In the construction of full-cycle mechanical hot water supply system, the circulating flow is to compensate for the hot water system is not water or less water when the hot water pipe to the surrounding heat, so that users get the required hot water temperature. Circulation pump is to ensure that circulating water flow in the normal flow of hot water system. At present, all countries choose the circulating pump of full-cycle mechanical hot water supply system to determine the pump selection parameters, all of them are based on keeping the water temperature of the hot water system as the basic starting point. However, the calculation methods of pump parameters are not the same.
1, foreign calculation method
One is: under the condition of satisfying 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 system and the return pipe, and determine the circulating flow Qb of the circulating pump; Head loss to determine the head of the circulating pump. Japan, the United States and other countries according to this method.
The other is: to meet the hot water supply system to allow the temperature drop conditions, according to the system of water pipe heat loss calculation system required circulation flow qx, and the system average hourly water 25% to 33% The system is 15% of the average hourly water consumption) as an additional circulation flow rate qf, the circulation pump flow rate Qb is determined by the sum of qx and qf, and qx and qf are calculated by calculating the head loss of the distribution pipe in the loop and qx through the calculation loop The sum of head losses of the return pipe determines the head of the circulating pump. According to this method, the former Soviet Union did not calculate the heat loss of the return pipe when calculating the circulating flow rate of some special pipe systems in the United States [1].
Obviously, the first method of pump selection is based on the consideration of the allowable temperature drop of the system and the full consideration of the heat loss of the return pipe to ensure the most unfavorable temperature requirements. In the designed pipe system, Outflow or a small amount of outflow case, can have the appropriate cycle of flow through the most adverse sections. The head of the circulating pump should be determined by calculating the head loss of the loop as the system does not flow. The second pump selection method is that the system of circulating flow is to ensure the most unfavorable point of the water temperature, so the calculation of circulating flow as long as the distribution pipe to meet the cooling requirements on it. However, when determining the pump flow is added additional circulating flow, and this additional calculation of circulating flow is based entirely on empirical estimates, and there is no actual theoretical basis.
2, the domestic calculation method
At present, the choice of circulating pump in China basically follows the method of the former Soviet Union, and only slightly adjusts the value of additional circulating flow. Current "Design Guideline for Building Water Supply and Drainage" 4.5.9 stipulates that in the full-time mechanical circulation hot water supply system, the circulating pump output should be the sum of circulating flow and additional circulating flow. The circulating flow refers to the minimum circulating flow rate required when the pipe network is not equipped with water so that the temperature of the water distribution point is not lower than the specified temperature. The additional circulation flow is to consider if only according to the circulation of circulating pump selection, when the hot water supply system when a large number of water, the system will reduce the circulation flow, water temperature at the water point is 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 use requirements, the general should be 15% of the design hour of water consumption.
3, some domestic scholars point of view
3.1 Circulation pump flow to determine the issue of focus is to add additional circulation.
Some people think that additional circulation should not be counted. The view is as follows:
(1) The circulation flow of full-cycle hot water circulation system is proposed according to the heat loss of supplementary water distribution network when the water distribution network is not allocated with water distribution. For a certain network, the total heat loss of the network is a fixed value. In case of no water distribution, the circulating flow must be able to meet the water temperature requirements of all water distribution points. In the water distribution condition, the circulating water distribution network will be the amount of water flow and circulation flow and the outlet temperature at the heater unchanged, the actual temperature drop of the pipe network will be lower than the design temperature drop, the furthest distribution point The outflow temperature will rise. Therefore, as long as the outflow rate of the circulating pump is the total circulating flow rate, any water distribution point in the hot water distribution network can flow out hot water no lower than the specified water temperature at any time [2], so the introduction of the additional circulating flow has no meaning . Admittedly, the total heat loss of a certain hot-water system pipe network is certain, but whether the heat loss has changed in the actual distribution process, and whether the flow rate must be the sum of water distribution and circulation flow , It is open to question.
(2) The temperature of the water point rises with the increase of the flow rate of the pipe flowing to this point, but has nothing to do with the use of this flow rate. With the increase of water consumption of hot water system, although the circulating flow of the system is reduced, the water temperature at the water distribution point will be higher than the prescribed temperature due to the corresponding increase of pipeline flow rate. Therefore, the basis for additional circulation can not be established [3]. This is undoubtedly correct for a single-loop system or for a pipe that dispenses water. However, in a multi-ring system, the non-outgoing section will cool as the circulating flow decreases.
â‘¢ additional circulating flow reflects the system part of the water distribution flow, its water flow from the water heater before the cold water system, its energy depends on the cold water system of the hydraulic pressure, so there is no need for circulating water pump to supply this part of the flow. Therefore, circulating pump delivery flow does not need to include this part of the flow [4]. This view is based on the premise that it is correct, but the norm has stated that the calculation of additional circulating flow is based on temperature requirements rather than on distribution considerations.
Others think that additional circulation should be added. The view is as follows:
â‘ calculated by the heat loss of the circulating flow are generally very small, in a multi-loop hot water pipe network, a very small circulation flow valve adjustment is difficult to balance the loss of the loop resistance, it will often produce near Short ring. In practical engineering, the selection of circulating flow should be larger than the theoretical calculation based on the heat pipe as well, generally should increase the safety volume of 50%, in a single loop hot water system and semi-cycle hot water circulation system Do not have to increase. This view is basically consistent with the norms, in the actual project this situation is entirely possible.
â‘¡ Additional circulation The meaning of the flow should be: due to the calculation of the circulating flow of some of the factors considered when the thermal effects of non-circulatory flow calculation accuracy, resulting in the calculated value is small, to make up for the calculation of the existence of this factor increased that part Flow [5], so determine the circulating pump flow should include additional circulating flow.
3.2 Determination of pump head Because some scholars and experts on the determination of circulating pump flow and the actual hot water system analysis is inconsistent, so there are some differences in the determination of circulating pump head, mainly in the following points:
â‘ to the circulating flow through the water distribution network and return pipe network head loss and additional circulating flow through the distribution network head loss and determine.
When part of the hot water system water distribution points, the water distribution pipeline in addition to circulation through the part of the water flow through the additional circulation flow, the water distribution pipeline generated hydraulic resistance greater than the individual flow through the circulation. To maintain normal circulation, the head of a circulating pump must contain energy that overcomes the increased hydraulic resistance due to passing this portion of the distribution flow. In other words, the circulation of the pump head both circulating flow, but also with this part of the distribution of water flow [4].
The purpose of adding additional circulating flow in the formula of circulating pump head is to increase the lift of the circulating pump so that when the outlet flow of some water points in the hot water pipe network reaches the selected additional circulating flow, Still can overcome the increase of additional circulating flow after the pipe network resistance, so that the hot water pipe network is still calculating the requirements of the circulating flow to the far ring, to ensure that other parts of the hot water temperature. In a single loop hot water pipe network, the additional circulating flow is generally assumed to occur in the pipe network end, in the multiple riser composed of multi-loop hot water pipe network, additional circulating flow is not all concentrated through the most adverse ring Road, but according to the proportion of heat pipes in the heat pipe network, the flow rate distribution, the additional circulation flow assigned to each hot water loop, and then according to formula circulating pump head [6].
â‘¡ to a certain amount of water distribution through the water distribution and return pipe network to determine the sum of head loss.
The head of the circulating water pump shall be determined by the sum of the pipe length along the pipeline and the local head loss when the pipe network is under a certain water distribution to ensure the safety and economic operation of the pipe network. The formula is:
hb = βhp + hx
Where β - water distribution calculation pipeline, backwater calculation pipeline head loss and only through the circulation flow head loss ratio hp-- distribution of water along the pipeline and local head loss, kPa
hx-- backwater calculation pipeline along the way and local head loss, kPa
In the calculation of pump head distribution water pipe network in a certain amount of water distribution and the determination of β, pending further study and summary [2].
â‘¢ to the circulating flow through the distribution network and return pipe network head loss and determine.
4, conclusions and recommendations
â‘ additional circulating flow should be defined as: In order to ensure the system in unfavorable conditions, such as short-circuit system, each non-outfall riser to meet the heat loss requirements and the need for increased flow. The only way to ensure that the water distribution system at any time to get the required hot water temperature.
â‘¡ circulating pump flow should be based on the sum of circulating flow and additional circulating flow to determine.
â‘¢ the determination of circulating pump head should be based on circulating flow and additional circulating flow in the system is not water, by calculating the head loss of the pipeline to determine.
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