ชื่อข้อมูลทางด้านเทคนิค : THERMAL STRESS GUIDE TECHNICAL DATA
Thermal Radiation
Man gains heat from the environment when the temperature of the materials of that environment (Walls, machinery, etc.) is higher than his skin temperature. This gain occurs by a process of heat exchange via radiation. No physical contact between man and the hot surface is necessary for this process to occur. The transfer occurs by the passage of electromagnetic waves through the air.
The level of thermal radiation to which man is being exposed may be measured directly in units of
Watts (or kilowatts) per square metre.
The rate of heat exchange between man and his environment by radiation depends on the gradient between the mean radiant temperature of the surroundings and the mean temperature of the skin.
The mean radiant temperature is a unit which summarises the surface temperature of the surroundings. In environments where the equipment around man is above 35 oC man will gain heat and his core temperature will rise. In cold environments there will be a loss of heat.
In very high level radiant fields it is not the elevation of core temperature which is of prime importance but the fact that a pain reaction or burns may result. These effects occur at skin temperatures in excess of 45 0C depending on the level and duration of the exposure.
The following table summarizes data relating to high radiant heat exposure.
Radiation Intensity
(kW.m-2) Time to Pain Time to Burn
1.5 indefinite exposure permitted
1.8 approx. 2 min 3-4 min.
2.2 – 3.0 30 sec. – 1 min. 1 – 2 min.
3.0 – 4.0 15 – 30 sec. 1 min.
4.0 – 6.0 10 sec. 30 sec.
6.0 – 10.0 5 – 10 sec. 10 – 20 sec.
> 10.0 severe burns in seconds – normal clothing may ignite
Reference:
- “Management Guide to Thermal Stress”, Shell Safety and Health Committee, December 1991
- “The Physiological and Pathological Effects of Thermal Radiation”, I. Hymes, Sept. 83, Safety and Reliability Directorate, UKAEA. Warrington.
Measurement of radiant temperature is performed using a black globe thermometer. The mean radiant temperature may then be determined using a monogram.
In the case of high level radiant sources the heat flux is measured directly using a radiation pyrometer which focuses incident radiation on a thermocouple. It is calibrated to produce a reading in units of, for example, kW.m-2