WO2008059524A2

WO2008059524A2 - Heat exchanger assembly

Info

Publication number: WO2008059524A2
Application number: PCT/IN2007/000460
Authority: WO
Inventors: Vivek Verma; Prateek Verma
Original assignee: Spray Engineering Devices Limited
Priority date: 2006-11-17
Filing date: 2007-10-05
Publication date: 2008-05-22
Also published as: WO2008059524B1; WO2008059524A3

Abstract

A heat exchanger assembly (101) is disclosed. The heat exchanger assembly (101) includes a heat exchanger (105) comprising of a bank of vertical parallel heat exchange tubes (106) of short height open at both ends. The tubes (106) are enclosed in a casing which has inlets for steam or other condensing medium. The heat exchanger (105) is arranged around the periphery of a chimney (103) of a natural draft tower (102) in a substantially circular plan. Means (110-112) are provided to spray the evaporative liquid from a particular height over the heat exchanger (105) co-current to the natural air draft. The heat exchanger assembly (101) operates as an improved evaporative condenser.

Description

HEAT EXCHANGER ASSEMBLY

FIELD OF INVENTION: The invention relates to an improved heat exchanger assembly for condensing steam or vapours or other condensing medium. More specifically it relates to heat exchanger assembly operating as an improved evaporative condenser.

BACKGROUND TO THE INVENTION (PRIOR ART):

A typical evaporative condenser arrangement for condensing steam or vapours consists of heat exchanger assembly comprising array of heat exchange tubes generally disposed horizontally, commonly referred to as indirect contact heat exchange section, in which the steam or vapours to be condensed are introduced through a distribution header. Evaporative liquid usually water is sprayed over the heat exchanger tubes through nozzles and forms a thin film over it Air is constantly passed over the condensing surfaces by fans. The evaporative liquid picks up heat from the external surface of tubes and surrenders it to the air by vaporizing a small fraction of its total mass. This process is termed evaporative. There is simultaneous transfer of heat and mass between both air and evaporative liquid as they come into direct contact with each other. The balance evaporative liquid drips from the indirect contact heat exchange tube surfaces to the sump below from where it is pumped again to be sprayed over the condensing surface. The evaporative liquid is cooled by passing it through an evaporative liquid cooling section comprising usually of a fill section either above or below the indirect contact heat exchange section. Evaporative condensing is still by far the most economical means to remove latent heat. The condensation in evaporative condensers is not fixed by upper temperature to which evaporative liquid in passing through a condenser is raised. This mode of heat transfer reduces the circulating cooling water requirement of the condenser to 10% of the requirement of surface condensers.

Another type of arrangement e.g. as described in US4969507 discloses a falling film air- cooled surface condenser comprising a plurality of spaced, generally parallel, plate heat exchange elements with channels there-between for vertical air flow, enclosed in a casing. Heat exchange elements are made up of substantially flat dimpled plates joined at their peripheries as well as at regularly spaced dimples. The vapour to be condensed is introduced into said elements by a transverse header and the cooling liquid (water) is distributed over the said elements to form a film. Fans are used to feed the air through the system either in a counter-current or a parallel-flow fashion. Use of vertical tubes in place of plates has also been proposed.

The arrangements described in the prior art evaporative condensers, for condensing steam or vapours suffer from the following disadvantages:

1. Requirement of Large Size: The surface area required to condense the steam is quite large especially in case of steam condensing inside the tubes since the specific volume of the steam at condenser pressure is very large and if the steam is to be passed through the condenser tubes, then the number of tubes (volume occupied by the tubes) required is extremely large and increases the capital cost of condenser. The tube material of greater strength has to be used to ensure structural integrity of the heat exchanger increasing both the capital cost and overall weight of the apparatus.

The height and/or width of the plates, as in US4969507, is to be kept large due to the design of the plates in order to maintain the plate sheets in proper spaced relationship with respect to one another and also provide adequate condensing surface. And if such a design is not provided for, the plates will have a tendency to chip in at reduced pressures. This again increases the overall cost & weight of the apparatus. Even if vertical tubes are used in US4969507, the length of the tubes will have to be kept more in order to provide sufficient retention time to detached water droplets for efficient heat transfer. In other cases also for practical commercial operation the tubes are provided in long lengths.

2. Increased Pumping Requirements: Due to increase in surface area, the pumping requirements for spraying or distributing cooling water increase. Providing an evaporative liquid cooling section comprising of fills either above or below the indirect heat exchange section increases the height of the apparatus requiring more pump capacity. The pumping requirements do not decrease even in cases where evaporative liquid cooling section and the indirect contact heat exchange section are provided separately & not in a overlying or underlying manner since the cooled evaporative liquid is pumped from the sump below the evaporative liquid cooling section for spraying over the indirect heat exchange section and the warmed evaporative liquid from the sump below the indirect contact heat exchange section is pumped to be sprayed over the evaporative liquid cooling section. 3. Higher Energy Expenditure: The energy which is spent to force or induce the air over the condensing surface is considerably high. The power required to blow the air in a counter-current fashion e.g. as in US4969507 and many other apparatuses is greater due to increased frictional losses owing to longer length of plates or tubes and resistance at air-liquid interface. In US4969507 the energy expenditure is still higher in order to maintain a velocity rapid enough to break large water droplets from the falling film and keep them suspended therein for achieving the object of enhanced heat transfer. The resistance which the current of air must overcome further increases when- blowing externally over an array of closely packed tubes generally disposed horizontally, in which steam or vapour is condensed inside the tubes. What is gained by use of a lower fan horsepower and improved airflow by spacing apart of these tubes is offset by loss in heat transfer area or increased size of the apparatus. In other cases in which fills are used in the evaporative liquid cooling section, more fan power is needed to draw the air through the fills. Use of fills also results in increased height of the apparatus besides the maintenance requirements.

4. Recirculation of Warm Air: In the prior art co-current arrangements, there is a very high probability of the warmed air to be sucked in again into circulation by the forced draft fans, resulting in decreased efficiency of the system.

5. Difficult mechanical removal of scale deposits: Besides being complex to make, the prior art heat exchangers are difficult to clean mechanically and inspect necessitating the use of acids which, apart from being very expensive, also affects the galvanising of the heat exchange surfaces and the housing.

6. Lower efficiency: The circulating water is cooled to a lesser extent thereby affecting the overall efficiency of the system especially in case of arrangement described in US4969507. In arrangements with an array of conventionally round tubes in which steam or vapour is condensed inside the tubes, the air does not blow over the rear portion of the tubes lowering the efficiency. The efficiency is further lowered in such cases where air is blown in a counter-current manner over a bundle of tubes disposed horizontally since the effective area for heat transfer is reduced due to the formation of condensate layer inside the tubes.

The present invention obviates the drawbacks of the prior art by providing a heat exchanger assembly of novel but simple construction. The said heat exchanger assembly operates as an improved evaporative condenser for condensation of steam or vapour and concentration of an evaporative liquid such as water, juice, effluent or other aqueous solution.

OBJECTS OF THE INVENTION:

The principal object of the invention is to provide an improved heat exchanger assembly which is energy efficient.

Another object of the invention is to provide a heat exchanger assembly which is considerably compact owing to novel features.

Still another object of the invention is to provide heat exchanger assembly which is economical owing to use of off-the shelf components, minimizing use of expensive materials e.g. bulky metal tubing and specific arrangement of the parts.

Yet another object of the invention is to provide a heat exchanger assembly which allows for convenient online mechanical cleaning of the heat exchange surfaces.

SUMMARY OF THE INVENTION:

The present invention provides a novel heat exchanger assembly for condensation of steam or vapours. The novelty of the invention lies in the specific arrangement of known components, resulting in synergistic beneficial effects which have not been disclosed in the prior art. In conventional heat exchangers, steam or vapours are passed through long metal tubes disposed horizontally and condensed by spraying evaporative liquid and mechanically forcing or inducing air over these tubes. However, in the present invention, steam or vapour is not passed through long metal tubes. Instead, steam or vapour is condensed on the shell-side of a welded-tube calandria comprising of a bank of vertical parallel tubes of short height enclosed in a casing. The calandria is arranged around the periphery of a natural draft tower and has a substantially circular plan. Evaporative liquid is sprayed over these tubes from a particular height co-current to the natural air draft, so that the cooling medium consists of evaporative liquid droplets, cooled air and also fresh

5 air. Such an arrangement results in a high heat transfer, resulting in an increased condensing coefficient. Hence, tubes of shorter height and less thickness of metal can be used, resulting in compactness and also economy of the overall heat exchanger. The evaporative liquid flowing through the hollow heat exchange tubes is collected in a sump at the bottom of the calandria and recirculated, while the warm air coming out from the

10 hollow tubes escapes through the tower.

The efficiency of the system is increased by the following factors:

(i) specific synergistic arrangement of different components resulting in beneficial effects of better energy efficiency, compactness of size, economy of 15 construction and operation and greater structural integrity;

(ii) effective cooling of the circulating evaporative liquid and that too without additional energy inputs, (iii) use of natural air draft in a co-current manner comprising of a mixture of air at or near wet bulb temperature and ambient air,

20 (iv) introduction of effectively cooled droplets of the evaporative liquid in the heat , exchange tubes which impinge all over the condensing surface augmenting heat transfer, . (v) enhancement of the natural air draft by spraying of evaporative liquid and short length of the heat exchange tubes. 25.

STATEMENT OF THE INVENTION:

A heat exchanger assembly for condensing steam or other vapours comprising a synergistic arrangement, in combination,

30 - a natural draft tower having a chimney part and apertured support means between the chimney part and the ground;

- regulated air inlets between the said apertured support means; ' - a heat exchanger; - inlet means for steam or other vapour into the heat exchanger;

- means for removing the condensate and non-condensable gases from the said heat exchanger;

- sump means for collecting discharged warmed evaporative liquid; - means for recycling the discharged evaporative liquid;

- means for spraying evaporative liquid in the form of fine droplets; means for retention of evaporative liquid sprayed over the heat exchanger;

- with or without means for distribution of cooling water to form a film on the inner surface of the heat exchanger tubes leaving a channel for vertical air flow co-current to the flow of the film of water; and

- means for entrainment separation; and wherein the said heat exchanger of the assembly comprises of vertical parallel tubes of short height arranged all around the periphery of the chimney part of the natural draft tower.

There is also provided a method of enhancing the performance of heat exchanger assembly operating as an evaporative condenser comprising introduction of cooled fine evaporative liquid droplets alongwith the mixture of ambient air and air at or near wet bulb temperature inside the heat exchanger tubes in a co-current manner employing natural draft causing increased heat transfer throughout the condensing surface due to impinging of the cooled evaporative liquid droplets all over the condensing surface in addition to use of effectively cooled evaporative liquid film and enhanced evaporative cooling provided by the air mixture coupled with the use of short heat exchange tubes enabling reduction of condensing surface and a lower condenser pressure. BRIEF DESCRIPTION OF THE DRAWINGS:

Fig. 1 shows the schematic representation of the heat exchanger assembly of the invention. Fig. 2 shows the perspective view of the heat exchanger assembly of the invention. Fig. 3 shows a sectional elevation of the heat exchanger assembly of Fig.2. Fig. 4 shows front sectional elevation of the heat exchanger assembly of Fig.1. Fig. 5 shows the plan view of the heat exchanger assembly of the invention. Fig. 6 shows front sectional elevation of the heat exchanger assembly of the invention with fans.

Fig. 7 shows another embodiment of the heat exchanger assembly of the invention with air inlets below the heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE DRAWINGS:

The present invention is directed to providing an improved heat exchanger assembly of novel but simple construction in which the novelty lies in specific synergistic arrangement of different components, resulting in beneficial effects of better energy efficiency, compactness of size, economy of construction and operation and greater structural integrity.

This has been made possible by incorporating the following features, in a specific synergistic arrangement, in the construction of the device:

1. Use of hollow, vertical parallel metal heat exchange tubes (calandria or shell-and-τube arrangement) of short height, open at both ends and arranged in a circular arrangement around the chimney part of a natural draft tower. The tubes are encased in an enclosure into which steam or other vapour enters.

2. Arrangement for spraying of evaporative liquid from a specific height above the upper open ends of the hollow metal heat exchange tubes co-current to the natural air draft, so that the 'cooling mixture' entering the tubes consists of evaporative liquid droplets, cooled air and fresh air. 3. Arrangement for outlet of the warmed air emerging from the bottom of the hollow metal heat exchange tubes, into a tower.

For greater clarity, details of the invention are further described. The heat exchanger assembly for condensing steam or other vapours comprises, in combination: i) a natural draft tower: having an upper chimney part and lower apertured support means between the chimney part and the ground; ii) regulated air inlets: provided in the apertured support means of the natural draft tower; iii) a heat exchanger: arranged around the periphery of the said chimney part. It is provided with support means between its lower level and the ground.

It consists of vertical parallel tubes of short height, preferably welded short-tube calandria, with the shell side enclosed in a casing and having one or more inlets for steam and means for removing the condensate and non-condensable gases; whereas both ends of the tubes are open to the atmosphere with the lower discharge end opening above the sump means for collecting the discharged warmed evaporative liquid; iv) means for recycling the discharged evaporative liquid; v) means for spraying evaporative liquid above the calandria; vi) means for preventing the sprayed evaporative liquid from spilling out from the calandria; vii) with or without means for distribution of cooling evaporative liquid to form a film on the inner surface of the heat exchanger tubes leaving a channel for vertical air flow, co- current to the flow of the film of water and viii) means for entrainment separation.

The novel features of the invention are:

1. Arrangement of a bank of vertical parallel tubes of short height, preferably a welded tube calandria or a welded honeycomb calandria, operating as an improved evaporative condenser around the chimney part of a natural draft tower.

2. Provision for effective cooling of circulating evaporative liquid without expenditure of additional energy, employing natural draft and reduced height of the evaporative liquid cooling section of the apparatus.

3. Natural flow of cooling air through the system in an unhindered/unobstructed manner. 4. Enhanced heat transfer characteristics due to presence of cooled fine evaporative liquid droplets within the tube in addition to the use of effectively cooled evaporative liquid to form the falling film and a mixture of air at or near wet bulb temperature and ambient air in a co-current manner, coupled with and enabling the use of short height heat exchange tubes, which further provide many other additional advantages.

These novel synergistic arrangements result in technical advantages as follows:

1. Availability of cooled evaporative liquid and air at or near wet bulb temperature and ambient air, resulting in better heat transfer characteristics increasing the energy efficiency.

2. Reduction in condensing surface both due to steam being on the shell side and presence of cooled fine evaporative liquid droplets and mixed air within the tube besides use of effectively cooled evaporative liquid.

3. Lower condenser pressure is achievable at the same wet bulb temperature due to effective cooling of circulating evaporative liquid and presence of cooled fine evaporative liquid droplets and mixed air inside the tubes.

4. Expenditure of energy by use of fans for forcing or inducing the air through the system is either completely eliminated or considerably reduced.

5. Nil probability of warmed air coming into circulation again.

6. The short height of tubes provides the following advantages: a) Reduced pumping requirements; b) Short tube height together with downward spraying of evaporative liquid above the calandria and co-current arrangement with reduced frictional losses, enhances the natural draft at an increased velocity; c) Easy to clean online and inspect owing both to short height and the tube ends being open to atmosphere; d) Increased condensing coefficient; e) Easy optimization of the heat transfer characteristics of the heat exchanger; and f) Greater structural integrity and reduced weight of the heat exchanger, which is also due to steam or vapours condensing on the shell side i.e. ' outside the heat exchange elements. 8. Easy removal of condensate and non-condensable gases.

9. Design simplicity - easy to make, operate/optimize and maintain.

Referring to Figs. 1 to 4, the novel heat exchanger assembly 101 operating as an improved evaporative condenser for condensing steam or vapours comprising of a natural draft tower 102 having a chimney part 103 and apertured support means between the lower end of the chimney part 103 and the ground providing regulated air inlet openings 104 which also serve as primary drift eliminators. The tower chimney 103 may be hyperbolic or of tubular or spiral configuration. Arranged around the external periphery of the said chimney part 103 is a heat exchanger 105 comprising of a bank of vertical parallel tubes 106 of short height, preferably a welded tube calandria. More preferably the heat exchanger comprises of a welded honeycomb calandria. The lower level of the heat exchanger 105 corresponds with the lower end of the chimney part 103 of the natural draft tower 102.

The shell side of the heat exchanger 105 is enclosed in a casing and has one or more inlets for steam S (shown in Fig. 1) and means for removal of condensate C and non- condensable gases N (shown in Fig. 1). The heat exchanger 105 is supported by support means 107 between its lower end and the ground. Both the ends of the heat exchanger tubes 106 are open to atmosphere, with the lower ends opening above the sump 108 into which the warmed evaporative liquid discharged from the said tubes is collected. The upper ends of the tubes 106 may or may not protude through the upper tube sheet 109 to some extent. Means (not shown) are also provided to add make-up liquid to the collected evaporative liquid in the sump 108 which is recycled and sprayed from at a certain distance above the heat exchanger (calandria) 105. The evaporative liquid is pumped into a common header 110 arranged around the chimney 103 and sprayed by means of spray nozzles 111 fixed on manifolds 112 radiating out from the common header 110. The spraying means are divided into various sections by partitions 113. Valves 114 are provided at suitable places i.e. at entry and exit of each section 113 to isolate any such section. The valves may be controlled manually or automatically. Regulated air inlets 115 are provided in the section between the spray nozzles 111 and the top level of the calandria. These regulated air inlets 111 also serve to prevent the spilling of sprayed evaporative liquid outside the calandria. Means for entrainment separation 116 are also provided. The said heat exchanger assembly 101 operates as an improved evaporative condenser.

When the assembly is in operation, the steam or vapours to be condensed are introduced into the shell side of the heat exchanger 105 by one or more inlets. Known means may be provided for proper distribution of the steam or vapour to be condensed and for the effective removal of the condensate and the non-condensable gases.

Evaporative liquid is sprayed from the spray nozzles 111 placed at a certain distance above the calandria 105. Air is pulled into the system by combined effect of the natural draft tower 102 and downward drag created by spray droplets. It flows through the tubes 106 in an unhindered manner with least possible resistance. The short height of the calandria tubes 106 coupled with spraying of evaporative liquid above them and co- current arrangement with reduced frictional losses further assists in maintaining natural draft at an increased velocity. This assists in an improved heat transfer.

The natural draft can be further enhanced by increasing the amount and pressure of evaporative liquid being sprayed from the nozzles 111. Alternatively, the natural air draft may be regulated by the number of nozzles operating. This eliminates or drastically reduces the requirement of a fan or fans 117 as shown in Fig. 6 to enhance the natural air draft which may be required at very high ambient air wet bulb temperatures. The energy expended for this is considerably less than the energy which has to be expended for systems employing air draft induced or forced only by the use of fan power, hi the latter case the heat exchanger 105 is arranged around an induced draft tower.

The sprayed evaporative liquid comes in contact with the ambient air being pulled into the tubes and is thereby cooled. The approach of such a cooling method is more than, that of a spray pond since air at an increased velocity as compared to that in a spray pond comes in contact with this evaporative liquid due to the natural draft provided by the chimney 103. The sprayed cooled fine evaporative liquid droplets fall over the tube plate 109 and overflow into the tubes 106 to form a thin film on their inner surface. A major part, however, of the cooled fine evaporative liquid droplets is carried along with the air directly into the tubes 106 and fill the tubes. The evaporative liquid is thus first reduced to near the wet bulb temperature and then partially evaporated on the condensing surface. The air that is blown over the condensing surface is the air that has cooled the evaporative liquid and this air is mixed with the cooled fine evaporative liquid droplets. Fresh air is pulled into the system continuously, by the natural draft provided by the chimney 103, from regulated air inlets 115 provided in the section between the spray nozzles 111 and top level of heat exchanger 105. This fresh

(ambient) air is mixed with the air that has cooled the evaporative liquid and is sucked into the heat exchanger tubes 106. This mixed air that is blown over the condensing surface takes up some moisture from the evaporative liquid during its cooling. This air may or may not be saturated.

It has been observed that better heat transfer is obtained through the condensing surface with a thin film of effectively cooled evaporative liquid over the condensing surface and presence of cooled fine evaporative liquid droplets and mixed air i.e. mixture of air at or near wet bulb temperature and ambient air, inside the tubes, hi the present invention, cooled fine evaporative liquid droplets which fill the tube besides aiding in formation of the film, also impinge upon the surface of the film all over the condensing surface creating turbulence further enhancing the heat transfer in addition to the evaporative cooling caused by the mixed air.

Increased heat exchange phenomenon has been achieved with simultaneous decrease in the height of the heat exchange sections, easy accessibility of the heat exchange surfaces for online cleaning, reduced weight of the heat exchanger while ensuring structural integrity and without expenditure of any additional energy. This is in sharp contrast to typical evaporative condenser arrangements comprising an array of tubes generally disposed horizontally whence the sprayed evaporative liquid droplets impinge only upon some portion of the heat exchange tubes and that too mainly in the upper part of the array, dissipating their momentum. In US4969507, the water droplets detached from the warmed falling film and suspended therein are cooled by the partially saturated air and fall to the bottom to be replaced by newly detached droplets.

In the present invention, the evaporative liquid is broken into fine droplets above (and not inside) the tubes 106 and is effectively cooled there itself. This obviates the necessity of retaining the droplets inside the tube for a considerable time to achieve effective heat transfer and also eliminates any fan power requirement to detach and suspend water droplets.

It is well known that water or other aqueous liquids can be cooled effectively by evaporative cooling either by breaking into small droplets or providing other means to increase its contact surface area. In US4969507 water has been described to fall from the heat exchange elements in the form of shower which provides insufficient surface area for effective cooling. Also the warmed water falling from the heat exchange elements at or towards the inner end will be cooled to a still lesser extent. This lowers the performance of the system since condenser pressure is a direct function of the cold water temperature. To overcome this disadvantage, it has been proposed to pass the warmed water through fills provided in an evaporative liquid cooling section either above or below the indirect contact heat exchange section. However, such arrangements have resulted in increased height of the apparatus. In cases where the fill section is placed below the indirect heat exchange section, it burdens the apparatus structure to support the mass of indirect heat exchange section at higher levels. And in cases where the fills section is located above the indirect contact heat exchange section, the latter becomes relatively inaccessible. A separately located fill section has the disadvantage of decreasing the plan area for indirect heat exchange section besides increasing the pumping requirements. Besides increasing the height, use of fill section also increases the weight of the apparatus and maintenance requirements.

All the above disadvantages have been overcome in the present invention since the evaporative liquid is sprayed at a certain distance above the calandria in the form of fine droplets which receive equivalent supply and quality of ambient air for effective cooling. This also enables the use of short height tubes. The height of the heat exchange sections is also considerably reduced and the heat exchange surfaces are easily accessible for maintenance. Furthermore, the need for maintenance of fill section is altogether eliminated.

The means for spraying evaporative liquid over the heat exchanger tubes 106 are divided into various sections by partitions 113. This is beneficial for online mechanical cleaning of the heat exchange surfaces. A section is isolated by closing the suitable valves 114 whence the heat exchanger tubes of that section can be cleaned mechanically. The quantity of evaporative liquid sprayed from the nozzles 111 is controlled by the operating condenser pressure or some other parameter either manually or automatically to conserve the evaporative liquid. The head at the spray nozzles 111 is preferably kept at a pressure to ensure laminar flow in order to reduce frictional losses.

Advantageously, such a type of heat exchanger 105 is a better choice for an application requiring high probability against leakage as in cases where vacuum is required to be maintained e.g. condensation of exhaust steam. Short height of the heat exchanger tubes 106 and condensation of steam on the shell side i.e. outside the tubes further give a twin advantage of adding significantly to the structural integrity of the heat exchanger and reducing the overall weight of the heat exchanger which has been the problem with most prior art evaporative condensers.

Figure 5 refers to the plan view of the heat exchanger assembly 101 of the invention. It shows the generally circular plan of the heat exchanger assembly 101 in which the heat exchanger comprising of a bank of vertical tubes 106 of short height enclosed in a casing is arranged around the chimney 103 of the natural draft tower. The evaporative liquid spraying means 111 are arranged on manifolds 112 radiating out from a common header

110. The evaporative liquid sprayed from the nozzles 111 falls over the top tube sheet 109 and overflows into the vertical tubes 106 to form a thin film. A majority of the sprayed evaporative liquid is directly carried into the heat exchanger tubes 106 along with the natural air draft created by the chimney 103. The spraying means are divided into sections by means of partitions 113 to facilitate online cleaning of the heat exchange surfaces. Air inlets 115 are also provided for allowing the entry of fresh air into the system. The mixed air and the cooled fine evaporative liquid droplets accentuate the heat transfer characteristics inside the tubes 106. The warmed evaporative liquid falls into the sump below whereas the air is sucked into the chimney 103.

The released warm humid air is passed through drift eliminators or other standard structures 116 before being released into the atmosphere through the natural draft tower 102 to minimize or eliminate any water droplets escaping along with the air draft. The regulated air inlets 204 also serve as primary entrainment separators. - In an embodiment of the present invention shown in Fig. 7, the support means 207 between the lower end of the heat exchanger 205 and the ground are apertured. The air inlets 218 are provided between the apertured support means 207 of the heat exchanger 205. They are regulated with means to control the flow of ambient air. In this embodiment twin means are employed to cool the re-circulating coolant evaporative liquid thereby enhancing the overall performance of the system. Firstly, when the coolant evaporative liquid is discharged from the lower end of the tubes 206 in the form of a shower, it comes in contact with the rapidly moving ambient air entering the space between the lower end of the heat exchanger 205 and the level of the water in the sump 208 from the regulated air inlets 218 provided all around the periphery between the apertured support means 207. Entry of ambient air at this place also helps in condensing water vapour in the air exiting from the calandria tubes 206. This helps in reducing the plume and conservation of the evaporative liquid. This partially cooled evaporative liquid is further cooled by atomizing it through spray nozzles 211 above the heat exchanger 205. Higher ambient air movement due to natural draft causes better cooling of evaporative liquid.

In this embodiment the air entering the tubes 206 from the upper ends is saturated to a lesser extent whereby the evaporative cooling caused by it inside the tubes is further augmented. Though in an unpredicted manner, the presence of cooled fine evaporative liquid droplets inside the tubes 206 which are entrained along with the air also enhance the heat transfer to a considerable extent in addition to the evaporative cooling caused by the mixed air. The lowered circulating cooling evaporative liquid temperature and enhanced heat transfer enables reduction of heat exchange surface and creates a lower pressure (high vacuum) at the same wet bulb temperature e.g. it creates a low back pressure for the turbine increasing the work of the turbine, thus enhancing the plant efficiency and reducing the steam flow for a given output. The efficiency of the system is also increased by use of natural draft instead of only forced or induced draft saving considerable recurring costs.

In another embodiment of the present invention one or more sections of the means for spraying evaporative liquid over the calandria tubes may be utilized for spraying a different evaporative liquid. Appropriate piping arrangements are made for the purpose. The sump is also correspondingly divided into two parts. Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Though the invention has been described with reference to condensation of steam or vapour yet other condensing medium can. be condensed in their place. The evaporative liquid used is usually water but juice, effluent or other aqueous solutions may also be used.

Example of Industrial Applicability:

Besides the application of the heat exchanger assembly of the invention for condensing exhaust steam in power generation plants using water as a coolant liquid, a very novel application this assembly can be made in industries in which steam or vapours from the process are rejected to the condensers and there is availability of some liquid or effluent to be treated. E.g., in sugar industry a considerable quantity of vapours from the evaporators and pans is rejected to the condensers. It has been observed that even the most efficient condensers used in sugar industry which are of multijet multispray type require forty times water to condense a given quantity of vapours. Further the amount of energy spent to circulate the cooling water and cool the warmed water discharged from the condensers is tremendous. By using the heat exchanger assembly of the invention, not only is the energy expenditure for circulating cooling water is drastically reduced but also the use of condensers and spray ponds can be completely eliminated by use of raw or mixed juice or effluent as a coolant liquid instead of water, saving a huge quantity of precious water. This not only gives the advantage of warming the raw or mixed juice but also increases the concentration of dissolved solids. Means are provided to remove the concentrated liquid or juice from the circulation. Means are also provided to effectively cap the heat exchanger without inhibiting the natural air draft. Further, in case of a sugar industry attached to a distillery, spent wash can be used as an evaporative liquid. The novel construction of the assembly allows for very easy online maintenance for mechanical cleaning of the calandria tubes.

Various modifications of the disclosed embodiments, as well as alternate embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit and scope of the present invention as described.

Claims

We CLAIM:

1. A heat exchanger assembly for condensing steam or other vapours comprising a synergistic arrangement, in combination, - a natural draft tower having a chimney part and apertured support means between the chimney part and the ground; regulated air inlets between the said apertured support means; a heat exchanger; inlet means for steam or other vapour into the heat exchanger; - means for removing the condensate and non-condensable gases from the said heat exchanger; sump means for collecting discharged warmed evaporative liquid; means for recycling the discharged evaporative liquid; means for spraying evaporative liquid in the form of fine droplets; - means for retention of evaporative liquid sprayed over the heat exchanger; with or without means for distribution of cooling water to form a film on the inner surface of the heat exchanger tubes leaving a channel for vertical air flow co-current to the flow of the film of water; and means for entrainment separation; and wherein the said heat exchanger of the assembly comprises of vertical parallel tubes of short height arranged all around the periphery of the chimney part of the natural draft tower.

2. A heat exchanger assembly according to claim 1, wherein the height of the lower level of the heat exchanger corresponds with that of the lower end of the chimney part of the natural draft tower.

3. A heat exchanger assembly according to claim 1, wherein the said heat exchanger comprising of vertical parallel tubes of short height is preferably a welded short-tube calandria and more preferably a welded honeycomb calandria.,

4. A heat exchanger assembly according to claim 1, wherein the said heat exchanger arranged all around the periphery of the chimney part of the natural draft tower is having a substantially circular plan.

5. A heat exchanger assembly according to claim 1, wherein the means for cooling & distributing the evaporative liquid include spray nozzles placed above the upper level of the calandria.

6. A heat exchanger assembly according to claim 5, wherein the spray nozzles are arranged on manifolds radiating out from a common evaporative liquid header.

7. A heat exchanger assembly according to claim 6, wherein the common evaporative liquid header is positioned around the periphery of the chimney.

8. A heat exchanger assembly according to claim 5, wherein the air draft is regulated by pressure of evaporative liquid at the spray nozzles.

9. A heat exchanger assembly according to claim 5, wherein the air draft is regulated by the number of nozzles operating.

10. A heat exchanger assembly according to claim 5 or 6, wherein the means for distributing evaporative liquid are divided into various sections by means of partitions.

11. A heat exchanger assembly according to claim 10, wherein a section can be isolated by regulating appropriate valves.

12. A heat exchanger assembly according to claim 10, wherein a different evaporative liquid is sprayed in one or more sections.

13. A heat exchanger assembly according to claim 12, wherein the sump is divided into parts corresponding to the section or sections spraying different evaporative liquids.

14. A heat exchanger assembly of claim 1, wherein the means for retention of evaporative liquid sprayed over the heat exchanger are air inlets to allow entry of ambient air into the system.

15. A heat exchanger assembly according to claim 1, wherein the chimney has fan or fans for enhancing air draft.

16. A heat exchanger assembly according to claim 1, wherein the means for cooling circulating evaporative liquid is at two stages, at the exit and at the entry of the heat exchanger tubes, without expenditure of additional energy.

17. A heat exchanger assembly according to claim 16, wherein the support means of the heat exchanger are apertured.

18. A heat exchanger assembly according to claim 16, wherein the apertured support means of the heat exchanger are provided with regulated air inlets.

19. A heat exchanger assembly according to any of the preceding claims 1 to 18, wherein the said heat exchanger assembly operates as an improved evaporative condenser due to the specific synergistic arrangement of different components and introduction of cooled fine evaporative liquid droplets along with the mixture of ambient air and air at or near wet bulb temperature inside the heat exchanger tubes in a co-current manner employing natural draft causing increased heat transfer throughout the condensing surface coupled with the use of short height heat exchange tubes and a lower condenser pressure.

20. A heat exchanger assembly substantially as herein described in the specification with reference to the accompanying drawings.