GB2105455A

GB2105455A - An apparatus for indirect cooling of fluids by a gas stream

Info

Publication number: GB2105455A
Application number: GB08127192A
Authority: GB
Inventors: Jaroslav Stanek; Zdenek Beran; Vaclav Michal; Stanislav Smejkal; Miloslav Hartman; Josef Lutcha; Josef Mika
Original assignee: VYZK USTAV CHEM ZARIZENI; Vyzkumny Ustav Chemickych Zarizeni
Current assignee: VYZK USTAV CHEM ZARIZENI; Vyzkumny Ustav Chemickych Zarizeni
Priority date: 1981-09-09
Filing date: 1981-09-09
Publication date: 1983-03-23

Abstract

An apparatus for indirect cooling of fluid by a gas stream in which the heat transfer rate at the gas side is enhanced by the presence of moving solid particles. The apparatus comprising a horizontal tube bundle 1, consisting of at least two tube layers, preferably arranged in such a way that the centre-lines of tubes from the upper layer is located vertically just above the centres of the pitch of the corresponding adjacent tubes from the layer below, the space between the tubes in the bundle being occupied up to one half, by solid particles 7 having a size greater than one third of the gap between the adjacent tubes in the layer, and in the plane parallel to the tubes centre-lines of the bundle's bottom layer a net 9 is located with apertures whose size is less than that of the size of the solid particles. The particles are moved by an upwardly flowing gas stream induced by a fan 4. Liquid from a distributing device 10 may be used to augment the cooling. <IMAGE>

Description

SPECIFICATION An apparatus for indirect cooling of fluids by a gas stream This invention relates to an apparatus for indirect cooling of fluid by a gas stream in which the heat transfer rate at the gas side is enhanced by the presence of moving solid particles.

For cooling fluids, such as liquids or condensing vapours and gases, it is a common practice to let the cooled substance flow within cooler tubes whose exterior surfaces are in the stream of a cooling gas. An example of such apparatus is an air cooler where the gaseous cooling fluid, air, is blown across the tube bundle within which the cooled substance flows. Mostly, the fluid inside the tubes is a liquid or condensing vapours. Heat transferred from the cooled fluid to the inner surface of a tube wall exhibits a relatively high heat transfer rate characterised by a heat transfer coefficient. The heat transfer coefficient values are, in this case, higher by an order of two or even three over those at a tube's exterior surface, where cooling gaseous substance flows.The heat transfer rate at a tube's outside is then a limiting factor and determinates the heat transfer from cooled to cooling substance.

A known method, being used for improving this state of affairs, is an enlargement of the tube's outside surface by fins. For common steel, it is possible to enlarge the tubes' outside surface by a factor of three, at the most by the known methods of cold rolling of fins. A greater surface enlargement factor, of up to twenty, by fins can be achieved only at the high price of the rather expensive ductile metals, mainly aluminium.

A disadvantage of finned tubes, apart from the high cost, is that operating problems such as fouling of fins, their difficult cleaning and for the thin and soft fins a high vulnerability to damage with the ensuing drop in their efficiency. A centrally unfavourable feature of coolers using gaseous cooling substances appears to be the low heat capacity of gases and in addition to this for air coolers there is a high cooling performance variation which is dependent on the ambient air temperature changes.

Another known method of overcoming the unfavourable great difference between the heat transfer rates at the outside and inside of tubes is the use of a fluidized bed of solid particles. In this case the tubes are immersed in a fluidized bed of sand, having a particle size of the order of 1 0-4m, or some similar matter which is suspended in the stream of the cooling gas.

In such a system the heat transfer rates at the tubes' outside are several times higher. The values of heat transfer coefficients of the order of 200 to 500Wm-2K-' have been published.

Further to that the fluidized bed can be sprayed on to by a liquid, usually water, and the cooling performance can be thus increased by direct evaporation of such a liquid. However, in order to ensure the stable hydrodynamic behaviour of the fluidized bed it is necessary to instal a so called distributing plate, which, by its hydrodynarhic resistance, causes a substantial increase of energy consumption needed for pumping cooling air through the cooler. Apart from this it is only practically possible to use a shallow bed which means only one layer of tubes can be immersed because of very high pressure loss over the whole bed.

An object of the present invention is to obviate or mitigate the aforesaid disadvantages.

According to the present invention there is provided an apparatus for indirect cooling of fluids by a gas stream comprising a fan or blower for pumping the cooling gas stream, input and output plenum chambers for the cooling gas, an input header for supplying fluid to be cooled and an output collector for the cooled fluid, a bundle of parallel tubes for conducting the fluid during cooling thereof, which bundle comprises at least two layers of tubes, a net beneath the bundle in a plane parallel to the layers of tubes in the bundle and a quantity of solid particles, between the net and the bundle of tubes, whose particle size is greater than the aperture size in the net.

This arrangement secures the high heat transfer rate at the cooling gas side, comparable with that of fluidized bed and yet substantially mitigates the drawback of the energy consumption needed for overcoming the hydraulic resistance of the bed.

The principle of the invention will now be further explained as follows: the horizontal tubes bundle, consisting of at least two layers of tubes, is preferably arranged in such a way so that the centre-lines of tubes from the upper layer are located vertically just above the centres of the pitch of the corresponding adjacent tubes from the layer below and the space between the tubes in the bundle is occupied up to one half, by solid particles having a particle size greater than one third of the gap between the adjacent tubes of the layer. In a plane parallel to the tubes centre-lines of the bundle's bottom layer a net is located with apertures whose size is less than that of the size of the solid particles.The tubes for cooling the fluid may be furnished with fins and a spraying device for evaporated liquid, preferably water, may be placed in the outcoming cooling gas plenum chamber or directly into the upper portion of the tube bundle. The solid particles size is preferably within the range of from 4 to 10 mm and the net's apertures from 3 to 9 mm.

The aforementioned arrangement causes an increase of the heat transfer rate at the cooling gas side by several times in comparison with a conventional air cooler with the ensuing higher specific cooling performance of the apparatus. For the majority of industrial applications it is possible to use either tubes with low fins or even bare tubes; this represents some saving of ductile metal-such as aluminium. Apart from this feature, the latent heat of sprayed liquid (water advantageously) the secondary cooling substance, contributes to the additional increase of the cooling performance. The secondary cooling substance can be also utilized for a partial elimination of the adverse effect of the usual variability of the ambient air temperature upon the cooler performance.

The effect of main features of the proposed arrangement is derived from the properties of spouted bed systems, which are being formed in the proposed apparatus, by the solid particles forming longitudinal fountains parallel with the tubes centre-lines. The solid particles of relatively low density function as turbulizers for the cooling gas stream and their motion contributes to the heat transfer between cooling gas and the tube wall surface. If a liquid, e.g. water, is even coarsely dispersed on to such a system this is then spread on the particles surface with ensuing intensive evaporation and subsequent temperature lowering of the cooling gas. The particle migration horizontally, and on a limited scale vertically, too secures an even temperature distribution and/or maintains suitable temperature gradients within the system.

A wind tunnel investigation on the test section with the area of 0.2 m2 and the pilot plant scale cooler of the 1 m2 plan view area, both built from a four layer tube bundle, allowed study of the system behaviour under conditions corresponding closely to an industrial operating environment.

The spherical particles, which formed the spouted beds, had the size of 6 to 7 mm and were of the density of approximately 750 kg.m-3. The superficial velocities of the cooling gas (air) were within the range from 2.5 to 4 m.s.-l (the considered velocity was that one in the free crosssection of the air stream and the air density was 1.2 kg.m-3). Condensing steam was inside the tubes. The values of heat transfer coefficients at the air side estimated from measured data by the heat balance were found within the range from 170 to 220 Wm-2K-'. The pressure losses over the entire tubes bundle sections ran from 250 to 350 Pa.Since the values of heat transfer coefficients usually found for conventional air coolers are around 50 Wm-2K-1 the increase from 200 to 300 percent has been achieved.

The present invention will now be described with reference to the enclosed drawing which shows an example of an air cooler for fluids and in which; Fig. 1 is a view of the cooler apparatus in a suitable housing which is partially cut away to reveal the layout of parts in a preferred embodiment of the invention.

Fig. 2 is a cross-section of part of the cooling tubes bundle.

The functional part of the cooler is the bundle of tubes 1 which are fixed in the header 2 of entering cooled fluid and the collector 3 of leaving fluid. A cooling gas (air) is pumped through the cooler by the fan 4 and flows subsequently through the intake chamber 5, between the tubes of the bundle and into the output plenum chamber 6. The space between the bundle's tubes is partially occupied by the spherical solid particles 7 which form the longitudinal spouted beds 8 in the cooling gas stream. The net 9 located in the bundle's bottom, prevents the particles from falling through into the cooling gas intake chamber 5. There is the liquid distributing device 10 for a secondary cooling liquid, advantageously water, located in the plenum chamber 6 of an outcoming cooling gas just above the top layer of cooling tubes 1.

The apparatus of this invention appears to be particularly suitable for fluid cooling by air where the required fluid stream target temperature is relatively low, around 40 to 800C. The cooler's increased performance secured by the latent heat of evaporated secondary cooling liquid finds its application expecially in those climatic localities which are characterised by the high ambient temperature of air. This is not meant to say that the apparatus cannot be used for cooling the fluid stream to a higher target temperature, for some cases of vapour condensation or with using some other cooling gas instead of air.

Claims

1. An apparatus for indirect cooling of fluids by a gas stream comprising a fan or blower for pumping the cooling gas stream, input and output plenum chambers for the cooling gas, an input header for supplying fluid to be cooled and an output collector for the cooled fluid, a bundle of parallel tubes for conducting the fluid during cooling thereof, which bundle comprises at least two layers of tubes, a net beneath the bundle in a plane parallel to the layers of tubes in the bundle and a quantity of solid particles, between the net and the bundle of tubes, whose particle size is greater than the aperture size in the net.

2. Apparatus according to claim 1, wherein the solid particles have a particle size greater than one third of the gap width between adjacent tubes in a layer in the bundle.

3. Apparatus according to claim 1 or claim 2, wherein the quantity of solid particles is sufficient to fill not more than half of the volume of the void between the layers of tubes.

4. Apparatus according to claim 1 or 2 or 3, wherein the layers of tubes are staggered such that a tube in a layer above a second layer lies over the space between two adjacent tubes in the second layer.

5. Apparatus according to claim 4, wherein the centre-line of a tube in an upper layer is located vertically above the pitch centre of two adjacent tubes in a lower layer.

6. Apparatus according to any one of claims 1 to 5, wherein the tubes are provided with fins.

7. Apparatus according to any one of claims 1 to 6, wherein a distributor for a secondary cooling liquid is located in the cooling gas output plenum chamber.

8. Apparatus according to any one of claims 1 to 6, wherein a distributor for a secondary cooling liquid is located in the upper portion of the bundle of tubes.

9. Apparatus according to claim 7 or claim 8, wherein the secondary cooling liquid is water.

10. Apparatus according to any one of the preceding claims, wherein the solid particles have a particle size within the range of from 4 to 10 mm and the apertures in the net have a size within the range of from 3 to 9 mm.

11. Apparatus for indirect cooling of fluids by a gas stream substantially as hereinbefore described with reference to and as shown in the accompanying drawing.