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 Part II

Thermal Garments

 

As we know very well, when the fiber absorbs moisture, heat is evolved. Apart from the moisture absorption property, this absorption of water causes the fabric to act as a heat reservoir, protecting the body from sudden changes of external conditions. Absorption also changes its structural features, cause the material to swell. This swelling results change in size, shape, stiffness and permeability of yarns and fabrics9.

Assessment of the protection requirements in cold environments requires information about the energy metabolism of the individual. Metabolic rate is related to the intensity of physical work and can be easily determined from measurements of oxygen consumptions.

Protection against cold is provided when the human body is in heat balance at acceptable levels of body temperatures. Our body temperature is quite stable. This is achieved by balancing the amounts of heat produced in the body with the loss5. Heat losses are equal to metabolic heat production.

 

in our body there are two sensors which govern the comfort feel in thermal sensing. Skin and hypothalamus are the sensors. Hypothalamus starts to execute its function while the body core temperature reaches exceeds 37˚c by providing necessary cooling action. Skin sensors are cold sensors which work to control the skin temperature, when it starts to fall below 33 – 34˚c. Conditions for both skin as well as core body should be maintained. Perception of cold or hot is totally depends on the individual persons. Various reports say black colored persons can withstand the hot climate but the resistant towards the cold climate is much lower when compared to the white colored persons. Like the same fat person and lean person. Fat persons are having high resistance towards the cold climatic conditions than the lean persons.

 

The protection against cold environment depend on the following main factors like metabolic heat, wind chill, thermal insulation, air permeability and moisture vapor transmission. The survival depends on the balance of heat losses due to the above said factors4.

 

Human beings are homoeothermic; our body always maintains the two different temperatures in our body system, body core temperature and skin temperature. Our body main core temperature is 37°c and our skin temperature closes to 33.4°c within the limits of ±1.5°c. In these temperatures body feels comfort, once tolerance of ± 4.5°c occurs body feels discomfort. In general body always maintains this temperature limits. In the body metabolism the body core temperature is maintained at a temperature of 37˚c. When the body becomes warm, the blood vessels vasodilate and increase in flow of blood occur. And our skin starts to sweat. While body attains cold, blood vessels restricted the flow of blood by vasoconstrict. This causes the increase in the internal heat generation by stimulating the muscles, due to this only we are getting shivering3.

 

 

The function of cloth is to support the thermoregulation of the body. If the environmental temperature is low, the clothing has to prevent too large heat release and on the other hand if the temperature is high it has to give better comfort by allowing the sweat to evaporate thro the garment. The heat and mass transfer through the garment should be as high as possible to avoid problems to the body metabolism. Our clothing should perform better at the lowest level of activity. As we have discussed our body generates heat during performing some work. So at rest our clothing should give better insulation property to avoid loss of heat, and during rest our body generates much lesser heat comparatively. On the whole, our clothing should give comfort in both cases by providing thermal insulation at rest and providing moisture vapor transmission while work.

 

Wind chill plays important role in heat loss process. Wind chill is determined by the number as factor. It purely depends on the temperature and velocity of wind. Around 80% of the total heat losses are due to wind chill effect4. Wind chill can be determined by the formula (1). Wind chill factor helps us to learn about the freezing time of exposed skin. Higher the wind chill factor freezing time for the exposed flesh will be lesser. Decrease in the air temperature and increase in velocity results increase in wind chill factor.

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