Unit 4: Science and Materials in Construction and the Built Environment. Thermal Comfort

Transcription

1 2.1 Introduction Thermal Comfort The thermal comfort of human beings is governed by many physiological mechanisms of the body and these vary from person to person. In any particular thermal environment it is difficult to get more than 50% of the people affected to agree that the conditions are comfortable! The body constantly produces heat energy from the food energy it consumes. This heat needs to be dissipated at an appropriate rate to keep the body at constant temperature. The transfer of the heat from the body is mainly by the processes of convection, radiation and evaporation. Evaporation transfers the latent heat we give to the water vapour given out on the skin (perspiration) and in the breath (respiration). The total quantity of heat produced by a person depends upon the size, the age, the sex, the activity and the clothing of the person. A further complication is the ability of the body to become accustomed to the surrounding conditions and to adapt to them. For example, everyone can tolerate slightly lower temperatures during winter. This adaptation can be influenced by the type of climate and the social habits of a country. Chapter 2 Page 1

2 2.2 Factors affecting thermal comfort The principal factors affecting thermal comfort can be conveniently considered under the headings below and are discussed further in the following sections. Personal variables Activity Clothing Age Sex Chapter 2 Page 2

3 Physical variables Air temperature Air movement Surface temperatures Humidity Chapter 2 Page 3

4 Activity The greater the activity of the body the more heat it gives off. The rate of heat emission depends upon the individual metabolic rate of a person and upon their surface area. People who seem similar in all other respects can vary by 10 to 20% in their heat output. The average rate of heat emission decreases with age. Table 2.1 lists typical heat outputs from an adult male for a number of different activities. The output from adult females is about 85% that of males. Table 2.1 Typical heat output of human body Activity Example Pictures Typical heat emission of adult male Immobile Sleeping 70 W Seated Watching television 115 W Chapter 2 Page 4

5 Light work Office 140 W Medium work Factory, dancing 265 W Heavy work Lifting 440 W Note: Adapted from the CIBSE Guide Clothing The amount of clothing worn generally depends on the season and can affect thermal comfort. In summer, we wear fewer clothes the heating is switched off in homes and the windows are often open for ventilation. In winter, we may wear some additional clothing indoors to feel warmer, particularly in older properties where there may be open chimneys, fireplaces and sash windows which do not seal effectively. The function of warm clothing is to trap a layer of warm air against your skin which heats up and makes you feel warmer, increasing your comfort. Chapter 2 Page 5

6 The Chartered Institution of Building Services Engineers has categorised clothing in an attempt to produce acceptable parameters. This classification measures clothing in terms of a scale called a clo value. One clo represents m 2 K/w of insulation to the body with typical values range from 1 to 4 clo. Table 2.2 illustrates how the amount of clothing worn affects the maintained temperature of a room. Table 2.2 Clothing values Clo value Clothing Pictures Typical comfort temperature when sitting ( o C) 0 clo Swimwear 29 o C 0.5 clo Light clothing 25 o C Chapter 2 Page 6

7 1.0 clo Suit, jumper 22 o C 2.0 clo Coat, gloves, hat 14 o C Room temperatures The temperature of the surrounding surfaces can affect the thermal comfort of people as much as the temperature of the surrounding air. This is because the rate at which heat is radiated from a person is affected by the radiant properties of the surroundings. For example, when sitting near the cold surface of a window the heat radiated from the body increase and can cause discomfort. A satisfactory design temperature for achieving thermal comfort needs to take account of both air temperatures and radiant effects. Different types of temperatures are described in the following chapter. Particular consideration will be taken to calculate the Inside Air Temperature, the Thermal Conductivity, U-values and Heat Loss, Thermal Resistances and Surface Resistances. Chapter 2 Page 7

8 Air Movement The movement of air in a room helps to increase heat loss from the body by convection and can cause the sensation of draughts. The back of the neck, the forehead and the ankles are the most sensitive areas for chilling. Air movements above 0.1 m/s in speed require higher air temperatures to give the same degree of comfort. For example, if air at 18 o C increases in movement from 0.1 m/s to 0.2 m/s then the temperature of this air needs to rise to 21 o C to avoid discomfort. The air movement rate is not the same thing as the air change rate and is not always caused by ventilation. Uncomfortable air movement may be due to natural convection currents, especially near windows or in rooms with high ceilings. Current building regulations dictate that air tests which test airtightness are undertaken on newly constructed domestic and commercial buildings. Airtightness is linked to the heat loss through ventilation of the property. Draught seals should be fitted to all openings to restrict thermal losses. During the summer months in the UK, windows are open for ventilation. Resulting local air movement within a room can cause problems and discomfort to the occupants of the room. If the warm air entering a room is not mixed with the cooler air, the room becomes hot nearer the ceiling and colder at floor level. A draughty room may also cause discomfort when cold air passes over the skin s surface, the skin s hair follicles are raised and shivering may occur as a reaction to the sudden cold environment. Remember! Many older properties have open fireplaces that act as natural ventilation and cause air movement within the building. Chapter 2 Page 8

9 A hot-wire anemometer and a Kata thermometer are devices used to measure air movement. Both devices make use of the cooling effect of moving air upon a thermometer. Hot-wire anemometer Mean and gust wind speeds are often measured in a wind tunnel with a delicate instrument called a hot-film (more robust, but lower frequency response) or hot-wire anemometer (less robust, but higher frequency response). The fine wire between the two supports transmits an electric current which varies with the cooling effect of air flowing over the wire. Frequency responses in the range 200 to 2000 Hz are common. Kata Thermometer plain bulb 38 to 35 C This simple instrument allows accurate measurement of air velocities and the cooling power of the air. Each thermometer has a stem with just two graduations corresponding to a drop of 3 C. The thermometer is heated and then allowed to cool, noting the cooling time. The air velocity is calculated using a formula and a resulting heat loss per unit surface area can be determined. The cooling thermometer is alcohol filled with a plain or silvered bulb. Silvered bulb patterns are used to overcome heat radiation from surrounding surfaces. Chapter 2 Page 9

10 Humidity Humidity is caused by moisture in the air. It is usually measured in relative humidity rather than actual humidity. Relative humidity is the ratio of the current humidity expressed as a percentage. Air that is totally saturated has 100% humidity it cannot hold any more moisture and so it may rain. Relative humidity within the range of 40% to 70% is required for comfortable conditions. High humidity and high temperatures feel oppressive and natural cooling by perspiration is decreased. High humidity and low temperatures cause the air to feel chilly. Low humidity can cause dryness of throats and skin. Static electricity can accumulate with low humidity, especially in modern offices with synthetic carpet, and cause mild but uncomfortable electric shocks. Ventilation In any occupied space ventilation is necessary to provide oxygen and to remove contaminated air. Fresh air contains about 21% oxygen and 0.04% carbon dioxide while expired air contains about 16% oxygen and 4% carbon dioxide. The body requires a constant supply of oxygen but the air would be unacceptable well before there was a danger to life. As well as being a comfort consideration the rate of ventilation has a great effect on the heat loss from buildings and on condensation in buildings. The normal process of breathing gives significant quantities of latent heat and water vapour to the air. Body odours, bacteria, and the products of smoking, cooking and washing also contaminate household air. In places of work, contamination may be increased by a variety of gases and dusts. A number of statutory regulations specify minimum rates of air-supply in occupied spaces. Recommended rates of ventilation depend upon the volume of a room, the number of occupants, the type of activity and whether smoking is expected. It is difficult therefore to summarise figures for air-supply but table 2.3 quotes some typical values. Chapter 2 Page 10

11 Table 2.3 Typical fresh air-supply rates Type of space Residences, offices, shops Restaurants, bars Kitchens, domestic toilets Recommended air-supply 8 litres/s per person 18 litres/s per person 10 litres/s per m 2 floor Chapter 2 Page 11