Refrigeration and Air conditioning

UNIT-7

Refrigeration System Operating Characteristics

General

Refrigeration systems must operate at all hours of the year, even when the building is unoccupied. Warmer weather tends to push refrigeration equipment to its capacity limit, thus creating a maximum operating kW and kWh. Evaporators - must be selected to provide the required cooling at all expected ambient conditions even with the maximum frost on the coils (i.e., just prior to defrosting). Evaporator coils used include two types of refrigeration systems: flooded evaporator and direct expansion. For direct expansion systems, two of the most commonly used refrigerant liquid metering devices are the capillary tube and the thermostatic expansion valve. In addition, proper provisions must be made for periodic defrosting of evaporator air-side surfaces. Defrosting may be accomplished using refrigerant compressor discharge hot-gas, water spray, or manually as selected to meet the user's objectives. Suitable drain connections should be provided to carry off the water resulting from defrost operations. Condensers - must be selected to operate at all outdoor weather conditions in the area. Air-cooled condensers must be supplied with the proper controls to permit operation at low outdoor ambient conditions. Water-cooled condensers may require water regulating valves to keep condensing pressure high enough to enable the thermal expansion valves to function. The type of condenser selected depends largely on the size of the cooling load, refrigerant used, quality and temperature of available cooling water (if any), and noise considerations. Water-cooled condensers require cooling water from an external cooling tower, or from a lake, well, river or other similar source. Once-through use of city water for condensing purposes is prohibited in most locations. Air-cooled condensers are the most popular since they avoid other problems of water acquisition, treatment and disposal. The trade-off may be higher electrical consumption. As seen here, the evaporative condenser is a combination of a water cooled condenser and an air-cooled condenser that rejects heat through the evaporation of water into an airstream traveling across a condenser coil. Compressors - must be sized to meet the varying needs of each application. Provision must be made to protect the compressor from liquid carry over from the evaporator, in addition to the normal safety controls (high and low pressure cutout. oil pressure, etc.). The most common type of compressor used for commercial refrigeration systems is the reciprocating compressor. Reciprocating compressor types include single-stage (booster or high state), internally compounded, and open, hermetic or semi-hermetic.

 

HEAT TRANSFER

The second important law of thermodynamics is that heat always travels from a warm object to a colder one. The rate of heat travel is in direct proportion to the temperature difference between the two bodies.

Assume that two steel halls are side by side in a perfectly insulated box. One ball weighs one pound and has a temperature of 400° F., while the second ball weighs 1,000 pounds and has a temperature of 390° F. The heat content of the larger ball is tremendously greater than the small one, but because of the temperature difference, heat will travel from the small ball to the large one until the temperatures equalize.

Heat can travel in any of three ways: radiation, conduction, or convection.

Radiation is the transfer of heat by waves similar to light waves or radio waves. For example, the sun's energy is transferred to the Earth by radiation. One need only step from the shade into direct sunlight to feel the impact of the heat waves, even though the temperature of the surrounding air is identical in both places. There is little radiation at low temperatures, and at small temperature differences, so radiation is of little importance in the actual refrigeration process. However, radiation to the refrigerated space or product from the outside environment, particularly the sun, may be a major factor in the refrigeration load.

Conduction is the flow of heat through a substance. Actual physical' contact is required for heat transfer to take place between two bodies by this means. Conduction is a highly efficient means of heat transfer as any service-man who has touched a piece of hot metal can testify.

Convection is the flow of heat by means of a fluid medium, either gas or liquid, normally air or water. Air may be heated by a furnace, and then discharged into a room to heat objects in the room by convection.

In a typical refrigeration application, heat normally will travel by a combination of processes, and the ability of a piece of equipment to transfer heat is referred to as the overall rate of heat transfer. While heat transfer cannot take place without a temperature difference, different materials vary in their ability to con-duct heat. Metal is a very good heat conductor, while asbestos has so much resistance to heat flow it can be used as insulation.

What is refrigerant:-

At any hardware store or electronics store you can buy a can of electronics duster. This is just a spray can that blasts a stream of gas when you pull the trigger. You use it to blow the dust out of tiny crevices in electronic circuits. Your teacher is going to bring some cans of electronics duster to class and you can feel for yourself what happens when you spray this stuff. If you spray it long enough, the can will get very cold. It can even get cold enough to give you frostbite.

If you're inquisitive, you may be asking "what gas does the electronics duster shoot out?" It so happens that the gas is a hydrofluorocarbon, or an HFC. Remember that HFCs are the family of compounds used to replace chlorofluorocarbons as refrigerants. It's obvious from feeling the can that HFCs can make things cold, but just how do they do that?

Heat and Changes of State

This is a change of state, of course. As you may remember, for every change of state there is a heat of transition. When a solid becomes a liquid, it absorbs heat in the process of melting. This is called the heat of melting. When a liquid becomes a gas, it absorbs heat in the process. This is called the heat of vaporization. This works backward, too. When gases condense to become liquids they give off heat, and when liquids freeze to become solids, they give off heat as well.

Heat of Vaporization and Your Refrigerator

A refrigerator works in the same way. In a refrigerator, an HFC is pumped through a tube called a coil, like you see in the animation below. In the coil, there is a plug with a small hole in it called a throttle valve. Because this opening is so small, pressure builds up behind the throttle valve, enough for the HFC to become a liquid. Slowly, the HFC passes through the throttle valve. On the other side of the throttle valve, the pressure is not as high. So the boiling point of the HFC drops low enough for the HFC to evaporate. As it evaporates, the HFC absorbs heat from its surroundings, specifically the inside of the refrigerator. The inside of the refrigerator then gets cold.

But there's more to this story. The HFC keeps moving through the coil. The coil passes to the outside of the refrigerator and to the compressor. The compressor puts pressure on the HFC, which condenses back into a liquid, and the whole process can start all over again.

What Makes a Good Refrigerant?

Why is it so hard to find a good refrigerant? To be a good refrigerant, a compound has to live up to a few requirements. Obviously, we want something that is nontoxic. We also want something that is unreactive. The refrigerant has to be stable for the lifetime of the refrigerator. In addition, we want a compound that is ozone-safe. But on top of all these criteria, we need a compound that has a low boiling point. So why not use nitrogen (N₂)? It's nontoxic (we breathe it all the time), the atmosphere is already full of it, and its boiling point is way down at -196°C. That's a little too low, as we'll soon see. We want a refrigerant to a have a low boiling point but not too low, because when a refrigerator is running, the refrigerant is constantly being boiled from a liquid to a gas, and then being condensed back into a liquid again. If the boiling point is too low, it will be hard to condense back into a liquid.

VAPOUR COMPRESSOR SYSTEM

Vapor compression refrigeration is the primary method used to provide mechanical cooling. All vapor compression systems consist of four basic components (plus the interconnecting piping): evaporator, compressor, condenser, and an expansion device. The evaporator and condenser are heat exchangers that evaporate and condense the refrigerant while absorbing and rejecting heat. The compressor takes the refrigerant vapors from the evaporator and raises the pressure sufficiently for the vapor to condense in the condenser. The expansion device controls the flow of condensed refrigerant at this higher pressure back into the evaporator.

Historically, the common refrigerants were R-11, R-12, R-22, and compounds in the R-500 series. With the CFC phaseout, new refrigerants have been developed to replace R-11 and R-12 in new equipment. These new refrigerants can also be used to retrofit existing equipment in many cases. However, these retrofits are not "drop-ins" and should be done by trained technicians.

Food processors often use ammonia (R-717). While potentially hazardous, ammonia is inexpensive and environmentally benign. Experts anticipate wider use of ammonia due to concerns over CFC phase-out. Interestingly, R-22 was developed as a safe alternative for cooling systems that would perform best at ammonia refrigerant characteristics.

The manufacturer selects the specific refrigerant used in any equipment to best match the cooling system design and size. The availability and cost of these refrigerants and the consequences of refrigerant leaks and disposal have become very serious concerns for today's building owners and the design community. Each of these issues is addressed in other areas of this interactive knowledge program.

Vapor Compression Systems - The Evaporator

The evaporator and condenser are both heat exchangers. Whether they move heat to or from air or water or refrigerant is merely a matter of design. On the design day the evaporator typically cools either:

1. Air returning from the building space (or outside air) to ~ 55 - 60°F

2. Water from about ~ 54°F as it returns from building air handlers to ~ 44°F.

In both cases the evaporator boils the selected refrigerant to provide this cooling. The pressure at which the refrigerant boils is exactly that which satisfies the energy balance of heat-in equals heat-out.

The refrigerant is circulated through numerous parallel paths. As the refrigerant flows and evaporates along these paths the pressure will drop as well. This in turn drops the temperature of the refrigerant as it evaporates. Consequently, properly designed direct expansion coils operate with the coldest refrigerant temperatures closest to the coil exit. However, the refrigerant temperature coming out of this coil is usually a little warmer than this to provide some level of superheat to be sure liquid refrigerant isn't leaving the coil and entering compressor (where it could cause mechanical failure in some designs).

Shell and tube heat exchangers commonly have water circulated through the tubes and refrigerant boiling around the tubes. There are also designs where refrigerant flows within the tubes and water flows over the tubes. Baffles are normally used in this case to direct water flow in a serpentine fashion to optimize heat transfer. Almost all large chillers use shell and tube evaporators with water flowing through the tubes.

 

 

Vapor Compression Systems - Evaporator Control

In comfort cooling applications, actual cooling loads are seldom at full load conditions. Capacity control is achieved in finned coil evaporators that directly chill air by splitting the coil into independent sections. The principal reason is to permit coil sections to be activated and deactivated to better match coil cooling capacity with compressor loading. The combination of smaller coil sections controlled by correspondingly sized expansion valves improves valve performance and part load humidity control.

Capacity control in shell and tube evaporators is usually handled using the return water temperature. For example, if the full-load temperature range for chilled water is from 44°F to 54°F, water returning at 50°F indicates the cooling load is about 60%. Liquid refrigerant is metered to the evaporator to match the load using an orifice plate system or an expansion valve. On large chillers, the expansion valve is pilot operated.

 

 

 

 

 

 

 

 

 

 

Vapor Compression Systems -
The Condenser

The refrigerant is recovered by condensing it in a heat exchanger using air or water to reject the heat. Air cooled condensers are most common in smaller sizes, up to about 200 ton capacity. Technically, there is no upper limit on the size of an air cooled condenser, but operating cost issues usually dictate water cooled units for applications over about 100 tons.

There are two water cooled designs: cooling towers and evaporative condensers. Both work on the principal of cooling by evaporating water into a moving air stream. The effectiveness of this evaporative cooling process depends upon the wet bulb temperature of the air entering the unit, the volume of air flow and the efficiency of the air/water interface.

Evaporative condensers use water sprays and air flow to condense refrigerant vapors inside the tubes. The condensed refrigerant drains into a tank called a liquid receiver. Refrigerant subcooling can be accomplished by piping the liquid from the receiver back through the water sump where additional cooling reduces the liquid temperature even further.

Cooling towers are essentially large evaporative coolers where the cooled water is circulated to a remote shell and tube refrigerant condenser. Notice the cooling water is circulating through the tubes while refrigerant vapor condenses and gathers in the lower region of the heat exchanger. Notice also that this area "subcools" the refrigerant below the temperature of condensation by bringing the coldest cooling tower water into this area of the condenser. The warmed cooling water is sprayed over a fill material in the tower. Some of it evaporates in the moving air stream. The evaporative process cools the remaining water.

The volume of water used by both evaporative condensers and cooling towers is significant. Not only does water evaporate just to reject the heat, but water must be added to avoid the buildup of dissolved solids in the basins of the evaporative condensers or cooling towers. If these solids build up to the point that they foul the condenser surfaces, the performance of the unit can be greatly reduced.

 

Defrosting

Defrosting is a procedure, performed periodically on refrigerators and freezers to maintain their operating efficiency. Over time water vapour in the air condenses on the cooling elements within the cabinet. It also refers to leaving frozen food at a higher temperature prior to cooking.

Defrosting a freezer

The resulting ice inhibits heat transfer out of the cabinet increasing running costs. Furthermore as the ice builds up it takes increasing space from within the cabinet - reducing the space available for food storage. Defrosting the unit is achieved by:-

• Temporarily removing all food from the cabinet.
• Turning off power to the unit.
• Leaving the doors to the unit open
• Waiting for the ice to melt and draining it appropriately. Using a towel is advisable when completing this step.

The process may be sped up by mechanical removal of ice, or the introduction of gentle heat into the cabinet. Placing a pan of hot water in the cabinet and closing it is an effective method. Using a fan to blow in room temperature air will also greatly speed up the melting process as well as help to evaporate the damp surfaces. Note that the fastest manual way is to use a vacuum cleaner: simply insert the hose into the exhaust port (nearly all are designed for this), and use the wand to blow on the coils; this method is much faster than any other.

Any mechanical removal of ice should be done gently so that the equipment is not damaged.

It is generally recommended that defrosting should be done annually.

Many newer units employ automatic defrosting (often called "frost-free" or "no frost") and do not require manual defrosting in normal use.

 

AC Compressor

Air conditioning is the cooling and air for comfort, the term can refer to any form of cooling, heating or ventilation that modifies the condition of air. An air conditioner is an instrument, system, or machinery designed to calm down the air temperature and humidity within an region, typically using a refrigeration cycle but sometimes using evaporation, commonly for comfort cooling in buildings and motor vehicles.

Humidity control
Air conditioning tools usually reduces the humidity of the air. From the processed air the coil evaporator condenses the water vapor, (much like an ice-cold drink will condense water on the outside of a glass), sending the water to a drain and removing water vapor from the cooled space and reducing the relative humidity. Since the human perspire to make himself cool by the evaporation of perspiration from the skin, drier air (up to a point) improves the comfort provided. The comfort air conditioner is designed to create a 40% to 60% relative humidity in the occupied space.

Relative Humidity

The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density.

The most common units for vapor density are gm/m3. For example, if the actual vapor density is 10 g/m3 at 20°C compared to the saturation vapor density at that temperature of 17.3 g/m3 , then the relative humidity is

Calculation

What is Humidification?

It is the artificial regulation of humidity in home environments, industrial environments, and health care applications such as artificial respiration. To be comfortable, people require a certain amount of ambient humidity -- not too high, and not too low. Adequate humidification in a manufacturing environment stabilizes moisture in wood, paper, and textiles, while preventing warping in glue joints. In all environments, humidification reduces fire risk and static electricity while making the area feel comfortable.

In humidification, two quantities are commonly used. Absolute humidification is expressed in grams of moisture per cubic volume of air, while the more commonly used relative humidification is expressed as a ratio between the amount of moisture currently in the air and the maximum moisture the air could hold before condensation occurs. A typical comfortable level of relative humidification is between 35% and 50%. Excess humidity can cause the growth of mold or fungus. Too little humidity can cause static discharge or the accumulation of unwanted dust, contributing to allergies.

Many humidifiers are cheap and require little maintenance. In industrial settings, they are often hung from the ceiling among duct work. Humidification is intimately tied to heating and cooling systems. The level of humidity in the air is also a function of the temperature. Therefore, humidity control systems are often integrated with cooling systems.

Dehumidifier

A dehumidifier is mostly a household appliance that reduces the level of humidity in the air, usually for health reasons, as humid air can cause mold and mildew to grow inside homes, which has various health risks. Relative humidity is preferably 30 to 50%.[1] Very high humidity levels are also unpleasant for human beings, can cause condensation and can make it hard to dry laundry or sleep. Higher humidity is also preferred by most insects, including clothes moths, fleas and cockroaches. Dehumidifiers are used in industrial climatic chambers for keeping certain level of humidity.

 

 

 

 

 

Dew point Control

Dew Point Control, LLC (DPC) is an equipment leasing company that provides an array of choices for hydrocarbon dew point control. Working within an industry that is in constant flux, DPC strives to match each customer's needs with the most cost efficient technology.

·         Meet transporting pipeline hydrocarbon dew point specifications.

·         Capture NGL liquid upgrade income.

·         Capture additional income by removing the "crude" component prior to NGL processing to avoid processing, transportation, fractionation and marketing fees.

·         Controlling gathering system liquid drip for efficiency and safety reasons.

·         Improve measurement volumes recorded by orifice custody transfer meters by removing the liquid buildup on the orifice plates.

·         Prove the potential of new gathering systems prior to the capital commitment of the deep liquid NGL recovery facility.

·         Short term lease to move gas to market during the construction phase of a deep liquid recovery NGL facility.

·         To provide additional short term capacity to a deep NGL liquid recovery facility by conditioning gas that bypasses the existing facility or leaning the NGL component of the gas entering the NGL liquid recovery facility.

 

Types of air conditioner equipment

Window and through-wall units

Room air conditioners come in two forms: unitary and packaged terminal PTAC systems. Unitary systems, the common one room air conditioners, sit in a window or wall opening, with interior controls. Interior air is cooled as a fan blows it over the evaporator. On the exterior the air is heated as a second fan blows it over the condenser. In this process, heat is drawn from the room and discharged to the environment. A large house or building may have several such units, permitting each room be cooled separately. PTAC systems are also known as wall split air conditioning systems or ductless systems.[5] These PTAC systems which are frequently used in hotels have two separate units (terminal packages), the evaportive unit on the exterior and the condensing unit on the interior, with tubing passing through the wall and connecting them. This minimizes the interior system footprint and allows each room to be adjusted independently. PTAC systems may be adapted to provide heating in cold weather, either directly by using an electric strip, gas or other heater, or by reversing the refrigerant flow to heat the interior and draw heat from the exterior air, converting the air conditioner into a heat pump. While room air conditioning provides maximum flexibility, when cooling many rooms it is generally more expensive than central air conditioning.

Evaporative coolers

In very dry climates, evaporative coolers are popular for improving comfort during hot weather. This type of cooler is the dominant cooler used in Iran, which has the largest number of these units of any country in the world, causing some to referring to these units as "Persian coolers." An evaporative cooler is a device that draws outside air through a wet pad, such as a large sponge soaked with water. The sensible heat of the incoming air, as measured by a dry bulb thermometer, is reduced. The total heat (sensible heat plus latent heat) of the entering air is unchanged. Some of the sensible heat of the entering air is converted to latent heat by the evaporation of water in the wet cooler pads. If the entering air is dry enough, the results can be quite comfortable; evaporative coolers tend to feel as if they are not working during times of high humidity, when there is not much dry air with which the coolers can work to make the air as cool as possible for dwelling occupants. Unlike air conditioners, evaporative coolers rely on the outside air to be channeled through cooler pads that cool the air before it reaches the inside of a house through its air duct system; this cooled outside air must be allowed to push the warmer air within the house out through an exhaust opening such as a open door or window.[7]

These coolers cost less and are mechanically simple to understand and maintain.

Portable air conditioners

Portable air conditioners (or PACs) are moveable units that can be used to cool a specific room in a home and do not require permanent installation.^[8] Warm air in the room is drawn in through inlets on the portable air conditioner. The air is circulated through the unit and is cooled by evaporator coils with refrigerant running through them and then blown out through the front. Remaining hot air in the unit is expelled and vented through the back with an exhaust hose.^[9] All portable air conditioners require exhaust hoses for venting.

Single Hosed Units

A single hosed unit has one hose that runs from the back of the portable air conditioner to the vent kit where hot air can be released. A single hosed portable air conditioner can cool a room that is 475 sq. ft. or smaller and has at most a cooling power of 12,000 BTUs.^[10]

Dual Hosed Units

Dual hosed units are typically used in larger rooms. One hose is used as the exhaust hose to vent hot air and the other as the intake hose to draw in additional air (usually from the outside). These units generally have a cooler power of 12,000-14,000 BTUs and cool rooms that are around 500 sq. ft.^[10] The reason an intake hose is needed to draw in extra air is because with higher BTU units, air is cycled in large amounts and hot air is expelled at a faster rate. This creates negative air pressure in the room, and the intake hose stabilizes the room's air pressure.^[9]

Split Units

Portable units are also available in split configuration, with the compressor and evaporator located in a separate external package and the two units connected via two detachable refrigerant pipes, as is the case with fixed split systems. Split portable units are superior to both single and dual hosed mono-portable units in that interior noise and size of the internal unit is greatly reduced due to the external location of the compressor, and no water needs to be drained from the internal unit due to the exterior location of the evaporator.

A drawback of split portable units compared with mono-portables is that a surface exterior to the building, such as a balcony must be provided for the external compressor unit to be located.

Heat and Cool Units

Some portable air conditioner units are also able to provide heat by reversing the cooling process so that cool air is collected from a room and warm air is released. These units are not meant to replace actual heaters though and should not be used to cool rooms lower than 50 °F (10 °C).

Central air conditioning

Central air conditioning, commonly referred to as central air (U.S.) or air-con (UK), is an air conditioning system which uses ducts to distribute cooled and/or dehumidified air to more than one room, or uses pipes to distribute chilled water to heat exchangers in more than one room, and which is not plugged into a standard electrical outlet.

With a typical split system, the condenser and compressor are located in an outdoor unit; the evaporator is mounted in the air handler unit. With a package system, all components are located in a single outdoor unit that may be located on the ground or roof.

Central air conditioning performs like a regular air conditioner but has several added benefits:

• When the air handling unit turns on, room air is drawn in from various parts of the building through return-air ducts. This air is pulled through a filter where airborne particles such as dust and lint are removed. Sophisticated filters may remove microscopic pollutants as well. The filtered air is routed to air supply ductwork that carries it back to rooms. Whenever the air conditioner is running, this cycle repeats continually.

• Because the condenser unit (with its fan and the compressor) is located outside the home, it offers a lower level of indoor noise than a free-standing air conditioning unit.

Mini (Small) Duct, High Velocity

A central air conditioning system using high velocity air forced through small ducts (also called mini-ducts), typically round, flexible hoses about 2 inches in diameter. Using the principle of aspiration, the higher velocity air mixes more effectively with the room air, eliminating temperature discrepancies and drafts. A high velocity system can be louder than a conventional system if sound attenuators are not used, though they come standard on most, if not all, systems.^[11]

The smaller, flexible tubing used for a mini-duct system allows it to be more easily installed in historic buildings, and structures with solid walls, such as log homes. These small ducts are also typically longer contiguous pieces, and therefore less prone to leakage. Another added benefit of this type of ducting is the prevention of foreign particle buildup within the ducts, due to a combination of the higher velocity air, as well as the lack of hard corners.^[12]

 

ELECTRICITY

Unit – 4

Electricity

Meaning and use:-  Fundamentally, electricity is the by-product of other energy resources.  It is product from energy reserves.  The conventional generation of electricity utilizes coal, oil, gas or nuclear energy to heat water, producing high temp. pressure steam.  The steam flows through an electric turbine generator.

Generator is the machine used to convert mechanical energy into electrical energy.  Generator is comprised of the essential elements of Faraday’s law to produce electrical power.

Faraday’s Law says three things must be present in order to produce electrical current:-

·          Magnetic Field

·         Conductor

·         Relative Motion

·         Conductor cuts lines of magnetic flux; a voltage is induced in the conductor.  Direction and speed is important in this activity.

 

Uses of Electricity

Electricity has invaded our lives and has become vital in almost all aspects of society today. The list of uses will fill a book but here are a few headings:

Transport Trains, buses, trams and cars all use electricity. Many use it as the motive power, meaning that electricity drives the wheels to make the vehicle move. Even gas and diesel powered vehicles use electricity to start the engines, control the engine and power the ancillary devices.

Home Heating, lighting, television, radio, computer, telephones all rely on electricity. Even wireless lights such as solar powered lamps will convert sight to electricity.

Communication As well as providing power for computers, cell phones, fixed phones, electricity is used as the medium for the transmission of signals. Even high speed optical fibers rely on an electrical signal at each end of the line. Without electricity, communication would be reduced to letters, flag waving and lighting fires and shouting at each other. None of the electricity free methods are as flexible as any that we are used to using today

Industry Manufacturing relies on electricity to drive virtually every moving part in a factory. Saws, cutters, conveyor belts, furnaces, chillers - whatever the process, electricity is involved somewhere.

Entertainment The MP3 player, the portable battery powered radio, memory stick are all accepted as part of our everyday lives. All rely on electricity to operate. Whether connected to a mains supply or battery, they all use electricity.

ADVANTAGE AS A TYPE OF ENERGY

Electricity

• Electricity is beneficial because it is clean, cheap, safe and a convenient source of energy.

• An advantageous use of electricity is restarting a person's heart.

• Its the most expensive to use for heating of any kind.

Conductors and Insulators

 

 

 

In a conductor, electric current can flow freely, in an insulator it cannot. Metals such as copper typify conductors, while most non-metallic solids are said to be good insulators, having extremely high resistance to the flow of charge through them. "Conductor" implies that the outer electrons of the atoms are loosely bound and free to move through the material. Most atoms hold on to their electrons tightly and are insulators. In copper, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons which propagates, "domino fashion" through the conductor.

Simply stated, most metals are good electrical conductors, most nonmetals are not. Metals are also generally good heat conductors while nonmetals are not.

INSULATORS

Most solid materials are classified as insulators because they offer very large resistance to the flow of electric current. Metals are classified as conductors because their outer electrons are not tightly bound, but in most materials even the outermost electrons are so tightly bound that there is essentially zero electron flow through them with ordinary voltages. Some materials are particularly good insulators and can be characterized by their high resistivities:

	Resistivity (ohm m)
Glass	1012
Mica	9 x 1013
Quartz (fused)	5 x 1016

This is compared to the resistivity of copper:

	Resistivity (ohm m)
Copper	1.7 x 10-8

Circuit Elements

Electric circuits are considered to be made up of localized circuit elements connected by wires which have essentially negligible resistance. The three basic circuit elements are resistors, capacitors, and inductors. Only these passive elements will be considered here; active circuit elements are the subject of electronics.

Electrical conductor

In science and engineering, a conductor is a material which contains movable electric charges. In metallic conductors, such as copper or aluminum, the movable charged particles are electrons (see electrical conduction). Positive charges may also be mobile in the form of atoms in a lattice that are missing electrons (known as holes), or in the form of ions, such as in the electrolyte of a battery.

All conductors contain electric charges which will move when an electric potential difference (measured in volts) is applied across separate points on the material. This flow of charge (measured in amperes) is what is meant by electric current. In most materials, the direct current is proportional to the voltage (as determined by Ohm's law), provided the temperature remains constant and the material remains in the same shape and state.

Most familiar conductors are metallic. Copper is the most common material used for electrical wiring. Silver is the best conductor, but is expensive. Gold is used for high-quality surface-to-surface contacts. However, there are also many non-metallic conductors, including graphite, solutions of salts, and all plasmas. See electrical conduction for more information on the physical mechanism for charge flow in materials.

Non-conducting materials lack mobile charges, and so resist the flow of electric current, generating heat. In fact, all non-superconducting materials offer some resistance and warm up when a current flows. Thus, proper design of an electrical conductor takes into account the temperature that the conductor needs to be able to endure without damage, as well as the quantity of electrical current. The motion of charges also creates an electromagnetic field around the conductor that exerts a mechanical radial squeezing force on the conductor. A conductor of a given material and volume (length × cross-sectional area) has no real limit to the current it can carry without being destroyed as long as the heat generated by the resistive loss is removed and the conductor can withstand the radial forces. This effect is especially critical in printed circuits, where conductors are relatively small and close together, and inside an enclosure: the heat produced, if not properly removed, can cause fusing (melting) of the tracks.

Since all non-superconducting conductors have some resistance, and all insulators will carry some current, there is no theoretical dividing line between conductors and insulators. However, there is a large gap between the conductance of materials that will carry a useful current at working voltages and those that will carry a negligible current for the purpose in hand, so the categories of insulator and conductor do have practical utility.

Thermal and electrical conductivity often go together. For instance, most metals are both electrical and thermal conductors. However, some materials are practical electrical conductors without being good thermal conductors.

 Conductor size

In many countries, conductors are measured by their cross section in square millimeters. However, in the United States, conductors are measured by American wire gauge for smaller ones, and circular mils for larger ones.

Conductor materials

Of the metals commonly used for conductors, copper has a high conductivity. Silver is more conductive, but due to cost it is not practical in most cases. However, it is used in specialized equipment, such as satellites, and as a thin plating to mitigate skin effect losses at high frequencies. Because of its ease of connection by soldering or clamping, copper is still the most common choice for most light-gauge wires.

Aluminium has been used as a conductor in housing applications for cost reasons. It is actually more conductive than copper when compared by unit weight, but it has technical problems related to heat and its coefficient of thermal expansion, which tends to loosen connections over time. It is the most common metal used in high-voltage transmission lines, in combination with steel. The surface of anodized aluminium does not conduct electricity.

Conductor voltage

The voltage on a conductor is determined by the connected circuitry and has nothing to do with the conductor itself. Conductors are usually surrounded by and/or supported by insulators and the insulation determines the maximum voltage that can be applied to any given conductor.

Voltage of a conductor "V" is given by

V = IR

where

I is the current, measured in amperes

V is the potential difference measured in volts

R is the resistance measured in ohms

An insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical equipment, also called insulators or insulation, intended to support or separate electrical conductors without passing current through themselves. The term is also used more specifically to refer to insulating supports that attach electric power transmission wires to utility poles or pylons.

Some materials such as glass, paper or Teflon are very good electrical insulators. A much larger class of materials, for example rubber-like polymers and most plastics are still "good enough" to insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).

Ampere:-

1. A unit of electric current in the meter-kilogram-second system. It is the steady current that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 2 × 10 ^-7 newtons per meter of length.
2. A unit in the International System specified as one International coulomb per second and equal to 0.999835 ampere.

Volt

 

- The volt (symbolized V) is the Standard International (SI) unit of electric potential or electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance. Reduced to SI base units, 1 V = 1 kg times m2 times s-3 times A-1 (kilogram meter squared per second cubed per ampere). voltage can be expressed as an average value over a given time interval, as an instantaneous value at a specific moment in time, or as an effective or root-mean-square (rms) value. Average and instantaneous voltages are assigned a polarity either negative (-) or positive (+) with respect to a zero, or ground, reference potential. The rms voltage is a dimensionless quantity, always represented by a non-negative real number. For a steady source of direct-current (DC) electric potential, such as that from a zinc-carbon or alkaline electrochemical cell, the average and instantaneous voltages are both approximately +1.5 V if the negative terminal is considered the common ground; the rms voltage is 1.5 V. For standard utility alternating current (AC), the average voltage is zero (the polarity constantly reverses); the instantaneous voltage ranges between approximately -165 V and +165 V; the rms voltage is nominally 117 V. Voltages are sometimes expressed in units representing power-of-10 multiples or fractions of one volt. A kilovolt (symbolized kV) is equal to one thousand volts (1 kV = 103 V). A megavolt (symbolized MV) is equal to one million volts (1 MV = 106 V). A millivolt (symbolized mV) is equal to one-thousandth of a volt (1 mV = 10-3 V). A microvolt (symbolized µV) is equal to one-millionth of a volt (1 µV = 10-6 V).

ohm

Resistances and reactances are sometimes expressed in units representing power-of-10 multiples of one ohm. A kilohm is equal to one thousand (10 ³ ) ohms. A megohm is equal to one million (10 ⁶ ) ohms. Fractional prefix multiplier s are seldom used for resistance or reactances; rarely will you hear or read about a milliohm or a microhm. Extremely small resistances and reactances are usually referred to in terms of conductance.

Ohm's Law

 

- Ohm's Law is the mathematical relationship among electric current, resistance, and voltage. The principle is named after the German scientist Georg Simon Ohm. In direct-current (DC) circuits, Ohm's Law is simple andlinear. Suppose a resistance having a value of R ohms carries a current of I amperes. Then the voltage across the resistor is equal to the product IR. There are two corollaries. If a DC power source providing E volts isplaced across a resistance of R ohms, then the current through theresistance is equal to E/R amperes. Also, in a DC circuit, if E volts appear across a component that carries I amperes, then the resistance of that component is equal to E/I ohms. Mathematically, Ohm's Law for DC circuits can be stated asthree equations: E = IR I = E/R R = E/I When making calculations, compatible units must beused. If the units are other than ohms (for resistance), amperes (for current), and volts for voltage), then unit conversions should be made before calculations are done. For example, kilohms should be converted to ohms, and microamperes should be converted to amperes.

 

Difference between AC and DC:-

 

Alternating Current vs. Direct Current

The figure to the right shows the schematic diagram of a very basic DC circuit. It consists of nothing more than a source (a producer of electrical energy) and a load (whatever is to be powered by that electrical energy). The source can be any electrical source: a chemical battery, an electronic power supply, a mechanical generator, or any other possible continuous source of electrical energy. For simplicity, we represent the source in this figure as a battery.

At the same time, the load can be any electrical load: a light bulb, electronic clock or watch, electronic instrument, or anything else that must be driven by a continuous source of electricity. The figure here represents the load as a simple resistor.

Regardless of the specific source and load in this circuit, electrons leave the negative terminal of the source, travel through the circuit in the direction shown by the arrows, and eventually return to the positive terminal of the source. This action continues for as long as a complete electrical circuit exists.

Now consider the same circuit with a single change, as shown in the second figure to the right. This time, the energy source is constantly changing. It begins by building up a voltage which is positive on top and negative on the bottom, and therefore pushes electrons through the circuit in the direction shown by the solid arrows. However, then the source voltage starts to fall off, and eventually reverse polarity. Now current will still flow through the circuit, but this time in the direction shown by the dotted arrows. This cycle repeats itself endlessly, and as a result the current through the circuit reverses direction repeatedly. This is known as an alternating current.

This kind of reversal makes no difference to some kinds of loads. For example, the light bulbs in your home don't care which way current flows through them. When you close the circuit by turning on the light switch, the light turns on without regard for the direction of current flow.

Of course, there are some kinds of loads that require current to flow in only one direction. In such cases, we often need to convert alternating current such as the power provided at your wall socket to direct current for use by the load. There are several ways to accomplish this, and we will explore some of them in later pages in this section.

Properties of Alternating Current

A DC power source, such as a battery, outputs a constant voltage over time, as depicted in the top figure to the right. Of course, once the chemicals in the battery have completed their reaction, the battery will be exhausted and cannot develop any output voltage. But until that happens, the output voltage will remain essentially constant. The same is true for any other source of DC electricity: the output voltage remains constant over time.

By contrast, an AC source of electrical power changes constantly in amplitude and regularly changes polarity, as shown in the second figure to the right. The changes are smooth and regular, endlessly repeating in a succession of identical cycles, and form a sine wave as depicted here.

Because the changes are so regular, alternating voltage and current have a number of properties associated with any such waveform. These basic properties include the following list:

Frequency. One of the most important properties of any regular waveform identifies the number of complete cycles it goes through in a fixed period of time. For standard measurements, the period of time is one second, so the frequency of the wave is commonly measured in cycles per second (cycles/sec) and, in normal usage, is expressed in units of Hertz (Hz). It is represented in mathematical equations by the letter 'f.' In North America (primarily the US and Canada), the AC power system operates at a frequency of 60 Hz. In Europe, including the UK, Ireland, and Scotland, the power system operates at a frequency of 50 Hz. 

• Period. Sometimes we need to know the amount of time required to complete one cycle of the waveform, rather than the number of cycles per second of time. This is logically the reciprocal of frequency. Thus, period is the time duration of one cycle of the waveform, and is measured in seconds/cycle. AC power at 50 Hz will have a period of 1/50 = 0.02 seconds/cycle. A 60 Hz power system has a period of 1/60 = 0.016667 seconds/cycle. These are often expressed as 20 ms/cycle or 16.6667 ms/cycle, where 1 ms is 1 millisecond = 0.001 second (1/1000 of a second). 
• Wavelength. Because an AC wave moves physically as well as changing in time, sometimes we need to know how far it moves in one cycle of the wave, rather than how long that cycle takes to complete. This of course depends on how fast the wave is moving as well. Electrical signals travel through their wires at nearly the speed of light, which is very nearly 3 × 10⁸ meters/second, and is represented mathematically by the letter 'c.' Since we already know the frequency of the wave in Hz, or cycles/second, we can perform the division of c/f to obtain a result in units of meters/cycle, which is what we want. The Greek letter (lambda) is used to represent wavelength in mathematical expressions. Thus,  = c/f. As shown in the figure to the right, wavelength can be measured from any part of one cycle to the equivalent point in the next cycle. Wavelength is very similar to period as discussed above, except that wavelength is measured in distance per cycle where period is measured in time per cycle. 
• Amplitude. Another thing we have to know is just how positive or negative the voltage is, with respect to some selected neutral reference. With DC, this is easy; the voltage is constant at some measurable value. But AC is constantly changing, and yet it still powers a load. Mathematically, the amplitude of a sine wave is the value of that sine wave at its peak. This is the maximum value, positive or negative, that it can attain. However, when we speak of an AC power system, it is more useful to refer to the effective voltage or current.
•  When we deal with AC power, the most important of these properties are frequency and amplitude, since some types of electrically powered equipment must be designed to match the frequency and voltage of the power lines. Period is sometimes a consideration, as we'll discover when we explore electronic power supplies. Wavelength is not generally important in this context, but becomes much more important when we start dealing with signals at considerably higher frequencies.

Why Use Alternating Current?

Since some kinds of loads require DC to power them and others can easily operate on either AC or DC, the question naturally arises, "Why not dispense entirely with AC and just use DC for everything?" This question is augmented by the fact that in some ways AC is harder to handle as well as to use. Nevertheless, there is a very practical reason, which overrides all other considerations for a widely distributed power grid. It all boils down to a question of cost.

DC does get used in some local commercial applications. An excellent example of this is the electric trolley car and trolley bus system used in San Francisco, for public transportation. Trolley cars are electric train cars with power supplied by an overhead wire. Trolley busses are like any other bus, except they are electrically powered and get their power from two overhead wires. In both cases, they operate on 600 volts DC, and the overhead wires span the city.

The drawback is that most of the electrical devices on each car or bus, including all the light bulbs inside, are quite standard and require 110 to 120 volts. At the same time, however, if we were to reduce the system voltage, we would have to increase the amount of current drawn by each car or bus in order to provide the same amount of power to it. (Power is equal to the product of the applied voltage and the resulting current: P = I × E.) But those overhead wires are not perfect conductors; they exhibit some resistance. They will absorb some energy from the electrical current and dissipate it as waste heat, in accordance with Ohm's Law (E = I × R). With a small amount of algebra, we can note that the lost power can be expressed as:

P_lost = I²R

The same reality of Ohm's Law and resistive losses holds true in the country-wide power distribution system. We need to keep the voltage used in homes to a reasonable and relatively safe value, but at the same time we need to minimize resistive losses in the transmission wires, without bankrupting ourselves buying heavy-gauge copper wire. At the same time, we can't use motor-generator pairs all across the country; they would need constant service and would break down far too often.

The answer is to use an AC power system and transformers. (We'll learn far more about transformers in a later page; for now, a transformer is an electrical component that can convert incoming AC power at one voltage to outgoing power at a different voltage, higher or lower, with only very slight losses.) Thus, we can generate electricity at a reasonable voltage for practical AC generators (sometimes called alternators), then use transformers to step that voltage up to very high levels for long-distance transmission, and then use additional transformers to step that high voltage back down for local distribution to individual homes.

When we examined DC circuit theory and discussed DC power losses above, we used capital letters to represent all quantities. This is standard nomenclature; capital letters are used to represent fixed, static values. Thus, we use a capital I to represent DC current and a capital V to represent DC voltage.

By the same token, we use the capital letters R, L, and C to represent the values of circuit components containing resistance, inductance, and capacitance, respectively. These are fixed values that are set at the time the component is manufactured.

On the other hand, AC circuits use constantly-changing voltages and currents. In addition, some circuits contain both AC and DC components, which must often be considered separately. Therefore, we typically use lower-case letters to designate instantaneous values of voltage, current and power. Thus, if you see an equation written as:

P = I × E

you know it refers to DC power, current, and voltage. On the other hand, an equation written as:

p = i × e

refers to the instantaneous power, current, and voltage of some AC signal at some specified instant in time.

There will also be cases where we must refer to an overall AC signal rather than one instantaneous value from it. In such cases, the use of upper- and lower-case letters may be adjusted so we don't have too many of one or the other in a single discussion. We will try to avoid unnecessary confusion by including subscripts in equations, or specifying in the text exactly what we are discussing.

open circuit, closed circuit, short ciruit

Open Ciruit:-

Any circuit which is not complete is considered an open circuit. A complete circuit which is not performing any actual work can still be a closed circuit. For example, a circuit connected to a dead battery may not perform any work, but it is still a closed circuit.

Close Ciruit:-

A circuit is considered to be closed when electricity flows from an energy source to the desired endpoint of the circuit. The open status of the circuit doesn't depend on how it became unclosed, so circuits which are manually disconnected and circuits which have blown fuses, faulty wiring or missing components are all considered open circuits.

short circuit A condition in any circuit of any size in which power jumps from the energy source to either a ground point or to the end of the circuit without actually completing the full circuit. Since this condition shortens the route travelled by the electricity, the condition is known as a short circuit.

 

Short Circuits:- Short circuits in energy transmission systems usually result in an overload which can result in anything from a harmless blown fuse to a lethal electrical fire. Most common short circuits that occur in household electrical service are potentially hazardous which is why fuses and breakers are so critical to safe electrical service. But a short doesn't necessarily indicate a hazard condition..

Calculating and Measuring Power Consumption

The design of an autonomous system always begins with the calculation of how much power is consumed. The easiest way to measure your device is a laboratory power supply that features a voltage and ampere meter. The nominal voltage provided by a lead acid battery typically varies between 11 Volts (empty) and about 14.5 Volt (charging, voltage at charging limit). You can tune the voltage at the laboratory power supply and see how much current the device draws at different voltages. If a laboratory power supply is not available, measurement can be performed by using the supply shipped with the device. Interrupt one cable that goes to the DC input of your device and insert an ampere-meter (or ammeter). Note that a ammeter will burn itself or your power supply if applied between the positive and negative terminal because it behaves like a simple cable between the probes -thus creating a short. Many ammeters have an unfused input, so exercise caution as they can be easily damaged.

The amount of power consumed can be calculated with this formula:

P = U * I

P being Power in Watts, U being voltage in Volts, I being current in Ampere. For example:

6 Watts = 12 Volts * 0.5 Ampere

The result is the rating of the device. If the device of the example is operating for an hour it will simply consume 6 Watt-hours (Wh), respectively 0.5 Ampere-hours (Ah). Thus the device will draw 144 Wh or 12 Ah a day.

To simplify things, I will use the nominal voltage rating of batteries for calculations and not take into account that the voltage provided by the battery varies depending on its state of charge. Batteries are rated at their capacity in Ah -so it is easier to calculate using Ah instead of Wh. A battery from a big truck has typically 170 Ah -thus a 100% charged truck battery would power the device for about 340 hours during a 100% discharging cycle.

Importance of Earthing:

Earthing or grounding is done for safety of equipment and human beings (including all animals and plants).

Equipment Safety:

The outer housing of an electrical equipment is earthed by directly connecting it to a earth grid or earth electrode, thereby providing a low resistance path to ground. In case of a fault involving earth the live part of the equipment gets connected with the low resistance earth path. This produces high earth fault current and the protective devices in the circuit disconnects the circuit from the power source thereby reducing further damage to the equipment.

Neutral of electrical equipment are also earthed for equipment safety. Like, neutral of generators in power plants are earthed through Neutral Grounding Resistor to limit the earth fault current. Three phase transformer's neutral are earthed to provide neutral point to supply single phase loads like lighting and small appliances.

Human Safety:

If a person touches an appliance which has an earth fault in it he will not get an electric shock as his body (standing on the earth) and the equipment's body are at the same potential provided the equipment is earthed properly. Thus proper earthing protects a person from getting electric shock.

Circuit symbols are used in circuit diagrams which show how a circuit is connected together. The actual layout of the components is usually quite different from the circuit diagram. To build a circuit you need a different diagram showing the layout of the parts on stripboard or printed circuit board.