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A Very Favorable Exchange

Oct. 27, 2011
A double slotter de-bottlenecks heat-exchanger throughput
Machining tube sheet grooves can be a long-reach, tight-quarters job. With the Ingersoll double grooving tool, tube sheet holes are created to ASME specs in a single step, and more than twice as fast. In two-inch thick carbon steel tube sheets, users report three-to-one reductions in cycle time versus single groovers, 30% longer tool life, complete elimination of burrs, and much lower tooling costs.
A double-grooving form tool creates ASME-compliant heat-exchanger tube sheet grooves in less than half the time.
Replaceable tip helps cushion the escalating cost of carbide. Only the tip needs to be replaced when the edges wear, not the entire tip. The tip-replacement repeatability is 0.0005 in., making offsetting and “touching off” completely unnecessary.

Driven by unprecedented demand for more efficient energy sources, most manufacturers of heat exchangers are playing catch-up. Without heat exchangers, process and thermal power-generating plants would come to a standstill. Solar and wind-power operation don’t need them, but the high-volume generating operations that are the foundation of the electrical grid cannot get along without heat exchangers.

In response to the high demand, several alert fabricators of familiar shell-and-tube heat exchangers have debottlenecked one of the most time consuming and repetitive operations in heat exchanger fabrication — tube sheet grooving. As a result, the grooves – literally hundreds of them in every heat exchanger – are completed in one-half to one-third the time, so the finished units can be shipped days or even weeks sooner.

“A two-thirds reduction in grooving cycle time can boost the effective capacity of a typical heat exchanger shop by 25%,” according to Mike Dieken of Ingersoll Cutting Tools, whose team developed the milling cutter that enabled such gains.

In a typical shell-and-tube heat exchanger the process fluid flows in one end of the pressure vessel, through the tubes, and out the other end, while the coolant circulates around the outside of the tubes between the two sheets. The tube sheets — huge steel discs at either end, riddled with holes to accommodate the tubes — seal off the process fluids from each other.

To forestall any leakage, every hole in the tube sheet must be of precise diameter and include two machined grooves, into which the tubes are “rolled in” or swaged. “Rolling in,” also covered by industry codes, is preferable to soldering or welding because it supports subsequent efforts at maintenance and tube replacement.

A critical aspect to Ingersoll’s tube-sheet grooving tool is that it machines both grooves in one step; the previous best practice was to mill the grooves one at a time.

One early user reported reducing cycle time from 18 seconds to 5 seconds per hole, while eliminating all post-deburring. These times describe an effort at grooving a two-inch thick carbon steel tube sheet with 3/4-inch holes. The user also reported that previously, burrs became a problem as soon as the single tool began to wear even the slightest.

Their previous tool of choice was a solid carbide single slotting cutter. Because of the rising cost of carbide, they tried to get more life out of each tool by gradually increasing the toolpath radius to offset the wear. This is what caused the burring. It became a matter of balancing deburring costs against tool-replacement costs.

“Burring is eliminated with the new double slotter because it uses a more durable carbide grade and also includes a deburring radius on one corner of the cutting edge,” explains Dieken. “Even if you expand the toolpath for wear compensation, the tool itself takes care of the burr.” One Gulf Coast user reports 30% longer tool life than before, with no deburring required.

In essence, the new grooving tool is a very sophisticated Ingersoll ChipSurfer double form-slotting mill. With basically one orbiting radial plunge action, the cutter simultaneously produces two identical code-compliant grooves. For a 3/4-inch hole, for example, a 5/8-inch milling cutter carves out two 0.125-inch flat-bottomed grooves spaced with a 0.250-inch gap and 0.030-inch maximum depth.

Like other tools in the ChipSurfer family, this cutter is a replaceable solid-carbide tip that screws onto a shank, made of either carbide or alloy steel. The repeatability with tip replacement is 0.0005 in., which eliminates dead cycle time for “touching off” or offsetting after each tip change.

Because of the tight quarters – only 1/8-inch of clearance between a 5/8-inch tool working in a deep 3/4-inch ID hole — “doubling up” wasn’t as simple as it may seem. “It is a long-reach operation in a high-aspect hole, which generates chips twice as fast as a single slotter and can double the lateral forces,” Dieken explained. As a result, he settled on a four-flute design that balances chip evacuation space with chip load per tooth, and a high-positive presentation geometry to minimize cutting forces.

The new replaceable-tip design also helps to cushion the rising cost of carbide. “A short tip uses less high-priced tungsten than a long carbide tool,” according to Dieken. “When the tool is worn, you replace the tip only, not the entire tip and shank.” He added that the 0.0005 inch repeatability — tip to tip, a recent refinement — simply enhances the value of replaceable-tip tooling for today’s cost-conscious shops.

One fabricator cited the tip costing 75% less than their solid-carbide single-tooth end mills, running 30% longer and reducing cycle time by more than 60%.

For two reasons specific to tube sheet-grooving Dieken recommended a carbide shank, with its extra rigidity, over its lower-cost alloy counterpart. “First, a tube sheet with a hundred holes is definitely a high added-value component. You can’t risk an out-of-tolerance condition on the hundredth hole and ruin the whole thing – or put it into service and risk a leak.” Second, a double cutter doing twice the work inherently will encounter higher lateral forces than a single cutter, he explained, so users will benefit from the more rigid carbide shank.

As a practical matter, most early users are backing off about 30% from Ingersoll’s recommended MRR, Dieken said. Even when they do, they come out ahead. That reported three-to-one productivity improvement at one fabricator was achieved despite a very high ‘ease-off’ from standard recommendations. “With more experience, I have a hunch they’ll ramp it up, but gradually,” he predicted.

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