WO1989011638A1

WO1989011638A1 - A district heating system with consumers having individual water meters and a water meter for use therein

Info

Publication number: WO1989011638A1
Application number: PCT/DK1989/000124
Authority: WO
Inventors: Thomas Bentsen
Original assignee: Unitec A/S
Priority date: 1988-05-17
Filing date: 1989-05-17
Publication date: 1989-11-30
Also published as: DK267688D0

Abstract

In district heating systems based on circulating hot water the inlet temperature at the consumers close to the heating station is noticeably higher than at remote consumers, and there occur considerable difficulties in achieving a correct measuring of the heat consumption by the individual consumers, as there must be accepted either an unfair measuring and accounting or considerably increased operation costs in order to achieve a just measuring and accounting. By the invention the desired justice is achieved without any associated increased operation costs, viz. by installation of a usual flow meter (19) at the individual consumers, said flow meter only being shunted through a pipe (24) in which there is a valve (27, 28), which is controlled by the temperature in such a manner that a decreased inlet temperature results in an increased water flow by passing the flow meter (19). The valve is controlled with such a characteristic that by consumers having a low inlet temperature there is compensated not only for the lower temperature, but also for the condition that the cooler water is exploited less efficiently, namely with an increased outlet temperature. The individual supply stretches can be in operation without any exterior by-pass connection, which gives an appreciable improvement of the overall economy of the heating system.

Description

A district heating system- with consumers having individual water meters and a water meter for use therein.

The present invention relates to a district heating system with at least one long supply stretch with con¬ sumer units connected in parallel, the heat consumption of the consumer units being measured with individual water flow gauges. Usual district heating systems and installations are based on circulating hot water, and accounting on the consumers is based on reading of ther¬ mal metering apparatus at each individual installation. There is used two different main principles for thermal metering apparatus, namely represented by calorie meters and quantity gauges respectively.

The calorie meter summarizes the heat quantity lead to the installation and subtracts the heat quantity returned from the installation to the heating station. This gives ideally a quite accurate measuring result with respect to the heat quantity used, independent of the water temperature at the inlet and outlet of the installation. Hereby a fair accounting to the consumers can be achieved, but the calorie meters are fairly ex¬ pensive and in practice they give occasion for an uneco¬ nomic operation of the heat plant. An economic operation requires namely that the temperature of the water returned to the station is as low as possible e.g. in the range of 30°C. When the heat quantity returned from the installations is automatically credited the respec¬ tive consumers, these are not incited to arrange the installations in such a manner that there is achieved an extensive cooling of the water received. The consumers are inclined to prefer inexpensive and small heaters, which generally are operated at high temperature, where¬ by the return water maybe only is cooled to e.g. 60°C. At such a relatively high temperature of the return water a considerable heat loss occurs in the return pipes, and the boilers at the heat station will operate with poor efficiency.

Quantity gauges in the nature of simple water meters will to the contrary incite the consumers to use the received heat energy as much as possible, as each consumer shall pay according to the water quantity regardless of the heat energy being well or poorly ex¬ ploited. For a good heat utilization the installation expenses can be higher than in the preceding case, but ideally the operation of the entire system will be sig¬ nificantly cheaper and require less overall energy.

Therefore the use of quantity gauges will generally be preferred because hereby a reasonably low temperature of the return water to the heating station will be se¬ cured, but unfortunately the use of said gauges combine other and rather essential problems, which are caused by the condition that a heat loss also occurs in the flow pipes from the heating station. Hereby consumers close to the station will receive the district heating water in a significantly warmer condition than the district heating water which is received by consumers at a longer distance from the station. Even though all consumers provide for a good cooling of the inlet water, it will be unfair if a consumer close to the heating station shall pay the same per ³ hot water as a consumer near the outer end of a supply pipe, where the temperature of the inlet water is significantly lower, such that said consumer is without the possibility to extract corre¬ spondingly much energy of the "consumed" water quantity.

Said problem with respect to a fair accounting by use of quantity gauges has already been overcome to a great extent thereby that a short circuit valve is ar¬ ranged, optionally thermostatically controlled at the outer end of each pair of flow and return pipes. A con- stant supply and return flow will be established through this valve irrespective of the occurring consumption such that the flow of district heating water especially at an occurring generally low consumption will not be cooled essentially during the flow to the outer end of the pair of pipes in question. It can hereby be secured that the different consumers along the pair of pipes can receive district heating water at approximately the same temperature, whereby an accounting based on the consump¬ tion of district heating water can be fairly accurate and just. In return the considerable problem occurs that the heating station shall pump out really unnecessary large quantities of district heating water only to com¬ ply with said fairness, and this requires both a con¬ structional and an operational expensive pump equipment, and moreover such pumping will imply that the return water will be operationally unnecessarily extra heated and thereby cause the said associated great losses.

The object of the invention is to provide a di¬ strict heating system of the above type, in which a fair consumption measurement can be achieved in the indivi¬ dual consumer units with the use of a simple water meter, and whereby further a significant operational saving at the heating station is obtainable.

The invention is based on the basic consideration that at places of consumption, where the temperature of the inlet water is relatively low, there can be provided a "fair" measuring by the quantity gauge if this, di¬ rectly or indirectly, is adapted to show, in a temper¬ ature controlled manner, an under-proportional reading the lower the inlet temperature is. At relatively low inlet temperatures the consumers in question will thus get compensation thereby that the quantity gauge in a controlled manner measures "too little" whereby the measuring can be representative for the effective heat supply. Here it also applies that it is up to the con- sumers to arrange the installation in such a manner that the energy supplied is exploited as well as possible such that the temperature of the return water is kept low. When by the compensated quantity measuring fairness is provided in the measuring, then the reason for ar¬ ranging the mentioned short circuit valves at the outer ends of the supply pipes is no longer there, and conse¬ quently neither an associated increased pump energy nor an associated raised temperature of the return water should be used, whereby the costs of the heating plant are reduced noticeably.

For realization of the invention it is an obvious possibility to provide a quantity gauge, the effective reading of which is currently corrected in accordance with the temperature of the flow medium, which could be obtained in many different manners, e.g. by a tempera¬ ture dependent gearing between the flow sensor of the gauge and its indicating means. However, by the inven¬ tion another solution is preferred, namely to use an entirely conventional water meter and on the other hand to make possible that in a thermostatically controlled manner a greater or smaller subflow of the district heating water can be brought to by-pass the meter. In the first mentioned case it would be a usable possibility that the meter was adjusted to true reading by an inter¬ mediate temperature, as there then could be procured a raised reading at increased temperatures and a reduced reading at lower temperatures, but by the mentioned preferred solution the meter should show a true reading at the highest occurring temperature, where the by-pas¬ sing of the meter is closed completely, while the meter will then read less and less all according to the by-pas¬ sing being gradually increased by decreasing tempera¬ tures.

A control of said by-pass flow can in an easy man¬ ner be obtained by arranging a temperature sensor in the flow or by-pass conduit, which activates an adjusting valve in the by-pass, whereby this valve should then only have such an opening characteristic that the by-pass and the associated reduced reading of the meter is made fair from an accounting point of view. Accord¬ ingly the adjustment should not only correspond to the lower inlet temperature but also take into account the condition that the relatively cool supply water is not exploited as well as the warmer water, as it will nor¬ mally be returned with a higher outlet or return temper¬ ature.

It should be noticed that from the publicly avail¬ able Danish patent application No. 4805/83 (priority 22 October 1982, Italy No. 23879 A/82) it is known to pro¬ vide a thermostatically controlled shunt of a dial meter in order to achieve a reading which is proportional with the inlet temperature of the hot water. This is to bene¬ fit a fair accounting in central water heating systems for sanitary use or for room heating, where the water is not always equally hot. Some consumers might be unfortu¬ nate to consume hot water at moments where the water temperature is lower than at other times, where other consumers tap hot water, and when the accounting is based on the consumed water quantity the latter consumer will get more heat for his money. Therefore it can be excellent to control the water measuring proportionally in accordance with the instant water temperature, but this has nothing to do with the problem here in ques¬ tion, which refers to different consumers along a wide-spread district heating system almost permanently having different individual operation conditions, as certain consumers must accept a lower inlet temperature than other consumers, this being a completely different problem than encountered with local central heating systems.

Seen in relation to the known art mentioned, it is the merit of the invention that it has been recognized that the unjust measuring which occurs by said wide¬ spread district heating networks can be remedied by use of a structurally similar technique, however with a completely different and just not proportional control characteristic, whereby is achieved not only justice aimed at for the individual consumers, but also a col¬ lective profit by an increase of the power factor of the district heating system which is completely unfamiliar for the known art referred to. In practice, with the invention, it will be an extraordinary significant result that the efficiency of a district heating system can be improved with over 5%, which under all circum¬ stances can be to the consumers' benefit quite apart from the fact that at the same time there is achieved an increased fairness in the incidence of the expenses.

To elucidate the special control characteristic of the measurements in a system according to the invention it should be mentioned that it applies almost in general that a 2°C decrease in the water inlet temperature at the individual consumers results in an increase of 1°C in the return water temperature. As it is the difference of temperature which is decisive for the heat consump¬ tion it will accordingly give a false result if there is adjusted proportionally in relation to the inlet temper¬ ature. In the characteristic it should - and may well - be taken into consideration that the heat consumption at a low inlet temperature will be even less than given only by the lower inlet temperature itself, or in other words that there should be opened further for the water by-passing the water meter in order for the measuring to be fair.

The invention will in the following be explained in more detail with reference to the accompanying drawing wherein:-

Fig. 1 shows schematically a conventional district heating system,

Fig. 2 is a similar schematic diagram of a system according to the invention, while

Fig. 3 more detailed shows a metering device at the inlet to a consumer installation located along one of the district heating system supply stretches.

The system shown in Fig. 1 comprises a heating station 2, from which extends a pair of main pipes 4 and 6, which are respective supply and return pipes for district heating water. Along the main pipes and various branch pipes 8 are connected consumer installations 10, each of which has an inlet pipe 12 and an outlet pipe 14. In each of the inlet pipes 12 is placed a thermal metering apparatus 16, while the outlet pipes 14 are connected direct to the return pipe 6.

It is common practice that at the outer end of each of the separate supply stretches there is mounted a thermostatically controlled by-pass valve 17, which valve directly connects the outer ends of the the re¬ spective supply and return pipes 4 and 6, e.g. as indi¬ cated with a closing temperature of 70°C. The consumer units are marked with F_x, F₂ , F₃, F_N and with

"80/30" it is indicated that the unit F-ι_ closest to the heating station 2 receives district heating water at 80°C and returns the water cooled to 30°C. Similarly is with "70/35" indicated that the outermost or remotest consumer on the supply pipe 4, 6 receives the inlet water at 70°C and returns the water at a temperature of 35°c. It is also indicated that the average supply tem¬ perature is 74°C and the average return temperature is 62.5°C, the return water being led back to the heating station 2 with a temperature of approximately 55°C.

It is correspondingly shown in Fig. 2 that by ex¬ ploitation of the invention the same situation "80/30" will occur at the consumer F-_ , while with the consumer F_N the situation "60/40" will occur, when the shunt valve 17 has been removed; the water pumped into the supply pipe 4 will thus only be the water actually used, and since this will amount to a weaker water flow the cooling of the water in the pipes will be more pro¬ nounced. However, the heat loss will be reduced because of the lower average temperature of the pipes. The re¬ duced heat loss will be much more distinct at the return side where the return water at the outer end of the stretch starts to be approximately 40°C as compared with formerly some 70°C. Inwardly towards the heating plant 2 the return temperature decreases due to mixing in of the colder return water from the other consumers and due to cooling from the earth, but the last contribution will be considerably less than in Fig. 1 because the water temperature is significantly lower. The drop in tempera¬ ture in relation to the earth will averagely be approxi¬ mately 14°C less than in Fig. 1, which means a reduction of the heat loss with approximately 23%.

The removal of the by-pass 17 from Fig. 1 further conditions a reduced pump pressure in the heating sta¬ tion.

With the use of water quantity measurements in the meters 16 as a basis for the consumer accounting the remotest consumer F_N in Fig. 1 will have to pay approxi¬ mately 43% more than the consumer ± for a given heat consumption this being a quite commonly occurring situa¬ tion. Also this could be made up for by use of the in¬ vention, but when it is possible by use of a similar arrangement to make fair measurements also in the system shown in Fig. 2, then there is hardly any arguments for using a system according to Fig. 1.

By the system according to Fig. 2 the consumer F_N shall, for a given heat quantity, use approximately 2 1/2 times as much district heating water as the consumer F-ι_, and the water meter here designated 18 shall ac¬ cordingly show 2 1/2 times too little. However, it is preferred to use a true reading meter and on the other hand arrange for 1 1/2 times as much water to be sup¬ plied by-passing the meter. In general it will be the inlet temperature which will determine how much water there should by-pass the meter, and accordingly there can be used the same measuring equipment at all con¬ sumers when said equipment comprises a temperature con¬ trolled measuring by-pass.

Such an equipment is shown in Fig. 3. It comprises a true reading water meter 19 which in series is con¬ nected with a flow chamber 20 having inlet and outlet stubs 21 and 22 and having a connecting branch 23 for a by-pass conduit 24 which leads to a T-fitting 25 on the other side of the water meter. Inside the housing 20 is located a temperature sensing bellow unit 26 which at its lower end forms a valve head 27 which cooperates with a valve seat section 28 at the internal end of the channel through the by-pass connecting conduit 23.

Accordingly there is a totally free flow connection through the water meter 19, this being important because for securing a fair accounting the flow resistance should be the same in all consumers' water meters, while on the other hand it will then be easy to determine the opening characteristic which the valve 27, 28 should have as a function of the temperature in order to ensure that a "fair" water flow can be brought to by-pass the water meter at respective different temperatures.

In practice it has been found that a cone angle of 45° for the valve head 27 is applicable for a suitable compensation if the bellow unit 26 has a linear charac¬ teristic in the actual temperature range, but the angle or generally the shape of the valve head 27, which also can be spherical, can be dependent on special operation parameters both for the system as a whole and in extreme cases also for the consumer installation, such that, the shape should be chosen based on thorough considerations by the experts.

It will be appreciated that the measuring equipment can be constructed by use of standard components with addition of the valve unit 20, which can be mounted in a very simple manner.

The valve unit 20 is adjusted so that the by-pass valve 27, 28 is just closed by the occurring temperature of the outlet water from the plant 2 , while it opens gradually all according to the sensor 26 in the dif¬ ferent installations being exposed to a lower tempera¬ ture.

As a further advantage should be mentioned that in the system according to the invention even some measur¬ ing errors will be compensated for, because for iden¬ tical heat consumptions all of the water meters will operate with the same speed and under uniform con¬ ditions.

Claims

C L A I M S ;

1. A district heating system having at least one long supply stretch with consumer units connected in parallel, the heat consumption of the consumer units being with individual water flow gauges characterized in that the flow gauges are arranged such that by a detec¬ tion of the temperature of the inlet water they produce an effective reading, which is correct at a given refer¬ ence temperature, but for temperatures decreasing there¬ from is superproportionally smaller in such a manner that for a given water consumption the reading is re¬ duced partly corresponding to this primary lowering of the temperature and partly additionally corresponding to the raise of the return water temperature as normally associated therewith, the latter contribution being provided as a result of an applied control characteristic, said supply stretch preferably being without any by-pass connection of its outer end.

2. A district heating system according to claim 1, characterized in that the water flow gauges are of the type that comprises a water meter, a by-pass across this meter, and a temperature sensing controller, which by decreasing temperature leads an increasing subquantity of the water through the by-pass, and that the control¬ ler cooperates exclusively with the by-pass such that the flow resistance in the pipe of the water meter is constant and independent of the position of the control¬ ler.

3. A water gauge for use in connection with a dis¬ trict heating system according to claim 1 and having a flowthrough part, which contains means for detecting the flow, and an indicator part having means to show the summarized flow, based on the flow acting on the detect¬ ing means, characterized in that the gauge further has a water temperature sensor and, operatively connected therewith, a regulator for adjusting the resulting read¬ ing of the indicator part in dependency of the water temperature, namely after a predetermined characteris¬ tic, which accounts for both the temperature of the inlet water and the normally therewith associated in¬ versely proportional temperature of the return water from any of the water meter supervised consumer units in order to generate a reading, which is proportional with the actual heat consumption.

4. A water gauge according to claim 3, where the flow-through part comprises two parallel conduits, one of which contains a water meter having a fixed gearing between the detecting means for the flow and the reading means in the associated indicator part, while the other is a by-pass which leads across the water meter, the regulator being a thermally controlled valve, which by decreasing water temperature causes an increasing sub¬ quantity of water to be delivered through the by-pass, characterized in that the control valve co-operates exclusively with the by-pass, namely by means of a valve head, which is arranged to control the flow of or in the by-pass in accordance with the desired characteristic, while the water meter conduit at both ends is in perma¬ nently open flow connection with the inlet and outlet of the water gauge.

5. A water gauge according to claim 4, character¬ ized in that the regulator valve with its associated thermal sensor unit is placed in a flow-through chamber having connecting stubs for connection to a standard water meter and to the inlet pipe leading thereto or therefrom, respectively, and a stub connection with the by-pass, the latter stub having internally a valve seat, with which the sensor unit cooperates by means of a conical or spherical valve head, which is shaped to achieve the desired flow characteristic for the water through the by-pass.