Split resistant tubular heat transfer member

The improved split resistant tubular heat transfer member of the present invention is directed to an elongated tube having a substantially circular transverse cross-section. The elongated tube hereof has an outer surface and an inner surface, and further has an outer diameter, a defined wall thickness, and an inner diameter. The tube inner surface has disposed therein a plurality of spiral grooves, defining and separating a corresponding plurality of spirally disposed fins extending from the inner diameter of the tube. The respective spirally disposed fins have an inverted substantially V-shape and have further an apex angle of approximately 28.degree.. In some such preferred embodiments the spiral grooves have a ratio of the cross-sectional area thereof to the depth thereof of approximately 0.01475 inches.

BACKGROUND OF THE INVENTION 
The present invention relates to heat exchangers in general and more 
particularly to an improved split resistant tubular heat transfer member 
through which refrigerant liquid flows and functions to evaporate or to 
condense, thereby respectively to accept heat from and to provide heat to 
a coolant fluid which is disposed in contact with the exterior of the 
tubular member. Yet further, the present invention is directed to a 
particularized structure for a tubular heat transfer member which provides 
resistance to splitting during the manufacture thereof, and does so while 
retaining its beneficial heat exchange characteristics. 
The improved split resistant tubular heat transfer member of the present 
invention is of the variety used in refrigeration and air conditioning 
systems utilizing an evaporator and condenser. Generally, the evaporator 
and condenser are comprised of a plurality of parallel tubes connected at 
the end to form a refrigerant circuit or circuits. A plurality of fins are 
connected in heat exchange relationship to the tubes and extend 
transversely of the tubes. In use, refrigerant is condensed in the 
condenser and evaporated in the evaporator. Liquid or air is passed over 
the condenser to condense the refrigerant fluid therein. Air passed over 
the evaporator is cooled. Cooled air from the evaporator may be used to 
cool the interior of a space, e.g. room to be cooled. 
In the above generalized procedure of refrigerating or air conditioning, 
the physical characteristics of the heat exchange tube determines the heat 
transfer efficiency. One certain type of heat transfer tubes which have 
found acceptance in the prior art utilize a multiplicity of rib-like 
projections, or "fins", disposed on the interior surface of the tube. In 
such heat transfer apparatus, a thin film layer of refrigerant liquid is 
maintained in contact with the interior surface of the tube, and in 
particular is disposed on the surface of the fins and the grooves 
therebetween. If the tube used in an evaporator application, this thin 
film layer is the subjected to evaporation. The multiplicity of rib-like 
fins increases the surface area available for evaporation and accordingly 
increases the efficiency of such evaporation. In some prior art ribbed 
tubing structures, the ribs are disposed in a spiral or helical 
disposition to cause a controlled degree of turbulence in the refrigerant 
liquid, which diminishes laminar flow and also serves to break up any 
insulating barrier layer of vapor from forming on the interior surfaces of 
the tube. 
Several prior art patents have made proposals for improvement of interior 
rib-containing tubular heat transfer members. Those prior art patents 
include: 
U.S. Pat. No. 4,044,797--Fujie 
U.S. Pat. No. 4,480,684--Onishi 
U.S. Pat. No. 4,545,428--Onishi 
U.S. Pat. No. 4,658,892--Shinohara 
U.S. Pat. No. 4,938,282--Zohler 
U.S. Pat. No. 4,921,042--Zohler 
U.S. Pat. No. 4,118,944--Lord, et al. 
These and other various tubular members of the prior art, including several 
different forms of interiorly disposed rib structures have increased 
somewhat the efficiency of refrigerant operation. However, such tubing has 
in several particulars been difficult or inefficient of manufacture, and 
has likewise resulted in a tendency to split the tube during manufacture. 
Rifle tube is used in the manufacture of heat transfer devices called 
"coils". The coils are constructed by placing tubes (aluminum or copper) 
through holes stamped into thin sheets of aluminum or copper. For assembly 
purposes the tube must be smaller than the holes in the sheets, but for 
heat transfer purposes the tube must be in intimate contact with the 
sheets. To achieve the intimate contact, a ball is forced through the tube 
after it is inserted into the sheets. The ball causes the OD of the tube 
to "expand" into intimate contact with the sheets. This is called the 
"expansion process". 
On smooth tube, the expansion process works well and causes few problems. 
However, with rifle tube the stress caused by the expansion process is 
increased in the thin part of the tube wall, causing the tube to split if 
there is even a minimal defect in the tube. It has been found by the 
applicants herein that, by increasing the amount of wall available (bottom 
wall to fin wall ratio) to accommodate the required expansion, the 
likelihood of the tube splitting can be reduced. 
In view of the above difficulties, defects and deficiencies of prior art 
structures, it is a material object of the improved tubular heat transfer 
member of the present invention to provide a novel structure having 
increased resistance to splitting during the manufacture thereof, while at 
the same time retaining the beneficial heat transfer characteristics of 
interiorly ribbed tubular heat transfer members. 
In addition, the improved split resistant tubular heat transfer member of 
the present invention has the further beneficial characteristic wherein 
the fins thereof hold their shape better during the expansion process, 
thus permitting the structure to retain a larger degree of its beneficial 
heat transfer characteristics after the expansion process than prior art 
tubing has been able to accomplish heretofore. 
It is a further object of the present invention to provide a versatile and 
novel tubular structure which may be utilized for evaporation and for 
condensation functions. These and other objects and advantages of the 
improved split resistant tubular heat transfer member of the present 
invention will become known by those skilled in the art upon a review of 
the following summary of the invention, brief description of the drawing, 
detailed description of preferred embodiments, appended claims and 
accompanying drawing. 
SUMMARY OF THE INVENTION 
The improved split resistant tubular heat transfer member of the present 
invention is directed to a structures having an enhanced interior surface 
thereof. This heat transfer tube interior surface enhancement, is directed 
to the form of a plurality of spaced ribs alternatingly disposed with a 
corresponding plurality of grooves. Suitable tubing for use in connection 
with the present invention has a thin side wall and is generally formed of 
refrigeration grade copper tubing. 
The improved structure of the split resistant tubular heat transfer member 
of the present invention provides an improved resistance to splitting 
during formation. The detrimental phenomenon of splitting occurs during 
the tube expansion process for formation of such group and rib structures. 
The cause of the splitting phenomenon is believed to be due to the 
necessity for sections of the tube between the fins to accommodate the 
stretch required by the expansion process, which necessarily causes 
increased stress to these areas of the wall. 
In particular, the improved split resistant tubular heat transfer member of 
the present invention is directed to an elongated tube having a 
substantially circular outside diameter and inside diameter. The elongated 
tube has an outer surface and an inner surface, and further has an outer 
diameter, a defined wall thickness, and an inner diameter. The tube inner 
surface has disposed thereon a plurality of spiral grooves, defining and 
separating a corresponding plurality of spirally disposed fins extending 
from the inner diameter of the tube. The respective spirally disposed fins 
have an inverted substantially V-shape and have further an apex angle of 
approximately 28.degree.. In a preferred embodiment, the spiral grooves 
have a ratio of the cross-sectional area thereof to the depth thereof of 
approximately 0.01475 inches.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring now to FIGS. 1 and 2 of the drawing, the improved split resistant 
tubular heat transfer member 10 of the present invention is directed to an 
elongated tube 12 having a substantially circular outside diameter and 
inside diameter defining there between transverse cross-section 13. 
Elongated tube 12 hereof has an elongated outer surface 14 and an inner 
elongated surface 16. The transverse cross-section 13 represents the wall 
thickness. Tube inner surface 16 has disposed therein a plurality of 
spiral grooves 24, defining and separating a corresponding plurality of 
spirally disposed fins 26 extending from inner diameter 22 of tube 12. 
Respective spirally disposed fins 26 have sloped sides 28,30 defining an 
inverted substantially V-shape and have further an apex angle a of 
approximately 28.degree.. In the embodiment of FIGS. 1 and 2, the spiral 
grooves 24 have a ratio of the cross-sectional area thereof to the depth 
thereof of approximately 0.01475 inches. 
As shown in FIG. 2, the distance along the curve of the inner surface of 
the tube between the termination of one fin, for example at slope side 30 
of an inverted v-shaped fin 26, and the beginning of the next fin, this 
distance is known in the art as the "bottom wall distance". The distance 
along the curve of the inner surface between the beginning of a fin and 
the termination of the same fin (i.e., the distance along the curve of the 
inner surface between the bottom portions of slope sides 28,30 of an 
inverted v-shaped fin 26 is known to those skilled in the art as the "fin 
wall distance". 
In a preferred embodiments of the improved split resistant tubular heat 
transfer member 10 of the present invention, elongated tube 12 has 
preferably approximately 50 of the spirally disposed fins 26, although one 
especially preferred embodiment has 53 such spirally disposed fins 26. 
However, the number of spirally disposed fins may vary depending upon 
other dimensions of heat transfer member 10. 
As shown in FIG. 2 and in these and other preferred embodiments, the apex 
angle of fin 26 is preferably asymmetrical with respect to a radius 32 of 
the circular transverse cross-sectional shape. Such radius 32 intersects a 
spirally disposed fin 26 to form respective angles of approximately 
13.degree. and approximately 15.degree. with regard to sloped sides 28,30 
of the inverted V-shaped fin 26. In such a manner, sloped sides 28,30 of 
the inverted V-shaped fin 26 do not in these preferred embodiments slope 
down at the same angle with respect to inner surface 16 of tubular member 
10. Accordingly, the shape of the several spiral grooves 24 between the 
spirally disposed fins 26 is that of an irregular trapezoid, as shown in 
FIG. 2. 
The structure of inverted substantially V-shaped fin 26 preferably has a 
substantially rounded apex 34. In a present embodiment, the ratio of the 
height of the spirally disposed fins 26 to inner diameter 22 of elongated 
tube 12 is approximately 0.023. The helical angle of the spirally disposed 
fins 26, and also the spirally disposed grooves 24 set forth therebetween, 
is approximately 20.degree., although in preferred embodiments a range of 
18.degree.-22.degree. may be utilizable. 
In some preferred embodiments and sizes, the pitch of the spirally disposed 
fins 26 is approximately 0.021 inches. The defined wall thickness 20 of 
elongated tube 12 is approximately 0.012 inches. Spirally disposed fins 26 
of the improved split resistant tubular heat transfer member 10 of the 
present invention may be separated along inner diameter 22 of elongated 
tube 12 by the distance of approximately 0.013 inches. Outer diameter 18 
of circular transverse cross-section 13 of elongated tube 12 is 
approximately 0.375 inches in such embodiments. 
The improved split resistant tubular heat transfer member 10 of the present 
invention may be formulated from refrigerant grade copper or other metal 
stock by means well known to those of ordinary skill in the art. In 
particular, in some of the useful methods of formation, a mandrel 
containing grooves and ridges thereon may be inserted within the inner 
diameter of a piece of smooth wall tubing for embossment of the mandrel 
grooves and fins onto the interior surface of the tubing by means of 
disposition of pressure on the exterior surface of the tubing. Such 
pressure on the exterior of the tubing may be brought about by means of 
ball bearings, roller bearings, or other apparatus such as disks disposed 
to revolve upon an arbor. In these embodiments, the exteriorly disposed 
ball bearings, roller bearings or disks displace the flowable metallic 
material of the tube wall, causing the material to deform downwardly and 
inwardly into the grooves of the mandrel structure in order to form an 
interior rib structure. According to known methods, the exterior surface 
of the tubular member may be smoothed with rollers or other suitable 
apparatus in order provide a finished and smooth wall outer surface. In 
such methods of tube formation, the end portions of the improved split 
resistant tubular heat transfer member may be left in an unworked 
condition to provide for ease of subsequent flaring for purposes of 
installation of such tubular member within a refrigerant system. 
In one preferred embodiment, the nominal outer diameter (O.D.) is 0.375 
inches, with a wall thickness of 0.012 inches, and an internal diameter 
(I.D.) of 0.35 inches. Other embodiments may have nominal outer diameters 
of 0.500 inches (1/2 inch) or 0.3125 inches (5/16 inch), with 
corresponding wall thickness. 
The basic and novel characteristics of the improved methods and apparatus 
of the present invention will be readily understood from the foregoing 
disclosure by those skilled in the art. It will become readily apparent 
that various changes and modifications may be made in the form, 
construction and arrangement of the improved apparatus of the present 
invention, and in the steps of the inventive methods hereof, which various 
respective inventions are as set forth hereinabove without departing from 
the spirit and scope of such inventions. Accordingly, the preferred and 
alternative embodiments of the present invention set forth hereinabove are 
not intended to limit such spirit and scope in any way.