to maximize module count while minimizing footprint, honeywell engineers developing the experion r300 process control system vertically stacked the i/o and controller modules. but vertical stacking created an overheating problem. cool air entering the bottom of the cabinet would grow warmer as it traveled upward from one module to the next. by the time it reached the upper portion of the stack, the air would be so hot that it would cause modules to overheat.

the system contains 36 modules stacked into three columns of 12 modules each. air enters through vents at the bottom of the cabinet and exhausts through the fans at the top. design specifications set the ambient air temperature at 50 c, while many components are rated at 70 c. to keep components from exceeding this limit, more fans and vents would be required, but this was not an option because fans reduce reliability, add noise, and pull in contaminants. additional venting on the sides was not acceptable because the equipment is often joined side by side.

with the obvious fixes off-limits, honeywell engineers reasoned that if each board were tilted at the appropriate angle, unheated air could enter at the bottom right side of the module, flow across it and exit at the top left side. each module, regardless of its vertical position in the stack, would be cooled by unheated air entering from the bottom.

since there was no time to build a physical prototype to prove the concept, engineers used coolit software. the coolit analysis predicted that if the modules were slanted 18 deg and stacked relatively close (1/4”) together in the vertical direction, cool air would reach each of the 36 modules. there would be no co-mingling of hot and cool air from one column to the next. verifying the design through physical prototyping would have taken a minimum of 2-3 months. time-to-market was critical and coolit delivered.

tilting the modules solved the airflow problem, but there was still one more thermal challenge. one i/o module dissipated almost twice as much heat as the others.��the problem module contained 16 high-heat-dissipating fets (field effect transistors) and heat from the fets was being transported via the copper traces across the length of the circuit board to other devices, negatively impacting their reliability.� engineers tried to fix the problem by spreading the fets evenly over the board surface, but coolit analysis predicted that some devices would still be subjected to excessive heat.�

the next proposal was to thermally isolate, as much possible, the high-heat-producing devices from the rest of the board. a design was developed in which all fets were separated on the board from other devices by a thermal barrier.��the concept worked. heat passing from the fets to the opposite end of the board was dramatically reduced, and coolit analysis verified that components on both sides of the barrier remained within their respective operating limits.

using computer modeling was essential. �it would take hundreds of boards to check out a design, and months to get them fabricated, assembled, and tested.� �they would have�been shipped to several honeywell sites around the world for complete system testing. by the time engineering had determined �there was a problem and redesigned the pwa, a minimum of six months would have passed and hundreds of boards would have been thrown away. developing this design using thermal simulation saved honeywell a minimum of 6 months and considerable money.

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coolit analysis shows that the tilted module design will deliver cool air to each module. in this scenario, the process control modules (right)�are mounted with optional third party equipment.

in the honeywell experion r300 system, 36 process control i/o and controller modules are vertically stacked in tilted side-by-side columns.