Refrigeration Plant & Components

Type of Refrigeration System  Most common type: vapour compression refrigeration system – liquid refrigerant throttled from high to low pressure on leaving the condenser and entering the evaporator – production of refrigeration effect in evaporator upon vaporisation of refrigerant – compression of vapour refrigerant in the compressor upon leaving the evaporator and entering the condenser – rejection of heat at the condenser through condensation of vapour refrigerant to liquid phase

Types of Refrigerant  Classifications depending on composition of refrigerants, e.g. chlorine, hydrogen, halogen – chlorofluorocarbons (CFC): e.g. CFC-11 and CFC-12 – halons (BFC): e.g. BFC-13B1 – hydrochlorofluorocarbons (HCFC): e.g. HCFC-22 – hydrofluorocarbons (HFC): e.g. HFC-134a – azeotropes: e.g. CFC/HFC-500 – inorganic compounds: e.g. ammonia (R-717)

Effects on Environment  Ozone depletion – reduction of shielding layer against UV causing increase in skin cancer and suppression of immune system reduction in crop yield and affecting micro-organisms in sea global warming – increased chance of depletion with presence of chlorine as a catalyst and higher degree of stability of refrigerant, e.g. CFC-11 and CFC-12  Green house effect – long wave radiation emitted from surface being trapped and absorbed by CFCs/ CO 2 in the troposphere leading to global warming – affecting distribution of crops/animals and sea level

Effects on Environment  Ozone Depletion and global warming potential  Phasing out/restricted use of CFC/BFC/HCFC

Types of Compressors  Positive displacement vs non-positive displacement – positive displacement type: increase of pressure by reduction in volume of compression chamber, e.g. reciprocating, screw, rotary and scroll compressors – non-positive displacement type: no reduction in volume of compression; mainly based on conversion of dynamic pressure to static pressure, e.g. centrifugal compressor

Types of Compressors  Types of construction: hermetic, semi-hermetic and open type

Types of Compressors  Reciprocating compressors – intake, compression and discharge strokes in cylinder – a no. of cylinders in a compressor with up to 200 TR

Types of Compressors  Reciprocating compressors (cont’d) – typical example of refrigeration plant – total refrigeration capacity from 50 TR to 500 TR with use of multiple compressors in a chiller plant

Types of Compressors  Screw compressors – typical construction

Types of Compressors  Screw compressors (cont’d) – operation – male rotor driven by a motor which enables the meshing female rotar to rotate with compression of trapped refrigerant – 50 TR to 500 TR for a compressor

Types of Compressors  Scroll compressors – construction

Types of Compressors  Scroll compressors (cont’d) – operation – 2 identical scroll: one stationary and one revolving eccentrically with compression of trapped refrigerant – low noise and vibration – small refrigeration capacity (up to 60 TR)

Types of Compressors  Rotary compressors – 2 types: rotating piston and rotating blade; stationary cylinder and revolving piston/rotor – up to 4 TR; quite and smooth in operation Rotating piston Rotating blade

Types of Compressors  Centrifugal compressors – construction Generally used for large installations (100 to 10,000 TR)

Compressor Comparison  Reciprocating vs other positive displacement compressors – more moving parts and maintenance – several cylinders implying balancing problem – less reliable and slightly lower efficiency – more noise and vibration problem – greater clearance volume lowering volumetric efficiency – step control instead of stepless control

Single Stage Refrigeration Cycle  p-h diagram Constant temp line Constant entropy (s) lineConstant pressure line Constant enthalpy (h) line

Single Stage Refrigeration Cycle  Ideal and practical refrigeration cycle – throttling process from 3 to 4: h 3 = h 4 – vaporisation process from 4 to 1 refrigeration effect: RE = h 1 – h 4, kJ/kg P h Subcooling Superheating

Single Stage Refrigeration Cycle  Ideal and practical refrigeration cycle (cont’d) – compression process from 1 to 2 ideal compression: no change in entropy (s 1 = s 2 ) specific work input: w = h 2 – h 1,kJ/kg – condensation process from 2 to 3 heating effect: HE = h 3 – h 2, kJ/kg – coefficient of performance of cooling – refrigeration capacity RC = m r (RE), W

Single Stage Refrigeration Cycle  Ideal and practical refrigeration cycle (cont’d) – coefficient of performance of heating – superheating prevent entry of liquid refrigerant to compressor to affect lubrication and damage components increase refrigeration effect be kept to a min. because of reduction in COP – subcooling increase in refrigeration effect

Single Stage Refrigeration Cycle  Actual refrigeration cycle – losses in system and compression process included – entropy increase s 2 ’’ > s 1 – determination of 2’’ using isentropic efficiency(  isen ) P h 2’ 2’’ Ideal compression: s 1 = s 2 ’

Single Stage Refrigeration Cycle  Plotting of single stage refrigeration cycle 2 4 Evaporator pressure: 0.2 MPa Condenser pressure: 1 MPa 1 Suction temperature: 10 o C Ideal compression3 Subcooling: 5 o C

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