Refrigeration Cycles MOHAMMAD FAISAL HAIDER LECTURER Department of Mechanical Engineering Department of Mechanical Engineering Bangladesh University of Engineering and Technology
Objectives Introduce the concepts of refrigerators and heat pumps and the measure of their performance. Analyze the ideal and actual vapor-compression refrigeration cycles. Discuss the operation of refrigeration and heat pump systems. Evaluate the performance of innovative vapor-compression refrigeration fi systems. 2
Refrigeration Refrigeration is the action of cooling, and in practice this requires removal of heat and discarding it at a higher temperature. Refrigeration is therefore the science of moving heat from low temperature to high temperature. In addition to chilling and freezing applications, refrigeration technology is applied in air conditioning and heat pumps. Refrigerators and Heat Pumps Refrigerators Take Heat from cooler place Transfer heat to hotter place Objective: Keeping low temp. at the cooler place Used in Tropical Countries/Deserts for Space Cooling Heat Pumps Take Heat from cooler place Transfer heat to hotter place Objective: e Keeping high temp. at the hotter place Used in Cold Countries for space heating 3
Sensible heat and latent heat If a change of enthalpy can be sensed as a change of temperature, it is called sensible heat. This is expressed epessedas specific c heat capacity, i.e. the change in enthalpy apy per degree ee of temperature change, in kj/(kg K). If there is no change of temperature but a change of state (solid to liquid, liquid to gas, or vice versa) it is called latent heat. This is expressed as kj/kg but it varies with the boiling temperature, and so is usually qualified by this condition. The resulting total changes can be shown on a temperature enthalpy diagram. 4
Refrigerators And Heat Pumps The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators. The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump pis to supply ppy heat (QH) to a warm medium. Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only. Performance: for fixed values of Q L and Q H 5
COP = ( Desired work)/ ( Net work done) =Q L / (Q H -Q L ) = (T L / (T H -T L ) 6
For fixed values of QL and QH. This relation implies that COPHP > 1 since COPR is a positive quantity. That is, a heat pump functions, at worst, as a resistance heater, supplying as much energy to the house as it consumes. In reality, however, part of QH is lost to the outside air throughh piping i and other devices, and COPHP may drop below unity when the outside air temperature is too low. When this happens, the system normally switches to the fuel (natural gas, propane, oil, etc.) or resistance-heating mode. The cooling capacity of a refrigeration system that is, the rate of heat removal from the refrigerated space is often expressed in terms of tons of refrigeration. The capacity of a refrigeration system that can freeze 1 ton (2000 lbm) of liquid water at 0 C (32 F) into ice at 0 C in 24 h is said to be 1 ton. One ton of refrigeration is equivalent to 211 kj/min or 200 Btu/min. The cooling load of a typical 200-m2 residence is in the 3-ton (10-kW) range. 7
The most efficient refrigeration cycle operating between T L and T H.But not a suitable model for refrigeration cycles because: (i) process 2-3 involves compression of a liquid vapor mixture - requires a compressor that will handle two phases, (ii) process 4-1 involves expansion of highmoisture-content refrigerant in a turbine. The Reversed Carnot Cycle Both COPs increase as the difference between the two temperatures decreases, i.e. as T L rises or T H falls. Schematic of a Carnot refrigerator and T-s diagram of the reversed Carnot cycle. 8
Ideal Vapor-compression Refrigeration Cycle Is the ideal model for refrigeration systems. The refrigerant is vaporized completely before it is compressed and the turbine is replaced with a throttling device. The most widely used cycle for refrigerators, A-C systems, and heat pumps. Schematic and T-s diagram for the ideal vapor-compression refrigeration cycle. 9
The ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process to make it a more realistic model for the actual systems. Steady-flow energy balance An ordinary household refrigerator. The P-h diagram of an ideal vaporcompression refrigeration cycle. 10
Actual Vapor-Compression Refrigeration Cycle An actual vapor-compression refrigeration cycle involves irreversibilities in various components - mainly due to fluid friction (causes pressure drops) and heat transfer to or from the surroundings. As a result, the COP decreases. Differences Non-isentropic compression; Superheated vapor at evaporator exit; Sub-cooled liquid at condenser exit; Pressure drops in condenser and evaporator. Schematic and T-s diagram for the actual vapor-compression refrigeration cycle. 11
Selecting the Right Refrigerant Several refrigerants may be used in refrigeration systems such as chlorofluorocarbons (CFCs), ammonia, hydrocarbons (propane, ethane, ethylene, etc.), carbon dioxide, air (in the air-conditioning of aircraft), and even water (in applications above the freezing point). R-11 11, R-12 12, R-22 22, R-134a 134a, and R-502 account for over 90 percent of the market. The industrial and heavy-commercial sectors use ammonia (it is toxic). R-11 is used in large-capacity water chillers serving A-C systems in buildings. R-134a is used in domestic refrigerators and freezers, as well as automotive air conditioners. R-22 is used in window air conditioners, heat pumps, air conditioners of commercial buildings, and large industrial refrigeration systems, and offers strong competition to ammonia. R-502 (a blend of R-115 and R-22) is the dominant refrigerant used in commercial refrigeration systems such as those in supermarkets. CFCs allow more ultraviolet radiation into the earth s atmosphere by destroying the protective ozone layer and thus contributing to the greenhouse effect that causes global warming. Refrigerants that are friendly to the ozone layer have been developed. Two important parameters to be considered - the temperatures of the refrigerated space and the environment with which the refrigerant exchanges heat. 12
Refrigerants Refrigerants are chemicals that act as the working fluid in Refrigeration cycles The names of the Refrigerants were initially developed by a company named DuPont for VCRS Chemical Formula R (Carbon atoms-1) (Hydrogen atoms+1) (Flourine atoms) R-11 = R011 = CCl 3 F and R-22= R022 = CHF 2 Cl 13
Innovative Vapor-compression Refrigeration Systems The simple vapor-compression refrigeration cycle is the most widely used refrigeration cycle, and is adequate for most refrigeration applications. The ordinary vapor-compression refrigeration systems are simple, inexpensive, reliable, and practically maintenance-free. However, for large industrial applications, efficiency (not simplicity) is the major concern. For some applications the simple vapor-compression refrigeration cycle is inadequate and needs to be modified. For moderately and very low temperature applications, some innovative refrigeration systems are used. The following cycles will be discussed: Cascade refrigeration systems Multistage compression refrigeration systems Multipurpose refrigeration systems with a single compressor Liquefaction of gases 14
Cascade Refrigeration Systems Some industrial applications require moderately low temperatures, and the temperature range they involve may be too large for a single vapor compression refrigeration cycle to be practical. A large temperature range also means a large pressure range in the cycle and a poor performance for a reciprocating compressor. One way of dealing with such situations is to perform the refrigeration process in stages, that is, to have two or more refrigeration cycles that operate in series. Such refrigeration cycles are called cascade refrigeration cycles. 15
Cascade Refrigeration Systems A two-stage cascade refrigeration cycle is shown. The two cycles are connected through the heat exchanger in the middle, which serves as the evaporator for the topping cycle and the condenser for the bottoming cycle. A two-stage A cascade compression refrigeration refrigeration system with system the same with refrigerant a flash chamber. in both stages. 16
Cascade Refrigeration Systems Assuming the heat exchanger is well insulated and the kinetic and potential energies are negligible, the heat transfer from the fluid in the bottoming cycle should be equal to the heat transfer to the fluid in the topping cycle. Thus, the ratio of mass flow rates through each cycle should be The coefficient of performance of the cascade system is 17
Multistage Compression Refrigeration Systems When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber (called a flash chamber) since it has better heat transfer characteristics. A two-stage compression refrigeration system with a flash chamber. 18
Multipurpose Refrigeration Systems with a Single Compressor Some applications require refrigeration at more than one temperature. This could be accomplished by using a separate throttling valve and a separate compressor for each evaporator operating at different temperatures. 19
Absorption Refrigeration Systems When there is a source of inexpensive thermal energy at a temperature of 100 to 200 C is absorption refrigeration. Some examples include geothermal energy, solar energy, and waste heat from cogeneration or process steam plants, and even natural gas when it is at a relatively low price. Ammonia absorption refrigeration cycle. 20
This system looks very much like the vapor-compression system, except that the compressor has been replaced by a complex absorption mechanism consisting of an absorber, a pump, a generator, a regenerator, a valve, and a rectifier. Ammonia vapor leaves the evaporator and enters the absorber, where it dissolves and reacts with water to form NH3 H2O. This is an exothermic reaction; thus heat is released during this process. TheamountofNH3thatcanbedissolvedinH2Oisinversely proportional to the temperature. Therefore, it is necessary to cool the absorber to maintain its temperature as low as possible, hence to maximize the amount of NH3 dissolved in water. The liquid NH3 H2O solution, which is rich in NH3, is then pumped to the generator. Heat is transferred to the solution from a source to vaporize some of the solution. The vapor, which is rich in NH3, passes through a rectifier, which separates the water and returns it to the generator. The high-pressure pure NH3 vapor then continues its journey through the rest of the cycle. 21
Absorption refrigeration systems (ARS) involve the absorption of a refrigerant by a transport medium. The most widely used system is the ammonia water system, whereammonia(nh 3 ) serves as the refrigerant and water (H 2 O) as the transport medium. Other systems include water lithium bromide and water lithium chloride systems, where water serves as the refrigerant. These systems are limited to applications such as A-C where the minimum temperature is above the freezing point of water. Compared with vapor-compression systems, ARS have one major advantage: A liquid is compressed instead of a vapor and as a result the work input is very small (on the order of one percent of the heat supplied to the generator) and often neglected in the cycle analysis. ARS are much more expensive than the vapor-compression refrigeration systems. They are more complex and occupy more space, they are much less efficient thus requiring much larger cooling towers to reject the waste heat, and they are more difficult to service since they are less common. Therefore, ARS should be considered only when the unit cost of thermal energy is low and is projected pojectedtoto remain lowrelative eat eto electricity. eect cty ARS are primarily used in large commercial and industrial installations. 22
The COP of actual absorption refrigeration systems is usually less than unity. Air-conditioning systems based on absorption refrigeration, called the absorption chillers, perform best when the heat source can supply heat at a high temperature with little temperature drop. The maximum COP of an absorption refrigeration system. 23