The cooling from evaporation of sweat
is a very common phenomenon. Based on this principle, the evaporative cooling
garments (ECGs) are developed where water is used as the evaporative liquid.
The large latent heat of evaporation of water, 2430 kJ/kg at 30C, promises
large cooling capacity. The concept of ECGs is to hold reserve water in the
cooling garment, which will evaporate on receiving heat either from the body or
from the outer environment. This will either raise the heat dissipation or
reduce the heat input to body, resulting in a reduced heat stress.
Evaporative cooling garment
It has been shown that with
the rise in ambient temperature the environmental heat will assist in
evaporation, and thus will reduce the duration of cooling. Low level of
humidity facilitates the evaporation process, whereas high amount of vapour
concentration in the air constrains the evaporation process. The light weight
of the laminate structure is one advantage for its use in cooling garments. The
main weight load will be of water, whose volume depends on preferred cooling
duration and environmental conditions.
The three layer quilted structure
(Figure 1) consists of an absorbent core which is basically a nonwoven fibrous
felt or batting, comprising of a typical blend of three types of fibre:
cellulose, a cross-linked polyacrylate co-polymer (superabsorbent fibre) and
polyolefin bonding fibre. The preferred composition of these three types of
fibre in the nonwoven structure is 40% polyacrylate, 30% cellulose and 30%
polyolefin. The nonwoven structure has a typical basis weight of 120 g/m2.
Cellulose fibres help in rapid uptake of water as well as a quick release,
giving an immediate cooling effect. The polyacrylate fibre is not as efficient
in wicking as cotton, but it absorbs and retains a large amount of water. The
strong hydrogen bonds in the polyacrylate fibre avoid draining of water from
the structure due to gravity and releases water vapour at a steady rate over
the period of time. On applying heat, the thermoplastic polyolefin fibres act
as an adhesive binder for the other fibres to stabilise the nonwoven structure.