. THERMAL COMFORT INDICES
Climatology, Landscape and Environmental studies. ROHIT KUMAR
ASSISTANT PROFESSOR MBS SPA 2016
2. THERMAL COMFORT • Our life cycle comprises Activity, fatigue and recovery.
• Recovery is essential to counter balance against mental and
physical fatigue through recreation, rest and sleep. •
This can be affected by unfavorable climatic conditions and
the resulting stress on body and mind causes discomfort, loss
of efficiency and breakdown of health. •
Thermal comfort is the condition of mind that expresses
satisfaction with the thermal environment and is assessed by
subjective evaluation (ANSI/ASHRAE Standard 55). •
The task of the designer is to create the best possible indoor
climate or even the environment for the users as they judge
the quality of design based on physical and emotional point of view.
3. INDICES OF THERMAL COMFORT •
Thermal comfort refers to the subjective feeling of temperature in an environment. •
Optimum levels of thermal comfort helps in maximizing productivity. •
Measurement of thermal comfort levels are complex and
many indices have been proposed over the years.
4. THERMAL COMFORT SCALE
• A single scale which combines the effects of various thermal
comfort factors (such as air temperature, humidity, air
movement and radiation) is called a THERMAL INDEX or COMFORT SCALE. •
The designer has to handle four such factors to understand
the effect of climatic conditions on the body’s heat dissipation process. • The factors: •
Air temperature • Humidity • Air movement • Radiation • Subjective factors.
5. THERMAL COMFORT SCALE
• To create such a scale, experiments were done in specially
built rooms where climatic conditions could be produced. •
The subjects were placed in the room and were asked to fill
questionnaires after each variation in the conditions according
to a set scale ranging from ‘very hot’ to ‘very cold’. •
The answers were then evaluated statistically and plotted on a
graph to find relationship among the factors. •
At least 30 or more scales were devised in this process.
6. EFFECTIVE TEMPERATURE •
It is defined as the temperature of a still and saturated
atmosphere which would, in the absence of radiation,
produce the same effect as the temperature in question.
• The first comfort scale was produced by Houghton and Yaglou
in 1923, working at the American Society of Heating and Ventilating Engineers. •
Their findings were plotted on a psychrometric chart, producing ‘equal comfort lines’. •
This new scale was named Effective Temperature (ET Scale).
7. EFFECTIVE TEMPERATURE •
The different factors determining thermal comfort – air
temperature, humidity and air movements are combined together into a single index –
Effective temperature • Effective temperature is the temperature in an environment
with 100% humidity and no air movements which will induce
the same level of thermal comfort as in the present situation •
For example, if the effective temperature is said to be 30°C, it
means that the thermal comfort is equivalent to one is an
environment with temperature 30°C, 100% humidity and no air movements •
But effective temperature does not take into consideration, the effect of radiant heat energy
8. EFFECTIVE TEMPERATURE
9. OPERATIVE TEMPERATURE •
This scale combined the effects of air temperature and radiation.
• The studies were carried out in specific regions with cool
conditions where the effects of humidity was small and the
rate of air movement was negligible. •
This scale was developed in USA by Winslow, Herrington and
Gagge, in principle very similar to equivalent warmth.
10. BIOCLIMATIC CHART •
Under over heated conditions when metabolic rates are low
(light work), which already produce discomfort, it was found
out that the DBT values correspond better with subjective
judgments than ET values. (Result of some Australian experiments).
• On this basis and similar other doubts, V. Olgyay concluded
that it is pointless to build a single figure index as each of the
four components can be controlled by different means. •
Thus a bioclimatic chart was created in terms of DBT and RH,
and by additional lines it was shown how comfort zone was
pushed up by the presence air movement and lowered by radiation.
11. BIOCLIMATIC CHART
12. REVISIONS OF ET SCALE • The ET scale was by ‘equal comfort lines’ drawn on the
psychrometric chart. • But it was found out that this method underestimates the
significance of moderate air movements at high temperatures
and at the same time overestimates the adverse effects of higher humidities. •
Keeping these observations in mind, a nomogram was
constructed which defines ET directly from DBT and WBT readings. •
Later, DBT was replaced with the Globe temperature (GT) to
account for radiant heat exchange.
13. DEFINTIONS NOMOGRAM • A graphic representation •
that consists of several lines marked off to scale and
• arranged in such a way that by using a straightedge to connect
known values on two lines an unknown value can be read at
the point of intersection with another line. MEAN RADIANT TEMPERATURE •
If all surfaces in an environment were uniformly at this
temperature, it would produce the same net radiant heat
balance as the given environment with its various surface temperatures. •
Device used is a globe thermometer.
14. GLOBE THERMOMETER •
An ordinary thermometer enclosed in a matt black painted
copper globe of 150mm diameter.
• It has an inertia of 15 min, but after this time, its reading gives
a combination of air temperature and the effect of any received or emitted radiation. •
If the air is warm and opposing surfaces are cold, radiation
from the globe will be emitted and the reading will be lower
than air temperature and vice versa.
15. CORRECTED EFFECTIVE TEMPERATURE •
The ET scale basically combined air temperature and humidity
(later air movement was added) but it lacked the effects of radiation.
• Thus, CET scale (Corrected Effective Temperature) was created by adding radiation effects.
• It is the most widely used and the most understood scale at
present and the results are accurate under most conditions.
17. FINDING THE CET • Measure the globe thermometer temperature • Measure the WBT
• Measure the air velocities with an anemometer. • Locate GT on the LHS of nomogram. •
Locate WBT on RHS. • Connect the two points with a line. •
Select the appropriate curve to the air velocity. •
Mark the point of intersection between air velocity and the drawn line. •
Read off the value of the short inclined line‐ CET.
18. CORRECTED EFFECTIVE TEMPERATURE: EXERCISE
• Find the CET when the • GT= 30 degree Celsius • WBT= 26 degree Celsius •
Air speed = 1.5 m/s
21. THE COMFORT ZONE • The range of conditions I which at least 80% of the people
would feel comfortable. • This has been determined on the CET nomogram and the
bioclimatic chart. Comfortableairspeedisbetween0.15m/sto1.5m/s.
22. Comfort zone within 22 to 27 degrees Celsius and air speed of 0.15 m/s to 1.5 m/s
23. CLIMATE ANALYSIS WITH CET •
The following figure shows the climate analysis of Islamabad. •
Like Delhi, it has composite climate with hot‐dry, war‐humid and cold dry periods. •
One day in each of the period were selected and plotted. •
From local information, it was found out that the comfort
limits are 18 and 24 degree C CET and this was marked. • There is considerable ‘under‐
heating’ in the cold season and constant ‘over heating’ in the hot‐dry season. •
Wind relief is unfeasible as it also carries dust.
24. CET analysis of Islamabad.
25. • The above is an Effective temperature histogram. • To construct an ET isopleth chart,
the temperature, humidity, MRT and wind velocity should be known for typical day at 2 hour
interval. This is rarely available, so it is estimated from mean max. and mean min. temp.
26. Mean max. temp. Mean min. temp.
29. BIBLIOGRAPHY •
Koenigsberger, O. H., Manual of Tropical Housing and building,
Orient Longman private limited, 1973.