GB2096317A - Heat metering - Google Patents

Abstract

In a device for the transfer and metering of thermal energy between a supply main and a consumer system, thermal contact between fluids flowing in supply and receiver ducts is effected by means of one or more thermosiphons (30), each containing a working fluid in contact with its vapour, and each having a condenser area (34) in the receiver duct, and an evaporator area (35) in the supply duct. Liquid condensate returns from the condenser area to the evaporator area through a liquid passage (42) which contains a flowrate meter (46) providing a pulsed output in which each pulse represents a quantum of fluid flow therethrough. The pulses are received by a counting and display device (48) calibrated to provide a direct reading of the quantity of thermal energy supplied. <IMAGE>

Description

SPECIFICATION Heat metering This invention relates to the transfer of thermal energy between a source and a consumer, and in particular to means by which the quantity of thermal energy transferred can be measured.

District heating schemes are coming increasingly into being, in which thermal energy generated centrally is supplied to individual consumer systems, for example to individual dwellings where it can be used for heating the interior thereof, and for the supply of hot water.

Similar schemes operate on an industrial scale where the consumer might be a factory or other industrial undertaking. Such schemes have a number of advantages, in that a constant ready supply of thermal energy can be available to the consumer, and especially in that low-grade thermal energy can be utilised which might otherwise be wasted. In the latter respect, the socalled combined heat and power schemes are particularly economical, in which heat produced as an incidental to power generation can be put to useful effect instead of creating a disposal problem. It is also possible that refrigerant could be supplied from a central source to individual consumer systems from which heat is to be abstracted, and the invention is also useful in relation to such arrangements.

In the most usual method of distribution, thermal energy is supplied to users by a system of main pipes carrying hot water, often under relatively high pressure. Of course, other heat transfer media such as steam can be used instead of water.

In order to provide a fair basis for charging related to consumption, the thermal energy transferred between the main and the consumer needs to be measured. The direct method of approach involves measurement of supply and return temperatures of heat transfer medium and its flowrate. The thermal energy consumed can then be calculated from the product of three factors, ie the temperature difference, the flowrate, and the time interval. An instrument which provides a reading of thermal energy consumed in this way is called a true heat meter.

In most practical cases the temperature difference and/or the flowrate will vary with time, and the instrument must then be capable of calculating the aforesaid product over short successive time intervals and summating the results. This makes for a relatively complex instrument, which can be simplified to some degree by keeping one or more factors constant (ie flowrate, and/or supply temperature, and/or return temperature). However, this in itself introduced further complexity as well as imposing constraints on the system which may have practical disadvantages. Such meters always need some form of flowrate meter which is required to measure the flowrate of a hot heat transfer medium which is usually impure and carrying dirt in suspension. Most flowrate meters are unsuitable for such an environment, and this gives rise to frequent failures.In many cases the flowrate meter is required to measure a range of flowrates down to a very low rates. For this purpose a very sensitive meter is required, but such a meter is particularly unsuitable for use with hot, dirty liquids.

As an alternative to the true heat meter, there are known a number of different types of "apportioning meter" which do not themselves measure heat consumption, but employ some means of indication which varies in accordance with the heat emission from a heat exchanger. If each consumer has a similar meter, the total quantity of heat used by a group can be apportioned among the individual consumers.

Some physical phenomena commonly exploited in this type of meter are evaporation of a liquid, metallic creep, thermocouple e.m.f. and variation with temperature of the resistance of certain electrical resistors. These devices still require some measurement of the total quantity of heat consumed by the group and tend to be rather rough and ready in their apportionment.

In addition, there are also known proportional meters in which a proportion of the hot liquid drawn from the supply main by a consumer is diverted into a measuring system. Such proportional meters are described for example, in UK Patent Nos 1199311; 701832672476 and 1 92610. These meters are very prone to inaccuracy because they rely upon a constant proportion of the hot liquid being diverted to the measuring system. This is exceedingly difficult to achieve where the flowrate of liquid drawn from the main is variable. In each of these proportional meters the heat in the diverted liquid is simply wasted, and hence to minimise this serious disadvantage it is important to keep the proportion diverted to the meter as small as possible. This compounds the difficulty of ensuring that a constant proportion is diverted.

The meters described in UK Patent Nos 1199311; 672476; and 192610 are all of a calorimetric type which cannot operate continuously and in which the quantity of hot liquid which can be metered at any one tapping is limited by the capacity of the meter. The meter must be allowed time to return to ambient temperature before a further tapping takes place, or further inaccuracy will result.

In UK Patents Nos 400821; and 251379, there are disclosed systems in which heat is distributed from a central source through a main which carries steam. Each consumer is provided with a heat exchanger through which heat is provided to the consumer's heating system. A liquid flowmeter measures the volume of liquid condensed from the main as the result of heat supplied to the consumer through his individual heat exchanger, and the volume of condensate can be related to the heat consumed. Such systems, however, suffer from a number of disadvantages. In order to provide an accurate indication, the steam supplied and the water returned must be at known, preferably constant temperatures, otherwise an un-metered flow of heat will take place as a result of unknown variations in supply and return temperatures.

Another difficulty is that the flow meter must be capable of registering accurately over a wide range of flowrates. Available meters capable of meeting this requirement are not normally sufficiently rigged to withstand working with hot, dirty liquids such as would be encountered in a steam main.

A need therefore exists for a true heat meter of reasonably simple form which is capable of reasonably accurate measurements, and which avoids the difficulties associated with requiring a sensitive flowmeter to measure the flowrate of hot dirty heat transfer medium.

In many instances, it is desirable to isoiate the main from the consumer's heating system, especially where the main is at high pressure, and this is commonly done by employing a heat exchanger through which heat is transferred from the main to the consumer's system without any fluid communication occurring between the two systems.

The Applicant has appreciated that by use of a particular form of heat exchanger, a very advantageous method of heat metering is made possible.

According to the present invention, a device for the transfer and metering the total quantity of thermal energy transferred between a supply main and a consumer system includes at least one thermosiphon comprising a hermetically sealed vessel containing a liquid in contact with its vapour and having a first heat transfer area of the vessel wall adapted to make thermal contact with a supply fluid can flow in the supply main, and a second heat transfer area of the vessel wall adapted to make thermal contact with the heat sink a consumer fluid which can flow in the consumer system, a vapour passage through which the vapour can flow between the first and second heat transfer areas, a liquid passage through which a return flow of liquid condensate can occur between the first and second heat transfer areas, and means for measuring the quantity of the liquid which flows through the liquid passage.

In the envisaged application of the invention the device will normally comprise a supply duct for connection into the supply main and a receiver duct for connection into the consumer system, said first heat transfer area being located adjacent the supply duct and said second heat transfer area being located adjacent the receiver duct.

In one useful arrangement the first heat transfer area of the vessel wall constitutes an evaporator area in which the liquid within the sealed vessel can be evaporated and the second heat transfer area of the vessel wall constitutes a condenser area in which vapour within the sealed vessel can be condensed.

Conveniently, the device comprises a thermosiphon having a plurality of separate evaporator areas and a plurality of separate condenser areas, the evaporator and condenser areas being connected by a common liquid passage, the separate evaporator areas of each thermosiphon being arranged to extend transversely with respect to the direction of flow in the supply duct, and the separate condenser areas of each thermosiphon being arranged to extend transversely with respect to the direction of flow in the receiver duct. The evaporator and condenser areas can also be connected by a common vapour passage if desired.

In another advantageous arrangement, the supply duct is split into a plurality of distinct supply duct portions which pass through the sealed vessel, each supply duct portion being defined at least in part by a wall which seals the interior of the portion from the interior of the sealed vessel and constitutes a part of the evaporator area.

The receiver duct can similarly be split into a plurality of distinct receiver duct portions which pass through the sealed vessel, each receiver duct portion being defined at least in part by a wall which seals the interior of the portion from the interior of the sealed vessel and constitutes a part of the condenser area.

The hot and cold fluids can be arranged to flow in any desired way through the supply duct portion and receiver duct portions respectively, ie in series or in parallel or in any desired combination of series and parallel groupings.

Advantageously there are provided two or more mutually independent thermosiphons having the evaporator areas of independent thermosiphons arranged successively along the intended direction of flow in the supply duct and the condenser areas of independent thermosiphons arranged successively along the intended direction of flow in the receiver duct.

The invention will now be described by way of example only with reference to the accompanying drawings, of which Figure 1 is a schematic longitudinal sectional elevation of a heat exchanger comprising a plurality of thermosiphons, Figure 2 is a schematic end elevational view in section of the heat exchanger shown in Figure 1, Figure 3 illustrates graphically the temperatures obtaining in the fluids when the heat exchanger of Figures 1 and 2 is operated in counter-current fluid flow, Figure 4 is a schematic end elevational view in section of a device for the transfer and metering of thermal energy in accordance with the invention, Figure 5 illustrates graphically the temperature distribution obtaining in a less preferred embodiment of the invention, Figure 6 is a sectional view of another device in accordance with the invention, and Figure 7 is a schematic longitudinal sectional elevation of an arrangement employing a plurality of devices as shown in Fig 6.

As shown in Figs 1 and 2, a heat exchanger comprises a duct 1 forming part of a supply main for carrying a flow of hot fluid and a receiver duct 2 above the duct 1 for carrying a flow of cooler fluid forming part of a consumer system such as a domestic heating circuit. The intended directions of flow of the hot and cooler fluids are indicated in Figure 1 by the respective arrows H and C. Heat can be transferred between fluids in the ducts 1 and 2 via a plurality of mutually independent thermosiphons 3. As seen in Fig. 2, the thermosiphons are arranged in rows extending transversely across the ducts, and as seen in Fig 1 there are a plurality of such rows disposed at successive locations longitudinally of the duct.

Each thermosiphon is in the form of a hermetically sealed cylindrical vessel containing a liquid in contact with its vapour (not shown)-water being a particularly suitable liquid where the ducts 1 and 2 are to carry water. The lower part 5 of the wall of each thermosiphon protrudes into and across the supply duct 1, and this area of the vessel wall constitutes a first heat transfer area which serves as an evaporator area. The upper part 4 of the wall of each thermosiphon protrudes into and across the receiver duct 2, and this area of the vessel wall constitutes a second heat transfer area which serves as a condenser area.

In use of such a heat exchanger, hot and cooler water flow in countercurrent through the respective ducts 1 and 2 as indicated by the arrows H and C. Water sealed within each thermosiphon vessel 3 boils within the evaporator area 5 as a result of heat transferred from the water in the duct 1. Vapour thus created rises up within the vessel 3 and condenses within the cooler condenser area 4 of the vessel wall, giving up its latent heat of vapourisation which is transferred through the wall to the cooler fluid in the duct 2. The condensate runs back under gravity down the sides of the vessel 3 into the evaporator area 5 where it can boil again. This is a conventional operation of a thermosiphon, in which it acts as a highly efficient thermal conductor.

In order to provide a measure of the quantity of thermal energy transferred, each thermosiphon is provided with means (not shown) for measuring the quantity of condensate returned to the evaporator. In practice this will most readily be achieved by providing separate flow paths for vapour and condensate. The volume of condensate descending to the evaporator is exactly related to the quantity of heat transferred from the duct 1 to the duct 2 by the equation:- Q=VLD where 0 is the quantity of heat transferred D is the density of the liquid in the vessel 3 L is the latent heat of vapourisation of the liquid in the vessel 3 V is the volume of condensate descending.

Thus, if the product LD can be assumed constant, the volume of condensate is in direct proportion to the quantity of heat transferred, and measuring that volume is a simple and effective way of determining the quantity of heat transferred.

A most suitable fluid for use in the thermosiphon vessel 3 is water, for which the value of LD varies slightly with temperature. The temperature of the water in the vessel 3 is always between the temperature of the hot fluid and that of the cooler fluid. Typically in use in a district heating scheme it might vary from 80 to 100 C, and in that range the product LD varies by 2%. If an average value were used, the maximum error from this cause would thus be only +1%.

Thus, by measuring only the quantity of liquid condensate flowing down through each vessel 3 during a given period, and summing the values for each vessel, an indication of the heat transferred is readily obtainable.

In order to obtain good heat transfer, an array of heat transfer elements, as for example in Figures 1 and 2, is desirable. However, the provision of separate flowmeter for each of such a large number of separate thermosiphons, and means to summate their readings, is inconvenient and expensive. In Figure 4 there is shown a heat exchange device comprising a single thermosiphon 30 having a plurality of distinct condenser areas in the form of cylindrical tubular elements 34, and a plurality of distinct evaporator areas in the form of cylindrical tubular elements 35. The evaporator areas 35 are arranged transversely across a supply duct 1 for hot water so that the hot water can flow between them for good heat transfer, and the condenser areas 34 are similarly arranged transversely across a receiver duct 2 for cooler water.The lower ends of the evaporators are connected to a common sump 41, and the lower ends of the condensers are connected to a common sump 40. The sumps 40 and 41 are interconnected through a single downcomer or liquid passage 42, so that all condensate from the condenser areas 34 drains through the sump 40, via the liquid passage 42, to the sump 41. The upper ends of the evaporators are connected by a common header space 43, and the upper ends of the condensers are connected by a common header space 44, the spaces 43 and 44 being connected by a single vapour passage 45. The condensers defined by the condenser areas 34, the evaporators defined by the evaporator areas 35, the sumps 40 and 41, the header spaces 43 and 44 and the passages 42, 45 collectively define a hermetically sealed vessel constituting the single thermosiphon 30.

A liquid flowrate meter 46 is located in the downcomer 42 to measure the fluid flow therethrough. The meter 46 is of a kind which produces a pulsed electrical output, each pulse representing a quantum of volume of the flow therethrough. The pulsed output is transmitted via an electrical connect 47 to a counting and display device 48, which counts the number of electrical pulses received, and displays the total on a scale calibrated in units of thermal energy consumed.

It is intended that the thermosiphon 30 should replace a transverse row of thermosiphons 3 such as those shown in Fig 2, so that only a single flowmeter is needed instead of one for each thermosiphon 3.

In the heat exchanger shown in Figs 1 and 2, there are eight transverse tows of the thermosiphons 3. Such an array will typically give temperature profiles as shown in Fig. 3 for counter-current flow where th is the temperature of the hot liquid in duct 1, tc is the temperature of the cold liquid in duct 2, and x represents distance in the longitudinal direction of flow in the duct 1.

Similar profiles will be obtained when each transverse row of thermosiphons 3 is replaced by a single thermosiphon 30. Each thermosiphon 30 must have its own flowmeter 46, but if the flowmeters are each calibrated to produce electrical pulses representing the same volume of liquid, connections 47 can feed the outputs of all flowmeters 46 to a common counting and display device 48. Although a plurality of flowmeters are required, a single counting and display device is sufficient.

For a pure countercurrent heat exchanger transferring the maximum quantity of heat, an infinite number of thermosiphons 30 would be required theoretically. However, considerable cost saving with very little loss of performance can be obtained by providing only three thermosiphons 30 of comparable surface area. In many systems, satisfactory performance may be achieved with only two such thermosiphons 30, with thus only two flowmeters required.

It might be thought that further savings could be achieved by having a plurality of thermosphons 30 modified to have a single liquid downcomer 42 common to them all. Only a single flowmeter would then be required for the entire device. This approach would not give very satisfactory results because as illustrated in Figure 5, the temperature of the liquid within the linked vessels 30 would assume the same value t, in each vessel 30. The temperature th of the hot liquid could not fall below ts and the temperature tc of the cooler liquid could not rise above ts no matter how many thermosiphons 30 where used. Thus, although only a single flowmeter would be required, the heat transfer would be inefficient as compared with the true countercurrent arrangement described with reference to Figures 1 to 4.

It will be apparent that the thermosiphon through which heat is conveyed from source to sink need not consist of tubular elements constituting the evaporator and/or condenser areas, nor need any such elements be vertically orientated. As a further example, Figure 6 illustrates an embodiment of the invention in which a shell boiler arrangement is used for both evaporator and condenser. Alternatively, a plate boiler or other suitable heat exchange device might be used.

As shown in Figure 6, a device for the transfer of heat from a supply fluid flowing in a supply main 51 to a consumer fluid flowing in a receiver duct 52 forming part of a consumer system comprises a boiler 53 and a condenser 54 each of shell and tube design.

The boiler 53 comprises a hot water box 55 having a flanged inlet connection into an upstream arm of the supply main 51 and a second hot water box 56 having a flanged outlet connection into a downstream arm of the supply main 51. The boxes 55 and 56 are connected together by a plurality of tubes 57, so that within the boiler 53 the supply main duct is split into a plurality of distinct supply duct portions constituted by the tubes 57.

The condenser 54 comprises a receiver water box 58 having a flanged inlet connection into an upstream arm of the receiver duct 52, and a second receiver water box 59 having a flanged outlet connection into a downstream arm of the receiver duct 52. The boxes 58 and 59 are connected together by a plurality of tubes 60, so that within the condenser 54 the receiver duct 52 is split into a plurality of distinct receiver duct portions constituted by the tubes 60.

Heat can be transferred from the boiler 53 to the condenser 54 by means of a thermosiphon comprising a boiler shell 61, a condenser shell 62, a vapour passage 63 and a liquid downcomer 64, the elements 61, 62, 63 and 64 together constituting a hermetically sealed vessel containing a liquid and its vapour.

The boiler shell 61 is bounded in part by walls of the boxes 55, 56 and the tubes 57 pass through the shell 61. Hot liquid (water) constituting a heat source flowing through the supply duct 51 can thus transfer heat to the thermosiphon fluid through those bounding walls of the boxes 55, 56, and the walls of the tubes 57, which walls thus constitute evaporator areas of the thermosiphon.

The condenser shell 62 is bounded in part by the walls of the boxes 58, 59 and the tubes 60 pass through the shell 62. Cooler liquid constituting a heat sink flowing through the receiver duct 52 can thus receive heat from the thermosiphon fluid through those bounding walls of the boxes 58 and 59, and the walls of the tubes 60, which walls thus constitute condenser areas of the thermosphon.

During the heat transfer process, liquid boils in the boiler shell 61, rises up the vapour passage 63, condenses in the condenser shell 62, and returns under gravity through the downcomer 64 to the boiler shell 61. This cycle is continuously performed by the thermosiphon fluid during the heat transfer process.

In order to provide a measure of the quantity of heat transferred, a flowmeter 65 is included in the downcomer 64, which provides a pulsed output via an electrical connection 66 to a counting and display device 67. The operation is the same as that of the flowmeter 46 and counting the display device 48.

In order to provide for more efficient heat exchange, a plurality of separate thermosiphons each for example as shown in Fig 6 and each comprising a boiler 53 and a condenser 54, can be provided as illustrated in Figure 7. In use the hot fluid constituting the heat source flows in the duct 61 in the direction of the arrow H, and the cooler fluid constituting the heat sink flows in counterflow thereto in the duct 62, in the direction of the arrow C. The temperature distribution along the ducts 61 and 62 will be substantially as shown in Figure 3, ie substantially as for a true countercurrent heat exchanger.

Although three thermosiphons as shown in Fig 7 will often provide the best compromise between performance and economy, arrangements with either two or four or more thermosiphons are possible. It should be noted that a separate flowmeter 65 is required for each thermosiphon, although it can be arranged that the outputs of all flowmeters are summed and displayed in a single counting and display device. This can be done readily where the flowmeters 65 provide a pulsed output, each pulse representing the same unit of volume.

It will be apparent to those skilled in the art that the present invention provides many advantages in the supply of metered quantities of thermal energy. Conventional heat meters have to measure not only volume, but also two temperatures, and a complicated integration of the product of the several variables must be performed. Their accuracy is usually limited by the accuracy with which the difference between the two temperatures can be measured, when the difference is small. The present invention provides a system in which no temperature measurement or complicated calculation is required.

Furthermore, the flowmeter required in the conventional heat meter has to cope with the hot dirty working fluid in the main or the domestic system. In the present invention, the liquid which is metered is the relatively small quantity recirculated continuously within the thermosiphon vessel. This liquid can be of a high degree of purity so that the adverse effect on the flowmeter is avoided, irrespective of the quality of the fluid in the main or domestic circuit.

Heat metering devices according to the present invention also have the effect of isolating the main from the consumer's circuit. This can be important where the main is at high pressure.

Claims (12)

Claims

1. A device for the transfer and metering of the total quantity of thermal energy transferred between a supply main and a consumer system, said device comprising at least one thermosiphon comprising a hermetically sealed vessel containing a liquid in contact with its vapour and having a first heat transfer area of the vessel wall adapted to make thermal contact with a supply fluid which can flow in the supply main, a second heat transfer area of the vessel wall adapted to make thermal contact with a consumer fluid which can flow in the consumer system, a vapour passage through which the vapour can flow between the first and second heat transfer areas, a liquid passage through which a return flow of liquid condensate can occur between the first and second heat transfer areas, and means for measuring the quantity of the liquid which flows through the liquid passage.

2. A device according to claim 1 comprising a supply duct for connection into the supply main and a receiver duct for connection into the consumer system, said first heat transfer area being located adjacent the supply duct and said second heat transfer area being located adjacent the receiver duct.

3. A device as claimed in claim 2 wherein the said first heat transfer area of the vessel wall constitutes an evaporator area in which the liquid within the sealed vessel can be evaporated, and the said second heat transfer area of the vessel wall constitutes a condenser area in which vapour within the sealed vessel can be condensed.

4. A device as claimed in claim 3 wherein the said at least one thermosiphon has a plurality of separate evaporator areas and a plurality of separate condenser areas, the evaporator areas being connected to the condenser areas by a common liquid passage, the separate evaporator areas of each thermosiphon being arranged to extend transversely with respect to the direction of flow in the supply duct, and the separate condenser areas of each thermosiphon being arranged to extend transversely with respect to the direction of flow in the receiver duct.

5. A device as claimed in claim 4 wherein the said separate evaporation areas are connected to the said separate condenser areas by a common vapour passage.

6. A device as claimed in claim 3 wherein the supply duct is split into a plurality of distinct supply duct portions which pass through the sealed vessel, each supply duct portion being defined at least in part by a wall which seals the interior of the said supply duct portion from the interior of the sealed vessel and constitutes a part of the evaporator area.

7. A device as claimed in claim 3 or claim 6 wherein the receiver duct is split into a plurality of distinct receiver duct portions which pass through the sealed vessel, each receiver duct portion being defined at least in part by a wall which seals the interior of the portion from the interior of the sealed vessel and constitutes a part of the condenser area.

8. A device as claimed in any one of claims 3 to 7 comprising two or more mutually independent thermosiphons arranged successively along the intended direction of flow in the supply main and the condenser areas of independent thermosiphons arranged successively along the intended direction of flow in the consumer system.

9. A device as claimed in any one preceding claim wherein the means for measuring the quantity of liquid which flows through the liquid passage comprises, in each liquid passage, a flowrate meter of a kind which produces a pulsed electrical output, each pulse representing a quantum of volume of the flow therethrough.

10. A device as claimed in claim 9 wherein the pulsed output of the or each liquid flowmeter is transmitted to a counting device which counts the number of electrical pulses received.

11. A device as claimed in claim 10 wherein the counting device is capable of a display representing the total number of electrical pulses on a scale calibrated in units of thermal energy consumed.

12. A device according to claim 1 and substantially as hereinbefore described.

1 3. A device substantially as hereinbefore described with reference to Figures 1 and 2, Figure 4, Figure 6 or Figure 7 of the accompanying drawings.

1 4. Any novel and inventive feature or combination of features disclosed herein.