ILLUMINATION, HEATING AND WELDING
1)
Define luminous flux.
It is defined as the total quantity of light
energy emitted per second from a luminous body. It is represented by symbol F
and is measured in lumens. The conception of luminous flux helps us to specify
the output and efficiency of a given light source.
2) What
is meant by candle power?
It is defined as the number of lumens given out
by the source in a unit solid angle in a given direction n. It is denoted by
CP.
CP=lumens/ω
3)
Define MHCP.
The mean of candle power in all directions in
the horizontal plane containing the source of light is termed as Mean
Horizontal Candle Power.
4)
Define utilization factor.
It is defined as the ratio of total lumens
reaching the working plane to total lumens given out by the lamp.
Utilization
factor= Total lumens reaching the working plane/Total lumens given out by the
lamp
5) What are
the laws of illumination?
Law of Inverse Squares:
Illumination at appoint is inversely proportional
to square of its distance from the point source and directly proportional to
the luminous intensity (CP) of the source of light in that direction.If a
source of light emits light equally in all directions be placed at the centre
of a hollow sphere, the light will fall uniformly on the inner surface of the
sphere. If the sphere be replaced by one of the larger radius, the same total
amount of light is spread over a larger area proportional to the square of the
radius.
Lambert’s cosine law:
The illumination at a point on a surface is
proportional to cosine of the angle which ray makes with the normal to the
surface at that point.
6) What is
meant by luminance?
It is defined as the luminous intensity per
unit projected area of either a surface source of light or a reflecting surface
and is denoted by L.
7)
Define space-height ratio.
It is defined as the ratio of horizontal
distance between adjacent lamps and height of their mountings.
Space-height ratio= Horizontal distance between
two adjacent lamps/Mounting height of lamps above working plane
8) What
is polar curve?
In most lamps or sources of light the luminous
intensity is not the same in all directions. If the luminous intensity, i.e.
the candle power is measured in a horizontal plane about a vertical axis and a
curve is plotted between candle power and the angular position,a curve is
obtained is called as horizontal polar curve.
The luminous intensity in all the directions
can be represented by polar curves. If the luminous intensity in a vertical
plane is plotted against the angular position, a curve known as vertical polar
curve is obtained.
9) Name the
various photometer heads.
1.
Bunsen
Head (or) Grease spot photometer
2.
Lummer-Brodhun
photometer head
There are two types of Lummer Brodhun heads
a)
Equality
of Brightness type photometer head
b) Contrast type photometer head
10) What are
all the sources of light?
·
According
to principle of operation the light sources may be grouped as follows. Arc
lamps
·
High
temperature lamps
·
Gaseous
discharge lamps
·
Fluorescent
type lamps
11) What
is stroboscopic effect of fluorescent tubes?
With a.c. supply frequency of 50 cycles per
second, discharge through the lamp becomes zero, 100 times in a second. Due to
the persistence of vision, our eyes do not notice this. If this light falls on
moving parts, they may appear to be either running slow or in the reverse
direction or even may appear stationary. This effect is called stroboscopic
effect.
12)
Define beam factor.
The ratio of lumens in the beam of a projector
to the lumens given out by lamps is called the beam factor. This factor takes
into account the absorption of light by reflector and front glass of the
projector lamp. Its values vary from 0.3 to 0.6.
13)
Mention the types of lighting schemes.
The distribution of the light emitted by lamps
is usually controlled to some extent by means of reflectors and translucent
diffusing screens or even lenses. The interior lighting schemes may be
classified as
·
Direct
lighting
·
Semi-direct
lighting
·
Indirect
lighting
·
Semi-indirect
lighting
·
General
lighting
14) What are
the drawbacks of discharge lamps?
Drawbacks of discharge lamps:
·
Take
time to attain full brightness.
·
High
initial cost and poor power factor.
·
Starting
requires trigger-starter.
·
Light
output fluctuates at twice the supply frequency.
·
The
flicker causes stroboscopic effect.
·
These
lamps can be used only in particular position.
15) What are
the requirements of lighting system?
The following factors are required to be
considered while designing the lighting scheme.
·
Illumination
level
·
Uniformity
of illumination
·
Colour
of light Shadows Glare
·
Mounting
height Spacing of luminaries
·
Colour
of surrounding walls.
16) What
are the advantages of electric heating?
The main advantages of electric heating over
other systems of heating such as coal, oil or gas heating are given below.
·
Economical
·
Cleanliness
·
Absence
of flue gases
·
Ease of
control or adaptation Automatic protection
·
Upper
limit of temperature
·
Special
heating features
·
High
efficiency of utilisation
·
Better
working conditions Safety
·
Heating
of non-conducting materials
17) Classify
the methods of electric heating.
Kinds of electric heating
A. Power frequency heating
a. Resistance heating
i) Direct resistance heating ii) Indirect
resistance heating iii) Infrared or Radiant heating
b. Arc heating
i) Direct arc heating ii) Indirect arc heating
B. High frequency heating
a. Induction heating
i)
Direct
induction heating
ii) Indirect induction heating
b.
Dielectric
heating
18) What is
meant by indirect resistance heating?
In this method, the current is passed through a
high resistance wire known as heating element. The heat produced due to I2 R
loss in the element is transmitted by radiation or convection to the body to be
heated.Applications are room heaters, in bimetallic strip used in starters,
immersion water heaters and in domestic and commercial cooking and salt bath
furnace.
19).What
is meant by (1) infra red /radiant heating? (2) Dielectric heating?
(1)When current possess through a resistive
element heat energyis produced and the same is dissipated in the form of
infrared radiation this is focused upon a body to be heated .e.g. to dry the
wet paint on an object.
(2)When a non metallic material is placed
between two electrodes at high voltage the dielectric loss is dissipated in the
form of heat which is used for heating purposes.
20) What are
the requirements of a good heating material?
1.
High
ressistivity
2.
Low
temperature coefficient of ressistance
3.
High
melting point
4.
Free
from oxidation
21) What are
the properties of heating element material?
·
The
material of the heating elements should posses the following desirable
properties for efficient operation and long life.
·
High
resistivity: It should have high specific resistance so that the overall length
to produce a certain amount of heat may be smaller.
·
High
melting point: It should have high melting point so that high temperatures can
be produced without jeopardizing the life of the element.
·
Free
from oxidation: It should be able to resist oxidation at high temperatures;
otherwise its life will be shortened.
·
Low
temperature coefficient: It should have a low temperature coefficient so that
resistance remains appreciably constant even with increases of temperature.
This helps in accurate control of temperature.
22) What are
the causes of failure of heating elements?
Principle causes are
·
Formation
of hot spots
·
General
oxidation of the element and intermittency of operation
·
Embrittlement
caused by grain growth
·
Contamination
of element or corrosion
23)
Write short note on infrared heating.
In radiant heating, the elements are of
tungsten operating about 2300°C as at this temperature a greater proportion of
infra-red radiation is given off.Heating effect on the charge is greater since
the temperature of the heating element is greater than in the case of
resistance heating. Heat emission intensities up to 7500 watts/sq.m can be
obtained leading to heat absorption up to 4300 watts/sq.m. This reduces the
time taken by various drying process.
24) What
is the basic principle of induction heating?
High frequency eddy current heating produced by
eddy currents induced by electromagnetic action in the metal to be heated.It
works on the principle of electromagnetic induction as same as a transformer.
It has a metal disc surrounded by a copper coil in which a.c supply is flowing.
The disc has a finite value of diameter and thickness and is spaced a given
distance from the coil and concentric to it. We find that a secondary current
is caused to circulate around the outer surface of the disc.
25) What are
the different types of resistance welding?
The different types are as follows
·
Butt
welding
·
Spot
welding
·
Projection
welding
·
Seam
welding
·
Percussion
welding
26)
Compare DC welding and AC welding.
27) What
is LASER welding?
LASER (Light Amplification by Stimulated
Emission of Radiation) welding is a welding process that uses the heat from a
laser beam impinging on the joint. The process is without a shielding gas and
pressure.
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