US2431228A

US2431228A - Heat exchange unit

Info

Publication number: US2431228A
Application number: US597428A
Authority: US
Inventors: Burgess Russell Harvey
Original assignee: Individual
Current assignee: Individual
Priority date: 1945-06-04
Filing date: 1945-06-04
Publication date: 1947-11-18
Anticipated expiration: 1964-11-18

Description

1947- R. H. BURGESS HEAT EXCHANGE UNIT Filed June 4, 1945 Fi'qzz.

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Patented Nov. 18, 1947 UNITED STATES PATENT OFFICE HEAT EXCHANGE UNIT Russell Harvey Burgess, Chicago, 111.

Application June 4, 1945, Serial No. 597,428

4 Claim's.

This invention relates to a heat exchange unit, the principles of which are applicable to a refrigerator evaporator unit, a refrigerator condenser unit, or a cooling unit in air conditioning systems and the like. More particularly, the invention relates to an evaporator unit for use in a refrigerating system employing a normally gaseous refrigerant.

In the conventional type of evaporator as used by the commercial refrigeration and/or air conditioning industry today, tubes of circular cross section are generally used for the flow of the refrigerant therethrough. In the gravity flow, direct expansion type of evaporator unit, the refrigerant in liquid state is introduced into the uppermost tube or tubes, for evaporation therein and for gravity flow of the refrigerant, while partly in liquid and partly in gaseous state, to lower rows of tubes. The heat transfer is effected through the walls of the tubes between the external flow of air or other gas about the tubes and the refrigerant within the tubes.

The factors that should be considered in evaporator design are the following:

1. Outside air film at prime surface.

2. Thermal conductivity of tube or prime surface.

3. Inside refrigerant film on prime surface.

4. Resistanceto air flow.

5..Velocity of air through or over evaporator tubes or units.

6. Fan load, in the case of forced convection jobs.

7. Area of prime surface.

8. Refrigerant-holding capacity of evaporator.

9. Free gas area inside evaporator.

10. Velocity of liquid through evaporator.

11. Velocity of gas through evaporator.

12. Oil flow restriction.

13. Gas flow restriction.

14. Liquid flow restriction.

If each of the foregoing factors be analyzed, it will be found that conventional evaportor designs violate some of the physical laws governing aerodynamic and thermodynamic principles as applied to a conventional gravity or forced convection commercial evaporator. One such violation is the use of round tubes without efficient liquid flow restriction, as the prime conductor in the evaporator unit, whether such tubes be connected in series or in parallel and regardless of how the refrigerant is fed to the evaporator.

From an aerodynamic viewpoint, the round or circular tube is inefficient as compared with a 2 streamlined tube, such as a tube of tear-drop shape in cross-section, since the round tube has about fifteen times the resistance to air flow that the streamlined tube has. By using a streamlined tube in the case of forced convection units,

the load on the fan is greatly decreased; Fur-' thermore, whether of the forced convection or natural convection type, the thermal transmission efilciency of heat from the air to the refrigerant in the case of streamlined tubes is greatly increased by reason of decreasing the air film resistance thermally on the exterior surfaces of the tubes. This latter is accomplished by eliminating dead air films surrounding the tube surface.

In any evaporator unit the liquid refrigerant must be converted from a liquid into a gas in passing through the unit, if of the direct expansion type. In the case of dichlorodifiouromethane, a pound of this refrigerant (under standard-ton conditions) in liquid state has a volume of 0.0111 cubic feet, whereas the same pound in gaseous state, at the same temperature and pressure in equilibrium with some of the compound in liquid state, has a volume of 1.485 cubic feet. In a direct expansion evaporator it is possible to occupy approximately 50% of the internal volume of the evaporator with liquid refrigerant during operation thereof. If the liquid were uniformly distributed throughout the evaporator, the velocity of the gas would be more than 130 times that of the liquid for the liquid for the same rate of flow by weight. Due, however, to the cohesive attraction of like molecules of any matter, there is a tendency for the gas to carry the liquid through the evaporator tubes at the same velocity as that at which the gas should travel. If this should happen there would be no evaporation of the liquid and therefore no refrigeration, unless the refrigerant entered the evaporator at a lower temperature than the temperature of the substance to be cooled.

If an evaporator is to accomplish the desired results, it is necessary, then, that the gas and liquid travel through the evaporator tubes at dif ferent velocities. The greater the difference between the two, the higher will be the rate of evaporation and the rate of heat transmission. If there is a free gas area above the surface of the liquid, a higher gas velocity will be obtained and there will be little danger of slugs of liquid being carried along with the gas through the tubes.

According to the principles of my present invention. advantage is taken of known thermodynamic and aerodynamic principles in order to obtain a higher thermal efliciency in the transfer of heat from the circumambient air or other gas and the refrigerant flowing through the evaporator tubes. Each of the tubes is streamlined to reduce its resistance to air flow. By the term "stream1ined is meant a surface so contoured as to offer the minimum amount of resistance to the flow of air over and past the surface. In general, a streamlined tube will be one having the shape of a tear-drop in cross-section. For instance, if W represents the greatest width of the section, then the greatest length of the section, L, should equal 3W. For most efficient spacing of the streamlined tubes, the minimum distance between laterally spaced tubes at the same elevation should be the same as the maximum width of the tube sections, and the minimum spacing between adjacent tubes in adjoining vertically spaced tiers should be about one-half such width, or

Further, in accordance with my present invention, the streamlined tubes are provided with end fittings having nipples Or extensions which may be circular or elliptical in cross section. Such extensions joined integrally to the tube ends and are connected at points adjacent the wider edges of the tubes, preferably so as to form continuations of the wider edges of the tubes thatare substantially flush with such edges. The end nipples or extensions of the tubes are thus spaced from the thinner edges of the tubes, which constitute the lower edges when the tubes are arranged in a refrigeration evaporator. By virtue of this arrangement, therefore, the liquid portion of the refrigerant is trapped in the lower portions of the tubes, while the gaseous portion of the refrigerant is free to pass at relatively high velocity from one tube to the next. Preferably, end fittings connect corresponding upper end portions of the tubes in vertically adjacent tiers so as to serve primarily for gas fiow communication between the tubes while causing the liquid refrigerant to be trapped in the lower portions of the tubes. There is therefore little likelihood of slugs of liquid refrigerant becoming entrained in the gas fiow through the tubes of the evaporator unit.

It is therefore an important object of this invention to provide a heat exchange unit, such as a refrigeration evaporator unit, in which the refrigerant tubes are streamlined and arranged in a plurality of horizontal tiers, with the tubes in vertically adjacent tiers staggered so as to present the minimum resistance to the flow of air downwardly over and past the tubes, whereby an efficient rate of heat transfer is obtained, both in natural convection and in forced convection installations.

It is a further important object of this invention to provide a refrigeration evaporator unit in which the refrigerant tubes are streamlined, with their wider edges constituting the leading edges, and to provide end fittings connecting the upper portions of the ends of such tubes, whereby liquid refrigerant is entrapped in the lower portions of the tubes and gaseous refrigerant is permitted to flow freely at a velocity independent of that of the liquid refrigerant from one tube to the next in the series.

It is a further important object of this invention to provide a refrigeration evaporator unit comprising a plurality of horizontal tiers of tubes,

with the vertically adjacent tubes arranged in staggered relationship and with the tubes each of a streamlined contour so as to offer the minimum resistance to air fiow through the unit, the tubes in vertically adjacent sections being connected individually by end fittings joining the top portions only of the tubes, thereby causing the liquid portion of the refrigerant to be largely trapped in the bottom portions of the tubes and insuring more rapid evaporation and a higher velocity of the gaseous portion of the refrigerant through the unit.

Other and further important objects of my invention will become apparent from a consideration of the following detailed description, taken in conjunction with the accompanying drawings.

On the drawings:

Figure 1 is an end elevational view of a refrigeration evaporator unit embodying the principles of my invention.

Figure 2 is a broken side elevational view of the unit.

Figure 3 is a longitudinal sectional view of one of the tubes with its end fittings.

Figure 4 is a perspective view of an end fitting for brazing or otherwise securing to an end of a tube.

Figure 5 is a perspective view of a modified form of end fitting.

The reference numeral It) indicates generally a refrigeration evaporator unit embodying the principles of my invention. While the invention will be described as applied to the construction of an evaporator unit, it will be understood that the principles of my invention are broadly applicable to all types of heat exchangers involving the flow of a fluid that is partly in liquid and partly in gaseous form through a closed path, such as defined by individuall connected tubes, for effecting a heat transfer between the fluid passing through the tubes and the circumambient air or gases passing into contact with the external surfaces of the tubes.

The refrigeration evaporator unit l0 comprises a plurality of horizontal tiers T, T1, T2 and T3 of tubes H. Said tubes II are arranged in laterally spaced relationship in the individual tiers, with the tubes of one tier staggered with relation to the vertically adjacent tubes in the next adjoining tiers. Each of the tubes l I is streamlined so as to offer the minimum of resistance to the flow of air, or other gas, through the unit H].

In steamlining the tubes and in arranging the tubes in spaced relation in the unit [0, certain mathematical formulae are closely adhered to for the purpose of reducing the air resistance of the unit to a minimum. For instance, if L designates the longer dimension of a section of a tube I I and W the greater width of a section of the tube ll, then the relationship existing can be expressed by the equation L=W.r3, or

By employing these mathematical relationships in the construction of a refrigeration evaporator of course, be downwardly,

tier of tubes.

. other gas, passing through the unit. The direction of the flow of air through the unit, l will, whether the uni-tis used ina natural convection system or in aforced convection system.- Accordingly, the wider upper edges l2 of the tubes I will constitute the leading edges in terms of the flow or air through'the unit.

Each' of the tubes consists of a straight,

open-ended length of tubing of the steamlined character already referred to.. The tubes may be formed of any of the materials commonly used in the construction of evaporator tubes, such as copper, brass, stainless steel and the like. ably, the tubes are formed of copper because of its high coefficient of heat transfer. The walls of the tubes are made as thin as a proper factor of safetywill permit. i r

Each of the tubes H is provided at its ends with an end fitting l3 or |4 (Figs 4 or 5) for connecting the tubes H to each other or into the refrigerant circulatory system of the refrigerator unit. As shown, the upper row .T of tubes II is provided at the front ends of the tubes with end fittings l3 and these end fittings are provided with caps l5 into which extend inlet tubes I6 for conducting a liquid refrigerant into said upper At their rear ends the tubes II in the upper tier T are connected through their end fittings |3 by means of their U-bends ll to the end fittings |3 of the tubes II in the nexttier Tl. Similarly, at their front ends, the tubes II in the tier T1 are connected to the tubes in the tier T2 by means of U-bends l1, and the tubesof tier T2 are likewise connected to the tubes in the tier T3 at the rear of the unit, The front ends of the tubes in the lowermost tier T3 are connected through a manifold |8 to a gas outlet and suction line I9.

With further reference to the end fittings l3, each of said end fittings comprises a streamlined socket portion 20 adapted to fit over an end of a tube H, and a cylindrical portion 2| extendof liquid refrigerant in the particular refrigeration system. The liquid refrigerant entering the upper'tierof tubes may be introduced at such a rate as .to fill each of the tubes in said upper tier T- toa-heightsuch as that indicated by the dotted line (Fig. .3) ,which may be slightly above the bottom wall of the end fitting extension 2|. Inqthat case; there would be some flow of liquid refrigerantffrom the tubesin the upper tier T through the end fittings l3 and U-bends into the tubes in the next lower tier T1. The major proportion of the cross-sectional areas of said extensions 2| and U-bendsv however, would Prefering from the larger end of the socket portion 20 and preferably forming a flush continuation of the upper wider edge of said socket portion. The socket portion 20 is adapted to be inserted over an open end of a tube II and to be brazed or otherwise integrally secured to said tube. Hydrogen brazing, silver soldering, or other method may be used to effect a good thermal bond between the socket portion 20 and the end of a tube Each end fitting l3 has an end wall 22 that closes an end of a tube II when the end fitting is secured in place.

A modified form of end fitting I4, illustrated in Figure 5, is provided with an extension 23 of elliptical cross-section, instead of circular crosssection. Said extension 23, like the extension 2|, has the major portion of its upper wall surface flush with the wider edge portion of the socket 20. Thus the extensions 2| and 23 are both adjacent the upper, or wider, edges of the tubes II be leftopen-for the flow of gaseous refrigerant formed by theevjaporation of the liquid refrigerantin the bottoms-of the tubes l.

There will, of course, becorrespondingly less refrigerant in liquid phase in the lower tiers T1,

Trand' Ta, and, in fact, if the fiow of refrigerant isproperly controlled, no liquid refrigerant will be permitted to pass out of the lower tier of tubes into the manifold l8, but will be trapped in the lower portions of the tubes in said lowermost tier T3. M a

There is thusla free gas area above the level 25 of the liquidrefrigerant' in each of thetubes II in each of .thetiersof theunit. By regulatingtherate oflfiow of refrigerant entering the evaporator and by maintaining the pressure on the surface of theliquid refrigerant in the tubes lowerthan the saturated vapor pressure at that particular. temperature, effective refrigeration can be produced. The lower vapor pressure referred to is maintained byeffecting a proper velocity of the gas through the unit, and this'is made possible by reasonof the fact that the end fittings are connected to theupper portions only of the tubes ll, so that there are no slugs of liquid refrigerant inthe gasiflow passages to slowdown the flow of gas therethrough.

It will thus be appreciated that I have provided an efficient construction and arrangement of tubes and end fittings for assembly into a refrigeration evaporator unit, whereby the unit may function with maximum efficiency of rate of heat 1. An evaporator unit for a normally gaseous liquid refrigerant, comprising a plurality of tiers of tubesof generally tear-drop cross-section, said tubes being arranged when said unit is mounted for operation with their rounded edges as the leading edges uppermost with respect to the fiow of air downwardly over and past said tubes and with the tubes in each tier laterally spaced at the same elevation and staggered with respect to tubes in verticall adjacent tiers, and end fittings individually connecting corresponding upper end portions of the tubes in vertically adjacent tiers, whereby said end fittings serve primarily for gas flow communication between said tubes while liquid refrigerant is trapped in the lower portions w of said tubes.

2. A heat exchange unit for the exchange of heat between a flowing body of an external gas and an internal fluid that is present in said unit in both gaseous and liquid states under the conditions of operation thereof, said unit comprising a plurality of tiers of tubes of generally tear-,

drop cross-section, said tubes being arranged 7 when said unitis mounted for operation with their rounded edges as the leading edges with respect to theiflow of said-external. gas .overand past said tubesand 'Withithe tubesin-"ea'ch tier laterally spacedat substantiallythe same elevation and staggered with respect to'tubesin vertically adjacent tiers,and=end fittings. individually connecting corresponding tuberendsof tubes-in vertically adjaeentstiers :atzthelevel of theccor- "responding leading edges :ibut "notincl'uding the levels of the trailing edgesof said tubes; whereby said end fittings serve primarily forifiow 'communication between leading edge portions of-said tubes of said'internal'fiuid-in one of'said'states 'while said fluid in the other of said states is trapped in the trailing edgeportions. ofsaid tubes.

3. Ina device of the characterdescribed; a heat exchange unit for causing a change of phase .of a fluid medium and having an inlet and an outlet, said .unit comprising.horizontaLtiers of lengths ofv tubing each .of generally roval cross section, said tiers overlying oneianotherand the tubing lengths of one tierbeing staggered with respect tothe tubing lengths Of'the next .tier.verticall spaced therefrom, end 'ifittings each connecting the upper portion only of one end'of a tubing length in .one row'to the corresponding end of a tubing length in thenext'row vertically spaced from said filStrlTlGIltiOHGdTQW; said tubing lengths forming traps for liquid. and the vvupper portion of said lengths andsaidfittings forming a passage for conducting. primarily theavapor. ortion of said mediumand extending from said said inlet to said outlet.

'4..In a device of thecharacter described, a

heat exchange unit for causinga change of phase of a fluid medium and having an inlet and :an outlet, said unitcomprising horizontal tiers of each of generally oval cross lengths of tubing section, said tiers overlying One anotherand the tubing lengths of one respect to'the tubing tier beingxstaggered with lengths .of the next tier vertically spaced therefrom end fittings each :5 .relatively constricted in cross sectional areaas compared to a tubinglength and each connect- :ing the upper portion only of one end of a tubing length. in one row to the corresponding end of a tubing length .in the 10 from saidfirstrmentioned row, saidtubing lengths next row vertically spaced RUSSELL HARVEY BURGESS.

REFERENCES CITED The following references are of'record in the Number 5 Number 20Lfile of this patent:

' UNITED STATES PATENTS Name Date Henshall eta]. .Aug.-30, 1932' Freeman Jan. 31, 1933 Whittle Sept. 30, 1941 .Compo Oct. 9, 1934 .Detrick .Dec. 23, 1890 Reeves May 1, 1934 Spofford Feb. 18, 1941 Trane -Mar. '24, 1931 Krackowizer Feb.,22, 1938 FOREIGN PATENTS 7 Country Date France May 11, 1914 Germany 1. Nov. 15, 1906 Great Britain Apr. 18, 1929