Water heaters are installed in closed heating schemes. In these schemes, the water heater plays the role of the boiler. Depending on the type of primary coolant, water heaters are divided into steam and water. In heating systems, high-speed water-cooled horizontal sectional shell-and-tube water heaters are traditionally used, the assembly scheme of which is shown in fig. 3.8. By design, the heat exchanger is a bundle of tubes in a pipe-casing. In the heat exchange tubes, the primary coolant moves - water from the heating network, and in the annular space - the secondary coolant - heated water from the heating system.
The industry produces water heaters of different types, differing mainly in the heating surface. Their outer diameter ranges from 57 to 325 mm, and from 4 to 151 tubes are located in the body of one section. A water heater is installed when heat is supplied to individual buildings in the basement or on the technical floor.
The choice of water heater is made in the following order.
According to the recommended speed of movement of the heated water Vр = 1 m / s, determine the recommended cross-sectional area of the heater tubes, m2,
, (3.12)
 | Fig. 3.8. The scheme of a high-speed five-section water heater: 1 - from the building's heating system; 2 - to the building heating system; 3 - supply of coolant from the external heat network; 4 - return of the spent coolant to the external heating network; 5 - heat exchange tubes | |
where G is the water consumption in the heating system, kg / h; r »1000 kg / m3 - the density of water.
According to the table. 3.5 a brand of water heater is selected with a close value of the area of the pipes. For the selected brand table. 3.5 for one section of the heater are the heating area f, the cross-sectional area of the tubes ftr.fakt.
Determined by the actual speed of the heated water, m / s,
. (3.13)
According to the table. 3.6 is determined by the heat transfer coefficient k , corresponding to the actual speed of the heated water V and the recommended speed of the heating water 1.0–1.5 m / s.
Table 3.5
Constructive characteristics of sections of high-speed water heaters according to GOST 27590
room water heater | Diameter enclosures mm | Length mm | Number of tubes | Surface heating f , m2 | Living area section, m2 | |
tube ftr | annular space fm.p | |||||
one | 57 | 2220 | four | 0.37 | 0,00062 | 0,00116 |
2 | 57 | 4220 | four | 0.75 | ||
3 | 76 | 2300 | 7 | 0.65 | 0,00108 | 0,00233 |
four | 76 | 4300 | 7 | 1.31 | ||
five | 89 | 2340 | 12 | 1.11 | 0,00185 | 0,00287 |
eight | 114 | 4424 | nineteen | 3.54 | 0,00293 | 0,005 |
9 | 168 | 2620 | 37 | 3.4 | 0,0057 | 0,0122 |
Table 3.6
The values of heat transfer coefficients k , W / m2 × ° С
Speed heated water, m / s | Heating water speed, m / s | |||
0.5 | 0.75 | 1.0 | 1.5 | |
0.5 | 1102 | 1276 | 1392 | 1508 |
0.75 | 1241 | 1450 | 1566 | 1740 |
1.0 | 1334 | 1566 | 1740 | 1972 |
1.5 | 1508 | 1798 | 2030 | 2320 |
Calculate the required heating surface area of the heater, m2,
, (3.14)
where Q is the calculated heat flux for heating the building, W, determined by the formula (2.1); k — heat transfer coefficient, W / (m2 × ° С); Dt is the average temperature pressure between the heating and heated water, ° C.
To determine Dt, first calculate the temperature difference between the heating and heated water at opposite ends of the heater when working with countercurrent
Δt = T1 - tg; Δ2 = T2 - t0, (3.15)
where T1, T2 are the temperatures of the heating and return water in the heating system; tг, tо - the same hot and chilled water in the heating system.
The larger of these differences is assigned the index " b ", the smaller - " m" and determine the average temperature of the pressure
. (3.16)
Number of water heater sections
, (3.17)
where f is the heating surface of one section.
The speed of movement of water in the water heater to avoid noise should not exceed 1.5 m / s, if this condition is not met, the heater section is connected in parallel. Characteristics of some heaters are given in table. 3.5.
| In recent years, lamellar water heaters are widely used, in which the plates are welded in pairs along the contour, forming a block (Fig. 3.9). Between the two welded plates there is a closed (welded) channel for the heating and heating water. From the outside, the twin plates are washed by heated water. | |
Fig. 3.9. Schematic diagram of the plate heater |
Example 6
The initial data: the building and the heating system to accept with the characteristics in accordance with examples 1, 2 and 3. The parameters of the coolant in the heating network T1 = 150 ° C, T2 = 70 ° C, in the heating network tg = 85 ° C,
tо = 60 ° С (such parameters are recommended when using water-to-water heat exchangers).
Decision. The calculated heat flux for heating the building is 53800 W, the coolant flow rate in the heating system is 1,406 kg / h:
m2
According to the table. 3.5 selected water heater brand number 1 with a living area of 0,00062 m2 tubes.
By the formula (3.13) is determined by the actual speed of the heated water:
m / s
According to the table. 3.6 is determined by the heat transfer coefficient k = 1500 W / m2 · ° C.
Is determined by the average temperature pressure:
Δ t 1 = 150 - 85 = 65 ° C;
Δ t 2 = 70 - 60 = 10 ° C;
° s
By the formula (3.14) is determined by the required heating surface
m2
When the resulting heating surface of the heater according to the formula (3.17) is calculated the required number of heater sections
.
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