Comfort Zone

Published on June 2016 | Categories: Documents | Downloads: 39 | Comments: 0 | Views: 225
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Comfort Zone

GOAL:
To understand how air temperature, humidity, mean radiant temperature (MRT), wind and sunshine affect human comfort.

OBJECTIVE:
You will be able to:

1. Describe ways that the human body reacts while seeking its comfort zone. 2. Explain how air temperature, mean radiant temperature, humidity, wind, and sun affect human comfort. 3. Use the comfort zone chart correctly.

 LESSON/INFORMATION:

The weather is always changing. As winter approaches, the rabbits shed old fur and grow a new heavier pelt, birds fly south, and bears hibernate. When trying to maintain a body temperature close to 98.6°F, humans have few natural controls with which to adapt to these changes in climate.

 The human body has three mechanisms to

maintain this narrow temperature range. The first is heat generated inside the body, the second is by gaining heat from surroundings, and the third is by losing heat to the surroundings. The body automatically makes constant changes to control these three mechanisms and regulate body temperature.

Body Heat Heat is continuously produced by the body due to metabolism, or the processes of food conversion and tissue building. Additional heat is produced by muscular activity, which varies from 70 watts while sleeping to 1100 watts for maximum heavy manual work. Of all the heat produced, 20% is utilised, and 80% must be dissipated, in order to maintain deep body temperature at 37°C. Any heat gained from the environment and from solar radiation must also be dissipated. The body can lose heat by convection, radiation and evaporation, and to a lesser extent by conduction. Convection is produced when heat is transferred from the body to the air adjacent to the skin or clothing, which rises and is replaced by cooler air. Radiant heat loss depends on the temperature of the body surface and the temperature of opposing surfaces. Evaporative heat loss depends on the rate of evaporation which depends on the humidity of the air.

The following are a few of the ways the body responds in order to stay within the comfort zone:

Increased muscle activity and a higher metabolic rate increases internal heat production.

Sweating -

Blood Flow -

Reduced flow to the hands, feet and skin surface in the winter to reduce heat loss to surroundings and an increase in blood flow to these areas in the summer to encourage heat loss.

Comfort Human beings can tolerate a fairly wide range of climatic conditions, but comfort in the climatic sense involves more than just avoiding the extremes of freezing to death and dying of heat exhaustion. Comfort depends on more than temperature; air temperature, humidity, radiation and air movement all produce thermal effects. Most climatic comfort indicators are objective, i.e. they can be measured, and acceptable ranges established quantitatively.

Factors Affecting Human Comfort Include:
Air

temperature is the most significant

ambient factor which affects our internal temperature and our level of comfort. But, it is not the only factor involved; air speed, humidity and mean radiant temperature must also be considered. Each of these four factors has a direct influence on the rate at which the body loses or gains heat to or from the surroundings.

 Air Temperature -

This affects temperature difference between the body and the surroundings, consequently affecting the rate of heat loss or gain by convection.  Air Speed This affects the rate at which the body loses heat by convection. The chill factor is one way to quantify the effects of air speed on heat loss. An air temperature of 35°F and a wind speed of 20 miles/hour combine to give a wind chill temperature of 11.2°F. This means that a body exposed to 35°F air and 20 mile/hour wind loses heat at the same rate as

 Mean Radiant Temperature (MRT) -

MRT is the average of the surface temperature of the surroundings with which the body can exchange heat by radiant transfer. Radiant heat transfer to and from the body is quite apparent when sitting near a fireplace (high MRT) or large cold window area (low MRT).
 Humidity -

Affects the rate at which the body loses heat by evaporation. During hot weather, high humidity

 The

diagram and which

shows the area of warmth humidity

the human body would classify as comfortable.
 The

higher lower

the the

relative humidity the room

 An easy way of describing the effect of air temperature, humidity, MRT,

wind and sunshine is the Bioclimatic Chart below.

The comfort zone can be pushed up by the presence of air movement, but lowered by higher levels of radiation. The results were obtained from a study of men in sedentary occupations, wearing clothing (suit, cotton underwear) in a warm climate.

 The air temperature is plotted on the vertical axis

and relative humidity on the horizontal axis. The shaded area near the center of the graph shows the combination of temperature and humidity which most humans would find comfortable during the summer if they are sitting in the shade. The dotted area shows the comfort zone for the winter. It is interesting to see that the human body can actually

 The climatic elements around the comfort zone are

shown by means of curves which indicate the nature of corrective measures necessary to restore the feeling of comfort at any point outside the comfort zone. For any point of known dry-bulb temperature and relative humidity which falls within the

boundaries of the comfort zone, no corrective

 For example, at dry-bulb temperature, 73°F, relative

humidity of 50%, no corrective measures are needed because this point falls within the comfort zone.
 At dry-bulb temperature, 78°F, relative humidity of

70%, it would require a wind speed of about 250 FPM to provide comfort.
 At dry-bulb temperature of 50°F, relative humidity of

55%, it would require 250 Btu/hr of sunshine to provide comfort.

Thanks…

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