MECH4880 Vapour Compression v2.pdf - MECH 4880 Vapour Compression Refrigeration Chris Menictas School of Mechanical and Manufacturing Engineering Vapour

MECH4880 Vapour Compression v2.pdf - MECH 4880 Vapour...

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MECH 4880 Vapour Compression Refrigeration Chris Menictas School of Mechanical and Manufacturing Engineering
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4.1 Vapour Compression Refrigeration The reversed Carnot cycle is the most efficient refrigerator (or heat pump) possible. If the refrigerant is a gas the Carnot cycle cannot be implemented in practice and the less efficient Brayton-Joule cycle has to be used. When the refrigerant is a vapour, then the reversed Carnot cycle can be made almost completely practical by operating in the liquid- vapour region - although may not be feasible in practice
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4.1 The Ideal Cycle The ideal cycle can be considered to be one of two forms . (a) The most commonly seen form is as follows. ref C c net H C q T COP w T T = = - ( ) 1 4 ref C q T s s = - ( )( ) 1 4 net cond ref H C w q q T T s s = - = - - Isentropic Compression Condensation Isentropic expansion Evaporation
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4.1 The Ideal Cycle (b) where we now have two compressors, an isentropic compressor and an isothermal compressor (see the figure below). CONDENSER EVAPORATOR 2c 3 4 1 2 Expander Isentropic Compressor Isothermal Compressor Constant entropy Constant temperature
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4.1 The Ideal Cycle The resulting T-s diagram is Isentropic Compression Isothermal Compression Condensation Isentropic expansion Evaporation More realistic, saturated vapour at suction point. Note different refrigeration capacity case (b) to case (a).
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4.1 The Ideal Cycle As before, we have that Where and from the First Law of Thermodynamics, i.e. \ ref c net q COP w = ( ) 41 1 4 1 4 ref c q q h h T s s = = - º - 23 41 net w q q = - ( ) ( ) ( )( ) 2 3 1 4 1 4 net H C H C w T s s T s s T T s s = - - - = - - C c H C T COP T T = - Evaporating T Condensing T
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4.1 The Ideal Cycle Both (a) and (b) are valid Carnot cycles, with exactly the same Coefficient of Performance, COP, but with different cooling capacities, q, and different power inputs. Case (a) is more realistic in that only one compressor is used, whereas in case (b), there are two compressors. Case (b) is more realistic in regard to the inlet conditions at suction, i.e. point 1.
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4.1 Theoretical (or Standard) Vapour Compression Cycle While the cycle shown before offers a high coefficient of performance, practical considerations require certain revisions. (i) Work of Compression In case (a) we saw that the compression is "wet" , viz. In this type of process "wet vapour" is taken from the evaporator and compressed isentropically until point 2 is obtained. Although this is possible and some early compressors did operate on this principle, there are however practical problems.
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4.1 Theoretical (or Standard) Vapour Compression Cycle These problems are: (1) During compression the droplets of liquid are vaporised by the internal heat transfer process which requires a finite amount of time. This time is not normally available and liquid droplets may become trapped in the head of the cylinder by the rising piston, possibly damaging the valves or cylinder head.
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