REFRIGERATION CYCLES ERT 206 THERMODYNAMICS OBJECTIVE Introduce the

REFRIGERATION CYCLES ERT 206 THERMODYNAMICS

OBJECTIVE • Introduce the concepts of refrigerators and heat pumps and the measure of their performance. • Analyze the ideal vapor-compression refrigeration cycle. • Analyze the actual vapor-compression refrigeration cycle. • Review the factors involved in selecting the right refrigerant for an application. • Discuss the operation of refrigeration and heat pump systems. • Evaluate the performance of innovative vapor compression refrigeration systems. • Analyze gas refrigeration systems. • Introduce the concepts of absorption-refrigeration systems.

REFRIGERATORS & HEAT PUMPS REFRIGERATORS OBJECTIVE: MAINTAIN REFRIGERATED SPACE AT TL BY REMOVING QL HEAT PUMPS OBJECTIVE: MAINTAIN HEATED SPACE AT TH BY ABSORBING QH COOLING CAPACITY OF REFRIGERATORS Rate of heat removal from refrigerated space

THE IDEAL VAPOR-COMPRESSION REFRIGERATION CYCLE ØEliminated the impracticalities of Reversed Carnot: a. Refrigerant is vaporized completely before compression b. Turbine is replaced by throttling device ØConsists of 4 main processes ØThe ideal vapor-compression refrigeration cycle involves an irreversible (throttling) process to make it a more realistic model for the actual systems, thus it is not internally reversible cycle. Superheated vapor 3 -4’ Isentropic turbine 3 -4 Thtrottling Δh = 0 1 -2 Isentropic compression Schematic and T-s diagram for the ideal vapor-compressionrefrigeration cycle.

üAll 4 components in vapor-compression refrigeration cycle are steady flow devices. üSteady-flow energy balance on a unitmass basis: The P-h diagram of an ideal vaporcompression refrigeration cycle. In P-h diagram, Process 2 -3 & 4 -1, constant Pressure Process 3 -4, Thtrottling Δh = 0 üCondenser & evaporator do not involve any work, compressor can be approxinated as adiabatic. üCOPs for refrigerators & heat pumps on vapor-compression refrigeration cycle: üWhere

EXAMPLE 1 A refrigerator uses R-134 a as the working fluid and operates on an ideal vaporcompression refrigeration cycle between 0. 14 & 0. 8 Mpa. If the mass flow rate of the refrigerant is 0. 05 kg/s, determine (a) the rate of heat removal from the refrigerated space and the power input to the compressor, [7. 1845 k. W, 1. 81145 k. W] (b) the rate of heat rejection to the environment [8. 996 k. W] (c) COP of the refrigerator [3. 9662] SOLUTION: Rate of refrigeration, power input, rate of heat rejection, COPR Assumption: Steady operating conditions, PE & KE negligible Analysis: 1. Draw the T-s diagram of the system 2. Determine the enthalpies at all 4 states (1 -2 Δs=0, 3 -4 Δh=0) 3. Solve all

ACTUAL VAPOR-COMPRESSION REFRIGERATION CYCLE üActual vapor-compression refrigeration cycle differs from the ideal one owing mostly to the irreversibilities that occur in various components fluid friction and heat transfer to or from the surroundings. Schematic and T-s diagram for the actual vaporcompression refrigeration cycle. 1 -2 Non-isentropic compression, Superheated vapor at 1, friction in compressor, Δs≠ 0, Win increase 2 -4 -5 Sub cooled liquid at condenser exit, P drops

SELECTION OF REFRIGERANT PARAMETERS TO CONSIDER 1) Temperatures of the 2 media which the refrigerant exchanges heat 2) Toxicity, flammability, chemical stability 3) Availability at low cost For heat pumps: Tmin & Pmin may be considerably higher HYDROCARBON CO 2, AIR, H 2 O AMMONIA Industrial & heavy sectors ADV low cost, high COP, low E cost, high heat transfer, no effect on ozone layer DISADV toxicity CFCs Low cost & versatile R-11: large capacity water chillers R-12: domestic refrigerators & freezers R-22: NH 3 competitors R-502: R-115 & R-22 blends, commercial refrigerators (supermarket) Fully haloganated CFCs damage ozone layer Developed R-134 a chlorine-free

HEAT PUMP SYSTEMS v. The most common energy source for heat pumps is atmospheric air (air-to- air systems). Water-source systems usually use well water and ground-source (geothermal) heat pumps use earth as the energy source. They typically have higher COPs but are more complex and more expensive to install. v. Both the capacity and the efficiency of a heat pump fall significantly at low temperatures. Therefore, most air-source heat pumps require a supplementary heating system such as electric resistance heaters or a gas furnace. v. Heat pumps are most competitive in areas that have a large cooling load during the cooling season and a relatively small heating load during the heating season. In these areas, the heat pump can meet the entire cooling and heating needs of residential or commercial buildings. A heat pump can be used to heat a house in winter and to cool it in summer by adding a reversed valve

GAS REFRIGERATION CYCLES Gas Refrigeration Cycle The Reversed Brayton cycle can be used for refrigeration. Internally reversible, executed in ideal gas refrigeration cycle T-s diagram Area under 4 -1 = QL Area enclosed by 12341 = Win Heat transfer not isothermal, low COP 2 desirable characteristics: simpler, lighter component & incorporated with Simple gas refrigeration cycle regeneration where

EXAMPLE 2 An ideal gas refrigeration cycle using air as the working medium is to maintain a refrigerated space at -18°C while rejecting heat to the surrounding medium at 27°C. The pressure ratio of the compressor is 4, determine (a) the maximum & minimum T in the cycle [106°C, -71°C] (b) the coefficient of performance [2. 05] (c) The rate of refrigeration for a mass flow rate of 0. 05 kg/s [2. 67 k. W] SOLUTION: Tmax & Tmin, COPR, Rate of refrigeration Assumption: Steady operating conditions, air as ideal gases with variable specific heats, PE & KE negligible Analysis: 1. Draw the T-s diagram of the system 2. Determine the enthalpies at all 4 states (1 -2 and 3 -4 isentropic for ideal gases) 3. Solve all

ABSORPTION REFRIGERATION SYSTEMS Absorption refrigeration is economical when there is a source of inexpensive thermal energy at a temperature of 100 to 200°C. EXAMPLES: 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. Absorption refrigeration systems (ARS) involve the absorption of a refrigerant by a transport medium. The most widely used system is the ammonia–water system, where ammonia (NH 3) serves as the refrigerant and water (H 2 O) as the transport medium. Major advantage: A liquid is compressed instead of a vapor and as a result the work input is very small and often neglected in the cycle analysis. ARS are often classified as heat-driven systems & primarily used in large commercial and industrial installations. ARS are much more expensive than the vapor-compression refrigeration systems since it is more complex and occupy more space, much less efficient & require 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 to remain low relative to electricity.

PRINCIPLE OF ARS üThe system is much like the vapor-compression cycle, except for the compressor which has been replaced by a complex absorption mechanism üAbsorption unit absorber, pump, generator, rectifier, regenerator, valve üPURPOSE increased the Pressure of NH 3 MECHANISM: 1. NH 3 vapor leaves evaporator enters the absorber, dissolves & reacts with H 2 O (exo rxn, heat released) 2. The NH 3+H 2 O solution pumped to generator, vaporize some solution by heat transfer to the solution 3. The vapor passes through rectifier to separate the water 4. High P NH 3 vpor enter condenser & the rest of the cycle 5. The hot NH 3+H 2 O solution passes through the regenerator, transfer some heat to rich solution from the pump 6. The solution is throttled to the absorber P.

The COP of ARS is defined as The MAX COP is determined by assuming that the entire cycle is totally reversible. Heat from the source (Qgen) transferred to Carnot HE & Woutput of this HE is supplied to Carnot Refrigerator Determining the maximum COP of an absorption refrigeration system. The COP of actual absorption refrigeration systems is usually less than 1.

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TUTORIAL III A refrigerator uses R-134 a as the working fluid and operates on an ideal vaporcompression refrigeration cycle between 0. 12 and 0. 7 Mpa. The mass flow rate of the refrigerant is 0. 05 kg/s. Show the cycle on a T-s diagram with respect to the saturation lines. Determine (a) The rate of heat removal from the refrigerated space & the power input to the compressor (b) The rate of heat rejection to the environment (c) The coefficient of performance