Abstract:
The freeze-protected heat exchanger comprises a fluid supply header for receiving a pressurized heated fluid and a drain chamber coextending with the fluid supply header for collecting and draining cooled fluid. A plurality of heat exchanger tubes extends radially from the fluid supply header and drain chamber, and each comprise outer and inner pipes. The outer pipe has a heat-conductive wall, a proximal end in fluid communication with the drain chamber and a distal closed end. The inner pipe is disposed coaxially within the outer pipe, has a proximal end in fluid communication with the fluid supply header and comprises a plurality of first orifices through which the inner pipe is in fluid communication with the outer pipe. At least one second orifice through which the drain chamber is in fluid communication with the fluid supply header opens in the drain chamber. In freeze-protected operation, heated fluid from the fluid supply header is supplied to the inner pipes, heated fluid from the inner pipes is transferred to the respective outer pipes through the first orifices, heat from the heated fluid in the outer pipes is transferred to the outside, for example to a flow of air, through the heat-conductive walls of the outer pipes, cooled fluid from the outer pipes is collected and drained through the drain chamber, the second orifice produces a jet of heated fluid in the drain chamber to prevent the formation of ice preferably in the area of the drain outlet, and heat from the fluid supply member is also transferred to the drain chamber by conduction and radiation. The invention also relates to a face and by-pass heat exchanger unit including the above described freeze-protected heat exchanger.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to heat exchangers, and more particularly to heat exchangers featuring anti-freeze protection of the condensate draining path. 
     2. Brief Description of the Prior Art 
     Although freeze protection is an important criteria in designing an air-cooled steam condenser, the systems of the prior art, after more than two decades of development, still present complex and costly solutions to that problem and/or are unable to prevent freezing under certain operating conditions. 
     A typical solution to reduce the risk of freezing in the tubes of a steam condenser is to use a bundle of more than one row of tubes successively traversed by the air flow. The first row is struck by the coldest air flow but only a portion of the steam supplied to the tubes can be condensed. The outlet of the first row is connected to the inlet of the next row which converts a further amount of steam into condensate but is contacted by preheated air. Hence, although the steam could be totally reduced to cooled condensate at that stage, freezing is prevented because of the higher temperature of the air flow striking that row. 
     Larinoff in U.S. Pat. No. 5,787,970 issued on Aug. 4, 1998 presents an improved solution based on that concept characterized by a mixed flow vertical tube bundle design, in which some of the tube rows conduct counterflow steam and condensate while others have parallel flow. The condensate is drained at the bottom of the bundle from a header connecting a parallel flow row to a successive counterflow row in the protected warm air zone and non-condensable gases are collected at the outlet header of the counterflow rows. 
     The main drawback of the above type of systems lies in their lower efficiency/cost ratio as the second pass tube rows provide less heat exchange than the others for a comparable size and manufacturing cost. Also, some risks of freezing in the condensate drain piping and in tubes next to the edges of the bundle are still present. Moreover, circulation of steam and condensate in counterflow may result in interaction between the two fluids that disrupts normal flow and heat transfer. U.S. Pat. No. 5,056,592 (Larinoff) issued on Oct. 15, 1991 offers a solution to that problem by providing baffling inside some of the tubes to channel and separate the upward bulk flow of steam and the downward flow of condensate. 
     Another approach based on a similar principle is to use two rows of U-shaped tubes connected to a common steam supply as described in U.S. Pat. No. 3,705,621 (Schoonman) issued on Dec. 12, 1972. The tubes are so disposed that the air flow is successively striking the hottest legs of the first and second rows and then the coldest legs of the second and first tube rows. 
     Similarly, U.S. Pat. No. 4,926,931 (Larinoff) issued on May 22, 1990 presents a system in which the tubes are so arranged that steam flows from the input headers to the exposed legs of the inner and outer tube rows, and returns as condensate through the tube legs located in the protected warm air region in the middle of the tube bundle. The air flow thus successively strikes the hottest legs of the outer tube row, the coldest legs of the same row, the coldest legs of the second tube row and finally the hottest legs of that second row. Such an arrangement provides better protection to the exposed tubes especially at the top and bottom faces of the bundle. Moreover, the condensate drain headers extending in the protected region parallel and next to the steam supply headers provide some protection against freezing of the condensate by radiation heating. However, this system has drawbacks similar to the above concepts, as to the efficiency/cost ratio and still offers limited freezing protection especially in the U-shaped portions connecting the two legs of the finned tubes. 
     Another solution of comparable efficiency is described in U.S. Pat. No. 5,765,629 delivered to Goldsmith on Jun. 16, 1998 and uses a second stage vent condenser disposed in the same plane as a first stage condenser, both comprising bundles of vertically oriented tubes. The first stage operates at a higher steam pressure and consequently is easily drained from condensate and non-condensable gases into a lower header with excess steam. This header is connected to the upper header of the second stage condenser and to a hydraulically balanced common drain pot below the lower header. Non-condensable gases from the second stage flow counter-currently to be vented near the upper header. In this arrangement, freezing is controlled by continuous purging of the tube rows to avoid steam back-flow in the tube rows thereby eliminating trapping of condensate and non-condensable gases. However, this system is maintaining a constant level of condensate in the drain headers and the drain pot which are subject to freezing, particularly on the second stage condenser side. 
     Some solutions of the prior art have been specifically addressing potential freeze-up of the condensate drain lines. For instance, U.S. Pat. No. 3,968,836 (Larinoff) issued on Jul. 13, 1976 discloses a heat exchanger wherein leg seals connecting with outlets from individual condensate outlet headers are enclosed within a drain pot which is heated by uncondensed vapor from one of the outlet headers. In U.S. Pat. No. 4,240,502 issued on Dec. 23, 1980, Larinoff brings some improvements to the latter system, including a small hole in the drain pipe to purge the drain pot when the steam condensing system is shut down and applying some insulating material on the portion of the outlet header extending outside of the heated drain pot. 
     In U.S. Pat. No. 5,145,000, (Kluppel) issued on Sep. 8, 1992, a steam condensing system similar to the above has a tank receiving the condensate drain line from the drain pot. A steam line from the source of steam which also feeds the condenser, is connected to the upper end of the tank section receiving the drain line for supplying steam above the condensate level in the tank section. The steam heats the condensate drain line in the tank section to avoid freezing of the condensate. 
     In U.S. Pat. No. 5,355,943 (Gonano) issued on Oct. 18, 1994, steam from the source supplying the condenser is again connected to the upper end of a tank section receiving a condensate overflow drain duct from a drain vessel. Condensate is rain-like spread falling in the duct while the steam supplied to the tank goes up along the duct in countercurrent with the condensate, thus heating it on its passage to finally be sucked with non-condensable gases through the top portion of the drain vessel. 
     Although the latter vapor condensing system arrangements of the prior art significantly contribute to prevent freeze-up of the heat exchanger tube bundles or condensate drain lines, considerable drawbacks still limit their use on the market. Principally, their relative complexity significantly increases the system manufacturing and maintenance costs, while some efficiency of the heat transfer is lost and most of these systems still present risks of freezing especially if they are operated outside of their optimal vapour pressure conditions. 
     There is thus a need for an improved air-cooled vapor condensing system providing freeze protection over a wide range of operating conditions as required in applications such as heating of buildings. 
     OBJECT OF THE INVENTION 
     The main object of the present invention is therefore to provide a freeze-protected heat exchanger which overcomes the limitations and drawbacks of the above described prior art. 
     SUMMARY OF THE INVENTION 
     More specifically, in accordance with the invention as broadly claimed, there is provided a freeze-protected heat exchanger comprising: 
     a fluid supply member for connection to a source of condensable heated fluid; 
     a drain chamber coextending with the fluid supply member, and comprising a drain outlet; 
     a plurality of heat exchanger tubes extending from the fluid supply member and drain chamber, each heat exchanger tube comprising: 
     a first pipe having a heat-conductive wall, and a proximal end in fluid communication with the drain chamber; 
     a second pipe coextending with the first pipe, and having a proximal end in fluid communication with the fluid supply member; and 
     at least one first orifice through which the first pipe is in fluid communication with the second pipe; and 
     at least one second orifice through which the drain chamber is in fluid communication with the fluid supply member. 
     In operation, heated fluid is supplied from the fluid supply member to the second pipes, heated fluid from the second pipes is transferred to the respective first pipes through the first orifices, heat from the heated fluid in the first pipes is transferred to the outside of the first pipes through the heat-conductive walls, cooled fluid from the first pipes is collected and drained through the drain chamber and drain outlet, and the second orifice produces a jet of heated fluid in the drain chamber to prevent the formation of ice in the drain chamber. 
     In accordance with preferred embodiments of the invention: 
     the second orifice opens in the drain chamber in the area of the drain outlet; 
     the first pipe comprises an outer pipe having the heat-conductive wall and a distal closed end, the second pipe comprises an inner pipe having an inner pipe wall and disposed within the outer pipe with a space between the inner and outer pipes, and the first orifice extends through the inner pipe wall; 
     the fluid supply member and the drain chamber are substantially elongated and coaxial to each other, and the heat exchanger tubes extend substantially radially from the fluid supply member and drain chamber. 
     the heat exchanger tubes are generally horizontal with a slight slope toward the fluid supply member and drain chamber to enable draining of the cooled fluid from the first pipes toward the drain chamber by gravity; 
     each outer pipe comprises at least one outer heat-conductive fin to enhance heat transfer from the heat-conductive wall of the outer pipe to the outside, this fin comprising a helical extruded fin integral with the outer pipe to further prevent dilatation of the outer pipe and thus prevent formation of ice in the outer pipe; 
     the coextending fluid supply member and drain chamber are elongated, the heat exchanger tubes are arranged in bundles distributed along the length of the fluid supply member and drain chamber, each bundle of heat exchanger tubes comprise a plurality of rows of heat exchanger tubes, the heat exchanger tubes comprise first and second sets of heat exchanger tubes, and these first and second sets are diametrically opposite to each other about the fluid supply member and drain chamber; 
     the coextending fluid supply member and the drain chamber are substantially elongated and vertical; 
     the drain chamber comprises a bottom end provided with the drain outlet through which cooled fluid collected by the drain chamber from the first pipes is drained; and 
     the fluid supply member comprises a header with a closed lower end proximate to the drain outlet, the lower end of the header being provided with the second orifice to produce the jet of heated fluid in view of preventing formation of ice in the region of the drain outlet of the bottom end of the drain chamber; 
     the fluid supply member comprises a heat-conductive wall located at least in part in the drain chamber to provide for transfer of heat from the heated fluid to the drain chamber in view of preventing formation of ice in the drain chamber; 
     each inner pipe has a distal end short of the distal closed end of the corresponding outer pipe, and the distal end of the inner pipe is open through at least one first orifice to transfer heated fluid from the inner pipe to the area of the outer pipe proximate to the distal closed end of the outer pipe; and 
     a plurality of first orifices are distributed along the first and second pipes of each heat exchanger tubes, and the fluid supply member comprises a header provided with a plurality of heated fluid inlets distributed along the header. 
     The present invention also relates to a freeze-protected heat exchanger comprising: 
     a fluid supply member for connection to a source of condensable heated fluid, the fluid supply member having a first heat-conductive wall; 
     a drain chamber comprising a drain outlet and enclosing at least a portion of the fluid supply member, the fluid supply member and the drain chamber being substantially elongated, and the fluid supply member extending longitudinally within the drain chamber; and 
     a plurality of heat exchanger tubes extending from the fluid supply member and drain chamber, each heat exchanger tube comprising: 
     a first pipe having a second heat-conductive wall, and a proximal end in fluid communication with the drain chamber; 
     a second pipe coextending with the first pipe, and having a proximal end in fluid communication with the fluid supply member; and 
     at least one orifice through which the first pipe is in fluid communication with the second pipe; 
     In operation, heated fluid is supplied from the fluid supply member to the second pipes, heated fluid from the second pipes is transferred to the respective first pipes through the orifices, heat from the heated fluid in the first pipes is transferred to the outside of the first pipes through the second heat-conductive walls, cooled fluid from the first pipes is collected and drained through the drain chamber and drain outlet, and heat from the heated fluid in the fluid supply member is transferred to the drain chamber through the first heat-conductive wall in view of preventing formation of ice in the drain chamber. 
     Preferably, the fluid supply member and the drain chamber are substantially coaxial to each other. 
     The present invention further relates to a face and by-pass heat exchanger unit comprising the above described freeze-protected heat exchanger. 
     Freezing is prevented by direct contact of heated fluid, for example steam, and cooled fluid, for example condensate in the first pipes and by heating of the drain chamber by radiation, conduction and/or convection provided by the fluid supply member, and by heated fluid jet(s) directed toward the lower end of the drain chamber to prevent formation of ice near the drain outlet of the drain chamber. 
     The present invention presents, amongst others, the following advantages: 
     freezing is prevented in the heat exchanger tubes as well as in the drain chamber and corresponding draining path; 
     the freeze-protected heat exchanger complies with face and by-pass system arrangements for building heating applications as well as in any other type of heat exchanger and can be easily retrofitted into a wide range of existing conventional system units of different types, capacities and sizes; 
     the freeze-protected heat exchanger is automatically drained from cooled fluid when shut-off; 
     the freeze-protected heat exchanger presents a good overall energetic efficiency and an improved capacity/size ratio; 
     the freeze-protected heat exchanger is economical to produce and maintain; 
     the freeze-protected heat exchanger featuring generally horizontally oriented heat exchanging finned tubes connected to a substantially vertical steam supply header; 
     the freeze-protected heat exchanger is functional with a single row of tubes or multiple parallel rows of tubes supplied by a common steam source through a common or separate headers; and 
     the freeze-protected heat exchanger is not subject to disturbance of the steam flow by countercurrent condensate flow. 
     The objects, advantages and other features of the present invention will become more apparent upon reading of the following non restrictive description of a preferred embodiment thereof, given by way of example only with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the appended drawings: 
     FIG. 1 a  is an isometric front view of a face and by-pass heat exchanger heating unit incorporating a freeze-protected heat exchanger according to the present invention; 
     FIG. 1 b  is an isometric rear view of the face and by-pass heat exchanger unit of FIG. 1 a;    
     FIG. 2 a  is an isomeric front view of a freeze-protected heat exchanger according to the present invention, in which fins of the outer tubes are not shown; 
     FIG. 2 b  is a front elevation view of the freeze-protected heat exchanger of FIG. 2 a;    
     FIG. 2 c  is a top view of the freeze-protected heat exchanger of FIG. 2 a;    
     FIG. 3 is a perspective, partly cross sectional view of an upper portion of the freeze-protected heat exchanger of FIGS. 2 a ,  2   b  and  2   c , showing steam distribution and condensate return paths; 
     FIG. 4 is a perspective, partly cross sectional view of a lower portion of the freeze-protected heat exchanger of FIGS. 2 a ,  2   b  and  2   c , showing condensate drain path and the heating steam jets; 
     FIG. 5 a  is a side elevational view of a preferred embodiment of coextending steam supply header and drain chamber forming part of the freeze-protected heat exchanger according to the invention; 
     FIG. 5 b  is an elevational, end view of the coextending steam supply header and drain chamber of FIG. 5 a ; and 
     FIG. 5 c  is a top plan view of the coextending steam supply header and drain chamber of FIGS. 5 a  and  5   b.   
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the appended drawings, similar reference numerals refer to similar parts throughout the various figures. 
     The preferred embodiment of the freeze-protected steam operated heat exchanger according to the present invention will now be described in detail referring to the appended drawings. 
     A face and by-pass heat exchanger unit  100  is illustrated in FIG.  1 . This face and by-pass heat exchanger unit incorporates the preferred embodiment of the freeze-protected heat exchanger  1  (FIG.  2 ). In this preferred embodiment, the freeze-protected heat exchanger  1  is steam operated. Of course, use of any other type of condensable heated fluid could be contemplated. The face and by-pass heat exchanger unit (FIG. 1) comprises a housing  10  in which the freeze-protected heat exchanger  1  (FIG. 2) is installed. 
     Referring to FIG. 1, housing  10  defines a pair of airflow passages  31  and  32  each provided with a remotely adjustable front set of air deflectors  11  (FIG. 1 a ) for: 
     directing a predetermined portion of the incoming air flow (see arrow  25 ) through bundles  5  of heat exchanger tubes  7  forming part of the freeze-protected heat exchanger  1  (better shown in FIGS. 2 a ,  2   b  and  2   c ; and 
     directing the remaining portion of the incoming air flow  25  toward by-pass zones such as  27  located between the bundles  5  of heat exchanger tubes  7 ; 
     and a remotely adjustable rear set of air deflectors  23  (FIG. 1 b ) for: 
     blocking passage of air through the by-pass zones; or 
     blocking passage of air through the bundles  5  of heat exchanger tubes by blocking the exit downstream these bundles  5 . 
     Each bundle  5  comprises at least one vertical row of generally horizontal heat exchanger tubes  7  connected at one end to generally vertical steam supply header  3  and condensate drain chamber  4 . 
     As better shown in FIGS. 2 a  and  3 , the steam supply header  3  is substantially cylindrical and extends substantially vertically and coaxially in the box-like condensate drain chamber  4 . The steam supply header  3  comprises an upper, threaded steam inlet connector  2 . Referring to FIG. 1 a , the steam supply header  3  and the box-like condensate drain chamber  4  are installed in a substantially central closed housing portion  12  of the face and by-pass heat exchanger unit  100 . 
     In FIGS. 1 a ,  1   b ,  2   a ,  2   b  and  2   c  diametrically opposite sets of superposed and substantially radially extending bundles  5  of heat exchanger tubes  7  are illustrated. However, it shall be deemed that in smaller units having less heating capacity, the housing portion  12  and the enclosed steam supply header  3  and drain chamber  4  may be located at one end of the unit  100  comprising a single set of superposed bundles  5  of heat exchanger tubes  7  extending substantially radially from supply header  3 . In this case, to improve distribution of the steam into the inner pipes  13  (FIG. 3) of the superposed bundles  5 , a plurality of steam inlets (not shown) can be provided in the side wall of supply header  3 . Preferably, these steam inlets will be distributed along the length of the header  3  and disposed diametrically opposite to the single set of superposed bundles  5  of heat exchanger tubes  7 . As described hereinafter and as illustrated in FIG. 3, each heat exchanger tube  7  is formed of an heat-conductive outer pipe  26  and an inner pipe  13 . 
     In this type of application, a substantially constant steam flow is established through the steam inlet connector  2  while the temperature of the air emerging downstream of the heat exchanger unit  100  is modulated according to the position of the cooperating series of air deflectors  11  and  23 . Both series of air deflectors  11  and  23  are connected together through connecting rods such as  24  and actuated through an external actuator such as an electric motor (not shown) to operate as follows: 
     in a face mode, the defectors  11  direct the incoming air flow toward the bundles  5  of heat exchanger tubes  7 , while the deflectors  23  block the by-pass zones; and 
     in a by-pass mode, the deflectors  11  direct the incoming air toward the by-pass zones, while the deflectors  23  block the exit downstream the bundles  5  of heat exchanger tubes  7 . 
     Intermediate positions of the deflectors  11  and  23  may be adopted by the face and by-pass heat exchanger unit  100  under the control of the external actuator so as to modulate the proportion of air flowing through the bundles  5  of heat exchanger tubes  7  and being heated by the heat exchanger  1 , thus controlling the average temperature of the air flow downstream the face and by-pass heat exchanger unit  100 . 
     The housing portion  12  provides some protection of the condensate drain chamber  4  against contact by incoming cold air and can be filled with insulating material to further improve insulating properties. FIGS. 2 a  and  2   b  illustrate a generally vertical condensate drain pipe  8  extending from the bottom of the condensate drain chamber  4 . FIGS.  2   a  and  2   b  also illustrate a threaded condensate outlet connector  9  of the condensate drain pipe  8 . 
     FIG. 3 illustrates the upper portion of the freeze-protected heat exchanger  1  showing the structure of the steam distribution and condensate return paths. Steam is supplied through the inlet connector  2  of the steam supply header  3 . The inner pipes  13  of the heat exchanger tubes  7  are each provided with two diametrically opposite series of orifices  14  distributed therealong. The inner pipes  13  extend generally horizontally and radially from the steam supply header  3  and are in fluid communication therewith (see openings such as  28 ). Each inner pipe  13  therefore extends through a wall of the condensate drain chamber  4  and is mounted in a corresponding outer pipe  26  coaxially therewith with an annular spacing between the inner  13  and outer  26  pipes. On the other hand, each outer pipe  26  is heat-conductive and provided with a rigid heat-conductive integral helical extruded fin  15  to enhance heat transfer from the heat-conductive wall of the output pipe  26  to the airflow  25 . Also, each outer pipe  26  has a distal closed free end  29  and a proximal end  30  opening in the condensate drain chamber  4 . More specifically, the proximal end  30  of each outer pipe  26  is connected to and extends through a side wall of the condensate drain chamber  4 , in fluid communication therewith. As illustrated, the inner pipes  13  extend into the respective outer pipes  26  up to a few inches short from the distal closed free ends  29 . These inner pipes  13  preferably comprise respective axial end orifices  21  to produce axial steam jets  22  toward the closed free ends of the respective outer pipes  26 . All the inner  13  and outer  26  pipes are slightly sloping downwardly toward the condensate drain chamber  4  to assure proper draining of the condensate  19  from the outer pipes  26  in the chamber  4  by gravity. A slope of the order of 2% fulfills this purpose. 
     Those of ordinary skill in the art will appreciate that the steam supplied by a steam source (not shown) through inlet connector  2  to the steam supply header  3  is distributed in the inner pipes  13  and subsequently transferred to the outer pipes  26  through the orifices  14  and  21 . Again, it shall be noted that in large units comprising many superposed bundles  5  of heat exchanger tubes  7 , more than one steam inlet can be provided along steam supply header  3  to better balance the distribution of steam into the inner pipes  13 . Upon contact with the inner side of the air-cooled wall of finned outer pipes  26 , heat from the steam is transferred to the airflow  25  through the finned outer pipes  26  and the steam condenses and flows by gravity as condensate  19  toward the drain chamber  4 , rain-like spread falling along the walls thereof toward the bottom  20  (FIG. 4) of that chamber. Each row of heat exchanger tubes  7  in such an arrangement provides about twice the heat-transfer capacity of a conventional U-shaped tube design, thus reducing the size and cost for a face and by-pass heat exchanger unit  100  of given capacity. 
     The internal volume and the walls of the condensate drain chamber  4  are submitted to some heating from the steam supply header  3 , thus preventing sub-cooling of the condensate and formation of ice in the chamber  4  or at the outlet (proximal ends  30 ) of the outer pipes  26 . Moreover, the rigid extruded fins  15  provide the outer tubes  26  with a high resistance to dilatation which contribute to further prevent formation of ice. Although integral, extruded fins  15  are preferred, use of some other fin configuration such as flat or corrugated plates, or flat or corrugated rectangular individual fins of an overlapped or footed “L” design could be contemplated with acceptable results. 
     FIG. 4 illustrates the lower portion of the freeze-protected heat exchanger  1  to show the structure of the condensate drain path. The condensate  19  dripping along the internal walls of drain chamber  4  hits the bottom  20  and flows through an inlet  18  of the condensate drain pipe  8  and is returned to the steam trap and remaining components of the system (not shown) via the threaded condensate outlet connector  9 . Two jets of steam  16   a  and  16   b  are respectively escaping from two small orifices  17   a  and  17   b  of diameter depending on the pressure of the steam supply, preferably provided in the bottom wall  31  of the steam supply header  3  and so positioned as to direct these steam jets  16   a  and  16   b  preferably toward the front (cold air side) corners of the bottom  20  of the condensate drain chamber  4  thus avoiding any build-up of ice at the bottom  20  and at the inlet  18  of the condensate drain pipe  8 . The orifices  17   a  and  17   b  also serve to drain the condensed steam from the steam supply header  3  when the steam-producing heating device (not shown) is shut-off and the steam flow  32  is interrupted at the steam inlet connector  2 . 
     Alternatively, more than two orifices such as  17   a  and  17   b  can be provided to produce more than two corresponding jets of steam such as  16   a  and  16   b.    
     In the case of the two orifices  17   a  and  17   b , these orifices can be positioned at a higher level on the vertical and cylindrical wall of the header  3  to both heat and prevent build-up of ice throughout the entire drain chamber  4 . In the case of a number of orifices larger than 2, the orifices can be distributed vertically on the vertical, cylindrical wall of the header  3  again to both heat and prevent build-up of ice throughout the entire drain chamber  4 . 
     Furthermore, a closure member (not shown) can be provided for manually or automatically controlling the opening and closing of the orifices as a function of different operating conditions such as external air temperature. 
     FIGS. 5 a ,  5   b , and  5   c  illustrate an alternative embodiment  50  of the freeze-protected heat exchanger  1  showing the structure of the steam distribution and condensate return paths. 
     The embodiment  50  of FIGS. 5 a ,  5   b  and  5   c  comprises a steam supply header  52  and a condensate drain chamber  53  formed of a vertical tube  55  with a closed top end  56 . A central vertical flat, heat-conductive wall  57  separates the vertical tube  55  into two halves of which one forms the header  52  and the other the drain chamber  53 . The steam supply header  52  has closed top and bottom ends, while the drain chamber  53  has a closed top end but a bottom end  54  open to form a drain outlet  58 . 
     Steam is supplied through an inlet connector  51  of the steam supply header  52 . As illustrated, inlet connector  51  is threaded for connection to a steam source (not shown). The inner pipes  13  of the heat exchanger tubes  7  are still provided with the two diametrically opposite series of orifices  14  (see FIGS. 5 c ) distributed therealong. The inner pipes  13  extend generally horizontally and radially from the steam supply header  52  and are in fluid communication therewith (see portions of inner pipes  13  extending through the drain chamber  53 ). Each inner pipe  13  therefore extends through a wall of the condensate drain chamber  53  and is mounted in a corresponding outer pipe  26  coaxially therewith with an annular spacing between the inner  13  and outer  26  pipes. On the other hand, each outer pipe  26  is heat-conductive and provided with a rigid heat-conductive integral helical extruded fin  15  to enhance heat transfer from the heat-conductive wall of the output pipe  26  to the airflow  25 . Also, each outer pipe  26  has a distal closed free end  29  and a proximal end  30  opening in the condensate drain chamber  53 . More specifically, the proximal end  30  of each outer pipe  26  is connected to and extends through a side wall of the condensate drain chamber  53 , in fluid communication therewith. As illustrated in FIG. 5 c , the inner pipes  13  extend into the respective outer pipes  26  up to a few inches short from the distal closed free ends  29 . These inner pipes  13  preferably comprise respective axial end orifices  21  to produce axial steam jets  22  toward the closed free ends of the respective outer pipes  26 . All the inner  13  and outer  26  pipes are slightly sloping downwardly toward the condensate drain chamber  53  to assure proper draining of the condensate from the outer pipes  26  in the chamber  53  by gravity. A slope of the order of 2% fulfills this purpose. 
     Those of ordinary skill in the art will appreciate that the steam supplied by a steam source (not shown) through the inlet connector  51  to the steam supply header  52  is distributed in the inner pipes  13  and subsequently transferred to the outer pipes  26  through the orifices  14  and  21 . Again, it shall be noted that in large units comprising many superposed bundles  5  of heat exchanger tubes  7 , more than one steam inlet such as  51  can be provided along steam supply header  52  to better balance the distribution of steam into the inner pipes  13 . Upon contact with the inner side of the air-cooled wall of finned outer pipes  26 , heat from the steam is transferred to the airflow  25  through the finned outer pipes  26  and the steam condenses and flows by gravity as condensate toward the drain chamber  53 , rain-like spread falling along the walls thereof toward the bottom end  54  and drain outlet  58  (FIG. 5 a ) of that chamber. Each row of heat exchanger tubes  7  in such an arrangement provides about twice the heat-transfer capacity of a conventional U-shaped tube design, thus reducing the size and cost for a face and by-pass heat exchanger unit  100  of given capacity. 
     The internal volume and the walls of the condensate drain chamber  53  are submitted to some heating through the heat-conductive wall  57  from the steam supply header  52 , thus preventing sub-cooling of the condensate and formation of ice in the chamber  53  or at the outlet (proximal ends  30 ) of the outer pipes  26 . Moreover, the rigid extruded fins  15  provide the outer tubes  26  with a high resistance to dilatation which contribute to further prevent formation of ice. Although integral, extruded fins  15  are preferred, use of some other fin configuration such as flat or corrugated plates, or flat or corrugated rectangular individual fins of an overlapped or footed “L” design could be contemplated with acceptable results. 
     The condensate dripping along the internal walls of drain chamber  53  hits the bottom  54  and flows through the drain outlet  58  and is returned to the steam trap or remaining components of the system (not shown) via this drain outlet  58 . Drain outlet  58  is threaded for connection to the steam trap or remaining components of the system. At least one jet of steam  59  escapes from a small orifice  60  of a diameter depending on the pressure of the steam supply, preferably provided in the lower portion of wall  57  of the steam supply header  52  and so positioned as to direct this steam jet  59  preferably toward a cold air side corner  61  of the bottom  54  of the condensate drain chamber  53  thus avoiding any build-up of ice at the bottom  54  and at the drain outlet  58 . The orifice  60  also serves to drain the condensed steam from the steam supply header  52  when the steam source (not shown) is shut-off and the steam flow is interrupted at the steam inlet  51 . 
     Alternatively, a plurality of orifices such as  60  can be provided to produce a plurality of corresponding jets of steam such as  59 . 
     In the case of the single orifice  60 , this orifice can be positioned at a higher level on the wall  57  to both heat and prevent build-up of ice throughout the entire drain chamber  53 . In the case of a plurality of orifices such as  60 , the orifices can be distributed vertically on the wall  57  again to both heat and prevent build-up of ice throughout the entire drain chamber  53 . 
     Furthermore, a closure member (not shown) can be provided for manually or automatically controlling the opening and closing of the single or plurality of orifices such as  60 , as a function of different operating conditions such as external air temperature. 
     Therefore, it will be apparent to those of ordinary skill in the art that the freeze-protected heat exchanger  1  of the present invention can be advantageously used for efficiently transferring heat from a steam flow  32  to an air flow  25  potentially below the freezing point of water, without causing damages or malfunctions due to freezing of steam condensate, thus overcoming the drawbacks of the prior art devices. 
     Although the present invention has been described hereinabove by way of a preferred embodiment thereof, this embodiment can be modified at will, within the scope of the appended claims, without departing from the spirit and nature of the subject invention. 
     For instance, it would be obvious for one of ordinary skill in the art to use the freeze-protected heat exchanger of the present invention with a different arrangement of bundles and rows of tubes, in a wide range of sizes and power capacities and/or to use two units forming a A-shaped condenser for condensing steam or other condensable heated fluid at the outlet of turbines in power plants. Moreover, the heat exchanger can be retrofitted into many types of existing units.