Optimization of a Cooling System:
the Cooling of Pilgered Seamless Tubes
W. E. Carscallen, J. Jeswiet (2), P. H.Oosthuizen
Received on January 19,1994
Summary.
This paper describes the design of a cooling system for Pilger Rolling. The design
changed a turbulent flow regime (a mist) t o a laminar one with an improved method of
delivery. Also, the new design led t o production improvements and increased capabilities in
the company; for instance: increased production rates 110-20%1, with an increase in quality;
extended tool life [mandrel and rollers], hence less downtime and increased productivity;
increased consistency in the product, hence less quality problems and rework; significant
improvements in the production environment - cleaner air in the factory.
Keywords: Cooling, Rolling, Pilgering
Introduction
The Pilger Rolling process, sometimes
called Pilgering, is used t o produce high quality
seamless steel tubing; usually for the chemical
and power generation industries. The North
American Market alone is worth 200-250
million dollars per year (1, 2). Large reductions
can be achieved in Pilger Rolling but only with
concomitant large forces and heat generation
during plastic forming (3). As with many other
forming processes, the cooling system in Pilger
Rolling is relatively primitive and provides an
opportunity for improvement t o the process.
The improvement in the environment alone
gave more than enough justification t o claim
the project a success. In this case it was an
unexpected but positive result, and although it
was not a requirement in the original project
specifications, the improvement created a more
comfortable environment for production personnel and also decreased production costs
because precautions necessary t o prevent
environmental pollution could be decreased.
Indeed, environmental factors increasingly
affect competitiveness worldwide (4) including
the redesign of processes: in this case the
coolant part of the process.
The Problem. Manufacturers operating
metal forming machines have found it is necessary t o design systems that will run faster and
longer while maintaining a high quality in order
t o remain competitive and survive in today's
market. In forging machines it is important t o
keep equipment cleaning problems t o a minimum, t o increase the period between cleaning
operations, and t o decrease the amount of
cleaning that is needed. More efficient cooling
methods are replacing old "coffee can"
methods t o assure proper protection of equipment and increase productivity. In tandem with
the necessity t o increase productivity is the
need t o decrease environmental pollution. This
can be done by reducing the amount of coolant
Annals of the ClRP Vol. 43/7/7994
used in the process and decreasing the dispersal of the coolant in the environment.
Cold Pilgering Mills are forming machines
which presently have limited output due t o
excessive overheating of the internal mandrel.
Indirect cooling of the internal mandrel is
necessary due t o the small contact area
involved in the process. Process heat affects
the coolant - "burns it" - and causes cleaning
problems, thereby causing quality problems and
reducing productivity. Excessive heating of the
coolant, due t o poor heat transfer, also
decreases tool life hence increasing production
costs. The problem, as posed by the company,
was t o construct a rigidly mounted nozzle
capable of directing oil flow t o a specific area
of the pilger mill working section. Therefore the
initial objective of this project was t o improve
the cold pilgering mill tool and workpiece
cooling system. The Pilgering Mill, in this case,
rolls stainless steel seamless tube.
The Process. The Cold Pilgering Mill i s a
two-high mill with non-uniform radius rolls. The
roll stand moves back and forth over hollow
tubing which has an internal mandrel within (5,
6 ) .The mandrel (plug), which is held in position
by a bar fixed t o the end of a drawbench,
controls the internal diameter and surface
finish. Figure 1 shows a mill cross section and
examples of plugs ( 6 ) ;the mill oscillates periodically, in a reciprocal manner, over the tube
and mandrel at 240 cycles per minute. Typical
tube sizes are 13.0 mm outside diameter with
a wall thickness of 0.40 mm (2). The reduction
usually achieved in one pass is 85%.
No direct cooling of the internal mandrel is
possible due t o extremely high loading and the
small diameter of the mandrel. The cooling is
usually accomplished by supplying coolant t o
the external surface of the workpiece in a
spraying manner as shown by Roberts (7). This
kind of cooling involves the turbulent flow of a
mixture of air and coolant and the flow is not
really directed t o a specific location. The exter223
nal coolant is usually a mineral oil with E.P.
additives in the case of stainless steel (6). The
mandrel has coolant applied t o it before insertion into the tubing prior t o rolling and thus is
the same as the external coolant. The temperature is a major consideration because in the
start o j
stroke
1 '
lower
rol]
Tube rolling section
8
the nozzle t o target distance must be
200mm or more.
Solution. Proper die cooling is not accomplished b y guesswork. It must be planned in
advance during the die design stage. However
this is often not the case, as with this installation. The method of delivery was an important
requirement and flow calculations could be
made and the results, obtained from changing
the cooling rkgime, could be easily observed.
Because of the high fluid viscosity, 200 t o 400
mm2/s, the flow was laminar. Also, recent
advances in convective heat transfer modelling
have shown that excellent cooling can be
achieved from a heated surface with a single
phase jet involving a concentrated, single
phase, laminar flow (8, 9, 10) where all the
lubricant is delivered t o a precise target area.
By directing the fluid t o a specific target area
the volume of coolant needed should also be
lower.
L e n d of
stroke
Figure 1. Pilger Rolling.
Lubrication System Design
General Nozzle Design Considerations. The
coolant design problem can be considered in
several parts; these include:
boundary areas it can be as high as 3OO0C and
will have an affect upon product quality, production rate and tool wear. The boundary
conditions are unknown (5).
In the Pilger mill used in this study, the
application of cooling oil t o the roll/tube interface, before redesigning the coolant nozzle, was
accomplished by flooding the area with t w o
streams of oil at a total rate of 300 I/min; one
stream at the entrance and one stream at the
exit. The coolant was delivered via a crude
arrangement of copper tubing and primitive
nozzles which were prone t o migrate from their
position because of mill vibrations and operators trying t o improve the .cooling. The
workpiece material in this case is austenitic
stainless steel, a high nickel content stainless
steel. The coolant has a kinematic viscosity of
200 t o 400 mm2/s. Because improved cooling
could increase production rates the mill engineers decided t o investigate t o possibility of
improving the cooling system.
The design requirements were:
h o w the coolant will be delivered? via
a general spray or a directed jet; is a
turbulent or laminar flow needed?
the space available for positioning of
the nozzle configuration and how t o
deliver the fluid from the nozzle t o the
area of cooling; a geometry question.
the losses that can be expected in the
nozzle; this is for pump requirements.
the distance lubricant jet must travel
horizontally from the nozzle exit t o the
target where cooling is needed.
the number of jets needed t o cover
the cooling area efficiently; this concerns flow rates.
the environmental considerations; an
increasingly important question.
the maximum coolant flow that can be
achieved with minimal oil misting;
related t o the environment.
the economic factors; lubricant cost
versus volume needed.
t o increase the cooling rates thereby allowing the production rates t o be increased.
to fix the nozzles so that they could not be
tampered with.
have concentric cooling on the deformation area.
The effectiveness of any coolant system
depends upon the method of delivery, which
inherently includes the nozzle configuration and
design; therefore this was the area t o which
attention was addressed.
Initial design requirements for the nozzle,
as listed previously, were twofold: first, t o
direct the lubricating oil, with a steady flow, a t
8
8
8
224
the external surface of the tube where unit
loading pressures were the highest; second, t o
maintain this direction, while eliminating direction changes due t o equipment vibration and
repositioning by operators. Due t o the high
viscosity of the coolant the flow was, by
necessity, laminar.
Without careful nozzle design, the flow
leaving the nozzle may be laminar but not
steady and thus can cause the jet t o break
d o w n leading t o droplet formation and dispersion. To avoid disintegration of the jet in the
atmosphere the flow must be made as steady
as possible before entering the atmosphere.
This can be done by accounting for all potential
losses in the nozzle and by accelerating the
flow over an appropriate length. Also the
orifice must be designed so that the minimum
shear occurs a t the jet wall (1 1). Shear at the
jet wall causes it t o disintegrate before it
strikes the target area.
Detailed Nozzle Design Considerations. There
are four important parts t o the internal nozzle
design (12); these are:
1.
the contraction ratio, which contributes t o
damping the unsteadiness of the flow.
2. the plenum size, which also plays a part in
damping unsteady flow.
3. preventing the occurrence of the vena
contracta by smoothing the entrance of
the nozzle.
4. the length of the nozzle which ensures
that there is no radial pressure gradient.
For the first part of the problem, the space
available for the nozzles was restrictive as can
be seen in figure 2. This gave rise t o the following possibilities for the nozzle arrangement:
t o have the nozzles move with the rolls; t o
have an annular nozzle arrangement around the
tube; t o completely change the philosophy of
cooling and spray everywhere. It was decided
to position a 360' nozzle around the inlet tube
guide as shown in figure 2. The nozzle was t o
be fixed and designed t o produce a flow which
would envelope the tube. It would also have
inserts which could be interchanged for different sized tube. A laminar flow and not a spray
was desired for reasons already stated.
For the second step, the problem is t o
keep the flow laminar and stable so that when
the jet enters the air it remains stable and
contiguous as it is delivered t o the targeted
cooling surface. This part of the analysis can
be considered in t w o sections. First as the
coolant enters the nozzle it must be accelerated, then secondly, all flow unsteadiness must
be eliminated before passing through the orifice
to become a laminar jet flow. For the acceler-
I
.*-
...-m-I-zrf\
1.6rnm x 4.76rnrn
'.
I
nozzle
I
Figure 2. An illustration of the nozzle with
a 360' ring of square jet orifices.
ation of the fluid, the bulk analysis of the
annular contraction, is analogous t o the flow
between parallel plates and the following
relations apply:
dP
--dx
dT
and
t-- P d U
'where
dY
dy
P a pressure, T P shear in the flow, p P dynamic
viscosity, unvelocity of the flow, x n coordinate
parallel t o flow direction, y e coordinate perpendicular t o flow direction. The velocity u and the
pressure P of the flow are governed by equations of motion and continuity.
For the most efficient nozzle ring design one with the lowest losses - a simple one
dimensional analysis can be carried out t o
determine losses. From Schlichting ( 1 31, one
can find there are three pressure drops which
have an effect upon the internal flow in the
nozzle:
(i) the drop associated with the entrance loss,
Ap-0.05- P P
2
(ii) the loss due t o acceleration of fluid while
2
becoming a developed flow, Ap-1.11
2
(iii)the loss associated with laminar flow and
hydraulicallysmooth boundaries, Ap---P P U 2
0 2
Adding these losses for a simple nozzle, one
,
then obtains: HL-hcnnoracc+hdcwlopingflow+hrsoulclcngth
where H, is the total head loss in the nozzle.
A n acceptable exit oil flow symmetry of the
nozzles indicates that the circumferential pressure gradients are much less than the radial
pressure gradients.
225
Results
The first design was based upon a numerical study of the stability of laminar flow in
square ducts by Tatsumi and Yoshimura (1 4).
It was based upon: one target area along the
length of the tube; a laminar flow with Re = 60;
six rectangular oil jets, each produced by a 1.6
mm by 4.76 mm nozzle; a lo:? contraction
ratio in the radial nozzle in which flow acceleration occurred - see figure 2. This configuration
produced a smooth, thin film laminar flow, 1.6
mm thick, over the tube surface as illustrated
in case 1 of figure 3. Also, once the flow
became attached t o the tube surface it
remained a thin film for 60 mm after which it
would disintegrate. However, due t o the reciprocating motion of the Pilger Rolls, the jets had
t o be targeted at more than one zone t o gain
increased cooling. The jets were aimed a t three
target areas t o improve the cooling. This led t o
a modified solution with three sets of square
nozzles, but this assembly was too bulky and
did not fit the mill.
The obvious solution was t o use smaller
round nozzles in a circular ring around the entry
zone of the tube. Calculations show the exit
area of the nozzle, with 18 nozzles, should be
flow quantity -18n-D 2
flow velocity
4
from which the approp-
riate drill diameter can be found. It must be
noted that using round holes also simplified the
manufacture of the nozzles considerably; the
cutting of the square ducts in the nozzle ring
had created a difficult problem in machining.
The three target areas are located 50 mm
from each other along the tube length. Each
target zone has a cone angle associated with it
as shown in the figure 3. The final jet configuration is shown in figure 4.
One final consideration is the need t o
compensate for gravity and hence droop in the
liquid jet trajectory. From calculation and then
by empirical fine tuning it was found that all oil
ports had t o be drilled with an upward tilt angle
of 2.5 degrees t o achieve proper attachment of
the flow along the tube. See figure 4.
Upon contact with the tube, each round jet
immediately became a 1.6 mm thick laminar
f l o w layer upon the tube surface.
Figure 3 illustrates t w o cases of actual jets
produced by t w o of the nozzles. In both cases
the centreline of each jet has been traced from
photographs of actual flows; this has been
done because of poor lighting conditions which
gave photographs of poor quality. Case 1
shows the rectangular jets for the first nozzle;
the six jets that were used in the first design.
T w o points of attachment can be observed.
The rate of cooling was not high enough for
226
point of attachment
CASE
1
Figure 3. Illustrations of laminar flow jet centrelines, showing nozzle cone angles and nozzle
tilt angles. Case 1 : 6 rectangular jets attaching
t o the tube surface with only one target zone.
Case 2: 18 circular jets attaching t o the
mandrel surface in a thin film; there are three
target zones (A, B and C) 50 mm apart.
EXIT VIEW
A B
NOZZLE CONE ANGLES 5.16, 3.94, 3.18 DEGREES
NOZZLE T I L T ANGLE 2.5 DEGREES
Figure 4. The final nozzle design. A indicates
nozzles aimed at zone A, B indicates nozzles aimed at zone B, etc. There are six
nozzles per zone and three zones 50 mm
apart.
this case and it was increased by changing t o
a design which consisted three sets of six jets,
each set targeted t o a specific area on the
tube. The concentric nozzles were all located in
a ring, which contained a total of 18 jets. The
point of attachment of the jets can be observed
in case 2. The flow remained a thin film for
more than 60 mm along the surface once
attachment of the oil t o the tube surface was
achieved. Cooling rates for case 2 were much
higher as measured by thermocouples placed
on the surface of the tubes a t the exit of the
Pilgering process.
From an environmental and oil usage point
of view, the flow rate was reduced from 300
Vmin t o 3 0 I/min; the oil flow was reduced t o
10% of the amount required previously giving
an added economic advantage. This appeared
t o provide sufficient cooling because the internal surface finish of the stainless steel tubing
was improved. There was a concomitant 10%
t o 20% increase in production rate due t o
improved cooling of the system. The oil mist
from the mill in the process area was reduced
significantly and this result in itself made the
project an unqualified success.
The life of the mandrels has also been
extended leading t o longer production periods
between mandrel changes.
Conclusions
The environmental impact of the pilger
rolling process was substantially reduced by
improving the cooling system with a properly
designed nozzle which was not an afterthought
but designed specially for this forming system.
The nozzle design for this cooling system
is state of the art and shows h o w proper
consideration of the nozzle cooling system can
lead t o significant improvements such as:
.
.
.
flow rates have been reduced t o 10% of the
original rates.
production rates have been increased by
10%t o 20% with a properly designed cooling
system.
increased efficiency has lead t o reduction in
cost of operating the system, for instance a
reduction in downtime.
Although the mill personnel have not
provided quantitative data regarding the tube
quality and operation of the cooling system,
they say the nozzles are working extremely
well and the tube quality has increased and is
more consistent. There has been a corresponding reduction in rework of tubes.
Because of the success of this project a
heat transfer model of the process is now
being developed.
Acknowledgements
The authors wish to acknowledge the
support of NRC and NSERC.
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