Human thermal comfort
- 1. HUMAN THERMAL COMFORT Ass. Prof. Kapil Wadkar (VPICOA, Indapur)
- 2. Energy-efficient buildings are only effective when the occupants of the buildings are comfortable. If they are not comfortable, then they will take alternative means of heating or cooling a space such as space heaters or window-mounted air conditioners that could be substantially worse than typical Heating, Ventilation and Air Conditioning (HVAC) systems.
- 3. Thermal comfort is difficult to measure because it is highly subjective. It depends on the air temperature Humidity radiant temperature air velocity metabolic rates, and clothing levels and each individual experiences these sensations a bit differently based on his or her physiology and state.
- 4. Definition “that condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation” A cold sensation will be pleasing when the body is overheated, but unpleasant when the core is already cold. Thus, sensation from any particular part of the skin will depend on time, location and clothing, as well as the temperature of the surroundings.
- 5. Factors in Human Comfort Metabolic rate (met): The energy generated from the human body Clothing insulation (clo): The amount of thermal insulation the person is wearing Air temperature: Temperature of the air surrounding the occupant Radiant temperature: The weighted average of all the temperatures from surfaces surrounding an occupant Air velocity: Rate of air movement given distance over time Relative humidity: Percentage of water vapor in the air
- 6. Environmental factors Air temperature This is the temperature of the air surrounding the body. It is usually given in degrees Celsius (°C). Radiant temperature Thermal radiation is the heat that radiates from a warm object. Radiant heat may be present if there are heat sources in an environment. Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment. Examples of radiant heat sources include: the sun; fire; electric fires; ovens; kiln walls; cookers; dryers; hot surfaces and machinery, molten metals etc.
- 7. Air velocity This describes the speed of air moving across the employee and may help cool them if the air is cooler than the environment. Air velocity is an important factor in thermal comfort for example: still or stagnant air in indoor environments that are artificially heated may cause people to feel stuffy. It may also lead to a build-up in odour moving air in warm or humid conditions can increase heat loss through convection without any change in air temperature physical activity also increases air movement, so air velocity may be corrected to account for a person's level of physical activity small air movements in cool or cold environments may be
- 8. Humidity If water is heated and it evaporates to the surrounding environment, the resulting amount of water in the air will provide humidity. Relative humidity is the ratio between the actual amount of water vapour in the air and the maximum amount of water vapour that the air can hold at that air temperature. Relative humidity between 40% and 70% does not have a major impact on thermal comfort. In workplaces which are not air conditioned, or where the weather conditions outdoors may influence the indoor thermal environment, relative humidity may be higher than 70%. Humidity in indoor environments can vary greatly, and may be dependent on whether there are drying processes (paper mills, laundry etc) where steam is given off.
- 9. Personal factors Clothing insulation Thermal comfort is very much dependent on the insulating effect of clothing on the wearer. Wearing too much clothing or PPE may be a primary cause of heat stress even if the environment is not considered warm or hot. If clothing does not provide enough insulation, the wearer may be at risk from cold injuries such as frostbite or hypothermia in cold conditions.
- 10. Personal factors Work rate/metabolic heat The more physical work we do, the more heat we produce. The more heat we produce, the more heat needs to be lost so we don’t overheat. The impact of metabolic rate on thermal comfort is critical. A person’s physical characteristics should always be borne in mind when considering their thermal comfort, as factors such as their size and weight, age, fitness level and sex can all have an impact on how they feel, even if other factors such as air temperature, humidity and air velocity are all constant.