Utilization of Electrical Energy List of contents

Transcription

1 Utilization of Electrical Energy List of contents Illumination: 1.1 Nature of light, visibility spectrum curve of relative sensitivity of human eye and wave length of light 1.2 Definition: Luminous flux, solid angle, luminous intensity, illumination, luminous efficiency, depreciation factor, coefficient of utilization, space to height ratio, reflection factor, glare, shadow, lux. 1.3 Laws of illumination simple numericals 1.4 Different type of lamps, construction and working of incandescent and discharge lamps their characteristics, fittings required for filament lamp, mercury vapour sodium lamp, fluorescent lamp, halogen lamp, neon lamp, compact filament lamp(cfl), LED Lamp, comparison of incandescent, fluorescent, CFL & LED 1.5 Calculation of number of light points for interior illumination, calculation ofillumination at different points, considerations involved in simple design problems. Illumination schemes; indoor and outdoor illumination levels 1.6 Main requirements of proper lighting; absence of glare, contrast and shadow 1.7 Awareness about time switches, street lighting, flood lighting, monument lighting and decorative lighting, light characteristics etc. Multiple choice Questions Electric Heating 2.1 Advantages of electrical heating 2.2 Heating methods: Resistance heating direct and indirect resistance heating, electric ovens, their temperature range, properties of resistance heating elements,domestic water heaters and other heating appliances, thermostat control circuit Induction heating; principle of core type and coreless induction furnace, their construction and applications Electric arc heating; direct and indirect arc heating, construction, working and applications of arc furnace Dielectric heating, applications in various industrial fields Infra-red heating and its applications (construction and working of two appliances) Solar Heating Multiple Choice Questions 1 P a g e

2 1 Illumination 1.1 Visibility spectrum curve of relative sensitivity of human eye and wave length of light Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light which is the visible spectrum that is visible to the human eye having wavelengths are between 4 x 10-7 m(4,000 Å or 0.4 m m) to 7 x 10-7 m(7,000 Å or 0.7 m m). ( Å stands for 'angstrom' and is equal to m.) 1.2 Various Terminology Luminous flux (in lumens) is a measure of the total amount of light a lamp puts out. The luminous intensity (in candelas) is a measure of how bright the beam in a particular direction The illumination (or illuminance) E of a surface is the luminous flux per unit area that reaches the surface: E = F/A Illumination = luminous flux / area Solid angle: Plane angle is subtended at a point in a plane by two converging straight lines and its magnitude is given by Solid angle = A r 2 Luminous efficiency : It is the ratio of luminous flux to power, measured in lumens per watt Depreciation factor: It is reverse of the maintenance factor and is defined as the ratio of the initial meter-candles to the ultimate maintained metre-candles on the working plane. Coefficient of utilization It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp Space to height ratio Spacing Height Ratio is defined as the ratio of the distance between adjacent luminaires (centre to centre), to their height above the working plane. Reflection factor the ratio of reflected light to the incident light is called the reflection factor. It s value always less than unity Glare It is a visual sensation caused by excessive and uncontrolled brightness. Glare is difficulty of seeing in the presence of bright light caused by a significant ratio of luminance A shadow is a dark area where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. 2 P a g e

3 A shadow is a dark area where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The lux (symbol: lx) is the SI derived unit of illuminance and luminous emittance, measuring luminous flux per unit area. It is equal to one lumen per square metre. 1.3 Laws of Illumination There are two laws of illumination: (a) Law of inverse squares (b) lamberts cosine law Law of inverse squares: Illuminance (E) at any point on a plane perpendicular to the line joining the point and source is inversely proportional to the square of the distance between the source and plane. E = I / d 2 Where, I is the luminous intensity in a given direction. the light source the surface appears dimmer Cosine Law of Illuminance: Illuminance at a point on a plane is proportional to the cosine of the I angle of light incident E= cosθ, I d 2 θ is the luminous intensity of the source, Ɵ is the angle between the normal to the plane, d is the distance to the illuminated point 1.4 Incandescent and discharge lamps Incandescent lamp technology uses electric current to Heat a coiled tungsten filament to incandescence. The glass envelope contains a mixture of nitrogen and a small amount of other inert gases such as argon Discharge lamps produce light by passing an electric current through a gas that emits light when ionized by the current Discharge lamps: (a) Sodium vapour lamp (b) Mercury Vapour lamps Fig. Incandescent lamp 3 P a g e

4 1.5 Sodium vapour lamp The lamp consists of a discharge tube having special composition of glass to withstand the high temperature of the electric discharge The lamp consists of a discharge tube having special composition of glass to withstand the high temperature of the electric discharge The temperature inside the discharge tube rises and vaporizes sodium. Sodium vapour has the highest theoretical luminous efficiency and gives monochromatic orange-yellow light. 1.6 Mercury Vapour lamps It is similar to construction of the sodium vapour lamp. The electric discharge first takes place through argon and this vaporizes The mercury drops inside the discharge tube. The space between two bulbs is filled with an inert gas. Mercury vapour lamps are used for lighting of secondary roads, car parking areas, parks and gardens, factory sheds, etc. 1.7 Fluorescent lamp A fluorescent lamp is a low weight mercury vapour lamp that uses fluorescence to deliver visible light An electric current in the gas energizes mercury vapor which delivers ultraviolet radiation through discharge process and the ultraviolet radiation causes the phosphor coating of the lamp inner to radiate visible light 1.8 Halogen Lamps & Neon Lamps Unlike incandescent lamps, halogen lamps use a halogen gas fill (typically iodine or bromine), to produced what is called a halogen cycle inside the lamp. In the halogen cycle, halogen gas combines with the tungsten that evaporates from the lamp. Halogen lamps are sometimes called quartz lamps because their higher temperature requires quartz envelopes instead of the softer glass used other incandescent lamps. A 4 P a g e

5 halogen lamp comes with a few modifications to eliminate this blackening problem of incandescent lamp Neon Lamps is cold cathode lamp and consists of a gas bulb filled with a neon gas with a small percentage of helium. It gives orange pink colored light. The efficiency of neon lamp lies between lumens/watt The power consumption is of the order of 5watts. Compact Filament Lamp (CFL) & LED Lamp CFLs produce light in the same manner as linear fluorescent lamps. Their tube diameter is usually 5/8 inch (T5) or smaller. CFL power ranges from 5 to 55 watts. LEDs are solid-state semiconductor devices that convert electrical energy directly into light LEDs can be extremely small and durable; some LEDs can provide much longer lamp life than other sources Comparisons Tungsten Filament Lamp and LED Tungsten Filament Lamp Voltage fluctuation has comparatively more effect on the light output. Luminous efficiency increases with the increase in the voltage of the lamp It gives light close to natural light. Therefore objects are properly seen high working temperature and heat radiations brightness is more initial cost per lamp is quite low. LED Light Emitting Diode bulbs High Efficiency Life hours High cost 6-8 watts normally Small in size Fluorescent Tubes Voltage fluctuation has comparatively low effect on light output Luminous efficiency increases with the increase in wattage and increase in length of tube. It does not give light close to natural light low working temperature heat radiation is low. Brightness is less. initial cost per tube is more 5 P a g e

6 1.10 Calculation of number of light points for interior illumination The number of lamps required in a particular place can be designed by following three methods: 1 Watt per square meter: This is a rough method. Watts per square meter are calculated on the basis of efficiency of lamp (lm/watt). 2. Inverse square law: This method is used in street light calculations. In this method law of illumination is used for this candle power of the 0lamps should be known. 3. Lumen per square meter method: This method is used for design of general lighting. In this method, lamp efficiency, Depreciation factor, utilization factor etc. are used Illumination schemes The interior lighting schemes may be classified as (i) direct lighting (ii) semi direct lighting (iii) semi-indirect lighting (iv) indirect lighting (i) Direct lighting: It is most commonly used type of lighting scheme. In this lighting scheme more than 90 % of total light flux Is made to fall directly on the working plane with the help of deep reflectors. it is most efficient but causes hard shadows and glare. It is mainly used for industrial and general out door lighting. (ii) Semi-direct lighting: In this lighting scheme % if the total light flux is made to fall downwards directly with the help of semi-direct reflectors, remaining light is used to illuminate the ceiling and walls. Such a lighting system is best suited to rooms with high ceiling where a high level of uniformly distributed illumination is desirable (iii) Semi-indirect lighting: In the lighting scheme % of total light flux is thrown upwards to the ceiling for defuse reflection and the rest reaches the working plane directly except for some absorption. It is mainly used for indoor light decoration purposes. (iv) Indirect lighting: In this light scheme more than 90% if total light flux is thrown upwards to the ceiling for diffuse reflection by using inverted or bowl reflectors. In such a system the ceiling acts as the light source, and the glare it reduced to minimum Main requirements of proper lighting: Proper Illumination Level Uniformity of illumination Colour of light Shadows: Glare 1.12 Street lighting The main objectives of street lighting are (i) To make the traffic and obstructions on the road clearly visible in order to promote safety and convenience. (ii) To make the street more attractive. (iii) To increase the community value of the street. 6 P a g e

7 The principle employed for street lighting is different from that of interior lighting. There are no walls and ceiling which reflect or diffuse light, hence only direct lighting scheme can be employed and hard shadows and high contrast cannot be avoided. Mercury vapour and sodium discharge lamps have been found to have certain particular advantages for street lighting purposes. The most important of these low power consumption for a given amount of light. Flood lighting Flood light means flooding of large surfaces with light from powerful projectors. It is employed to serve one or more of the following purposes. For enhancing beauty of building at night such as public places, ancient buildings For illuminating railway yards, sports stadiums, car parks, construction sites, quarries etc. Flood lighting it is necessary to concentrate the light from the light source into a narrow beam. The particular type of reflector and its housing used for concentrating the light into narrow beam is known as flood light projection. Multiple Choice Questions Q1 Carbon arc lamps are commonly used in (A) domestic lighting (B) street lighting (C) cinema projectors (D) photography. Q2 Light waves travel with a velocity of (A) 3 x cm/s (B) 3 x cm/s (C) 3 x cm/s (D) 3 x cm/s Q3 Candela is the unit of (A) Luminous flux (B) Luminous intensity (C) Wavelength (D) None of the above. Q4 The unit of luminous flux is (A) steradian (B) candela (C) lumen (D) lux. Q5 Illumination level required for precision work is around (A) 50 lm/m 2 (B) 100 lm/m 2 (C) 200 lm/m 2 7 P a g e

8 (D) 500 lm/m 2. Q6 Which of the following lamp gives nearly monochromatic light? (A) Sodium vapor lamp (B) GLS lamp (C) Tube light (D) Mercury vapor lamp. Q7 The illumination level in houses is in the range (A) lumen/m 2 (B) lumen/m 2 (C) lumen/m 2 (D) lumen/m 2 Q8 One lumen per square meter is the same as (A) One lux (B) One candela (C) One foot candle (D) One lumen meter. Q9 Lumen/watt is the unit of (A) Light flux (B) Luminous intensity (C) Brightness (D) Luminous efficiency. Q 10 A zero watt lamp consumes (A) no power (B) about 5 to 7 W power (C) about 15 to W power (D) about 25 to 30 W power. Q11 Melting temperature of tungsten is (A) 2000 K (B) 2500 K (C)2655 K (D) 3655 K. Short/long Answer types questions Q1 Define luminous intensity, reflection factor, coefficient of utilization. Q2 Explain working principle Incandescent and discharge lamps 8 P a g e

9 2 ELECTRICAL HEATING Electric heating is any process in which ELECTRICAL ENERGY is converted to HEAT ENERGY. Electrical heating is based on the principle of that when electric current passes through a medium heat is produced. Let us take the case of solid material which as resistance R ohms and current flowing through it is I amps for t seconds than heat produced in the material will be H=I²Rt Joules. 2.1 Advantages of Electrical Heating over other Methods of Heating Clean and atmosphere / Free from dirt. No pollution / No flue gas is produced Response quickly Accurate Controlled temperature can made easily Comparatively safe Localized application Overall efficiency is much higher Uniform heating Highest efficiency of utilization Cheap furnaces Mobility of job Transfer of Heat Conduction: This phenomenon takes place in solid, liquid and gas. Heat transfer is proportional to the difference of temperatures between two faces. No actual motion of molecules. Convection: This phenomenon takes place in liquid and gas. Heat is transferred due to actual motion of molecules Convection: This phenomenon takes place in liquid and gas. Heat is transferred due to actual motion of molecules 9 P a g e

10 Classification of Electrical Heating 1 Power Frequency heating/ ohmic heating a) Resistance Heating b) Arc Heating both are of direct and indirect types 2 High Frequency heating a) Induction Heating b) Di-electrical Heating c) Infrared Heating 3 Microwave Heating RESISTANCE HEATING This method is based upon the I²R loss. Whenever current is passed through a resistor material heat is produced because of I²R losses. The generation of heat is done by electric resistor carrying current. There are two methods of resistance heating. They are: (i) Direct Resistance heating and (ii) Indirect Resistance Heating Direct Resistance heating Electric current is passed through the body (charge) to be heated Mode of heat transfer is Conduction. Example - Resistance Welding Indirect Heating Electric current is passed through highly resistive material(heating element) placed inside an oven. Heat produced due to I²R loss in the element is transmitted to the body Mode of heat transfer is Conduction &/or Convection &/or Radiation Example Room Heater CAUSES OF FAILURE OF HEATING ELEMENTS Formation of hot spots. Oxidation Corrosion Mechanical failure 10 P a g e

11 Fig. Resistance Indirect Heating Electric ovens It essentially consists of a high resistive material through which an electric current is passed placed in a chamber made of heat insulating material. The element may be in the form of strip or wire and is placed on the top, bottom of the oven depending upon circumstances. Resistance ovens are used for various purposes such as heat treatment of metals, drying, backing of pottery materials, cooking of food etc. The temperature of oven can be controlled by controlling (i) voltage or current (ii) time and (iii) resistance. The automatic control of temperature can be obtained by providing thermostat which will operate a switch to OFF or ON the circuit as soon as the temperature exceeds or fall below the adjusted value. Domestic water heaters Most of electric water heating is done by immersion heaters which consists of resistance coils placed in slotted cylinders of ceramic material. The material used for resistance coil is nichrome wire coated with magnesium oxide for preventing oxidation of the element which heat up the water due to I 2 R loss in it. Properties of resistance heating elements Low temperature coefficient High melting point Free from oxidation High resistivity 11 P a g e

12 2.2.2 Induction heating Induction heating is based on the principle of transformers. There is a primary winding through which an a.c current is passed. The coil is magnetically coupled with the metal to be heated which acts as secondary. An electric current is induced in this metal when the a.c current is passes through the primary coil. The following are different types of induction furnaces: 1. Core type (low frequency) induction furnaces. 2. Coreless type (high frequency) induction furnaces Coreless type induction furnace In this furnace, heat developed in the charge due to eddy currents flowing through it. When primary coils are excited by an alternating source, the flux set up by these coils induce the eddy currents in the charge. The direction of the resultant eddy current is in a direction opposite to the current in the primary coil. These currents heat the charge to melting point and they also set up electromagnetism forces that produce a stirring action to the charge. 12 P a g e

13 2.2.3 Electric arc heating The heating of matter by an electric arc. The matter may be solid, liquid, or gaseous. When the heating is direct, the material to be heated is one electrode; for indirect heating, the heat is transferred from the arc by convection, or radiation. Electrodes used in arc furnaces: 1. Carbon electrodes 2. Graphite electrodes 3. Self-baking electrodes Types of arc heating furnaces: 1. Direct arc furnaces 2. Indirect arc furnaces Direct arc furnaces When supply is given to the electrodes, two arcs are established and current passes through the charge, as shown in Fig. As the arc is in direct contact with the charge and heat is also produced by current flowing through the charge itself, it is known as direct arc furnace. In indirect arc furnace, the arc strikes between two electrodes by bringing momentarily in contact charge in this furnace is heated not only by radiation from the arc between electrode tips but also by conduction from the heated refractory during rocking action; so, the efficiency of such furnace is high. 13 P a g e

14 2.2.4 Dielectric Heating Dielectric heating, also known as electronic heating, RF (radio frequency) heating, and high-frequency heating, is the process in which a radio frequency alternating electric field, or radio wave or microwave electromagnetic radiation heats a dielectric material. We understand dielectric heating as the generation of thermal energy (heat) in a nonconducting material by the application of an electromagnetic force or field t it. This is the way a microwave oven heats things placed in it Infra-red heating & Solar Heating In this method of heating, heating elements consist of tungsten filament lamps together with reflectors to direct the whole of heat emitted on to charge (material to be heated). The lamps are operated at 2300 degree celcius there by giving a large amount of infrared radiations and the reflectors are plated with rhodium which prevents the leakage of heat through the chamber. The lamps used are rated between ,000 watts as 250V Solar Heating: In solar water heating collectors capture and retain heat from the sun. This heat is then transferred to a liquid. Heating of the sun trapped using the green house effect. Solar radiation is energy in the form of electromagnetic radiations from the infrared to the ultraviolet example solar cooker, solar water heater etc. 14 P a g e

15 Multiple Choice Questions Q1 Eureka is an alloy of a. Nickel and chromium b. Nickel and copper c. Iron, chromium and aluminium d. Nickel, chromium and aluminium Q2 A good heating element should have a. High resistivity and low melting point b. Low resistivity and high melting point c. High resistivity and high melting point d. Low resistivity and low melting point Q3 Direct resistance heating method is used in a. Salt bath furnaces b. Resistance welding c. Electrode boiler for heating water d. All of these Q4 The heat can be transferred from high temperature body to low temperature body by a. Conduction method b. Convection method c. Radiation method d. All of these Q5 The efficiency of heating is more for a. Solid fuels b. Oil heating c. Gas heating d. Electric heating Short/Long Answer type questions Q1 Define Heating and advantages of heating Q2 Explain resistance heating with its types Q3 Explain Dielectric heating in detail. 15 P a g e