WO1982002004A1

WO1982002004A1 - Thermally controlled mixer and apparatus and methods of operating same

Info

Publication number: WO1982002004A1
Application number: PCT/US1980/001623
Authority: WO
Inventors: Tribos Inc Armorflite; Desider G Csongor; Donald H Avery; Stanley Harrison
Original assignee: Tribos Inc Armorflite
Priority date: 1980-12-08
Filing date: 1980-12-08
Publication date: 1982-06-24
Also published as: EP0065947A1

Abstract

A thermally controlled mixer apparatus includes a fluid conduit (2) of the extruder barrel class, a plurality of thermal control vanes (6), (8), (10) supported in the barrel and means (22) for circulating a thermal control fluid through the thermal control blades to induce temperature changes in fluid materials conducted through the mixer apparatus. The fluid conduit is formed with slots (4) (5) occurring in staggered relationship and the thermal control vanes consist of tubular bodies located in angularly disposed relation to one another so as to present flow diverting surfaces for diverting fluid material passing through the fluid conduit in successively differing directions. The means for circulating a thermal control fluid comprises a reservoir (20) of fluid whose temperature may be selectively regulated and pump means (22) for moving the fluid from the reservoir into the thermal control blades. The control vanes may be made of a heat conductive material and when utilized without a thermal control fluid may function to conduct heat away from heated plastic material in the extruder barrel. A non-mixing heat exchanger (120) and modular arrangements of mixers and heat exchangers are also disclosed. Finally, novel control systems are presented.

Description

THERMALLY CONTROLLED MIXER AND APPARATUS AND METHODS OF OPERATING SAME

Description

Technical Field This invention relates to plastic extrusion systems and has. particular application to static mixers in such systems

Background

Conduit means for receiving and mixing together plastic materials prior to introduction into a forming die is well known in the art and one type of mixer apparatus is commonly referred to as static mixture apparatus in which vanes are inter¬ posed in the path of flow of pressurized plastic materials. In these prior art structures it is customary to employ fixed diverter vanes of vary¬ ing shapes by means of which desirable mixing is carried out. As a example of such devices there may be cited Patent Nos. 3,460,580, 4,093,188, 3,045,984 and 3,243,318. However, in these prior art structures the diverter vanes tend to present wear problems and require replacement from time to time, a procedure which may be expensive and time consuming. In addition, when plastic materials to be mixed are of relatively high viscosities, un¬ desirably large amounts of energy are required to force the plastic materials thru the conduit means. Furthermore, where the plastic materials tend to adhere rather tenaciously to the diverter vanes. as may occur, it becomes necessary to clean the vanes in some way as for example by heating or by forcing a cleaning fluid thru the extruder barrel or by removing the shaft from the barrel in order to expose the vanes to cleaning operations.

In jlastic extrusion systems, the plastic is often « melted in a screw-type extruder which places the plastic in shear and thus heats, melts and mixes the plastic. The extruder operates most efficiently with the plastic at high temperatures. The melted plastic is then forced through a die which forms the plastic. At the die, sufficient heat must be removed from the plastic to bring the temperature of the plastic below the set point. The rate that material can be processed is limited by the time required to remove sufficient heat at the die; thus the material introduced into the die should be close to the set point for fast processing. This is made difficult by the high operating temper- ature required by the extruder.

Disclosure of the Invention

The present invention is concerned with an improved mixer apparatus fpr use in mixing together plastic materials which are to be introduced into a forming die.

A chief object of the invention is to provide an improved static mixer apparatus in which vanes are located through a cylindrical barrel in an angu¬ larly disposed relationship to one another and in which the control vanes are of hollow construction to constitute a plurality of tubular members through which a thermal control fluid may be circulated thereby to induce temperature changes in plastic materials which are passed through the mixer appa¬ ratus. 5 Another specific object of the invention is to devise methods of controlling viscosities of plas¬ tic materials to be mixed.

Another object is to provide a method of mixing plastic materials contained in mixing 0 barrel which have been brought to a desired temp- ^' erature prior to being moved along the barrel.

Still another object is to provide a method of heating or cooling a plastic mass in a barrel in which heating or cooling may be carried out uniformly throughout the diametrical cross section of the mass to minimize thermal gradients.

It is also an. object of the invention to devise a mixer apparatus having thermal control vanes which are of a composite construction wherein grooved or tubular portions are combined with wear resistant portions.

It is still further an object of the inven¬ tion to provide for the adjustment of final out¬ put temperature of the plastic without varying the ^" speed of output itself and to further provide means for adjusting the temperature of the plastic mass independently of rotative extruder screw speed. The foregoing objectives are in accordance with the invention achieved by combining a slotted barrel member with thermal control vanes of hollow construction which are of a wear resistant nature. The vanes may take several different forms and may be connected to a reservoir for holding a thermal control fluid which can be introduced into the vanes to induce either heating or cooling. The control vanes may be made of a heat conductive material and may also be activated by electrical means to induce desirable heating.

Brief Description of the Drawings

Fig. 1 is a perspective view illustrating the static mixer apparatus of the invention partly bro- ken away to show in more detail a thermal control vane mounted therein.

Fig. 2 is an end elevational view of the mixer apparatus with, thermal control vanes mounted therein and having thermal control fluid conduits connected thereto.

Fig. 3 is a diagrammatic view illustrating means for supplying a thermal control fluid to vanes in the mixer apparatus, a portion of which is indicated in cross section. Fig. 4 is a cross section taken on the line 4-4 of Fig. 3.

Fig. 5 is a cross section taken on the line 5-5 of Fig. 4.

Fig. 6 is a detail perspective view of.the-- closure component shown, in Fig. 1.

Fig. 7 is a detail perspective of a modified form of closure component.

Fig. 8 is a fragmentary cross sectional view of the mixer barrel and vane structure of Fig.-7. ^" Fig. 9 is a fragmentary perspective view of another modification of vane structure. Fig. 10 is a detail cross sectional view of another form of connector means for a thermal control vane.

Fig. 11 is a cross sectional view of a mixer barrel enclosed within a manifold to provide a passageway for circulating a thermal control fluid.

Fig. 12 is a fragmentary perspective view of a portion of the vane of Fig. 11.

Fig. 13 is a perspective view of'a portion of a composite vane structure having wear resistant- components combined with tubes.

Fig. 14 is a detail perspective view of a vane construction in which one component has formed grooves. : • Fig. 15 is a detail perspective of another modification of vane means.

Fig. 16 is a longitudinal sectional^' iew of a heat exchanger module which may be used to control the temperature of the plastic without mixing. Fig. 17 is a cross sectional view of the heat exchanger of Fig. 16.

Fig. 18 is a cross sectional view of another heat exchanger module utilizing heat pipes.

Fig. 19 is a cross sectional view of one of the heat pipe elements of Fig. 18.

Fig. 20 is one example of a modular heat exchanger and mixer configuration.

Fig. 21 is another modular configuration in which heat exchanger and mixer modules are arranged alternately in series.

Fig. 22 is yet another modular configuration in which two heat exchanger units precede a mixer module. Fig. 23 is one embodiment of a temperature control system for controlling the amount of heat transfer from the plastic flowing through a mixer.

Fig 24 is yet another embodiment of a control system.

Fig. 25 is a still further embodiment of a control system in which flow of the temperature control fluid is controlled.

^* Fig. 26 is another embodiment in which the flow of the temperature control fluid is controlled.

Preferred_Modes of Carrying Out the Invention Referring more in detail to the drawings, numeral 2 denotes a static mixer cylinder or barrel of the class commonly employed in extruding or ix- ing plastic materials prior to entry into a form¬ ing die. Formed in the barrel 2 are a plurality of pairs of slots which in one preferred form are .arranged to extend in angularly disposed relation to the central axis of the barrel. Each pair of slots also occurs in an angularly disposed relation¬ ship to other pairs and one pair is denoted by the numerals 4 and 5 (Fig. 1) .

In accordance with the invention, a plurality of thermal control vanes are mounted in respective pairs of slots so as to extend through the space de¬ fined by the barrel wall. These thermal control vanes are designed to induce temperature changes in material contained within the barrel when the vanes are activated by suitable means for heating or cooling portions of the vanes. Activation of the vanes may be carried out for example by forming the vanes with heater elements therein and passing an electrical circuit through the vanes. In another preferred mode of activating the vanes there may be employed vanes characterized by a hollow construction to constitute tubular mem¬ bers through which a thermal control fluid may be introduced, and if desired circulated, to induce either heating or cooling. When the vanes are thus activated heating and cooling effects are trans¬ mitted from the vanes to plastic materials located around the -vanes in the barrel member ahd thus heating and cooling takes place uniformly through¬ out the mass of contained plastic material so that no problem of temperature gradients is experienced. In.Fig. 1 the thermal control vanes of the invention are illustrated as denoted by the numerals 6, 8 and 10, in fully inserted position in the bar¬ rel 2. One other vane 12 is shown removed from the slots 4 and 5 together with a vane connector here¬ inafter described in more detail.

It will be noted that each of the vanes extends in an angularly disposed relationship to one another in positions such that outer sides of the vanes present diverting surfaces against which plastic materials moving through the barrel are diverted a- long successively differing paths of travel as well as undergoing, concurrently, either heating or cooling. In Fig. 2 the vanes 6, 8, 10 and 12 are fur- ther illustrated and as will be apparent from an inspection of this figure they are arranged to ex¬ tend angularly with respect to one another and with respect to the central longitudinal axis of the barrel 2. However, it may be desired to position one or more of the vanes in a manner such that no intersection with the central longitudinal axis of the barrel occurs.

In utilizing the thermal control vanes as di¬ verting means for mixing plastic materials it is desirable to provide diverter surfaces of relatively large size and therefore in the preferred form of tubular structure a shape of rectangular.cross sec¬ tion has been provided. However, it is intended that the vanes may have other cross sectional shapes as circular, square, triangular and the like.

In combination with the thermal control vanes shown in Figs. 1 and.2 there is further provided means for supplying a thermal control fluid and introducing such fluid into the several vane mem¬ bers. Fig. 3 illustrates diagrammatically a means for supplying a thermal control fluid to the vane 8 in a manner such that a circulation of the fluid is carried out. As shown in Fig. 3 a reservoir for thermal control fluid is denoted by the numeral 20 from which fluid may be pumped by pump means 22 through conduit means 24 into vane 8 and returned through conduit 26 to provide for either a contin¬ uous or intermittent flow suitable for acheiving a desired heating or cooling action.

It will be understood that the pump means 22 indicated diagrammatically in Fig. 3 is shown con- nected to the inlet conduit 24 for vane 8 and this connection is intended to be illustrative of con¬ necting pump 22 with inlet conduits communicating with other thermal control vanes. Numeral 30 de¬ notes an inlet conduit for vane 10 and numerals 32 and 34 refer to inlet conduits for vanes 6 and 12 respectively. Similarly, numerals 26, 36, 38 and 40 refer to outlet conduits for vanes 8, 10, 6 and 12 respectively.

As one suitable means for sealably connecting the inlet and outlet conduits with the barrel 2 there may be employed connector plates detachably secured to the outer peripheral surface of the barrel 2 and indicated by numerals 14 and 42, 44 and 46, 48 and 50, and 52 and 54.

In Figs. 3-5 there is illustrated in more detail and on a larger scale the mounting of one pair of connector plates 14 and 42 on the barrel 2 to communicate with the thermal control vane 8. The connector plates are formed of a curved shape similar to that of the outer surface of barrel 2 and are of a size suitable for overlying the slots 4 and 5. Each of .the plates is formed with openings through which are located threaded fastening as 58 and 60 which are threaded into the barrel as shown. Each of the connector plates is further formed with fluid passageways centrally disposed there— through and inner sides of the plates are recessed to provide spaces into which ends of the control vanes may be fitted in an angularly disposed relationship as suggested in Fig. 5. Surrounding the recessed por¬ tions are sealing ring grooves in which are received sealing ring means such as O-rings. Numeral 62 denotes one of the fluid passageways shown in Fig. 6. Numeral 64 denotes a sealing ring shown in cross section in Fig. 4 and indicated in dotted lines in Fig. 5. By means of this arrangement the several plates are secured in sealed relationship agains the barrel 2 and it will be understood that the sealing means may be varied to deal with thermal control fluids furnished at varying pressures and rates of flow where a circulation of fluid is de¬ sired to be carried out.

As shown in^* Figs. 7 and 8, it may be desired to provide a connector plate as 66 having an open¬ ing into which a vane 8 may slideably engage into contact with a sealing ring 70. This connector plate may be secured by screws 72 and 74 as earlier described. In Figs. 9 and 10 a modified structure is shown which includes a connector plate 76 se¬ cured to barrel 2' by a threaded fastening 78 and a locking nut arrangement 80.

In another desirable modification of the inven¬ tion there may be provided a manifold member as 84 (Fig. 11) having an inlet and outlet conduit 82 and 86. The manifold member 84 is mounted around a barrel member 88 in spaced relation thereto as indicated in Fig. 11, and in this arrangement of parts to thermal control vanes as 90 are of a size to project thru the barrel 88 and abut against the manifold 82. Opposite extremities of the vane 90 are formed with apertures as 92, 94 etc. A thermal control fluid is introduced thru inlet conduit 84, passes through the annular spaces between the manifold 82 and barrel 88, and is conducted into apertures 92, then through the vane 90 and out of the aperture 94. In Fig. 13-15 there are further illustrated vane structures which are suitable for use where a very high wear factor is encountered in a mix¬ ing operation. These structures may be of a compo- site nature and include a hollow vane characterized by a plurality of tubes as 96 (Fig. 13) the oppo¬ site sides of which are attached, for example by welding 98 or other suitable means, rod elements as 100 and 102 formed of a very hard wear resis- tant material. ^*,*,. .

In another desirable arrangement a vane body 104 may be milled or otherwise formed with grooves as 106 to provide fluid passageways and overlying these grooves 106 is a welded cover plate 108 of a relatively greater wear resistant character. It may also be desired to construct a vane 110 .TFig. 15⁾ through which a thermal control fluid may be passed.

In all of the vanes of the invention now dis- closed it may be seen that it is readily possible to carry out a mixing operation without having to deal with a pre-hardened material and with the material heated or cooled to any desired temper¬ ature. Also, in such a mixing operation final te p- erature adjustments may be made without interrupt¬ ing the normal operation of an extruding screw which is being actuated to enhance the passage through the die. This final temperature is norm¬ ally less than the optimum temperature for pro- cessing in the barrel. The vanes may be utilized in some cases without a thermal control fluid to conduct heat away from a heated plastic mass in a barrel outwardly to the barrel wall.

As an example of a thermal control fluid there may be cited silicone oil which may be heated through a temperature range of from 70° to 400° F. To increase the surface area provided by any single vane in the mixer, a number of parallel vanes may be connected between each slot in the barrel. For example, each vane shown in Fig. 1-3 may be replaced by three parallel vanes, each vane being 3/8 of an inch thick and being spaced 3/8 inch.

In addition to serving the purpose of a con¬ ventional static mixer, the mixer described above serves the additional function of warming plastic which has been allowed to set in a mixer to make that plastic sufficiently fluid for flow of the plastic during a start up period. Further, once the extruder system is in operation, the mixer serves to lower the temperature of the hot plas¬ tic received from the extruder to a temperature at which the plastic forced through a die will hold its shape. Thus the heat exchanger and mixer modules described separate the extruder, which for the sake of product quality and uniformity should operate at a higher temperature, and the die. The arrangement of the vanes to form a mixer minimized temperature gradients within the plastic

Figs. 16 and 17 illustrate a heat exchanger module which may be used to control the temperature of the plastic but which does not serve the function of a mixer. Specifically, the module includes a barrel 120 through which the plastic may flow. Top and bottom cavities 121 and 123 are formed in that barrel and closed by caps 122 and 124 to form top and bottom manifolds. A number of parallel vanes are fitted into the barrel 120, and conduits 128 in those veins connect the manifolds 121 and.123. ^' Heat transfer fluid for heating or cooling the plastic is introduced into an inlet port 130, distributed to the various conduits 128 and passed through the varies to the opposite manifold 123. The liquid is then drawn out through the outlet port 132.

Fig. 18 illustrates another heat exchanger module in which the vanes are flat heat pipes 134 which extend between heat sinks 136 and 138. These heat pipes include wicks 140 and are evacuated but for a low vapor pressure fluid as in conventional heat pipes. The heat pipe fluid is vaporized with heat transfer from the plastic. That vapor provides a high conductivity thermal path to the end heat sinks where the vapor condenses, thereby releasing heat through the heat sinks. The condensed fluid is carried by the wicks back into the barrel region to again be vaporized.

The use of modular heat exchangers and mixers as described increases design flexibility. Any number of heat exchangers and mixers can be con¬ nected in series in a configuration most suited to the particular extruder application. For example. as shown in Fig. 20, a single heat exchanger may precede a mixer to cool the plastic before that plastic enters the mixer.

Figs. 21 and 22 show other configurations in which heat exchangers 142 and mixers 144 are ar¬ ranged. The mixers are preferably of the heat exchanger type as described above but may be con¬ ventional mixers.

As noted, a silicone oil is the preferred heat transfer fluid. By controlling either the flow rate of that oil or the temperature of the oil

_* entering the heat exchanger or mixer varies, one can control the amount of heat transferred to or from the plastic to maintain a desired output to the die. Fig. 23 is one example of a control sys¬ tem for controlling the temperature of the oil at a constant flow rate. This system includes a temperature sensor 146 near the input of the mixer or heat exchanger to provide a signal T-- on line 148.- This signal is compared to a set point T_g_ from a manual input 149. The difference signal _IN is modified by a constant and a time constant factor in circuit 150. The constant K is determined experi¬ mentally and serves to predict the changes in temp- erature of the heat transfer oil necessary to obtain the desired set point.. The constant is dependent on the heat transfer characteristics of the plastic and the surface area and other heat transfer char¬ acteristics of the vanes. In one successful con- figuration, this constant is about equal to 6. The time constant factor is e — _'' _ in which τ is the time required for the plastic to flow through the section.

' -^' A The output of circuit 150 is compared again to the set point and the resultant signal 152 is compared to the oil temperature indicated by a sensor 154. The final signal.Δ then controls the 5 heater or cooler in the oil bath 156. To cool the oil, water is passed into heat exchange relationship with th≤ oil and is quickly vaporized in a quick transfer of heat to the water. An electric heater is used to provide initial heating of the oil 0 during start up and for compensating for overcooling of the oil. An indicator 158 is provided to indi¬ cate the temperature of the plastic at the output of the mixer. High and low.temperature limits sensors 160 and 162 are also provided. 5 • Fig. 24 illustrates another electrical controller which offers greater precision in the control. In addition to sensing the temperature of the plastic at the input of the mixer and the temperature of the oil which enters the mixer, the temperature of the oil leaving the mixer is sensed. The difference in oil temperatures is proportional to the heat trans¬ fer from the oil to the plastic. By comparing the plastic temperature at the input with the set point one provides an indication of the heat transfer which is required. Comparing the required heat transfer.with that provided by the oil, as indicated by the change in temperature of the oil, one can more precisely control the temperature of the oil to obtain the necessary heat transfer. The con- ^* stant K by which the difference in plastic temper¬ ature from the set point is modified in circuit 164 is roughly equal to the ratio of the specific

^\_ H E.Λ heat, flow rate products of the plastic and oil. Again a time constant is provided by a circuit 166 to minimize the effects of transients.

Fig. 25 shows another approach to controlling the heat transfer to the plastic. In this case, the flow rate of heat transfer oil is controlled by a valve 168. This valve is in turn controlled by a proportional controller 169 such as the Foxboro controller. The controller 169 provides a control signal proportional to the ratio of the set point minus the plastic tempera¬ ture at the output of the mixer to the change in temperature of the oil. Thus, as in the embodi¬ ment of Fig. 24, the heat transfer to the plastic is controlled by monitoring the heat transfer from the oil.

Fig. 26 is another control system^* in which a proportional controller 170 controls the valve 172 to control the heat transfer fluid flow. In this case, the system also responds to the input temper¬ ature of the plastic according to the function

<^TPI-^TSP>

ΔT_Λ ^{X T}PO~^TSP.

As in other embodiments, the oil is pumped by a pump 176 from a reservoir 174 the temperature of which is controlled by cooling coils 178 and a heater 180.

The control of the heat transfer from the plastic to the mixer or heat exchanger in any of the above control systems provides for a more con- stant temperature and thus more constant viscosity and back pressure in the mixer. Surges common to ' extruders are thus filtered out for a more uniform and efficient extrusion of the plastic material through the die.

Claims

In a method of introducing a flow of material into a cylindrical body and exerting pressure to advance the material progressively along the cylindrical body the steps which include interposing in the path of flow of the material thermal control means and activating the ther¬ mal control means to induce predetermined temperature changes in the said material.

The invention of Claim 1 in which the thermal control means is activated before the material starts to advance along the cylindrical body.

3. The invention of Claim 1 in which the thermal control means is activated while the material is advancing.

4. The invention of Claim 1 in which the thermal control means is activated after portions of the material are no longer advancing.

5. A method according to Claims 2, 3, and 4 in which the material is a mixture of plastic substances and the cylindrical body is an extruder barrel in which the plastic sub- stances are mixed together. 1 6. The invention of Claim 1 in which the thermal

2 control means consist of tubular members pre-

3 senting flow diverting surfaces and the thermal

4 means are activated by conducting a heating

5 fluid through the tubular member.

17. The invention of Claim 1 in which the thermal

2 control means consist of tubular members pre-

3 senting flow diverting surfaces which occur

4 in angularly disposed relation to one another

5 and the thermal means is activated by con-

6 ducting a cooling fluid through the tubular

7 member.

18. The invention of Claim 1 in which the thermal

2 control means consist of flow diverting vanes

3 which have supported thereon electrical con-

4 ductor means and the thermal control means

5 is activated by passing an electrical current

6 thru the electrical conductor means.

19. The invention of Claim 1 in which the thermal

2 control means includes a fluid receptacle hav-

3 ing a thermal control fluid therein and the 4* thermal control fluid is circulated thru the 5 thermal control means.

110. Fluid conduit apparatus for receiving a flow

2 of material, said fluid control apparatus

3 including a cylindrical body having opposite

4 side wall portions formed with spaced apart

5 slots and thermal control means supported in the slots for inducing temperature changes in material contained in the cylindrical body.

11. The invention of Claim 10 in which the thermal control means present flow diverting surfaces arranged in angularly disposed relation to one another for diverting material forwardly of the cylindriqal body in successively differ- ing directions.

12. The invention of Claim 10 in which the thermal control means includes vanes formed with passageways therein and means for introducing a fluid into the passageway to induce changes in temperature in the same material.

13. Static mixer apparatus including a barrel member formed with slots therein, a plurality of flow diverting vanes mounted in the slots in angularly disposed relationship to one another, said vanes being formed with passageway ex- truding structure, and means connected to the vanes for introducing a thermal control fluid into the passageways.

14. The invention of Claim 13 in which the means for inducing a thermal control fluid includes a reservoir in which the fluid is contained and pump means for circulating fluid from the reservoir thru the passageways.

- 15. An interface between an extruder and an output die, that interface comprising a modular con- figuration of mixer and heat exchanger units, each heat exchanger unit comprising heat trans- fer flow conduits passing through a plastic transfer barrel in heat exchange relationship with the plastic in the barrel.

16. The invention of Claim 15 in which the mixer modules comprise heat transfer fluid conduits which extend through a plastic transfer barrel in angularly disposed relation to one another.

17. In an extruder system comprising an extruder and a dye, the improvement of a heat exchange unit between the extruder and die, the heat exchange unit comprising heat transfer fluid conduits extending through a plastic transfer barrel and means for controlling the tempera- ture and/or fluid flow of heat transfer fluid through those conduits in reponse to a sensed temperature of the plastic to provide controlled cooling of the plastic from the high tempera- ture at the output of the extruder to a desired lower temperature at the die.

: 1 18. The invention as claimed in Claim 17 wherein

2 the means for controlling the temperature

3 and/or fluid flow of the heat transfer fluid

4 responds to the temperature differential be-

5 tween heat transfer fluid entering and heat

6 transfer fluid exiting the conduits through

7 the barrel, that temperature differential

8 providing an indication of the heat transfer

9 from the heat transfer fluid into the plastic.

119. The invention as claimed in Claim 17 further

2 comprising a heat transfer fluid reservoir for

3 providing fluid to the conduits through the

4 barrel, that reservoir including means for

5 both heating and cooling the heat transfer

6 fluid.

120. A method of operating an extruder and die at ? respective optimum temperatures comprising

3 providing a heat transfer unit between the

4 extruder and dye, that heat transfer unit

5 comprising heat transfer fluid conduits ex- tending through a plastic transfer barrel and controlling the temperature and/or flow of heat transfer fluid through those conduits to cool the plastic flowing therethrough an 0 amount necessary to provide the proper temper- 1 ature differential between the extruder and 2 die.

C"PI 21. The invention as claimed, in Claim 20 wherein the temperature or flow of the heat transfer fluid is controlled by the difference between the temperatures of the heat transfer fluid entering and exiting the heat exchanger unit.