In auto repair workshops, welding, cutting, exhaust system maintenance and dismantling of old components generate large amounts of heat. The essential protective function of heat resistant fire suits lies in intercepting, attenuating and blocking heat energy. Heat transfers in three basic modes: thermal radiation, thermal convection and thermal conduction. In repair workshops, these three heat transfer modes usually act simultaneously. The garment design, fabric structure and tailoring process are all built to control heat transfer paths, slowing heat penetration through fabric toward human skin and preventing thermal burns. Understanding heat transfer rules is the key principle behind heat-resistant protective garments.

　　Thermal radiation constitutes the primary heat source in repair workshops. Hot molten slag and red-hot workpieces from welding emit infrared radiation, transferring thermal energy through air in the form of electromagnetic waves without any medium. Contact with hot objects is unnecessary; workers can feel intense radiant heating when standing a certain distance away from heat sources. Modified outer fabric of fire suits is designed to reflect part of radiant heat and reduce heat absorption by textiles. Without anti-radiation modification on fabric surfaces, most infrared energy will be absorbed by fibers, raising fabric temperature and enabling continuous inward heat transfer. On fabric structure, low-absorption, high-reflection outer coatings reflect a large portion of infrared radiation outward and reduce heat entering inside the garment. During repair operations, higher heat-source intensity or shorter distance increases radiant heat flux and speeds up heat accumulation inside the suit.

　　The second mode is thermal convection, transferring heat through flowing hot air and hot flue gas. During welding, hot air rises and carries high-temperature particles to form thermal airflow inside workshops. Hot airflow hitting the garment surface continuously delivers heat to the fabric. The dense outer layer of suit fabric blocks hot airflow from penetrating fabric pores and reduces inward convective heat transfer. Fabric pore design balances breathability and thermal airflow blocking: excessively large pores allow hot airflow penetration and rapid heat ingress; overly compact pores trap sweat vapor generated by human bodies, causing internal heat buildup, heat stress, stuffiness and reduced physical stamina that compromises operational safety.

　　The third type, thermal conduction, transfers heat through direct contact with hot surfaces. In repair workshops, when workers accidentally touch red-hot exhaust pipes, hot workpieces or slag fragments, heat conducts directly from hot solids to suit fabric and then toward the skin. The thermal conductivity of fiber materials determines conduction speed. Low-conductivity aramid fibers, paired with trapped static air inside fabric layers, greatly slow thermal conduction. Air features low thermal conductivity; static air trapped between multi-layer fabrics serves as the most efficient and economical barrier against conductive heat. However, when protective garments are squeezed or folded, trapped air is squeezed out, insulation capacity drops sharply and conductive heat transfer accelerates.

　　Heat accumulation inside garments is another easily overlooked factor in thermal management. After absorbing radiant and convective heat, fabric temperature rises and the fabric itself becomes a heat reservoir. Even when workers temporarily move away from heat sources, heat stored inside fabric continues releasing inward. Under intermittent working patterns in repair shops, workers repeatedly approach and leave welding torches, causing fabrics to absorb and store heat cyclically. After multiple cycles, accumulated thermal energy builds up continuously. Once total stored heat exceeds the critical threshold, burn risks remain even without direct heat exposure. Therefore fabric heat capacity is a vital parameter. The fabric must absorb limited heat without unlimited heat storage and gradually release trapped heat during work breaks to avoid concentrated heat release that burns skin.

　　Heat control of the whole suit relies on coordinated multi-layer fabric functions. The outer layer reflects radiant heat and resists impact from high-temperature particles to reduce heat absorption. The middle barrier film blocks hot airflow and liquid molten droplets and cuts off convective and contact heat transfer paths. The fluffy inner insulation felt captures static air and reduces conductive heat transfer speed to delay inward heat penetration. Heat load varies across body zones, so the suit adopts zoned design: full three-layer composite material with thickened insulation is used for high heat-exposure areas including chest, shoulders and arms, extending heat penetration time. Lightweight double-layer material is applied at armpits, waist and joints to control heat ingress while lowering total garment weight and improving mobility for disassembly and repair tasks.

　　Heat-related performance tests quantify the suit’s ability to block heat, with the core index being Thermal Protective Performance (TPP). Test equipment simulates flame and radiant heat sources found in repair workshops and records the time required for heat to penetrate fabric and reach simulated skin. Higher TPP values mean stronger heat-blocking capability. Tests also record temperature-rise curves to observe heat transfer rates and heat accumulation and release processes. Besides TPP tests, repeated thermal cycle aging tests simulate workshop usage and examine whether heat-blocking performance degrades after repeated heating and cooling cycles. Long-term use, fabric carbonization, coating abrasion and compressed insulation layers weaken heat interception and reduce TPP values, leading to protection failure.

　　During field wear, environmental conditions alter heat transfer. High ambient workshop temperature and poor air circulation slow outward heat dissipation of garments and accelerate internal heat accumulation. Humidity also influences heat transfer; sweat fills fabric pores, raises fiber thermal conductivity and speeds heat conduction. When repair technicians wear the suit for work, continuous operating duration must be controlled to avoid prolonged heat absorption and accumulation. Operators should take short breaks in ventilated zones between tasks to allow garments release stored heat and lower internal temperature. Once fabric suffers ablation or tearing, multi-layer structures are broken, heat transfer channels open and protective capacity drops drastically, requiring immediate withdrawal from service.

　　Maintenance directly affects the garment’s ability to control heat transfer. Strong alkaline detergents and high-temperature drying are forbidden during cleaning. High heat damages the fabric’s anti-radiation coating and compresses inner insulation felt, reducing static air content and lowering thermal insulation performance. Each washing cycle causes minor degradation of thermal protection indicators. When reaching the maximum laundering lifespan, fire suits must be scrapped and cannot be reused for repair work.

　　In summary, heat resistant fire suits are not merely “high-temperature resistant”. They intervene in three heat transfer paths — thermal radiation, convection and conduction via fabric construction, reflecting, blocking and delaying heat penetration. Auto repair workshops feature complex and variable heat source intensity, and heat accumulation effects cannot be ignored. The core design of the suit extends the time window for heat penetration and gives operators a safety margin during short high-temperature exposure. In daily use, controlling continuous working time, regular fabric inspection and standardized cleaning and maintenance all preserve the garment’s heat-blocking performance, reduce burn hazards during high-temperature repair operations and protect maintenance staff.