Hot Water Circulation and Additional Flow
Sheng Guorong
ChongQing Architectural Design Instiute
Add:No.31 Renhe Street, Yuzhong District, Chongqing Zip Code:400015
Tel:023-63621179 Fax:023-63856935

ABSTRACT By analyzing the process of hot water circulating and the implication of additional flow,the weakness,lying in the definition of additional flow and in the calculation formula of circulating-pump head and circulationg-pump flow in current code,is pointed out. The implication of additional flow is also discussed.A new opinion about calculationg the pump head and the pump flow is brought brought forward.
KEYWORDS circulating flow, additional flow, circulating-pump flow,circulating-pump head
1 Requirements of the code
In curent Code for Design of Building Water Supply and Drainage(GBJ15-88),there are mainly two stipulations to the circulating-pump in a central heating flow,the circulating-pump head should as 15% of the maximum hourly hot-water consumption;Secondly,the circulating-pump head should be counted as Hb=(1+qf/qx)2.hp+hx[formula?],in which Hb is the circulationg-pump head is the circulation flow;qf is the additional flow,hp and hx are the head loss in distributing pipes and in is the circulationg flow;qf is the additional flow,hp and hx are the head loss in distributing pipes and in returning pipes respectively. In the code,the additional flow is definged as follows: In a mechanical circulating hot-water flow should be considered when calculating the head loss in the distributing pipes, so as to prevent the circulation from being destroyed by the partial water consumption. This water flow is called as an additional flow.
In recent years, many queries focusing on the implication and calculation the additional flow are put forward,but this seemed to have taken no effect on the code. In this paper, these queries are to be discussed.
According to the code, the formula for calculating the circulating flow is :qx=(3600Qs)/(C.?t),where Qs is the heat loss,which can be counted as 5%to 10%of the hourly heat consumption;C is the specific heat of water;?t is the temperature difference. It‘s obvious that the circulating flow is figured out on the assumption that there is no consumption occurring in the hot water system, the purpose of circulating is to maintain the temperature of the system.
2. One opinion about the implication of additional flow
According to formula?, when we calculate the circulating-pump head, the head loss in distributiong pipes should be counted as (1+qf/qx)2.hp,this indicates that not only the circulating flows, but also the additional flow passes through the distributing pipes;while, the head loss in returning pipes is still hx, this shows that the additional flow doesn‘t pass through the returning pipes. Inthe definition, it‘s said that the additional flow should be considered "when calculating the head loss in the distributing pipes".Where the distributing pipes are mentined but not are the returning pipes,this also indicates that the additional flow passes through only the distributing pipes but not the returning pipes.So it seems reasonable to think in this way: the additional flow arises from the partial water consumption, and it approximately reflects the small quantity of consumption. As shown in figure1, the flow is replenished by cold water pipe. Since it is an approximate reflection of partial water consumption, certainly it passes(or partially)through the distributing pipes. It‘s also clear that it doesn‘t pass through the returning pipes, because it‘s consumed, it doesn‘t take part in the circulation really, it can‘t be called as an additional circulating flow. From figure 1, it also can be found that the additional flow is supplied from behind the circulating pump but not from before the pump, it doesn‘t pass through the pump. The core of this opinion is that the additional flow passes through only the distributing pipes but neither the returning pipes nor the punmp,which is deduced from the code. But this doesn‘t agree with another part of the code, which requires the pump flow to be the summation of circulating flow and additional flow. Since the additional flow doesn‘t pass through the pump,why must the outflow of the pump be the summation of the circulating flow and the additional flow as described in the code? A contradiction appears.
Somebody says figure 2 can work, because the additional flow passes through both the pump and the distributing pipes but not the retuming pipes, which is in accordance with the code. But that manner is not always feasible in most cases. In that manner, because the pump has to be running all the time and the outflow might range between one-tap-flow and a design flow, it‘s difficult to choose a centrifugal pump to meet the request.
3. Another opinion about the implication of additional flow
According to the code, the additional flow is utilized to prevent the circulation from being destroyed. So it appears logical to think in this way:Because the circulating flow is figured out on the assumption that there is no consumption in the hot water system, it can be called as a "theoretical circulating flow"to which is resorted to make up for the heat loss.When it‘s averaged to several vertical pipes ,the circulating flow in each pipe may be very little. In order to keep the circulation from being destroyd by partial consumption ,an extra flow should be added to the theoretical circulating flow ,thus a "factual circulating flow‘is gained. Because the flow in each pipe is enlarged, the influence of the partial consumption is remarkably reduced. Consequently, the circulation effect is ensured. This extra flow is just the "additional circulating flow".Being understood in this way, the additional flow is not an approximation of the partial consumption but an extra flow added to the theoretical circulating flow.It passes through the pump, the heat exchanger, the distributing pipes and the returning pipes, therefor it takes part in the circulation really and can be called as an additional circulating flow. Since it passes through the returning pipes, certainly the head loss in returning pipes ought to be counted as (1+qf/qx)2.hx but not hx yet.Thecore of this opinion is that the additional flow passes through the entire pipe-loop of the pump should be summation of the circulating flow and the additional flow",but it conflicts with formula(1) in calculating the head loss in returning pipes. This opinion can‘t make the code justify itself none the less.
4 Comparison of the two opinions
The implications of "additional flow"are quite different in above two opinions. According to the former one ,the cold water pipe replenishes the additional flow, and this flow is consumed in the distributing pipes, so it seems more exact to call the flow an "additional distributing flow"(qpf)than to call it an "additional circulating flow".To the latter opinion ,the additional flow does take part in the circulation, it can be called as an " aadditional circulating flow"(qxf).The two different implications are both referred to in the code, but they are not told from each other. If it‘s necessary to keep the circulation from being destroyed and properly figure out the head loss in the pipe-loops,both the two kinds of additional flows ought to be taken into account, namely Hb=[1+(qpf+qxf)/qx]2.hp+(1+qxf/qx)2.hx[formula?].If the factual circulating flow is shown as qsx(namely qsx=qpf+qxf),the head loss in distributing pipes and returning pipes are shown as hsp and hsx respectively, from formula? the following formula is derived; Hb=(1+qpf/qsx)2.hsp+hsx[formula ?].formula?resembles formula ? formally.
5 Examples
In a section of a hotel, the parameters are as follows:n=280beds, q=200l/bed.d, Tr=60?,T1=7?,?t=10? and Kh=5.73, thus Qh=13.4m3/h;If Qs is estimated as 7% of the maximum hourly heat consumption, then qx=5.0m3/h. When qx is averaged to the 10 vertical pipes, the circulating flow in each pipe is 0.5m3/h. As the diameter of inlet and outlet are DN50 and DN25 respectively, the velocities of inlet and outlet are Vi=0.07m/s and Vo=0.28m/s respectively. Considering the qf which is counted as 15% of the Qh according to the code,the total factual circulating flow is enlarged to be 0.7m3/h?then the velocities are Vi=0.10m/s and Vo=0.40m/s.
In a section of a hospital, each original parameter is the same as above except Kh=2.66. Then the results are Qh=6.2m3/h and qx=2.3m3/h, accordingly Vi=0.03m/s and Vo=0.13m/s. Appended with the qf, the velocities are Vi=0.05m/s and Vo=0.18m/s.
From above, it‘s easy to find that in a hotel, no matter the additional flow is considerd or not ,the velocity in each vertical pipe changes little; with the same Qh, the more are the vertical pipes, the more will the velocity decrease. Ina hospital, as the Kh is much smaller than in a hotel, the velocities are much lower even if the additional flow is considered.
6. Factors influencing the effect of circulation
What on earth is the disadvantage of a too low velocity? As we know, although each pipe-loop is designed to be nearly alike, the circulating velocities may be very different ,we have to make them nearly identical by the conditioning of the valves. If the velocities are too low,it will be very difficulty to ensure a certain quantity of flow passing through the pipes and avoid any short-circuiting. Unfortunately, from above example of a hospital, we can see that even if the additional flow is considered according to the code,the disadvantage is not avoided. That means the destruction of circulation can‘t avoided.
If the circulating velocity is not very low, will the partial water consumption destroy the circulation ? Let‘s analyze it referring to figure 1. If there‘s a consumption near the inlet of a vertical pipe(as at point A),because the pipe diameter is relatively big enough ,the circulating flow in this pipe won‘t be influenced remarkable, the circulation of the whole pipes is not destroyed .If the consumption is near the outlet of the pipe(as at point B), because the pipe diameter is relatively small,the circulating flow in this pipe will must decrease and the circulation of the pipe seems destroyed. But the total flow of the pipe increases,the heat loss is compensated for, the "destruction"doesn‘t matter; and the circulating flows in other pipes increase a little. In the same way ,if the consumption is at the middle of the pipe(as at point C),the total flow during point A and point C increases, and the total flow (circulating flow )during point C and point B decreases a little. It is clear that the nearer is the consumption point to the inlet ,the less is the influence to the circulation; the nearer is it to the outlet, the shorter is the influenced pipe.
Above analysis shows: although the partial consumption may reduce the circulating flow in a vertical pipe, the total flow of the pipe won‘t decrease. So the heat loss can be compensated for, the destruction doesn‘t harm the purpose of circulation. The more is the consumption, the better is the circulation effect. So long as the circulation can be ensured with no partial consumption, even if the partial consumption occurs, the effect of circulation can be ensured all the same. What really matters is how to ensure the circulation with no consideration of the partial consumption.
7 Conclusions
The additional flow is just adopted to ensure an appropriate factual circulating flow so that the circulation can be ensured; therefor the determination of an additional flow is of significant sense.From my point of view, when we determine the outflow of a circulating pump,the theoretical circulating flow should be figured out first according to the code,then the circulating velocity of each pipe must be verified. If it‘s not very low, the additional flow needn‘t be considered. Otherwise, if it‘s too low, the additional flow should be added to the circulating flow, thus we can get a factual circulating flow that can be the outflow of the pump. the pump head needn‘t be counted as formula?. So an appropriate circulating velocity determines the additional flow. What velocity can be regarded as an appropriate one? This depends. As in above example of a hotel, the outlet velocity of 0.3m/s or so (counted according to the code)is acceptable; it‘s also proved feasible by many other practical projects. But it‘s not the case in a hospital, the additional flow should be determined according to an appropriate velocity(for instance, 0.3m/s) but not to the code.
All in a word, the circulating velocity determines if an additional flow should be taken into account; and the quantity of the additional flow should also be determined by nthe verification of the velocity so as to get an appropriate factual circulating flow. Only in this way can the effect of circulation be ensured.