In more forgiving processes, steam tube tracing sometimes gets the nod because it is thought to be adequate and economical. What can be cheaper than to wrap a few feet of plain copper tubing around the pipe, then run a hot fluid through it? Then, the system starts up and the operator accepts the results, however good or bad. What the operator may forget is the energy bills he will be paying thereafter, not to mention limitations in temperature control as well as increases in scrap rates, downtime and repair costs, and incomplete melt-out after shutdown.
That was exactly the situation at a Midwestern insulation manufacturer that uses asphalt as a binder in the laminating process. The asphalt bonds the vapor barrier to the fiberglass mat. The existing system used a patchwork of fully jacketed piping and steam tube tracing to keep the asphalt hot and flowing. Flow alternated between liquid and blobs, which was deemed to be okay, even though it spoiled a couple of thousand dollars a month worth of product.
The asphalt transfer system, involving about 1,200' of 3" dia. piping, distributes the hot asphalt to five processing lines. Valves permit shutoff of the asphalt at any idle processing line. Process temperature in the old system ranged between 150 and 350oF (66 to 177oC), depending on ambient temperatures, location in the system and the plant's operating rate. The supply tank and the first 200' of piping are outdoors; the balance is indoors.
In an effort to reduce plant-wide energy costs, senior management converted the entire plant from steam to a closed hot-oil system. The conversion was triggered by an outside study that projected that switching heating media would reduce energy costs by about 40 percent.
The conversion meant the plant engineers in the asphalt-handling section had to find another way to keep the asphalt hot and flowing. Their original idea for a replacement system was to simply run hot oil through the existing steam lines. However, a study by Varo Engineers, Columbus, Ohio, revealed that the entire existing system, a combination of both jacketed piping and trace tubing, needed replacement. The insulation was in bad shape and both the unjacketed and jacketed asphalt piping leaked. Energy losses as well as mounting maintenance and housekeeping headaches would have cut into the expected savings. It could have kept working, but at a very high cost.
Start From ScratchSo, the search began for a completely new thermal maintenance system. The company and Varo narrowed the choice down to either a new tube trace system or bolt-on jacketing. Fully jacketed piping was ruled out as unnecessarily expensive overkill.
Estimates from Mid Ohio Mechanical, Granville, Ohio, showed that the bolt-on system would cost about 40 percent less than fully jacketed piping and fit in a smaller physical envelope. Importantly, it also would offer a four-hour or less warmup/recovery time, not the several hours (or days, in some cases) required for tube trace.
Richard Herron, a mechanical project engineer at Mid Ohio, based his recommendations on both his estimates and previous experience with both types of systems. “True, copper tubing is inexpensive, but fitting it snugly to pipe in the field is a very uncertain, skill-intensive operation, which can really run up the total installed cost,” he said. “Also, once the line is up and running, heat transfer efficiency depends greatly on how closely the tubing contacts the pipe. At best, it is 'point contact.'”
“Greater contact area is the main reason for the difference in thermal efficiency between tube trace and bolt-on jacketing. Bolt-on jacketing contacts the pipe over its total area while copper tubing makes contact only at the tangent point,” said Chris Grice, an engineer at Controls Southeast Inc. (CSI), Charlotte. A known contact area establishes the predictable thermal performance and explains the faster recovery time and higher thermal efficiency of bolt-on jacketing.
The operator agreed with the bolt-on recommendation, so Mid Ohio Mechanical completed the installation of the $180,000 piping and thermal maintenance project.
“All the piping is covered with ControTrace bolt-on pipe jacketing from CSI,” noted Mike McGuire an engineer with CSI. “The new valves are jacketed with ControHeat bolt-on jacketing from CSI to fit each particular valve. Installation time for the entire jacketing system was about three weeks and was concurrent with the schedule for the process piping itself.”
New System, Close UpThe asphalt supply system operates as a closed circuit with individual loops running off the main line to serve each of the five production lines. It uses 200 gal of heating oil. Temperature is maintained at 369oF +/-5oF (187oC +/-2.78oC). Shutoff valves on either side of the dispensing nozzles in each loop make it easy to turn the asphalt supply on and off to match laminating line production cycles. When the line is running, so is the asphalt, and vice versa.
The jacketing system, designed by CSI, matches the asphalt supply system in operating flexibility. When the asphalt is turned off to a particular loop, so is its heating oil, reducing energy costs. Moreover, hot oil is fed into the system at 17 separate areas to keep temperature consistent throughout.
The system maintains exactly 369oF throughout the active parts of the system, regardless of outside temperatures or which laminating lines are running. Production personnel can restart a cold, idle asphalt line and be back up to temperature in less than 4 hr.
Energy and maintenance savings aside, the company is saving a substantial amount of money per month in reduced scrap. The asphalt flows so uniformly that all scrap associated with blobs and chunks of asphalt have disappeared. PH
For more information on Controls Southeast Inc.'s bolt-on jacketing: Call (704) 588-3030 or visit www.csiheat.com.