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Flashcards by Alice Kimpton, updated more than 1 year ago
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Created by Alice Kimpton
over 4 years ago
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| Question | Answer |
| What is the definition of thermal comfort? | “Thermal comfort is that condition of mind which expresses satisfaction with the thermal environment.” “If a change occurs such as to produce discomfort, people react in ways which tends to restore their comfort.” |
| Why did groups look into thermal comfort? | There were two main research groups in the 1970s looking into thermal comfort, due to the oil crisis (oil became expensive) and people needed to create more energy efficient buildings. Fanger published a book which proved groundbreaking to this science. |
| Scale of being too cold: | . 37C - normal . 36C - shivering . 35C - hypothermia, intense shivering, blue/grey skin . 34C - Sever shivering, loss of movement of fingers, confusion . 33C - Moderate/severe confusion, sleepiness, depressed reflexes . 32C - medical emergency: hallucinations, delirium, complete confusion, extreme sleepiness . 31C - comatose, no or slight reflexes, shallow heart rate and breathing . 28C - severe heat disturbances, may appear dead . < 26C - certain death |
| Scale of being too hot: | . > 44C - certain death . 43C - normally dead, serious brain damage . 42C – comatose, delirium, convulsions . 41C – medical emergency: fainting, vomiting, headache, confusion, hallucinations . 40C – fainting, dehydration . 39C – pyrexia: sweating, flushed, red . 38C – sweating, uncomfortable, hungry . 37C – normal (depending where you measure) |
| What are the 2 models and what does each do? | 1. Heat balance model (static or consistency model) by Fanger 2. Model of conceptual thermal adaption by Nicol and Humphreys “The first approach suggests that comfort is universally definable states of affairs, the other is a social cultural achievement” |
| Definition of the heat balance model: | “Heat balance models view a person as a passive recipient of thermal stimuli and are premised on the assumption that the effects of a given thermal environment mediated exclusively by physics of heat and mass exchanges between the body and environment” Widely used by: . ISO 7730: ergonomics of thermal environment . CIBSE guide A . ASHRAE standard 55 |
| The heat balance model only applies to non-free running buildings i.e. buildings with aircon or heating. What else does it consider? | . Only considers heat transfer between a person and their environment . The body has thermoregulatory system to maintain a constant internal body temperature i.e. body must balance the heat or loss to from the environment . There is a balance of metabolic heat produced and he transferred to the surroundings . Heat transfer occurs by combination of conduction, convection, radiation and evaporation, where evaporation is transpiration through the skin which is counterbalanced by the others. |
| Show table of key comfort variables: | |
| Name the key physical and environmental comfort variables: | Physical: 1. Activity level (metabolic rate) 2. Thermal resistance of clothing (conductive heat loss) Environmental: 1. Air temperature (convective heat loss) 2. Mean radiant temperature (radiant heat loss) 3. Air velocity (convective heat loss) 4. Relative humidity (evaporated loss, perspiration) |
| What can effect the natural convective boundary layer? | 1. Posture affects boundary layer (if in a small ball you have smaller surface area) 2. Boundary layer of 18 cm face (standing only) 3. Maximum velocity of 0.5m/s 2 cm away from the skin (faster equals more exchange) 4. Expiratory flow from nose and mouth is taken upwards avoiding short-circuiting (air from mouth is called a jet) |
| Activity level: - What depends on activity level? - What is 1 met =? - Average surface area of an adult - Thus 1 met also equals? | - Metabolic heat production depends on activity level and is measured in the unit ‘met’: - 1 met = 58.2 W/m2, being the average metabolic rate for a person seated at rest. - The average surface area of an adult is 1.8 m2. - Thus 1 met is about 100W of heat emission |
| Activity level: - Examples of metabolic activity | - Seated e.g. office work: 1 met = 58.2 W/m2 - Light machine work: 2 met = 116 W/m2 - Heavy machine work e.g. factory: 3 met = 175 W/m2 - i.e. increaseing generating of thermal energy by body. - Note that 1 met is 50 kcal/hour and that the surface area of an average human body is about 1.8 m2. (CIBSE Guide A table 1.4 typical values). |
| Thermal insulation of clothing: - What is clo? - What does 1 close equal and what is an example? | - The unit used is the clo which expresses the total thermal resistance from the skin to the outer surface of the clothing. - 1 clo = 0.155m2 oC/W - A clothing ensemble which is approximately represented by 1 clo consists of underwear, blouse/shirt, slacks/trousers, jacket, socks and shoes (i.e. a suit). |
| Thermal insulation of clothing: - Typical clo values | - Light summer clothing (long light-weight trousers, open-neck shirt with short sleeves) 0.5 clo - Typical business suit 1.0 clo - Business suit plus cotton over-coat 1.5 clo - i.e. increasing thermal resistance of clothing and reducing heat transfer from the body by conduction, convection, and radiation See CIBSE Guide A Table 1.3 for detailed list |
| Air temperature (dry-bulb): - How important? - What does it affect? - What is free and forced convection? | - The most important parameter, because we are sensitive to variations in temperature - Affects convective heat loss from the skin and clothing surface - At low air speeds (<0.1m/s), free convection dominates - At higher air speeds (>0.1m/s), forced convection dominates |
| Air temperature (dry-bulb): - Another influence of air temperature is from dry reparation heat loss, how does this arise and what is it proportional too? | – this arises from the difference between the temperatures of inhaled and exhaled air (consider breathing on a very cold day). We usually heat up the air in our breath to around 37C. – this temperature difference is proportional to the heat given to the air by the human body through the lungs |
| Mean radiant temperature: - What does it represent? - How sensitive? - What does this mean? | - Represents the mean temperature of surrounding surfaces sending heat by radiant heat transfer to the human body, or receiving radiant heat from it. - We are as sensitive to radiant heat transfer as we are to convective heat transfer at low air velocities. - Thus mean radiant temperature is as important as air temperature |
| Mean radiant temperature: - As velocity increases... - Examples? | - When air velocity increases, we become less sensitive to surfaces. - Example include cellars and caves, we would still be comfortable in them on a hot day due to their surfaces. |
| Air velocity: - What does it influence? - When does forced convection? - Excess heat? - What is freshness? - What areas are most sensitive? | - Combined with air temperature, influences convective loss. - At air velocities > 0.1 m/s, forced convection is likely and heat removal depends on both temperature and velocity - Excess heat removal in this way constitutes a draught - Small and varied air movements contribute to the comfortable condition of freshness (opp stuffiness). Remeber the way air moves can be refreshing i.e. not always dependent on the temperature of the air, e.g. fan doesn’t cool down the air but moves it around. - Ankles and back of neck most sensitive to moving air. |
| Humidity: - What does it affect, and therefore..? - When it is most significant? | - Affects rate of evaporation of moisture from skin. Therefore affects latent heat losses of body - Significant mainly in heat stress conditions when body depends on latent heat dissipation Note: latent is when there is a change in phase i.e. liquid to gas, sensible is when there is no change in phase. |
| Humidity: What are the 3 humidity moisture evaporation processes? | 1. Skin diffusion (how moisture is absorbed by the skin): happens all the time and is quite small 2. Latent respiration heat loss: a function of difference of moisture content between inhaled and exhaled air 3. Sweat secretion: the emergency process for dissipating heat when air temperature approaches body temperature |
| Humidity: - Rule of thumb for relative humidity? - If humidity is too high? - If humidity is too low? | - In comfort conditions we do not notice changes in relative humidity between 40% and 70%. - If too high, problems with fungi and dust mites may occur; also higher risk of condensation on surfaces - If too low, can cause irritation in upper respiratory tract due to rapid evaporation of mucous; also risk of irritation for contact lens wearers; and static electric shocks are more likely. |
| Write expression which expresses the balance of metabolic heat production with the conductive, convective, radiant and evaporative heat dissipation. This was produced by Fanger in his 1970 book Thermal Comfort. | |
| What are the 3 requirements for comfort? | - The body should be in thermal equilibrium (metabolic transfers are in balance) with its surroundings as described by the above equation. - The skin temperature should be at a certain value – e.g. 34oC for sedentary, but less for higher activity levels - There should be a certain perspiration rate – e.g. zero for sedentary, but increasing at higher activity levels (no sweating) |
| Conditions for comfort: | - The comfort equation with the imposed limits of skin temperature and perspiration rate can be solved to give a range of conditions in which thermal comfort should be achieved - Tested by Fanger on North American subjects in laboratory tests - Verified by on students in the Technical University of Denmark. |
| Operative temperature: - What is it? - What does it take into account? | Operative (dry resultant) temperature – a simple commonly used index – mean of radiant and dry-bulb air temperatures • Takes into account air velocity, air temperature and mean radiant temperature • Does not take into account humidity because we usually are insensitive to quite large variations |
| Operative temperature: Equation | |
| Operative temperature: - What does it verify? - What about at higher air velocities? - In a well insulated room? - What is dry-bulb temperature? | • Verifies the equal significance of air and mean radiant temperatures at this air velocity • At higher velocities air temperature becomes more important than mean radiant temperature • In a well insulated room (thermally light-weight building), the surfaces are almost as warm as the air so that tr ≈tai ≈tc • Dry-bulb air temperature is the comfort index – Within the usual range of acceptable RH [%] and air velocity [m/s]. |
| Air velocity in occupied zone: - Too high or low? - Guide? - Rule of thumb? | - Too high and the zone is perceived as being draughty - Too low and occupants struggle to • BSEN ISO 7730 (Table A.5) gives maximum air velocities in the occupied zone of an office (category C) – Winter 0.21m/s – Summer 0.24m/s • Take 0.25m/s as a rule-of-thumb maximum air velocity in the occupied zone |
| Mean radiant temperature: - What is it? - At the centre of the room...? - What should it not go below? | • The average effect of radiation from surrounding surfaces. • At the centre of the room it is equal to the mean surface temperature • Should be kept near air temperature and not more than 3oC below it |
| Mean radiant temperature: Equation | |
| Predicted mean vote (PMV): - What is it? - How is it derived? - What is it used for? - What does it predict? - What does it permit? | • a more sophisticated index covering a wider range of conditions (also based on the work of Fanger). • Empirically derived – A total of 1396 subjects were questioned about their comfort conditions in controlled test chambers (in Denmark and earlier in the USA) • Used for the practical assessment of the thermal environment • Predicts the mean thermal sensation vote on a standard scale for a large group of people • Permits any combination of the FOUR environmental variables and activity/clothing levels. |
| Predicted mean vote (PMV): Scale | -3 cold -2 cool -1 slightly cool 0 neutral (comfortable) +1 slightly warm +2 warm +3 hot |
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