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21. EFFECTIVE TEMPERATURE (ET)

Effective temperature by designed by Houghton and Yaglou in the year 1923

It is one of the comfort scales or thermal indices * i.e. a single scale or value which

combines the effect of different components of climate like air temperature,

humidity, air movement but does not consider radiation.

It is defined as the temperature of a still, saturated atmosphere, which would, in the

absence of radiation, produce the same effect as the atmosphere in question. It thus

combines the effect of dry air temperature and humidity.

Corrected Effective Temperature (CET) is a modification of ET scale and it includes

radiation effects to measure ET.

A nomograph* can be used to find out the effective temperature if the WBT (wet

bulb temperature) and DBT (Dry bulb temperature) are known along with the air

velocity.

*A thermal index is a measure of atmospheric stability/instability

* Nomo graph is a diagram representing the relations between three or more

variable quantities by means of a number of scales, so arranged that the value of

one variable can be found by a simple geometrical construction.

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Left hand verticle scale = Dry bulb temperature (DBT )

Right hand scale = wet bulb temperature ( WBT)

Long curvilinear lines between 2 scales = Effective temperature (ET) or Corrected

Effective Temperature ( CET )

Short curvilinear lines= air velocity

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22. PSYCHROMETRIC CHART

A Psychrometric charts are graphic representations of the properties of air and is a

study of moist air and changes in its conditions.

Relationships of

1) Relative humidity (curved lines)

2) Dry bulb temperature (vertical lines )

3) Absolute humidity (horizontal bold lines)

4) Wet bulb temperature (inclined lines)

5) Vapour Pressure (horizontal dotted lines)

is given by a Psychometric chart.

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23. SOLAR ALTITUDE ANGLE (Φ)

Solar altitude angle is the verticle angle at the point of observation between the

horizon plane and the line connecting the sun with the observer . (Denoted by Φ,

phi)

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24 SOLAR AZIMUTH ANGLE (α)

The azimuth of an object is the angular distance along the horizon to the location of

the object. By convention, azimuth is measured from north towards the east along

the horizon

It can be defined as

The angle at the point of observation measured on a horizontal plane in a northerly

direction and a point on the horizon circle where it is intersected by the arch of a

vertical circle going through to the zenith and the suns position.

Azimuth is usually measured in degrees (°).

The azimuth angle is like a compass direction with North = 0 or 360 degrees and

South = 180 degrees, East 90 and West 270 degrees

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25. SHADOW ANGLE PROTRACTOR

Is a semi-circular protractor with two sets of lines.

Radial lines marked 0 at the centre, to - 90 to the left and + 90 to the right, to

give readings of the horizontal shadow angle (HSA)

Arc lines which coincide with the altitude circles along the middle, but then deviate

and converge at the two corners of the protractor and give readings of the vertical

shadow angle ( VSA)

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26. SHADOW ANGLE

Shadow angles express the sun's position in relation to a building face of given

orientation. The shadow angles help us design an efficient shading system for each

latitude and orientation.Shadow angle are of two types

1) Vertical shadow angle

2) Horizontal shadow angle

Vertical Shadow Angle (ε)

The vertical shadow angle or “profile angle” characterises a horizontal shading

device and is measured in a plane perpendicular to the building face. It is the

altitude of the sun projected to this surface.

When the sun is parallel to the normal of the surface then the solar altitude is equal

to the vertical shadow angle. The Vertical shadow angle will be larger than the

Altitude angle, when the sun is not perpendicular to the building face

When the sun is sideways, its altitude angle will be projected, parallel with the

building face, onto the perpendicular plane and the Vertical shadow angle will be

larger than the Altitude angle.

A larger overhang means a lower vertical shadow angle

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Horizontal shadow angle (δ)

Horizontal shadow angle (δ) characterizes a vertical shading device and is the

difference between solar azimuth and wall azimuth I.e the same as horizontal

component of angle of incidence. A larger fin means a smaller HAS angle

When the HSA is between +/- 90o and 270o, then the sun is behind the facade, the

facade is in shade, there is no HSA.

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27. SHADING DEVICES

Heat flows through windows can be a very important component of heat gains and

losses in a building. In the winter, heat can be lost by conduction or gained by

radiation. In the summer, heat can be gained by conduction or by radiation. The

most effective strategy to reduce heat gains by radiation is by shading.

Shading the Window is possible to shade the windows in three forms :

•External shading devices,

•Internal shading devices

•With the glass pane itself.

Each of these systems has advantages and disadvantages.

External Shading Devices

An external shading system has the advantage of blocking the solar radiation before

the sun penetrates the building, but has the disadvantage of exposure to the climatic

elements for maintenance. The size and position of these external shading elements

can be calculated so as to cover the windows on the most problematic hours.

External shading systems are usually classified as:

1) Vertical devices

2) Horizontal devices

3) Egg crate devices

VERTICAL DEVICES

Vertical devices consist of louver blades projecting fins in a vertical position. The

horizontal shadow angle measures their performance. Narrow blades with close

spacing may give the same shadow angle as broader blades with wider spacing.

Using shadow angle protractor, the shading mask of a given device can be

established. For vertical devices this is the characteristic sector shape.

These devices are most effective when the sun is to one side of the elevation, such

as eastern or western elevation.

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HORIZONTAL DEVICES

Horizontal device can be canopies, horizontal louver blades or externally applied

venetian blinds. Their performance will be measured by a vertical shadow angle.

The shading mask is of segmental shape. These will be most effective when the sun

is opposite to the building face considered and at a high angle, such as north and

south facing walls. To cover low angles the device would have to cover the window

completely permitting only downward view.

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EGG CRATE DEVICES

Egg crate devices are a combination of horizontal and vertical shading devices. The

many types of grille-blocks and decorative screens fall in to this category. These can

be effective for any orientation depending on detail dimensions