US 3895675 A
A heat exchanger for use in intermittent flow situations in breathing apparatus is disclosed that includes a plurality of thermally conductive rod members extending through a septum member that divides a duct into a plurality of counter flow passages. The exchanger is uniquely adapted to efficient heat exchange in intermittent gas flow situations by the provision of a septum member that is relatively low in thermal energy storage capacity and relatively low in thermal conductivity in directions parallel to flow in the passages.
Description (OCR text may contain errors)
United States Patent 1 Rein et al.
[ 1 BREATHING GAS HEAT EXCHANGER  Inventors: Charles R. Rein; Charles B. Jones.
Jr., both of Panama City. Fla.
 Assignee; The United States of America as represented by the Secretary of the Navy, Washington. DC.
 Filed: Aug. 15,1973
 Appl. No.: 388,704
 US. Cl. 165/164; 128/212; 165/179  Int. Cl F281 1/42; F28d 1/09  Field of Search 165/164, 46, 179, I65, 165/66; 2/2.l R; 128/146. 212
 References Cited UNITED STATES PATENTS 2,610,038 9/1952 Phillips 165/66 X 3,409,075 11/1968 Long .1 165/179 3,477,504 11/1969 Colyer et a1. 165/164 FOREIGN PATENTS OR APPLICATIONS 231,124 5/1944 Switzerland 165/179 1 July 22, 1975 Primary E.raminerCharles .1. Myhre Assistant ExaminerTheophil W. Streule, Jr. Attorney. Agent, or Firm-Richard S. Sciascia; Don D. Doty; Harvey A. David  ABSTRACT A heat exchanger for use in intermittent flow situations in breathing apparatus is disclosed that includes a plurality of thermally conductive rod members extending through a septum member that divides a duct into a plurality of counter flow passages. The exchanger is uniquely adapted to efficient heat exchange in intermittent gas flow situations by the provision ofa septum member that is relatively low in thermal energy storage capacity and relatively low in thermal conductivity in directions parallel to flow in the passages.
4 Claims, 6 Drawing Figures 3.895575 PATENTEDJUL 22 ms FIG: 4
BREATHING GAS HEAT EXCHANGER STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefor.
FIELD OF THE INVENTION This invention relates to conservation of thermal energy in breathing apparatus. for example for underwater use. and more particularly to an improved. intermittent or alternate flow heat exchanger therefor.
One of the more pressing problems related to life supporting breathing apparatus is the need for maintaining the user's body within the temperature range for comfort and well being. In the case of diving in cold waters. a diver needs to conserve body heat wherever possible. However. a substantial amount of body heat is normally lost to respiratory gases. cold breathing gas being breathed in. and warmed gas being breathed out. Ev en in closed circuit systems. wherein most of the gas is rc-hreathed after regeneration by carbon dioxide removal and oxygen enrichment. heat is dissipated during the regeneration. In severe situations the core temperature of the diver can be rapidly decreased until the diver is incapacitated. Continued exposure is fatal.
The problem is accentuated in breathing apparatus wherein supplies of breathing gases. e.g.. oxygen. are stored in liquified condition. This provides advantages of small volume required for storage. absence of high storage pressures and the associated heavy pressure vessels. and the possibility of controlling the partial pressure of gaseous oxygen in a mixed gas breathing mixture within the limits necessary for life support at various depths by controlling the temperature of the liquid oxygen. One example of such an apparatus is found in US. patent application Ser. No. 337.782, filed Mar. 5. 1973 now US. Pat. No. 3,831,594 granted Aug. 27. 1974. entitled Life Support System. and assigned to the assignee hereof.
In such apparatus. which are normally closed circuit devices with respect to the diluent portion (eg. helium of the breathing mixture. oxygen gas from the liquid oxygen supply is utilized for oxygen enrichment of exhaled gases to maintain a desired oxygen partial pressure in the mixture to be breathed. Because the gaseous oxygen developed from the liquid oxygen supply is quite cold. it is necessary to warm the breathing gas mixture of which it becomes a part. To this end. it has been proposed to effect an exchange of heat from exhaled gases to the oxygenated breathing gas mixture. thereby conserving thermal energy. In doing so. an important factor resides in the intermittent flow nature of movement of the gases through the heat exchanger to be used. As will be discussed more fully in the following description of a preferred embodiment. the heat exchanger must minimize any capacity to store heat in its travel from the warming gases to the warmed gases because to do so in the case of intermittent gas flow results in an efficiency reducing heat filter effect. Also. any tendency for heat to travel parallel to the direction of fluid flow must be minimized as such a tendency constitutes a leak or loss of thermal energy to the systcm.
DISCUSSION OF THE PRIOR ART Various heat exchange devices have been devised that recognize the use of a dividing wall or septum member between fluids flowing in opposite directions parallel to the wall. and incorporating thermally conductive elements such as fins or rods extending through the wall and into the fluid streams on either side thereof. Examples of such heat exchangers are found in U.S. Pat. No. 2.294.137 to W. A. Spofford. US. Pat. No. 2.873.354 to T. E. Shoup. and US. Patv No. 3.103.971 to H. A. Freyholdt. These patents. however. each provide a dividing wall that is characterized by heings formed of or incorporating. metallic heat conducting material that encourages the flow of heat in directions parallel to the fluid flow. and also that serves as a heat capacitance that would be markedly detrimental to heat exchange efficiency were the fluid flow to be intermittent. For these reasons. heat exchangers according to the principles exemplified by those patents are inefficient for use in an intermittent flow. life support system such as those referred to above.
SUMMARY OF THE INVENTION The invention aims to overcome the aforementioned disadvantages of the prior art heat exchangers as they pertain to use in a low pressure gradient. intermittent flow system such as a cryogenic underwater breathing apparatus or other life support system.
Accordingly. it is a principal object to provide an improved intermittent flow heat exchanger.
Another object is the provision of an efficient and compact heat exchanger for utilizing heat from fluid in a first stream to warm fluid in a second stream. when the streams are characterized as intermittently flowing.
Yet another object is the provision of an improved heat exchanger that is unusually well suited to use in an environmental life support system of the closed or semiclosed types in that it is efficient. lightweight. compact. and inexpensive.
As another object the invention aims to accomplish the foregoing through the provision of a duct that is divided into a plurality of flow passages by one or more septum members that are formed of a material that is relatively thermally insulating and is free of any components or portions that would tend to encourage transfer of heat therealong in a direction parallel to the direction or directions of flow of heat exchange fluids. and a plurality of heat transfer members such as metallic fins or rods extending through the septum members into conductive relation with fluid standing or flowing in said passages.
Other objects and many of the attendant advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description. when considered in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWING FIG. I is a longitudinal view. partly in elevation and partly in section of an improved heat exchanger embodying the invention;
FIG. 2 is an end elevational view of the heat exchanger of FIG. I;
FIG. 3 is a fragmentary sectional view. on an enlarged scale. taken along section line 3-3 of FIG. 2;
FIG. 4 is an enlarged fragmentary sectional view taken substantially along section line -l4 of FIG. 3;
H6. 5 is an enlarged fragmentary sectional view taken substantially along section line 5-5 of FlG. l'. and
HG. 6 illustrates a method of manufacture according to the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT In the form of the invention illustrated in the draw ings and described hereinafter. there is provided an improved intermittent flow heat exchanger. generally indicated at l0 in FIG. I. that is particularly well suited to use in breathing apparatus wherein cryogenic gas supplies are employed and heat exchange must take place between gases that are moved in a periodic manner. as by breathing. Heat exchanger 10 comprises. in this example a duct 12 that may be described as an oblate cylinder in shape and. as may be seen from the end view of FIG. 2. comprises curved wall portions. Duct 12. which may have other sectional configurations if desired. is formed of a rigid material having thermal in sulating properties as compared to metal. The pressure differential that exists across the walls of duct 12 in the intended use are quite small. although the entire exchanger may be operated in an environment where pressures are substantial. such as in deep diving apparatus. A suitable material for the walls ofduct l2. consid ering its relatively low thermal conductivity as com pared to metals is fiberglass reinforced plastic. usually a polyester resin.
Duct 12. which has a generally uniform cross section throughout its length. is divided by one or more septum members into a plurality of gas carrying passages. In the present example two such septum members 14 and I6 are provided in spaced. parallel relation to one another. and span the width of the duct. The walls of the duct. and the spaced septum members thereby define three parallel gas passages. a control passage 18. and two side passages and 22. When in use. passage 18 carries intermittently flowing gases in the direction indicated by arrow 26. while passages 20 and 22 carry intermittently flowing gases in the directions of arrows 28 and 30. respectively.
Passing through septum members 14 and 16, and extending into and across passages 18, 20, and 22 are a plurality of elongate. spaced. parallel heat conducting members in the form of metal wires or rods 34. Rods 34 are. in the present embodiment. of circular section and are disposed in evenly spaced rows and columns as is best shown in FIG. 4. In one practical working embodiment for a cryogenic breathing apparatus. a duct 12 was used having a length of about 12 inches. a minor transverse dimension of about 3.5 inches. and a major transverse dimension of about 4.5 inches. Solid copper wire rods 34 were used having diameters of 0.043 inch and disposed at the corners of squares that were 0.10 inch on a wide. thereby being arranged in evenly spaced rows and columns. The rod density was. there fore. on the order of l00/sq. inch.
As will be made apparent later in this description. a particularly important feature of the invention is that septum members [4 and 16 are formed ofa material of low thermal conductivity and of low thermal capacity (specific heat). Suffice it to say for now that these two characteristics. conductivity and capacity. have respectively to do with the ability of the material to ass thermal energy and the ability to store thermal energy. A suitable septum member material for the mentioned working embodiment was found to be flat. fiberglass reinforced polyester plastic panels of the type used for small electrical circuit boards and having preformed holes of the desired spacing through which rods 34 were inserted with a force fit.
It will be noted from FIG. 5. that the ends of rods 34 enter a small distance into the wall ofduct [2. whereby the rods are structurally supported against becoming bent or distorted. while at the same time the rod ends are insulated against thermal leakage to the exterior of heat exchanger 10.
A preferred method of manufacture of the heat exchanger comprises a plurality of steps including inserting thermally conductive rod members through one or more thermally insulating panels in a predetermined pattern so as to provide a septum member and conductive rod member subcombination 40 (see FIG. 6]. wherein the opposite ends of said rods 34 define loci of predetermined curved surfaces. Another step is to prepare a mole 42 having a mold surface 42a of substan tially the same curvature as said predetermined curved surfaces by applying on said mold surface reinforcing liber and plastic resin materials 44 in an uncured state. The septum member and conductive rod subcombination 40 is then placed as shown in FIG. 6 with the ends of rods 43 embedded and in the plastic resin material which is then allowed to harden or cure into a selfsustaining shape as the wall portion 121: of duct 12.
The septum member and conductive rod member combination. together with the wall portion in which the rod ends are embedded. is removed from mold 42 which is then prepared with more uncured fiber and resin materials. The opposite ends of rods 34 are then embedded in the fiber and plastic materials which are allowed to harden so as to form another curved wall portion 12b of duct 12.
Thereafter. opposite flat wall portions 12c and 12d of duct 12 are formed in a similar manner by using a plane molding surface to complete the duct walls with plane portions and 12d. The resulting structure is light. strong enough for the purpose contemplated. and very effective for intermittent gas flow heat exchange.
MODE OF OPERATION As mentioned earlier. one of the notable features of the invention. and one by which it is distinguished from other heat exchange structures. is the use of dividing walls or septum members that are low in heat capacity and heat transmission. Consider the use of the heat exchanger 10 in an intermittent flow system such as a breathing apparatus. The efficiency of a typical rapid breathing cycle may be taken as having a duration of 1.6 seconds. with the breathing flow occurring during the first 0.8 seconds and substantially no flow during the remaining 0.8 seconds.
This intermittent breathing flow. in the types of equipment to which heat exchangers according to this invention are applicable. is applied to the passages 18, 20. and 22 in either of two ways depending upon the location of the heat exchanger with respect to one or more breathing or gas accumulation bags in the circuit. Without going into the details of the basic circuits both of the open and the closed types which are well known to those skilled in the art to which the invention pertains. the first situation in which the invention may be advantageously used is where the gases in passage 18 and in passages 20 and 22 are substantially synchronized in their periods of flo and non-flow. The second situation in which the invention may be advantageously used is where the gases in passage l8 and the gases in passages 20 and 22 alternate in their periods of flow and non-flow. It will be recognized that each situation may be characterized as intermittent flow. Accordingly the first situation may be conveniently identified as simultaneous passage intermittent flow and the second situation as alternate passage intermittent flow.
Considering first simultaneous passage intermittent flow, during the flow-half of the breathing cycle heat is transferred by convection from the warmer gas in passage 18 to rods 34. Meanwhile, heat is carried by conduction along the rods, through the septum members 14, I6, and transferred by convection to the cooler gas moving in counter flow in passages 20, 22. During the non-flowing half of the cycle. heat continues to be transferred from the warmer to the cooler gas wholly by conduction from gas to rod. along the rod, and rod to gas. The system is most efficient during the flow half of the cycle. Now, if the septum members were provided with a high heat storage capacity, thermal energy received by the rod members in passage 18 would tend to enter storage in the septum members during the flow half of the cycle and to leave such storage during the non-flowing half cycle. However, the thermal energy transfer effected from such storage would be between zones of lesser thermal potential than occurs during the flow-half cycle. The net result would be a time smoothing and essentially heat transfer blocking filter action. Hence thermal storage capacity within the septum members is undesirable.
[n the alternate passage intermittent flow situation the presence of heat storage capacity in the septum member would be even less desirable as the heat transfer blocking filter effect would be applied sequentially to each side of the exchange cycle.
While the invention has been described with reference to the presently preferred embodiment having a central passage for flow in a first direction, and two side passages for flow in the opposite direction, it will be recognized that the principle involved may be used to advantage with as few as two passages or with more than three passages.
From the foregoing, it will be appreciated that the invention represents a departure from existing heat exchanger construction that is advantageous in the rather unique situations of intermittent flow found in breathing apparatus of the character referred to.
Obviously, other embodiments and modifications of the subject invention will readily come to the mind of one skilled in the art having the benefit of the teachings presented in the foregoing description and the drawings. For example. the thermally conductive elements could be in the form of flat strips or fins, or rods 34 could be provided with projections or fins to increase their effective area. it is. therefore. to be understood that this invention is not to be limited thereto and that said modifications and embodiments are intended to be included within the scope of the appended claims.
What is claimed is:
1. An intermittent flow heat exchanger for use in a breathing system wherein gas to be breathed is to be heated to higher temperatures by exchange of heat from gas that has been breathed. and wherein said gases are each moved intermittently so as to have flow periods and non-flowing periods, said exchanger comprising:
an elongated duct having walls formed of rigid. thermally insulating material. said duct having a substantially uniform cross section from end-mend;
septum means. disposed in said duct. for dividing space within said duct into a plurality of passages extending lengthwise of said duct. said septum means comprising a thin panel formed of rigid. thermally insulating material free of relatively heat conductive material lying in a plane extending parallel to said directions of flow.
a plurality of elongated. thermally conductive metal wires extending through said septum means at right angles thereto, and transversely across said passages said metal wires being received in pre-formed holes extending through said panel. and effectively obturating said holes;
said panel together with the portions of said metal wires that are covered thereby being characterized as having low thermal capacity whereby thermal energy applied to said metal wires in a first passage during one of said flow periods for said gas that has been breathed is conducted. substantially without capacitive delay, to said gas to be breathed in a second of said passages.
2. A heat exchanger as defined in claim I, and
said wires have their opposite ends embedded in said duct walls. whereby they are supported against distortion and insulated against heat loss therefrom to the exterior of said duct.
3. A heat exchanger for intermittent gas flow heat exchange in breathing apparatus. said heat exchanger comprising:
an elongated duct having a uniform cross-section along its length. said duct comprising walls formed of rigid, thermally insulating material;
first and second septum members, each formed of panels of rigid, thermally insulating material. said septum members being in spaced, parallel relation and dividing said duct into first, second, and third parallel passages;
a plurality of wire, heat conducting fins each having a uniform cross-sectional configuration from end to end and each extending through both of said septum members at right angles thereto, and across said first, second, and third passages, said fins each having its opposite ends embedded in said duct walls;
said septum members being further characterized by lack of heat conductive and heat capacitive capabilities in zones penetrated by, or in thermal communication with, said fins.
4. A heat exchanger as defined in claim 3. and
said wire, heat conducting fins are in spaced. parallel relation to one another and are arranged in columns and rows of equal spacing; and
said wire, heat conducting fins having a density of about per square inch of said septum members.