REFRIGERATION and AIR CONDITIONING Short note of mechanical engineering student.

REFRIGERATION & AIR CONDITIONING

Air conditioning—deals with artificial tampering of the conditions of air that may involve cooling as well as heating coupled with ventilation, filtration and air circulation.

Refrigeration—means the artificial withdrawal of heat, producing in a substance or within a space a temperature lower than that which would exist under the natural influence of the surroundings.

Application of refrigeration—(a) Ice making, (b) Preservation and transportation of perishables, (c) Special industrial processes, (d) Air conditioning.

Application of air conditioning—(a) For human comfort (b) Industrial air conditioning

PSYCHROMETRIC TERMS

Psychometric—is that branch of engineering science, which deals with the study of moist air i.e. dry air mixed with water vapor or humidity.

Dry air—the pure dry air is a mixture of a number of gases such as nitrogen, oxygen, carbon dioxide, hydrogen, argon etc.

Moist air—it is a mixture of dry air and water vapor. The amount of water vapor, present in the air, depends upon the absolute pressure and temperature of the mixture.

Saturated air—it is a mixture of dry air and water vapor, when the air has diffused the maximum amount of water vapor into it. However, when the saturated air is cooled, the water vapor in the air starts condensing, and the same may be visible in the form of moist, fog or condensation on cold surfaces.

Degree of saturation—it is the ratio of actual mass of water vapor in a unit mass of dry air to the mass of water vapor in the same mass of dry air when it is saturated at the same temp.

Humidity—it is the mass of water vapor present in 1 kg of dry air, and is generally expressed in terms of g/kg of dry air. It is also called specific humidity or humidity ratio.

Absolute humidity—it is the mass of water vapor present in 1 m³ of dry air, and is generally expressed in terms of g/m³ of dry air.

Relative humidity—it is the ratio of actual mass of water vapor in a given volume of moist air to the mass of water vapor in the same volume of saturated air at the same temperature and pressure.

Specific humidity—it can be defined as the ration of mass of water vapor to the mass of dry air in a given volume of the air-vapor mixture.

Dry bulb temperature—it is the temperature of air recorded by a thermometer, when it is not affected by the moisture present in the air.

Wet bulb temperature—it is the temperature of air recorded by a thermometer, when its bulb is surrounded by a wet cloth exposed to the air.

Dew point temperature—it is the temperature of air recorded by a thermometer, when the moisture present in it begins to condense.

Dew point depression—it is the difference between the dry bulb temperature and dew point temperature of air.

Sensible heating—the heating of air, without any change in its specific humidity, is known as sensible heating.

Sensible cooling—the cooling of air, without any change is its specific humidity, is known as sensible cooling.

Humidification—the addition of moisture to the air, without change in its dry bulb temperature, is known as humidification.

Dehumidification—the removal of moisture from the air, without change in its dry bulb temperature, is known as dehumidification.

Cooling with dehumidification—this process involves lowering of both the air temperature and the specific humidity. This process is commonly used in summer air conditioning in which air passes over a cooling coil. When moist air is cooled below its dew point, vapor is condensed from the air resulting in simultaneous cooling and dehumidification. The final relative humidity is generally higher than that at the start.

Heating and humidification—in this process, which is reverse of the cooling and dehumidification process, air is simultaneously heated and humidified. While both the final specific humidity and temperature rise, the relative humidity may be lower or higher than the initial value.

Sensible heat factor—the ratio of the sensible heat to the total heat is known as sensible heat factor. (Briefly written as SHF or sensible heat ratio written as SHR)

By-pass factor—a little consideration will show that when air passes over a coil, some of it (say x kg) just by-passes unaffected while the remaining (1-x) kg comes in direct contact with the coil.

The bypass factor may be defined as follows:     

Where,              T_la    is the DBT of air leaving the coil

                        T_ea     is the DBT of air entering the coil

                        T_coil   is the DBT of the coil surface temperature

REFRIGERATION HARDWARE

Compressors—the main functions of a compressor are:

• Removing the refrigerant vapor from the evaporator
• Increasing the pressure of the refrigerant vapor through the process of compression, and simultaneously increasing the temperature of the vapor

Condenser—is the component in the refrigeration cycle where heat is removed and rejected. Condenser are thus heat exchangers designed to get rid of the heat absorbed by the refrigerant in the evaporator and the heat of compressor added by the compressor. The condenser receives the hot, high pressure gas from the compressor and cools it to remove first the superheat and then the latent heat, so that the refrigerant will condense back to a liquid.

Evaporators—are heat exchangers in which a refrigerant is evaporated at low temperature and pressure for the purpose of removing heat from the refrigerated space or material.

Defrosting—when an evaporator operates at a temperature below 0⁰C, the exchanger surface progressively becomes covered with a layer of frost, produced by moisture condensation on the cold surfaces, the moisture being deposited by the circulating air on the heat exchanger surface. Frost is undesirable from operational standpoint for two reasons:

• Thick layers of frost act as insulation, greatly reducing the heat transfer capability of the evaporator and this condition becomes critical when the frost bridges the gap between adjacent fins, thus also greatly reducing the surface area available for HT
•  In forced convection coils the frost reduces the air flow.

Expansion devices—the main purpose of the expansion device is twofold:

• It must reduce the pressure of the liquid refrigerant
• It must regulate the flow of refrigerant to the evaporator

Receiver—this device basically acts as a storage space and surge tank for the liquid refrigerant in a refrigerating system. During operation it provides storage space for excess liquid refrigerant to accommodate large load changes. During the shutdown period, the whole charge is pumped into the receiver and isolated by using the valves, thus preventing the loss of refrigerant.

Filter-drier—the device is installed in the liquid line to remove foreign particles as well as water. It also prevents foreign particles from clogging the orifice of the expansion valve. The common desiccants used in the drier are silica gel and activated alumina.

Oil separator—the purpose of the oil separator is to separate the lubricating oil from the refrigerant and return the same to compressor. The detrimental effects of oil in the evaporator are:

• It thickness and gets coated on the inside walls of the evaporator (reduce the HT)
• Often the wax separates out from the oil at low temperature
• Boiling temperature of the refrigerant gets changed due to oil, resulting in more work for the compressor.

Accumulators—the device is installed in the suction line to ensure that no liquid refrigerant from the evaporator reaches the compressor. It is essential to be used in a system where there is inherent vulnerability to liquid flood back.

A refrigerant is any substance, which acts as a cooling agent by absorbing heat from another body or substance.

Common refrigerants are—(a) Ammonia, (b) Carbon dioxide, (c) Water, (d) Sulphur dioxide, (e) Methyl chloride, (f) Halocarbon refrigerants are—(1) Refrigerant-11(CCl₃F), (2) Refrigerant-12, (3) Refrigerant-22, (4) Refrigerant-134a.

COP—it is the ratio of rate of refrigeration to the rate of heat supply to the generator.

TOR or TR—a ton of refrigeration is defined as the amount of refrigeration effect produced by the uniform melting of one ton (1000kg) of ice form and at 0⁰C in 24 hours. Since the latent heat of ice is 335kJ/kg, therefore one ton of refrigeration,

1TR = 1000*335 kJ in 24 hours; = (1000*335)/ (24*60) = 232.6 kJ/min; = 3.5 kW

Comfort chart—could then be constructed by drawing lines through the points at which the majority of people equally clothed and equally active reported the same feeling of comfort. These lines are called the effective-temperature (ET). The summer comfort zone for most people and spans the range of effective-temperatures from around 19 to 24⁰C. The winter comfort zone for most individuals and spans the range of effective temperature from 17 to 22⁰C.

Alignment Circle—is the point on Psychometric Chart, where Dry Bulb Temperature is 26°C and relative humidity is 50%.           

POWER PLANT ENGINEERING SHORT NOTE.

POWER PLANT ENGINEERING

Calorific value—of fuel is defined as the amount of heat produced when unit quantity of fuel is completely burnt under standard conditions. The calorific value is expressed as kcal/kg the calorific value of a fuel can be classified in two ways: (a) Higher calorific value (HCV), (b) Lower calorific value (LCV)

Higher calorific value—is the total amount of heat produced when unit quantity of fuel is burnt completely and the products of combustion have been cooled to room temperature generally 15⁰C.

Lower calorific value—is the net amount of heat produced when unit quantity of fuel is completely burnt and the products of combustion are not cooled to room temperature but are allowed to escape.

Power plant—is an assembly of equipment that produces and delivers mechanical and electrical energy. Electrical equipment of a power station includes generators, transformers, switch gears and control gears.

Requirement of plant design—the factors to be kept in view while designing a power station are follows: (a) economy of expenditure, (b) safety of plant and personnel, (c) reliability, (d) efficiency, (e) ease of maintenance, (f) good working conditions, (g) min^m transmission loss.

Connected load—it is the sum of ratings in kW of equipment installed in the consumers premises.

Maximum demand—it is the maximum load which a consumer uses at any time.

Demand factor—it is defined as the ratio of maximum demand to connected load.

Load curve—it is graphical representation between load in kW and time in hours.

Load factor—it is defined as the ratio of average load to maximum demand.

Plant capacity factor—it is defined as the ratio of actual energy produced in kWh to the maximum possible energy that could have been produced during the same period.

Plant use factor—it is defined as the ratio of energy produced in given time to the maximum possible energy that could have been produced the actual number of hours the plant was in operation.

Diversity factor—it is defined as the ratio of sum of individual maximum demand to the simultaneous maximum demand of a system.

Load duration curve—represents re-arrangement of all the load elements of chronological load curve in the order of descending magnitude.

Power plant capacity—depends upon the following factors: (a) max^m demand of consumers at present, (b) type of load, (c) future load conditions, (d) availability of fuel, (e) total cost of power plant, (f) possibility of inter connecting the power plant to other power plants.

Principles of power plant design—while designing a power plant the following factors should be considered: (a) low capital cost, (b) reliability of supplying power, (c) low maintenance cost, (d) low operating cost, (e) high efficiency, (f) low cost of energy generated, (g) reserve capacity to meet future power demand, (h) simplicity of design.

Types of loads—the various type of loads are as follows:

Residential load—it includes domestic lights, power needed for domestic appliances such as radio, television, water heaters, refrigerators and small motor for pumping water.

Commercial load—it includes lighting for shop, advertisements and electrical appliances used in shops and restaurants etc.

Industrial load—it consists of load demand of various industries.

Municipal load—it consists of street lighting, power required for water supply and drainage purposes.

Irrigation load—electrical power need for pumps driven by electric motors to supply water to fields is included in this type of load.

Traction load—it includes tram cars, trolley, buses and rail-ways.

Depreciation cost—is the amount to be set aside per year form income to meet the depreciation caused by the age of service, wear and tear of machinery. The most commonly used methods are as follows: (a) straight line method, (b) sinking fund method.

Energy rates (tariff)—it is desirable to charge the consumer according to his maximum demand in kW and the energy consumed in kWh. The tariff chosen should recover the fixed cost, operating cost and profit etc.

Types of tariff—the various types of tariffs are as follows: (a) flat demand rate, (b) straight line meter rate, (c) step meter rate, (d) block rate tariff, (e) two part tariff, (f) three part tariff.

Flat demand rate—it is based on the number of lamps installed and a fixed number of hours of use per month or per hour. It is expressed by the expression: Y=DX

Straight line meter rate—according to this energy rate the amount to be charged from the consumer depends upon the energy consumed in kWh which is recorded by a means of kWh meter. It is expressed in the form: Y=EZ

Block rate tariff—according to this tariff a certain price per units is charged for all or any part of block of each unit and for succeeding blocks of energy the corresponding unit charges decrease.

Two part tariff—in this tariff the total charges are based on the maximum demand and energy consumed. It is expressed as   Y=DX+EZ

Three part tariff—according to this tariff the customer pays some fixed amount in addition to the charges for maximum demand and energy consumed. The fixed amount to be charged depends upon the occasional increase in fuel price, rise in wages of labors etc. it is expressed by the expression: Y=DX+EZ+C

Where,  Y = Total amount of bill for the period considered.

            D = Rate per kW of maximum demand.

            X = Maximum demand in kW.

            E = Energy rate per kW.

            Z = Energy consumed in kWh during the given period.

C= Constant amount to be charged form the consumer during each   billing period.

Plant performance and operation characteristics

Boiler, turbines, generators etc, of a power station should work efficiently. Some curves are plotted to observe their performance. The various curves used are as follows:

Input output curve—is a graphical representation betⁿ the net energy output (L) and input (I).

Efficiency curve—the ratio of output of power station to input is called efficiency. The efficiency curve is obtained by plotting efficiency against output.

Heat rate curve—the ratio of input to output is known as heat rate (HR). Heat rate curve is obtained by plotting values of heat rate against corresponding value of output.

Incremental rate curve—incremental rate is obtained by plotting values of IR against corresponding values of output. Incremental rate is defined as   IR=dI/dL

Types of dust collectors—the various types of dust collectors are as follows:

(a) Mechanical dust collectors. (b) Electrical dust collectors.

Mechanical dust collectors—are sub-divided into wet and dry types. In wet type collectors also known as scrubbers water sprays are used to wash dust from the air. The basic principles of mechanical dust collector’s are—(1) By increasing the cross-sectional area of duct through which dust laden gases are passing, the velocity of gases is reduced and causes heavier dust particles to fall down. (2) Changing the direction of flow of flue gases causes the heavier particle of settle out. (3) Sometimes baffles are provided to separate the heavier particles.

Electrical dust collectors—it has two sets of electrodes, insulated from each other, that maintain an electrostatic field between them at high voltage. The flue gases are made to pass between these two sets of electrodes. The electric field ionizes the dust particles that pass through it attracting them to the electrode of opposite charge. The other electrode is maintained at a negative potential of 30,000 to 60,000 volts. The dust particles are removed from the collecting electrode by rapping the electrode periodically.

Draught—is defined as the difference absolute gas pressure at any point in a gas flow passes and the ambient atmospheric pressure. The various types of draught systems are as follows:

(a)     Natural draught, (b) Mechanical draught, (c) Steam jet draught.

The purpose of draught is as follows:

§  To supply required amount of air to the furnace for the combustion of fuel.

§  To remove the gases products of combustion.

Boiler—is a closed vessel in which water is converted into steam by the application of heat. Thermal energy released by combustion of fuel is transferred to water which vaporizes and gets converted into steam at the desired pressure and temperature. A boiler should fulfill the following requirements: (a) safety, (b) accessibility, (c) capacity, (d) efficiency, (e) it should be simple in construction and its maintenance cost should be low, (f) its initial cost should be low, (g) the boiler should have no joints exposed to flames, (h) the boiler should be capable of quick starting and loading.

Boiler mountings and accessories

Mountings are the component used for the safety of boiler. Example: feed cheek valve, steam stop valve, safety valve, blow off valve, water level indicator, pressure gauge, fusible plug.

Accessories are used to operate it efficiently. Example: feed pump, air pre-heater, super-heater, draft equipment to supply air to furnace.

Boiler performance—the performance of a steam boiler may be expressed in terms of the followings: (a) heat release per cubic meter of furnace volume, (b) efficiency, (c) heat transferred per square meter of the heating surface area per hour, (d) rate of combustion is kcal/m² of the grate area per hour for solid fuel, (e) amount of steam produced per hour.

Diesel power plant—is suitable for small and medium outputs. It is used as central power station for smaller power supplies and as a standby plant to hydro-electric plants and steam power plant. The diesel power plants are commonly used where fuel prices or reliability of supply favor oil over coal, where water supply is limited, where loads are relatively small, and where electric line services is unavailable or is available at too high rates. Diesel power plants in common use have capacities up to about 5 MW.

Engine performance

IMEP—in order to determine the power developed by the engine. From the area of indicator diagram it is possible to find an average gas pressure which while acting on piston throughout one stroke would account for the network done. This pressure is called indicated mean effective pressure (IMEP)

IHP—the indicated HP of the engine can be calculated as: IHP= (P_mLANn)/ (4500*k)

BHP—is defined as the net power available at the crankshaft. It is found by measuring the output torque with a dynamometer.

FHP—the difference of IHP and BHP is called FHP. It is utilized in overcoming frictional resistance of rotating and sliding parts of the engine.

Indicated thermal efficiency—it is defined as the ratio of indicated work to thermal input.

Brake thermal efficiency—it is defined as the ratio of brake output to thermal input.

Mechanical efficiency—it is defined as the ratio of BHP to IHP.

Nuclear power plant—as large amount of coal and petroleum are being used to produce energy, time may come when their reserve may not be able to meet the energy requirements. Thus there is tendency to seek alternative sources of energy. The discovery that energy can be liberated by the nuclear fission of materials like uranium, plutonium has opened up new sources of power of great importance. The heat produced due to fission of U and Pu is used to heat water to generate steam which is used for running turbo-generator.

Nuclear fuel—fuel of a nuclear reactor should be fissionable material which can be defined as an element or isotope whose nuclei can be caused to undergo nuclear fission by nuclear bombardment and to produce a fission chain reaction. It can be one or all of the following U²³³, U²³⁵ and Pu²³⁹

Hydro-electric power plant—water is the cheapest source of power. It served as the source of power to our civilization in its earlier days in the form of water wheels.

Gas turbine power plant—has relatively low cost and can be quickly put into commission. It requires less space. This plant is of smaller capacity and is mainly used for peak load service.

TERMS AND DEFINITIONS

Work ratio—it is defined as the ratio of network output to the total work produced in the turbine.

Thermal efficiency—it is defined as the ratio of network output the total fuel energy input.

Air ratio—it is defined as the amount of air entering the compressor inlet per unit of network output of the turbine.

Pressure ratio—it is defined as the ratio of absolute pressure at the compressor outlet to the absolute atmospheric pressure at compressor inlet.

Compressor efficiency—is defined as the work required for ideal compression to the actually required by the compressor for a given pressure ratio.