1335 Suction pressure valve throttling valve

This valve is incorporated in the compressor output suction line to limit the maximum suction

Adjusting Piston nut

Intake vapour from evaporator

Spring

Limiting pressure

- Bellows

Adjusting Piston nut

Intake vapour from evaporator

Spring

- Bellows

1 Pin

Spring

Flat valve (a) Valve fully open

Outlet vapour to compressor suction valves

Fig. 13.10 Suction pressure regulating valve (throttling valve)

Flat valve (a) Valve fully open

Outlet vapour to compressor suction valves

1 Pin

Spring

Limiting pressure

(b) Valve partially open

Fig. 13.10 Suction pressure regulating valve (throttling valve)

pressure generated by the compressor thereby safeguarding 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 combined 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 compressor (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 compressor 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 compressor 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 passage 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 piston. 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 compressor discharge

From evaporator coil

To compressor suction

To compressor discharge

Slave piston &

cylinder

Compressor discharge valve

From condenses coil

To compressor discharge

Slave piston &

cylinder

Compressor discharge valve

From condenses coil

Compressor .suction valves

From compressor discharge

To compressor evaporator suction coil

Compressor .suction valves

From compressor discharge

From evaporator coil

To compressor suction

From compressor discharge

To compressor evaporator suction coil

(a) Cooling cycle

(b) Heating cycle

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 compressor discharge valve closes the condenser to compressor suction passage and opens the evaporator to the compressor suction pressure.

Heat/defrost cycle mode (Fig. 13.11(b)) Energizing 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 compressor 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 compressor suction double valve closes the evaporator to the compressor suction passage and opens the condenser to compressor suction pressure.

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 normally 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 connections 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 substance has microscopic holes for the liquid refrigerant 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 expansion 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 compressor's lubrication oil in the following way:

1 During the cycle of suction and discharge refrigerant 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 components in the compressor will therefore increase.

Contaminated vapour/liquid mixture from receiver

Desiccant dehydrating material

Dry clean refrigerant to expansion valve

Contaminated vapour/liquid mixture from receiver

Desiccant dehydrating material

Dry clean refrigerant to expansion valve

Coarse filter

Fig. 13.12 Adsorption type liquid line drier

Separated oil return to compressor crankcase

Fig. 13.13 Oil separator

2 When the refrigerator is switched off the now static refrigerant in the evaporator may condense and enter the suction line and hence the compressor 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 separates 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 refrigerant 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 partitions 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 operating, 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.

The receiver is a container which collects the condensed 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 expansion valve.

This device is situated in the liquid line on the output 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.

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