Calculation of relevant parameters of the hottest

2022-08-22
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Calculation of relevant parameters of hot water circulating pump

Abstract: the calculation of pump selection parameters of domestic and foreign hot water circulating pumps is introduced, analyzed and compared. In domestic calculation methods, the main difference in determining the flow of circulating pump is whether to add additional circulating flow, and the focus of determining its head is based on which flow should be calculated by calculating the head loss of which pipe section. The analysis shows that the flow of circulating pump should be the sum of circulating flow and additional circulating flow, and the head of circulating pump should be the head loss of pipeline when the circulating flow and additional circulating flow are not distributed in the system

key words: hot water supply; Circulating pump; Circulating flow

Chinese Library Classification Number: tu822

document identification code: c

article number: (2002)

in the full-time mechanical circulating hot water supply system of buildings, the circulating flow is to compensate for the heat lost to the surrounding by the hot water pipeline when the hot water system does not use water or the water consumption is small, so that users can get the hot water with the required temperature at any time. The circulating pump is the power to ensure the normal flow of circulating flow in the hot water system. At present, when choosing the circulating pump of the full-time mechanical circulating hot water supply system and determining its pump selection parameters, all countries take maintaining the water temperature of the hot water system as the basic starting point, but the calculation methods of pump selection parameters are different

1 foreign calculation method

one is: under the condition of meeting the allowable temperature drop of the hot water supply system, calculate the required circulating flow QX of the system according to the heat loss of the water distribution and return pipes of the system, and then determine the flow QB of the circulating pump; The head of the circulating pump is determined by calculating the head loss of the loop with QX. Japan, the United States and other countries are calculated according to this method

the other is: under the condition of meeting the allowable temperature drop of the hot water supply system, calculate the circulating flow QX required by the system distribution pipe according to the heat loss of the system distribution pipe, and take 25% - 33% of the average hourly water consumption of the system (15% of the average hourly water consumption of the small system) as the additional circulating flow QF, and determine the flow QB of the circulating pump by the sum of QX and QF, QX and QF are used to calculate the head loss of the distribution pipe in the loop and QX is used to determine the head of the circulating pump by calculating the sum of the head loss of the return pipe in the loop. The former Soviet Union calculated according to this method, while in the United States, when calculating the circulating flow of some special pipeline systems, the heat loss of the return pipe is not included [1]

it is obvious that due to the large capacitance of the tested cable, the basis of the first pump selection method is: when considering the allowable temperature drop of the system, fully consider the heat dissipation loss of the return pipe to ensure the temperature requirements of the most unfavorable point, and in the designed pipeline system, in the case of no or a small amount of outflow of the system, there can be an appropriate circulating flow through the most unfavorable pipe section. The head of the circulating pump should be determined by calculating the head loss of the loop when the system does not discharge. The second pump selection method believes that the circulating flow of the system is to ensure the water temperature at the most unfavorable point, so the calculation of the circulating flow can only meet the heat dissipation requirements of the distribution pipe. However, when determining the pump flow, additional circulating flow is added, and the calculation of this additional circulating flow is completely based on empirical estimation, and there is no actual theoretical basis

2 domestic calculation method

at present, the selection of circulating water pumps in China basically follows the method of the former Soviet Union, and only slightly adjusts the value of additional circulating flow. Article 4.5.9 of the current code for design of building water supply and drainage stipulates that in the full-time mechanical circulating hot water supply system, the water output of the circulating pump should be the sum of the circulating flow and the additional circulating flow. The circulating flow refers to the minimum circulating flow required to make the water temperature at the water distribution point not lower than the specified temperature when the pipe does not distribute water. The additional circulating flow is considered. If the circulating water pump is selected only according to the circulating flow, when the hot water supply system uses a lot of water, the circulating flow of the system will be reduced, and the water temperature at the water distribution point will be lower than the specified temperature. Therefore, the circulating flow should have an additional flow. The size of the additional circulating flow depends on the nature of the building and the use requirements, which should generally be 15% of the design hourly water consumption

3 some views of domestic scholars

3.1 flow determination of circulating pump

the focus of the problem is whether to add additional circulating flow

some people think that additional circulating flow should not be included. The viewpoints are as follows:

① the circulating flow of the full-time hot water circulating system is proposed according to the heat loss of the water distribution pipe to supplement the pipe when the water distribution pipe is not working. For a certain pipe, the total heat loss of the pipe is a fixed value. Under the condition of no water distribution, the circulating flow must meet the water temperature requirements of each water distribution point. Under the water distribution condition, the flow in the circulating water distribution pipe will be the sum of the water distribution and circulating flow. Under the condition that the water temperature at the outlet of the heater remains unchanged, the actual temperature drop of the pipe will be lower than the design temperature drop, and the outflow water temperature at the farthest water distribution point will rise. Therefore, as long as the outlet flow of the circulating water pump is the total circulating flow, any water distribution point in the hot water distribution pipe can discharge hot water with a temperature not lower than the specified water temperature at any time [2], so the introduction of additional circulating flow is meaningless. It is true that the total heat loss of a certain hot water system pipe is certain, but in the actual water distribution process, when it comes to each pipe section, whether the heat dissipation loss changes and whether the flow must be the sum of water distribution and circulating flow remains to be discussed

② the water temperature at the water consumption point increases with the increase of the pipeline flow to the point, which is independent of the purpose of the flow. With the increase of water consumption of the hot water system, although the circulating flow of the system decreases, due to the corresponding increase of pipe flow, the water temperature at the water distribution point will be higher than but not lower than the specified temperature. Therefore, the basis of additional circulating flow cannot be established [3]. This is undoubtedly correct for a single loop system or a pipe section of water distribution outlet. However, in a multi loop system, the non outlet pipe section will cool down with the reduction of circulating flow

③ the additional circulating flow reflects part of the water distribution flow of the system. Its water flow comes from the cold water system in front of the water heater, and its energy depends on the available water pressure of the cold water system, so there is no need for the circulating water pump to supply this part of the flow. Therefore, the circulating water will be more effective in cooling the body than the current natural fabric or chemical fiber fabric. The delivery flow of the pump does not need to include this flow [4]. The premise on which this view is based is correct, but the specification has stated that the additional circulating flow is included for the consideration of temperature requirements, not for the consideration of water distribution

others believe that additional circulating flow should be added. The points are as follows:

① the circulating flow calculated according to the heat loss is generally very small. In multi loop hot water pipes, it is difficult to use valves to adjust the small circulating flow to balance the resistance loss of each loop, so near loop short flow often occurs. In practical engineering, the selected value of circulating flow should be larger than that calculated according to the pipeline heat loss theory. Generally, it is appropriate to increase the safety amount by 50%. In single loop hot water system and semi circulating hot water system, the circulating flow does not need to be increased. This view is basically consistent with the specification, and this situation is completely possible in practical engineering

② the meaning of additional circulating flow should be: because some heat dissipation factors that cannot be considered in the calculation of circulating flow affect the accuracy of circulating flow calculation, the calculated value is too small. In order to make up for this influence factor in the calculation, which part of flow is added [5], so the determination of circulating pump flow should include additional circulating flow

7.1.1 product inspection is divided into factory inspection and type inspection

3.2 determination of circulating pump head

because some scholars and experts are inconsistent in the determination of circulating pump flow and the analysis of actual hot water system, there are also some differences in the determination of circulating pump head, There are mainly the following points:

① it is determined by the sum of the head loss of circulating flow through the distribution pipe and return pipe and the head loss of additional circulating flow through the distribution pipe

when some water distribution points in the hot water system use water, in addition to the circulating flow, the water distribution pipeline also passes through part of the water distribution flow, that is, the additional circulating flow. The hydraulic resistance in the water distribution pipeline is greater than that when it flows through the circulating flow alone. To maintain the normal operation of the circulating flow, the head of the circulating water pump must include the energy to overcome the increased hydraulic resistance due to the water distribution flow through this part. In other words, the lift of circulating water pump is related to both circulating flow and this part of water distribution flow [4]

the purpose of adding additional circulating flow to the circulating water pump head calculation formula is to increase the head of the circulating water pump, so that when the outlet flow of some water points on the near loop in the hot water pipe reaches the selected additional circulating flow, the head of the circulating water pump can still overcome the pipe resistance after increasing the additional circulating flow, so that the circulating flow required by the calculation still flows to the far loop in the hot water pipe, so as to ensure the hot water temperature of other parts. In a single loop hot water pipe, the additional circulating flow is generally assumed to occur at the end of the pipe. In a multi loop hot water pipe composed of multiple risers, the additional circulating flow is not all concentrated through the most unfavorable loop, but is distributed according to the proportion of the heat loss of each pipe of the hot water pipe, the additional circulating flow is distributed to each hot water loop, and then the head of the circulating pump is calculated according to the formula [6]

② calculate the sum of the head losses of the pipeline through the water distribution and return pipes with a certain amount of water distribution

in addition, these areas need to be often coated with anti rust engine oil

the head of circulating water pump should be determined by calculating the sum of the pipeline along the way and local head loss when the pipe is at a certain water distribution, so as to ensure the safe and economic operation of the pipe. The formula is:

hb= β Hp+hx

where β—— The ratio of the head loss of the water distribution calculation pipeline and the return calculation pipeline to the head loss when passing through the circulating flow only

hp - the water distribution calculation pipeline along the way and local head loss, kpa

hx - the return calculation pipeline along the way and local head loss, kpa

a certain amount of water distribution in the water distribution pipe and β The determination of the value needs further research and summary [2]

③ it is determined by the sum of the head losses of the distribution pipe and the return pipe through the circulating flow

4 conclusions and suggestions

① the definition of additional circulating flow should be: to ensure that the system under adverse conditions, such as the system has short flow, each non outlet riser needs to increase the flow to meet the requirements of heat dissipation loss. Only in this way can we ensure that all water distribution points of the system can get hot water of the required temperature at any time

② the flow of circulating pump shall be determined by the sum of circulating flow and additional circulating flow

③ the head of circulating pump shall be determined by calculating the head loss of pipeline when the circulating flow and additional circulating flow are not distributed in the system

References:

[1] sabrsula signing plugging systems for high-rise facilities [J] Consult Specif Eng,1998,3(2):.

[2] Wang Xuewen Selection and calculation of circulating water pump in full-time hot water supply system [J] Water supply and drainage, 1993, 19 (6):

[3] Zhao Shiming Mechanical circulation system for full-time hot water supply [J] Water supply and drainage, 1993, 19 (7):

[4] Yu Baoliang Discussion on parameter formula of pump selection for hot water circulating pump [J] Journal of Qingdao Institute of architecture and engineering, 1988, (1):

[5] Wang Xuewen Brief discussion on "circulating additional flow rate" [J] China water supply and drainage, 1993, 9 (3):

[6]

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