CA1311467C - Heat exchanger - Google Patents

Abstract

Heat Exchanger Abstract A very simple and inexpensive heat exchanger comprises a closed vessel V having thermally conduct-ing walls and helium gas within the vessel. The helium gas is a heat transfer medium between a source of heat or cold and the vessel walls and the vessel walls are a heat transfer medium between the helium gas and a fluid in contact with the walls to be heated or cooled.

Description

Descriotion eat Exchan~er Technical Field The present in~ention relates to the heating or cooling of fluids (liquids or gases) efficientl~ and rapidly and, in particular, to a heat exchanger suit-- able for various uses, such as heating water or gen- -erating steam for general use or for space h~ating.

Backqround Art In most cases i~ which a fluid is being h~ate-d or cooled i~ is kept physically separa~e frol~ the mediu~ that is supplying heat to it or receiving he~t from it by confinement within a vessel or condui~. ?
Like~ise, the msdium supplying heat to or receivin~
heat from the fluid is also usually confined to the vessel or-conduitO The vessels cr conduits or both constitute a heat èxchan~er.
M~st heat exchansers use a tube or a sys~em OL
tu~es as the ~arrier between the rluid ~ein~ heats~
or ?~oled ar.d the heating or cooling mediu.~. Ofte.
it is necessaryt in order to maximize the erflcienc~
of the heat transfer, to provide a co~pLica~ed sy~em of baffles and tu~s and to ~mploY t~.~b~s that a~e constructed to enhar.ce the rate o~ heat trf1nsfe~, for example, by inclusicn of rib3, ins, corrugatlons or the like. For durability falxly cos~.ly metals, .sucn as copper, are often used~ The comple~it~ o the structure ~nd t~e high cost CL t~e matQrial~ r~a~e eFf iciQr.~; long~l~sting heat exchangers very e~Ypensiv ~any d~vices that are in wiâespr~ad use and emplov h~a~ exchallge~ ha~e reiativel~ io~ ~rficlen-- cies. ~or example, residential furnaces an~ hot Wat'r heaters fueled ~y na~ural gas ~r oil have o~erall ~: ~

-2~ 7 efficiencies of only a~out S0%. ~atural gas and oil fired equipmeRt could be made con~ider~bly mor~ ef~i-cient u~ing presently available technology in th~
design of he2t exchanger~, but only by conqid~rably 5 increa~ing the complex~ty and th~ 8iZ~ O~ th~ equlp~
ment and rnaking it much more costly.

There is prov ided, in accordanc~ with the peesenJc lnventionf a heat exchanger tha~ i5 o ~ry simple lû con~truction and that can, therefore, be manufactursd at relatively low cost. I~ ::an also p~ovlde a su~s~can tial improveA~ent in efficiency withou~ substantially increasing the cost, particularly when compared ~h pre~ently known hea'c exchanger~., A heat exchanger according to the invention has a closed vessel defined by thermally conducting external walls and heat source within the vessel in spaced relation from the external walls. The vessel. contains helium gas at an initial filling pressure at ambient temperature of not less than 200 kPa, the helium gas being the sole medium for transfer of heat from the heat source to the thermally conducting external wallsO
In o~ form the vessel of th~ h~at exchang~r i~
tubular in ~hat it has external and int~rnal ~hermalîy 25 conducting walls. Either ~he internal or ~xternal wall is the heat transf~r mediusl~ between the fluid belng hea.ed or cooled and the helium gas a~d the other is a heat, transf~r medium betwe~n th~ ~ourc~ of heat or cold and the heliu~ gas. For example~ the 30 internal wall may serve as a conduit through which a flowin~ fluid ~ource o~ heat i~ conducted and by which heat is tran~ferred to the helium ~n th~ vessel.
~ n another ~orm o~ the presen~ vention the helium filled vessel is at least partly received 35 withln a container. P~ fluid to or ~rola which hea~ i~
~o be transferred is supplled t!lrough one or ;nor~
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inlets into the container and, having been hea-ted, is renoved from one or more outlets remote from the inle~
or inlets.
In one embodiment of the present invention the vessel is tubular and includes an internal wall defin-in~ a condui~ that is adapted 'o receive a flow of a hot fluid to be cooledv The external wall of the vessel is surrounded by a container wall, -thus provid-ing a passage for flow of a fluid to which heat is tG
be ~ransferred~ In this embodiment both the intern~1 and external walls o~ the ves~el containing the helium participate in the heat tran~er between the two fluids.
Such a heat exchanger is especially useful in equip~
ment in which it is desirable for safety reasons to have a double wall barrier bet-~een the two flui~.~s.
hazardous fluid flowing within the intern~3. c~nduit of the vessel is contained by the outer wall of the vessel if the i~ner wall should rupture.
- The invention may incorporate one or more of the ~ollowing additional features. T~7here ~he heat ex~han-ger vessel i5 of the tubular ~orm and is employed to tran~fer heat bet~een a fluid fiowing wit~ln the inter-nal wall (or conduit) and a fluid flowins along the external wall, the two fluids flow in opposite direc-tions. The conduit of the ~7essel may have flutesextending inwardly and lengthwise to enhance ~he hea~
flux bet~een ~all and the fluid flowlng along ~he wall. The vessel may have transverse dividers d~fin-ing a multiplicity of se~arate adjacen~ compartments to prevent eirculation of the helium along the length of the heat exchanger and thereb~ pro~o~e a larger temperature gradient along the iength.
One important advantage of ~he inv2ntion is ~hat ~h~ h~l'um ga~ ln th~ ves~oi providas a coi~paxati~eL~
high rate of heat transfer~ ~elium has, a~ong all ga~es, a very high coefficient of thermal condu~tivity.

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Accordingly, heat is transferred very rapidly to the walls OL the vessel and thence through ~he walls ln~o the fluid medium to which the heat is being transferred.
A hea~ exchanger according to the invention makes it possible to transfer heat ~rom a very high tempera-ture source acting over a fairly small area to a la~ge heat transfer area, namely the external walls OL the helium containing vessel. The temperature o~ the vessel walls from which the heat is transferred to the fluid to be heated is substantially less than tne temperature o~ the source and, indeed, can be designed for optimum heat transfer conditions between the Yes-sel walls and the fluid. For example, effe~tive hea~
~ransfer to liquid is best accomplished under tempera-lS ture conditions under which film boiling doe~ no~occur. The inherent abili~y of the present inven~ion to transfer heat from a high tem~erature con~entrated qource to a relatively lower temperature ~arrier wall of large area with inexpensive equipment ls an impor- -tant advantage.
Helium is ~Tery light in weight. Whi~e lt i5 possible with metals and some li~uids to ~rovide a heat transfer between a concentrated high temperature area and a large surf~ce a.ea the equi~ment envolved is heavy and expensive. ~he simple construction and the Light weight of heat exchang-rs em~odying ~he present invention facilitate the manufacture and installation of e~uipmen~ utilizing the inven!cion.
The invention has nu~.erous uses in both consumer 30 products and industrial products, pa.ticularly for hea~ e~change between fluids at greatly different temperatures, in applications where two or more barriers between the two fluids are re~uired or ~eslra~le ~a ~ ere lt is _~slrab'e ~;o .~a~e minimum storage capacity. A particularly important use of a heat e~changer emDodying the present i~vention is i~

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water heatin~ or steam generating for general use and for spac~ heating for individual residences, apartments, hotels, motels and o~fice and institutional buildings.
~ot water heaters utilizing ~he heat exchanger of the S invention are economical to make and easy to install and make possible the use of comparatively small units located close to the place where the hot water will be used. Both capital investment and operating costs can-be saved by locating small water heaters in various places in a building, ~hereby eliminating expensive long distribution systems and th~ heat losses that occur in such systems.
For a better understanding of the inventlon reference may be mad~ to the following descrlption of an exemplary embodi~ent ta~en in conjunction s~ith the figures oE the accompanying drawings.

Brief DescriDtion of ~he Drawings - Fiq. 1 is a schematic diagram illustrating the principle of opera~ion of a heat exchan~er embodyin~
the present invention, as applied to a vPry sim~le electric 'neater;
Fig. 2 is another schematic diagram depictin~ in a generaiized way a heat exchanger according to ~he present invention for trans~erring heat from a ho~
fluid to a cold fluid;
Fig. 3 is a side cro s-sectional view of a ver~
simple and inexpensive heat exchanger or pre coo3.ins a hot re~rigerant in a refriger~tion syste~;
Fig. 4 is a side cross-sectional vie~.~ or an ~ 30 electric ~ater heater embodving a he2t exchanger in : accordance with the present invention;
Fig, 5 is a side cross~sectional view o~ a qas~
f ired hv~ wa-~er ~lPa~er 4na~ employs a neat e~changer according to the present inven~ion; and , -6- ~ 3 1 ~ ~ ~

Fiq. o ls a fragmentary cross~sectional view of the internal wall of ~he heat exchanger shown in - ~ig. 5~

Modes for CarrYinq Out the Invent on A very simple form of a heat e~changer embodying the present invention, ~s shown diagramatically in Fig. 1, is a small electrically energized space heater.
It comprises a closed vessel V that is filled with helium gas, preferably ai an absolute p~essure of about 200kPa to about 700kPa t~ilopaScal). Although the vessel may be of a~y shape, it is desirable tO
make it o~ circular cylindrical shape for ease of manufacture and uniform heat transfer in all ~irec-tions radially with respect to the cen~ral axi~.
Thus, the vessel V show~ i~ Fig. 1 comprises a cir-cular cylindr ica' external wall and top and bottom walls. One or more elec~rical heating elements H are - suitably ~ounted within the vessel. The heating ele- .
- ments may be of very simple low cost con~truction~
for exampla, a ceramic support thound witn a hare resistance wire heating element. T~e ele~trical leads are connected ~o a source of _~r~ent.
Upon energiza~lon ~he heatin~ elem-~n~. E re2c:~es a very high temperature Tl~ The heat is conducted by the helium gas radi~lly outwardly in all dlre~tion~, as represented by the arrowed line~ within the vessel.
As mentioned above, helium has a relatively hlgh co-e~ficient of thecmal ~onductivity, as colr.pared to air and other gases. Accordingly, the hea~ cf the heating element is trans~erred qulte r.~idI~v to tne waLls ~L
the vessel V~ In the case of a space heater ambient air from below tne hea~er at a tem~erature T~ is drawn o_~w~rdly b~ ~he cor-~:ectl~e flow induc~d by the hot walls of ~he vessel, as represented by the arrowed Lines designated T~ i~ Fig. 2~ ~s the a~r curren~

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flows over the ex~ernal wall of the vessel, the air is heated to a temperature T3 and rises in the direc-tion represented by the arrows T3. The hot w~lls of the vessel also radiate heat into the space, The schematic iLlus~ration in Fig. 1 is represen tative of the basic principle of operation of all heat exchangers embodying the present invention. The helium gas contained in the closed vessel Y accepts heat.~rom any suitable source.located inte~nally of the vessel, as represented by the electric heating element H in Fig. 1. The internal source of heat may be a hot liquid or gas that is conducted -through one or more pipes within the vessel or passing completely through an internal conduit in the vessel. Heat i3 transferred from the source within ~he vessel by the helium gas to the external walls o the ~essel. ~rh~
vessel may be partly or entirely enclosed within an outer container C, as represented in Fig. 1 by a cir-cular cy~ indrical shell having one or more inlets and outlets spaced apart from one another and defining a passage P for a flow of a gas or liquid that is to be heated by the hot walls of the vessel. a.ccordingly, the invention operates by acceptance by the hellum of hea~ fxom a source at a temperature Tl, the tr~nsfe~
OL that heat by the helium to the w211s of .ha ~essel and transfer of the heat through the walls o the vessel to a fluid ~liquid or aas) rlo~ing in contact with the walls of the vessel, the temperatu.re of the fluid being raised from T2 to ~3.
~ig. 2 illustrates ~che~.atically some preferred characteristics of heat exchangers embod~ins th~
present invention, as applied ~o equipment ~r trans-ferring heat from a higher temperature liquid or gas ~o a iower e~p~ratur~ liquid or gas. I'he h~iu~ ~a~
~ 35 is contained ~ithin a tubular vess~1 ~hichr a3 men-: tioned above, may b~ of any suitable ~hape but is , . .. . .

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prerera~ly a clrcular c~linder. For purposes of the present description, it may be assumed tha~ ~ig~ 2 depicts a vessel V having a circular cylindrical external ~all EW, a circular cylindrical internal 5 wall rw, and annular top and bottom walls TW and BW.
The vessel shown in Fig. 2 is, m~reover, subdivided into a multiplicity or individual annular chambers by separator plates S suitably joined to the internal and external walls. The s~parator plates ~inimize convective heat trans~er alo~g the length of the vessel, thus increasing the end-to-end tPmperature gradientO Accordingly, it is not necessary to ha-Je a gas tight connection between the ~lates and the walls of the vessel.
~ hot Eluid at a temperature Tl is introduced into and passes through the passage defined bi~ the internal wall IW of the vessel. Heat is transferred from the hot fluid to the internal wall. Accordingly, the fluid leaves the passage at a temperature T2 below the temperature Tl. The heat accepted by the in~ernal wall o~ the heat exchanger is transferred radially by conduction through the heliu~ gas to the external wall, by con-~ection currents o~ the heliu~. gas and ~y radiation. A fluid to be heated is su?plied at ~he cold end of the heat exchanger at a temperature T3, flows along the external ~all EW of the chamber and exits the heat exchanger at a higher tem~erature T4.
In ~05t cases the flo~ of the fluid ~o be heated is confined by a con~ainer C that recei~es part or all of the helium containing ~Jessel and ~hat is repre-s~nted in Fig ~ diagramatlcally by a cylln~rlcal shell.
An important advantage of the invention i~ the ability to ~ransfer heat from a very high ter,lpera~ure 3s sourc~ to an ou~put heat trans~er surace of large area. The heat exchanger inhere~tly distribu-tes hea~

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_g_ received over a relatively s~all area from a high temperature source and distributes the heat over the large area output surface constituted by the external walls of the vessel. In making such distribution, the temperature of the output surface is inherently substantially reduced from the temperature o the source, assumLng, of course, that heat is transferred away from the output surfac~ of the heat exchanger.
- The ability of the heat exchanger to distribute heat over a large surface is of advantage in applications of the invention where the cool fluid cannot for one reason or another be subjected to a high t~mperAture, such as che~ical degradation or unwanted vaporization.
The heat exchanger can be designed so that the cool liquid is not exposed to a temperature higher than a predetPrmined safe vaLue.
An example o~ the usefulness of this char~cter-istic is the space heater described briefly above. A
-space heater can be de~igned to distribute the heat input over a large enough area thatr given the heat transfer characteristics between the surface of ~he extern~l walls of the vessel and the ~ir, the ex~ernal walls do not reach a temperature 'nigh enough to be hazardous. Indeed, the outer surface may be kept well below the temperature t~at would cause severe discom~ort to someone who touched the heater, In the case of most liquids, it is desirable to avoid film boiling of the liquid a~ the external sur-~ace of the vessel. A heat exchanger embodyin~ the present invention can be designed to trans~er hea~ at below the boiling point or within the range of nucleate boiling at the surface.
Other advantases o~ the invention are evident f~om the v~.y Sil~pl~ heat exch~nger showll in Fig. 3 3; of the drawings. A length of pipe 50 defines the intern~l wall of a tubula~ closed vessel 52 and , .

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~- -' 10- ~3~ 7 serves as a passage for Elow o~ a hot liquid. For example, it is desirable in refrigeration systems to pre-cool the hot compressed refrigerant, such as ~reon.
In order to make efficient use of the heat content of the refrigerant, the cold side of the heat exchanger is potable water used in the building The heat exchanger in this application is used as a hot water heater to supply part of or all of the requirements of the building for hot wat~r. 3uilding codes re~uire .
a double barrier between the r2frigerant and the pot-able water, so that if the refrigerant conduit of the heat exchanger ruptured and released refrigerant, a second barrier will prevent the refrigerant Erom enter-ing the hot water supply. A heat exch~nger embodying ~5 the present invention meets that requirement.
In particular the tubular vessel 52 provides ~he required double barrier between the ~efrigerant and the water by means of the external wall 53 and the conduit 50. The heat exchanger vessel 52 containing helium under pressure is closed ~t each end by an annular end pla~e 54 ~elded at the inner and oute~
diameters to ~he internal and external walls. Sur-roundlng the heaL~ ~ch~nger vessel 52 is a container de~ined by a cylindrical ~all 5O and annular end pla~es 58 welded at the inner and outer diameters to the ~alls 50 and 53. r~ater to be heated is supplied ~o the annular passage defined b~ the container and sur-rounding the heat exchanger ~re55~1 through an inle~
59, flows through the annular passage, as represented by the arrowed lines, and exit3 through an outle~ 60.
~eat is transferred from the refrigerant flowing through the conduit 50 to the helium, the helium transfers the heat to the external wall 53 and the ~ L~ h~ h~ ex~e~n~
Fig. 4 illustrates the ~se of a heat exchanser accordLng to the present inv2ntion in an electr~c :~

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~ 7 heater for heating a fluid. It comprises an outer container 10 having a circular cylindrical side wall 12, a top wall 14 and a bottom wall 16. A closed vessel 18, which consists o~ a circular cylindrical side wall 20, a top wall 22 and the bottom wall 16 of the container, receives any suitable number of elec-trical resistance heating elements 24. Each heating element comprises a ceramlc support and 2 helical wirlding of conven~ional nicrome wire. Preferably, - 10 the wires of the heating elements 24 are connected in series by a sys~em of bus-bars ~not shown), and suit-able electrical connectors 26 conduct ~lectric current ~o the first element and ~rom the last element in the series. The use of heating elements connected in series enables heavier gage nicrome wire ~o be used, thereby ensuring long liEe, but parallel wired elements and the possibility of having two or more groups of series wired elements, with the groups wired in paral-lel is, of course~- entirely feasible. ParalLel wired electrical heating elements also present the possibil-ity o~ providing variable heat output by varying the number of elem~nts that are switched on at any polnt in time in response to suitable controls.
It is considered preferable for the con~ainer 10 and vessel 1~ ~o be entirely of wPlded construction fo~ assurance against leakage, and t~i5 iS the case with the embodiment shown in the drawing. It is, nonetheless, envisioned that tne he~ting unit (i.e., the vessel 18 with electrical heating ~lements ~4) can be constructed so that it can be remlove~ from the container 1~. There are va~ious ways that ~ill be readily apparent to those skilled in the art a~ a matter of ordinary engineeriny skill of making the heating unit remo-~2ble. ~or som~ t~p~s o~ serv ce, notabl~ those in which deposits build up within ~he container because of the characteristics of 'he fllli2r . ~

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the abillty to re~ove the heating unit from the con-~ainer may also be desirable in order ~o provide access to the container for thorough cleaning frcm time to time. ~or service with liquids, it will usually be 5 desirable to provide a valved drain outlet (not shown) in the bottom of the container.
Fluid to be heated is supplied to the container 10 through an inle~ 26 in th~ top adjacent the outer wall 12.. A manifold distri~ution sy~tem (not.shown) 10 may be interposed be~ween the inlet and the annular space between the side wall 12 of the container and the side ~all 20 of the vessel, or multipl~ inlets can be provided to distribute the incoming fluid relatively evenly around the upper part o~ the con~
tainer. The annular space between the walls 12 and ~0 is subdivided into an inlet chamber ~0 and an ou~let chamber 32 by a circular cyclindrical baf,le 28 ~hat extends nearly the entire distan~e from the top wall 14, to which it is welded, to the bottom ~0 wall 1~ of the c~ntainer. The fluid enter ing the inlet 26 is compelled by the ba~fl~ ~8 t~ flow do~n through the inlet chamber to the ~ottom of the cor.-tainer and then turn and flow up~ardly through the ou~let chamber to the top of the container. rrhe then heated fluid flows radially inwardly toward the axis of the container and enter~ a o~tlet pipe tha~ extends vertlcally throllgh the vessel 13 and exits through the bottom wall 16 o~ ~he container to an outle~ 36.
~he vessel 18 contains helium gas at a suitable pressure, preferably in the range of from about 20~
KPa to about 700 ~Pa. The vesgel 18 may ~e, ~ut need not be, evacuated before being charged with heliu~.
The helium atm~sp'nere within the vessel provides ~OL
rapid transf~} 9L heat ron~ the electricai resistance heating elements 2~ in the vessel to the wall~ o the ~essel 18 and the pipe 34.

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As ~he ~luid entering the inlet 26 flows down throug~ ~he inlet chamber 30 betwe~n the baffle 28 and the wall 12 of the container it is gradually preheated, inasmuch as the fluid flowing up thxough outlet chamber 3Z ~lows in direct contact with the hot outer wall 20 of the vessel and transfers some of the heat it receives from the vessel wall out to the baf~le which, in turn, trans~ers it to the fluid flo~-ing through the.inlet chamber. The fluid flowing up 10 along the hot outer wall 20 of the vessel is rapidly heated in the relatively thin channel defined bet~een the wall 20 and the baf fle 28 . The wall 20 provides a very large surface area to which heat is trans-ferred from a suitable number of heating elements 24 within the vessel very rapidly by the helium atmos-phere. Thus, ~he heater is ideally suited for 5ub~
stantially instantaneous heating of a fluid. Heat losses from the heater are kept to a minimum, because the incoming fluid provides an insulating barrier.
Inasmuch as heat transfer is a function of the dif-ference in the temperatures on opposite sides o a barriar, the loss of heat through the outer wall 12 is '~ept low because the incoming ~luid i~ only slightly heated, as compared to the much higher temperature of 2~ the ~luid flowing through the ou~let cham~er. This natural barrler of the incoming fluid in the chamber 30 contri~utes to the high efficiency of the unit.
It is pre~erable for the outlet chamber to be very thin in order to promote turbulant flow of the fluid, and ihe turbulence ~nd the surface area ~ay ~e increased by providing a corrugated ou~e.r wall on the vessel, or ~y providin~ fin~ or other devices for promoting turbu'ence.
~pon. -2~chin the ~op ~ the out~et cha~er 32, the fluid flows inwardly across the ~op of the vesselr ~here it receives`additional heat, and ~hen passes -14~

through the outlet pipe 34, again receiving heat from the wall of the pipe to which heat is rapidly and eff2ctively transferred ~y the helium atmosphere within the ~essel.
The heating elements 24 may be controlled by any suitable thermostatic control sy~tem, preferably on~
which ~.easures temperature of the incoming fluid near the inlet by means oE a thermo-couple 38, measuxes the t~mperature o~ the fluid af~er it has been sub-stantialLy heated, such as by a thermo-couple 40, and turns the elements 24 on and off in accordance with some integrated value that takes into account both incoming and outgoing temperatures. Many such systems are known in the art and are shown schematic~lly in the drawing by means of the block 42 labelled "con-troller." The abili~y of the helium at~osphere within the vessel tQ transfer heat rapidly to the walls along which the fluid passes and by which the fluid is heated improves the response-rate of the control system.
~he embodiment lllustrated in the dra~inq i3 an instantaneous type unit, inasmu~h as it has ~irtually no storage capacity. It can be controlled to maintain the temperature of the fluid in the region of the ther.~o~couplQ 40 somewh2t heated b~t not heat~d to ~he output Se~perature. When fluld is demanded from the outlet 36, the .hermo~couple 38 detects a dr~p in temperature and the controller 42 s~itches on the heating elements 24. In a matter of a ~ew seconds the helium atmosphere ~1ithin the vessel begins ~rans~
ferring heat from the heating elements to the wall ~0 and the pipe 3~, and the fluid flowing from the outle-t becomes rapidly ho~ter until it attains a desired t~nperature. ~he heating element is Snen controLled primarily by the thermo-couple 40 to cycle the heating elements on and off and maintain a fairl~ const2nt .

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temperatl1re ~f the fluid cominy from the outl~t 36.
When fluid is no longer drawn from the heater, the thermo-couple 38 will detect an incr~ase in tempera-ture indicative of the fact that cold fluid is no longer entering through the inlet 26, and such indica-tion is processed in the con~roller 42 and shuts off the heating elements.
The embodiment o Fig. 4 is applicable to storage . . type liquid heating equipment/ which e~uipment may incorporate designs known in the prior art insofar as temperature control, possible recycli~g of fluid from the container through the heater and similar design factors. It is also well-suited for heating process gases, as is a unit having pipes or ducts th~t pass 1~ through the spacQ within the vessel. The side of wall the vessel mav be corrugated in the longitudinal direction for greater strength and surface area for a given overall size and weight.
. Figs. 5 and 6 also illustrate a fluid heater th~t employs a heat exchanger cons~xuct~d in accor-dance with the preser.t invention. The he.~ter com-prises a pul~e-type combustion chamber 101 o~ a type that is known ,er se. .~ir is injected ~hrough an inlet 11~ to the cham~e~ 101, and natural gas is injec~ed through the inlet 11'. An igniter initiates an explosion of the ruel-gas mixture in the chamber.
The ~orce of the exp~osion drives the hot combustion products out of the chamber and down through a pas~age 102 defined by the internal wali 120 of a closed ves-sel 122. The vessel has a circula. cylindrical oute wall 113 and is s~bdivided into a multiplicity of annular compartments by plates llO~ The ~-essel can be assembled by welding the plates 110 to the inner wall l~ and t~n slidi~lg the outer wall 118 on~o ; 35 the welded inner ~ssembly. If desired, seal~s can be ~ installed between the outer perimeters of the pl~tes -16- ~ 3 ~

llO and the ou-ter ~all 118, ~hough it i5 probably unnecessary to do so inasmuch as the purpose of the plates is only to inhibi~ convective flow o helium gas within the vessel 122 and thereby increase the S temperature gradient along the length of the heat exchanger.
The hot combustion products flow down from the combustion chamber 101 through the passage 102 in a state of very high turbulanc~, due to the explosive force by which they are propelled. To enhance the heat transfer between the hot combustion produc~s a~d the internal wall 120, the internal surface oE the wall 12Q is fluted, as shown in Fig. 6. In addition, the wa].l l~0 is of fairly large cross~sectional thick-lS ness so that it acts as a heat sink for heat received from the hot gases so ~hat heat is stored in the wall during periods between the combustion pulses. The ho~ gases of combustion are rapidly cooled as they ~low down through the passage 102 and reach the bottom at a relati~ely low temperature. Condensate from ~he combustion process is collected in an exhaust Plen~lm at the bottom of the hea~er and drains through an outlet 107, throl~gh a trap (not sho~n) and ther. to a ~aste line~ Exhaust gases e~:it through an ouLle t 105.
2S The vessel 1~2 i~ surrounded on the sides and top by a container having a cylindrical side wall 113 and an annular top wall 113a. The container defines with the exterior oE the vessel 122 a thin Passage 114. A fluid to be heated is in.roduced through an 33 inlel lOd to the chamber 114. ~s the fluid flows upwardly through th~ chamber, it receives hea~ from the large s~rface area of the hot external ~all 11 of the vessel~ which in turn rapidly receives heat ~rom the ir.ternai wall i20 ~r~nsferred tnrough the helium to the external wall. Hot fluid is discharged f rom the hea~er through an outle~ lQ3.

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A second tubular closed container 115 partly surrounds the inner container 113. Fluid is delivered to an inlet 111 to the annular passage within the container 115 and receives heat by transfer from the f}uid in the passage 114 ~hrough the wall 113. Thus, the fluid in the outer container is also heated as it passes up the annular chamber, and hot fluid is di3-charged through an outlet 112.
A highly advantageous use for the fluid heater shown in Figs. 5 and ~ is as a combined furnace and hot water heater. Hot water produced by the inner chamber and delivered through the outlet 103 can be distributed to hot water convectors throughout the space being served by the furnace, ~his water being at a relatively high temperature. The water dis-charged thxough the outlet 112 will be at a somewhat lower temper~ture, particularly when there is a d~and for both heating water and hot water for general pur-pose use in the building. The system~ can ~e opera~ed 2~ simultaneously or separately under the control of suitable thermostatic controls for the burner. When there is no demand for heat, water in the lnner con-tainer does not circulate and functions merely as a heat transfer medium to tran~fer heat from the vessel to tne outer con~ainer, so less input heat is needed.
The fluid hea~er shown in Figs~ S and 6 can be modified t~ employ a pul~e combustion burner unit located at the bottom. In addition, other types OL
fuel bu~ners can be substituted for the pulse combus-tion unit. Among the advantages OL this unit is the simplicity and resulting low c05t oS m~ing i~ and comparativ~ly high ef-iciency, especially tYith ~
pulse combustion unit -~hich enables substantially all o~ ~he h~ cont~llt of ~he fuel to ~e transf~i r~ so that very llttle heat goes out with the exhaust com bustion sases.

., -i8- ~ 3~

The above-descri~ed embodiments of the invention are intended to be merely exemplary, and numerous variations and modifications,will be apparent to those skilled in the art without departing from the spirit of the 5cope of the invention. All such variations and modificatiorls are intended to be included within the scope of the invention, as defined in the appended claims.
.

' :' - , - . .

~ 31l~
3 ,1 6-diketo-cholest- 1 ,4-diene-26-oic acid ~6 hydroxycholesterol 34. The use as defined in claim 33 wherein s ud pharmaceutically acceptable carrier comprises a penetratiGn-enhancing compound.

35. The use as defined in claim 34 wherein said penetration-enhancing compound is selected from the group of compounds represented by the structural formula:
o }~' 11 (C~ N (C~ R

., wherein R' is H or a lower allcyl group, m is 3-7, n is 0-17 and R is -CH3, phenyl or substituted phenyl or o --N ~CH2)m 36. The use as defined in claim 3~ wherein when m is 3 and R is -CH3, then n is not 0-6.

37. The use as del'ined in claim 36 wherein said penetration-enhancing compnund is 1-n-dodecylazacycloheptan-2-one.

38. ~ The use as defined in clalm 35 wherein sa~d oxygenated cholesterol is 26-hydroxycholesterol.
.
39. The use as defined In claim 35 wherein said oxygenated cholesierol is cholest- 1 ,4-diene-26-ol-3-one.

3~

~:

Claims (9)

1. A heat exchanger having a closed vessel defined by thermally conducting external walls and a heat source within the vessel in spaced relation from the external walls, characterized in that the vessel contains helium gas at an initial filling pressure at ambient temperature of not less that 200 kPa, the helium gas being the sole medium for transfer of heat from the heat source to the thermally conducting external walls and in that the external walls are adapted to transfer heat to a fluid outside the vessel.

2. A heat exchanger according to claim 1 and further characterized in that the vessel is tubular and is further defined by an internal wall of thermally conducting material and the heat source is contained within the internal wall.

3. A heat exchanger according to claim 2 and further characterized in that the heat source is a flowing fluid and the internal wall is a portion of a conduit that has an inlet and an outlet for receiving and descharging, respectively, the flowing fluid.

4. A heat exchanger according to claim 1 and further characterized in that a container at least partly contains the vessel and in that the container has an inlet and an outlet for receiving and discharging a fluid to be heated.

5. A heat exchanger according to claim 1 and further characterized in that the heat source is one or more electrical heating elements.

6. A heat exchanger according to claim 1 and further characterized in that the vessel is tubular and includes an internal wall defining a conduit for passage in heat exchange relation therewith of a gaseous source of heat, in that there is a container at least partly surrounding the vessel and having a wall defining with the external wall of the vessel an annular passage, and in that the container has an inlet and an outlet for receiving into and discharging from the passage a liquid to be heated.

7. A heat exchanger according to claim 6 and further characterized in that the inlet and outlet are located to provide a flow of the liquid through the passage in a direction opposed to the flow of hot gas through the conduit.

8. A heat exchanger according to claim 6 and further characterized in that the internal wall has flutes extending inwardly and lengthwise to enhance the heat flux between the gas and the internal wall.

9. A heat exchanger according to claim 6 and further characterized in that the vessel has transverse dividers defining a multiplicity of separate adjacent compartments in the vessel to promote a large temperature gradient along the path of the gas flow through the conduit.