FR2492067A1 - Exothermic gas-liq. heat exchanger - has vertical cylindrical cooling coil between inner and outer sheets and cross-flow gas baffles - Google Patents

Abstract

The heat exchanger consists of a vertical cylindrical vessel in which is installed a cooling coil. The coil comprises an assembly of three or four overlapping vertical tubular helices forming an annular tube nest. The tube nest is encased in an outer and inner sheet, perforated to allow the passage of gas into the coil at the same time preventing the egress of solution. Weak solution is introduced at the top of the vessel through a series of sprays and cascases over the tube nest. The cooling water in the tubes flows upwards from the bottom to the top. The gas enters at the top of the vessel and passes laterally through the perforated casings mixing intimately with the weak solution. Enriched solution is drawn off at the base of the vessel.

Description

The invention relates to the absorption of a gas in a liquid in the case where the heat given off is appreciable, as occurs in particular in absorption refrigeration units. It is then necessary to remove this heat as the gas is absorbed, otherwise, the temperature of the solvent increasing, the solubility at constant pressure is reduced.

The intimate contact of the gas and the liquid must be ensured and, at the same time, the latter must be cooled.

The usual production of absorbers, that is to say horizontal bundles of straight tubes, has several faults: -Geometry poorly suited to heat exchange giving a high temperature pinch -Reduced path of the liquid on the exchange surface n ' not allowing a high exchange coefficient.

 -Bad velocity profile of the gas stream because, due to absorption, the flow rate varies considerably along the course which, also being that of the liquid, must keep a constant section.

These faults explain why in industrial absorbers
Ammonia / Water, for example, the concentration does not increase by more than about 10 in a single enclosure.

The present invention therefore proposes simultaneously to provide the following two functions: - Cooling of the absorbent liquid flowing in thin film in the gravity field along walls on the other side of which a cooling fluid circulates against the current, water for example.

- Contact of the gas and the solvent extended throughout the path of the liquid, this can be obtained provided that the paths of the gas and the liquid are dissociated.

For this, the lean solution is introduced into the upper part of an enclosure, the largest dimension of which is vertical, and it is caused to run in a thin film on a cooled surface placed in the enclosure; the gas is channeled so as to penetrate laterally into the conduits through which flows
the liquid, so that its circulation speed is
substantially identical from top to bottom of the device.

In a preferred arrangement (fig. I), the enclosure is a
vertical cylinder in which there is a tube bundle
wound in vertical axis helices. Inside the tubes
the coolant flows from bottom to top. The
concentric propellers are nested so as to present
the flow of the solvent from the series of obstacles arranged in
The vertical spacing of two consecutive rows
ves constitutes a series of horizontal conduits in which
the gas circulates in cross current with respect to the liquid.
gas can be introduced into these conduits either following a
centrifugal direction from the central well required by the
minimum bending radius, in a centripetal direction
by providing an annular space between the outer propeller and
the ferrule, this solution ensuring at best a speed of
constant progression since the sectioh of passage is reduced
from the periphery to the center.

The confinement of the flow of liquid on the sheets
can for example be ensured by a cylindrical plate with
perforations allowing the gas to pass and returning the liquid to the tubes.

It should be noted that for the clarity of the drawing the collectors
water inlet and outlet have not been shown; in
addition to avoid overlapping the turns they have
been drawn apart without respect for scale, nesting
real is that shown schematically in FIG. 2.

A variant of this provision is constituted by the em
contiguous pounding of vertical layers each by
folding an accordion tube so that the branches
pins are horizontal. Two consecutive layers
are shifted vertically by half a step so that the
horizontal branches of the pins are staggered and
the support is only carried out at the ends. The solvent flows vertically and the gas penetrates horizontally as in
the previous provision. (Fig. 3)
When trying to achieve absorption units of
reduced capacity, the production of tubular exchange surfaces becomes too expensive because the shaping remains significant and, on the other hand, the volume occupied by the exchanger becomes relatively greater; it is therefore advantageous to produce the device with an Easter heat exchanger.

In a preferred arrangement, the cooling fluid circulates in a duct constituted by a pair of slightly spaced vertical plates, the edges of which are rap = roche to ensure the closing. A series of pairs of plates thus formed will be arranged, providing intervals between them in which the solvent and the gas flow vertically.

Figure 4- shows an example of an embodiment: the plates comprise, on the absorption side, added or preferably stamped outgrowths, arranged in staggered rows with, preferably, a greater vertical spacing step bare the horizontal step. The contiguous stacking of several pairs of plates supported on their contiguous protrusions constitutes series of obstacles on the path of the liquid, flowing vertically, and, on the other hand, horizontal conduits for the path of the gas.

To take account of the fairly high pressures encountered, for example with ammonia, and of the variations between off and on, the geometry of the plates must contribute to resistance in both directions; this is obtained on the one hand by the protuberances and, on the other hand with the aid of spacings on the side opposite to the protuberances which can be either attached, or preferably obtained by a crossed ribbing of two opposite plates.

The communication of the cooling conduits with one another can be carried out by known devices which may or may not be removable.

The arrangement of FIG. 5-, perret the use of flat plates with attached gutters, for example nar welding; at least part of the edge of these is folded to provide a space between them and the opposite plate allowing passage to the runoff of the liquid. The spacers delimiting the cooling conduits may be made of resilient non-conductive material. The stack @ e can be maintained by tie rods.

your arrangement represents in Fig. 6- allows a simplified realization valid for net capacities.

The exchanger, which is also the enclosure, consists of a vertical double-walled tube N in the interval between which the coolant circulates.

The solvent is introduced with a rotation component so that it travels in a helical path. I, e gas is introduced in the same way, but with a reverse step.

It is possible to maintain the speed of the gas by taking a slightly conical tube whose section decreases downwards.

Claims (8)

 I- Absorption device with external cooling allowing the continuous evacuation of the heat released by the absorption of a gas in a liquid and characterized in that in a confinement enclosure the circulation of the liquid and of the gas carries out two different routes from each other, the passage section and the length of which can be adapted independently to the characteristics of the flow of each of the flows which flow in cross current with respect to each other while the coolant, separated from the liquid by a thin wall allowing heat exchange, circulates against the latter. 2- Device according to claim I, the exchange surface of which consists of a bundle of tubes, smooth or not, wound in helices so as to constitute a coil with a vertical axis and on the surface of which the solvent flows, the coolant circulating in an ascending direction in the tubes, the gas penetrating radially between the rows of tubes. 3- Device according to claim 2, wherein the liquid is channeled on the tubes by a vertical sheet metal provided with perforations in cups allowing only the gas to pass. 4- Device according to claim I, wherein the exchange surface consists of a stack of vertical plies of tubes folded in accordion, two consecutive pins, with horizontal branches, being vertically offset by a tubular half-step so that the support is only reachable on the curved ends while providing free circulation between the staggered tubes, vertically for the liquid, horizontally for the gas. 5- Device according to claim It where a single tube with vertical axis and double envelope constitutes the enclosure and the exchanger, the cooling fluid circulating from bottom to top in the envelope while the liquid and the gas are introduced to the end of the central part located above, their access conduits being oriented so as to impart to them an opposite rotational movement so that they describe descending helical trajectories in opposite steps. 6- device according to claim T, wherein the exchange surface consists of pairs of vertical and parallel plates in which circulates bottom in hnrat the cooling fluid and on the surface of which the solvent trickles while the gas circulates horizontally. 7- Device according to claim 6, wherein the plates have regularly spaced outgrowths which by pressing on those of the opposite plate constitute in the interval of two pairs of plates series of obstacles to the flow of liquid and also horizontal conduits for the gas path. 8- Device according to claim 6, wherein the plates are provided with horizontal gutters notched so as to let the liquid flow between the gutter and the opposite plate, the gas entering horizontally.