Refrigeration and Air Conditioning Objectives of the lesson: 1. Define refrigeration and air conditioning 2. Introduce aspects of various natural refrigeration methods, namely: Use of ice transported from colder regions Use of ice harvested in winter and stored in ice houses Use of evaporative cooling 3. Introduce historical aspects of various artificial refrigeration methods, namely: Vapour compression refrigeration systems, including Domestic refrigeration systems Air conditioning systems Vapour absorption refrigeration systems 4. Applications of refrigeration in: Food processing, preservation and distribution Chemical and process industries Comfort air-conditioning 5. Applications of air conditioning, namely: o Industrial, such as in textiles, printing, manufacturing, photographic, computer rooms, power plants, vehicular etc. o Comfort commercial, residential etc. At the end of the lesson the student should be able to: Identify various natural and artificial methods of refrigeration List salient points of various refrigeration techniques, and Name important landmarks in the history of refrigeration List various applications of refrigeration and air conditioning List typical conditions required for various food products, processes etc
Definition Introduction The term refrigeration may be defined as the process of removing heat from a substance under controlled conditions. It also includes the process of reducing and maintaining the temperature of a body below the general temperature of its surroundings. In other words, the refrigeration means a continued extraction of heat from a body whose temperature is already below the temperature of its surroundings. Fundamental Units The measurement of physical quantities is one of the most important operations in engineering. Every quantity is measured in terms of some arbitrary, but internationally accepted units, called fundamental units. Derived Units Some units are expressed in terms of other units, which are derived from fundamental units are known as derived units e.g. the unit of area, velocity, acceleration, pressure etc. Thermodynamic system The thermo dynamic system (or simply known as system) may be broadly defined as a definite area or a space where some thermodynamic process is taking place. It is a region where our attention is focused for studying a thermodynamic process. A little observation will show that a thermodynamic system has its boundaries and anything outside the boundaries is called its surroundings as shown in the fig. These boundaries may be fixed like that of a tank enclosing a certain mass of compressed gas, or movable like the boundary of a certain volume of liquid in a pipe line.
Thermodynamic systems may be classified into the following three groups: Closed system; Open system; Isolated system The systems are discussed, in details, as follows: 1. Closed system. This is system of fixed mass and identity whose boundaries are determined by the space of the matter (working substance) occupied in it. A closed system is shown in Fig. The gas in the cylinder is considered as a system. If heat is supplied to the cylinder from some external source, the temperature of the gas will be increase and the piston will rise. As the piston rises, the boundary of the system moves. In other words, the heat and work energy crosses the boundary of the system moves. In other words, the heat and work energy crosses the boundary of the system during this process, but there is no addition or loss of the original mass of the working substance. It is thus obvious, that the mass of the working substance which comprises the system is fixed. Thus, a closed system does not permit any mass transfer across its boundary, but it permits transfer of energy (heat and work). 2. Open system. In this system, the mass of the working substance crosses the boundary of the system. Heat and work may also cross the boundary. Fig. shows the diagram of an air compressor which illustrates an open system. The working substance crosses the boundary of the system as the low pressure (L.P) air enters the compressor and leaves the high pressure (H.P) air. The work crosses the boundary of the system through the driving shaft and the heat is transferred across the boundary from the cylinder walls. Thus an open system permits both mass and energy (heat and work) transfer across the boundaries and the mass with in the system may not be constant. 3. Isolated system. A system which is completely uninfluenced by the surroundings is called an isolated system. It is a system of fixed mass and no heat or work energy cross its boundary. In other words, an isolated system does not have transfer of either mass or energy (heat or work) with the surroundings. An open system with its surroundings (known as an universe) is an example of an isolated system.
Laws of Thermodynamics The following three laws of thermodynamics are important from the subject point of view: 1. Zeroth Law of Thermodynamics. When two systems are each in thermal equilibrium with a third system, then the two systems are also in thermal equilibrium with one another. This law provides the basis of temperature measurement. 2. First Law of Thermodynamics. (a). The heat and mechanical work are mutually convertible. According to this law, when a closed system undergoes a thermodynamic cycle, the net heat transfer is equal to the net work transfer. In other words, the cyclic integral of heat transfer is equal to the cyclic integral of work transfers. Mathematically, Ф δq = Ф δw Where symbol Ф stands for cyclic integral (Integral around a complete cycle), and δq and δw represent infinitesimal elements heat and work transfers respectively. It may be noted that δq and δw are expressed in same units. (b) The energy can neither created nor destroyed through it can be transformed from one form to another. According to this law when a system undergoes a change of state (or a thermodynamic process), then both heat transfer and work transfer takes place. The net energy transfer is stored with in the system and is known as stored energy or total energy of the system. Mathematically, δq δw = de The symbol δ is used for a quantity which is inexact differential and symbol d is used for a quantity which is an exact differential. The quantity E is an extensive property and represents the total energy of the system at a particular state. 3. Second Law of Thermodynamics The second law of thermodynamics may be defined in many ways, but the two common statements according to Kelvin-Planck and Clausius are as follows: According to Kelvin-Plank It is impossible to construct an engine working in a cycle process, whose sole purpose is to convert heat energy from a single thermal reservoir into an equivalent amount of work In other words, no actual heat engine, working on a cyclic process, can convert whole of the heat supplied to it, into mechanical work. It means that there is degradation of energy in the process of producing mechanical work from the heat supplied. Thus the Kelvin-Planck statement of the second law of thermodynamics is sometimes known as law of degradation of energy. According to clausius statement It is impossible for a self acting machine, working in acyclic process, to transfer heat from a body at a lower temperature to body at a higher temperature without the aid of an external agency. In other words heat cannot flow itself from cold body to a hot body without the help of an external agency (i.e. without the expenditure of mechanical work).
Air Refrigeration Cycles Introduction In an air refrigeration cycle, the air is used as refrigerant. In olden days, air was widely used in commercial applications because of its availability at free of cost. Since air does not change its phase i.e. remains throughout the cycle, therefore the heat carrying capacity per kg of air is very small as compared to vapour absorbing systems. Units of Refrigeration The practical unit of refrigeration is expressed in terms of tone of refrigeration (briefly written as TR). A tone of refrigeration is defined as the amount of refrigeration effect produced by the uniform melting of one tone (1000 kg) of ice from and at 00C in 24 hours. Since the latent heat of ice is 335 kj/kg, therefore one tone of refrigeration, In actual practice, one tone of refrigeration is taken as equivalent to 210 kj/min or 3.5 kw (i.e. 3.5 kjs). Coefficient of Performance of a Refrigerator The coefficient of performance (briefly written as C.O.P) is the ratio of heat extracted in the refrigerator to the work done on the refrigerant. It is also known as theoretical coefficient of performance. Mathematically, Theoretical C.O.P = Q/W Where, Q = Amount of heat extracted in the refrigerator (or the amount of refrigeration produced, or the capacity of a refrigerator), and W = Amount of work done.
Notes: For per unit mass, C.O.P = q/w The coefficient of performance is the reciprocal of the efficiency (i.e. 1/ η) of a heat engine. It is thus obvious, that the value of C.O.P is always greater than unity. The ratio of actual C.O.P to the theoretical C.O.P is known as relative coefficient of performance. Mathematically, Relative C.O.P = Actual C.O.P/Theoretical C.O.P Difference between a Heat Engine, Refrigerator and Heat Pump
A simple vapor compression refrigeration system consists of the following equipments: i) Compressor ii) Condenser iii) Expansion valve iv) Evaporator. The vapour at low temperature and pressure enters the compressor where it is compressed isentropically and subsequently its temperature and pressure increase considerably. This vapour after leaving the compressor enters the condenser where it is condensed into high pressure liquid and is collected in a receiver tank. From receiver tank it passes through the expansion valve. Here it is throttled down to a lower pressure and has a low temperature. After finding its way through expansion valve it finally passes on to evaporator where it extracts heat from the surroundings or circulating fluid being refrigerated and vapourises to low pressure vapour.
Simple Vapour Absorption System Some liquids like water have great affinity for absorbing large quantities of certain vapors (NH3) and reduce the total volume greatly. The absorption refrigeration system differs fundamentally from vapor compression system only in the method of compressing the refrigerant. An absorber, generator and pump in the absorption refrigerating system replace the compressor of a vapor compression system. Figure shows the schematic diagram of a vapor absorption system. Ammonia vapor is produced in the generator at high pressure from the strong solution of NH3 by an external heating source. The water vapor carried with ammonia is removed in the rectifier and only the dehydrated ammonia gas enters into the condenser. High pressure NH3 vapor is condensed in the condenser. The cooled NH3 solution is passed through a throttle valve and the pressure and temperature of the refrigerant are reduced below the temperature to be maintained in the evaporator. The low temperature refrigerant enters the evaporator and absorbs the required heat from the evaporator and leaves the evaporator as saturated vapor. Slightly superheated, low pressure NH3 vapor is absorbed by the weak solution of NH3 which is sprayed in the absorber as shown. Weak NH3 solution (aqua ammonia) entering the absorber becomes strong solution after absorbing NH3 vapor and then it is pumped to the generator through the heat exchanger. The pump increases the pressure of the strong solution to generator pressure. The strong NH3 solution coming from the absorber absorbs heat form high temperature weak NH3 solution in the heat exchanger. The solution in the generator becomes weak as NH3 vapor comes out of it. The weak high temperature ammonia solution from the generator is passed to the heat exchanger through the throttle valve. The pressure of the liquid is reduced to the absorber pressure by the throttle valve.
Comparison between Vapour Compression and Absorption system S. No Details Vapour Compression Vapour Absorption 1 Working Principle Refrigerant is compressed Refrigerant absorbed and heated 2 Refrigerant Freon -12 & Freon 22 Ammonia (NH 3 ) 3 Capacity Limited to 1000 tones of refrigeration for a compressor unit Above 1000 tons. (Not suitable for small capacities 4 C.O.P High (4 5),but very low at part loads 5 Energy Input to Change the condition of refrigerant from the evaporator 6 Mechanical energy input Mechanical Energy (High grade energy) Refrigerant vapour is compressed to high pressure, so mechanical energy input is more 7 Refrigerant refilling Simple Difficult Low (1.1), but same as full and part loads Heat energy (Low grade energy) Pump has to only circulate the refrigerant. Therefore, mechanical energy input to run the pump is less 8 Leakage of refrigerant More chances of leakage of refrigerant from the system due to high pressures 9 Noise More noise due to the presence of the moving parts of the compressor 10 Operating cost High, since the electrical energy is expensive due to the use of compressor 11 Maintenance High, due to the presence of compressor 12 Liquid traces Liquid traces of refrigerant in suction pipe damages compressor No question of leakage of refrigerant from the system, as there is no compressor Quiet in operation, as the only moving part of the system is a pump with small motor Low, because thermal can be supplied from sources other than electrical energy. Also electrical energy required to run the pump is very less Less Liquid traces of refrigerant present in piping at the exit of the evaporator pose no danger
Refrigerants Introduction The refrigerant is heat carrying mediums which during their cycle (i.e. Compression, condensation, expansion and evaporation) in the refrigeration system absorb heat from a low temperature system and discard the heat so absorbed to a higher temperature system. The natural ice and a mixture of ice and salt were the first refrigerants. In 1834, ether, ammonia, sulphur dioxide, methyl chloride and carbon dioxide came into use as refrigerants in compression cycle refrigeration machines. Most of the early refrigerant materials have been discarded for safety reasons or for lack of chemical or thermal stability. In the present days, many new refrigerants including halo-carbon compounds. Hydro-carbon compounds are used for air conditioning and refrigeration applications. The suitability of refrigerant for a certain application is determined by its physical, thermodynamic, chemical properties and by various practical factors. There is no one refrigerant which can be used for all types of applications i.e. there is no ideal refrigerant. If one refrigerant has certain good advantages, it will have some disadvantages also. Hence a refrigerant is chosen which has greater advantages and less disadvantages. Desirable Properties of an Ideal Refrigerant We have discussed above that there is no ideal refrigerant. A refrigerant is said to be ideal if it has all of the following properties: Low Boiling point High critical temperature High latent heat of vaporization Low specific heat of liquid Low specific volume of vapour of Non-corrosive to metal Non-flammable and non-explosive Non-toxic Low cost Easy to liquefy at moderate pressure and temperature Easy of locating leaks by odour or suitable indicator, and Mixes well with oil. The standard comparison of refrigerants, as used in the refrigeration industry, is based on an evaporating temperature of -150C and a condensing temperature of +300C.
Classification of Refrigerants 1. Primary Refrigerants & 2. Secondary refrigerants The refrigerants which directly take parting the refrigeration system are called primary refrigerants where as the refrigerants which are first cooled by primary refrigerants and then used for cooling purposes, are known as secondary refrigerants. The primary refrigerants are further classified into the following four groups: 1. Halo carbon refrigerants, 2. Azeotrope refrigerants, 3. Inorganic refrigerants, and 4. Hydro-carbon refrigerants. These above mentioned refrigerants are discussed, in detail in the following pages. Halo-carbon Refrigerants The American society of Heating, Refrigeration and Air-conditioning Engineers (ASHRAE) identifies 42 halo-carbon compounds as refrigerants, but only a few of them are commonly used. The following table gives some of the commonly used halo-carbon refrigerants: Commonly used halo-carbon refrigerants
Azeotrope Refrigerants Organic Refrigerants Hydro-carbon Refrigerants
Air Conditioning System The Maintenance of controlled atmosphere according to the requirements in a space is known as Air-conditioning. The controlled atmosphere which gives maximum comfort to the human beings is known as comfort air-conditioning. The controlled atmosphere which is required for the manufacturing process is known as industrial air- conditioning. Comfort air-conditioning is further subdivided into summer air- conditioning and winter air conditioning. The air cooling and dehumidification used in summer is known as summer air conditioning and heating and humidification used in winter is known as winter air conditioning. Psychrometry Psychrometry and Psychrometric Properties The Science which deals with the study of the behavior of air and water vapour mixture is known as Psychrometry. The properties of water vapour and air mixture are known as Psychrometric Properties. Dry Air Psychrometric Properties The pure dry air is a mixture of number of gases such as nitrogen, oxygen, and hydrogen and carbon dioxide. Density of air is 1.293kg/m3 at 1.01325 bar 273oK. Pure dry air does not exist in nature as it contains some impurities and some moisture all the time. Moist Air It is a mixture of dry air and water vapour. The amount of water vapour present in air depends on the pressure and temperature of the mixture. Saturated Air It is the mixture of dry air and water vapour when air has diffused maximum amount of water vapour into it. This means air has reached its moisture carrying capacity to the fullest and no more addition is possible. Degree of Saturation (µ) It is the ratio of actual mass of water in a unit mass of dry air to the mass of water vapour in unit of saturated air at same temperature and pressure. Specific Humidity (w) It is the mass of water vapour present in 1kg of dry air and is expressed in terms of gram per kg of dry air.
Humidity Ratio It is the same as specific humidity, i.e., mass of water vapour, present in unit mass of dry air. Absolute Humidity It is the mass of water vapour present in 1m3 of dry air and is expressed in terms of grams /m3 of dry air. It can be expressed in terms of grains per cubic meter of dry air. 1kg of water vapour = 15,430 grains. Relative Humidity It is the ratio of actual mass of water vapour in a given volume of moist air to the mass of water vapour in the same volume of saturated air at the same temperature and pressure. Specific Volume (v) It is defined as the ratio of volume per unit mass of moist air. Dry Bulb Temperature (DBT) or Td It is the temperature of air recorded by a thermometer when its bulb is surrounded by a wet cloth exposed to air. Wet Bulb Depression (WBD) It is the difference between dry bulb temperature and wet bulb temperature at any point. Enthalpy Enthalpy of moist air is equal to enthalpy of dry air plus enthalpy of water vapour associated with dry air. Dew Point Temperature (DPT) or Tdp It is the temperature of air recorded by a thermometer when water vapour in air begins to condense or dew is formed. Psychrometer It is an instrument to measure DBT and WBT of moist air.
Equipments used in Air Conditioning System Circulation fan. The main function of this fan is to move air to and from the room. Air conditioning unit. It is a unit, which consists of cooling and dehumidifying processes for summer air conditioning or heating and humidification processes for winter air conditioning. Supply duct. It directs the conditioned air from the circulating fan to the space to be air conditioned at proper point. Supply outlets. These are grills, which distribute the conditioned air evenly in the room. Return outlets. These are the openings in a room surface which allow the room air to enter the return duct. Filters. The main function of the filters is to remove dust, dirt and other harmful bacterias from the air. Classification of Air Conditioning Systems The air conditioning may be broadly classified as follows: 1. According to the purpose Comfort air conditioning system, and Industrial air conditioning system, 2. According to the season of the year Winter air conditioning system, Summer air conditioning, and Year-round air conditioning. 3. According to the arrangement of equipment Unitary air conditioning system, and Central air conditioning system. In this chapter, we shall discuss all the above mentioned air conditioning systems, one by one.
Window Air- Conditioner Figure shows the schematic diagram of a window air-conditioner. The window airconditioner works on the vapour compression refrigeration cycle. The basic components of the system are compressor, condenser, a capillary tube and an evaporator, in addition, filters, accumulators, motors, fan and electrical controls from the rest of components. The high pressure gas (refrigerant), which comes out of the compressor, is condensed in the condenser using ambient air as a coolant. The liquid refrigerant is expanded in a capillary tube and the n enters the evaporator. Here, it evaporates and causes the refrigerating effect. Then fan blows the fresh air over the evaporator coil, which cools the air. The cool air is supplied the room. Meanwhile, refrigerant from the evaporator enters the compressor to be compressed and the cycle is repeats itself. Window Air conditioning System
The return air from the room is exhausted by the evaporator fan by blowing air over the condensed coil, where it picks up heat from the refrigerant. The compressor used in a window air-conditioner is hermetic type, which is sealed in a casing with a motor as shown in the figure. The condenser and evaporator fans are both propeller type. The condenser coil is a continuous copper coil with aluminum fins. The capillary tube is 40cm long, usually with 0.75mm diameter. Evaporator coil is also made of copper with aluminum fins. Dampers and louvers are provided for intake and exhaust on both sides of the air-conditioner casing. A one tone window air-conditioner circulaets4m3/min of air in the room. Some fraction of air (25%) is fresh; where as some fraction (75%) of air is recirculated. Filters are used to filter air entering the A/C unit. The filter is plastic foam or metallic mat type. The controls used by the unit are (1) Thermostat Control: This is used to set the room temperature. Usually it is a bimetallic strip. (2) Master Control It is an electrical switch used to stop or start the compressor motor. The installation of the unit is done either in the wall or by using external brackets or locating it inside the room with exhaust flush to the window. In either case, the condenser should be facing outside and the evaporator should be facing inside. It is advisable to install the unit for a minimum sun exposure as sun s rays can diminish the efficiency of the condenser. The air louver inlets should not be blocked and provision for draining of the condensate (like a pipe) should be provided. Maintenance of the window air-conditioner should include periodic cleaning, replacing filters and air seals and lubrication of fans. Split Type Room Air-Conditioner Split Air conditioning System These are preferred for offices, schools, auditoriums, etc. The idea of split units came from the fact that air-conditioning system operation was noisy. In order to eliminate the noise, compressor and condenser were located at remote location and only evaporator coil and a fan were located near the application. This division or splitting of the air- conditioning unit gave the equipment its name. The given figure shows the schematic of a split unit. Usually split units are wall mounted. They are preferred due to their noiseless operation. The refrigerant liquid lines and suction lines are line gas they connect the compressor at the remote location to the split unit evaporator. This increases cost of the system. This operation is similar to the window air-conditioner as the basic cycle of operation is vapour compression refrigeration. The refrigerant is compressed, condensed, expanded at the remote site and evaporated only in the room evaporator. The air is blown over the evaporator coil and that causes cooling.
Advantages of the Units are Noiseless operation Easy Maintenance Less Vibration Compact unit Aesthetic quality of unit is high Disadvantages of the Units are High cost of unit Longer refrigerant lines and thus more leakage COP of unit is poor Control over operation is difficult
Central Air conditioning System The physical size of the coil and filter bank are two main constraints in the development of the central air-conditioning systems. These two factors limit the tonnage of the equipment to about 100 tons capacity. The blowers used are of high capacity delivering 6000m3/min of air. Such systems are used for offices, multi stored buildings and supplied to the conditioned space by: An individual evaporator. Circulating chilled water. Air-circulation from a central location. The Advantages of Central Air-Conditioning System Ease of control Better accessibility for maintenance Cost of running equipment is less. Noise and vibration can be reduced.
Domestic Refrigerators Summer Air-conditioning system
Winter Air-conditioning system Year Round Air-conditioning system
Comparison of unitary and Central Air-Conditioning System APPLICATION OF AIR-CONDITIONING The applications of air-conditioning are quite diverse. What was once considered as a luxury has become a necessity of day to-day life. It is difficult to summarize the diverse applications and few important areas are listed below. Air-conditioning of theatres cinema houses. Air-conditioning of hotels, restaurants Air-conditioning of aircraft Air-conditioning of automobiles buses, cars. Railway air-conditioning Air-conditioning of hospitals
Air-conditioning of department store, amusement parks Air-conditioning of offices, homes Air-conditioning in textile industry. Air-conditioning of computer centers Air-conditioning in photographic industry Museum air-conditioning/bank air-conditioning Marine air-conditioning Television studio air-conditioning 1. Define refrigeration. Two Mark Questions Refrigeration is the process of reducing and maintaining the temperature of a body below the general temperature. 2. What is a refrigerator? Refrigerator is equipment used to reduce and maintain the temperature below atmospheric temperature by removing the heat from the space continuously. 3. Define refrigerant. Refrigerant is a fluid which absorbs the heat from the body and rejects the heat at high temperature. 4. Give some examples for refrigerant. Ammonia, carbon-di-oxide, Freon-12, Chloro fluoro carbon, Methyl chloride etc. 5. Define COP. COP is the ratio of heat extracted and work input. Coefficient of performance of a refrigerator (COP) = Heat extracted/ Work input 6. Give some properties of a good refrigerant. It should have low freezing point and boiling point. It should be easily liquefied. It should have high COP. It should absorb high latent heat.
. State the function of a compressor Compressor is used to compress the low pressure vapour refrigerant. 8. Mention some applications of refrigeration. In preserving food, fruits and drugs. Used in refineries. Manufacturing of ice. In manufacturing industries. 9. Define air conditioning. Air conditioning is the process of conditioning the air according to the human comfort irrespective of external conditions. 10. Define relative humidity. It is the ratio of water Vapour in a given volume of air at a given temperature, to the mass of water Vapour present in the same volume under same temperature of air when it is fully saturated. 11. Define DBT The temperature of air measured by the ordinary thermometer is called dry bulb temperature. 12. Define WBT The temperature of air measured by the thermometer when it is covered by wet cloth is known as wet bulb temperature. 13. Mention the types of air-conditioning. Comfort air conditioning. Industrial air conditioning. Key terms Refrigeration Air conditioning VCRS, VARS Refrigerant R12, R22 Window air conditioner Split air conditioner DBT, WBT Humidity Psychrometry
Objective Questions 1) The highest temperature during the cycle, in vapour compression refrigeration system occurs after (a) compression (b)condensation (c)expansion (d)evaporation 2) Heat is rejected by the refrigerant, during vapour compression refrigeration cycle in (a) Evaporator (b) condenser (c) throttle valve (d) compressor 3) A domestic refrigerator capacity may be approximately (a)1 ton (b)0.1 ton (c)5 ton (d)10 ton 4) A domestic window type air conditioner capacity may be approximately (a)1 ton (b)0.1 ton (c)5 ton (d)10 ton 5) In vapour compression refrigeration system the heat is absorbed from the environment to be cooled by (a) Compressor (b) throttle valve (c) evaporator (d) condenser 6) The order in which main components of vapour compression system are arranged is (a) compressor-evaporator-condenser-throttle valve (b) evaporator-condenser-throttle valve-compressor (c) compressor-condenser-throttle valve- evaporator (d) condenser-compressor-throttle valve-evaporator 7) In a domestic vapour compression refrigerator, the refrigerant commonly used is (a)ammonia (b) CO2 (c) Freon-12 (d)all of the above 8) The refrigerant commonly used in vapour absorption system is (a) Water (b) ammonia (c) freon-12 (d) aqua-ammonia 9) The lowest temperature during the cycle, in a vapour compression refrigeration system occures after (a)compression (b)evaporation (c)condensation (d)expansion 10) The air conditioning system selection (a) Should provide correct temperature, humidity, air movement and noise level, otherwise it is rejected (b) should be most economical otherwise it is rejected (c) Should be able to take up the peak load of the year, otherwise it is rejected (d) should satisfy the customer's concept of environment desired, otherwise it is rejected Summary In this lecture the student is introduced to different methods of refrigeration, both natural and artificial. Then a brief history of artificial refrigeration techniques is presented with a mention of the pioneers in this field and important events. The working principles of these systems are also described briefly.