How to Increase the Production of Heat Exchangers

Only solar and wind plants don’t need them. Driven by an unprecedented demand for more efficient energy sources, most providers of heat exchangers are playing catch-up. But in response, 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.

The result is that 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 percent,” says Mike Dieken of Ingersoll Cutting Tools (Rockford, IL), whose team developed the milling cutter that enabled such gains.

HEAT EXCHANGER ANATOMY
In a typical shell and tube heat exchanger (see Figure 1) 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 tubesheet 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 facilitates maintenance and tube replacement later on.

DOUBLE GROOVER: 3-TO-1 IMPROVEMENT
The key to the new tubesheet grooving tool is that it machines both grooves in one step. The previous best practice was to mill them one at a time. One early user reports reducing cycle time per hole from 18 seconds to 5 seconds while eliminating all post-deburring. These are the times for grooving a two-inch thick carbon steel tubesheet with ¾ in 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 this 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 percent longer tool life than before, with no deburring required.

The new grooving tool is essentially a very sophisticated ChipSurfer double form-slotting mill. With basically one orbiting radial plunge action, the cutter simultaneously produces two identical code-compliant grooves. For a ¾ in hole, for example, a 5/8 in milling cutter carves out two 0.125 in flat-bottomed grooves spaced with a 0.250 in gap and 0.030 in maximum depth.

This cutter uses a replaceable solid carbide tip that screws onto a shank, either carbide or alloy steel. 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 in of clearance between a 5/8 in tool working in a deep ¾ in 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,” states Dieken. In response, 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.

PRICE CUSHION ON COSTLY CARBIDE
This new replaceable-tip design also helps cushion the rising cost of carbide. “A short tip uses less high-priced tungsten than a long carbide tool,” notes Dieken. “When the tool is worn, you replace the tip only, not the entire tip and shank.” He adds that the 0.0005 in repeatability tip to tip, a recent refinement, simply enhances the value of replaceable-tip tooling in today’s shop environment. One shop cites the tip costing 75 percent less than their solid carbide single tooth end mills, running 30 percent longer and reducing cycle time by more than 60 percent.

For two reasons specific to tubesheet grooving, Dieken recommends a carbide shank, with its extra rigidity, over its lower-cost alloy counterpart. “First, a tubesheet 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 will inherently encounter higher lateral forces than a single cutter, so users will benefit from the more rigid carbide shank.”

As a practical matter, most early users are backing off about 30 percent from the recommended MRR, adds Dieken. “But even when they do, they come out ahead, because that reported 3-to-1 productivity improvement at one shop was achieved despite a very large easing off from standard recommendations. With more experience, I have a hunch they’ll ramp it up, but gradually.”

Ingersoll Cutting Tools, 845 S. Lyford Road, Rockford, IL 61108-2749, 815-387-6600, Fax : 815-387-6968, [email protected], www.ingersoll-imc.com.