Behavior of Apparel Fabrics During Convective and Radiant Heating

Personal protective equipment (PPE) recommended for arc flash is not always designed for arc flash exposure. The purpose of this paper is to warn of the dangers posed by using the improper materials in arc flash exposures until standards have caught up on this issue.

The table below shows a representative range of everyday textiles along with some of the measurements of importance in establishing their response towards convective and radiant heating[1]:

Properties of Fabric Table

 

Times to ignition or melting of the 20 fabrics in Table above were reported by Wulff[2], [3] for different incident heat fluxes. The Wulff’s data have been used to develop a methodology by which ignition and melting times may be forecast. A semi-empirical relationship between ignition/melting time and radiative heat flux has been derived[1]:

[NF0] = -1 / NBi * ln(1 – NBi / [qxrad]) + a * [qxrad]^b * (1 – NBi / [qxrad])^-1, Equation 1
where [NF0] is the non-dimensional destruction time of the fabric (that is, time to ignition or melting) and is given by:

[NF0] = (k/l) * t / (pl * c), Equation 2

 

where:

(k/l) – average thermal conductance, W / (m^2 * C);
t – ignition/melting time, sec;
pl – mass/unit area, kg / m^2;
c – average specific heat, W * sec / (kg * C).

NBi is the Biot number which is defined as the ratio of the average convective heat transfer coefficient of the fabric to the average thermal conductance of the fabric. It is obtained experimentally for each fabric.

The non-dimensional radiative heat flux [qxrad] is given by:

[qxrad] = ads * W0 / (k/l) / (Tim – T0), Equation 3 READ MORE