WO2001011295A1

WO2001011295A1 - A generator for an absorption chiller

Info

Publication number: WO2001011295A1
Application number: PCT/GB2000/002938
Authority: WO
Inventors: Robert James Tucker; David Anthony Clark; Jeffery David Sadler; Aik Beng Lua
Original assignee: Lattice Intellectual Property Limited
Priority date: 1999-08-06
Filing date: 2000-07-31
Publication date: 2001-02-15
Also published as: AR025853A1; MXPA02001278A; GB0018536D0; EP1206669A1; JP2003506659A; GB2355059A; BR0013078A; CN1378634A; GB9918581D0

Abstract

A generator (4) for an absorption chiller (2) comprising a container (42) with an inlet (68) for a solution of absorbent and refrigerant; a heater (46) to generate a refrigerant vapour from the solution; a first outlet (72) for the refrigerant vapour; and a second outlet (70) for the non-vapourised liquid wherein a barrier is provided within the container (42) between the inlet (68) and a second outlet (70) and the barrier is arranged such that for non-vapourised liquid to leave through the second outlet (70) it must pass under a first portion (80) of the barrier and over a second portion (82) of the barrier to ensure that non-vapourised liquid with greater concentration of absorbent leaves through the second outlet (70). The barrier also provides a calm area (78) between itself and the second outlet (70) for level measurements which may be used to control the flow of fluids into the container (42).

Description

A GENERATOR FOR AN ABSORPTION CHILLER

The present invention relates to a generator for an absorption chiller for separating refrigerant and absorbent from a solution of the two.

An absorption chiller circuit supplies liquid refrigerant to an evaporator. The refrigerant in the evaporator absorbs heat from its surroundings to provide a desired cooling effect and undergoes a phase change from liquid to vapour. The vapourised refrigerant is then absorbed by an absorbent to form a solution of the two. The solution is supplied to a generator in which the two are separated, generally by boiling off the refrigerant as vapour which is then condensed and supplied to the evaporator again to continue to provide the desired cooling effect. The separated absorbent is used to absorb the vapourised refrigerant from the evaporator.

However, the separated absorbent from the generator still contains a considerable amount of refrigerant so that it is not able to absorb as much vapourised refrigerant from the evaporator as is desirable. Thus the circuit requires more absorbent to compensate for this, producing a larger absorbent chiller circuit with bigger conduits, greater absorbent cost and higher running costs. It is also desirable to ensure that the generator does not run out of refrigerant-absorbent solution otherwise it is liable to become damaged by continued heating to boil off refrigerant. According to a first aspect of the present invention a generator for an absorption chiller comprises a container with an inlet to receive solution of absorbent and refrigerant; a first outlet for refrigerant vapour to leave the container; and

a second outlet for the non-vapourised liquid to leave the container; wherein a barrier is provided within the container between the inlet and the second outlet and the barrier is arranged such that for non-vapourised liquid to leave through the second outlet it must pass under a first portion of the barrier and over a second portion of the barrier.

The portion of the non-vapourised liquid that contains most absorbent, forming the most concentrated absorbent solution will be the most dense and so will settle at the bottom of the container. By the barrier being arranged such that non-vapourised liquid must pass under the first portion of the barrier, only the more concentrated absorbent solution is passed to the second outlet. The size of the gap through which the non-vapourised liquid passes under the first portion of the barrier determines the concentration of the non-vapourised liquid removed.

By the barrier being arranged such that the non-vapourised liquid must pass over the second portion of the barrier, the container will not run out of refrigerant-absorbent solution during use. Fluid in the portion of the container on the inlet side of the barrier will be turbulent because of the continued delivery of more solution through the inlet, the boiling-off of refrigerant vapour and the possible delivery and withdrawal of the solution for other purposes. It is thus very difficult to determine the level of solution in the container. However, as solution only passes to the second outlet side of the barrier by passing under the first portion of the barrier, the portion of the container on the second outlet side of the barrier contains a relatively calm volume of solution. A viewing means such as a clear panel is preferably provided in the container wall on the second outlet side of the barrier to view the level of solution in the container.

Non-vapourised solution with a lower concentration of absorbent than that at the bottom of the container may be withdrawn from a higher portion of the container as described in the following detailed description.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic view of an absorption chiller formed according to the invention with the regenerator shown in perspective;

Figure 2 is a side elevation of the regenerator in Figure 1 with a side casing panel and insulation removed; Figure 3 is a plan view of the regenerator in Figure 2:

Figure 4 is a side view partly in section of the upper and lower tanks, heat exchange tubes and thermo-syphon passages of the regenerator in Figure 2;

Figure 5 is a view on arrow V in Figure 4;

Figure 6 is a plan view of Figure 4;

Figure 7 is a section on line VII- VII in Figure 4;

Figure 8 is a section on line VIII- VIII in Figure 4;

Figure 9 is a view on arrow IX in Figure 4 of a top portion of the regenerator components in Figure 4;

Figure 10 shows an enlargement of region X in Figure 4;

Figure 11 diagramatically shows a fuel gas supply and control train layout of a fuel gas burner used to heat the regenerator in Figure 1 ; and

Figure 12 are graphs representing efficiency and heat exchange tube wall temperature in a generator illustrated in Figures 1 to 11 having eleven rows of heat exchange tubes and heated by a fuel gas burner rated at 350kW. In the drawings like references identify the same or comparable parts.

With reference to Figure 1, an absorption chiller 2 comprises a regenerator 4 supplying refrigerant in its vapour or gaseous phase along line 6 to a condenser 8 (known per se) and supplying concentrated liquid absorbent along line 10 via one-way valve 12 and a pump 14 to an absorber 16 (known per se). From the condenser 8 liquid refrigerant is supplied to an expansion arrangement 18 (known per se) in line 20 and thence the refrigerant enters evaporator 22 (known per se). Line 24 carries refrigerant vapour to the absorber 16 in which the refrigerant dissolves in the absorbent to form a weak solution of absorbent containing refrigerant. That solution is conveyed via one-way valve 26 in line 28 and a pump 30 to the regenerator in which the weak solution becomes concentrated absorbent by boiling-off the refrigerant.

Preferably the refrigerant is water (H₂O) in which case the refrigerant vapour on line 6 can be steam, and the liquid absorbent is lithium bromide (LiBr solution/H₂θ) though other refrigerant and absorbent combinations may be used, for example ammonia as refrigerant and water as absorbent.

Now with reference to Figures 1 to 10 aspects of regenerator 2 will be described in more detail. The regenerator 2 comprises a base frame 32 supporting an outer casing 34 of parallel piped shape (shown in dotted line in Figures 5 to 7) and a lower tank 36 of substantially rectangular cross-section having a flat top 37 from which ascend a plurality of substantially vertical heat exchange tubes 38A and 38B, tubes 38A having cylindrical, plain outer surfaces whilst the tubes 38B have heat collecting formations formed by fins 40. The heat exchange tubes 38 A, 38B open through a flat base 41 of an upper tank 42 of substantially rectangular cross-section and greater volume than the lower tank 36.

Between the casing 34 and the structure comprising the tanks 36 and 42 and the heat exchange tubes 38 A, 38B is heat insulating material having an upper inner face 44A (Figure 2), lower inner face 44B (Figure 2), and two opposite side inner faces 44C and 44D (Figure 5) defining between them a combined combustion chamber and flue 46 also defined in part by surfaces of the tank top 37 and tank bottom 41. The heat insulation may comprise one or more layers of suitable material, for example ceramic fibre board and/or ceramic blanker and/or Rock wool. The heat exchange tubes 38A, 38B are substantially wholly within the combustion chamber 46. At an upstream or front end 48 of the casing 34 is a gas burner 50 of a pre-mix type having an electrically driven fan or impeller propelling combustion air pre-mixed with a fuel gas to a burner outlet orifice or combustion surface which may be disposed within a vertically elongate substantially rectangular frame 52 (Figure 2) within the casing 34 having longer side walls 54 (only one shown, Figure 2). If desired the aforesaid burner outlet orifice may comprise a metal fibre burner. Externally of the casing 34, a downstream flue path within a rectangular tube 56 leads from combustion chamber 46. From the above it will be understood that the heat exchange tubes 38 A, 38B are in cross-flow relation with, more particularly at a right-angle to, flow direction X of hot heating gas or products of combustion through the combustion chamber 46 from the burner 50. It can also be seen that the heat exchange tubes 38 A, 38B are arranged in a plurality of rows, in this particular example eleven rows, spaced one from another along the flow direction of the hot products of combustion of each row extending transversely to the direction of flow X of the combustion products - there being at least two heat exchange tubes per row, in this example four heat exchange tubes per row. With respect to flow direction X, the finned heat exchange tubes 38B are disposed at or towards the downstream end of the array of tubes 38A, 38B, whereas the plain heat exchange tubes 38A are upstream of the finned heat exchange tubes in the array. In the example there are seven rows of plain heat exchange tubes 38A and four rows of finned heat exchange tubes 38B.

Two thermo-syphon tubes 58A and 58B are disposed substantially vertically and extend from the lower tank 36 to the upper tank 42 and open into each. As will be understood from Figures 5 to 7 the thermo-syphon tubes 58 A, 58B are surrounded by the heat insulating material which screens the tubes from the combustion chamber 46 and opposes heat transfer from the combustion chamber to the thermo-syphon tubes. With respect the direction X of products of combustion flow in the combustion chamber 46, 60 and 62 are upstream ends respectively of the tanks 36 and 42 and 64 and 66 are the respective downstream ends. The thermo-syphon tubes 58A and 58B open into the upper and lower tanks 42, 36 adjacent to the respective upstream ends 62, 60. With respect to direction X, the thermo-syphon tube 58B is downstream of the tube 58A and opens into the lower tank 36 about substantially mid-way therealong and opens into the upper tank 42 nearer to the upstream end 62 then the downstream end 66. However, the number of thermosyphons, their size and inlet-outlet positions can all be adjusted.An inlet tube 68 to supply weak refrigerant/absorbent solution from line 28 (Figure 1) to the upper tank 42 opens thereinto opposite to the entrance to the thermo- syphon tube 58 A (see Figures 4 and 5). An outlet tube 70 for carrying off concentrated absorbent solution to line 10 (Figure 1) leads from the downstream end 66 of the upper tank, and an outlet tube 72 to carry off refrigerant vapour or gas to the line 6 (Figure 1) leads from the top of the upper tank. The unit comprising upper and lower tanks 36, 42, the heat exchange tubes 38A, 38B and the thermo-syphon tubes 58 A, 58B can be formed of metal, for example carbon steel. However because the absorbent used may be corrosive, it may be preferred to form the aforesaid unit of corrosion resistant metal, for example cupro-nickel.

With weak refrigerant/absorbent supplied to the upper tank 42 continuously through the inlet 68 and with the burner 50 operating, the weak solution descends to the lower tank 36 through the thermo-syphon tubes 58 A, 58B and then ascends to the upper tank through the heat exchange tubes 38 A, 38B. As the solution rises in the heat exchange tubes 38 A, 38B the refrigerant boils off to a vapour which leaves through the outlet 72, whereas the remaining concentrated absorbent leaves the upper tank through the outlet 70. The heat exchange tubes 38 A, 38B may be substantially half full of aforesaid boiling off vapour.

As mentioned, in this example there are eleven rows of heat exchange tubes 38A, 38B which may be identified as rows 1 to 11 in which row 1 is, relative to direction X at the upstream end of the array of heat exchange tubes and row 11 is at the downstream end. Thus the plain tubes 38 A form rows 1 to 7 and the finned tubes 38B form rows 8 to 11. The products of combustion tend to be hotter at the upstream end of the array of heat exchange tubes 38 A, 38B than at the downstream end. To ensure a more even extraction of heat along the flow path X, the tubes 38B are finned to increase therein ability to extract heat from the relatively cooler downstream combustion products. It will thus be appreciated that the position of the thermo-syphon tube 58A tends to receive the initially input weak solution from the inlet and feed it to a position in the lower tank 32 from which the solution is more likely to ascend through upstream heat exchange tubes 38 A, say tube rows 1, 2, and 3, which are exposed to the hottest combustion products.

A vapour permeable demister pad 74 which may be of metal mesh or fibre is disposed in front of the entrance to the outlet pipe 72, and in front of or below the pad is a baffle plate 76 to prevent upward surges of solution hitting the pad or entering the outlet pipe.

A calm zone is established in the upper tank 42 at its downstream end and in front of the entrance to the outlet pipe 70. The calm zone 78 is designed to reduce the chance of absorbent in a turbulent state entering the outlet 70 and to increase the chance of only the more concentrated absorbent being supplied to the outlet. In this example the calm zone comprises two substantially vertical baffle plates 80 and 82 extending across the upper tank 42. The taller first baffle plate 80 is spaced at 84 (see Figure 10) from the floor of the upper tank 42 and somewhat higher than the space 84 is the second baffle plate 82 acting as a weir. However, the second baffle plate 82 could be replaced by the provision of the outlet 70 being a suitable distance above the bottom 41 of the tank 42 as shown in Figure 10. Since the more concentrated absorbent tends to be lower in the upper tank 42, only such more concentrated absorbent can pass through the space 84 and over the baffle plate 82 to the outlet 70. The size of space 84 may be variable to adjust the concentration of absorbent delivered to outlet 70, by for example the use of pins to secure first baffle place 80 at one of a number of possible heights. A space is preferably provided at the top of the first baffle plate 80 to permit vapour to leave the calm zone. It is difficult to determine the level of fluid in the upper tank 42 because of the turbulence produced by continued delivery of solution through inlet 68, the boiling off of refrigerant vapour, the withdrawal of fluid through thermosyphon tubes 58A and 58B and the delivery of fluid by heat exchange tubes 38A and 38B. However the calm zone 78 between plate 80 and outlet 70 presents a relatively calm surface from which the fluid level in the upper tank 42 can be determined. This fluid level may be determined by looking through a viewer in the side of the tank 42, such as a see through vertically extending panel made from glass or plastic, for example. Alternatively or additionally, one or more level sensors may be provided in the calm zone 78 to determine the fluid level in the tank 42. There is preferably a plurality of level sensors arranged in a vertical series on the inside wall of the upper tank 42 in the calm zone. Fluid level information from sensors may be used for flow control and operation of the generator.

Since the combustion products passing down flue pipe 56 may still contain recoverable heat, the pipe 56 may be lined with heat insulating material and may contain a further heat exchanger 86 exposed to the flue gases, this further heat exchanger acting as a regenerator/economiser/pre-heater. Heat exchanger 86 may be a tube in serpentine form or a plurality of serpentine forms disposed side by side and connected for liquid to flow through them in succession from one serpentine arrangement to the next, the vertical straight tube lengths in the or each serpentine form being in cross-flow relation with flue gases flow direction. An inlet to the heat exchanger 86 is indicated at 86A and an outlet at 86B.

It is preferred that the further heat exchanger 86 be used to pre-heat the weak refrigerant/absorber solution delivered by the pump 30. To this end the section of line 28 between points a and b in Figure 1 is omitted and the line 28 extended by a section 28A leading to the inlet 86A. From the outlet 86B another section of line 28B leads to the inlet tube 68.

When the apparatus uses the further heat exchanger 86, an H₂O/LiBr solution and a 350kW burner 50 burning fuel gas, for example natural gas, provided with about 20% excess combustion air, the following operational conditions may be obtained. The boiling temperature of the solution may be about 160°C, the concentrated solution supplied to outlet 70 may be about 64% LiBr salt, and the velocity of the mixture entering the upper tank 42 from the heat exchange tubes 38 A, 38B may be about 1.5m/s. The temperature of the flue gases in the flue 56 may be about 210°C. Shown in Figure 12 is a variation in heat exchange tube wall temperature for the tubes in row 1 to the tubes in row 11 and how the cumulative efficiency of the generator may progressively (and relatively uniformly) increase along the array of heat exchange tubes from one row to next. The pressure within the upper tank 42 may substantially approach or be 0.5 barg.

In the drawings, reference 88 indicates a union for mounting a pressure relief valve on the upper tank 42, reference 90 indicates access tubes to receive liquid level sensors inserted into the calm zone of the upper tank, and reference 92 indicates a normally closed drain passage. Sight glasses enabling a view of the main interior of upper tank 42 and the calm zone 78 are indicated at 94 and 96 respectively. Temperature sensors may be provided in the lower and upper tanks 36, 42. In the above description supply of weak refrigerant/absorbent solution is to the upper tank 42 through the inlet pipe 68, instead that pipe 68 may be blocked-off or omitted and the weak solution from line 28 or line 28, 28A, 28B be supplied to the lower tank 36 through an inlet pipe 98 opening into the lower tank opposite to the lower opening of the thermo-syphon tube 58 A.

A package burner 50 rated at 350kW has been referred to above. Package burners of different ratings may be used for example from a few kW to MW ratings.

In a gas supply system for the package burner 50 in Figure 11, the burner comprises a fan or impeller 100 driven by an electric motor 102 to draw combustion air along a duct 104 in which the air pre-mixes with fuel gas from a gas supply line 106 before supply to the burner outlet orifice. An electrical control (not shown) comprises a motor control 108, a pressure switch 110 observing the output pressure of the air/fuel gaseous mixture from the impeller 100, a pressure switch 112 observing the pressure of the fuel gas supplied, solenoid controlled valves 114 and 116, and an air/fuel ratio controller 118 arranged to respond to signals representative of air pressure in the vicinity of an orifice plate 120 in the duct 104. A manual valve 121 has to be open before any fuel gas may be supplied. If the pressure of the supplied gas observed by the pressure switch 112 falls outside a pre-determined range the control may operate one or other of the solenoid valves 114, 116 to shut off gas supply to the burner. Should the pressure observed by the switch 110 fall outside a pre-determined range the control may operate to shut one or other of the valves 114, 116 and may also stop the motor 102. The control may be responsive to a demanded firing rate at the burner and thus operate the motor control 108 so that the impeller 100 speed is varied to supply combustion air in desired quantity. Alternatively the control 108 may be omitted and the impeller 100 driven at a constant speed, the amount of combustion air supplied being varied by operation of the throttle valve 122, in the duct 104, driven by a throttle motor 126 in accordance with signals thereto initiated by the control.

Instead of using a package burner 50, some other means of generating hot gases to heat the heat exchange tubes 38 A, 38B may be used, for example hot exhaust gases from a gas turbine.

By choosing a thermo-syphon tube or tubes as the solution transfer means and/or by locating the thermo-syphon tube or tubes outside the hot gas flow path, there is no or at least little or a reduced heat transfer from the combustion gas to the tube or tubes promoting better thermo-syphon action. Weak solution can enter the top chamber and a lower volume of solution can be used.

In addition, by locating the thermo-syphon tube or tubes outside the hot gas flow path, there is greater freedom in the choice of the size, shape and position of the tube or tubes. By carefully selecting the appropriate design features a weak solution can be circulated/selected and its flow rate controlled. When the thermo-syphon tube or tubes are located outside the hot gas flow path it or they is/are easier to install in the generator, the tube or tubes can be used for flow level measurements and the overall weight of material can be reduced.

Alternatively the plates 80, 82 forming the calm zone 78 could be positioned anywhere in the top tank 42, for example at the front, on either side or any combination of positions.

Claims

1. A generator for an absorption chiller comprising a container with an inlet to receive a solution of absorbent and refrigerant; a heater to heat the solution to generate a refrigerant vapour; a first outlet for the refrigerant vapour to leave the container; and a second outlet for the non-vapourised liquid to leave the container; wherein a barrier is provided within the container between the inlet and the second outlet and the barrier is arranged such that for non-vapourised liquid to leave through the second outlet it must pass under a first portion of the barrier and over a second portion of the barrier.

2. A generator according to claim 1, wherein the first portion of the barrier is a first plate, the length of which extends substantially perpendicularly to the direction in which non-vapourised liquid is arranged to leave the container through the second outlet and a space is provided between the lower portion of the first plate and the base of the container.

3. A generator according to claim 2, wherein the first plate extends from one side of the container to the other.

4. A generator according to claim 2 or claim 3, wherein the height of the space between the lower portion of the first plate and the base of the container is variable to adjust the concentration of non-vapourised liquid leaving the container.

5. A generator according to any of claims 2 to 5, wherein a space is provided between the top of the first plate and the top of the container.

6. A generator according to any of claims 2 to 5, wherein the second portion of the barrier is a second plate the length of which is substantially parallel to the first plate, the second plate extending from the bottom of the container to a desired height.

7. A generator according to claim 6, wherein the second plate extends from one side of the container to the other.

8. A generator according to claim 6 or claim 7, wherein the second plate extends upwardly to a height above the height of the lower end of the first plate.

9. A generator according to any of claims 2 to 5, wherein the second outlet is arranged above the base of the container such that the portion of the container wall between the second outlet and the base of the container acts as the second portion of the barrier.

10. A generator according to any of the preceding claims, wherein a viewer is provided in the wall of the container to view the level of liquid in the container between the barrier and the second outlet.

11. A generator according to any of the preceding claims, wherein the container is provided with a level sensor at a particular height between the barrier and the second outlet to determine whether or not liquid in the container is at the height of the level sensor.

12. A generator according to claim 11, wherein the container is provided with a plurality of level sensors, each arranged at a particular height between the barrier and the second outlet to determine the level of liquid in the container.

13. A generator substantially as hereinbefore described with reference to the accompanying drawings.

14. A method of operating a generator of an absorbtion chiller, the generator comprising a container with an inlet to receive a solution of absorbent and refrigerant; a first outlet for refrigerant vapour to leave the container; a second outlet for the non-vapourised liquid to leave the container and a barrier provided within the container between the inlet and the second outlet and the method comprising passing non-vapourised liquid under a first portion of the barrier, over a second portion of the barrier and through the second outlet.