Thermal_Comfort_lesson_9

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Thermal Comfort: 

Thermal Comfort

Thermal comfort: 

Thermal comfort Thermal comfort is that condition of mind that which expresses satisfaction with the thermal environment .Thermal environment is those characteristics of the environment which affects a person's heat loss. In terms of bodily sensations, thermal comfort is a sensation of hot, warm, slightly warmer, neutral, slightly cooler, cool and cold.

Metabolic Rate : 

Metabolic Rate (a) degree of muscular activities, (b) environmental conditions, and (c) body size.

Mechanical Work: 

Mechanical Work External work developed by the human body is positive while mechanical impact acted on the human body is said to be negative .

Evaporative Heat Loss : 

Evaporative Heat Loss Evaporative heat loss consists of two components: Respired Vapour Loss, Latent Respiration Heat Loss , (ii) Convective or Sensible Respiration Heat Loss ,

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Evaporative Heat Loss from Skin Surface, Evaporative heat loss from skin surface has two parts: Evaporative Heat Loss by Skin Diffusion , (ii) Heat Loss due to Regulatory Sweating ,

Dry Heat Exchange : 

Dry Heat Exchange Dry heat exchange represents the heat exchange between the human body and the environment through convective and radiative heat transfer .

Effect of Clothing Insulation: 

Effect of Clothing Insulation Thermal Insulation of Clothing The addition of thermal resistance due to clothing affects heat transfer mechanisms between the human body and the environment. "Clo" value is a numerical representation of a clothing ensemble's thermal resistance, 1 Clo = 0.155 m2K/w.

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(b) Evaporative Resistance of Clothing The evaporative resistance is a measure of moisture permeability which affects the latent heat transfer from the skin through the clothing layer and affects evaporative heat loss from skin surface .

Heat Transfer Mechanisms : 

Heat Transfer Mechanisms Heat is transferred from a high temperature zone towards a low temperature zone via three different modes of heat transfer . Conduction Convection Radiation

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Thermal insulation is therefore designed to control the different component of heat transmittance: Conduction through building fabric Convection via air movement Thermal radiation

Conduction: 

Conduction Conduction depends on molecular activity and requires the presence of matter . Example is heat transfer through wall

Heat conduction : 

Heat conduction Heat conduction is a very intuitive concept: it represents the spontaneous transfer of heat through matter, in order to even out temperature differences. This explains why a metal bench or chair "feels" cold, while a wooden one "feels" warm.

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Metal is a very good heat conductor and dissipate body heat through its mass. Wood on the contrary is a poor heat conductor. For the same reason, a metal spoon in boiling water quickly becomes hot , while a wooden one stays cool at the other extremity. Materials used for thermal insulation must have a very small heat conductivity to be efficient.

Convection: 

Convection Convection takes place by bulk transport and the mixing of macroscopic parts of hot and cold elements of a fluid. This also includes the transfer of heat between a solid surface and a fluid. Convection heat transfer is of two types: Natural/Free Convection (b) Forced Convection

Heat convection : 

Heat convection Convection represents the transfer of heat by circulation or movement of the hot particles to cooler areas. This is another intuitive concept as we know that hot air or hot water rises. Air or water surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current .

Heat convection current triggered by a radiator and in boiling water : 

Heat convection current triggered by a radiator and in boiling water

Radiation : 

Radiation Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object's temperature .

Need for thermal comfort: 

Need for thermal comfort The study of thermal comfort is very important because it is correlated not only with occupants' comfort, but also with energy consumption . Indoor air quality and thermal comfort are two important aspects of indoor environmental quality that receive considerable attention. Ventilation is crucial to indoor air quality.

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From the physiological point of view, thermal comfort occurs when there is a thermal equilibrium in the absence of regulatory sweating between the heat exchange between the human body and the environment.

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Factors Influencing Thermal Comfort Environmental factors: Air Temperature (Dry-Bulb) Relative Humidity Air Velocity Radiation (Mean Radiant Temperature) Personal factors: Metabolic Rate Clothing Insulation

Air Temperature (Dry-Bulb): 

Air Temperature (Dry-Bulb) This is the temperature of the air surrounding the body. It is usually given in degrees Celsius (°C) or degrees Fahrenheit (°F). 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.

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Dry Bulb Temperature - T db The Dry Bulb temperature, usually referred to as air temperature, is the air property that is most common used. When people refer to the temperature of the air, they are normally referring to its dry bulb temperature

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Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment. Our skin absorbs almost as much radiant energy as a matt black object, although this may be reduced by wearing reflective clothing. Examples of radiant heat sources include: the sun; fire; electric fires; furnaces; steam rollers; ovens; walls in kilns; cookers; dryers; hot surfaces and machinery, molten metals etc.

Air Velocity : 

Air Velocity Air velocity is an important factor in thermal comfort because people are sensitive to it 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 odor Moving air in warm or humid conditions can increase heat loss through convection without any change in air temperature.

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If the air temperature is less than skin temperature it will significantly increase convective heat loss. Physical activity also increases air movement, so air velocity may be corrected to account for a person's level of physical activity.

Relative Humidity: 

Relative 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 vapor in the air and the maximum amount of water vapor that the air can hold at that air temperature.

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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 climatic conditions outdoors may influence the indoor thermal environment, relative humidity may be higher than 70% on warm or hot humid days. may be high

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High humidity environments have a lot of vapor in the air, which prevents the evaporation of sweat from the skin. In hot environments, humidity is important because less sweat evaporates when humidity is high (80%+). The evaporation of sweat is the main method of heat loss in humans. When vapor-impermeable PPE is worn, the humidity inside the garment increases as the wearer sweats because the sweat cannot evaporate. If an employee is wearing this type of PPE (e.g. asbestos or chemical protection suits etc) the humidity within the microclimate of the garment may be high

Personal factors: Clothing Insulation : 

Personal factors: Clothing Insulation Clothing, by its very nature, interferes with our ability to lose heat to the environment. Thermal comfort is very much dependent on the insulating effect of clothing on the wearer. Clothing is both a potential cause of thermal discomfort as well as a control for it as we adapt to the climate in which we live and play.

Metabolic Rate: 

Metabolic Rate The work or metabolic rate, is essential for a thermal risk assessment. It describes the heat that we produce inside our bodies as we carry out physical activity. 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. When considering these factors, it is also essential to consider a person's own physical characteristics.

Metabolic Rate : 

Metabolic Rate (a) degree of muscular activities, (b) environmental conditions, and (c) body size.

Comfort Zones : 

Comfort Zones The comfort zones are intended to provide acceptable thermal environment for occupants wearing typical indoor clothing and at a near sedentary activity . Acceptable thermal environment is an environment which at least 80% of the occupants would find thermally acceptable.

Local Thermal Comfort : 

Local Thermal Comfort

Vertical Air Temperature Difference : 

Vertical Air Temperature Difference

Heat Exchange between the Human Body and the Environment  : 

Heat Exchange between the Human Body and the Environment Human Response to heat Cold Environment:     shivering Hot Environment:   sweat

An overhang, for a south facing window, is working by shading the window during summer, when the sun is high in the sky, while letting the sunlight in, in winter, when the sun is low in the sky. : 

An overhang, for a south facing window, is working by shading the window during summer, when the sun is high in the sky, while letting the sunlight in, in winter, when the sun is low in the sky.

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Some psychological triggers, such as certain colors, also seem to affect comfort, and "state of mind" can have major effects on individual comfort sensations. Comfort zones, therefore, are very generally defined as the zone in which 80 percent of the population will experience the sensation of Thermal comfort.