5 10 15 20 25 30 IMPROVED SPIRAL PLATE HEAT EXCHANGER FIELD OF INVENTION The present invention relates to heat transfer equipment and more specifically it relates to an improvement in the use of spiral plate heat exchangers to process problematic fluids. BACKGROUND OF INVENTION Various industries use heat exchangers to transfer heat from one material to another. In many cases the fluid on one side of the exchanger is air, water (or steam), refrigerant, or heat transfer oil because these material have desirable properties in terms of flow, heat transfer flexibility, flammability or availability. They are also well documented materials with a variety of test data publicly available. Heat integration is a way of economizing heating and cooling use in a facility, such that waste heat is used for heating, and cooler streams that need heat are used for cooling. Black oil streams, a common target of heat integration, have less known, tested and consistent properties than standard heat transfer fluids. In some cases, these properties (for example high viscosity) can be difficult in the heat transfer context. In the oil sands industry, a variety of streams with available heat are in the form of slurries and other heterogeneous mixtures. These streams present challenges, as they need to be handled within a velocity envelope for proper hydraulics. These fluids also contain some larger particulate matter, which limits the degree to which heat exchanger channel size can be adjusted. Spiral Plate Heat Exchangers (SPHE) present an intriguing solution to a variety of heat exchanger challenges. They allow for single channel flow, which permits use with velocity sensitive systems (fouling, slurries, and/or shear thinning). SPHE can also allow for true counter current or true cross-current flow. The basic configuration of a spiral heat exchanger is disclosed in US 4,128,125. Improvements to this configuration are encompassed in a novel design allowing for compact use of an intermediate fluid is disclosed in US 4,577,683. Alternatively, US 7,147,036 discloses a cross flow design that allows for a “less than one pass” flow through the spirals that results in both fluids being in cross flow. US 4,611,655 teaches a heat exchanger including a plurality of cylindrical walls that are joined together to form at least one annular chamber. A plurality of tubes that spiral through the chamber, each of the tubes forming a coil and the coils being axially separated. A fluid is moved in one direction through the coiled tubes, and simultaneously another fluid is moved through the chamber, thereby producing a transfer of heat between the two fluids. In the instance where the heat exchanger is used as a boiler or steam generator, the fluid in the tubes is water or water vapor, and the fluid in the chamber is hot exhaust gases from a burner 1 CA 2969595 2017-06-025 10 15 20 25 30 mounted at the center of the exchanger. US Patent No. 4,546,826 teaches a spiral heat exchanger comprising: a vessel; a plurality of concentric sheet metal spirals situated in said vessel and having a longitudinal axis and defining a first and a second spiral flow channels for respective first and second fluids for effecting a heat exchange there¬ between; a first and a second closure plate extending perpendicularly to said axis and covering said spirals at opposite spiral edges thereof; mounting bars pressing the closure plates against axially opposite edges of said spirals; said first closure plate defining a first axially oriented opening and said second closure plate defining a second axially oriented opening providing, in said first flow channel, an inlet and an outlet for said first fluid; means defining a radially oriented inlet and a radially oriented outlet in said second flow channel for said second fluid. The improvement is said to be wherein the vessel comprises a cylindrical vessel shell entirely surrounding said spirals and further wherein said first closure plate has a radially outer circumferential edge surrounded by said first axially oriented opening and said second closure plate has a radially inner circumferential edge surrounding said second axially oriented opening. This configuration results in said first axially oriented opening communicating with a radially outer part and said second axially oriented opening communicating with a radially inner part of said first flow channel. Despite the various known SPHE configurations, a challenge that has been identified with these exchangers is that geometrically, the only hydraulic degrees of freedom available in the conventional design are: channel width and a choice between countercurrent flow and cross flow. This can present a problem when the fluids on both side of the exchanger are multiply constrained - by particulate sizing, fluid properties, relative quantities and/or other characteristics. This can often be the case in heat integration applications where the efficiency of the system requires that two “difficult fluids” (fluids/fluid streams with constraining properties) have to be exchanged directly against one another without the use of an intermediate fluid. The inventors have designed an improved SPHE capable of handling fluids that have multiple constraints as described above. Accordingly, fluids that have settling particulate, abrasives, larger particles, high viscosity and non-Newtonian viscosity can exchange their heat in this format. SUMMARY OF THE INVENTION According to one aspect of the present invention, there is provided a spiral heat exchanger comprising: 2 CA 2969595 2017-06-025 10 15 20 25 30 35 - a vessel of generally cylindrical shape comprising a longitudinal axis; and a first and a second extremity; - a first fluid inlet; - a first fluid outlet . - a second fluid inlet - a second fluid outlet; - at least two concentric walls disposed inside said vessel and having substantially the same length as the vessel; said at least two concentric walls defining a first concentric annular channel chamber and a second concentric annular chamber; said first concentric annular channel chamber being fluidly connected to said first fluid inlet and first fluid outlet and said second concentric annular channel chamber being fluidly connected to said second fluid inlet and second fluid outlet; said first annular channel chamber and second annular channel chamber being hydraulically separated from one another, said first annular channel chamber and second annular channel chamber defining together a spiral pack; - each of said first and second concentric annular chamber comprising a first end located at said first extremity of the vessel and a second end located at said second extremity of the vessel; wherein the heat exchanger is adapted to direct the flow of a first fluid in a radial flow from the first fluid inlet to the first fluid outlet along the first concentric annular channel chamber and to direct the flow of a second fluid from the second fluid inlet in a longitudinal direction along the first concentric annular chamber of the vessel and radially towards the middle of the vessel in a circuitous path from the second fluid inlet towards the second fluid outlet, thereby creating a cross-current heat exchanging flow. Preferably, the concentric annular chambers result in fluid pathways that successfully convey two fluids through the spiral pack. According to a preferred embodiment the fluid flows identified and outside to inside can be alternated to inside to outside, based on the needs of the exchange. A series of cross-current exchanges can achieve and nearly countercurrent performance in the system, by having the two fluids move in opposite directions through the exchanger. Preferably, the spiral heat exchanger further comprises a plurality of baffles arranged perpendicularly to the longitudinal side of the concentric wall and adapted to provide a seal between a first section of the longitudinal channel chamber from a second section immediately adjacent to said first section when in contact with another concentric wall. 3 CA 2969595 2017-06-025 10 15 20 25 30 Preferably also, the spiral heat exchanger further comprises a plurality of spaced apart extended tabs arranged on each one of said longitudinal edge in an alternating pattern and adapted to redirect the longitudinal flow in the opposite direction. According to a preferred embodiment of the present invention, the spiral heat exchanger further comprises a plurality of spaced apart studs located on an exterior side of each concentric wall, said studs adapted to provide an inter-wall spacing between the first and second concentric walls and define said channel chambers. Even more preferably, both the studs and the baffles have the same height perpendicularly from the surface of the concentric wall. According to a preferred embodiment of the present invention, the tabs located on alternating edges located at said first and second extremities of one of the concentric walls and adapted to direct the longitudinal flow within the annular chamber from one direction to the opposite direction and wherein the tabs are adapted to bridge one annular chamber section adapted for longitudinal flow of a fluid to the next adjacent annular chamber section while maintaining a chamber spacing equal to or greater than the inter-wall spacing provided by the studs and baffles. Preferably, the spiral heat exchanger further comprises closure plates located at each one of said extremities, said closure plates being adapted to seal the vessel from the exterior. According to a preferred embodiment of the present invention, the adjoining edges at each extremity of said concentric walls of the second annular concentric channel chamber are sealed together to seal the second annular chamber from the first annular concentric channel chamber. According to another aspect of the present invention, there is provided a use of a spiral heat exchanger, said use comprising: - providing a spiral heat exchanger according to the present invention; - introducing a first fluid into the first fluid inlet; - introducing a second fluid into the second fluid inlet; - circulating both first and second fluid at a flow speed sufficient to provide a desired heat exchange and hydraulic performance between the two fluids; - removing the first and second fluids from the heat exchanger through said first and second fluid outlets. CA 2969595 2017-06-025 10 15 20 25 30 According to another aspect of the present invention, there is provided a method to extract heat from a first fluid by a second fluid, said method comprising: - providing a spiral heat exchanger according to the present invention; - providing a first fluid having a first initial temperature; - providing a second fluid having a second initial temperature; - injecting said first fluid into the first fluid inlet of the spiral heat exchanger; - injecting said second fluid into the second fluid inlet of the spiral heat exchanger; - circulating the first fluid radially into the first annular channel chamber; - circulating the second fluid longitudinally and increasingly radially and outwardly into the second annular channel chamber; - recovering the first fluid from the first annular channel chamber through the first fluid outlet and - recovering the second fluid from the second annular channel chamber through the second fluid outlet; - optionally, measuring the temperature of the first and second fluid recovered from the spiral heat exchanger. According to one aspect of the present invention, there is disclosed the use of baffles and channel connections with a spiral exchanger to modify a cross current flow path such that the spiral unit can be tuned to the constraints of two fluid streams simultaneously. To that end, once the constraints have been laid out, the two desirable channel geometries are identified. One channel will be run “along the channel” (i.e. in the spiraled direction). The other channel will be run “across the channel” (perpendicular to the spiral). In the channel run across, some of the studs will be replaced with a baffle plate of the same length, isolating the section. At the channel ends, these isolated sections are connected with end plates. The result is a continuous channel in cross flow with a velocity envelope tuned to the fluid and an area to channel length relationship freed from the constraints of its counterpart fluid in the spiral flow channel. BRIEF DESCRIPTION OF THE FIGURES The invention may be more completely understood in consideration of the following description of various embodiments of the invention in connection with the accompanying figure, in which: Figure 1 is a perspective side view with a cutaway portion to illustrate the flow of the two fluids within the spiral heat exchanger according to a preferred embodiment of the present invention; 5 CA 2969595 2017-06-025 10 15 20 25 30 35 Figure 2 is a cross-sectional view of the channel connections as found in a spiral heat exchanger according to the present invention; Figure 3 is a side view of a heat exchanger comprising two spiral heat exchangers according to the present invention illustrating the flow of fluids; Figures 4a and 4b are top views of the plates according to an embodiment of the present invention prior to rolling them together; Figure 4c is a perspective side view of a plate comprising both baffles and studs; Figure 5A is a schematic top view of the spiral heat exchanger according to a preferred embodiment of the present invention illustrating the fluid entrances exits; and Figure 5B is a schematic top view of the spiral heat exchanger according to a preferred embodiment of the present invention illustrating the fluid entrances exits. BRIEF DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION According to a preferred embodiment of the present invention as illustrated in Figure 1, the spiral heat exchanger comprises: - a vessel of generally cylindrical shape comprising a longitudinal axis; and two extremities; - a first fluid inlet; - a first fluid outlet - a second fluid inlet - a second fluid outlet; - at least two concentric walls disposed inside said vessel ; said at least two concentric walls defining a first concentric annular channel chamber and a second concentric annular chamber; said first concentric annular channel chamber being fluidly connected to said first fluid inlet and first fluid outlet and said second concentric annular channel chamber being fluidly connected to said second fluid inlet and second fluid outlet; said first annular channel chamber and second annular channel chamber being hydraulically separated from one another; - each of said first and second concentric annular chambers comprising a first end located at a first extremity of the vessel and a second end located at a second extremity of the vessel; wherein the heat exchanger is adapted to direct the flow of a first fluid in a radial flow from the first inlet 6 CA 2969595 2017-06-025 10 15 20 25 30 35 to the first outlet along the first concentric annular channel chamber and to direct the flow of a second fluid from the second inlet in a longitudinal direction around the first concentric annular chamber of the vessel and radially from the second fluid inlet towards the second fluid outlet, thereby creating a discreet countercurrent heat exchange from a series of cross-current flows. Figure 2 depicts an example of channel connections. This represents half of the spiral in cross¬ section. It can be seen that one to one, one to two and two to two channel connections have been used in this case (such that 18 channels- as defined by baffles) have been connected into 15 passes. Figure 3 is an example of a typical physical arrangement for this type of spiral heat exchanger. A stack of multiple spirals in a single main pressure envelope can be used to approach counter current performance with a series of cross current units. This is similar to how evaporation and condensation service spirals are assembled. Figures 4a and 4b illustrate the baffle (1) and tab (2) layouts for the two plates (3 and 4) according to an embodiment of the present invention. Figure 4c provides a perspective view and highlights the details of the baffle/plate attachment. According to a preferred embodiment of the present invention, the spiral plate heat exchanger has a modification adapted to provide for segregated cross flow channels allowing the independent modification of velocity and flow length for the two fluids being handled in the exchanger. The tabs when folded and welded become equivalent to a continuous closure plate. According to a preferred embodiment of the present invention, at least one of said closure plates is formed of a plurality of circular sectors. The resulting plate has an entrance and exit profile similar to that shown in Figures 5A and 5B. Sealing of the channel chambers As best illustrated in Figure 1, the first channel chamber (5) is sealed preferably by welding the outer wall edges (6) of the first annular channel chamber with the inner wall edges (7) of the first annular channel chamber located at each extremity (9)(10) together in order to form a radially continuous seal (8). This yields a concentrically-shaped gap corresponding to the second channel chamber (If). The sealing of the second annular channel chamber (12) is accomplished by providing an extension piece (13) which is shaped to straddle one sealed edge (corresponding to the sealed edge of the first annular channel chamber, 14) and to connect, at an extremity, a portion of the second annular channel chamber (5) separated by the previously 7 CA 2969595 2017-06-025 10 15 20 25 30 mentioned sealed edge of the first annular channel chamber (15). The extension piece should preferably have sufficient length to overshoot the second annular chamber and not create a channel restriction Typically a spiral exchanger channel is sealed by welding the tab of plate that extends from the edge beyond the studs to the adjacent piece of tab (the tab is continuous). This is the case for one channel of this exchange. The other channel has extended tabs that can be folded right over the seal channel and connected to the next open channel. The ends are also sealed to the baffle ends to create a completely sealed unit. Typically, the simpler spiral exchangers are designed to have both fluids flow radially along the annular channel chambers in such a way as to provide counter current flow. However, as discussed previously, the drawbacks of such a standard system will prevent any potential use with fluids having the constraining properties which are one of the targeted fluids by the present invention. Preferably, the SPHE has two channels chambers designed to provide as little impediment as possible to the flow of both fluids within the heat exchanger, According to an embodiment of the present invention, the second annular chamber is designed to have the second fluid flowing there-through longitudinally along the width (or length) of the walls and then move closer towards the center of the heat exchanger (i.e. radially) once it reaches an extremity of the wallflow towards or away from the centre is achieved in the end chambers of the channel, rather than along the channel. At that point, it flows longitudinally in the opposite direction until it reaches the opposite extremity, it will then move inwardly and flow again in the opposite direction until it reaches the center of the heat exchanger where the fluid outlet connected to the second annular channel chamber is reached and exits the heat exchanger at that point. All the while the fluid in the first annular channel chamber proceeds to flow radially towards the center of the heat exchanger in a circular path. According to an alternative embodiment, while the second fluid moves along the same pathway, the first fluid can enter the heat exchanger at the center thereof and proceed to flow outwardly until it reaches the first fluid outlet and exits therefrom. According to yet another alternative embodiment, the second fluid moves from the center of the heat exchanger along a longitudinal path inside the second channel chamber and radially outwards as described previously until it reaches the second fluid outlet, the first fluid can enter the heat exchanger at the center thereof and proceed to flow outwardly until it reaches the first fluid outlet and exits therefrom. 8 CA 2969595 2017-06-02According to yet another alternative embodiment, the second fluid moves from the center of the heat exchanger along a longitudinal path inside the second channel chamber and radially outwards as described previously until it reaches the second fluid outlet, the first fluid can enter the heat exchanger from the side and then flow inside the heat exchanger following a concentric path until it reaches the first fluid outlet 5 located near the center of the heat exchanger and exits therefrom. It will be appreciated that variations of the above disclosed and other features and functions, or alternatives thereof, maybe desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein 10 maybe subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. 9 CA 2969595 2017-06-02