Pneumatic and Linear Heat Detection: Industrial Early Warning Systems

Standard smoke detectors don't work in high-ceiling warehouses, cable tunnels, or petroleum tanks. For these environments, linear heat detection (LHD) and pneumatic detection monitor temperature along a cable or tube. NFPA 72 governs these systems. Types, principles, and application details below.

Why Linear Detection?

• Point detectors: fixed location; miss a fire starting 5-10 m away
• Very large or irregular spaces: point detector count explodes
• Explosive atmosphere (ATEX): electrical detectors risky
• Linear solutions: sensitive at every point along the cable/tube

Digital LHD Cable

Two conductors separated by heat-sensitive polymer. At a set temperature (68°C, 88°C, 105°C), polymer melts, conductors touch, alarm fires.

• Pro: Simple, cheap, ATEX-compatible
• Con: Alarm only (no temperature reading)
• Typical: conveyor, cable gallery, cold storage
• Single-shot: triggered section must be replaced

Analog LHD

Thermistor or thermopile array inside the cable. Panel tracks temperature continuously, performs trend analysis, fires fixed + rate-of-rise alarms.

• Pro: Temperature monitoring + false-alarm reduction
• Con: More expensive than digital
• Typical: data center hot aisle, LPG storage

DTS (Distributed Temperature Sensing)

Fiber optic cable; temperature reading every 1 m. Cable length up to 30 km.

• Laser pulse → backscatter → Raman scattering analysis
• ±1°C accuracy, 1 m location resolution
• Pro: Pinpoints fire within 1 m
• Con: Expensive equipment, fiber expertise required
• Typical: HV cable tunnel, LNG tank, rail tunnel

Pneumatic Detection

Air inside a copper or stainless steel tube; heat expands air, pressure rises, pressure switch alarms.

• Rate-of-rise: rapid heating + pressure sensor
• Fixed temperature: eutectic fuse
• Pro: Fully passive, no power, ATEX-ready
• Con: Tube maintenance, leak risk
• Typical: petroleum tank, turbine enclosure, aircraft hangar

Design Rules

• Cable/tube 40-50 cm below ceiling (smoke accumulation zone)
• Max spacing: LHD 7.5 m (type I), 5 m (type II), 3 m (type III)
• Minimum bend radius at turns
• Mechanical protection: forklift, rodent damage
• Calibration: annual test with heat gun

Turkey Application

LHD is widespread in Turkish refineries, petrochemical plants, LNG terminals, and highway tunnels. DTS still expensive — limited to critical pipelines and tunnels. Pneumatic systems less common due to import cost and digital LHD alternative.

Conclusion

Industrial early warning is an engineering problem point detectors can't solve. LHD, DTS, pneumatic — each best in a different scenario. Correctly sizing the site (ATEX, temperature, length) is the key to device selection. NFPA 72 provides the framework; the engineer adapts it.

Detection system selection in MEP Calc

LHD/DTS/pneumatic comparison, cable layout schedule, ceiling distance calc.

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