133 Refrigeration system components

A description and function of the various components 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 evaporator 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 refrigerant 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 evaporator coil.

The reciprocating compressor completes a suction and discharge cycle every revolution; the outward 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 commences 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 V1 to V2, the vapour refrigerant 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 P1 to 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 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 P1 to P2 through a volume reduction from V3 to V4 at which point the cylinder pressure

Discharge line

Suction line

Low pressure vapour refrigerant from evaporator

High pressure vapour refrigerant to condenser i

Cylinder head Piston ring

Piston Gudgeon pin

Connecting rod

Cylinder wall

Crankshaft

Crankcase

Sump

Discharge line

Suction line

Low pressure vapour refrigerant from evaporator

Crankcase

Sump

(a) Piston at TDC both valves (b) Piston on downward closed high pressure vapour suction stroke vapour trapped in discharge line and refrigerant drawn into clearance volume 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

P

1 Vap°urfcharge 4

\

P

Vapour intake

Clearance 2j volume

Swept volume

> 3

! : ! i 1 1 1

Vi (TDC) V2

V4

Fig. 13.8 Reciprocating compressor pressure-volume cycle

(attached to output side of evaporator) (cold)

Adjustment screw

Feeler Lp bulb

(attached to output side of evaporator) (hot)

(attached to output side of evaporator) (cold)

(a) Valve closed

Fig. 13.9 (a and b) Thermostatic expansion valve

Adjustment screw

Feeler Lp bulb

(attached to output side of evaporator) (hot)

(b) Valve open equals the discharge line pressure; the final cylinder volume reduction therefore from V4 back to V1 will be displaced into the high pressure discharge line at the constant discharge pressure of P2 (see Fig. 13.8).

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 chamber atmosphere.

The evaporator consists of copper, steel or stainless 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).

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 convection to the surrounding atmosphere.

A condenser consists of a single tube shaped so that there are many parallel lengths with semicircular 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 convection 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 surrounding atmosphere. This process is known as forced air convection.

13.3.4 Thermostatic expansion valve

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 refrigerant, 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 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 refrigerant 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 output from the evaporator. This cycle of events is a continuous process in which the constant superheated 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 chamber now requires a higher remote feeler bulb pressure to open the expansion valve.

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