Advanced Vehicle Technology Episode 3 Part 10 pps

Oil separator Receiver Sight glass Drier Expansion valve Fan Evaporator coil Evaporator unit Condenser coil Condenser unit Vee cylinder compressor Starter motor In-line four cylinder diesel engine Coupling and clutch Fig. 13.4 Heavy duty diesel engine shaft driven compressor refrigeration unit Sensible heat Latent heat of evaporation Super heat Refrigerant absorbs heat, converting to vapour Refrigerant rejecting heat, converting to liquid Refrigerant begins to boil (vaporize) Refrigerant completely boiled to a saturated vapour Subcooled temperature Superheat temperature Saturated temperature Refrigerant temperature (°c) Heat increase (J) Fig. 13.5 Illustrative relationship between the refrigerant's temperature and heat content during a change of state 572 Latent heat of evaporation (Fig. 13.5) This is the heat needed to completely convert a liquid to a vapour and takes place without any temperature rise. Superheated vapour (Fig. 13.5) This is a vapour heated to a temperature above the saturated temperature (boiling point); superheating can only occur once the liquid has been completely vaporized. 13.2 Principles of a vapour±compression cycle refrigeration system (Fig. 13.6) 1 High pressure subcooled liquid refrigerant at a typical temperature and pressure of 30  C and 10 bar respectively flows from the receiver to the expansion valve via the sight glass and drier. The refrigerant then rapidly expands and reduces its pressure as it passes out from the valve restric- tion and in the process converts the liquid into a vapour flow. 2 The refrigerant now passes into the evaporator as a mixture of liquid and vapour, its temperature being lowered to something like À10  C with a corresponding pressure of 2 bar (under these conditions the refrigerant will boil in the evap- orator). The heat (latent heat of evaporation) necessary to cause this change of state will come from the surrounding frozen compartment in which the evaporator is exposed. Condenser coil Discharge line (high pressure) Superheated vapour 60 10 bar (high pressure) C° Superheated vapour 8 2 bar (low pressure) C° Oil separator Compressor Refrigerant rejects heat to surrounding atmosphere Suction line (low pressure) Evaporator coil Saturated vapour –10 2 bar (low pressure) C° Frozen storage chamber Refrigerant absorbs heat from surrounding frozen storage space Saturated liquid 40 C 10 bar (high pressure) ° Remote feeler bulb Receiver Sight glass Liquid line Drier Liquid/vapour mixture 40 C 10 bar° Subcooled liquid 30 C 10 bar° Expansion value Liquid/vapour mixture – 10 C 2 bar (low pressure) ° Saturated vapour 40 10 bar (high pressure) C° 6 5 4 7 8 1 2 3 Fig. 13.6 Refrigeration vapour±compression cycle 573 3 As the refrigerant moves through the evaporator coil it absorbs heat and thus cools the space surrounding the coil. Heat will be extracted from the cold storage compartment until its pre-set working temperature is reached, at this point the compressor switches off. With further heat loss through the storage container insula- tion leakage, doors opening and closing and additional food products being stored, the com- pressor will automatically be activated to restore the desired degree of cooling. The refrigerant entering the evaporator tube completes the evaporation process as it travels through the evaporator coil so that the exit refrigerant from the evaporator will be in a saturated vapour state but still at the same temperature and pressure as at entry, that is, À10  C and 2 bar respectively. 4 The refrigerant is now drawn towards the compressor via the suction line and this causes the heat from the surrounding air to superheat the refrigerant thus raising its temperature to something like 8  C; however, there is no change in the refrigerant's pressure. 5 Once in the compressor the superheated vapour is rapidly compressed, consequently the super- heated vapour discharge from the compressor is at a higher temperature and pressure in the order of 60  C and 10 bar respectively. 6 Due to its high temperature at the exit from the compressor the refrigerant quickly loses heat to the surrounding air as it moves via the discharge line towards the condenser. Thus at the entry to the condenser the refrigerant will be in a satur- ated vapour state with its temperature now low- ered to about 40  C; however, there is no further change in pressure which is still therefore 10 bar. 7 On its way through the condenser the refrigerant saturated vapour condenses to a saturated liquid due to the stored latent heat in the refrigerant transferring to the surrounding atmosphere via the condenser coil metal walls. Note the heat dissipated to the surrounding atmosphere by the condenser coil is equal to the heat taken in by the evaporator coil from the cold storage compartment and the compressor. 8 After passing through the condenser where heat is given up to the surrounding atmosphere the saturated liquid refrigerant now flows into the receiver. Here the increased space permits a small amount of evaporation to occur, the refrig- erant then completes the circuit to the expansion valve though the liquid line where again heat is lost to the atmosphere, and this brings the refrig- erant's temperature down to something like 30  C but without changing pressure which still remains at 10 bar. 13.3 Refrigeration system components A description and function of the various compon- ents incorporated in a refrigeration system will be explained as follows: 13.3.1 Reciprocating compressor cycle of operation (Fig. 13.7(a±d)) Circulation of the refrigerant between the evapor- ator and the condenser is achieved by the pumping action of the compressor. The compressor draws in low pressure superheated refrigerant vapour from the evaporator and discharges it as high pressure superheated vapour to the condenser. After flowing through the condenser coil the high pressure refriger- ant is now in a saturated liquid state; it then flows to the expansion valve losing heat on the way and thus causing the liquid to become subcooled. Finally the refrigerant expands on its way through the expansion valve causing it to convert into a liquid-vapour mix before re-entering the evapor- ator coil. The reciprocating compressor completes a suc- tion and discharge cycle every revolution; the out- ward moving piston from TDC to BDC forms the suction-stroke whereas the inward moving piston from BDC to TDC becomes the discharge stroke. Suction stroke (Fig. 13.7(a and b)) As the crank shaft rotates past the TDC position the piston com- mences its suction stroke with the discharge reed valve closed and the suction reed valve open (Fig. 13.7(a and b)). The downward sweeping piston now reduces the cylinder pressure from P 1 to P 2 as its volume expands from V 1 to V 2 , the vapour refrig- erant in the suction line is now induced to enter the cylinder. The cylinder continues to expand and to be filled with vapour refrigerant at a constant pressure P 1 to the cylinder's largest volume of V 3 ,thatisthe piston's outermost position BDC, see Fig. 13.8. Discharge stroke (Fig. 13.9(c and d)) As the crankshaft turns beyond BDC the piston begins its upward discharge stroke, the suction valve closes and the discharge valve opens (see Fig. 13.7(c and d)). The upward moving piston now compresses the refrigerant vapour thereby increasing the cylinder pressure from P 1 to P 2 through a volume reduction from V 3 to V 4 at which point the cylinder pressure 574 Discharge line Suction line Low pressure vapour refrigerant from evaporator High pressure vapour refrigerant to condenser Cylinder head Piston ring Piston Gudgeon pin Connecting rod Cylinder wall Crankshaft Suction reed valve Valve block Crankcase Sump Discharge reed valve (a) Piston at TDC both valves closed high pressure vapour trapped in discharge line and clearance volume (b) Piston on downward suction stroke vapour refrigerant drawn into cylinder (c) Piston at BDC both valves closed, cylinder filled with fresh vapour refrigerant (d) Piston on upward discharge stroke, suction valve closed discharged valve open, compressed vapour refrigerant pumped into discharge line Fig. 13.7 (a±d) Reciprocating compressor cycle of operation Vapour discharge 1 4 P 2 P 1 Pressure (bar) Clearance volume 2 V a p o u r e x p a n s i o n Vapour intake Swept volume V a p o u r c o m p re s s i o n 3 V (TDC) 1 V 2 V 4 Volume V (BDC) 3 Fig. 13.8 Reciprocating compressor pressure-volume cycle 575 equals the discharge line pressure; the final cylinder volume reduction therefore from V 4 back to V 1 will be displaced into the high pressure discharge line at the constant discharge pressure of P 2 (see Fig. 13.8). 13.3.2 Evaporator (Fig. 13.6) The evaporator's function is to transfer heat from the food being stored in the cold compartment into the circulating refrigerant vapour via the fins and metal walls of the evaporator coil tubing by convection and conduction respectively. The refrigerant entering the evaporator is nearly all liquid but as it moves through the tube coil, it quickly reaches its saturation temperature and is converted steadily into vapour. The heat necessary for this change of state comes via the latent heat of evaporation from the surrounding cold cham- ber atmosphere. The evaporator consists of copper, steel or stain- less steel tubing which for convenience is shaped in an almost zigzag fashion so that there are many parallel lengths bent round at their ends thus enabling the refrigerant to flow from side to side. To increase the heat transfer capacity copper fins are attached to the tubing so that relatively large quantities of heat surrounding the evaporator coil can be absorbed through the metal walls of the tubing, see Fig. 13.15(a and b). 13.3.3 Condenser (Fig. 13.6) The condenser takes in saturated refrigerant vapour after it has passed though the evaporator and compressor, progressively cooling then takes place as it travels though the condenser coil, accordingly the refrigerant condenses and reverts to a liquid state. Heat will be rejected from the refrigerant during this phase change via conduction though the metal walls of the tubing and convec- tion to the surrounding atmosphere. A condenser consists of a single tube shaped so that there are many parallel lengths with semi- circular ends which therefore form a continuous winding or coil. Evenly spaced cooling fins are normally fixed to the tubing, this greatly increases the surface area of the tubing exposed to the con- vection currents of the surrounding atmosphere, see Fig. 13.15(a and b). Fans either belt driven or directly driven by an electric motor are used to increase the amount of air circulation around the condenser coil, this therefore improves the heat transfer taking place between the metal tube walls and fins to the sur- rounding atmosphere. This process is known as forced air convection. 13.3.4 Thermostatic expansion valve (Fig. 13.9(a and b)) An expansion valve is basically a small orifice which throttles the flow of liquid refrigerant being pumped from the condenser to the evaporator; the immediate exit from the orifice restriction will then be in the form of a rapidly expanding re- frigerant, that is, the refrigerant coming out from the orifice is now a low pressure continuous liquid- vapour stream. The purpose of the thermostatic valve is to control the rate at which the refrigerant passes from the liquid line into the evaporator and Diaphragm Tapered valve Outlet to evaporator Inlet from condenser Feeler bulb (attached to output side of evaporator) (cold) (a) Valve closed (b) Valve open Adjustment screw Spring External equalizer to suction line Inlet from condenser Feeler bulb (attached to output side of evaporator) (hot) External equalizer to suction line Effective expansion orifice Outlet to evaporato r Fig. 13.9 (a and b) Thermostatic expansion valve 576 to keep the pressure difference between the high and low pressure sides of the refrigeration system. The thermostatic expansion valve consists of a diaphragm operated valve (see Fig. 13.9(a and b)). One side of the diaphragm is attached to a spring loaded tapered/ball valve, whereas the other side of the diaphragm is exposed to a refrigerant which also occupies the internal space of the remote feeler bulb which is itself attached to the suction line tube walls on the output side of the evaporator. If the suction line saturated/superheated temperature decreases, the pressure in the attached remote feeler bulb and in the outer diaphragm chamber also decreases. Accordingly the valve control spring thrust will partially close the taper/ball valve (see Fig. 13.9(a)). Consequently the reduced flow of refrigerant will easily now be superheated as it leaves the output from the evaporator. In contrast if the superheated temperature rises, the remote feeler bulb and outer diaphragm chamber pressure also increases, this therefore will push the valve further open so that a larger amount of refrig- erant flows into the evaporator, see Fig. 13.9(b). The extra quantity of refrigerant in the evaporator means that less superheating takes place at the out- put from the evaporator. This cycle of events is a continuous process in which the constant super- heated temperature control in the suction line maintains the desired refrigerant supply to the evaporator. A simple type of thermostatic expansion valve assumes the input and output of an evaporator are both working at the same pressure; however, due to internal friction losses the output pressure will be slightly less than the input. Consequently the lower output pressure means a lower output saturated temperature so that the refrigerant will tend to vaporize completely before it reaches the end of the coil tubing. As a result this portion of tubing converted completely into vapour and which is in a state of superheat does not contribute to the heat extraction from the surrounding cold chamber so that the effective length of the evaporator coil is reduced. To overcome early vaporization and superheating, the diaphragm chamber on the valve-stem side is subjected to the output side of the evaporator down stream of the remote feeler bulb. This extra thrust opposing the remote feeler bulb pressure acting on the outer diaphragm cham- ber now requires a higher remote feeler bulb pres- sure to open the expansion valve. 13.3.5 Suction pressure valve (throttling valve) (Fig. 13.10(a and b)) This valve is incorporated in the compressor output suction line to limit the maximum suction Intake vapour from evaporator Adjusting nut Piston Spring Valve seat Bellows Pin Spring Outlet vapour to compressor suction valves Flat valve (a) Valve fully open Limiting pressure (b) Valve partially open Fig. 13.10 Suction pressure regulating valve (throttling valve) 577 pressure generated by the compressor thereby safe- guarding the compressor and drive engine/motor from overload. If the maximum suction pressure is exceeded when the refrigeration system is switched on and started up (pull down) excessive amounts of vapour or vapour/liquid or liquid refrigerant may enter the compressor cylinder, which could produce very high cylinder pressures; this would therefore cause severe strain and damage to the engine/electric motor components, conversely if the suction line pressure limit is set very low the evaporator may not operate efficiently. The suction pressure valve consists of a com- bined piston and bellows controlled valve subjected to suction vapour pressure. When the compressor is being driven by the engine/motor the output refrigerant vapour from the evaporator passes to the intake port of the suction pressure valve unit; this exposes the bellows to the refrigerant vapour pressure and temperature. Thus as the refrigerant pressure rises the bellows will contract against the force of the bellows spring; this restricts the flow of refrigerant to the compres- sor (see Fig. 13.10(a)). However, as the bellows temperature rises its internal pressure also increases and will therefore tend to oppose the contraction of the bellows. At the same time the piston will be subjected to the outlet vapour pressure from the suction pressure valve before entering the compres- sor cylinders, see Fig. 13.10(b). If this becomes excessive the piston and valve will move towards the closure position thus restricting the entry of refrigerant vapour or vapour/liquid to the com- pressor cylinders. Hence it can be seen that the suction pressure valve protects the compressor and drive against abnormally high suction line pressure. 13.3.6 Reverse cycle valve (Fig. 13.11(a and b)) The purpose of this valve is to direct the refrigerant flow so that the refrigerant system is in either a cooling or heating cycle mode. Refrigerant cycle mode (Fig. 13.11(a)) With the pilot solenoid valve de-energized the suction pas- sage to the slave cylinder of the reverse cycle valve is cut off whereas the discharge pressure supply from the compressor is directed to the slave pis- ton. Accordingly the pressure build-up pushes the piston and both valve stems inwards; the left hand compressor discharge valve now closes the From compressor discharge To condenser coil From compressor discharge From evaporator coil To compressor suction Compressor discharge valve From condenser coil To compressor discharge Slave piston & cylinder Compressor suction valves From compressor discharge To coil evaporator To compressor suction (a) Cooling cycle (b) Heating cycle Fig. 13.11 (a and b) Reverse cycle valve 578 compressor discharge passage to the evaporator and opens the compressor discharge passage to the condenser whereas the right hand double com- pressor discharge valve closes the condenser to compressor suction passage and opens the eva- porator to the compressor suction pressure. Heat/defrost cycle mode (Fig. 13.11(b)) Energiz- ing the pilot solenoid valve cuts off the compressor discharge pressure to the slave cylinder of the reverse cycle valve and opens it to the compressor suction line. As a result the trapped refrigerant vapour in the slave cylinder escapes to the com- pressor suction line thus permitting the slave piston and both valves to move to their outermost position. The left hand compressor discharge valve now closes the compressor discharge to the condenser passage and opens the compressor discharge to the evaporator passage whereas the right hand com- pressor suction double valve closes the evaporator to the compressor suction passage and opens the condenser to compressor suction pressure. 13.3.7 Drier (Fig. 13.12) Refrigerant circulating the refrigerator system must be dry, that is, the fluid, be it a vapour or a liquid, should not contain water. Water in the form of moisture can promote the formation of acid which can attack the tubing walls and joints and cause the refrigerant to leak out. It may initiate the formation of sludge and restrict the circulation of the refrigerant. Moisture may also cause ice to form in the thermostatic expansion valve which again could reduce the flow of refrigerant. To overcome problems with water contamination driers are nor- mally incorporated in the liquid line; these liquid line driers not only remove water contained in the refrigerant, they also remove sludge and other impurities. Liquid line driers are plumbed in on the output side of the receiver, this is because the moisture is concentrated in a relatively small space when the refrigerant is in a liquid state. A liquid line drier usually takes the form of a cylindrical cartridge with threaded end connec- tions so that the drier can be replaced easily when necessary. Filter material is usually packed in at both ends; in the example shown Fig. 13.12 there are layers, a coarse filter, felt pad and a fine filter. In between the filter media is a desiccant material, these are generally of the adsorption desiccant kind such as silica gel (silicon dioxide) or activated alumna (aluminium oxide). The desiccant sub- stance has microscopic holes for the liquid refriger- ant to pass through; however, water is attracted to the desiccant and therefore is prevented from moving on whereas the dry (free of water) clean refrigerant will readily flow through to the expan- sion valve. 13.3.8 Oil separator (Fig. 13.13) Oil separators are used to collect any oil entering the refrigeration system through the compressor and to return it to the compressor crankcase and sump. The refrigerant may mix with the com- pressor's lubrication oil in the following way: 1 During the cycle of suction and discharge refriger- ant vapour periodically enters and is displaced from the cylinders; however, if the refrigerant flow becomes excessive liquid will pass through the expansion valve and may eventually enter the suction line via the evaporator. The fluid may then drain down the cylinder walls to the crank- case and sump. Refrigerant mixing with oil dilutes it so that it loses its lubricating properties: the wear and tear of the various rubbing com- ponents in the compressor will therefore increase. Contaminated vapour/liquid mixture from receiver Desiccant dehydrating material Dry clean refrigeran t to expansion valve Fine filter Felt pad Coarse filter Fig. 13.12 Adsorption type liquid line drier 579 2 When the refrigerator is switched off the now static refrigerant in the evaporator may condense and enter the suction line and hence the com- pressor cylinders where it drains over a period of time into the crankcase and sump. 3 Refrigerant mixing with the lubricant in the crankcase tends to produce oil frothing which finds its way past the pistons and piston rings into the cylinders; above each piston the oil will then be pumped out into the discharge line with the refrigerant where it then circulates. Oil does not cause a problem in the condenser as the temperature is fairly high so that the refrigerant remains suspended; however, in the evaporator the temperature is low so that the liquid oil separ- ates from the refrigerant vapour, therefore tending to form a coating on the inside bore of the evaporator coil. Unfortunately oil is a very poor conductor of heat so that the efficiency of the heat transfer process in the evaporator is very much impaired. After these observations it is clear that the refrig- erant must be prevented from mixing with the oil but this is not always possible and therefore an oil separator is usually incorporated on the output side of the compressor in the discharge line which separates the liquid oil from the hot refrigerant vapour. An oil separator in canister form consists of a cylindrical chamber with a series of evenly spaced perforated baffle plates or wire mesh parti- tions attached to the container walls; each baffle plate has a small segment removed to permit the flow of refrigerant vapour (Fig. 13.13), the input from the compressor discharge being at the lowest point whereas the output is via the extended tube inside the container. A small bore pipe connects the base of the oil separator to the compressor crank- case to provide a return passage for the liquid oil accumulated. Thus when the refrigerator is operat- ing, refrigerant will circulate and therefore passes though the oil separator. As the refrigerant/oil mix zigzags its way up the canister the heavier liquid oil tends to be attracted and attached to the baffle plates; the accumulating oil then spreads over the plates until it eventually drips down to the base of the canister, and then finally drains back to the compressor crankcase. 13.3.9 Receiver (Fig. 13.6) The receiver is a container which collects the con- densed liquid refrigerant and any remaining vapour from the condenser; this small amount of vapour will then have enough space to complete the condensation process before moving to the expan- sion valve. 13.3.10 Sight glass (Fig. 13.14) This device is situated in the liquid line on the out- put side of the receiver; it is essentially a viewing port which enables the liquid refrigerant to be seen. Refrigerant movement or the lack of movement due to some kind of restriction, or bubbling caused by insufficient refrigerant, can be observed. 13.4 Vapour±compression cycle refrigeration system with reverse cycle defrosting (Fig. 13.15(a and b)) A practical refrigeration system suitable for road transportation as used for rigid and articulated vehicles must have a means of both cooling and Perforated battle plates Vapour + oil flow path High pressure vapour refrigerant + oil From compressor To evaporator High pressure vapour refrigerant Separated oil return to compressor crankcase Fig. 13.13 Oil separator From receiver Liquid line drierTo Inspection glass Fig. 13.14 Sight glass 580 Reverse expansion valve – cold (closed) Filter Fins Condenser fan cvo cvc Discharge line Oil separator Reverse cycle valve Suction line Suction pressure valve (throttling valve) Suction valve Suction port Discharge valve Compressor Discharge port Pilot solenoid valve (closed) Remote feeler bulb Remote feeler bulb Evaporator coil Drier Thermostatic expansion valve (open) Fins Evaporator fan Sight glass Check valve open cvo Receiver cvc 2 4 5 1 cvc 3 Condenser coil (a) Refrigeration cycle Fig. 13.15 (a and b) Refrigeration system with reverse cycle defrosting 581 [...]... check valves (2), (3) and (5) are closed for the cool cycle 13. 4.2 Heating and defrosting cycle (Fig 13. 15(b)) With constant use excessive ice may build up around the evaporator coil; this restricts the air 5 83 14 Vehicle body aerodynamics The constant need for better fuel economy, greater vehicle performance, reduction in wind noise level and improved road holding and stability for a vehicle on the move,... transition point (Fig 14 .10( a and b)) A boundary layer over the forward surface of a body, such as the roof, will generally be lamina, but further to the rear a point will be reached called the transition point when the boundary layer changes from a lamina to a turbulent one, see Fig 14 .10( a) As the speed of the vehicle rises the transition point tends to move further to the front, see Fig 14 .10( b), therefore... valve – cool (closed) cvo 2 cvo 3 Fins Fins Condenser fan Evaporator fan cvc 4 Oil separator Reverse cycle valve Suction pressure valve (throttling valve) cvo 5 Suction valve Suction port Discharge port Check valve closed cvc 1 Pilot solenoid valve (open) Discharge valve Compressor Receiver (b) Heating and defrost cycle Fig 13. 15 Contd 582 Sight glass defrosting the cold compartment The operation of such... Fig 13. 15(b) Subcooled high pressure liquid refrigerant is permitted to flow from the receiver directly to the now partially opened reverse thermostatic expansion valve (due to the now hot remote feeler bulb's increased pressure) The refrigerant expands in the reverse expansion valve and accordingly converts to a liquid/vapour; this then passes through the condenser via the open check valve (3) in... restriction around the evaporator coil automatically triggers defrosting of the evaporator coil before ice formation can reduce its efficiency A manual defrost switch is also provided 13. 4.1 Refrigeration cooling cycle (Fig 13. 15(a)) With the pilot solenoid valve de-energized and the compressor switched on and running the refrigerant commences to circulate through the system between the evaporator and condenser... prompted vehicle manufacturers to investigate the nature of air resistance or drag for different body shapes under various operating conditions Aerodynamics is the study of a solid body moving through the atmosphere and the interaction which takes place between the body surfaces and the surrounding air with varying relative speeds and wind direction Aerodynamic drag is usually insignificant at low vehicle. .. rolling resistance over a typical speed range A vehicle with a high drag resistance tends only marginally to hinder its acceleration but it does inhibit its maximum speed and increases the fuel consumption with increasing speed Body styling has to accommodate passengers and luggage space, the functional power train, steering, suspension and wheels etc thus vehicle design will conflict with minimizing... ce tan ir a low ) 25 sis is res ( CD =0 rolling resistance 0 0 40 80 120 Vehicle speed (km/h) Fig 14.1 Comparison of low and high aerodynamic drag forces with rolling resistance 584 160 Outer layer Full velocity of air flow Thickness of boundary layer V5 Parabolic rise in air layer velocity from inner to outer boundary layer V4 V3 V2 V1 Viscous shear Surface of body Inner layer Fig 14.2 Boundary layer... Boundary layer velocity gradient Airstream Direction of plate drag Flat plate Fig 14 .3 Viscous resistance reading Spring scale Rollers Apparatus to demonstrate viscous drag the body then takes place within this boundary layer via the process of shearing of adjacent layers of air When air flows over any surface, air particles in intimate contact with the surface loosely attach themselves so that relative... there will be more resistance to air movement compared with a smooth surface 14.1.2 Skin friction (surface friction drag) (Fig 14 .3) This is the restraining force preventing a thin flat plate placed edgewise to an oncoming airstream being dragged along with it (see Fig 14 .3) , in other words, the skin friction is the viscous resistance generated within the boundary layer when air flows over a solid surface . bar (high pressure) ° Remote feeler bulb Receiver Sight glass Liquid line Drier Liquid/vapour mixture 40 C 10 bar° Subcooled liquid 30 C 10 bar° Expansion value Liquid/vapour mixture – 10 C 2 bar (low pressure) ° Saturated vapour 40 10 bar (high pressure) C° 6 5 4 7 8 1 2 3 Fig. 13. 6 Refrigeration. discharge pressure of P 2 (see Fig. 13. 8). 13. 3.2 Evaporator (Fig. 13. 6) The evaporator's function is to transfer heat from the food being stored in the cold compartment into the circulating refrigerant. evaporator coil can be absorbed through the metal walls of the tubing, see Fig. 13. 15(a and b). 13. 3 .3 Condenser (Fig. 13. 6) The condenser takes in saturated refrigerant vapour after it has passed