Page 216 - Industrial Ventilation Design Guidebook

S. 1 THERMAL COMFORT 1 77

or

2
where A D is body surface area in m . The subscript D refers to the DuBois
6
equation commonly used for calculating the area of the skin:

where m = mass (kg), h = height (m). Surface areas are generally in the range
2
of 1.4 to 2.2m .
In some activities metabolic energy may be converted to useful work
(force • distance). At steady state the rate of doing work P = force • distance/
time and the thermal losses must balance with metabolism:

and if the rate of work is expressed as a thermal efficiency, 17 = P/M, then Eq.
(5.5) simplifies to

Example
Determine the met level of a person who bicycles up a 150 m-high hill in
10 minutes. The person weighs 75 kg and is 182 cm tall. The bicycle weighs
10 kg.
work of cycling up the hill = force • distance

The work is accomplished over a period of 10 minutes, so

Cycling with the legs is rather efficient and it can be reasonably assumed that
the thermal efficiency (77) is about 20%. Thus

This energy, normalized per unit of body surface area (M/A D) where

is

Expressed in terms of met:

Since this activity is greater than about 7 met, the effort of breathing may
make it difficult to talk during the climb.

Physiological Temperature Regulation
For most situations and conditions in daily life, the human can be repre-
sented adequately by a simple model that is helpful for understanding human
7
thermal regulation. The model has two thermal compartments (Fig 5.1). The