Refrigeration Flashcards

Question

Why high pressure cut out is fitted ?

Answer 1

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

Answer 2

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.

Answer 3

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

Answer 4

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.

Answer 5

* 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.

Answer 6

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.

Answer 7

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.

Answer 8

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

Answer 9

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.

Answer 10

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.

Answer 11

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.

Answer 12

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.

Answer 13

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.

Answer 14

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.

Refrigeration Flashcards

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