Advanced Vehicle Technology Episode 3 Part 10 pps

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

separator

Drier Expansion

Evaporator coil Evaporator

unit

Condenser coil

Condenser unit

Vee cylinder compressor Starter

motor In-line four cylinderdiesel 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 temper

Superheat temper

Saturated temperature

Heat increase (J)

Fig 13.5 Illustrative relationship between the refrigerant's temperature and heat content during a change of state

Latent heat of evaporation (Fig 13.5) This is

the heat needed to completely convert a liquid to

avapourandtakesplacewithoutanytemperaturerise

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 30Cand

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 10Cwith

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

7

8

3

Fig 13.6 Refrigeration vapour±compression cycle

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, 10Cand 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 8C; 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 60Cand 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 40C; 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 30C 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 revolusuc-tion; the out-ward moving piston from TDCto BDCforms the suction-stroke whereas the inward moving piston from BDCto TDCbecomes the discharge stroke

Suction stroke (Fig 13.7(a and b)) As the crank shaft rotates past the TDCposition 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 P1 to P2 as its volume expands from V1to V2, 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

P1to the cylinder's largest volume of V3, that is the piston's outermost position BDC, see Fig 13.8

Discharge stroke (Fig 13.9(c and d)) As the crankshaft turns beyond BDCthe 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 P1to P2through a volume reduction from V3to V4at which point the cylinder pressure

line

Suction line

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

P2

P1

Clearance volume

2

V p u r e p n sion

Vapour intake Swept volume

V ap

our

com pre

ssio n

3

V (TDC)1 V2 V4

Volume

V (BDC)3

Fig 13.8 Reciprocating compressor pressure-volume cycle

equals the discharge line pressure; the final cylinder

volume reduction therefore from V4back to V1will

be displaced into the high pressure discharge line at

the constant discharge pressure of P2(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

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 evaporator

Fig 13.9 (a and b) Thermostatic expansion valve

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)

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

Fig 13.11 (a and b) Reverse cycle valve

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 refrigerant to expansion valve

Fine filter Felt pad Coarse filter

Fig 13.12 Adsorption type liquid line drier

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

Inspection glass

Fig 13.14 Sight glass

Reverse expansion

valve – cold

(closed)

Filter

Fins

Condenser

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

cvc 3 Condenser coil

(a) Refrigeration cycle Fig 13.15 (a and b) Refrigeration system with reverse cycle defrosting