Human comfort is defined by the “ASHRAE”, i.e. ‘American Society of Heating, Refrigeration and Air Conditioning Engineers’. Human comfort is defined as, “the condition of mind, which expresses satisfaction with thermal environment”. Thus, human comfort provides a human-friendly environment by controlling properties of air and its circulation. Human comfort depends upon the physiological and psychological conditions of human beings. Perception (Definition) of comfort varies from person to person, However, there are certain conditions of temperature and humidity of air, at which, everybody feels comfortable.
CONCEPT OF THERMAL EXCHANGE OF BODY WITH ENVIRONMENT
Human body is a machine producing work and consuming food, i.e. calories intake. Human body cannot tolerate wide variation in living temperatures. A human body fee s comfortable thermodynamically, when
Heat produced by the metabolism of human body = Heat dissipated to the surroundings + Heat stored in human body by raising the temperature
The human body maintains its thermal equilibrium with the environment by means of three modes of heat transfer i.e. Evaporation, Radiation and Convection. These modes can be represented by the following equation.
QM = QE ± QR ± QC ± QS
Here,
QM = Metabolic heat produced within the body in kJ/hr
W = Useful rate of working in kJ/hr
QM – W = Heat to be dissipated to atmosphere in kJ/hr
± QS = Stored energy in temperature rise of body tissue (positive, when temperature of tissue increases and negative, when temperature of tissue decreases) in kJ/hr
QE = Evaporative heat loss due to water evaporating at skin surface in kJ/hr
± QR = Heat loss or gain by radiation (positive, when heat is lost to the surrounding, and negative, when it is gained from the surrounding) in kJ/hr
± QC = Heat loss or gain by conduction and convection (positive, when heat is lost to the surrounding and negative, when heat is gained from the surrounding) in kJ/hr.’
Fig. 1: Thermal exchange of body with environment
Metabolic rate is defined as, “the rate, at Which, the body produces heat”. The value of metabolic rate depends upon a number of factors such as individual’s health, his physical activity and his surrounding environment. The metabolic heat produced depends upon the rate of food energy consumption in body. A man doing fast (i.e. not to eat or religious fasting) will produce less metabolic heat, as compared to others on same day. If QE, QR and QC are high and positive, then QS will become negative. Thus, QM is low and hence, human being feels sick. Old, weak or fasting man feels more cold, when exposed to low temperature and high draft air (high velocity air). A man gets fever, when Internal body activities increase QM to such an extent that, QS becomes positive for given QE, QR and QC. The stored energy QS has the maximum and minimum limits, Crossing the imit leads to death. Human body temperature exceeding 40.5°C (i.e. 104.9°F in Fahrenheit) and falling below 36.6°C (i.e. 98°F) is dangerous. For a normal man, the normal body temperature is 37°C (i.e. 98.6°F), when QS = 0. There is some kind of thermostatic control in human body, which tries to maintain temperature of human body at the normal level of 37°C (i.e. 98.6°F).
Sensible heat transfer means a process of heat exchange, where temperature change can be physically sensed and measured, whereas, in latent heat transfer, the temperature change cannot be sensed. But, phase change takes place. Solid, liquid and gases are the existing phases. An example of phase change is conversion of water at 0°C into ice at 0°C.
During winter period, the outside environment temperature is lesser as compared to body temperature and so there is a sensible heat transfer from body to environment. Due to this, the temperature of body falls down and human body feels cold effect. In contrast to this, during summer period, the outside environment temperature is higher as compared to body temperature. This leads to heat transfer from surrounding to human body and so there is a rise in temperature of human body and human body fee s warm or hot effect. These are the cases of sensible heat exchange.
When during summer period, due to hotness and wet conditions (i.e. more humidity conditions found in Mumbai) there could be a lot of sweat coming from human body. If now there is any evaporation of sweat taking place (i.e. phase change of sweat droplets in to vapour form), then it will take heat from the body and so body temperature is maintained. The heat taken away for evaporation of sweat particles is latent heat of vapourisation. Therefore, this is a case of latent heat exchange.
What are the factors affecting on human comfort?
The factors affecting human comfort must be considered, systems for human comfort. They are,
Temperature of Air
The objective of an air conditioning system should be to remove air temperature. This is essential for human comfort. Proper control of temperature of air surrounding the body removes physiological stress thereby giving greater comfort and improved physical health. Human body acts as a heat engine, which converts thermal energy (calories) in to mechanical energy to perform work with approximately 20% efficiency. Some amount of thermal energy is lost to atmosphere in the form of heat. This increases the temperature of surrounding.
Humidity of Air
Too much humidity will cause discomfort. Places, like Mumbai, are humid and hot, since moisture content of air is very high due to Mumbai’s proximity to sea. But, R.H. (Relative Humidity) should not be more than 60% for any conditioned space. Thus, in summer, in humid places, the idea is to remove moisture, but, in winter, in dry places, exactly the reverse (i.e. moisture addition) is required. Humidification process involves increasing moisture content of air. R.H. (Relative Humidity) around 30% – 40% is suitable for winter conditions. Extremes of humidity (i.e. too low humidity or too more humidity) lead to undesirable physiological relations. Also, adverse effects are seen on the properties of many substances in the conditioned space such as clothing and furniture in particular.
Motion of Air
When low velocity air comes in contact with human skin, the air dissipates heat from the skin surface and gives a feeling of pleasantness. Air velocity should not exceed 5 m/s for any air conditioning system. However, if a high velocity air or turbulent air motion is employed, it is uncomfortable for occupants. This is because, increased velocity leads to increase in heat removal at a faster rate. This causes discomfort for human beings due to very very low skin temperature below the temperature of air surrounding the body (i.e. atmospheric temperature). The human fee s very cold and may become sick. Air distribution is also important. Air should be uniformly distributed in the conditioned space.
Purity of Air
Air contains toxic gases, small particles, suspended solids, bacteria, smoke, viruses etc. All these are harmful to human life. people feel uncomfortable, While breathing contaminated air, even if, it is within acceptable ‘temperature and humidity range’. Therefore, proper filtration, cleaning and purification of air is necessary to keep it away from dust, dirt and other properties. Purifying chemicals and sprays should be used to maintain a standard air quality.
Odour Control
Air contains gases, fumes and smoke, which add their peculiar odour to air making it unpleasant. Thus, odour control becomes necessary. Air is sprayed with deodorants, freshness chemicals and air purifying sprays. They impart (give) a pleasant smell to air, making conditioned space comfortable.
Air Quantity
It is essential to decide the percentage of each, i.e. amount of re-circulated air and the amount of outdoor fresh air .e. air sucked from atmosphere). Usually 15% fresh outdoor air is added to maintain enough amount of oxygen in air. This fresh air also helps to dilute carbon dioxide. Remaining 85% of air is re-circulated through the conditioned space.
Hot and Cold Surfaces
Hot and cold surfaces in a space may cause discomfort to occupants. During summer season, ceiling is warmer than the room air, which causes discomfort by radiation. During winter season, big glass windows act as cold radiators. In the above conditions, ceiling and g ass windows represent hot and cold surfaces respectively. Air conditioning system should be designed to regulate air supply on these surfaces and minimise discomfort of occupants.
Air Stratification
When air is heated, its density decreases and it rises up from ground floor to the ceiling of the room. Cold air from top areas located near the ceiling of room occupies this space of ground floor. In simple words, the hot air of ground floor is replaced by comparatively cold air existing near the ceiling of room. This results in variation of temperatures (gradient) from ground floor to ceiling. This is uncomfortable for occupants. The unwanted movement of air to produce this variation of temperatures (gradient) is known as air stratification. Thus, ‘air stratification’ may be defined as, “the unwanted movement of air producing undesirable variation of temperatures within an enclosed space like room”. Thus, the air conditioning system should be designed to avoid stratification by introducing sufficient amount of air changes per hour.
Effective Temperature.
Body Temperature Mechanisms.
