Abstract:
An evaporator having an external vessel and an inner cavity, and a heat exchanging tube assembly, the external vessel comprising an opening closable by a hatch, the heat exchanging tube assembly being insertable into and removable from the inner cavity through the opening. In oil production and in other industrial processes, waste water is often produced which requires treatment. This water may be contaminated with amongst other contaminants Oil &amp; Grease, minerals, Silica, and organic contamination.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to an evaporator for treating industrial waste water or produced water of an oil or gas production plant. 
       BACKGROUND OF THE INVENTION 
       [0002]    In oil production and in other industrial processes, waste water is often produced which requires treatment. This water may be contaminated with amongst other contaminants Oil &amp; Grease, minerals, Silica, and organic contamination. It is desirable to treat the water, both for possible recycling into the industrial process or to reduce or minimize disposals and environmental effects, often in accordance with environmental regulations. 
         [0003]    As part of such as treatment process, it is known to pass the water through an evaporator. This produces a stream of distillate, or purified water, and a blowdown of water with a much higher concentration of contaminants. Such evaporators have a heat transfer area, an array of heat exchange tubes or plates, and water to be treated is sprayed over these tubes or plates in a “falling film” configuration. Where contamination is present, scaling and fouling deposits form on the heat exchanging surfaces which reduce the thermal efficiency and the water treatment capacity of the evaporator and increase its energy consumption. 
         [0004]    Conventionally, to deal with such scaling and fouling in evaporators, it is necessary to take the evaporator out of operation and perform chemical washes inside the vessel which has a limited efficiency in scaling and fouling removal. Unlike evaporators, heat exchangers may be disassembled to remove heat exchange tubes bundle in order to be cleaned in an external cleaning device. This may either be an ultrasonic bath or a high flow-rate spray system, in which a suitable treatment fluid is sprayed onto the tubes at a high rate to remove the scaling. Such cleaning processes are themselves expensive, and also impose costs on the industrial plant in terms of lost production during shutdown of the evaporator 
       SUMMARY OF THE INVENTION 
       [0005]    According to a first aspect of the invention is provided an evaporator for treating industrial waste water or produced water, having an external vessel and an inner cavity, and a heat exchanging tube assembly, the external vessel comprising an opening closable by a hatch, the heat exchanging tube assembly being insertable into and removable from the inner cavity through the opening. 
         [0006]    The hatch may comprise a first upturned lip, and a wall adjacent the hatch may comprise a second upturned lip, wherein the first and second upturned lips may abut when the hatch closes the opening. 
         [0007]    The hatch may comprise a first connection plate, and a wall adjacent the hatch may comprise a second connection plate, wherein a plurality of connection elements may engage the first connection plate and second connection plate to maintain the hatch in a closed position. 
         [0008]    The connection elements may comprise a plurality of bolts. 
         [0009]    The connection elements may be mounted such that the total vector of forces on the hatch and the adjacent wall extends generally longitudinally of the connection elements. 
         [0010]    A sealing element may be engaged between the first connection plate and second connection plate. 
         [0011]    The heat exchanging tube assembly may comprise a first end tube sheet, a second end tube sheet and a plurality of heat exchange tubes received in the first end tube sheet and second end tube sheet. 
         [0012]    The evaporator may comprise support elements disposed in the cavity engage the first end tube sheet and second end tube sheet. 
         [0013]    The evaporator may further comprising a lifting frame, the lifting frame having a first end part for connection to the first end tube sheet, a second end part for connection to the second end tube sheet, and a rigid frame extending between the first end part and second end part, wherein the lifting frame may be releasably connectable to the heat exchanging tube assembly to permit it to be inserted into or removed from the inner cavity. 
         [0014]    The evaporator may comprise a plurality of openings, each opening closable by a hatch 
         [0015]    According to a second aspect of the invention, there is provided an evaporator having a heat exchanging tube assembly, and a first plurality of nozzles disposed to introduce water to the heat exchanging tube assembly during normal operation, the evaporator comprising a second plurality of nozzles, the second plurality of nozzles being disposed to introduce cleaning fluid to the heat exchanging tube assembly during cleaning operation. 
         [0016]    The evaporator may comprise a first main feed connected to the first plurality of nozzles and a second main feed connected to the second plurality of nozzles. 
         [0017]    The first plurality of nozzles may be located to form a falling film of water over the heat exchanging tube assembly, and the second plurality of nozzles may be offset from the first plurality of nozzles. 
         [0018]    The evaporator may comprise an external vessel having an opening closable by a hatch, and the first main feed and second main feed may be mounted on the hatch. 
         [0019]    At least some of the first plurality of nozzles and the second plurality of nozzles may be mounted on the hatch. 
         [0020]    The second plurality of nozzles may further comprise nozzles to direct fluid at the sides of the heat exchanging tube array. 
         [0021]    According to a further aspect of the invention there is provided an evaporator comprising an external vessel having a longitudinal axis, the vessel comprising a plurality of longitudinal extending walls, said longitudinally extending walls being curved in a plane transverse to said longitudinal axis 
         [0022]    The plurality of walls may comprise a base wall and opposed side walls 
         [0023]    The base wall and opposed side walls may be formed integrally. 
         [0024]    The opposed side walls may alternatively be welded to the base wall. 
         [0025]    The evaporator may comprise an opening closable by a hatch. 
         [0026]    The hatch may comprise a curved top wall. 
         [0027]    The hatch may comprise a first upturned lip, and a wall adjacent the hatch may comprise a second upturned lip, wherein the first and second upturned lips may abut when the hatch closes the opening. 
         [0028]    The hatch may comprise a first connection plate, and a wall adjacent the hatch may comprise a second connection plate, wherein a plurality of connection elements may engage the first connection plate and second connection plate to maintain the hatch in a closed position. 
         [0029]    The connection elements may comprise a plurality of bolts. 
         [0030]    The connection elements may be mounted such that the total vector of forces on the hatch and the adjacent wall extends generally longitudinally of the connection elements. 
         [0031]    A sealing element may be engaged between the first connection plate and second connection plate. 
         [0032]    In any embodiment of the invention, the evaporator may comprise a single preassembled module which can be transportable using a standard vehicle. 
         [0033]    The evaporator may comprise a compressor having an inlet. 
         [0034]    The evaporator may comprise a demister to remove entrained water droplets before the inlet. 
         [0035]    The demister may comprise a plurality of horizontal louvers. 
         [0036]    The demister may comprise a plurality of horizontal knitmesh elements. 
         [0037]    The demisters may be washed thorough a designated spraying system, by caustic or other chemicals 
         [0038]    The evaporator may comprise a plurality of effects, each effect comprising a heat exchanging tube assembly and an associated plurality of nozzles. 
         [0039]    The evaporator may comprise a plurality of ultrasonic transducers to permit ultrasonic cleaning of the heat exchanging tube assembly 
         [0040]    The evaporator may comprise an inner cavity and a deflector plate to retain fluid within the inner cavity to immerse the heat exchanging tube bundle to permit ultrasonic cleaning. 
         [0041]    According to a further aspect of the invention there is provided an oil production apparatus, the oil production apparatus comprising a water recovery apparatus, the water recovery apparatus comprising an evaporator array according to any preceding aspect of the invention. 
         [0042]    According to a further aspect of the invention there is provided a method of cleaning a heat exchanging tube or plate array for an evaporator, comprising the steps of removing a hatch which closes an opening provided in an external vessel of the apparatus, removing the heat exchanging tube or plate array, cleaning the heat exchanging tube or plate array, inserting the heat exchanging tube or plate array in the external vessel through the opening, and closing the hatch. 
         [0043]    According to a further aspect of the invention there is provided method of cleaning a heat exchanging tube array for an evaporator, where the evaporator has a first nozzle array for introducing produced water and a second nozzle array, the method comprising the step of introducing a cleaning fluid to the heat exchanging tube array through the second nozzle array. 
         [0044]    According to a further aspect of the invention there is provided method of cleaning a heat exchanging tube array for an evaporator, where the evaporator has a plurality of ultrasonic transducers and an external vessel defining an internal cavity, the method comprising the steps of introducing fluid in to the internal cavity to immerse the heat exchanging tube array, and operating the ultrasonic transducers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0045]    Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings wherein; 
           [0046]      FIG. 1  is a perspective view of an evaporator assembly embodying the present invention, 
           [0047]      FIG. 2   a  is a further perspective view of the evaporator assembly of  FIG. 1 , 
           [0048]      FIG. 2   b  is a perspective view of an evaporator vessel of the evaporator assembly of  FIG. 1 , 
           [0049]      FIG. 3  is a perspective view of a hatch of the evaporator assembly of  FIG. 1 , 
           [0050]      FIG. 4  is a section on line  4 - 4  of  FIG. 1 , 
           [0051]      FIG. 5  is a view on an expanded scale of part of  FIG. 4 , showing a seal assembly of the evaporator of  FIG. 1 , 
           [0052]      FIG. 6   a  is an illustration of the forces acting on the cross-section of  FIG. 4  in negative-pressure operation, 
           [0053]      FIG. 6   b  is an illustration of the forces acting on the cross-section of  FIG. 4  in positive-pressure operation, 
           [0054]      FIG. 7  is a perspective view of the evaporator assembly of  FIG. 1 , illustrating removal of a bundle of heat exchanging tubes, 
           [0055]      FIG. 8  is a partial longitudinal section of he evaporator assembly of  FIG. 1 , 
           [0056]      FIG. 9  is a view in more detail of parts of  FIG. 8 , 
           [0057]      FIG. 10  shows the view on a larger scale of  FIG. 9  in a second operating position, 
           [0058]      FIG. 11  is a further view on a larger scale of part of  FIG. 8 , 
           [0059]      FIG. 12  is view of a further part of  FIG. 8  on a larger scale, 
           [0060]      FIG. 13  is a diagrammatic side view of a further embodiment of an evaporator, 
           [0061]      FIGS. 14   a  and  14   b  comprise a longitudinal section through a further evaporator assembly embodying the present invention, 
           [0062]      FIGS. 15   a  and  15   b  form a plan view of the evaporator assembly of  FIGS. 14   a  and  14   b , and 
           [0063]      FIG. 16  is a diagrammatic illustration of alternative cleaning processes. 
       
    
    
     DETAILED DESCRIPTION 
       [0064]    With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
         [0065]    Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated n the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0066]    Referring now to  FIG. 1  to  FIG. 4 , an evaporator assembly  10  embodying the present invention is illustrated. As shown in the figures, the assembly  10  includes an evaporator  11  and a compressor  12  which receives steam from the evaporator through an inlet pipe  13 , and returns steam to the far end of the evaporator  11  via pipe  14  to circulate steam through the heat exchanging tube assembly or bundle  22  within the evaporator  11 . Heat exchanging tubes may be made of stainless still, aluminum, titanium or other metals or alloys. internal or external coating materials may be practiced in order to coat internal or external heat exchanging tube&#39;s surface to better withstand working and cleaning corrosive conditions. Ceramic on metal coating may be practiced. Aluminum tubes may be coated with alumina coating, followed by a sealer coating to seal the alumina pores. Such a sealer improves the alumina resistance to corrosion and better protects the aluminum tubes. In addition, the alumina coating, together with the sealer reduces the tubes surface roughness and therefore reduces amount of foaling and scaling deposits. The evaporator  11  comprises an outer vessel  20  which has an internal cavity  21 . As illustrated in  FIG. 2 , the heat exchanging tube bundle  22  is received in this cavity  21 . The upper part of the vessel  20  has an opening or hatchway  23  which in operation is sealingly closed by hatch  24 . The hatch  24  is held in place by a plurality of bolts  25  in a configuration discussed in more detail below. 
         [0067]    At either end of the evaporator vessel  20  are provided a first end part  26  and a second end part  27 , each provided with a corresponding inspection hatch  26   a,    27   a,  respectively. End part  27  houses a demister to remove water droplets from the steam stream. In addition, end part  27  may house a third nozzles array connected to and fed by a third feeder tube. This third and optional nozzle array may be dedicated to clean the demisters and knit mesh. An example demister will be discussed in more detail below. 
         [0068]    Referring now to  FIG. 3  and  FIG. 4 , the hatch  24  will be discussed in more detail. In  FIG. 3 , the hatch  24  is shown upside down, i.e. with that surface which faces towards the cavity  21  in operation shown here uppermost. The hatch  24  is provided with first header tube  30  and second header tube  31 . Mounted on the underside of the hatch  24  are a plurality of secondary header ribs  32 , each of which is connected to at least one further nozzle  33 . Each secondary header rib is connected to only one of the first main header tube  30  or second main header tube  31 . Accordingly, the nozzles  33  are separated into two pluralities of nozzles  33 , a first plurality  33   a  which is in flow communication with the first header tube  30  and a second plurality  33   b  which is in flow communication with the second header tube  31 . The first plurality  33   a  of nozzles  33  is configured to spray produced or waste water on top of the heat exchanging tubes bundle in order to produce a falling film around the heat exchanging tube bundle  22  in known manner. As is known, the arrangement of the nozzles  33  in the first nozzle array  33   a  is designed to optimally produce a falling film of water and provide optimal heat transfer. 
         [0069]    The second array  33   b  of nozzles  33  is configured to spray the heat exchanging tube bundle  22  with a suitable cleaning or descaling fluid. It might be envisaged that further nozzles may be provided to at least one side of the heat exchanging tube bundle  22  in flow communication with the second main header  31  to use supply cleaning fluid to the side of the heat exchanging tube bundle  22 . 
         [0070]    Preferably, the second array  33   b  of nozzles  33  is located in such a way as to not disturb or interfere with the optimal placing of the first array  33   a  of nozzles  33 . This may be achieved, for example, in the present case by the nozzles  33  of the second nozzle array  33   b  being laterally or longitudinally offset from the nozzles  33  of the first nozzle array  33   a.  The nozzles  33  of the second nozzle array  33   b  may be adapted to suit the nature of the cleaning fluid, for example by being made of an appropriate alloy to accommodate a cleaning fluid comprising a concentrated acid with a low pH. 
         [0071]    In a number of alternatives, it will be apparent that a hatch  24  may be provided without the second main header  31  and without a second nozzle array. In this example, the heat exchanging tube bundle  22  may be cleaned either by removing the bundle  22  and subjecting it to an external cleaning process as discussed in more detail below, or by supplying a cleaning fluid through the first main header  30  and the plurality of nozzles  33   a.  In a further alternative, it will be apparent that an evaporator may be provided with an array of nozzles  33  having first and second main headers  30 ,  31 , but these need not necessarily be provided as part of or in combination with a hatch  24  and indeed in such circumstances the hatch  24  may be omitted altogether. 
         [0072]    As discussed in more detail below, advantageously the evaporator  11  is manufactured as a single and preassembled module with such dimensions that it can be transported as an integral unit on a single standard vehicle. In this example, by ‘standard vehicle’ is meant a truck or other transporter within the applicable legal size and/or weight limits, preferably without requiring modification to transport the evaporator  11 . Advantageously, the evaporator  11  can be transported as a single unit to a site and installation assembly required that the evaporator  11  and the associated compressor  12  be mounted on appropriate, modular frames or supports  17 ,  18 , respectively. Installing a preassembled and transportable evaporator unit may reduce construction efforts and complexity in the target operational industrial site. 
         [0073]    Referring now to  FIG. 4 , a section through the evaporator assembly  11  is shown in more detail. The evaporator vessel  20  is supported on transverse base parts  27 . The evaporator vessel  20  is made up of four similar concave walls, a base wall  40 , side walls,  41 ,  42 , and an upper wall  43  provided by the hatch  24 . In this example, the walls  40 ,  41 ,  42 ,  43  extend longitudinally of the evaporator vessel  20  and are curved in the plane transverse or perpendicular to the longitudinal axis of the vessel (while being essentially or substantially parallel to the longitudinal axis over the majority of their lengths). The “curved square” geometry is desirable because it maximises the volume available within the evaporator  11  while keeping its dimensions small enough to enable the evaporator  11  to be transported on a single vehicle. A completely circular cross-section would be strongest but would result in a loss of internal volume. Similarly, a square cross-section would maximise the possible internal volume but would be weakest during both the negative and positive pressure operations of the evaporator and vulnerable to failure at wall edges. In the present example, the evaporator vessel  20  has a width of 2557 mm, a height of 2569 mm and a length of 8924 mm. The vessel  20  has a weight of about 7500 kg, the hatch  24  weighs about 2000 kg and the heat exchanging tube assembly  22  weighs about 6000 kg. Consequently, the evaporator  11  may be transported as a single complete preassembled module. 
         [0074]    The base wall  40  and side walls  41 ,  42  may be formed integrally from a single metal sheet, by appropriate bending and folding. Alternatively, the side walls  41 ,  42  may be formed separately and welded to the base wall  40 . The heat exchanging tube bundle  22  is located in the cavity  21  between side deflectors  44 . The side deflectors  44  are spaced from the side walls  41 ,  42  to define steam suction channels  46 ,  47  respectively. Vapour created in the heat exchange tube bundle  22  is evaporated from the bottom of the vessel cavity  21  into the side steam suction channels  46 ,  47  and are then drawn towards the compressor. Where the evaporator is provided with a second nozzle array  33   b,  additional nozzles of the second nozzle array  33   b  may be mounted on the side deflectors. 
         [0075]    To provide a strong, sealing and releasable connection between the hatch  24  and side walls  41 ,  42 , a closure assembly is provided generally shown at  50  in  FIG. 5 . In  FIG. 5 , the junction between the top wall  43  provided by the hatch  24  and the right concave side wall  42  is shown. At its edge, the upper wall  43  is provided with an upturned lip  51  which is supported and held in place by a plurality of transversely extended flanges  52 . Connection plate  53  is mounted to the upper edge of the flanges  52  and extends over the junction between concave walls  42 ,  43 . 
         [0076]    In a similar manner, side wall  42  has an out-turned lip  54  which extends a shorter distance than lip  51 . Out-turned lip  54  is held in place by a plurality of mildly extending flanges  55 , which have a further connector plate  56  which extends longitudinally of the opening  25 . Disposed between the upper and lower connection plates  53 ,  56  is a V-shaped seal  57  and a spacer  58 . Bolts  59  pass through the upper and lower connection plates  53 ,  56  and are releasably held in place by nuts  60  received on the bolts  59 . Bolts  59  are located at spaced intervals around the edges of the hatch  24 , for example every 10 cm. The seal  57  generally extends around the periphery of the hatch  24  to ensure a complete seal. The notch  61  in the V-shaped seal  57  faces towards the out-turned lip  51 . In the event of any pressurized fluid escaping through the junction between the lips  54 ,  51 , the arms on either side of the notch  61  will be forced against the respective upper and lower plates  53 ,  56 , ensuring a good seal. 
         [0077]    As illustrated in  FIGS. 6   a  and  6   b , the orientation of the bolts  59  is selected such that the axis of the bolt extends along the product of the forces acting on the respective adjacent side wall  41 ,  42  and top wall  43 . As shown in  FIG. 6   a , when the evaporator is operating at a negative pressure with respect to the ambient pressure, each of the walls  40 ,  41 ,  42 ,  43  experiences an inward force. The longitudinal axis of the bolts  59  is aligned with the sum of the forces acting on the two walls meeting at the respective joint. Similarly, as shown in  FIG. 6   b  when the evaporator is at its operating positive pressure of approximately 1 atmosphere, the walls  40 ,  41 ,  42 ,  43  experience an outward force and the axes of the bolts  59  are aligned with the total force vector acting at the joint as a result of the forces on the adjacent walls. 
         [0078]    Removal and insertion of the heat exchanging tube bundle  22  will now be discussed with reference to  FIG. 7  to  FIG. 12 . The heat exchanging tube bundle  22  contains a plurality of closely spaced heat exchange tubes  70  (shown as a block in  FIG. 7  for clarity) supported at either end by tube sheets  71 ,  72 . It will be apparent that, in the preferred example, there are no transversely extending supports or sheets between end tube sheets  71 ,  72  as it is desirable that this region be clear to permit longitudinal movement of steam within the heat exchange bundle  22 . In addition, the tubes need not be rigidly mounted to one or more of the tube sheets  71 ,  72 . For example, it is known to receive the end of each tube in a rubber mount, for example an H-shaped rubber mount, which is then received in a hole in the respective tube sheet  71 ,  72 . 
         [0079]    To allow the heat exchange tube bundle  70  to be safely lifted, a lifting frame  74  is preferably provided. The lifting frame  74  has a first end part  75 , for connection to the first tube sheet  71 , and a second end part  76  for connection to the second tube sheet  72 . The first and second end parts  75 ,  76  are rigidly interconnected by a suitable frame  77 . By connecting the first end part  75  to the first tube sheet  71  and the second end part  76  to the second tube sheet  72  the components of the heat exchange bundle  70  are held in a fixed, rigid orientation and can be safely removed from the evaporator vessel  20  or inserted into the vessel  20 . 
         [0080]    To assist in accurately locating the heat exchanger tube bundle  22  in the evaporator vessel  20 , a plurality of support and alignment points are provided. A support bar  78  is provided on the first tube sheet  71  whilst each of the first and second tube sheets  71 ,  72  have a downwardly extending bottom part  71 A,  72 A respectively. As seen in  FIG. 9  and  FIG. 10 , the evaporator vessel  20  has a slidably movable inner end wall  80 . A guide pin  81  extends downwardly from the bottom end of the inner end wall  80  and is slidably movable in a slot  82  provided in a horizontally extending flange  83  which is supported by support wall  84 . At its lower end, the inner end wall  80  has an engagement part  85  which includes an adjustable seal  86 . The projection of the seal beyond the engagement part  85  may be controlled using a screw  87 . 
         [0081]    As shown in  FIG. 9 , when the inner end wall  80  is in the engaged position, the seal  86  abuts the side face of the lower part  71   a  of the tube sheet  71 , providing a seal between the tube sheet  71  and the inner end wall  80  and providing a lateral force acting on the tube sheet  71  to assist in holding the heat exchange bundle  22  in place. As shown in  FIG. 10 , when it is desired to remove or insert heat exchange tube bundle  22 , the inner end wall  80  is moved to the left as shown in the figure, disengaging the seal  86  and end part  71   a  of the tube sheet  71  and allowing movement of the heat exchange bundle  22 . 
         [0082]    As seen in  FIG. 11 , a transverse flange  90  with an upwardly extending seal element  91  engages and supports the support bar  78  to support the heat exchange tube bundle  22  at a particular vertical orientation. As shown in  FIG. 12 , a bundle support  92  extends upwardly from the base of the reactor vessel  22 . The bundle support  92  has a notch  93  at an upper part thereof which receives the lower part  72   a  of the second tube sheet  72  in a correct orientation. 
         [0083]    As will be discussed in more detail below, in one alternative it may be desirable for ultrasonic cleaning to take place within the evaporator vessel  20  without needing to remove the heat exchanger tube bundle  22 . Ultrasonic cleaning operates by inducing cavitation in an immersing liquid to break scaling deposits. Immersing liquid may contain cavitation nucleus such as mall air bubbles or particles as known to the skilled man in the art. With reference to  FIG. 13  and  FIG. 14A  this may be achieved by providing a suitable apparatus as part of the evaporator vessel  20 . As shown in  FIG. 13 , a plurality of ultrasonic transducer elements  100  are shown. These may be provided in any suitable orientation or configuration. They may comprise ultrasonic transducers which are permanently mounted on a surface of the evaporator vessel  20 , or may be removably mounted thereon, or alternatively the elements  100  may comprise ports within which the transducers may be received or mounted. To enable ultrasonic cleaning to function, the heat exchanging tube bundle must be immersed in a liquid bath. Accordingly, as shown at  101  in  FIG. 14A , a suitable deflector is provided which is configured such that when the cavity  21  is filled with a sufficient supply of liquid, the deflector will enable the heat exchange tube bundle  22  to be immersed in liquid, and with suitable ultrasonic transducers  100  cleaned ultrasonically. In addition to the transducers  100 , or alternatively, transducers may be installed internally of the evaporator, for example on the tube sheets  71 ,  72 , the deflectors  44  or even along or between the heat exchanging tubes  70  of the bundle  22 . 
         [0084]    As shown in  FIG. 14   a ,  FIG. 14   b ,  FIG. 15   a  and  FIG. 15   b , an evaporator  11 ′ may be provided which has two or more effects. The embodiment of  FIG. 1  has a single effect but it will be apparent that a plurality of effects may be provided with any suitable configuration of the evaporator, or indeed series of evaporators sequentially connected with suitable compression. In the example of  FIG. 14  and  FIG. 15 , separate heat exchange tube bundles  22  are provided, and separate hatches  24  corresponding to each effect are provided. It will be apparent that multiple effects could be provided under a single hatch such as that shown in  FIG. 1 . 
         [0085]    With reference to  FIG. 8  and  FIG. 14A , a demister is generally shown at  110 . The purpose of the demister  110  is to remove entrained droplets of water from the steam stream before it passes to the compressor. The demister  110  comprises a plurality of horizontally extending demister knit mesh elements  111  supported by suitable flanges  112 . Beneath the demister knit mesh elements  111 , horizontally mounted demister louvers  113  are provided. The horizontal orientation permits a higher number of louvers to be introduced providing a higher effective surface area for the demister. Where a deflector  101  is provided, the more efficient demister  110  may help compensate for steam-baffling effects of the deflector  101 . Water droplets connected on the demister  110  drop into the produced water. In order to maintain demister  110  clean and effective, a third and independent nozzle array may be added. 
         [0086]    The evaporator assembly described herein is thus advantageous in a number of respects. Firstly, in the transport and installation of the evaporator, the compact “square-circle” design permits a suitably large heat exchange tube bundle having a large surface area to be used, whilst maintaining the strength of the evaporator vessel but also permitting the evaporator to be moved on a single vehicle. Installation of the evaporator and the associated compressor is simply a matter of conveying both to the site and mounting the evaporator and compressor on suitable frames or bases. In the embodiments where a hatch is provided, maintenance and on-line cleaning of the evaporator is noticeably enhanced. 
         [0087]    The alternative methods of cleaning the heat exchanging tube bundle are illustrated in  FIG. 16 , after suspending operation of the evaporator as shown at step  200 . When external cleaning of the heat exchanger tube bundle  22  is required, shown as method (A), the bundle  22  may be simply removed by removing the hatch  24  (step  201 ) and lifting the bundle  22  out of the evaporator vessel  20 , shown at step  202  and as illustrated in  FIG. 7 . The bundle  22  can then be transferred to an ultrasonic or high-flow cleaning system in straightforward manner as shown at step  203 . It might be envisaged that a second, clean and operational, bundle  22  may be placed in the evaporator vessel  20  immediately to allow operation to resume as quickly as possible without waiting for the original bundle  22  to be cleaned, or the original cleaned bundle can be reintroduced, as shown at step  204 . The hatch  24  is replaced in a sealing manner and the bolts  59  reattached (step  205 ) and evaporator operation can then resume (step  206 ). 
         [0088]    In the embodiments where a second main feed  31  and second nozzle array  33   b  are provided, the bundle  22  may be cleaned in situ without removal. As shown at method B, the tube bundle  22  may be cleaned simply by supplying a suitable cleaning fluid, for example an acid solution with a relatively low pH, through the second main feed  31  and second nozzle array  33   b  (step  207 ). Cleaning fluid may be supplied at a suitable high rate to remove scaling and deposits from the bundle  22  and allow the bundle  22  to be cleaned without removal from the vessel  20 . Following cleaning, operation can resume at step  206 . 
         [0089]    In the further alternative, shown as method C the bundle  22  may be cleaned in situ by filling the vessel  20  with liquid such that the bundle  22  is immersed, at step  208 . An ultrasonic cleaning process using suitable ultrasonic transducers  100  mounted on and/or within the evaporator vessel  20  is shown at step  209 . 
         [0090]    In any of the alternatives, it will be apparent that the loss of production or operational time is kept to a minimum and cleaning and maintenance of the evaporator  11  and the heat exchanging tube bundle  22  performed quickly and efficiently. 
         [0091]    It will be apparent that suitable evaporators may be provided having some or all of the features of any of the embodiments and alternatives discussed herein. For example, it may be apparent that an evaporator vessel  20  may be provided having the curved wall configuration but with an otherwise conventional heat exchanger configuration or an evaporator may be provided with a hatch  24  with no second main feed  31  or second nozzle array  33   b.  Alternatively, an evaporator may be provided with first and second main feed  30 ,  31  and first and second nozzle arrays  33 A,  338 , but without the provision of a hatch  24 . Other combinations of features may be provided from any of the examples herein as needed or as advantageous. The evaporator may be provided with any suitable number of effects, whether one or two as shown in the embodiments here or, more, for example four, where a suitable compressor is provided. 
         [0092]    It is particularly envisaged that an evaporator and evaporator assembly as described herein is suitable for use as part of a water recovery apparatus at an oil production plant, to clean produced water, but it will be apparent that the evaporator may be used as part of any other industrial plant or process. 
         [0093]    In the above description, an embodiment is an example or implementation of the invention. The various appearances of “one embodiment”, “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. 
         [0094]    Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. 
         [0095]    Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above. 
         [0096]    The invention is not limited to those diagrams or to the corresponding description. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. 
         [0097]    Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belong, unless otherwise defined.