Control system for manufacturing enhanced tubes

A method and control system for manufacturing externally enhanced evaporator tubes. A pressure signal indicative of the pore size on the external surface of the enhanced evaporator tube is provided to a microcomputer system which processes the signal indicative of the pore size and compares the signal to a predetermined pore size signal to determine when the pore size is within a selected tolerance. The microcomputer generates a control signal to adjust the enhancing process if the microcomputer system determines that the pore size of the manufactured tube is different than the selected pore size.

BACKGROUND OF THE INVENTION 
This invention relates generally to enhanced evaporator tubes, and more 
particularly, to a method and closed loop control system for manufacturing 
externally enhanced tubes. 
In an evaporatory of certain refrigeration systems a fluid to be cooled is 
passed through heat transfer tubing while refrigerant in contact with the 
exterior of the tubing changes state from a liquid to a vapor by absorbing 
heat from the fluid within the tubing. The external and internal 
configuration of the tubing are important in determining the overall heat 
transfer characteristics of the tubing. For example, it is known that one 
of the most effective ways of transferring heat from the fluid within the 
tube to the boiling refrigerant surrounding the tube is through the 
mechanism of nucleate boiling. 
It has been theorized that the provision of vapor entrapment sites or 
cavities on a heat transfer surface cause nucleate boiling. According to 
this theory the vapor trapped in the cavities forms the nucleus of a 
bubble at or slightly above the saturation temperature, and the bubble 
increases in volume as heat is added until surface tension is overcome and 
the vapor bubble breaks free from the heat transfer surface. As the vapor 
bubble leaves the heat transfer surface, liquid enters the vacated volume 
trapping the remaining vapor and another bubble is formed. The continual 
bubble formation together with the convection effect of the bubbles 
traveling through and mixing the boundary layer of superheated 
refrigerant, which covers the vapor entrapment sites, results in improved 
heat transfer. A heat exchange surface having a number of discrete 
artificial nucleation sites is disclosed in U.S. Pat. No. 3,301,314. 
It is known that a vapor entrapment site or cavity produces stable bubble 
columns when it is of the re-entrant type. In this context, a re-entrant 
vapor entrapment site is defined as a cavity or groove in which the size 
of the surface pore or gap is smaller than the subsurface cavity or 
subsurface groove. Heat transfer tubes having re-entrant type grooves are 
disclosed in U.S. Pat. Nos. 3,696,861 and 3,768,290. 
It has been discovered that an excessive influx of liquid from the 
surroundings can flood or de-activate a re-entrant type vapor entrapment 
site. However, a heat transfer surface having subsurface channels 
communicating with the surroundings through surface openings or pore 
having a specified "opening ratio" have been found to provide good heat 
transfer and prevent flooding of the vapor entrapment site or subsurface 
channel. 
In regard to the interior surface configuration of a heat transfer tube, it 
is known that providing an internal rib on the tube may enhance the heat 
transfer characteristics of the tube due to the increased turbulence of 
the fluid flowing through the ribbed tube. 
As disclosed in U.S. Pat. Nos. 4,425,696 and 4,438,807 assigned to the 
present assignee and incorporated by reference herein, an internally and 
externally enhanced heat transfer tube, having an internal rib and an 
external helical fin (creating a subsurface channel) communicating with 
the surrounding liquid through surface openings (pores) is manufactured by 
a single pass process with a tube finning and rolling machine. According 
to the disclosed process a grooved mandrel is placed inside an unformed 
tube and a tool arbor having a tool gang thereon is rolled over the 
external surface of the tube. The unformed tube is pressed against the 
mandrel to form at least one internal rib on the internal surface of the 
tube. Simultaneously, at last one external fin convolution is formed on 
the external surface of the tube by finning discs on the tool gang. The 
external fin convolutions form subsurface channels therebetween. The 
external fin convolutions also have depressed sections above the internal 
rib where the tube is forced into the grooves of the mandrel to form the 
rib. A smooth roller-like disk on the tool arbor is rolled over the 
external surface of the tube after the external fin convolution is formed. 
The smooth roller-like disc is designed to bend over the tip portion of 
the external fin so that it touches the adjacent fin convolution and forms 
an enclosed subsurface channel. However, the tip portion of the depressed 
sections of the external fin, which are located above the internal rib, 
are also bent over but do not touch the adjacent convolutions, thereby 
forming pores which provide fluid communication between the fluid 
surrounding the tube and the subsurface channels. 
The performance of the foregoing tube is critically dependent upon the 
external enhancement of the tube. It is therefore important to maintain a 
consistent subsurface channel size and pore size during the manufacturing 
process. Normal variations in subsurface channel size and surface pore 
size do occur, however, due to tool wear, material variations in the tube, 
dimensional variations in the tube lengths, and machine tolerances. In 
order to account for these variables and maintain a consistent pore size, 
it is necessary to measure the pore size on each tube produced and adjust 
the finning machine to maintain the correct subsurface and pore sizes. 
However, the prior methods of checking the pore size in an enhanced tube 
and adjusting the finning machine were very laborious and expensive 
processes, and were very difficult to use in a manufacturing process. For 
example, one method was to have an operator randomly select a manufactured 
tube and optically check the pore size of the selected tube under a 
microscope. Another method was to take a photograph of a tube and using an 
image analyzer compare the area of the pores in a selected area to the 
area of the pores in a reference photograph. After determining the size of 
the pores, the operator would then adjust the finning machine to 
compensate for any variations in the desired pore size. However, these 
methods were time consuming and did not provide the quality and quantity 
of tubes necessary for a manufacturing process. 
Thus, there is a clear need for a method and control system for 
manufacturing enhanced tubes that would, to a large extent, overcome the 
inadequacies that have characterized the prior art. 
SUMMARY OF THE INVENTION 
A closed loop electronic control system for manufacturing enhanced tubes 
has been developed. This control system is characterized by at least one 
pressure transducer which measures the average pore size on the enhanced 
tube surface and transmits an output signal corresponding to the size of 
the pores to a microcomputer which analyzes the pressure transducer 
signals and sends an output signal to a programmable controller, which in 
turn controls a servo motor for adjusting the finning machine to maintain 
the correct cavity size. 
Accordingly, it is an object of the present invention to provide a method 
and control system which measures the average pore size of an enhanced 
tube surface and automatically adjusts the finning machine to maintain the 
correct cavity size. 
Another object of the present invention is to provide a method and control 
system which can inspect 100% of the enhanced tubes and adjust the finning 
machine to maintain the correct cavity size on every tube produced. 
A further object of the present invention is to provide a method and 
control system which would reduce down time of the finning machines due to 
mechanical adjustments of the finning heads. 
These and other objects of the present invention are obtained by a novel 
method and control system for measuring the pore size on an enhanced 
evaporator tube and automatically adjusting the finning machine to 
maintain the correct pore size. The control system comprises at least one 
pressure transducer that provides an electrical analog signal to a signal 
conditioner which amplifies and linearizes the signal and feeds it to a 
microcomputer. The microcomputer compares the input signals to a reference 
signal and generates an output signal to a programmable controller which 
generates a digital signal to be fed to a motion controller. The output 
signal of the motion controller is fed to a servo motor which in turn 
moves a linear actuator to adjust the finning head. Thus, the present 
invention measures the average pore size of an enhanced tube and adjusts 
the finning machine to maintain the correct pore size. 
The various features of novelty which characterize the invention are 
pointed with particularity in the claims annexed to and forming a part of 
this specification. For a better understanding of the invention, its 
operating advantages and specific objects attained by its use, reference 
should be had to the accompanying drawings and descriptive matter in which 
there is illustrated and described a preferred embodiment of the invention 
.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The embodiment of the present invention described below is specially 
designed for use with enhanced evaporator tubes because these tubes have a 
critical dimension which must be precisely controlled in order to maintain 
good heat transfer performance. These enhanced tubes are designed for use 
in an evaporator of a refrigeration system having a fluid to be cooled 
passing through the tubes and having refrigerant, which is to be 
vaporized, in contact with the external surfaces of the tubes. Typically, 
a plurality of heat transfer tubes are mounted in parallel and connected 
so that several tubes form a fluid flow circuit and a plurality of such 
parallel circuits are provided to form a tube bundle. Usually, all of the 
tubes of the various circuits are contained within a single shell wherein 
they are immersed in the refrigerant. The heat transfer capability of the 
evaporator is largely determined by the average heat transfer 
characteristics of the heat transfer tubes. Thus, the size of the 
subsurface channels and pores on the surface of the tube are critical. 
Therefore, it is important to maintain a consistent subsurface channel 
size and pore size during the manufacturing process of the enhanced 
evaporator tube. 
Referring now to the drawings, FIG. 1 is a diagrammatic representation of a 
finning station for manufacturing enhanced tubes in accordance with the 
principles of the present invention. The finning station 10 includes an 
electronic control cabinet 12, a feed station 14, a finning head section 
16, an ejection section 32, and a pore measurement section 18. The 
electronic control cabinet includes a microcomputer, a programmable 
controller, and an operator console 22. The microcomputer determines 
whether the process is within control tolerances and the programmable 
controller performs logic execution, timing, sequencing, and calculations 
for the finning operation. The feed section 14 generally includes two 
similar parallel mandrels 24 (the two mandrels are generally in the same 
horizontal plane, thus, the rearward mandrel is not shown in the Figure) 
typically supported by a plurality of support arms 26 and positioned by 
piston means 28. Accordingly, the operator will load a blank tube on the 
front and rear mandrels 24 and cycle the feed section 14 such that one 
mandrel, e.g. the front mandrel, will drop down and move the blank tube 
along the longitudinal finning axis 29 into the finning head section 16. 
When the blank tube is completely enhanced the mandrel will retract to its 
original position while ejection means, e.g. eject wheels, in the ejection 
section 32, will engage the enhanced tube and send it into the pore 
measurement section 18. Once the enhanced tube is completely into the pore 
measurement section 18 the enhanced tube is matingly engaged by measuring 
apparatus 40 for measuring the pore size on the surface of the evaporator 
tube. A fixed reference means 50 provides a reference pressure drop. Once 
the front mandrel is in its original position, the rear mandrel will drop 
down and the control system will adjust the finning machine and the 
enhancing process will repeat itself. 
FIG. 2 is a schematic illustration of a finning head for the manufacture of 
enhanced tubes having a closed loop control system for operating the 
finning head in accordance with the principles of the present invention. 
The closed loop control system comprises a pore measurement section 18 of 
a finning station into which a finned tube is ejected after manufacture, 
and compressed air is blown through the pores of the enhanced tube 
resulting in a pressure drop across the pores. The resulting pressure drop 
is sensed by a plurality of pressure transducers 41. The analog output 
signal from the pressure transducers 41 is fed to a signal conditioner 43 
which amplifies and linearizes the output signal and feeds it to 
microcomputer 45. The computer uses standard statistical process control 
methods to control manufacturing tolerances that are needed to meet 
minimum average heat transfer performance of the tubes. The computer 
monitors changes in the process means for average pore size which may 
result from process drift or a sudden change in a critical finning 
variable. If a change in the process is needed, microcomputer 45 provides 
an electrical analog signal to the programmable controller 47. The 
programmable controller 47 processes the received electrical signals 
provided by the microcomputer 45, according to preprogrammed procedures, 
and generates a digital electrical signal which is then provided to motion 
controller 49. The motion controller 49 processes the received electrical 
signal provided by the programmable controller 47 and generates a position 
signal output. The output of the motion controller 49, a current signal, 
is fed to servo motor 58 which in turn moves the linear actuator 59 which 
adjusts the finning head to its new position. After the finning head is 
adjusted to its new position a new tube is enhanced and the closed loop 
control system will repeat itself. 
As shown in FIG. 3, the finning head section includes a finning head 50 
having a plurality of tool arbors 52 and a tube locating device 53, which 
accurately positions the end of the blank tube within finning head 50 
prior to the start of the finning process. Each of the tool arbors 52 
includes a tool gang arrangement having a plurality of finning discs 54 
and rollers, well known in the art, cooperating with the mandrel to 
produce the enhanced tube. The finning discs 54, which are skewed at an 
angle to the longitudinal finning axis 29, inherently move the enhanced 
tube through the finning head section 16 to the ejection section 32. When 
the blank tube is completely enhanced the finning head 50 of the finning 
head section 16 will open, i.e. the tool arbors 52 will move radially 
outward due to the servo motor 58 coacting with camming surface 59, and 
the mandrel will retract to its original position. Further, as more 
clearly shown in FIG. 1, after the mandrel is retracted to its original 
position the ejection means, e.g. eject wheels, in the ejection section 
32, will engage the enhanced tube and move it into the cavity measurement 
section 18 where the closed loop control system will measure the pore size 
of the enhanced tube and according to procedures will position the tool 
arbors for optimum tube geometry of the next tube to be manufactured. 
There is a range in which the microcomputer will control the average pore 
size in the closed loop manufacturing process. This range is caused by the 
differences in material properties and dimensions of the blank tubes used. 
Accordingly, the process uses a distribution of the average pore size for 
a plurality of tubes in determining the average overall heat transfer 
performance of the enhanced evaporator tubes in order to meet the minimum 
average heat transfer performance of the enhanced tube. FIG. 4 shows a 
typical enhanced evaporator tube 30 consisting of subsurface channels 35 
communicating with the surroundings of the tube through the pores 34. The 
measuring apparatus 40 comprises a rectangular block 42 and a flexible 
insert 44 having an arcuate longitudinal channel therein whereby the 
flexible insert matingly engages with the surface of the enhanced tube 30. 
Flexible insert 44 acts like a gasket against the surface of the enhanced 
tube. Thus, when air is blown into chamber 45 through inlet 47, and the 
flexible insert 44 is sealed against the surface of the enhanced tube, the 
air in chamber 45 enters pores 34 in the surface of the tube within a 
projected area of the chamber 45 and flows through corresponding 
subsurface channels 35 and out pores 34 outside the projected area of the 
chamber to the surroundings. The measuring apparatus 40 thus measures the 
average pores size on the tube. This average pore size measurement is 
directly related to the boiling heat transfer coefficient of the tubes. 
The present closed loop control system for the manufacture of enhanced 
evaporator tubes more closely controls the operation of the finning head, 
as opposed to the prior mechanical adjustments for operator control, and 
inspects all of the tubes that are produced and automatically adjusts the 
position of the finning head to maintain the correct pore size. Thus, the 
closed loop control system requires no operator interaction. 
In operation, after each tube is enhanced it is ejected into the pore 
measurement section 18 where at least one measuring apparatus 40 clamps 
down on the tube. Compressed dry air is then blown through the pores and 
the resulting pressure is sensed by the pressure transducer and read by 
the microcomputer. After a number of tubes have been processed, the 
microcomputer applies statistical process control procedures to determine 
whether or not a change in the finning head position is required. If a 
change in head position is required, the microcomputer sends a signal to 
the programmable controller indicating the required change. Small changes 
in the finning head position, accomplished by the servo motor and linear 
actuator, are sufficient to alter the pore size and bring the process back 
into proper tolerance limits. 
Of course, the foregoing description of a method and control system for 
manufacturing enhanced tubes is directed to a preferred embodiment, and 
various modifications and other embodiments of the present invention will 
be readily apparent to one of ordinary skill in the art to which the 
present invention pertains. Therefore, while the present invention has 
been described in conjunction with a particular embodiment, it is to be 
understood that various modifications and other embodiments of the present 
invention may be made without departing from the scope of the invention as 
described herein and as claimed in the appended claims.