BCIT 5th class boiler engineer quiz 6 Flashcards
Superheated refrigerant is refrigerant at saturation pressure, whose temperature is above the
a) saturation temperature.
b) compressor suction temperature.
c) sensible temperature.
d) fusion temperature.
e) conduction temperature.
a) saturation temperature.
B02 CH15 Q006
When a vapour is heated above its boiling temperature or saturation temperature it is said to be superheated vapour.
Mechanical refrigeration depends on the fact that
a) boiling point decreases as pressure increases.
b) heat flows from a colder to a hotter substance.
c) boiling point increases as pressure increases.
d) liquid refrigerants have high boiling points.
c) boiling point increases as pressure increases.
B02 CH15 Q004
The boiling point of water and the condensing point of its vapour can be raised by increasing the pressure in the vessel, and can be lowered by decreasing the pressure. We can see that a refrigerant vapour can be condensed at a high temperature as long as its pressure is also high enough. In order to expel the heat from the refrigerant, using normal air or water, we must therefore first increase its pressure. The high pressure vapour can then be condensed at a correspondingly higher temperature.
Ammonia refrigerant
a) will not mix with water.
b) is miscible only with hot lubricating oil.
c) will dilute the oil in the crankcase.
d) is not miscible with refrigerant lubricating oil.
e) is very miscible with refrigerant lubricating oil.
d) is not miscible with refrigerant lubricating oil.
B02 CH16 Q012
Miscibility: The ability of a refrigerant to be dissolved into the oil and vice versa is called miscibility. Refrigerant is in contact with lubricating oil in the crankcase and on the cylinder walls of reciprocating compressors and it will carry some of this oil into other parts of the system. Most refrigerants in the Freon group are miscible with oil. However, Freon-22 is only miscible under certain conditions. Ammonia (R-717) is not miscible with oil.
The actual amount of heat absorbed from the refrigerated medium by a mass unit of refrigerant in changing from a liquid at a given condensing pressure and temperature to a gas at a given evaporating pressure and temperature is called the
a) evaporation constant.
b) net chilling ratio.
c) miscibility ratio.
d) net refrigerating effect.
e) coefficient of cooling.
d) net refrigerating effect.
B02 CH16 Q010
The actual amount of heat absorbed from the refrigerated medium by a mass unit of refrigerant in changing from a liquid at a given condensing pressure and temperature to a gas at a given evaporating pressure and temperature is called the net refrigerating effect.
Freon 22 refrigerant
a) is a heat transferring medium.
b) is corrosive on copper.
c) is used widely in large packing plants.
d) is corrosive on aluminium.
e) is explosive when mixed with air.
a) is a heat transferring medium.
In the standard refrigeration cycle, VAPOURIZATION takes place at a temperature of
a) -15°C.
b) -7°C.
c) -12°C.
d) +30°C.
e) +12°C.
a) -15°C.
B02 CH16 Q008
Standard Conditions: To get a true indication of the difference in thermodynamic properties of refrigerants and their performance in a system, it is necessary to compare them under the same temperature conditions. These conditions are known as “standard conditions” and indicate a standard evaporator temperature of -15°C (5°F) and a standard temperature of 30°C (86°F) for the condenser and for the liquid supplied to the regulating valve.
A desirable quality of a refrigerant is
a) highly flammable for use in absorption systems.
b) a high boiling point at atmospheric pressure.
c) an offensive odour to easily detect leaks.
d) a high condensing pressure.
e) a high latent heat capacity.
e) a high latent heat capacity.
B02 CH16 Q004
Liquid refrigerants should possess certain characteristics which make them suitable for use in refrigerating systems. The ideal refrigerant should have:
- A low boiling point at atmospheric pressure
- A high latent heat capacity: that is, it should require a large amount of heat to convert it from a liquid to a gas after its boiling point has been reached
- A fairly low condensing pressure
- An inoffensive odour
- A nonpoisonous nature
- A noncorrosive action on metals
- A nonflammable and nonexplosive nature when mixed with air
- A reasonable price
The low-pressure side of a refrigeration system refers to the:
a) receiver outlet, regulating valve and evaporator.
b) compressor discharge and the condenser.
c) downstream side of regulating valve, evaporator and compressor suction.
d) condenser outlet, receiver and upstream side of regulating valve.
e) compressor discharge, condenser and receiver.
c) downstream side of regulating valve, evaporator and compressor suction.
B02 CH17 Q004
The low pressure side of the system includes the downstream side of the regulating valve, the evaporator, the suction of the compressor, and all interconnected tubing and piping. The pressure on this side is determined by the evaporation temperature required to obtain the proper refrigeration effect.
The heat given up by the brine in an indirect refrigeration evaporator must equal the heat
a) produced by the compressor.
b) absorbed by the condenser.
c) picked up by the refrigerant.
d) absorbed by the cooling water.
e) given up by the condenser.
c) picked up by the refrigerant.
B02 CH17 Q005
The indirect system is one in which a liquid, such as brine or water, is cooled by the refrigerant. This brine or water is then circulated by means of a pump to the material or space being refrigerated. The figure shows the arrangement of an indirect system. The evaporating coils are located within a tank of brine and, as the refrigerant is evaporating within the coils, the brine is cooled. The cold brine is then pumped through brine coils that are located within the space to be refrigerated. The brine absorbs the heat from this space and then circulates back to the brine tank to be cooled once again.
A refrigeration system which circulates brine to cooling coils in the cold rooms is said to be
a) flooded.
b) indirect.
c) wet.
d) dry expansion.
e) direct.
b) indirect.
B02 CH17 Q006
The indirect system is one in which a liquid, such as brine or water, is cooled by the refrigerant. This brine or water is then circulated by means of a pump to the material or space being refrigerated. The figure shows the arrangement of an indirect system. The evaporating coils are located within a tank of brine and, as the refrigerant is evaporating within the coils, the brine is cooled. The cold brine is then pumped through brine coils that are located within the space to be refrigerated. The brine absorbs the heat from this space and then circulates back to the brine tank to be cooled once again.
In a rotating sliding vane refrigeration compressor, a good seal is maintained between the vanes and the cylinder by
a) an eccentric mechanism.
b) high pressure refrigerant vapour.
c) a gear mechanism.
d) centrifugal force.
e) low suction pressure.
d) centrifugal force.
B02 CH18 Q005
The rotating vane compressor consists of a cylindrical rotor having radial slots into which sliding vanes fit. The rotor is mounted eccentrically in the cylinder so that it nearly touches the wall between suction and discharge ports. Heads or end plates close the cylinder off and hold the rotor shaft. During the operation the vanes move back and forth in the slots of the rotating rotor when they follow the contour of the cylinder wall but the edges are held firmly against the wall due to centrifugal force so that a good seal is maintained.
Calculate the piston displacement in cubic metres per minute for the following single stage, single-acting reciprocating compressor:
Stroke = 12 cm
Diameter = 10 cm
Speed = 1100 rev/min.
a) 1.027 cu m/min.
b) 0.654 cu m/min.
c) 1.017 cu m/min.
d) 0.937 cu m/min.
e) 1.037 cu m/min.
e) 1.037 cu m/min.
B02 CH18 Q007
Stages = 1
Stroke = 12cm = 0.12 m
Diameter = 10 cm = 0.1m
Speed = 1100 rev/min
V = A x L x N x R
V = 0.7854 x D2 x L x N x R
= 0.7854 x (0.1)2 x 0.12 x 1 x 1100
= 1.037 cu m/min. (ans.)
Centrifugal refrigeration compressors increase the pressure by
a) using rotary screws.
b) trapping the vapour in a pocket and reducing its volume.
c) accelerating the vapour.
d) increasing the temperature of the vapour.
e) increasing the volume of the vapour.
c) accelerating the vapour.
B02 CH18 Q007
In the centrifugal compressor, the vapour is given high velocity by means of a rapidly rotating impeller. Then the high velocity vapour travels through specially shaped passages of increasing cross-sectional area where the high velocity is converted to pressure.
The refrigeration condenser used most extensively in ammonia installations is the
a) atmospheric condenser.
b) double pipe condenser.
c) shell and coil condenser.
d) shell and tube condenser.
e) air cooled condenser.
d) shell and tube condenser.
B02 CH19 Q005
The shell and tube condenser consists of a welded steel shell containing a number of straight tubes fastened in the tube sheets which close the shell. The condenser is fitted with water boxes to which the cooling water supply and return pipes are connected, and which direct the water through the tubes in two or more passes. The hot refrigerant vapour enters at the top of the shell and condenses by coming in contact with the cool outer surface of the tubes. As the vapour condenses, the refrigerant drains down and collects on the bottom of the shell as a liquid. This type of condenser is used extensively in ammonia installations of all sizes, as well as in medium and large air conditioning installations using other types of refrigerants.
Refrigerant condensers
a) remove the latent heat from the gas.
b) are always water cooled.
c) cool the liquid refrigerant leaving the compressor.
d) keep the refrigerant from flooding the compressor.
e) remove only the sensible heat from the gas.
a) remove the latent heat from the gas.
B02 CH19 Q003
The function of the refrigeration condenser is to cool the hot compressed refrigerant vapour until it condenses to a liquid. To do this, the condenser must remove both the heat absorbed in the evaporator by the vapour, and the heat of compression added to it in the compressor.
Direct-expansion evaporators with more than one refrigerant circuit should be equipped with
a) a liquid distributor.
b) a fusible plug.
c) an emergency discharge valve.
d) an accumulator.
e) a high pressure cut-off device.
a) a liquid distributor.
B02 CH20 Q006
When a direct-expansion type evaporator has more than one refrigerant circuit in it, the liquid refrigerant entering the evaporator should be evenly distributed to all circuits. This is the function of the distributor which is placed in the liquid line directly downstream from the expansion valve.
A pressure relief device commonly used in smaller refrigeration systems is the
a) torsion bar safety valve.
b) atmospheric relief valve.
c) fusible plug.
d) weighted relief valve.
e) pop safety valve.
c) fusible plug.
B02 CH20 Q016
Two of the most common pressure relief devices used in a refrigeration system are the spring-loaded relief valve and the fusible plug. The fusible plug type relief device consists of a connection filled with a metal alloy designed to melt at a specific temperature. Because temperature of a refrigerant is related to pressure, refrigerant can be released by sensing the temperature before a dangerous pressure is reached. The plug melts when the rated temperature of the relief device is reached, allowing the entire refrigerant charge to escape. A new plug and a new refrigerant charge will then be required before the system can be put back into operation. Fusible plugs are commonly used in smaller systems. Since the loss of refrigerant in large systems is quite costly, the use of spring-loaded relief valves is more economical and more practical in such systems.
When copper tubing is used in a refrigerant application
a) it may only be connected using flanged joints.
b) it may only be connected using screwed joints.
c) it may only be connected by soft soldering.
d) only Types K or L should be used.
e) it must be at least Schedule 80.
d) only Types K or L should be used.
02 CH20 Q008
Several precautions pertaining to piping should be taken by a maintenance technician. Some of these are:
- Use proper material for piping. For all refrigerants except ammonia, refrigerant lines up to 100 mm in outside diameter may be either copper or steel. Steel should be used for all larger piping but wrought iron pipe is sometimes used due to its greater resistance to corrosion. The piping should be lap welded or seamless. Schedule 80 pipe should be used when the diameter of steel piping is 25 mm or less while schedule 40 may be used above this size, except that liquid lines over 40 mm should be schedule 80. Only type K or L copper tubing are suitable for refrigerant lines.
- Use a minimum number of fittings to avoid excessive pressure drop and to reduce the possibility of leaks.
- Special precautions should be taken when making soldered connections to use the proper solder and the correct soldering technique.
- Keep all piping clean to minimize corrosion.
- Horizontal lines should slope downward in the direction of refrigerant flow. The minimum recommended slope should be 4.2 mm per metre (0.5 in. for each 10 ft).
The gauge glass of a refrigerant receiver should be equipped with
a) coloured glass for ease of viewing.
b) a remote valve closing mechanism.
c) an electric lighting system.
d) an accumulator column to dampen pulsations.
e) safety type self-sealing valves.
e) safety type self-sealing valves.
B02 CH20 Q012
Receiver Gauge Glass:
The receiver should be fitted with a gage glass so the level of liquid can be readily seen. This is important since the amount of liquid present in the receiver is indicative of the efficiency of the system. The gage fittings should be of the safety type: that is, in the event of the glass breaking, the connection should be self-sealing in order to prevent loss of refrigerant, as shown in the figure.
The charging valve on a refrigeration system is used to:
a) add refrigerant to the system.
b) add oil to the compressor crankcase.
c) isolate the high and low pressure sides of the system.
d) isolate the condenser when charging the system.
e) remove non-condensable gases from the system.
a) add refrigerant to the system.
B02 CH20 Q014
Purge Valve and Charging Valve:
The purge valve and charging valve are usually packed or diaphragm type angle valves. The purge valve is used to vent non-condensable gases from the system to atmosphere. The charging valve is used to charge the system with refrigerant. The open ends of these valves are usually capped to prevent escape of refrigerant from the system when the valves are not in use.
To prevent leakage along the stem of a packed valve, refrigeration valves are often equipped with
a) a safety ball-check mechanism.
b) a sealed cap.
c) a leakoff connection to the compressor crankcase.
d) a grease nipple for injection of additional packing.
e) a secondary packing gland.
b) a sealed cap.
B02 CH20 Q010
As an added precaution against leakage along the valve stem, many packed valves are also equipped with a valve cap that covers and seals the valve stem. The cap must be removed before the valve can be operated.
Atmospheric natural draft cooling towers operate most effectively in large open spaces and where
a) wind velocity is relatively constant.
b) a windbreak shelters the tower.
c) a large body of water is nearby.
d) relative humidity is high.
e) the tower is not in direct sunlight.
a) wind velocity is relatively constant.
B02 CH21 Q004
Natural draft cooling towers are subdivided into:
- Atmospheric towers
- Chimney towers (which are used mainly in large generating stations)
Atmospheric towers are those in which the air movement through the tower is dependent on atmospheric conditions. They operate effectively only in locations where there are relatively constant winds and large open spaces.
During cooling tower operation, the amount of water that evaporates is approximately
a) 1%
b) 5%
c) 10%
d) 15%
e) 20%
b) 5%
B02 CH21 Q002
A cooling tower is a device that uses a combination of heat and mass transfer to lower the temperature of the cooling water. The actual transfer of heat is accomplished by a sensible heat transfer (heating the air passing through the tower), and by the removal of latent heat as a result of the evaporation of about 5% of the water. As this small portion of water evaporates, the heat required is drawn from the remaining water, thus cooling it.
Cooling tower distribution nozzles should be cleaned at least
a) once per shift.
b) weekly.
c) monthly.
d) every three months.
e) daily.
c) monthly.
B02 CH21 Q006
To ensure continuous, effective operation of the tower, a monthly inspection schedule should be implemented. This should include:
- Cleaning the louvres, piping, and nozzles to ensure that they are free of scale, algae, and dirt.
- Cleaning the water basin and checking for leaks.
- Cleaning the suction screen and checking that the float control valve is operating properly.
Water droplets entrained in the air leaving a cooling tower are removed by
a) surge louvres.
b) cyclone separators.
c) drift eliminators.
d) chevron scrubbers.
e) intake baffles.
c) drift eliminators.
B02 CH21 Q008
During operation, the fan forces air at a low velocity horizontally through the packing and then vertically against the downward flow of the water that occurs on either side of the fan. Water entrained in the air is removed by the drift eliminators located at the top of the tower. Vibration and noise are minimal since the rotating equipment is built on a solid foundation. The fans handle mostly dry air, greatly reducing erosion and water condensation problems.
Sensible heat removal from a substance
a) results in a change of state from liquid to solid.
b) is the sum of the latent heat plus heat of evaporation.
c) causes a drop in temperature.
d) cannot be read on a thermometer.
e) results in a change of phase from liquid to vapour.
c) causes a drop in temperature.
B02 CH15 Q005
Sensible heat is heat energy transferred between the surface and air when there is a difference in temperature.
The temperature at which a refrigerant boils is dependent upon the
a) heat content.
b) total evaporator heating surface.
c) boiling index.
d) absolute volume.
e) pressure acting on the surface.
e) pressure acting on the surface.
B02 CH15 Q003
In a closed vessel, the boiling point of water and the condensing point of its vapour can be raised by increasing the pressure in the vessel, and can be lowered by decreasing the pressure.
Ammonia refrigerant
a) is not miscible with refrigerant lubricating oil.
b) will dilute the oil in the crankcase.
c) is very miscible with refrigerant lubricating oil.
d) is miscible only with hot lubricating oil.
e) will not mix with water.
a) is not miscible with refrigerant lubricating oil.
B02 CH16 Q012
Miscibility: The ability of a refrigerant to be dissolved into the oil and vice versa is called miscibility. Refrigerant is in contact with lubricating oil in the crankcase and on the cylinder walls of reciprocating compressors and it will carry some of this oil into other parts of the system. Most refrigerants in the Freon group are miscible with oil. However, Freon-22 is only miscible under certain conditions. Ammonia (R-717) is not miscible with oil.
In the standard refrigeration cycle, VAPOURIZATION takes place at a temperature of
a) -7°C.
b) -15°C.
c) +12°C.
d) -12°C.
e) +30°C.
b) -15°C.
B02 CH16 Q008
Standard Conditions: To get a true indication of the difference in thermodynamic properties of refrigerants and their performance in a system, it is necessary to compare them under the same temperature conditions. These conditions are known as “standard conditions” and indicate a standard evaporator temperature of -15°C (5°F) and a standard temperature of 30°C (86°F) for the condenser and for the liquid supplied to the regulating valve.
The actual amount of heat absorbed from the refrigerated medium by a mass unit of refrigerant in changing from a liquid at a given condensing pressure and temperature to a gas at a given evaporating pressure and temperature is called the
a) evaporation constant.
b) miscibility ratio.
c) net chilling ratio.
d) coefficient of cooling.
e) net refrigerating effect.
e) net refrigerating effect.
B02 CH16 Q010
The actual amount of heat absorbed from the refrigerated medium by a mass unit of refrigerant in changing from a liquid at a given condensing pressure and temperature to a gas at a given evaporating pressure and temperature is called the net refrigerating effect.
Freon 22 refrigerant
a) is corrosive on aluminium.
b) is a heat transferring medium.
c) is used widely in large packing plants.
d) is explosive when mixed with air.
e) is corrosive on copper.
b) is a heat transferring medium.
B02 CH16 Q006
A desirable quality of a refrigerant is
a) a high latent heat capacity.
b) highly flammable for use in absorption systems.
c) an offensive odour to easily detect leaks.
d) a high boiling point at atmospheric pressure.
e) a high condensing pressure.
a) a high latent heat capacity.
B02 CH16 Q004
Liquid refrigerants should possess certain characteristics which make them suitable for use in refrigerating systems. The ideal refrigerant should have:
* A low boiling point at atmospheric pressure
* A high latent heat capacity: that is, it should require a large amount of heat to convert it from a liquid to a gas after its boiling point has been reached
* A fairly low condensing pressure
* An inoffensive odour
* A nonpoisonous nature
* A noncorrosive action on metals
* A nonflammable and nonexplosive nature when mixed with air
* A reasonable price
Refrigeration compressors
a) raise the refrigerant vapour temperature above the condensing water temperature.
b) must have water jackets.
c) determine the degree of superheat of the refrigerant leaving the evaporator.
d) must be placed outside the building.
e) raise the vapour pressure on discharge side of the evaporator.
a) raise the refrigerant vapour temperature above the condensing water temperature.
B02 CH17 Q003
To remove the latent heat from the refrigerant vapour in order to condense it seems simple, but there is one problem. If the latent heat in the refrigerant vapour is to flow to the condensing medium, the temperature of the vapour must be higher than that of the medium. However, the vapour leaving the evaporator is usually considerably cooler than the condensing medium. Therefore, in order to make it possible for the heat in the vapour to be absorbed by the condensing medium, the condensing point (saturation temperature) of the vapour must be raised above the temperature of this medium. This is the function of the compressor.
A secondary refrigerant commonly employed in the cooling coils beneath an arena’s ice surface is
a) calcium chloride brine.
b) ammonium chloride.
c) Freon 12.
d) ammonia.
e) water.
a) calcium chloride brine.
B02 CH17 Q007
The indirect refrigeration system is commonly used for skating and hockey rinks in which the brine is cooled to about -11°C (12°F) to provide the freezing effect for the ice surface. The brine used for this service is usually a solution of calcium chloride which can be subjected to temperatures as low as -50°C (-60°F) without freezing.
The heat given up by the brine in an indirect refrigeration evaporator must equal the heat
a) produced by the compressor.
b) absorbed by the cooling water.
c) given up by the condenser.
d) absorbed by the condenser.
e) picked up by the refrigerant.
e) picked up by the refrigerant.
B02 CH17 Q005
The indirect system is one in which a liquid, such as brine or water, is cooled by the refrigerant. This brine or water is then circulated by means of a pump to the material or space being refrigerated. The figure shows the arrangement of an indirect system. The evaporating coils are located within a tank of brine and, as the refrigerant is evaporating within the coils, the brine is cooled. The cold brine is then pumped through brine coils that are located within the space to be refrigerated. The brine absorbs the heat from this space and then circulates back to the brine tank to be cooled once again.
The theoretical mass of refrigerant moved by a compressor is 8.28 kg/min. If the volumetric efficiency is 75%, the actual mass moved is
a) 6.21 kg/min.
b) 2.07 kg/min.
c) 33.12 kg/min.
d) 8.28 kg/min.
e) 11.04 kg/min.
a) 6.21 kg/min.
B02 CH18 Q008
Transposing:
Actual mass = volumetric eff. x theoretical mass
= 0.75 x 8.28 kg/min
= 6.21 kg/min (Ans.)
To prevent damage due to entering liquid, reciprocating refrigeration compressors are equipped with
a) spring-loaded safety heads.
b) suction line rupture discs.
c) a quick-closing suction valve.
d) discharge line rupture discs.
e) a discharge relief valve.
a) spring-loaded safety heads.
B02 CH18 Q004
It is possible that under certain conditions liquid refrigerant may enter the compressor through the suction. To prevent damage to the compressor by the hydraulic pressure resulting from trapped liquid at the end of the compression stroke, compressors are often fitted with a safety head.
Valves commonly used in reciprocating refrigeration compressor heads are
a) poppet valves and ring valves.
b) disc valves and ball valves.
c) flexible ring and butterfly valves.
d) nonflexible ring and ball valves.
e) flexible reed and disc valves.
e) flexible reed and disc valves.
B02 CH18 Q003
Two basic types of compressor valves are commonly used. They are flexible reed and disc valves
In a double tube refrigerant condenser, the
a) refrigerant and water flow in opposite directions.
b) hottest refrigerant leaves at the point the cooling water enters.
c) refrigerant and water flow in the same direction.
d) cooling water flows in the outer tube and refrigerant in the inner tube.
e) cooling water must be at a higher pressure than the refrigerant.
a) refrigerant and water flow in opposite directions.
B02 CH19 Q004
A double tube condenser consists of a small tube contained concentrically within a larger tube as shown in Fig. Cooling water flows through the inner tube of the double tube condenser while refrigerant flows in the opposite direction in the space between the inner and outer tubes. This type has the lowest efficiency of the three designs, resulting in quite a large condenser for a given capacity. It is sometimes used as a booster condenser with air cooled condensers during periods of peak loading.
The refrigeration condenser used most extensively in ammonia installations is the
a) shell and tube condenser.
b) air cooled condenser.
c) shell and coil condenser.
d) atmospheric condenser.
e) double pipe condenser.
a) shell and tube condenser.
B02 CH19 Q005
The shell and tube condenser consists of a welded steel shell containing a number of straight tubes fastened in the tube sheets which close the shell. The condenser is fitted with water boxes to which the cooling water supply and return pipes are connected, and which direct the water through the tubes in two or more passes. The hot refrigerant vapour enters at the top of the shell and condenses by coming in contact with the cool outer surface of the tubes. As the vapour condenses, the refrigerant drains down and collects on the bottom of the shell as a liquid. This type of condenser is used extensively in ammonia installations of all sizes, as well as in medium and large air conditioning installations using other types of refrigerants.
Direct-expansion evaporators with more than one refrigerant circuit should be equipped with
a) an emergency discharge valve.
b) a fusible plug.
c) a high pressure cut-off device.
d) an accumulator.
e) a liquid distributor.
e) a liquid distributor.
B02 CH20 Q006
When a direct-expansion type evaporator has more than one refrigerant circuit in it, the liquid refrigerant entering the evaporator should be evenly distributed to all circuits. This is the function of the distributor which is placed in the liquid line directly downstream from the expansion valve.
The gauge glass of a refrigerant receiver should be equipped with
a) safety type self-sealing valves.
b) a remote valve closing mechanism.
c) coloured glass for ease of viewing.
d) an accumulator column to dampen pulsations.
e) an electric lighting system.
a) safety type self-sealing valves.
B02 CH20 Q012
Receiver Gauge Glass:
The receiver should be fitted with a gage glass so the level of liquid can be readily seen. This is important since the amount of liquid present in the receiver is indicative of the efficiency of the system. The gage fittings should be of the safety type: that is, in the event of the glass breaking, the connection should be self-sealing in order to prevent loss of refrigerant, as shown in the figure.
The charging valve on a refrigeration system is used to
a) remove non-condensable gases from the system.
b) add oil to the compressor crankcase.
c) add refrigerant to the system.
d) isolate the condenser when charging the system.
e) isolate the high and low pressure sides of the system.
c) add refrigerant to the system.
B02 CH20 Q014
Purge Valve and Charging Valve:
The purge valve and charging valve are usually packed or diaphragm type angle valves. The purge valve is used to vent non-condensable gases from the system to atmosphere. The charging valve is used to charge the system with refrigerant. The open ends of these valves are usually capped to prevent escape of refrigerant from the system when the valves are not in use.
A pressure relief device commonly used in smaller refrigeration systems is the
a) fusible plug.
b) torsion bar safety valve.
c) weighted relief valve.
d) pop safety valve.
e) atmospheric relief valve.
a) fusible plug.
B02 CH20 Q016
Two of the most common pressure relief devices used in a refrigeration system are the spring-loaded relief valve and the fusible plug. The fusible plug type relief device consists of a connection filled with a metal alloy designed to melt at a specific temperature. Because temperature of a refrigerant is related to pressure, refrigerant can be released by sensing the temperature before a dangerous pressure is reached. The plug melts when the rated temperature of the relief device is reached, allowing the entire refrigerant charge to escape. A new plug and a new refrigerant charge will then be required before the system can be put back into operation. Fusible plugs are commonly used in smaller systems. Since the loss of refrigerant in large systems is quite costly, the use of spring-loaded relief valves is more economical and more practical in such systems.
To prevent leakage along the stem of a packed valve, refrigeration valves are often equipped with
a) a safety ball-check mechanism.
b) a leakoff connection to the compressor crankcase.
c) a sealed cap.
d) a secondary packing gland.
e) a grease nipple for injection of additional packing.
c) a sealed cap.
B02 CH20 Q010
As an added precaution against leakage along the valve stem, many packed valves are also equipped with a valve cap that covers and seals the valve stem. The cap must be removed before the valve can be operated.
When copper tubing is used in a refrigerant application
a) it may only be connected using flanged joints.
b) it must be at least Schedule 80.
c) only Types K or L should be used.
d) it may only be connected by soft soldering.
e) it may only be connected using screwed joints.
c) only Types K or L should be used.
B02 CH20 Q008
Several precautions pertaining to piping should be taken by a maintenance technician. Some of these are:
- Use proper material for piping. For all refrigerants except ammonia, refrigerant lines up to 100 mm in outside diameter may be either copper or steel. Steel should be used for all larger piping but wrought iron pipe is sometimes used due to its greater resistance to corrosion. The piping should be lap welded or seamless. Schedule 80 pipe should be used when the diameter of steel piping is 25 mm or less while schedule 40 may be used above this size, except that liquid lines over 40 mm should be schedule 80. Only type K or L copper tubing are suitable for refrigerant lines.
- Use a minimum number of fittings to avoid excessive pressure drop and to reduce the possibility of leaks.
- Special precautions should be taken when making soldered connections to use the proper solder and the correct soldering technique.
- Keep all piping clean to minimize corrosion.
- Horizontal lines should slope downward in the direction of refrigerant flow. The minimum recommended slope should be 4.2 mm per metre (0.5 in. for each 10 ft).
During cooling tower operation, the amount of water that evaporates is approximately
a) 1%
b) 5%
c) 10%
d) 15%
e) 20%
b) 5%
B02 CH21 Q002
A cooling tower is a device that uses a combination of heat and mass transfer to lower the temperature of the cooling water. The actual transfer of heat is accomplished by a sensible heat transfer (heating the air passing through the tower), and by the removal of latent heat as a result of the evaporation of about 5% of the water. As this small portion of water evaporates, the heat required is drawn from the remaining water, thus cooling it.
Atmospheric natural draft cooling towers operate most effectively in large open spaces and where
a) the tower is not in direct sunlight.
b) a large body of water is nearby.
c) relative humidity is high.
d) wind velocity is relatively constant.
e) a windbreak shelters the tower.
d) wind velocity is relatively constant.
B02 CH21 Q004
Natural draft cooling towers are subdivided into:
- Atmospheric towers
- Chimney towers (which are used mainly in large generating stations)
Atmospheric towers are those in which the air movement through the tower is dependent on atmospheric conditions. They operate effectively only in locations where there are relatively constant winds and large open spaces.
Water droplets entrained in the air leaving a cooling tower are removed by
a) intake baffles.
b) drift eliminators.
c) cyclone separators.
d) chevron scrubbers.
e) surge louvres.
b) drift eliminators.
B02 CH21 Q008
During operation, the fan forces air at a low velocity horizontally through the packing and then vertically against the downward flow of the water that occurs on either side of the fan. Water entrained in the air is removed by the drift eliminators located at the top of the tower. Vibration and noise are minimal since the rotating equipment is built on a solid foundation. The fans handle mostly dry air, greatly reducing erosion and water condensation problems.
Cooling tower distribution nozzles should be cleaned at least
a) weekly.
b) daily.
c) monthly.
d) once per shift.
e) every three months.
c) monthly.
B02 CH21 Q006
To ensure continuous, effective operation of the tower, a monthly inspection schedule should be implemented. This should include:
- Cleaning the louvres, piping, and nozzles to ensure that they are free of scale, algae, and dirt.
- Cleaning the water basin and checking for leaks.
- Cleaning the suction screen and checking that the float control valve is operating properly.
