Heat exchange – Flow passages for two confined fluids – Interdigitated plural first and plural second fluid passages
Reexamination Certificate
2002-09-05
2004-11-30
Leo, Leonard R. (Department: 3753)
Heat exchange
Flow passages for two confined fluids
Interdigitated plural first and plural second fluid passages
C165S139000
Reexamination Certificate
active
06823934
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a heat transfer plate, according to the preamble to claim
1
, and a plate pack for use in a plate heat exchanger. The invention further relates to a plate heat exchanger consisting of such plates and plate packs respectively.
BACKGROUND ART
A plate heat exchanger comprises a plate pack consisting of a number of assembled heat transfer plates forming plate interspaces between them. Usually, every second plate interspace is connected with a first inlet duct and a first outlet duct, each plate interspace being arranged to define a flow area and to convey a flow of a first fluid between said inlet and outlet ducts. Correspondingly, the other plate interspaces are connected with a second inlet duct and a second outlet duct for a flow of a second fluid. Thus, the plates are in contact with one fluid through one of their side surfaces and with the other fluid through the other side surface, which allows a major heat exchange between the two fluids.
Modern plate heat exchangers have heat transfer plates, which in most cases are made of thin sheet that have been pressed and punched to obtain their final shape. Each heat transfer plate is usually provided with four or more “ports” formed by through holes being punched in the plate. The ports of the different plates define said inlet and outlet ducts, which extend through the plate heat exchanger transversely of the plane of the plates. Gaskets or any other form of sealing means are arranged around some of the ports alternately in every second plate interspace, and in the other plate interspaces around the other ports to form the two separate ducts for the first and the second fluids respectively.
Since the fluid pressure levels attained in the heat exchanger during operation are considerable, the plates need to have a certain stiffness so as not to be deformed by the fluid pressure. The use of plates made of sheet bars is possible only if these are somehow supported. As a rule, this is solved by the heat transfer plates being designed with some kind of corrugation so that the plates abut against each other in a large number of points. The plates are fixed to each other between two stiff end plates in a “rack” and thereby form stiff units with flow ducts in each plate interspace. To obtain the desired contact between the plates two different types of plates are manufactured, which are sandwiched in such manner that the plates in the heat exchanger are alternately of a first kind and of a second kind.
A modern example of a plate heat exchanger of this type is disclosed in U.S. Pat. No. 5,226,474. This plate heat exchanger is intended for evaporation of a liquid fluid taken in through a central inlet at the lower edge of each plate and discharged from the plate heat exchanger, in the form of vapour and concentrated liquid fluid, through an outlet located at one upper corner of the plate. The second fluid is taken in in the form of vapour through an inlet located at the other upper corner of the plate and discharged in the form of condensate and residual vapour through two outlets located at the two lower corners of each plate.
The manufacture of a plate heat exchanger of this type requires two different types of plates, which means that two sets of pressing tools are needed, which in turn implies big investments. The need for two different types of plates also means that large storage space is needed, both for the finished plates and for the pressing tools. Furthermore, the tools have to be changed in connection with the pressing of the plates.
Ever since the manufacture of plate heat exchangers with heat transfer plates made of sheet bars started, a solution which means that only one type of plate is needed has been in demand in the industry, since this would be more cost-effective.
There are some cases today where only one type of plate is used, for example in applications subject to two fundamental design requirements: on the one hand that inlets and outlets for each heat exchanging fluid can be located at the same lateral edge of the plates and, on the other, that the plates can be designed in such manner that size of the inlets and outlets is the same for the respective fluids. An example of such a plate is disclosed in U.S. Pat. No. 4,359,087.
In such specific cases, it is ensured that the ports and sealing elements of the plates, such as weld-prepared ridges and/or gaskets, are arranged symmetrically about a symmetry line located in the centre of the plate between the inlet and the outlet of the two fluids and extending transversely of the main flow direction of the fluids. The plates of the heat exchanger are arranged in such a way that every second plate is rotated or “flipped” through 180 degrees about its symmetry line. The location requirement concerning the inlets and outlets is due to the fact that the location of the sealing means relative to the ports that define the inlet and outlet ducts has to be the same for all plates. Locating the inlets and outlets this way means, however, that only part of the plate surface is used efficiently for heat transfer, since great flow rate differences arise between a partial flow taking the shortest way from the inlet to the outlet and a partial flow describing on its way from the inlet to the outlet a curved path via the opposite edge of the plate.
There are other applications where, as a standard solution, only one type of metal plate is used, but different types of gaskets in every second interspace, to constitute the whole plate heat exchanger. In this type of structure, every second plate is rotated through 180 degrees in the plane about a centre line extending, accordingly, perpendicularly to the plane of the plate. This means, just as in the above case, that the different ports have to be of the same size. In this plate design, different kinds of stiffening means in the form of special gaskets or weigh belts are also often used. However, this entails additional costs for manufacture and mounting of the stiffening means. In addition, these stiffening means often have a detrimental effect on the functioning of the heat exchanger since they interfere with the flow in an undesired way.
A further example of prior art is to be found in GB-A-2,121,525, which discloses a plate heat exchanger in the form of an evaporator provided with two different types of plates in a plate pack.
There are, however, a large number of applications where the above special cases are not applicable. For example, they cannot be used when one or both fluids undergo a phase transformation. There are evaporation and/or condensation processes, for example, where a liquid is transformed into vapour and vapour is transformed into liquid, which requires different sizes of the inlets and outlets for the respective fluids (see the above-mentioned U.S. Pat. No. 5,226,474).
Thus, there is no general technique for reducing to only one the number of plate types in one and the same plate heat exchanger. The attempts at solving the problem proposed have either been limited to very special applications or require special stiffening means and gaskets resulting in a more expensive and poorer construction, which means that the economic benefit of only one type of plate is lost.
There is above all a great need for a reduced number of plate types in various forms of condensation and evaporation applications, since these require relatively large plates in order to achieve an efficient heat exchange even if one of the fluids is in the vapour phase. The need is even more pronounced in connection with large-scale industrial operation processes.
SUMMARY OF THE INVENTION
The object of the invention is to provide a solution to the above problems.
More specifically, the primary object of the invention is to provide a heat exchanging plate, which is constructed in such a way that a plate heat exchanger can be manufactured at the lowest possible cost, the flow of each of the two heat exchanging fluids in the plate heat exchanger being as evenly distributed as possible in the
Alfa Laval Corporate AB
Fish & Richardson P.C.
Leo Leonard R.
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