Thermal Comfort
By Rashed K. Al-Shaali
1
Thermal balance and comfort
•
The human body continuously produces
heat by its metabolic processes.
• production can be of two kinds:
1.
basal metabolism, due to biological
processes which are continuous and nonconscious
2.
muscular metabolism, whilst carrying out
work, which is consciously controllable
(except in shivering).
• The heat output of an average body is
often taken as 100W, but it can vary from
about 70W (in sleep) to over 700W in
heavy work or vigorous activity (e.g.
playing squash). This heat must be
dissipated to the environment, or else the
body temperature will increase. This deepbody temperature is normally about 37◦C,
whilst the skin temperature can vary
between 31 and 34◦C.
M± Rd ± Cv ± Cd − Ev = ΔS
M = metabolic heat production
Rd = net radiation exchange
Cv = convection (incl. respiration)
Cd = conduction
Ev = evaporation (incl. in
respiration)
ΔS = change in stored heat.
2
Definition and Factors of Comfort
•
comfort is defined as ‘the condition of mind that expresses satisfaction with the
thermal environment, it requires subjective evaluation’. This clearly embraces
factors beyond the physical/physiological.
environmental :
personal:
contributing factors:
air temperature
metabolic rate (activity)
food and drink
air movement
clothing
acclimatization
humidity
body shape
radiation
subcutaneous fat
age and gender
state of health
3
Environmental Factors of Comfort
•
•
•
Air temperature is the most important
environmental factor, measured by the dry
bulb temperature (DBT). This will
determine the convective heat dissipation,
together with any air movement. In the
presence of air movement the surface
resistance of the body (or clothing) is much
reduced.
Air movement is measured by its velocity (v,
in m/s) and it also affects the evaporation
of moisture from the skin, thus the
evaporative cooling effect.
Humidity of the air also affects evaporation
rate. This can be expressed by relative
humidity (RH, %), absolute humidity or
moisture content (AH, g/kg), or vapor
pressure (p, in kPa).
About 20 -24 °C
See the Psychrometric Chart
People tolerate up to 0.8 m/s
according to ASHRAE, but other
studies showed that people
tolerated up to 2 m/s with
naturally ventilated buildings in
regions that are hot and humid*
About 30 -70 %
See the Psychrometric Chart
* See B. Givoni, Passive Low Energy Cooling of Buildings. Wiley, 1994.
4
Environmental Factors of Comfort
•
•
Radiation exchange will depend on the mean
temperature of the surrounding surfaces
(weighted by the solid angle subtended by
each surface), referred to as the mean radiant
temperature (MRT) or on the presence of
strong monodirectional radiation, e.g. from
the sun.
The mean radiant temperature cannot be
measured directly, but it can be approximated
by globe temperature measurements. The
globe thermometer (GT) is a mat black copper
sphere, usually of 150 mm diameter, with a
thermometer located at its centre. Positioned
in a room, after equilibrium is reached (in 1015 minutes) the globe will respond to the net
radiation to or from the surrounding surfaces.
If radiation is received, then GT>DBT; GT<DBT
indicates that the surrounding surfaces are
cooler than the air, radiation is emitted. In still
air MRT = GT, but a correction for air
movement of v velocity (in m/s) is possible
Globe thermometer
5
Metabolic Rate Effect on Comfort
Metabolic rates at different activities
met
W/m²
450
400
350
300
250
200
150
100
50
0
8
7
6
4
3
2
1
very heavy work
heavy work
steady medium work
medium light work
very light work
standing, sedentary work
seated, at rest
reclining, lying in bed
0
1 met = 58.2 W/m²
6
met
5
sleeping
•
The personal factors include the metabolic
rate (activity level), which in turn may be
influenced also by food and drink, and the
state of acclimatization. Short-term
physiological adjustment to changed
conditions is achieved in 20 - 30 minutes, but
there are also long term, endocrine
adjustments which may extend beyond six
months, which constitute the acclimatization
process.
Both the vasomotor and evaporative
regulation mechanisms are subject to
acclimatization. In hot climates - for example the volume of blood circulating can be
increased by up to 20%, to maintain a
constant vasodilation. Sweat secretion rate
also increases over a period of several weeks.
It is believed that the forward section of the
hypothalamus gland regulates these changes
through a complex neuro-endocrine process
W/m^2
•
Clothing effect on Comfort
• Clothing is one of the dominant factors affecting heat dissipation. For the
purposes of thermal comfort studies a unit has been devised, named the
clo. This corresponds to an insulating cover over the whole body of a
transmittance (U-value) of 6.45 W/m²K (i.e. a resistance of 0.155 m²K/W).
Insulation of clothing in clo units
7
Body Heat and Comfort
As the graph shows, the human body
depends more on convection and
radiation to lose heat generated when the
ambient air temperature is around the
comfort zone, but when the temperature
rises above the comfort zone the body
core temperature increases , then the
hypothalamus calls for changes in the
blood distribution system to minimize
blood circulation around the core, and
because blood carries heat, the blood
flow towards the skin increases which
result in an increase of the sweat glands
and eventually evaporation. When air
molecules pass by the skin, it absorbs
heat and reduces skins temperature.
B. Stein, J. S. Reynolds, Mechanical and Electrical Equipment for Buildings, 9th Edition. Wiley, 2005, page 39
8
Comfort under different conditions
Allard, F., Natural Ventilation in
Buildings: A Design Handbook. 1998,
London: James & James.
Khedari, J., et al., Thailand Ventilation
Comfort Chart. Energy and Buildings, 2000.
32: p. 245-249
9
Comfort and more variables
10