Cornell University Ergonomics Web

DEA3500: Ambient Environment: Thermal Conditions

Thermal Conditions

FUNDAMENTALS

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)

THE BODY

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 32F to 212F 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.

TABLE 2.1 Metabolic Rates for Typical Tasks

Activity Metabolic rate, a (met units), b
Reclining 0.8
Seated, quietly1.0
Sedentary activity (office, dwelling, lab, school)1.2
Standing, relaxed1.2
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
ASHRAE

a) For whole-body average heat production in watts and Btu per hour (see course text)

b) One met = 58.2 W/m2 = 18.4 Btu/h ft2 In this room with 40-50 bodies, waste body heat alone is equivalent to a 4-5 Kw fire burning! In most buildings the problem is one of cooling, not heating, for much of the year.

Thus far we have discussed people as heat sources. Now let us look at measures of heat in the environment.

How is heat output measured for people?

At rest, 20-30% body heat is produced by muscles. During strenuous exercise for about 1 minute the heat output from muscles can be 40 X that from all other tissues. The degree of muscular activity is one of the most important ways in which the body regulates its temperature.

MEASURING THERMAL CONDITIONS

Fundamentals of Psychrometry (Once mastered, can understand thermal comfort & ventilation (HVAC))

Atmospheric influences on our sensation of thermal conditions depend on the interaction of heat, moisture, and air. The study of the interaction of these components is termed psychrometry (the study of moist air)

Heat (enthalpy) = sum of internal energy of a body, and the product of its volume x pressure)
Enthalpy = sensible heat + latent heat
Sensible heat = the type of heat that increases the temperature of the air e.g. an electric fire.
Latent heat = the heat that is present in increasing moisture in the air e.g. when boil a kettle or use steam humidifier. (To evaporate 1 pound of water at 212F requires 1061 Btu which is approximately 6 x energy required to heat 1 pound of water from 32F to 212F). This moisture in the air doesn't necessarily change air temperature but the heat energy it contains can be released when this moisture condenses (latent heat of vaporization).

Air - as air temperature rises, its volume increases and its capacity to hold moisture increases.
Warmer air is less dense (because of increased volume) and it rises.
As air temperature decreases so its volume decreases and its capacity to hold moisture decreases.
Colder air is more dense (because of decreased volume) and it falls.

Moisture - the amount (mass) of moisture present in air at a given volume and temperature is termed the absolute humidity or moisture content. More commonly, we talk about the humidity ratio or relative humidity of the air.

MEASURING THERMAL FACTORS

Measuring temperature

Air temperature (ta) - conversion from degrees C  (C) to degrees F (F)
(C/5 x 9) + 32
0 32 100 212
Typically measured by a mercury-in-glass thermometer.
To estimate the air temperature of a room this should be taken at a central location and at about face level (avoiding bright sunlight or other asymmetrical heat sources). Vertical effects are especially problematic in buildings.

Mean radiant temperature (MRT) is the average temperature of the surfaces in a cubical room. Mean radiant temperature may be higher or lower than the air temperature in a room. Mean Radiant Temperature (tr) is the uniform temperature of the surface of an imaginary enclosure where the radiant exchange of heat between this enclosure and a man would be equal to the radiant exchanges in the actual environment.

Plane Radiant Temperature (tpr) is the uniform surface temperature of an enclosure in which the incident radiant flux on one side of a small plane element is the same as in the actual environment.

Radiant Temperature Asymmetry ((delta) D tpr) is the difference between the plane radiant temperature of the two opposite sides of a small plane element.

Operative temperature - an average of the air temperature and the MRT. The operative temperature is usually made using a globe thermometer placed at trunk level.

MEASURING MEAN RADIANT TEMPERATURE

Globe thermometer.

 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

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.

VAPOR PRESSURE

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 20C and 40% RH what will be the dew point?
i) By moisture content method: at 20C air can hold 17.118 g/m3 of water.
40% of 17.118 g/m3 = 6.847 g/m3
Air at 5.2C can hold 6.847 g/m3 and this is the dew point.

ii) By SVP method: At 20C SVP = 2338 N/m2
40% of 2338 = 935 N/m2
from table, 935 N/m2 = SVP for 6C
dew point approximately 6C (which is slightly above actual dew point).

MEASURING HUMIDITY

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 = 20C
dew point temperature = 8C
Air at 20C can contain 17.118 g/m-3. However, since the dew appeared at 8C, this is equal to the temperature at which the air would be saturated.
Air at 8C can contain 8.215 g/m-3 (from table)
%RH = 8.215/17.118 x 100 = .48 x 100 = 48%

Digital thermometers/hygrometers

Air velocity
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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