IMPROVEMENTS RELATING TO RECTIFIER COLUMNS FOR VAPOUR GENERATORS

Abstract: A rectifier column (10) for a vapour generator means of a heat trans­ fer system having a reflux inlet (12) and modulating means (11, 13, 14, 15, 16) at said inlet responsive to a condition representative of the quality of the vapour being generated so as to control the flow of reflux into the column thereby maintaining optimum vapour generation. In one arrangement the modulating means comprises a temperature sensitive actuator (15), e.g. a wax filled bellows (16) which is situated in the path of the generated vapour at the output of the column.

Description: This invention relates to a heat-transfer apparatus comprising a fluid circulatory system and a vapour generator means having a rectifier column in which one component of a heat transfer medium is separated from a mixture of components as a vapour and purified. In particular it relates to the known procedure of adding a condensate of the purified component, known as reflux, to the rectifier column to aid the purification process of the vapour. As in the present invention, in prior rectifier columns of vapour generators used in closed heat-transfer systems, vapour, generated by the so-called reboiler, ascends the column past trays which collect intermediate concentrations of mixture by achieving an equilibrium in each tray between vapour ascending through the column and a liquid condensate descending through the tray system. Heat and mass are transferred in each tray by inducing turbulent contact of liquid and vapour phases so that ascending vapours are enriched with volatile components from the descending liquid whilst less volatile gases condense to dilute the descending liquid, thereby ensuring that ascending vapour is progressively concentrated whilst descending liquid is progressively diluted. In some prior systems a portion of the purified vapour provided by the column is condensed within the column by means of an internal heat exchanger to form the liquid condensate that is fed to the top tray and which passes down through the tray system to establish the aforesaid equilibrium conditions, so purifying the ascending vapour. This condensed vapour, termed reflux, directly affects product quality. Some other systems recover condensate from an external condenser and use that as the reflux. An example of a system using externally generated reflux is disclosed in US-A-2 556 753 in which discrete batches of reflux are returned to the rectifier column at rates which are determined by the flow of condensate from an external condenser. However, the quantity of reflux influences the quality of the vapour. If the quantity of reflux is too low, vapour concentration cannot occur to the required extent and the purity of the rectified vapour is unacceptably low, but if the quantity of reflux flow is too high, then the overall quantity of product, though of high quality, falls. Moreover heat is wastefully transferred from the reboiler to the rectifier column condenser. With some prior columns, in order to preserve the heat energy supplied to the system, fluids from elsewhere in the system are supplied to the said heat exchanger as the coolant. The generation of reflux is uncontrolled and there is usually a surfeit thereof. It is desirable to optimise this reflux in a rectifier column. US-A-2 556 753 and US-A-3 658 655 on the other hand disclose systems in which reflux is added to a respective still under the control of prescribed programs whereby there is provided predetermined time period ratios. These systems are directed towards the generation of distillation products. Accordingly the invention provides a heat-transfer apparatus comprising a fluid circulation system and a vapour generator means which generates vapour from a solution comprising a mixture of components and includes a vapour rectifier column surmounting a boiler, the vapour rectifier column comprising a vertical array of vapour-exchange trays having the purpose of progressively purifying the vapour as the latter ascends the said column, wherein said fluid circulation system includes reflux means, comprising a vapour condenser situated externally of the vapour generator means for condensing vapour purified in the rectifier column, and a duct coupled back to the vapour rectifier column for feeding reflux thereto, characterised in that the supply of reflux to the column is regulated by a valve controlling the passage of the reflux through the duct, which valve is operated in response to temperature sensing means situated so as to react to the temperature of the vapour at the top of the vapour rectifier column. Hereinafter the invention is further described in relation to a number of embodiments illustrated in the accompanying drawings, wherein:- Figure 1 shows in diagrammatic form part of the rectifier column of a boiler showing the reflux liquid input incorporating a flow modulating valve; Figure 2 illustrates an alternative valve arrangement in the reflux column of Figure 1; Figure 3 shows a further arrangement of the reflux column having a modified form of reflux modulating valve; Figure 4 shows yet another form of modulating valve for the rectifier column; Figure 5 shows a typical rectifier column in section and including yet a further form of modulating valve for modulating the flow of reflux liquid; Figure 6 shows the construction of the modulating valve of Figure 5 in greater detail; and Figure 7 shows a novel heat pump system embodying the rectifier column fitted with a modulating valve according to the invention. In the arrangements of Figures 1-3, reflux flow from a condenser 9 into the rectifier column 10 is controlled by a slidable piston 11 which progressively opens or closes an orifice 12 in the side wall of a cylinder 13 in which the piston slides. Thus the piston and the cylinder arrangement take the form of a spool valve. The piston is connected to a rod 14 which is moved by means of an actuator 15. The latter may take several forms and may be variably positioned. In the arrangement of Figure 1 the actuator is a bellows 16 which contains an expansible medium such as wax and which is housed within the rectifier column and situated in the environment of the output vapour. Increase of temperature within the column in the vicinity of the bellows causes a greater opening of the reflux valve and hence an increased reflux, conversely a drop in temperature reduces the reflux input into the column. With the normal solutions in use, e.g. ammonia in water, a drop in temperature occurs as the ammonia content in the solution is increased; thus there is provided a positive feedback control for the device. By controlling the reflux in this way it is possible to operate the rectifier column automatically to provide optimum conditions. The valve shown in Figure 2 has essentially a similar action to that previously described but with a different actuator 15. In this case it comprises a set of bimetal strips or discs 20 coupled face to face so as to provide a movement of the actuating member as the temperature varies in the rectifier column which is equal to the summed movement of all the strips or discs. In Figure 3 a somewhat different physical arrangement is shown. The spool valve, though illustrated adjacent to the rectifier column, may in fact be remote therefrom. The bellows 16 is contained in a housing 22 which may be sited at a position which is intermediate the rectifier column 10 and the reflux condenser 9 and it may also be at ambient temperature. The bellows is linked to a sensor bulb 23, which is shown contained within the rectifier column, by means of suitably connecting tubing 24 and the whole actuator system is filled with an expansible wax. When the temperature of the bulb changes the wax within its expands or contracts, thereby causing wax to be forced into the bellows or removed therefrom via the aforesaid tubing and causing actuation of the spool valve. This arrangement facilitates a simplification in the construction of the rectifier column, but the latter is not the only possible location for the bulb, as there are other locations within systems in which rectifier columns are used, where the temperature of fluids are related to the quantity of reflux required at any instant for optimum performance; downstream of an expansion valve in a heat pump is one such position. A further alternative, this time again situated in the rectifier column, is shown in Figure 4. It comprises a post 25 extending into the interior of the rectifier column, attached at one end to a plug 26 in the reflux inlet and having a free end. An apertured disc closure member 27 co-operates with an orifice in the plug 26 and is rotatably mounted upon the post. A helical element 28 formed from a bimetal strip has a first end attached to the disc at a radial position thereon and a second end attached to the free end of the post. As the temperature within the rectifier column changes so the diameter of the helical element changes, resulting in the rotational displacement of the end coupled to the disc. By such means the aperture in the plug is obstructed to a greater or a lesser degree by the disc, thus modulating the flow of reflux from the condenser 9. Referring now to Figures 5 and 6, there is shown a typical rectifier column 10 fitted with yet a further form of reflux modulating valve. This is essentially an orifice 30 defined by a tube 31 which may be a simple sealing push fit into the reflux inlet duct 32 which enters the rectifier column through its vertical wall 33. A threaded collar 34 (best seen in Figure 6) is carried on a corresponding threaded portion of the tube and to the collar is attached a dogleg support arm 35 which tends to overhang the said orifice. A bimetal strip 36 is attached to the dogleg of this support arm at one end which at its other end co-operates with orifice 30 in the tube 31 to control the flow of reflux liquid therethrough. The bimetal strip is mounted with an orientation which causes it to engage the tube when the temperature around the reflux input falls and to move away therefrom when the temperature rises. In this way because the temperature falls with increasing ammonia content in the solution in the trays, the cooler vapour passing the bimetal strip tends to close the valve, and as the temperature rises due to reduced ammonia the valve tends to open to admit further quantities of reflux. The operation of the valve may be adjusted by rotating the collar on the tube, which by virtue of the threaded engagement, causes it to move in an axial direction therealong. Conveniently, once set, it is locked in position by means of a set screw (not shown). Also conveniently the collar may take the external form of a nut (not shown) for easy adjustment. Sealing engagement of the tube 30 in the reflux inlet duct 32 is provided by means of a pair of "O" rings 37, 38, which are respectively partially housed in circumferential slots 39, 40 provided on the external surface of the tube. These "O" rings engage the inner surface of the reflux inlet duct 32 when the valve is inserted therein. An example of a novel advantageous heat pump system which has been realised by the use of a present reflux valve is disclosed in Figure 7 which illustrates the aspects of the system concerned with the transportation of the system fluid mediums. A pump 100 pumps a strong solution from the absorber 102 to the rectifier column 10 via a heat receiving coil 105 in the condenser 9 and a weak/strong solution heat exchanger 108, on both occasions picking up heat from other fluids in the system. This pumped solution gives off a volatile fraction in the rectifier column and the remainder drains as a weak solution into the generator 110 where it receives further heat and as a consequence releases even more vapour. The heated but weakened solution is ducted to the pump as its working fluid, passing thereto via the heat exchanger 108 where some of its sensible heat is transferred to the aforesaid strong solution passing in the opposite direction. From the working chamber of the pump the weak solution flows into the input of the absorber 102 to complete the circulation of the solution. Vapour leaving the rectifier column 10 is ducted to the condenser 9 where its latent heat is removed. The condensate is split at the condenser, a fraction thereof being returned as reflux to the rectifier column 10, whilst the major proportion flows through an expansion valve 112 from there into an evaporator 114 where it is revaporised. The vapour is ducted to the absorber 102 where it enters into solution in the soluble medium of the system. For optimum operation the split in the condensate has to be controlled. This control is performed by the reflux valve 116 situated at the reflux inlet to the rectifier column. The control is exercised in one of the manners hereinbefore described. The use of a single condenser 9 to provide both reflux and the liquid phase to the main system simplifies the construction of the apparatus and therefore reduces costs. It is made possible only by the presence of reflux valve 116. The purpose in feeding the strong solution through the heat exchanger coil 105 in condenser 9 and through the solution heat exchanger 108 is to preserve and make use of the heat supplied to the system via the generator 110 as long as possible. Analysis shows that, as a general proposition, the greater the work done by the heat supplied through the boiler, the greater is the heat transferred from the environment to the point where it is put to use.

Claims: A heat-transfer apparatus comprising a fluid circulation system and a vapour generator means which generates vapour from a solution comprising a mixture of components and includes a vapour rectifier column (10), the vapour rectifier column comprising a vertical array of vapour-exchange trays (8) having the purpose of progressively purifying the vapour as the latter ascends the said column, wherein said fluid circulation system includes reflux means, comprising a vapour condenser (9) situated externally of the vapour generator means for condensing vapour purified in the rectifier column, and a duct (7) coupled back to the vapour rectifier column for feeding reflux thereto, characterised in that the supply of reflux to the column is regulated by a valve (11, 13, 26, 31, 116) controlling the passage of the reflux through the duct (7), which valve is directly operated in response to temperature sensing means (16, 20, 23, 28,) situated so as to react to the temperature of the vapour at the top of the vapour rectifier column. Apparatus according to Claim 1 wherein said valve comprises a spool valve (11). Apparatus according to Claim 2 further comprising a bellows (16) forming the functional part of an actuator (15) for said spool valve (11). Apparatus according to Claim 3 wherein said bellows (16) contains an expansible wax substance and is situated within said rectifier column (10) so as to sense the temperature of the output vapour. Apparatus according to Claim 3 wherein the bellows (16) is situated externally of the rectifier column (10) and is coupled by means of a duct (24) to an expansion bulb (23) situated in the fluid circulatory system at a position at which the temperature of the fluid is functionally related to the quantity of reflux required to maintain optimum conditions of vapour generation. Apparatus according to Claim 1 wherein said valve is actuated by a linear arrangement of bimetallic elements (20) which act serially in response to the temperature of the output vapour to position the valve. Apparatus according to Claim 1 wherein said valve comprises a disc (27) which is angularly positioned by a bimetallic element (28) which, in response to changes in temperature of the output vapour, turns the disc (27) about an axis (25) so as to open or close the valve. Apparatus according to Claim 1 wherein said valve comprises a cantilevered bimetallic strip (36) which acts so as to open or close an orifice (30) forming the source of reflux into the rectifier column (10).

Inventors: HOLMES, PAUL, HOUGHTON, FRED
Applicants: BRITISH GAS PLC
Application year: 1988
Main classification: B01D3
IPC codes: B01D3, B01D5