Industrial PLC Controller Cooling in Offshore Oil Platform Control Room

Scenario

Retrofit of redundant Allen-Bradley ControlLogix 5580 PLCs aboard a North Sea oil platform. The control room is air-conditioned but experiences transient ambient spikes during HVAC failure or summer heat waves. Space is extremely constrained (rack-mounted 1U chassis), and vibration/marine corrosion preclude fans or liquid cooling. Critical constraint: junction temperature must never exceed 105°C to avoid firmware throttling or watchdog timeouts during safety-critical sequences.

Given Data

• Maximum Junction Temperature (t_j_max): 105°C (manufacturer’s absolute max for sustained operation)
• Ambient Temperature (t_a): 55°C (verified worst-case during 72-hr HVAC outage test)
• Power Dissipation (p_diss): 28 W (measured peak across CPU, I/O, and communication modules)

Calculation

$$ R_{\text{th}} = \frac{T_{j,\text{max}} - T_a}{P_{\text{diss}}} = \frac{105^\circ\text{C} - 55^\circ\text{C}}{28,\text{W}} = \frac{50}{28} = 1.79,^\circ\text{C}/\text{W} $$

The Thermal Resistance Calculator returns: r_th = 1.79 °C/W.

Result and Decision

A copper-aluminum hybrid heat sink (base: 3 mm thick copper for spreading, fins: extruded aluminum) with 1.2-mm fin pitch and nickel-plated finish (for salt corrosion resistance) delivered 1.72 °C/W in accelerated life testing (55°C ambient, 2g vibration, 95% RH). Combined with a 0.1 mm graphite thermal pad (0.5 °C·cm²/W effective interface resistance), total system Rth was 1.77 °C/W — within 1.1% of target. Existing rack airflow (1.2 m/s forced convection from adjacent servers) was leveraged as a design input, validated via thermal imaging.

Lesson

For space-constrained industrial electronics, interface resistance dominates total thermal path — investing in low-resistance TIMs (e.g., graphite pads or metal-filled greases) often yields greater ROI than larger heatsinks; always measure actual interface resistance under operational vibration and thermal cycling, not just datasheet values.