DEA3500: Ambient Environment: Thermal Conditions
One of earliest reasons for building was to create shelter from elements. The desire to keep dry and warm/or cool (depending on climate) has generated a variety of architectural forms which have evolved to increase the impermeability of the building envelope to natural conditions and through environmental engineering, allow us to create our own interior environmental conditions. Basic model of thermal conditions (insert model)
Our living bodies generate heat because we are homiothermic (warmblooded) creatures. The rate at which heat is produced depends primarily on our metabolic rate.
Metabolic rate = our ability to generate heat is mostly a function of our level of muscular activity. Some of the energy generated by muscular activity will be directly translated into work (force x distance) and the excess energy will be dissipated as heat.
Met units - Each of us in this classroom is producing about 1 met (1 unit of metabolic rate) of waste heat.
Because, as we shall see, heat exchange with our environment is primarily via the skin, the met unit is defined in terms of both heat energy and surface area.
1 met = 58.2 w/m2 (SI units)
= 18.4 Btu/h/ft2
(i.e. 58.2 x 3.412/10.76 = 198.5784/10.76 = 18.4
1 watt = 3.423 Btu/h
1 m2 = 10.76 ft2
1 Btu = amount of heat required to increase temperature of 1 pound (1 pint) of water by 1 DEGREE F = heat produced by 1 standard wooden match. Every square foot of body gives off heat of about 19 matches/hour.
To increase temperature of 1 pound of water from 32°F to 212°F requires 180 Btu (i.e. 212-32=180)
SURFACE AREA OF BODY
Du Bois area: The surface area of skin of an "average" adult is 1.8 m2 (1.8 x 10.76 = 19.368 ft2) The total heat production of an "average" person at rest per hour is 58.2 x 1.8 = 104.76 = 105 watts (18.4 x 19.368 = 356.37 = 356 Btu's per hour).
The Du Bois area normally varies between 1.3 m2 (14 ft2) and 2.2 m2 (23.7 ft2) and in any setting the heat produced by sedentary adults will vary between about 75.66 watts (271 Btu's) for 1.3 m2 and 128 watts (459 Btu's) for 2.2 m2.
|Activity||Metabolic rate, a (met units), b|
|Sedentary activity (office, dwelling, lab, school)||1.2|
|Light activity, standing (shopping, lab, light industry)||1.6|
|Medium activity, standing (shop assistant, domestic work, machine work)||2.0|
|High activity (heavy machine work, garage work)||3.0|
This consists of a thin-walled copper sphere painted black containing a thermometer with its bulb at the center of the sphere (typically of diameter 150 mm). The globe thermometer is suspended and allowed to reach thermal equilibrium with its surroundings (usually 20 minutes). With a far-inside globe, equilibrium time is 6 minutes, and using a thermocouple instead of a mercury thermometer the time is 10 minutes. The equilibrium temperature depends on both convection and radiation transfer, however by effectively increasing the size of the thermometer bulb the convection transfer coefficient is reduced and the effect of radiation is proportionally increased. In equilibrium the net heat exchange is zero.
Because of local convective air currents the globe temperature (tg) typically lies between the air temperature (ta) and the true mean radiant temperature (tr). The faster the air moves over the globe thermometer the closer tg approaches ta. NB If there is zero air movement, tg = tr.
MEASURING SURFACE TEMPERATURE (thermal conductivity)
All surfaces are made of materials which conduct heat at varying rates (thermal conductivity). Our thermal sensations are not good indicators of surface temperature but rather we sense the rate of heat loss or gain e.g. in a thermally stable setting a tile floor will feel colder than a carpeted floor even though they have the same surface temperature because tile has a higher thermal conductivity than carpet. Surface temperatures can be measured by thermometers placed in direct contact with the surface of interest. Surfaces can be a significant source of discomfort.
Humidity (absolute humidity) refers to the dampness/wetness in the air in the form of water vapor, that is, the mass of water vapor present in a unit volume of air (moisture content).
In S.I. units it is expressed in grams of water per cubic metre of air or space. (nb 454 grams = 1 lb/ 1m3 = 1.308 yd3 = .027 oz/yd3). On a normal day humidity remains fairly constant but it is relative humidity which changes considerably.
RELATIVE HUMIDITY is of more practical importance.
RH = ratio of mass of water vapor present in air at a temperature/ maximum water vapor content of that air at that temperature.
Relative humidity is the ratio of the prevailing partial pressure of water vapor to the pressure of saturated water vapor at the prevailing temperature. Usually talk of %RH If the air contains its maximum water vapor it has a %relative humidity of 100% and is said to be SATURATED. This situation is very unusual inside buildings except at very cold surfaces e.g. breath on cold mirror.
Dew Point is the temperature at which atmospheric water vapor starts to condense when the air is cooled - major problem for condensation in buildings.
As air temperature falls at night the maximum vapor content of the air falls although the actual vapor content remains constant, and the relative humidity increases.
When the air cools sufficiently that the maximum vapor content = actual vapor content then RH = 100% and water begins to condense from the air to give dew, especially at ground level (because ground is colder than surrounding air). This air temperature is called the dew point (whether inside or outside). When air temperature continues to fall dew freezes to give frost. (Inside buildings typically get dew or frost on coldest surfaces e.g. windows).
As air temperature rises so the maximum vapor content rises and as air temperature increases at a constant moisture content, relative humidity decreases. (RH is measured using sling psychrometer (whirling hygrometer) or a hygrometer). to be described later.
The molecules of a liquid like water are in a constant state of motion. As temperature increases so the movement becomes more hectic e.g. note bubbling/spitting at surface of boiling water, and eventually some break loose into air. These molecules create a pressure (vapor pressure) in the air space above the liquid and as the temperature of the liquid increases so the vapor pressure increases (e.g. boiling pan of water may lift the lid off). For any liquid there is a maximum pressure for any temperature and this is called the SATURATED VAPOR PRESSURE (SVP). SVP = 100% humid air, above this surplus water vapor condenses out. Explain SVP table.
Knowledge of air temperature, relative humidity, and either moisture content or SVP allows easy calculation of the dew point.
e.g. If air is at 20°C and 40% RH what will be the dew point?
i) By moisture content method: at 20°C air can hold 17.118 g/m3 of water.
40% of 17.118 g/m3 = 6.847 g/m3
Air at 5.2°C can hold 6.847 g/m3 and this is the dew point.
ii) By SVP method: At 20°C SVP = 2338 N/m2
40% of 2338 = 935 N/m2
from table, 935 N/m2 = SVP for 6°C
dew point approximately 6°C (which is slightly above actual dew point).
Hygrometers (sometimes called psychrometers) These instruments measure relative humidity. The most commonly used instruments are:
Wet and dry bulb hygrometers (whirling hygrometers, sling psychrometers)
Consists of a dry bulb (Td) and a wet bulb (Tw). The two thermometers are read and the difference noted.
Td - Tw = Tdiff The wet bulb temperature will typically be lower because the water takes heat from its surroundings (including the thermometer bulb) to supply latent heat for water evaporation (latent heat of vaporization). From the dry bulb temperature and the temperature difference, the percentage relative humidity can be found from a table. This is a quick and accurate way of measuring RH. Other devices for measuring RH include:
Dew point hygrometer - consists of a plain glass tube about 25 mm in diameter with a highly polished nickel cap. To use this:
i) the air temperature is taken.
ii) ether is poured into the tube to a depth of 25 mm (to cover the thermometer bulb).
iii) air is blown through the ether which causes this to evaporate (ether is a very volatile
liquid which boils at blood temperature).
iv) the temperature at which dew starts to appear on the cap is noted.
Suppose air temperature = 20°C
dew point temperature = 8°C
Air at 20°C can contain 17.118 g/m-3. However, since the dew appeared at 8°C, this is equal to the temperature at which the air would be saturated.
Air at 8°C can contain 8.215 g/m-3 (from table)
%RH = 8.215/17.118 x 100 = .48 x 100 = 48%
Velocity of air at a point in a space. Measured in ft/min. or m/sec.
NB 1 fpm = .00508 m/s
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