Refrigeration

Question 1

what are the desirable properties of a refrigerant?

Answer 1

1. low boiling point (otherwise operation at a high vacuum becomes necessary)
2. low condensing pressure (to avoid a heavy machine, plant scaling’s and to reduce the leakage risk)
3. high specific enthalpy of vaporisation (to reduce the quantity of refrigerant in circulation and lower machine speeds,, sizes, etc).
4. low specific volume in the vapour phase, which reduces the size of the plant required and increases efficiency
5. high critical temperature (temperature above which vapour cannot be condensed by isothermal compression)
6. noncorrosive and non-solvent (pure or mixed)
7. stable under working conditions
8. non-flammable and non-explosive (pure or mixed)
9. no action with oil (the fact that most refrigerants are miscible may be advantageous, i.e. allows removal of oil film, lowers pour point
10. easy leak detection
11. nontoxic (non-poisonous and non-irritating)
12. cheap, easily stored and obtained

Question 2

Why back pressure valve is fitted ?

Answer 2

It is fitted at the outlet of vegetable room to prevent under cooling of cargo.

Question 3

What are the safety devices in refrigeration system on ships?

Answer 3

1.L. P cut-out on compressor suction side: Set at a pressure corresponding to 5 °C below the lowest expected evaporating gauge reading

2.H.P cut-out on compressor discharge side: Set at a pressure corresponding to 5 °C above the highest expected evaporating gauge reading

3.Lube oil low pressure cut-out: Oil pressure usually set at 2 bar above crankcase pressure

4.Cooling water L .P cut-out in condenser side

5.Safety spring loaded liquid shock valve on compressor cylinder head

6.Bursting disc on cylinder head,
between inlet and discharge manifold

7.Bursting disc on Condenser, [if fitted]

8.Relief valve on Condenser; air purging valve on condenser

9.Master solenoid valve : to prevent liquid being entered into Compressor, when the plant is standstill, especially in Large Plant

Question 4

What is critical temperature?

Answer 4

Critical temperature is the temperature above which it is impossible to liquefy the refrigerant regardless of pressure.

Question 5

Difference between primary and secondary refrigerant?

Answer 5

Primary refrigerant is a medium which is used in a vapour compression cycle to remove heat from a low temperature region and transfer it to a high temperature region. It undergoes changes of physical state during its working cycles. Secondary refrigerant is a substance which is transferred heat by conduction and convection. Secondary refrigerants does not undergo refrigeration cycle but are used as a medium for cooling.

Question 6

What is the principle of refrigeration?

Answer 6

The principle of refrigeration is to provide a liquid at a suitable pressure so that it will boil and extract heat from the medium to be cooled at the particular temperature required.

Question 7

what is the purpose of a refrigeration system?

Answer 7

Refrigeration systems transfer heat from a lower temperature region to a higher temperature region, using a working fluid called refrigerant. The refrigerant undergoes phase changes from liquid to vapour and back to liquid as it circulates through the system, absorbing and releasing heat.

Question 8

What is the difference between a subcooled or undercooled liquid and a saturated liquid?

Answer 8

Compressed liquid (subcooled liquid or undercooled liquid) is a liquid existing at a temperature lower than saturation temperature (boiling point) for a given pressure, whilst a liquid exactly at saturation temperature is a saturated liquid. Saturated liquid is a liquid that is about to vaporize. An example would be water at atmospheric pressure, which is a subcooled liquid at 77 degrees Celsius and a saturated liquid at 100 degrees Celsius.

Question 9

What refrigerant is mostly used on ships?

Answer 9

The refrigerant used on ships is mostly Freon based and there are various types. R12 is common but tended to be replaced with R22. Because these refrigerants are CFC based (chlorofluorocarbon) and have a damaging effect on the ozone layer in the atmosphere, they are in turn being replaced with R134A or R417A which causes zero ozone depletion. Other options include carbon dioxide and ammonia, both previously widely used as refrigerants, but potentially dangerous if incorrectly handled.

Question 10

What are examples of leakage tests which can be carried out for various refrigerants?

Answer 10

CO2 – Soap and water solution

NH3 – Wet litmus paper ( Red to Green ); Sulphur candles, which gives off white dense smokes when contact with ammonia.

Freon – Soap and water solution / Halide torch /Electronic leak detector (buzzer sound)

Question 11

What are 5 parameters which should be observed to establish if the refrigeration plant is operating correctly?

Answer 11

• Correct oil level in the compressor,
• Suction and discharge pressures of the compressor are correct,
• Seawater inlet and outlet temperatures of the condenser are correct,
• Acceptable range of fridge room temperatures,
• No excessive frost at the thermostatic valves.

Question 12

What is Starting Sequence for a refrigeration plant?

Answer 12

1. Check oil level in the compressor’s sight glass
2. Check all gauges on compressor and condenser are working properly
3. Open air purge plug in front and rear covers of condenser water side to prevent water hammering
4. Ensure that the condenser cooling water valves-inlet and outlet, are fully open
5. Open the suction valve of the compressor till halfway and fully open the discharge valve
6. The crankcase heater on the compressor to be started at least 6 hours before the operation
7. Fully open the refrigeration inlet and outlet valves for condenser
8. Fully open the liquid outlet valve
9. Fully open the stop valve before and after the back pressure regulating valve provided in the vegetable room
10. Fully open the stop valve for the suction side of other rooms
11. Start the cooling water pump for condenser and purge the air from it
12. Check if the settings of HP and LP cut-outs are proper and then start the compressor
13. Gradually open the suction valve fully while checking the suction pressure, taking care that liquid doesn’t flow into the compressor
14. Switch on the ozone generator in the rooms where fitted
15. If any knocking noise is heard from the crankcase or excessive foaming of oil is detected, immediately throttle the suction valve
16. Increase the capacity gradually before allowing the next level, so that the compressor is adjusted to new conditions
17. Check the oil return pipe from the oil separator is warm
    “Check the temperature of oil return piping between oil separator and compressor. If it is slightly hot than the ambient temperature, plant is in normal operation.

Question 13

What is the Stopping Sequence for a refrigeration plant?

Answer 13

1. If the refrigeration system is to be shut down for a prolonged period, it is essential to pump down the system and isolate the refrigerant gas charge in the condenser. Leaving the system with full refrigerant pressure in the lines increases the tendency to lose charge through the shaft seal.
2. Shut the liquid outlet valve on the condenser and the outlet from the filter.
3. Run the compressor until the low-pressure cut-out operates. The refrigerant gas will be condensed and will remain in the condenser as the condenser outlet valve is closed.
4. After a period of time the suction pressure may rise, in which case the compressor should be allowed to pump down again. This procedure should be repeated until the suction pressure remains low and the compressor does not start again automatically.
5. Stop compressor and shut the compressor suction and discharge valves.
6. Close the cooling water inlet and outlet valves and drain the condenser of water.
7. The compressor discharge valve should be marked closed and the compressor motor isolated, in order to prevent possible damage.
8. Trip the breaker for the compressor “In case the temperature is below 0 °C while stopping the condenser, it is necessary to drain the condenser off water to prevent damage to the tubes and the cover.”

Question 14

What Maintenance has to be carried out for a refrigeration system?

Answer 14

The basic everyday maintenance of marine refrigeration machinery includes the following.

• Maintain level and quality of oil in compressor – change if required.
• Maintain the quantity of refrigerant in the system – add or remove as required.
• Check for air in the system – purge if required.
• Keep coils of evaporator and condenser clean.
• De-ice the evaporator regularly.
• Keep belt drives / flexible couplings tight – adjust when required.
• Check holding down bolts – tighten when required.
• Keep to planned maintenance schedules.

However, if a fault occurs, then additional maintenance would be required.

Question 15

(a) Sketch a simple refrigeration system showing the FOUR major components. (8)

(b) State the condition of the refrigerant between EACH component in the sketch of Q2(a). (8)

Answer 15

a.) see EOOW ORAL Sketch Pack for drawing

b.) Evaporator - Compressor = low pressure superheated gas.

Compressor - Condenser = high pressure superheated vapour.

Condenser - Thermostatic Expansion Valve = Sub cooled high pressure liquid.

Thermostatic Expansion Valve - Evaporator = low pressure liquid & gas mixture

Question 16

(a) Describe the operation of a domestic refrigeration plant, referencing the refrigerant condition at the main components. (12)

Answer 16

Refrigerant is added into the suction side of the compressor and then compressed from low pressure superheated gas to a high pressure superheated gas. Then the high pressure superheated gas travels to the condenser which removes the sensible and latent heat and thereby converting the Refrigerant to a high-pressure subcooled liquid. Undercooling prevents the refrigerant flashing of to a gas before it reaches the evaporator. The high-pressure subcooled liquid then passes across the expansion valve where it drops in pressure and temperature and becomes a low-pressure liquid and gas mixture. The refrigerant then enters the evaporator coil. The relatively warm air of the refrigeration chamber is blown across the cool evaporator coil by fans causing the cold refrigerant to absorb the heat from the air and evaporate to become a low pressure superheated gas.

Question 17

Explain the function of an expansion valve?

Answer 17

The expansion valve is the regulator through which the refrigerant flows from the high-pressure side of the system to the low-pressure side. Its throttling effect dictates the compressor delivery pressure which must be sufficient to give the refrigerant a saturation temperature which is higher than the temperature of the cooling medium. The pressure drop through the regulator causes the saturation temperature of the refrigerant to fall, so that it will boil at the low temperature of the evaporator. As the liquid passes through the expansion valve the pressure drop makes its saturation temperature fall below its actual temperature. Some of the liquid boils off at the expansion valve, taking latent heat from the remainder and causing its temperature to drop. This also prevents liquid refrigerant being entered into the compressor. The expansion valve throttles the liquid refrigerant and maintains the pressure difference between the condenser and evaporator, while supplying refrigerant to the evaporator at the correct rate. It is thermostatically controlled in most of the systems.

The pressure drop across the regulator valve is directly related to the temperature of the refrigerant at the outlet from the evaporator, and the regulator is adjusted at installation of the plant so that the gas leaves the evaporator just slightly superheated. A small quantity of system gas is sealed in a bulb and capillary tube, with the bulb fastened to the refrigerant outlet from the evaporator and the capillary tube connected to the upper side of a bellows unit in the regulator. Temperature changes in the refrigerant gas leaving the evaporator are sensed by the gas in the bulb-capillary system expanding or contracting. This will move the bellows, which will alter the opening of the valve by means of the pushpins. Reduced flow through the evaporator coils will result in a higher outlet temperature which will expand the fluid in the bulb and capillary, increase the pressure on the top of the bellows and open the regulator valve to pass more refrigerant liquid. A flooded evaporator will reduce the outlet temperature of the refrigerant and the regulator will close in to reduce refrigerant flow. The equalizing line connection from the evaporator outlet is fed to the inside of the bellows, but this pressure is more than balanced by the pressure in the bulb capillary tube acting on the top of the bellows. This greater pressure is in result of saturation temperature, plus some superheat. This overcomes the spring force which is tending to close the regulator. A correctly operating expansion valve will have frosting on the outlet side but not on the inlet side.

Question 18

What are the types of expansion valve?

Answer 18

• Thermostatic control
• Electronic control
• Manual control

Question 19

Explain the function of the Evaporator?

Answer 19

The refrigerant enters the evaporator coil, at a temperature lower than that of the surrounding secondary coolant (air or brine) receives latent heat and evaporates. Later the heat is given up in he condenser when the refrigerant is again compressed and liquefied. For a small refrigerator the evaporator cools without forced circulation of a secondary coolant. In larger installations, the evaporator cools air or brine which are circulated as secondary refrigerants. This part, usually, comprise finned tube coils encased in a metal housing and at least one fan to circulate air over the coils. In large evaporators the coils are formed by steel tubes and fins, galvanised externally for protection against corrosion. In small evaporators, the coils are formed by copper tubes and fins. When operating at temperatures below 0 degree C, the evaporator (air cooler) dehumidify the air causing frost to collect on the surfaces of the coils. This restricts heat transfer and air flow which results in reduced capacity of the cooler and must be removed. Most common defrosting methods are (a) hot gas defrosting and (b) electric defrosting. The drier trays and lines fitted under the cooler to collect the defrost water are also heated during the defrost cycle in order to prevent re-freezing of the water. The defrost cycle may be initiated either manually or by an automatic device. The fans are stopped during the defrost period to prevent the heat of defrost and water being blown into the refrigerated space.

Question 20

Explain the function of the Condenser.

Answer 20

In the condenser the refrigerant is liquefied by being sub cooled to below the saturation temperature relating to the compressor delivery pressure, by circulating seawater (or air for domestic refrigerators). Latent heat, originally from the evaporator, is thus transferred to the cooling medium. The liquid refrigerant, still at the pressure produced by the compressor, passes to the receiver and then to the expansion valve. Shell and tube type condenser is the standard type of condenser used, with seawater circulating through the tubes and refrigerant condensing in the shell. Condensers are designed to resist corrosion, erosion and fouling. A typical condenser’s tubes are made of aluminium brass which are expanded into copper alloy clad mild steel tube plates. Cast iron, cast bronze or mild steel end covers may be employed and the water velocity is chosen to resist erosion. Corrosion plugs of iron are often fitted in the end covers to reduce the Galvanic corrosion from the seawater. An air purge connection is fitted on top of the shell to facilitate the purging of air and other non-condensable gases. Some larger condensers may have two liquid refrigerant outlets, one at either end. This is to allow the refrigerant to drain away easily when the vessel is pitching or rolling.

Question 21

Briefly describe the function of the following three components which can be added to a refrigeration system.

Oil separator.
Drier
Liquid line sight glass

Answer 21

Oil separator.

Oil separators are essential for screw compressors and may be fitted with other types depending on design. The oil separator is fitted in the compressor discharge line after the compressor. It is fitted to remove as much of the oil carried over by the refrigerant as it leaves the compressor and return that oil to the compressor crankcase. Oil and gas enter the separator where the heavier oil collects at the bottom, more oil is separated by a sudden change in direction, when enough oil has collected its level rises and operates a float valve that returns the oil to the crankcase.

Drier

A drier chamber containing activated alumina pellets or silica gel is necessary to remove any water from the refrigerant. The chamber is located after the liquid receiver and will have a by-pass arrangement to allow for re-charging without shutting down the refrigeration plant. Moisture in the system can freeze inside the expansion valve and disrupt its operation. Most systems have renewable drier units and the state of the gas is indicated by the sight gas moisture indicator.

Liquid Line Sight Glass

These are fitted on the liquid line after the condenser, they are used to give an indication of the system refrigerant charge (bubbles of vapour in the sight glass will indicate an undercharge). They normally also include a moisture indicator, which will change colour if moisture is detected in the refrigerant.

Question 22

what is the procedure for stopping the refrigerant compressor?

Answer 22

Close the condenser liquid outlet valve and the outlet from the filter.

Allow the compressor to pump down the system to the condenser so that the low-level pressure cut-out operates.

Isolate the compressor motor.

Close the compressor suction valve.

Close the compressor discharge valve.

Close the inlet and outlet valves on the cooling water supply to the condenser.

Switch on the crankcase heater
It is important to note that a fully closed suction valve with the compressor running might cause.
foaming of the lubricating oil in the crankcase.

Question 23

Why Refrigeration Compressor Takes Suction from Crankcase?

Answer 23

Unlike air compressors, it is common in reciprocating type refrigeration compressors to take suction from crankcase. Outlet from the evaporator coils is led to the compressor crankcase.

This is done due to the following advantages.

1.Since crankcase is pressurized, no air can enter the system.

2.No refrigerant gas is wasted by even small blow pasts from the compressor pistons.

3.Refrigerant gas is miscible with oil. This property helps the gas to bring the oil in the system back to the compressor. In some designs, oil in the refrigerant gas drips inside the crankcase before leading it for compression.

Question 24

What is short cycling ?

Answer 24

A condition of a compressor unit repeatedly running for a few second and then cutting out.

Question 25

Why high pressure cut out is fitted ?

Answer 25

It is fitted to shut down the compressor in the event of high pressure. After remedy the fault, it must be reset manually.

Question 26

with reference to a compressor what kind of maintenance is carried out for the following

oil filter

suction filter

Answer 26

Oil Filter

The oil filter should be cleaned at regular intervals. Please note in this connection that often the filter must be cleaned already after a short operating period following the initial start-up. This is a consequence of the tiny dirt particles that will be coming from the plant during its first operating period. Clean the oil filter in a suitable dissolvent and blow clean with pressurized air before refitting.

Suction Filter

Between suction stop valve and compressor a fine-meshed filter has been fitted. The purpose of this filter is to prevent that impurities from the plant are conveyed with the gas flow into the compressor. Clean the suction filter at regular intervals as stated in the section on servicing the reciprocating compressor. On cleaning the filter dismantle suction stop valve by removing screws. The filter and gaskets can be removed without the use of any tools. Clean the filter in a suitable solvent and blow clean with pressurized air.

Question 27

With reference to compressors list the Possible Causes of Failure for a Mechanical Seal

Answer 27

Lack of lubrication (insufficient oil supply, high refrigerant concentration in the oil)

Heavy wear of driving parts (high proportion of dirt in the oil)

Overheating (hardening and cracking of O-rings, oil carbon)

Strong vibration (insufficient fixing of coupling or drive pulley, drive not smooth enough, coupling or drive pulley displaced)

Belt tension too high

Question 28

What is the function of a Shaft Seal or Mechanical Seal for a compressor?

Answer 28

Reciprocating compressors are fitted with high quality shaft seal, which consists of a rotating part and a stationary unit. A routine inspection of the shaft seal is not normally necessary because it tends to be highly reliable. Shaft seals prevent liquid escaping from a pump’s rotating or reciprocating shaft.

Question 29

What is the purpose of the built in Heating Rods in the crankcase for Refrigeration Compressor?

Answer 29

• The purpose of the heating coil or rod is to keep the oil in the crankcase warm even during standstill of the compressor. This ensures a low content of refrigerant in the oil.
• Too much refrigerant in the oil makes it loose its lubricating properties. This may lead to damage of the movable parts in the compressor.
• Further, the danger exists that the oil, during start-up of the compressor, foams so vigorously that the lubricating pressure will disappear.
• Before start-up the heating rod should be switched on for at least 8 hours.
• The heating coil or rod must not be switched on if the oil level in the vessel is below minimum in the sight glass. While the compressor is operating, it is usually switched off.
• Further, remember to switch off the heating rod if the compressor crankcase is opened for inspection.

Question 30

State how liquid is prevented from returning to the compressor. (4)

Answer 30

To stop liquid returning to the compressor, a solenoid is connected to a thermostat inside the evaporator. As the room is cooled to a set temperature, the solenoid shuts off the thermostatic expansion valve, stopping the flow of refrigerant in the system. Solenoid valve is a liquid stop valve. The valve is either fully open or closed. It is a electromagnetic cut in cut out device when the room temperature reaches the desire point, the pressure switch in the room shuts the solenoid valve which stops the refrigerant flow into the evaporator. The compressor will continue to run briefly but will be stopped once there’s a drop in pressure. If compressor stops due to a fault, there is a master solenoid valve which will close to prevent liquid refrigerant from entering compressor and causing damage.

Question 31

With reference to a refrigeration system, state the indications of the following fault:

air in the system; (8)

Answer 31

1. Air in the refrigeration system increases the total head pressure, which will equal the refrigerant condensing pressure plus the pressure of air in the condenser. This higher pressure results in the compressor having to work harder with the potential for the associated higher temperatures causing oil problems. Damage can also be caused to compressor valve plates, valve seats, head gaskets etc. by the higher pressures and temperatures.
2. High Discharge Temperatures — These are caused by high compression ratios. High heats of compression are associated with high compression ratios. High compression ratios are associated with high condensing (head) pressure. The compressor has to compress suction vapours through a greater pressure range; thus, more heat is generated.
3. High Condensing (Head) Pressures — High head or condensing pressures are generated from the air taking up condensing surface volume at the top of the condenser. Because the air stays at the top of the condenser and doesn’t condense, it leaves a smaller condenser to condense, and sub cool the refrigerant.
4. High Condenser Sub cooling — The elevated condensing temperatures and pressures make the subcooled liquid in the bottom of the condenser hotter. Now there is more of a temperature difference between the subcooled liquid and the ambient to where heat is rejected. This will increase the rate of heat transfer from the subcooled liquid because the temperature difference is the driving potential for the heat transfer to take place. The higher sub cooling does not necessarily mean there is more liquid at the condenser’s bottom, it just means there is more cooling of the same amount of liquid to make the temperature difference greater. Remember, condenser sub cooling is a temperature difference between the liquid temperature at the condenser outlet and the condensing temperature.
5. High Condenser Splits — Because the air is sitting at the top of the condenser, causing elevated condensing pressures and temperatures, the temperature difference between the surrounding ambient and the condensing temperature will be high. This temperature is defined as the condenser split.
6. High Compression Ratios — The higher condensing (head) pressures will cause the compression ratio to increase, causing low volumetric efficiencies and loss of capacity.

Question 32

With reference to a refrigeration system, state the indications of the following fault:

undercharge of refrigerant gas. (8)

Answer 32

1. The compressor is running hot, and the performance of the compressor falls due to high temperature at the suction side of the compressor.
2. The suction and discharge pressure for the compressor is low.
3. Large vapour bubbles in the liquid sight glass.
4. Low gauge reading for condenser.
5. low current value for the compressor motor
6. In-effective cooling or No liquid visible within condenser sight-glass
7. Partial blockage of the refrigerant at the filter or drier or evaporator may cause undercharging.
8. Low discharge pressure and Evaporator High superheated temp at evaporator or low temperature at condenser

Question 33

With reference to a refrigeration system

a) Describe how air is removed from a system (8)

Answer 33

a.) Outlet valve from condenser is closed and compressor used to pump refrigerant down into condenser until the compressor cuts out on low pressure and the majority of the refrigerant charge has accumulated in the condenser. after allowing settling time for the air to rise to the top of the condenser a vacuum pump is then attached to a recovery bottle which is connected to the top of the condenser to remove the air. The recovery bottle is a device that collects and stores the excess or contaminated refrigerant from the system. The recovery bottle has a valve that allows refrigerant to enter or exit, and a float switch that automatically shuts off the recovery machine when it reaches 80% of its capacity. The recovery bottle also has a label that indicates the type and amount of refrigerant it contains. Connecting the vacuum pump to a recovery bottle ensures no refrigerant gas escapes to atmosphere.

To correctly use the recovery bottle, follow these guidelines:

Use only approved recovery bottles that are compatible with the type of refrigerant you are using. Do not mix different types of refrigerants in the same bottle, as this can cause chemical reactions, pressure changes, and performance issues.

Check the label on the recovery bottle before using it. Make sure that it matches the type of refrigerant you are using, and that it has enough space to accommodate the amount of refrigerant you are transferring. If the label is missing or damaged, do not use the bottle.

Open the valve on the recovery bottle and turn on the recovery machine. Monitor the weight reading on the scale and the pressure reading on the gauge. Do not exceed 80% of the capacity of the bottle, as this can cause overfilling and possible explosion of the bottle. The float switch will automatically shut off the recovery machine when it reaches 80% of its capacity, but you should also keep an eye on it as a backup.

Close the valve on the recovery bottle and turn off the recovery machine when you are done transferring refrigerant. Disconnect the outlet hose from the bottle and cap it with a protective cap.

Update or replace the label on the recovery bottle with accurate information about the type and amount of refrigerant it contains. Store or transport the recovery bottle in a safe and secure place, away from heat sources, sparks, flames, or direct sunlight.

Question 34

Describe how and where refrigerant gas is added to a system (8)

Answer 34

Recharge the system with refrigerant gas on low pressure side of the compressor, which is the suction side. Using a compound metre gauge set, attach a charging line and pump to the suction side of the compressor. Then throttle the liquid discharge valve from the charging bottle to flash off the liquid refrigerant so it can enter the compressor as a superheated gas. ensure no liquid refrigerant enters the compressor, as it causes damage because you can’t compress a liquid. You may have a ‘flash off’ chamber in the charge line which prevents liquid passing to the compressor. Start the compressor, monitor the condenser sight glass to prevent overfilling of refrigerant. Once filling is complete the quantity of gas and refrigerant type is logged in refrigerant systems and gas record book. The volume of refrigerant added is calculated from the change in weight of the charge bottle.

Question 35

describe the common refrigeration system faults

Answer 35

Common system faults:

1. Undercharge of refrigerant: If the charge of refrigerant is insufficient, it is difficult to maintain the correct working levels of liquid in the condenser and in the evaporator. A refrigerator cannot operate correctly if it is undercharged because the refrigerating capacity depends directly on the mass of refrigerant in circulation. If the evaporator is undercharged then the effective coil surface which is absorbing heat by the evaporation of the liquid refrigerant is much reduced. The liquid evaporates so quickly that the vapour, in passing through the remaining portion of the coils, becomes too highly superheated before leaving the evaporator. This, combined with the reduced charge of refrigerant, causes the compressor to run at higher temperatures. Symptoms are lack of frost on the suction pipe and lengthy running times.
2. Overcharge of refrigerant: If there is too much refrigerant in the system the liquid level in either the condenser or the evaporator will be too high. If the excess is in the condenser there will be insufficient condensing surface, and the pressure will be excessive. If it is in the evaporator, complete evaporation cannot take place, the compressor will run cold and compression knock may occur due to liquid refrigerant entering from the evaporator. Provided that the charge can be accommodated in the condenser without raising pressure above the safe limit, it is better to be overcharged than undercharged.
3. Air in the system: This can be detected by the jumping of gauge pointers and high discharge gauge reading (high condenser pressure). Air will cause the compressor to overheat, and the temperature and pressure in the condenser will be too high. Air being lighter than the refrigerant will collect at the top of the condenser. The remedy is to pump the charge into the condenser with the liquid stop valve shut and, after waiting a few minutes, to blow off the air from a convenient fitting or purge valve at the top of the condenser. During this operation, which is known as purging, the pressure should not exceed recommended levels.
4. Oil contamination: This can be detected by incorrect condenser and evaporator temperature differentials and excess frost on the suction pipe. Oil may produce partial fouling of the condenser or evaporator and result in a reduction of heat transfer owing to the formation of an oil film of low conductivity. Some oil will always be carried round the circuit, but the amount should be kept to a minimum.
5. Moisture in the system: This will cause choking of the expansion valve with ice which can cause a drop in pressure in the evaporator. The drier should be checked with the drying chemicals being replaced if required. This will cause choking of the expansion valve with ice. Every precaution must be taken to prevent moisture entering the circuit, e.g. by sealing pipe connections, etc, during maintenance. After refit the circuit must be thoroughly dehydrated by creating a high vacuum for several hours, and charging should always be done with a drier in the charging line as well as in the circuit itself.
6. Dirty condenser or insufficient cooling water: This will show as a high discharge gauge reading and incorrect condenser temperature differential readings. If condenser tubes or suction strainer are clogged, cleaning will be required. The operation of the cooling pump should also be investigated.

Question 36

what is ozone layer?

Answer 36

Ozone is made up of three oxygen atoms joined together. The ozone layer is located in the stratosphere, which is about 10-25miles above the earth’s surface. Its function is to reflect back into space the ultraviolet radiation that comes from the sun. Any reduction in this protective ozone layer would mean that more harmful radiation would reach the surface of the earth. The ozone layer is created and destroyed by natural processes. The ozone layer has been found to be depleted by certain natural processes such as Sunspots and stratospheric winds. Its destruction has increased due to the development and release of chlorofluorocarbons (CFCs) into the atmosphere. The natural production of Ozone couldn’t keep up with CFCs, and so the ozone layer was reduced. Since then, it was discovered that CFCs are ozone depleting substances (ODS), and the use of gases containing CFCs has therefore been stopped in most parts of the world. Natural production of ozone layer occurs because Ozone forms when sunlight breaks apart oxygen molecules (O2) and free oxygen atoms then bond with other O2 molecules forming O3. Ozone is destroyed when it reacts with other molecules, such as nitrogen, hydrogen, chlorine, and bromine.

Question 37

How do CFCs harm the ozone layer?

Answer 37

CFCs are very stable chemical that aren’t broken down by water and other chemicals in the lower part of the atmosphere. Meaning they remain intact until they reach the stratosphere where they are struck by intense radiation from the sun. Then CFCs decompose under the sun’s UV radiation, and chlorine is released, which interacts with the ozone layer and is harmful to it and leads to the ozone layer being slowly depleted.

Question 38

What chemicals are most harmful to the ozone layer?

Answer 38

Most harmful chemicals to ozone layer are CFCs, carbon tetrachloride, methyl bromide, methyl chloroform and halons. As well as Hydrochlorofluorocarbon (HCFC), which is a compound composed of hydrogen, chlorine, fluorine and carbon atoms. It being used temporarily as a replacement. Introducing hydrogen means that a chemical compound can break down in the lower part of the atmosphere before it reaches the stratosphere. certain portion of HCFC molecules released to the atmosphere will reach the stratosphere and be destroyed there by photolysis (light-initiated decomposition). The chlorine released in the stratosphere then participates in ozone depleting reactions.

R11 (CClF ),
R12 (CCl2F2),
R22 ( CHClF2)
are all CFC gases so are all harmful

Refrigerants that do not affect Ozone layer:

NH3, =ammonia
R134A - HFC gas

Ammonia does not harm atmospheric ozone. Ammonia is a natural refrigerant. It is not a halocarbon like many of the synthetic refrigerants on the market. When halocarbons are released into the atmosphere, they eventually reach the stratosphere and the ozone layer.
Because R134A has no chlorine in its molecule, it has a zero ozone-depletion potential (ODP) and doesn’t deplete the stratospheric ozone layer. its a HFC gas. The hydrofluorocarbons are refrigerants that contain no chlorine and are not harmful to the ozone layer. However, their impact on global warming is very large compared with traditional refrigerants.