US2927877A - Heat exchanger - Google Patents

Description

March 8, 1960 Vw. s. NORMAN HEAT EXCHAGER Filed June 1. 1954 3 Sheets-Sheet 1 INVENTO/ Wann-f ,4W-0R Arsys March'8, 1960 w. s. NORMAN 2,927,877

f HEAT EXCHANGER Filed June 1, 1954 3 Sheets-Sheet 2 gr/VVENTo/Q W. S. NORMAN HEAT EXCHANGER Mr'h s, 1960 3 Sheets-Sheet 3 Filed June 1. 1954 uw um mw New IN VEN TO/ www AWM/M440 Wrth? .constructional purposes. exhibits its greater strength under compressionV and is Vdominantlyv of carbon-carbon linkages.

United safes Parent o Claims priority, application Great Britain January 14, 1949 2 Claims. (Cl. 154-75) The present application is a continuation-in-part of '2,927,877 Patented Mar. 8, 1960` metal'conduits are inappropriate; for example, one or other of the uids may have some action on metal such Y as a `corrosive action, or the metal may react upon one of the fluids either directly or catalytically. Hence for some purposes'tubular heat exchangers are made from carbon or graphite. They are necessarily more expenk sive than those made from the commoner metals, and

my prior similarly entitled application Serial No. 120,480.y

tiled October 10, 1949, now abandoned.

This invention relates to improvements in heat exchangers which are constructed or substantially constructed of elementary carbon. Elementary carbon possesses specially advantageous properties for the exi' change of thermal energy between fluids, especially where one or both of the fluids is an active chemical substanceY with strongly corrosive or solvent action. In general,

metallic elements and alloys have high thermal 'conduc tivity, but with the exception ofthe noble metals they have poor resistance to attack by `corrosive chemical agents. Non-metals, metallic oxides, silicates and other materials commonly used for handling corrosive liquids .and gases have poor thermal conductivity and are therefore inappropriate for use asrnaterials of construction for heat transfer apparatus. They also have in general poor mechanical strength. Elementary carbon has ther-v mal conductivity of the same' order asthemetallic elements but is very muchrnore resistant to corrosion than those metals which are freely and cheaply available for On the other hand, carbon brittle. It is therefore not well adapted to the construction of tubular heat exchangers of thevkind commonlyy used when-the material or" construction is metallic. The

term carbon as used in this specification refers to 1 all forms of carbon in which the molecular structure is prewhat is commonly called amorphous carbon and also graphite, whether natural or produced by the subjection of amorphous carbon to high temperatures including such partially graphitised carbons as are produced in the temperature range from 1700 to 2500 C. It also includes material predominantly carbon inwhich carbon particles are bonded by some organic or inorganic binding agent.

There are many industrial processes in which heat exchangers are used to transfer heat from one moving iluid to another moving fluid, the effect being to increase or decrease the temperature of any one fluid according to whetherit enters the heat exchange system at a lower or at a higher temperature than that ofthe other fluid. One type of heat exchanger incornmon ,use comp-rises an assembly of plates andV is known.` as the plate type. Another type, which is perhaps in most. common use today, is the tubular type, one .example of which comprises a numberof parallel tubes each about y6 feet longflocated between two headers. A uid is made to'ow through the tubes, and the tubes are traversed externally by the other uid, the elect of which is to heat'or to cool, as the case may be, the uid passing within the tubes. n

Owing to the cheapness of metal tubes and their high thermal conductivity and mechanical strength even with walls as thin as 1/32, it is customary to make tubular heat exchangers of metal wherever operating conditions permit.

This, includes they are much more susceptible to damageunder transport, handling, and loperating conditions. Moreover, for

adequate mechanical strength, the Wall thickness of the tubes may have to be three or more times that of metal tubes of the same internal diameter, although it isvobvious that the thinner the wall the better the heat transfer.

Vit isknown to make a heat exchanger by forming passages-through a metal block, one liuid ilowing through a setfor group of parallel conduits while the other fluid flows through another set or group of parallel conduits adjacent to and crossing the first set or group.

, lnrnetal heat exchangers this block principle of constructions does not appear to have achieved anycommercial success, probably because themetal tubular type was cheaper and of quite adequate strength.

One main object of this invention is to produce a heat exchanger ,unit composed of carbon or graphite in a strong, tough and coherent form. Another object is to provide specic means of producing a heat exchanger unit "of the appropriate type.` A third object is to provide physical methods of building or making the foraminous body.

YThe invention provides a method of manufacturing a low porosity carbon heat exchanger body comprising assembling a plurality of carbon plates to form a block,

said plates being grooved to provide passages traversing said block, impregnating the assembled plates and therebetween with a liquid organic resinrand heat-hardening said resin to reduce the porosity of the platesand bond them together. A l

The carbon plates can be composed of carbon produced from finely sub-divided bituminous coal by the process` described in.U.S. Patent No. 2,461,365. Y In an alternative form the carbon plates are made from `'coal or coke or a mixture c-onsisting predominantly of coal or coke and a minor proportion of a carbon-producing binder, such for example as pitch, lafter which the plates are tired to carbonize` said binder and impart strength and toughness to the tired plates. Y

In both cases if the iinal temperature of thel heating operation ,does not exceed l600 C. the product Will be practically pure carbon and if the final temperature's increased to that necessary for electrographitization, sayV 2250 C. or above, the carbon will be in the form` ofv graphite.k Either form of carbon may be employed' in There are however circumstances in which this invention.

, There are various ways in which the heat. exchanger block with its cross-how passages may be produced or built up.v a

In one method hat plates with the appropriate grooves may iirst be formed.k as described hereinafterv and the Vrequired number of such plates may be cemented tor gether in the requiredfrelation to one another to form the complete foraminous block. vThereafter the block'fis impregnated with a liquid organic resin. Alternatively, the cementing or bonding operation may be conducted simultaneously with aninipregnating operation. i

Headers for introduction and discharge of the liuids may themselvesbe formed of carbon or graphite parts moulded and redby techniques analogous to those above referred to. The headers may be joined to the body byk a carbonaceous cement and may be impregnated. The headers may be of metal if the lmetal is not aiected by the heat exchange fluids;`

A block type of heat exchanger comprising a 12" cube with approximately 250 3/s" holes in each direction has the same heat transfer surface as tubes, each 6 long `and of 1 internal diameter.

The liquid organic resin may be converted into carbon. Thus it may consist of a mixture of furfural with concentrated sulphuric acid (with or without finely divided carbon) or it may be an artificial resin which when raised to a high temperature is carbonised. g The process .of simultaneous cementing and impregnation may consist of clamping the unimpregnated plates together in the correct alignment and then impregnating by evacuation, submerging in a liquid resin and applying pressure to force the liquid into the pores. The liquid resin having been poured out of the passages the resin (which has penetrated into the joints between adjacent carbon plates) is cured or hardened by baking and a strong and permanent bond is formed between the .,plates.

Alternatively the carbon plates may be cemented together before the above impregnating procedure is followed.

" In an alternative method of impregnating, the liquid Vresin is supplied under pressure to passageways traversing the assembled block, draining and curing following.

This procedure may be followed by the vacuum impregnation described above. Y

The liquid organic resin may be any suitable resinous material which in the unpolymerised or partly polyvmrerised state is sufficiently fluid to permeate the porous voids in the carbon, and which can be polymerised to form a hard cement after the impregnation. Suitable resins areY described in the examples given hereinafter.

In carrying out the method of the invention the following points are of practical importance:

(a) Before impregnation the assembled plates should be thoroughly dried (eg. by heating for at least an hour `at 125 C).

(b) It is necessary toensure that the carbon plates shall be held lrmly together throughout the impregnation and curing steps and especially the latter, bearing in mind the diierential expansion of the carbon and theA Vmetal bolts of the cover plates when heat is applied. It is also desirable that the completed heat exchanger body when ready for use shall be under compression between the cover plates. Thus, when simultaneous bonding and impregnation are performed it is desirable that the bolts shall be tighter than when pre-cementing is used, since in the latter case the cement is suicient 4to hold the carbon plates together during polymerisation of the resin.

A suitable inal torque loading for each of four 1 inch diameter corner bolts for the cover plates of a heat eX- changer block comprising a-15 inch cube is 80 ft.lbs., the block being ready for use at this loading. In the manufacture of the block by simultaneous bonding and impregnation, it has been found that a suitable bolt loading as soon as the plates are assembled is one of at least 175 ft.lbs. and that no adjustment of the bolts need be made until after polymerisation when adjustment is made to the required loading of 80 ft.lbs. In using pre-cementing, the carbon plates are preferably held with the bolts adjusted to hand tightness until the cement has hardened, when the bolt loading is adjusted to 80 ft.-1bs. Impregnation and curing follow and thereafter any necessary final adjustment of the bolts to ensure a loading of 80 ft.lbs. is made.

(c) The Viscosity of the liquid impregnating resin should preferably be in the range of 30 to 100 centipoises. (Between 30 and 40 centipoises is usually most suitable.)

(d) In vacuum impregnation, the vacuum should be the highest practicable. The vacuum should not if avoidable be above 10 mm. of mercury absolute and is preferably about 3 mm. of mercury absolute.

(e) The vacuum treatment should generally be for at least one hour.

(f) The pressure applied to force the liquid into the pores is suitably about 50 lbs. per square inch but higher pressures may be used.

(g) The time of immersion under pressure should usually be for at least one hour. V

(h) Before polymerisation excess resin should be permitted to drain from the block and ample time should be allowed for this (eg. about l2 hours).

(i) The heating of the block for polymerisation of the impregnating resin should be done at a rate such as to avoid loss of resin. If the block is heated to high temperatures too rapidly, resin exudes to the surface of the block thus weakening the resulting structure and impairing its heat transfer properties. The temperature should be slowly and gradually increased to the required linal-value. A suitable heating schedule for a cashew nut shell resin is:

24 hours at 50 C. 36 hours at 100 C. 24 hours at 120 C.

(j) To ensure sufficiently low porosity, two separate impregnating stages may be employed, the second impregnation following the curing of the resin of the preceding stage. Both impregnating stages may involve vacuum treatment, or one stage may involve vacuum treatment while in the other stage liquid resin is forced into the passages of the block. Different resins may conveniently be used in the two stages.

(k) The guide to whether impregnation has been adequate is whether the heat exchanger body leaks when tested (e.g. by immersion in water and the supply of pressure air to the passages in the body). y

It is preferred that each carbon plate has on one side a rst set of equally spaced parallel grooves and on the other side a second set of equally spaced parallel grooves disposedat right angles to the first set, the lirst and second sets of grooves, respectively, of adjacent plates being in register with one another to provide two series of parallel passages traversing the block at right angles to one another.

One construction of heat exchanger made in accordance with this invention will now be described with reference to the accompanying drawings in which:

Figure 1 is a plan;

Figure 2 is a section on the line AA of Figure 1; and

Figure 3 is a section on the line BB of Figure 1;

The heat exchanger body proper is built up from a series of graphite plates 12 which are good heat conductors. Each plate 12 has on one side a first Set of equally spaced parallel semi-cylindrical grooves 13 and on the other side a second set of equally spaced parallel semi-cylindrical grooves 14 lying in a direction at right angles to the direction of the first set 13, the alternate contiguous plate 12 being so arranged that'the first sets of grooves lie in register with the first set of grooves in a contiguous plate and the second set of grooves in register with the second set of grooves in a contiguous plate so as to form first and second sets of cylindrical passages at right angles to one another. As shown in Figures 1 and 2 the passagesformed by the grooves 14 open at their ends into the chambers 15 and 16 of cast iron headers-17. and 18. The header 18 is provided with steel inlet and outlet pipes 19 and 20 for the first fluid which in this vcase is a huid which does not react with the iron or steel.

In the same way, as will be seen from Figure 3, the cylindrical passages formed by vthe semi-cylindrical grooves 13 open at their ends into headers 21 and 22 made of carbon. The hollow spaces 23 and the header 21 are sealed by glands 24,and similarly the hollow spaces 25 and the carbon header 22 are sealed by glands 26 and thecarbon header I22 is provided with the carbon -nletxpipe27` and a carbon outletpipe 28 for the luirl whichrpasses through the passages 13, which in this ease is a tluid4 which might have a corrosive action or other reaction -.with metal. The graphite plates 12, which are good conductors of heat,` conveniently have at the: top and" bottom thicker plates 29 ofV carbon with extemalcast iron cover plates 30. As shown in Figure 3 the carbon headers 21 and 22 rhave cast iron cover plates 31. The cover vplates() and 31 are held int position byY bolts 32 .andnuts 33'.. Y

In the above described construction, the headers were of' carbon or of cast iron and the cover plates were of eastl iron. It is to be understood that other solid mate- -rials may be used for such headers and cover plates such asy earthenware, vulcanised rubber, resin coated metal, artificial resin.

Itr is preferred to make each graphite plate by moulding from very finely pulverised coal with or without 'a small proportionl of very finely dividedpulverised coke or graphite and/or finely divided sulphur and after moulding to the required dimensions (allowing for sub-V sequent shrinkage, e.g. 11% linear shrinkage) is embedded inV coke fliers and fired under non-oxidising con- 'ditions to a temperature of 800 to l000 C. at such a rate of temperature rise as to avoid intumescence as described in United States patent specification No. 2,461,- 365 or 2,493,383. Thereafter each plate is heated vto a temperature kof 2500" C. in an electric furnace. The conditions at this stage of theprocess are controlled in` such a way that the carbon is not fully converted into graphite but into an intermediate form which has a high thermal conductivity (about 75% of that of natural graphite) but a higher strength and hardness than fully Agraphitised carbon.

Each plate is ground flat on each side in a grinding machine and each edge of each plate 'is ground straight and to the correct dimensions. I

Grooves are formed in the plates by grinding, using a machine with a rotary grinding wheel the peripheral edge of which conforms to the shape of the groove required in the carbon plate. Thus for example when semi-cylindrical grooves are tobe formed a grinding wheel having an edge of semi-circular section is employed.

There follow three examples of methods of manufacturing a low porosity carbon heat exchanger lbody in. ac-

oordance with the invention, the body being substantially as shown in the drawings and made of 15 inch square i Example 1 (a) The graphite plates are assembled in the appropriate order, as set out above, to form a block.

(b) The cover plates 30 and 31 `are positioned and the whole assembly subjected to a compressive load by the four bolts 32 connecting the cover plates. As set out hereinbefore, it is desirable that a compressive load shall be retained throughout the subsequent processing' and a suitable torque loading on the bolts has been found to be `at least 175 ft.lbs.`

(c) The assembly is dried to remove any residual moisture at a temperature of 125 C. for 2 hours.

l(d) The assembly is placed in a suitable autoclave and a vacuum of at least 10 mm. of mercury absolute is applied for a period of l hour.

(e) A liquid mixture consisting of by weight, 90%

cashew nut shell liquid and 10% diethyl sulphate as a polymerisation catalyst is prepared, the mixture being,

heated to 70 C. to reduce theV viscosity into the range 30-40 centipoises.

(f) The resin mixture is then introduced into the auto# clave in sufficient quantity-to immerse the block completely with suicient excess to allow a volumetric absorption of 30% of the volume of the graphite block.

(g) Air pressure of 50 lbs. per square inch is then applied within the autoclave for a period of 2 hours.

(h) The air: pressure is released gradually over a.

period of about half an hour and the block is removed from rthe resin. The excess resin is allowed to drain from the block for a period of 12 hours.

(i) The assembly is placed in an oven, a slight internal air circulation being preferred, Yand isjsubjected to the following heating stages:

24 hours at 50 C. 36 hours at 100 C. 24k hours at 120 C.

50% furfuralv 50%Y furfuryl alcohol 0.4 ccs. of N/ 1 H2SO4 per lb. of mixture yA similar impregnation technique is carried outas in stages (d) to (i) with the following exceptions:

(l) The resin is used at room temperature (about 17 C.) and not pre-heated as the viscosity is low enough for T,the purpose required.

(2) The curing or polyme isation cycle (i) is modified to:

8 hours at 50 C. 12 hours at 100 C.

(k) The torque loading is adjusted to 80 ft. lbs.

` l Example 2V (a) A cement mixture is prepared ofthe following composition:

92% cashew nut shell liquid 8% para-formaldehyde The resin should have a viscosity of 300 centipoises at 20 C. Y l

(b) The cement is applied to the surfaces of the plates and the complete stack is assembled, the bottom cover plate being used as the base for that operation. It is preferred to use a jig to hold the plates in alignment whilst they are being stacked and the cement is preferably rolled on to the plates by means of al rubber roller.

(c) The top cover plate is placed in position and the four bolts are adjusted to hand tightness.

(d) The cement is allowed to pre-set for 12 hours at 20 C.

(e) The bolt loading is then adjusted to ft.lbs.

y() The complete assembly is then heated at 80 C. for 12 hours to set the cement.

(g) Operations (d) to (k) of Example 1 are then kperformed.

Example .3

(a) Operations (a) to (f) of Example 2 are rst carried out.

(b) A pair of cast iron headers are bolted to opposite Yfaces of the block, a connection being provided in one header4 for the` introduction of resin via a hand pump,

(e) The headers are removed and the unit allowed to drain for 12 hours.

(f) The curing cycle is then carried out as described under (i) in Example l.

(g) A second vacuum impregnation is then carried out as described under (d) to (i) in Example 1.

V(h) The torque on the bolts is then adjusted to 80 ft.lbs.

After the iinal step in the above examples, the heat exchanger body is tested for the possibility of a leak. One pair of headers is bolted up and the header outlet is closed and the header inlet is subjected to substantial air pressure (eg. 50 pounds pervsquare inch), the body is immersed in water and is examined for any trace of air leak and in theV event of a leak appearing re-impregnation is effected on the lines set out in any of the above examples. 1

Before impregnation in the above Examples 2 and 3, it is preferred that the faces of the block shall be machined flat to ensure close tting of the headers. In the case of Example 1 the faces of the block are machined flat after impregnation and heat hardening.

I claim:

1. A method of manufacturing a low porosity carbon heat exchanger body comprising assembling a plurality of carbon plates to form a block, clamping said assembled plates together, said plates being grooved to provide passages traversing said block, and with the plates so assembled and clamped together impregnating the assembled platesV and therebetween` with a liquid `orgaiait: resin, and heat-hardening saidY resin to .reduce `the porosity ofthe plates and bond them together.Y

2. A method of manufacturing a low porositycarbon heat exchanger body comprising assembling a plurality of carbon plates to form a block, clamping said assembled plates together, each of said plates having on one side a first set of equally spaced parallel grooves and on the other side a second set of equally spaced parallel' grooves disposed at right-angles to the first set, the iirst and second setsV of grooves, respectively, of adjacent plates being in register with one another to provide two series of parallel passages traversing said block atv right angles to one another, and with the plates so assembled and clamped together, impregnating the assembled plates and therebetween with a liquid organic resin, and heat-hardening said resin to reduce the porosity of' said plates and bond them together.

' References Cited in the file of this patent` UNITED STATES PATENTS

Claims (1)

1. A METHOD OF MANUFACTURING A LOW POROSITY CARBON HEAT EXCHANGER BODY COMPRISING ASSEMBLING A PLURALITY OF CARBON PLATES TO FORM A BLOCK, CLAMPING SAID ASSEMBLED PLATES TOGETHER, SAID PLATES BEING GROOVED TO PROVIDE PASSAGES TRAVERSING SAID BLOCK, AND WITH THE PLATES SO ASSEMBLED AND CLAMPED TOGETHER IMPREGNATING THE ASSEMBLED PLATES AND THEREBETWEEN WITH A LIQUID ORGANIC RESIN, AND HEAT-HARDENING SAID RESIN TO REDUCE THE POROSITY OF THE PLATES AND BOND THEM TOGETHER.

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