FR2475918A1 - METHOD AND DEVICE FOR INCREASING THE CALORIFIC POWER OF EVAPORATOR - Google Patents

Abstract

ACCORDING TO THIS PROCEDURE, WITH A VIEW TO GIVING MAXIMUM THERMAL POWER, THE MASS SPEED IS MODIFIED ALONG THE TUBES 1 OF THE EVAPORATOR, BROUGHT THROUGH THE CALOPORANT FLUID, SO AS TO OBTAIN, WHILE LOCATED ALONG THE EVAPORATOR, A APPROXIMATELY UNIFORM SURFACE HEAT LOAD OF THE SURFACE OF TUBES 1, AND THIS, USING INSERTS 2, 2, 2 PROVIDED IN THE TUBES BETWEEN INLET 7 AND OUTLET 8 OF THE FLUID. APPLICATION IN PARTICULAR TO TUBULAR BEAM EVAPORATORS.

Description

24759 1 8

  The invention relates to a method and a device

  to increase the heat transfer power of evapo-

  including tube bundle evaporators.

  It is known that in practice in the case of evaporators, the coolant enters each evaporator tube with a vapor head of between about 0% and%, the term "vapor head" denoting the ratio of the flow rate mass of steam at total mass flow. At the end of the evaporator tube, the heat transfer fluid is completely vaporized, that is to say that the vapor

is equal to 100%.

  As vaporization increases, the speed of

  It also increases along the tube under consideration and both the pressure drop dp / dl and the heat transfer coefficient a have different values along the tube. It is furthermore known that when the length of the tube, which is traveled, increases and therefore when the

  mass velocity increases, the average coefficient of trans-

  thermal mission to increase. The total pressure loss

  Ap also increases sharply and this reduces the difference

  this of average temperature As in the evaporator. Since the heat transfer power is proportional to a x AE, a determined optimal length of the tube is obtained for a given case of use. This optimal length of the tube can however not be respected in many technical cases of use, for reasons

construction or manufacturing.

  In order to improve the heat transfer power,

  it is already known to double the cross-section of

  for the heat transfer fluid, after a certain

  length of tube, creating bifurcations. This provision

  In many cases, this may lead to an improvement, but does not allow real optimization,

  to maximize the heat transfer power

  nomic. Various types of inserts inside the tube that are at least partly

  the length of the tube carrying the coolant.

  With these inserts, we can influence the speed of

  coolant and can, therefore, improve the heat transfer power. However, in the case of these inserts internal to the tube and made for example in the form of propellers, a disadvantage lies mainly in the fact that

  these inserts cause the appearance of flow conditions em-

  scrambled and confused, which prevent a calculation of the power

  heat transfer of the entire evaporator.

  The present invention aims to create a method of the type defined above, to obtain, in the desired manner, the highest heat transfer power

  possible, regardless of the length of the tube.

  This problem is solved according to the invention

  thanks to the fact that in order to maximize the power of

  With this heat transfer, the mass velocity along the evaporator tubes is changed so that a superficial heat load is provided at any location.

  at least approximately uniform on the surface of the tube.

  The heat transfer power can be maximized

  especially when heat is transmitted by

  pure vection of the tube wall to the coolant. The boiling zone, which in evaporators of a prior type of construction, appeared in the low vapor range and stressed a substantial part of the tube length due to the relatively low thermal transmittance ,

  is avoided (see document VDI-Warmeatlas, para.

phes Ha, Hb).

  According to the invention, the mass velocity and the equivalent hydraulic diameter of the

  in order to obtain, along the tube, an optimum value

  aunt for the pressure gradient and for the coefficient of

thermal transmission.

  The method according to the invention is implemented by using inserts internal to the tubes and extending at least over a portion of the length of the tubes carrying the coolant, and this so that the free cross section of the flow in each tube is increased, by at least one insert in the flow direction of the coolant, from a surface of minimum value

  leaving at least the heat transfer fluid essential-

  in liquid form to a surface of maximum value.

  allowing it to pass at least the coolant essential-

in the form of steam.

  When determining the size of the enlargement

  of the flow-free cross-section, the criteria of the method according to the invention for obtaining the maximum heat transfer power are taken into account, and preferably on a calculation basis, since it is precisely an essential advantage of the invention must be

  seen in the fact that because of the simple and definite geometry

  and flow states, which are therefore very clear, the evaporators become accessible to the calculation. However, this should not exclude the fact that the desired maximum of heat transfer power is calculated by taking into account

  account of the data of the invention, by carrying out tests.

  As inserts inside the tubes, it is preferable

  closed displacement or displacement organs,

  but in principle, with a view to producing a device designed

  according to the invention, any types of discharge members may also be suitably used, provided that the free cross-section of the flow of the flow can be pre-determined in the required manner.

coolant along the tube.

  In practice it may be advantageous to use delivery members consisting of several elements

  of tubes succeeding each other directly and maintained

  coaxially in the respective tube conveying the

  heat transfer fluid, said tube elements having a di-

  meter decreasing in a stepped way that can be predefined

  born, in the direction of the flow of heat transfer fluid.

  As an advantageous variant from the point of view of manufacture and assembly, it is also necessary to mention

  the use of individual delivery devices

  with different cross-sectional areas

  each other, and which can be arranged in the

  rectilinear sections of a tube made in a serpentine

crossing a pack of slats.

  By way of example, hereinafter and schematically illustrated in the accompanying drawings, there are described several examples of

  accomplishment of the object of the invention.

  Figure 1 is a schematic representation of a first embodiment of the invention. Figure 2 is a schematic representation of a

  second embodiment of the invention.

  According to Figure 1, an evaporator tube 1 is routinely guided in the manner of a coil, through a pack of slats. In the straight sections of this tube 1 are inserted displacement or discharge members 2, 2 ', 2 "made in the form of closed tubes at their ends, while a fixing in the coaxial position with respect to the tube 1 is ensured. by appendices

  5 which are preferably made by local deformations

  the limited of the tube. Since the elbows of the tube

  participate insignificantly in the heat transfer

  that it is not necessary to have at that place

pushing organs.

  The delivery members 2, 2 ', 2 ", which possess

  different sizes, that is to say of which the trans-

  versales have different surfaces, determine, -for the

  penetrating heat transfer fluid at 7, transverse sections

  flow free -variable throughout the length of the tube. Thus at the beginning of the tube 1, the discharge member 2 determines an annular slot 3 having a relatively small width, which has the effect of increasing the mass velocity (kg / m 2, s) or the flow velocity and reducing the equivalent hydraulic diameter. This contributes to increasing the heat transfer coefficient and the

pressure drop.

  We adjust the width of the slot or the trans-

  flow free verqle in order to obtain the gradient

  optimum dp / dl pressure and the average transmission coefficient

  Thermal mission has maximum possible.

  So that this optimal value of the gradient of pres-

  also be ensured in the later part of the

  tube, it is necessary to increase the free cross-section of

  taking into account the increasing vaporisation, which is obtained by means of a reduction in the cross-sectional area of the delivery members 2 'and 2 ",

  which follow one another in the direction of the flow of

  carrier. This principle can be used successfully for relatively long lengths of tubes, which allows, for example, already for a vapor head of about

  %, to dispense with the use of an additional body

  repression. This is shown in Figure 1 -with the last section 4 of the tube, in which there is

  no need to have an organ of repression and

  which escapes, in 8, the heat transfer fluid practically

completely vaporized.

  Instead of using individual organs of refu-

  have different cross-sectional areas

  of each other, it is also possible to use,

  according to another variant embodiment of the invention,

  inserts internal to the tube and which are again made

  in the form of closed delivery members, but which consist of several successive tube elements

  9, 10, 11, 12 having stepped diameters. This is represented

  schematically in Figure 2.

  Likewise in the case of this embodiment, a flow-free cross-section is thus obtained which increases in the direction of the flow,

  the increase of this free cross-section of

  and the decrease in the corresponding area in sec-

  cross section of the delivery member being realized

  in accordance with the design and dimensional requirements

  provided by the process according to the invention.

  The principle of the predetermination of trans-

  free flow direction for the heat transfer fluid sent to an evaporator tube, can be not only implemented by means of discharge members selected in a particular way, but it is also possible

  to connect in parallel. at least on separate sections

  of the tube in question. evaporator, a circuit

  of the coolant, and the heat transfer fluid

  circulating in this reduced supplementary circuit, in the

  considered, the flow-free cross-section for the heat transfer fluid sent to the

  total in the tube considered of the evaporator, and that of a

  which may be predetermined by the design of the

additional circuit.

  In all practical and

  in all practical implementations of the process in accordance

  In the invention, however, it must be ensured that the mass velocity is varied so that at any location along the evaporator an approximately uniform heat load is applied to the surface of the tube, since only in this way can one, in the desired way, increase

  maximally the heat transfer power,

  according to the present invention.

  According to an alternative embodiment of the invention, each insert 2 can be made in the form of a discharge member whose cross section is in the form of a star.

Claims (13)

  1. Process for increasing heat output   evaporators, including tube bundle evaporators,   re, by means of an influence on the flow of the coolant, said influence being implemented   at least a part of the length of the tubes and   in the sense of a decrease in the mass velocity of the coolant, characterized in that, in order to maximize the thermal power, the   mass velocity along the tubes (1) traversed by the   to obtain, in any location along   the evaporator, a surface heat load appro-   approximately uniformly of the surface of the tubes (1).   2. Method according to claim 1, characterized in that the mass velocity and / or the equivalent hydraulic diameter of the tubes (1) is adjusted so as to maintain, at any location along the tube (1), a pressure gradient having a optimum value and a heat transfer coefficient with an optimum value and which   create a maximum average surface heat load.   3. Process according to claims 1 and 2 taken   as a whole, characterized in that the mass velocity is regulated so as to always be in the   field of convection heat transfer.   4. Method according to claims 1 to 3 taken   as a whole, characterized in that at least one   part of the tube (1) is equipped with an additional circuit   for the heat transfer fluid.   5. Method according to claim 4, characterized   in that partial sections of the tube (1), which succeeds   tooth in the direction of the flow of heat transfer fluid, are equipped with separate additional circuits   others and dimensioned differently.   6. Device for implementing the method   according to one of claims 1 to 5, subject to   the use of inserts (2, 2 ', 2 ") extending over at least a part of the length of the tubes (1) carrying the coolant, characterized in that the free cross section of flow of each tube (1) is increased by at least one insert (2) in the direction of fluid flow   coolant, from a surface of minimum value (3)   passing at least the heat transfer fluid essentially in liquid form to a maximum-value surface (4) passing at least the heat transfer fluid essential- in the form of steam.   7. Device according to claim 6, characterized in that the increase of the free cross-section   of flow corresponds at least essentially to the   of the vapor titer of the two-phase flow is established health.   8. Device according to one of claims 6 or 7,   characterized in that each insert (2) is made under the   form of a closed delivery member.   9. Device according to one of claims 6 or 7,   characterized in that each insert (2) is made under the   form of a delivery device whose cross-section dirty is star-shaped.   10. Device according to one of claims 8 or   9, characterized in that each insert (2) has over its entire length at least substantially the same cross-sectional area and that successive inserts in the direction of flow of the coolant have cross-sectional areas decreasing one by one. report to other. -   11. Device according to claim 8, characterized in that each insert (2) has over its entire length   a cross-sectional area, decreasing.   12. Device according to any one of the   cations 1 to 11, characterized in that each insert (2) is held in the tube (1) by supports (5) made by   deformed parts of said insert.   13. Device according to claim 6, characterized in that each discharge member is constituted by a plurality of tube elements (9, 10, 11, 12) succeeding each other directly to each other, connected closely together and held in a manner coaxial in the respective tube (1) conveying the coolant, the diameter   said tube elements decreasing in a stepwise manner.   to be predetermined, in the direction of flow of the coolant.