroyizdat, 1990.
SNiP 2.04.01-85*
Building regulations
Internal plumbing and sewerage of buildings.
Systems of internal cold and hot water supply
WATER PIPES
8. Calculation of the hot water plumbing network
8.1. Hydraulic calculation of hot water systems should be made on estimated flow hot water
Taking into account the circulation flow, l / s, determined by the formula
(14)
where is the coefficient taken: for water heaters and the initial sections of systems up to the first standpipe according to the mandatory appendix 5;
for other sections of the network - equal to 0.
8.2. The circulation flow of hot water in the system, l / s, should be determined by the formula
(15)
where is the coefficient of circulation misalignment;
Heat losses by pipelines of hot water supply, kW;
Temperature difference in the supply pipelines of the system from the water heater to the most remote draw-off point, °C.
The values and depending on the hot water supply scheme should be taken:
for systems that do not provide for the circulation of water through the risers, the value should be determined by the supply and distributing pipelines at = 10 ° C and = 1;
for systems in which water circulation is provided through water risers with variable resistance of circulation risers, the value should be determined by supply distributing pipelines and water risers at = 10 ° C and = 1; with the same resistance of sectional nodes or risers, the value should be determined by the water risers at = 8.5 ° C and = 1.3;
for a water riser or a sectional unit, heat losses should be determined along the supply pipelines, including the annular jumper, taking = 8.5 ° C and = 1.
8.3. Pressure loss in sections of pipelines of hot water supply systems should be determined:
for systems where it is not required to take into account the overgrowth of pipes - in accordance with clause 7.7;
for systems taking into account the overgrowth of pipes - according to the formula
where i - specific pressure loss, taken in accordance with the recommended application 6;
The coefficient taking into account the pressure loss in local resistances, the values of which should be taken:
0.2 - for supply and circulation distribution pipelines;
0.5 - for pipelines within heating points, as well as for pipelines of water risers with heated towel rails;
0.1 - for pipelines of water risers without heated towel rails and circulation risers.
8.4. The speed of water movement should be taken in accordance with paragraph 7.6.
8.5. The pressure loss in the supply and circulation pipelines from the water heater to the most remote water or circulation risers of each branch of the system should not differ for different branches by more than 10%.
8.6. If it is impossible to link the pressures in the pipeline network of hot water supply systems by appropriate selection of pipe diameters, it should be provided for the installation of temperature controllers or diaphragms on the circulation pipeline of the system.
The aperture diameter should not be less than 10 mm. If, according to the calculation, the diameter of the diaphragms must be taken less than 10 mm, then it is allowed to provide for the installation of valves for pressure control instead of the diaphragm.
The diameter of the openings of the control diaphragms is recommended to be determined by the formula
(17)
8.7. In systems with the same resistance of sectional units or risers, the total pressure loss in the supply and circulation pipelines within the limits between the first and last risers at circulation flow rates should be 1.6 times higher than the pressure loss in the sectional unit or riser when the circulation is misadjusted = 1.3.
The diameters of the pipelines of the circulation risers should be determined in accordance with the requirements of clause 7.6, provided that, at the circulation flow rates in the risers or sectional assemblies, determined in accordance with clause 8.2, the pressure loss between the points of their connection to the distribution supply and collection circulation pipelines does not differ more than 10%.
8.8. In hot water supply systems connected to closed heating networks, pressure losses in sectional units at the estimated circulation flow should be taken as 0.03-0.06 MPa (0.3-0.6 kgf / sq. cm).
8.9. In hot water supply systems with direct water intake from pipelines of a heating network, pressure losses in the pipeline network should be determined taking into account the pressure in return pipeline thermal network.
The pressure loss in the circulation ring of the pipelines of the system at the circulation flow should, as a rule, not exceed 0.02 MPa (0.2 kgf / sq. cm).
8.10. In shower rooms with more than three shower nets, the distribution pipeline should, as a rule, be provided as a loop.
One-way hot water supply may be provided for collector distribution.
8.11. When zoning hot water supply systems, it is allowed to provide for the possibility of organizing natural circulation of hot water at night in the upper zone.