The phrase air-conditioning is commonly used to suggest cooling, but it also refers to the process of conditioning the air to the desired level through heating, cooling, humidifying, dehumidifying, cleaning, and deodorizing. The purpose of a building’s air-conditioning system is to ensure total thermal comfort for its occupants. As a result, in order to develop an effective air-conditioning system, we must first understand the thermal features of the human body.

Cells are the basic blocks of living beings, like microscopic factories that execute diverse duties essential for organ-ism existence. The human body is made up of around 100 trillion cells with an average diameter of 0.01 mm. Every second, millions of chemical events occur in a normal cell, during which some molecules are broken down, energy is released, and other molecules are produced. Metabolism is the high degree of chemical activity in the cells that keeps the human body temperature at 37.0C while completing the required biological tasks. Metabolism is the process through which meals such as carbs, fat, and protein are burned. Nutritionists often represent the metabolizable energy content of foods in terms of capitalized calories. One calorie equals one kilocalorie = 4.1868 kJ.

The rate of metabolism at rest is known as the basal metabolic rate, and it is the rate of metabolism required to keep a body executing essential biological activities like breathing and blood circulation at zero external activity level. The metabolic rate can also be viewed as the rate at which a body consumes energy. The basal metabolic rate for a typical guy (30 years old, 70 kg, 1.73 m tall, 1.8 m2 surface area) is 84 W. That is, at a rate of 84 J/s, the body converts chemical energy from food (or body fat if the person hasn’t eaten) into heat, which is subsequently dispersed to the surroundings. When performing a vigorous exercise, the metabolic rate increases with the degree of activity and can approach 10 times the baseline metabolic rate. That is, two persons moving vigorously in a room may contribute more energy to the space than a 1-kW resistive heater. While sitting in a classroom, an average male creates 108 W of heat while reading, writing, typing, or listening to a lecture. An average man’s maximal metabolic rate is 1250 W at age 20 and 730 W at age 70. The equivalent rates for women are almost 30% lower. Trained athletes’ maximum metabolic rates can surpass 2000 W.

The table shows metabolic rates per unit of body surface area during various occupations. D. DuBois calculated the surface area of a bare body in 1916. 

where m is the body mass in kilograms and h is the height in meters. Clothing may increase a person’s visible surface area by up to 50%. The metabolic rates in the table are accurate enough for most uses, however, there is significant ambiguity at high activity levels. More precise figures may be obtained by measuring the rate of respiratory oxygen consumption, which ranges from around 0.25 L/min for an average resting man to more than 2 L/min for really strenuous employment. Because the external mechanical effort done by the muscles is so little, the whole energy released during metabolism may be considered to be discharged as heat (in perceptible or latent forms). Furthermore, most activities, such as walking or riding an exercise bicycle, convert work to heat through friction.

Part-5: Thermal comfort air.

Part-4: Adaptations and acclimation mechanisms of Heat in the Human Body.

Part-3: Cold adaptation in humans.

Part-2: Thermal comfort in buildings.

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