FR2554571A1 - Method of heat exchange between a hot and cold fluid using a fluid mixture as the heat-carrying medium and producing a circulation of the heat-carrying medium by capillary attraction - Google Patents

Abstract

The method transfers heat from a hot to a cold fluid by means of a heat-carrying medium with at least two non-azeotropic constituents contained in a loop circuit. The hot fluid 2, 3 gives up its heat in an exchanger I and this heat is used to vaporise the heat-carrying medium, which is then condensed in an exchanger II giving up its heat of condensation to the cold fluid 5, 6. The heat-carrying medium is transferred from exchanger II to exchanger I by capillary attraction 4. Usable in the recovery of heat from industrial effluent or discharges from thermal processes, e.g. smoke from a furnace.

Description

The method according to the invention aims to allow the transfer of heat from a hot fluid to a cold fluid, and more particularly to allow the recovery of available heat from a hot fluid to transmit it to a cold fluid that it is necessary to reheat.

In many cases, such a heat exchange cannot be carried out under satisfactory conditions by directly putting the hot fluid and the cold fluid in exchange relationship in a heat exchanger. This is in particular the case when these two fluids circulate in conduits distant from one another, the fact of bringing them into contact in an exchanger which can lead to a bulky and expensive installation, or even to pressure drops unacceptable. This can happen in particular for exchanges between gases circulating at relatively low pressures.

 I1 is known in such a case to use a heat transfer agent such as water, glycol water or liquid organic fluids of high boiling temperature, circulating in an exchange loop, the heat transfer fluid being heated by the hot fluid in a first heat exchange zone and heating the cold fluid in a second heat exchange zone separate from the first.

Such a system requires the permanent operation of a circulation pump, which requires maintenance to have reliable operation over a long period. On the other hand, none of the fluids used is entirely satisfactory. Water used without antifreeze cannot be used in winter in most applications; glycol water which avoids this drawback has characteristics of high viscosity, detrimental to heat transfer, and of corrosion. Finally, heavy organic fluids are expensive and also have a high viscosity.

 I1 is known on the other hand, that a heat transfer can be carried out by vaporization and condensation of a fluid such as water or an organic fluid. However, such a technique is not suitable for the exchange of heat between fluids whose temperature changes during the exchange and in particular can not be implemented if the temperature intervals according to which the hot fluid and the fluid evolve. cold partially overlap.

The object of the invention is to describe a process allowing a high heat recovery rate without consuming mechanical energy and which can be used even at low temperatures without involving the risk of freezing provided that a suitable heat transfer fluid is chosen. . In particular, the invention describes a method of transferring heat from a hot fluid to a cold fluid which makes it possible to be able to work with partial recovery of the ranges of evolution of the temperature of the hot fluid and of the cold fluid, therefore with better heat recovery rate.

The method according to the invention for transferring heat from a relatively hot fluid A to a relatively cold fluid B is characterized in that a heat transfer fluid is maintained in a substantially isobaric continuous circuit, forming a loop and comprising in series at at least two distinct heat exchange zones (I) and (II), capillary suction means on one of the circuits joining the exchanger (I) to the exchanger (11), said heat transfer fluid comprising at least two constituents capable of evaporating at least partly in a temperature range located at least partly below the temperature of the fluid A and of condensing
at least in part in a temperature range located at least in
part above the temperature of fluid B, at least two of these
constituents not forming an azeotrope therebetween, and in that
passes the fluid A into the exchange zone (I) and the fluid B into the exchange zone (II), each in heat exchange relation with said heat transfer fluid, said capillary suction means
allowing the liquid phase formed by condensation of
zone (II) zone (I).

The present process is illustrated in Figures 1 and 2.

Figure 1 illustrates a first embodiment of the invention.

Figure 2 illustrates the application of the process to the control of ventilation in residential premises.

With the arrangement according to the invention, the liquid heat transfer fluid rises through the capillary suction means (4), evaporates at least partially in the exchanger (I), descends at least partially in the state of vapor through the conduit (1) and condenses in the exchanger (II) to be taken up by the capillary suction means.

The fluid A and the fluid B circulate in the respective exchangers (I) and (II), preferably overall against the current of the heat transfer fluid contained in the circuit forming a loop.

By capillary suction means means any means for moving a liquid from bottom to top in a pipe, without exerting on it a pressure of external origin. These means will be, for example, one or more capillary tubes (of small diameter) or one or more tubes filled with a porous material.

An essential characteristic of the process of the invention lies in the fact that no mechanical device is necessary, the
transfer of the mixture between the exchange zones I and II taking place
naturally of itself, without requiring external mechanical energy,
under the action of capillary forces. This feature allows
easily build a sealed circuit without risk of leakage from the
mixing and avoid maintenance and reliability issues related to
the use of a compressor or a pump.

In other words, the method according to the invention for the transfer of
heat from a relatively hot fluid A to a relatively fluid
cold B is characterized in that a fluid is maintained
coolant in a closed circuit with two successive zones
separate gradient I and II heat exchange interconnected, on one side by at least one continuous pipe and on the other
by at least one capillary pipe or containing a material
continuous porous, said closed circuit not containing mechanical means for circulating fluids, said heat transfer fluid comprising at least two distillable constituents, not forming an azeotrope therebetween, the method comprising the steps
following:
(a) the mixture in the liquid phase is gradually vaporized at least partially with the temperature of the mixture being raised by heat exchange preferably generally against the current with a first external fluid introduced at a temperature higher than that of the start of vaporization of said mixture and which gives it heat in the first heat exchange zone I,
(b) the vapor phase obtained during step (a) is sent by
the pipe continues in the second heat exchange zone II
without undergoing compression or expansion,
(c) the vapor phase mixture is condensed progressively with lowering of the temperature of the mixture by heat exchange, preferably generally against the current, with a second external fluid introduced at a temperature lower than that at the start of condensation of said mixture and which receives the heat in the second exchange zone II, (d) the liquid phase obtained during step (c) is brought into contact with the porous material, which allows it to return by capillarity to the first zone of heat exchange without undergoing compression or expansion, steps (b) and (d) being carried out without substantial heat exchange with the outside and the average level of the exchange zone I being higher than the average level of exchange zone II.

The process is shown diagrammatically with reference to FIG. 1. The non-azeotropic mixture, which circulates in the closed circuit shown in FIG. 1, arrives in the liquid state, by capillary action, through the conduit 4 in the exchange zone I, in which it is placed in heat exchange connection, by indirect contact generally against the current, with a first external fluid which arrives via line 2 and leaves via line 3. The mixture is gradually vaporized, at least in part, at during this stage with a rise in temperature which corresponds at least in part to the vaporization interval of said mixture. In this way it is therefore possible to change the temperature of the mixture according to a change in temperature parallel to the change in temperature of the external fluid which cools between the inlet and the outlet of the exchange zone. exchange conditions, it is desirable to select the mixture so that the vaporization interval is as close as possible to the interval of variation of the temperature of the external fluid and it is advantageous to carry out the exchange in conditions as close as possible to the exchange against the current. When the heat exchange takes place with air or with a gas, the exchange battery will preferably be designed to allow a mixed exchange mode against the current / cross currents.

The vapor phase of non-azeotropic mixture obtained is evacuated by itself through the free passage duct 1 in the exchange zone II in which the non-azeotropic gas mixture is brought into heat exchange relationship by indirect contact , generally against the current, with a second external fluid which arrives via line 5 and leaves via line 6. The mixture is condensed progressively during this stage with a lowering of temperature which corresponds at least in part to the interval of condensation of said mel-angel. The entire circuit is substantially isobaric, the pressure variations being only related to pressure losses due to the circulation of the mixture. Under these conditions the condensation interval is the same as the vaporization interval and, during the 'condensation step the mixture follows in opposite direction (lowering instead of raising the temperature) an evolution substantially identical to the temperature evolution followed during the vaporization step. During the said condensation step, the mixture cools while the external fluid heats up. It is also advantageous to carry out this exchange under conditions as close as possible to the exchange against the current.

The liquid phase obtained rises by capillary action in line 4 and thus returns to the exchange zone I, without undergoing compression or expansion.

The non-azeotropic mixture used must comprise at least two constituents which do not form an azeotrope therebetween, characterized by boiling temperatures different from at least 15 C (under atmospheric pressure) and preferably at least 30 C , each of said constituents being present in a proportion of at least 5% (for example 5% to 95% and 95% to 5% in the case of two constituents) by mole and preferably at least 10% by mole.

The mixtures used can be mixtures of two, three (or more) constituents (separate chemical compounds). The constituents of the mixture can be hydrocarbons, the molecule of which comprises a number of carbon atoms, for example
from 3 to 8, such as propane, normal butane, isobutane, 'normal pentane, isopentane, neopentane, normal hexane, isohexane, normal heptane, isoheptane, normal octane and
isooctane as well as aromatic hydrocarbons such as benzene and toluene and cyclic hydrocarbons such as cyclopentane and cyclohexane.

The mixture used can also be a mixture of halogenated fluids of the "freon"type; among these fluids, there may be mentioned, trifluoromethane CHF3 (R23), chlorotrifluoromethane CClF3 (R13), trifluorobrbmomethane CF3Br (R13B1), chlorodifluoromethane
CHClF2 (R22), chloropentafluoroethane CClF2 -CF3 (R115), dichlorodifluoromethane CCl2F2 (R12), difluoroethane CH3CHF2 (R152a), chlorodifluoroethane CH3-CClF2 (R142b), dichlorot2 R21), trichlorofluoromethane CCI3F (R11), trichiorotrifluoroethane CCl2FCClF2 (R113), dichlorohexafiuoropropane (R216).

At least one of the constituents of the mixture could be an azeotrope of halocarbon compounds, having the property of behaving like a pure fluid; among the main azeotropes which can be used, there may be mentioned - R500: azeotrope of R12 / R152a (73.8% / 26.2% by weight) - R501: azeotrope of R22 / R12 (75% / 25% by weight) - R502: R22 / R115 azeotrope (48.8% / 51.2% by weight - R503: R23 / R13 azeotrope (40.1% / 59.9% by weight) - R504: R32 / R115 azeotrope (48.2 % / 51.8% by weight) - R505: azeotrope of R12 / R31 (78.0% l22.0% by weight) - R506: azeotrope of R31 / R114 (55, P44.9% by weight)
Other types of mixtures are mixtures comprising water and at least one second water-miscible constituent such as mixtures formed of water and ammonia, mixtures formed of water and
an amine such as methylamine or ethylamine, mixtures
formed of water and an alcohol such as methanol, the mixtures formed
water and a ketone such as acetone.

In the case which has been described in relation to FIG. 1, the exchange zone I through which the hot fluid passes is located above the exchange zone II through which the cold fluid passes. Under these conditions, the liquid phase condensed at the outlet of the exchange zone II is sucked by capillarity towards the exchange zone I. An important criterion for selecting the nonazeotropic mixture will be the surface tension of the liquid phase condensed in the zone II.

To raise the liquid phase, the method according to the invention consists in filling the conduit 4 with a porous filling
which can be produced, for example, using a metallic fabric, glass fibers or sintered or unsintered metallic powders. It is also possible to install a bundle of capillary tubes. The diameter of the pores obtained must be smaller the higher the height to be wound up and be, for example, in the range 0.01 to 0.2 mm.

The porous filling must extend over at least part of the vaporization zone occurring in the exchanger I, so that the vaporization of the fluid at the liquid-vapor interface causes the continuous suction of the liquid phase by capillarity.

The following two examples describe the implementation of the invention in specific cases.

EXAMPLE 1
It represents the application of the technique according to the invention, in the case of a house heated by air and equipped with a double-flow system. The house, equipped with controlled mechanical ventilation, has a living area of 150 m2; the volume of air renewed is approximately 360 -m3 / h. The capillary suction transfer loop is shown in Figure 2.

An air extractor (EXA) placed under the roof, for example, draws air from the house at 200 C; it is cooled to a temperature of 80 C in the evaporator (El) of the transfer loop (BT). The Frequent power in (E1) is 1.56 kW; in general, a device for discharging the condensates resulting from the cooling of the extracted air is provided. The fresh air arrives, in the lower part of the loop, on the battery (E2) supplied by a helical fan (VH); the fresh air flow identical to the extracted air flow is injected through a duct in the lower part of the house.

For example, the fresh air is heated through (E2) from 0 C to 120 C; the heat transfer agent is a mixture of difluorochlomethane (R22) and dichlorotetrafluoromethane (R114), with molar fractions 0.80 and 0.20. The mixture is selected to vaporize and condense according to a temperature profile of around 120 C; as a first approximation, the vaporization of the mixture in (E1) and its condensation in (E2) are assumed to be total; the vaporization and condensation ranges vary between 40 C and 160 C, while the average system pressure is close to 5 bars.

The application described implements, in practice, two batteries made up of several layers of finned tubes allowing a mixed heat exchange against the current / crossed currents corresponding overall to a counter current. The main pressure losses of the installation are located in the evaporator (E1) and the condenser (E2); the pressure drop of the vapor phase along the connecting tube (T1) can be made low, by an adequate dimensioning; the capillary materials of the tube (T2) (and in particular, the diameter of the pores) will be chosen to ensure sufficient capillary pressure. In practice, this will be between 1 kPa and 50 kPa, for example.

The average operating pressure will be greater than atmospheric pressure and less than 30 bars, preferably between 1 and 15 bars.

EXAMPLE 2
The technique proposed by the invention can also find industrial applications, whenever a direct thermal transfer between effluents or thermal discharges and the fluid to be heated is not possible.

Consider, for example, the case of heat recovery from the smoke from an oven; these are cooled from 3000 C to 200 C, in the upper part of the exchange loop, previously described in principle; in the lower part of the loop, the fresh air necessary for combustion or for the reaction is preheated, for example, from 500 C to 150 "C.

By way of example, as a heat transfer agent, a mixed fluid composed of propane and para-xylene is used, the molar composition of propane of which is 0.75; the above mixture has a temperature at the end of evaporation of 2250 C and a temperature at the end of condensation of 1250 C; its temperature range in vaporization or condensation equal to 1000 C is therefore identical to the temperature variation of each of the external fluids.

The total pressure drop in the exchange zones will be, for example, between 10 and 30 kPa and will be compensated by the capillary force. More generally, for an application of the above type, it is possible to replace propane, isobutane, butane or pentane; the second constituent of the mixture may be an aromatic type benzene, toluene, xylene (ortho, meta or para) or alkylbenzene; one can also envisage a ternary combination of the preceding compounds.

Claims (9)

 said heat transfer fluid comprising at least two constituents capable of evaporating at least partly in a temperature range located at least partly below the temperature of the fluid A and of condensing at least partly in a temperature range located at at least partly above the temperature of the fluid B, at least two of these constituents not forming an azeotrope therebetween, and in that the fluid A is passed through the exchange zone I and the fluid B in the exchange zone II, each in heat exchange relation with said heat transfer fluid, said capillary suction means making it possible to pass the liquid phase formed by condensation from zone (iI) to zone (I).  (I) at the exchanger (II), capillary suction means,  in a substantially isobaric continuous circuit, forming a loop and comprising in series at least two distinct heat exchange zones (I) and (II) and, on one of the circuits joining the exchanger  1- Heat exchange process between a hot fluid and a cold fluid, characterized in that a heat transfer fluid is maintained  2- A method according to claim 1, in which the  fluid A and fluid B, each in its respective zone of exchange of  heat, generally against the flow of the heat transfer fluid.  3- A method according to claim 1 or 2, wherein the means  capillary suction tubes are made of porous material  comprising pores with a diameter of 0.01 to 0.2 mm.  4- A method according to any one of claims 1 to 3, in  which the capillary suction means extend over at least  part of the exchanger (I), so that the vaporization of the  heat transfer fluid causes continuous suction of the phase  liquid by capillarity.  5- A method according to any one of claims 1 to 4, in  which the constituents of the heat transfer fluid are hydrocarbons  halogenated. 6- A method according to any one of claims 1 to 4, wherein the constituents of the heat transfer fluid are hydrocarbons. 7- A method according to any one of claims 1 to 4, wherein the constituents of the heat transfer fluid are water and ammonia. 8- A method according to any one of claims 1 to 7, wherein the heat transfer fluid consists of two separate chemical compounds with boiling points differing by at least 150 C at the working pressure, the molar proportions of the two constituents being 5 to 95% and 95% to 5% respectively. 9- A method according to any one of claims 1 to 8, wherein the pressure, in the circuit forming a loop, is between 1 and 15 bars.