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The Case of the Reluctant Radiator
Myron J. Boyajian, President, Engineering Consultants -- Design News, May 12, 2008
In the day, a major forklift manufacturer redesigned a forklift model for a product launch. This engine-powered unit received new controls, structural and cosmetic changes and a revised engine.
The Scene of the Crime
Development went smoothly until one pre-production truck overheated during a rapid cycle, dock operation.
The Investigation
Puzzlement reigned as there were no prior overheating incidents. However, all testing was done on the outside test track, so we concluded that hard-running indoors stressed a possibly marginal cooling system. We noted a last minute change; assured by a supplier that the fin-and-tube count and heat transfer rates were unchanged, the project engineer substituted new radiators on the pre-pro trucks without additional tests. Now, designers and our lab crew met to ferret out the cause of overheating. While our meeting went on, covering such items as anemometers to measure air flow through the radiator and thermocouples to measure coolant and radiator in/out air temperatures, I sort-of mentally zoned out, going back a year or so to recall a particular experience.
On a hazy, almost windless late summer day, a thick white cloud arose in my backyard. In awe, my son and I watched the thick, dense, roiling, billowing cloud rise like the djinni in the classic movie, “The Thief of Baghdad.” The cloud rose to about 40 ft and gently pushed by a soft westbound wind, started to move through our neighborhood. Holding its shape, it crossed backyards and streets, then covered a home, crossed another yard and then drifted across a four-lane thoroughfare that divided our village. On our bicycles, my son and I followed the lazily drifting cloud at about walking speed. We could hear neighborhood doors opening and slamming and shouts of, “What's going on?” as we pedaled westward. Almost with a life of its own, the dense cloud slowly drifted across the highway, bringing auto traffic to a grinding halt, while wide-eyed drivers called out in puzzlement. Finally, in the open area of the highway, with the urging of the breeze, the cloud began to break up as auto traffic resumed. Blinking through tears of laughter, we rode home. For our neighbors, this was a mystery to talk about at their next barbeque, but for us, this was another backyard experiment, ranking up there with constructing a pulse jet and running a lawn mower engine with an oxy-acetylene fuel source until the piston blew out of the exhaust port like confetti at a Columbus Day parade. So, what caused the cloud? A few days before this incident, our plant maintenance lads were cleaning up and pitching out unused materials and worn and broken parts. Among the discards were two dozen unused smoke bombs of the type used to determine obstructions in sewers, drains and vents. With the shop foreman's blessing, I took home the eventual source of our mystery cloud.
This short reverie led me to suggest that a judiciously placed smoke bomb, along with well-located view ports, would give us a real-time, dynamic view of air flow. I opined there was air recirculation around the fan blade tips reducing the net radiator air flow and a smoke test could prove it. The project engineer disagreed and ordered formal anemometer and thermocouple tests to proceed. I told the engineer that the lab had a modest discretionary budget and I would conduct the smoke test at no charge to his project budget. Glowering, he neither agreed nor disagreed and concluded the meeting.
We did conduct the air flow and temperature tests, but the smoke test beckoned. My trusty lieutenant for this test was a greybeard who was not only our union shop steward, but an excellent mechanic; a fellow who would have been a fine technician or engineer in another life.
We drilled some view ports, set up the lights, and touched off the smoke bomb and watched as the smoky air between the spinning pusher-type fan and the radiator was beaten into a hazy froth, demonstrating a lack of smooth air flow through the radiator. Test data, including the hated smoke test, led the project engineer to compare the changed radiator to the original radiator.
The Smoking Gun
He concluded the fan shroud of the new radiator was too short to cover enough of the projected length of the fan blades to permit good air circulation through the radiator. A new radiator with the correct shroud solved the problem. This experience taught us all to be open to new ways to conduct tests while not abandoning classic test procedures, and yes, make sure new parts really match the specs!
| Author Information |
| Myron J. Boyajian, P.E., (mboyajian@sbcglobal.net) is president of Engineering Consultants, a consulting service for forensic and design activities. Cases presented here are from his actual files. |
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