BCIT 5th class boiler engineer quiz 7 Flashcards
For ventilation, a forced air furnace for a house typically utilizes
a) 50% outdoor air and 50% recirculation.
b) 100% outdoor air.
c) a fixed percentage of 10% outdoor air.
d) a variable rate of outdoor air, depending upon time of year.
e) 100% recirculation air.
c) a fixed percentage of 10% outdoor air.
B02 CH22 Q010
In Fig.(b) the unit is operating on approximately 90% recirculation and a fixed 10% of outside air which is drawn through a connection from the return air duct to outdoors. This “minimum outdoor air” arrangement is typically utilized with a forced air furnace to provide ventilation for a house.
In air conditioning systems, the type of air intake arrangement in which “free cooling” is achieved is
a) the 100% recirculation arrangement.
b) the fixed percentage of outdoor air arrangement.
c) the by-pass air arrangement.
d) the mixed air arrangement.
e) the 100% outdoor air arrangement.
d) the mixed air arrangement.
B02 CH22 Q012
The mixed air system is the most commonly used arrangement in central and combined systems for larger buildings. When the unit is operating on full outdoor air, all of the return air must be released from the building. The mixing dampers are usually automatically controlled to provide a mixed air temperature of approximately 12.5°C (54.5°F), which is the desired supply air temperature to the building for cooling purposes. The amount of outdoor air for a 12.5°C (54.5°F) mixed air temperature is reduced to approximately 15% during the coldest winter days, but increases as the outdoor temperature rises, and is 100% at 12.5°C . In this temperature range, the desired 12.5°C supply air temperature for building cooling can be achieved by the operation of the mixed air dampers, so mechanical cooling is not required. This is called “free cooling” and results in a significant saving in energy consumption compared to systems which do not have this feature.
A basic forced air furnace installed in most residential homes can provide
1. humidification.
2. fresh air mixing.
3. air filtration.
4. dehumidification.
5. air heating.
a) 2, 3 and 5.
b) 1, 2 and 3.
c) 1, 2 and 5.
d) 1, 2 and 4.
e) 3, 4 and 5.
a) 2, 3 and 5.
B02 CH22 Q020
The most common central system is the forced air furnace installed in many residences. The furnace is normally located in the basement and provides conditioned air which is ducted to each room in the house. The basic furnace includes low efficiency filters, direct drive or belt driven fan, and indirect natural gas-fired heating section, as shown in Fig. An outdoor air duct is normally connected to the return air ductwork to provide a fixed amount of outside air into the system. High efficiency electronic air filters, water humidifiers, and a split system, direct expansion cooling coil with remote air-cooled condensing unit may also be added to the furnace to provide satisfactory environmental conditions on a year round basis. Forced Air System with Natural Gas Heating Furnace
The dual duct air conditioning system
a) maintains a hot deck temperature at 12.5°C year-round.
b) employs acoustic insulation in mixing boxes as a standard feature.
c) is suitable only for a single-zone installation.
d) is suitable for small buildings such as schools.
e) provides variable air volume to each zone.
b) employs acoustic insulation in mixing boxes as a standard feature.
B02 CH22 Q030
Due to the high velocity design of dual duct systems, special attention must be given during design to minimize noise transmission into occupied areas. Therefore, silencers may be installed in each duct main, and acoustic insulation is a standard feature of mixing boxes.
A small air conditioning unit with DX coils, condenser, compressor, fans and controls all located in a closed casing that only provides air filtration, cooling, ventilation and dehumidification is called a
a) window air conditioner.
b) packaged air conditioner.
c) fan coil unit.
d) central conditioning unit.
e) rooftop air conditioner.
a) window air conditioner.
B02 CH22 Q016
Window Air Conditioner is a packaged unit that utilizes a direct expansion refrigeration system to provide cooling and dehumidification in a local area. It is designed to supply conditioned air directly to the space without ductwork, and is normally installed in a window or exterior wall opening. It requires no piping connections and can usually be plugged into a standard wall outlet for electrical power supply.
A high velocity terminal reheat system differs from a low velocity system in the
1. supply air temperature
2. supply air velocity.
3. size of supply air duct.
4. lack of a terminal reheat box.
5. increased fan size.
a) 1, 2 and 4 only.
b) 1, 3 and 5 only.
c) 2, 3 and 5 only.
d) 2, 3 and 4 only.
e) 1 and 2 only.
c) 2, 3 and 5 only.
B02 CH22 Q024
Terminal Reheat System - High Velocity:
This is a variation of the low velocity, terminal reheat system in which the size of supply ductwork is reduced by utilizing higher air velocities for distribution throughout the building. This reduces ductwork costs and building space requirements to accommodate the ducts. However, more powerful supply fans with larger motors are required to develop higher supply air pressures. Also, a terminal reheat box is required in the branch duct to each zone to reduce the air flow to low velocity, and to maintain a constant supply air volume in spite of air pressure fluctuations in the high velocity ducts. The boxes consist of a volume regulator, reheat coil, and acoustic insulation. Each box is rated for a maximum capacity, but actual air flow can be set by manually adjusting a volume controller on the exterior of the box.
The outdoor enthalpy control refers to the use of outdoor air enthalpy content to control
a) mixed air temperature.
b) supply air temperature.
c) relief air temperature.
d) exhaust air temperature.
e) return air temperature.
a) mixed air temperature.
B02 CH22 Q014
When the outdoor temperature exceeds the 24°C (75°F) return air temperature it is more energy efficient to operate on recirculation, so the dampers adjust to a 10% or 15% minimum outside air position. In some climatic locations, the dry bulb and wet bulb temperatures of the outdoor and return air are measured, and this “changeover” occurs when the outdoor air enthalpy exceeds the return air enthalpy, thus resulting in further energy savings.
In the single zone constant air volume system, space conditions are maintained by adjusting supply air temperature in response to a thermostat located in the
a) supply air duct.
b) outdoor space.
c) room or return air duct.
d) mixed air duct.
e) fresh air duct.
c) room or return air duct.
B02 CH22 Q022
Single Zone, Constant Air Volume System:
Space conditions are maintained by adjusting supply air temperature with a return air or room thermostat.
The space thermostat of each zone on a simple multizone system
a) controls the mixing box.
b) controls the hot water temperature to the coil.
c) controls the chilled water temperature to the coil.
d) regulates the total make-up air into the system.
e) regulates the modulating damper of the mixing compartment.
e) regulates the modulating damper of the mixing compartment.
B02 CH22 Q026
A space thermostat in the zone acts through an operator to modulate the dampers and adjust the mixture of hot and cold air, thus satisfying space conditions by varying supply air temperature.
To compensate for a large pressure drop in a dry expansion evaporator, the thermal expansion valve may be equipped with
a) an equalizing line.
b) a solenoid-operated by-pass line.
c) a hand-operated by-pass expansion valve.
d) a secondary bellows assembly.
e) an auxiliary spring mechanism.
a) an equalizing line.
B02 CH23 Q006
To compensate for a large pressure drop in an evaporator, the thermal expansion valve is equipped with an equalizing line which connects the underside of the diaphragm or bellows with the outlet of the evaporator instead of the inlet. The reduction in the downward force of the thermal element is now compensated for by a reduction in upward force exerted by evaporator pressure. The valve will now admit enough liquid to maintain the superheat at the outlet at 5°C to 6°C and to provide for maximum evaporator efficiency. A diagram of an evaporator equipped with a thermal expansion valve with an equalizing line is shown in the figure.
The amount of refrigerant charge is critical in a system equipped with a
a) thermostatic expansion valve.
b) high-pressure float valve.
c) low-pressure float valve.
d) hand expansion valve.
e) automatic expansion valve.
b) high-pressure float valve.
B02 CH23 Q008
High-Pressure Float Valve: Like the low-pressure float valve, the high-pressure float valve is also a liquid level operated control valve. However, the float is located on the high pressure side of the system and is operated by the liquid refrigerant level on that side. The figure shows a cross-sectional view of this valve.
A compressor capacity control that results in no reduction in power consumption and may cause compressor overheating is the
a) evaporator damper method.
b) cylinder bypass method.
c) cylinder unloader method.
d) speed control method.
e) hot gas bypass method.
e) hot gas bypass method.
B02 CH23 Q014
The figure shows a simple hot gas bypass capacity control. When a reduction of compressor capacity is required, a solenoid valve located in the bypass line is energized by the pressure or temperature at the compressor inlet, allowing some hot gas to go directly into the suction line.
This type of capacity control has several disadvantages. There is little or no reduction in power consumption when the bypass line is open, and excessive compressor overheating can occur. The hot gas bypass is used alone only on small compressors. It is often used in conjunction with other types of capacity control when it is necessary to provide capacity control down to 0% loading or when it is necessary to unload a compressor before starting. It may also be used with centrifugal compressors.
Refrigeration compressor cylinder unloaders are activated by sensing
a) evaporator pressure.
b) condenser pressure.
c) receiver pressure.
d) compressor suction pressure.
e) compressor discharge pressure.
d) compressor suction pressure.
B02 CH23 Q012
Compressor Unloader:
As the compressor suction pressure falls to a preset value, a pressure switch, sensing low-pressure on the suction side, energizes a solenoid valve to open and admit condenser pressure to the unloader piston. This pressure moves the unloader piston downward to depress the suction valves and holds them in the open position. Refrigerant vapour is drawn into the cylinder during the suction stroke and is discharged back to the suction line during compression. When the suction pressure rises to a certain value, the pressure switch de-energizes the solenoid valve causing the unloader piston to return to its normal position; allowing the suction valves to become operational again.
During periods of minimum loading, freeze-ups in liquid chillers are prevented by means of
a) a hot-gas bypass from the compressor discharge.
b) an evaporator electric low-pressure float switch.
c) a solenoid valve at the evaporator inlet.
d) a chilled-liquid flow control.
e) an evaporator pressure regulating valve.
e) an evaporator pressure regulating valve.
B02 CH24 Q010
Evaporator Pressure Regulating Valve:
In many cases the evaporator pressure must not be allowed to drop below a certain level. Very low evaporator pressure results in a correspondingly low evaporator temperature which causes frosting of coils or freeze-ups in water chillers during periods of minimum loading. An evaporator pressure regulator prevents an overly low pressure and low temperature condition from occurring in the evaporator while the compressor is running. The regulating valve is installed at the evaporator outlet.
A refrigerant flow control that employs a hygroscopic sensing element is a
a) humidistat.
b) flooded evaporator level regulator.
c) thermostat.
d) pressurestat.
e) purge flow regulator.
a) humidistat.
B02 CH24 Q006
Humidity Actuated Control (Humidistat)
In some air conditioning systems, refrigeration is used to lower the humidity of the air by condensing the moisture on the cold surfaces of the evaporator or dehumidifier. The refrigerant flow to the evaporator is then controlled by a humidistat similar to that shown in the figure. When used to control dehumidification, the control is often referred to as a dehumidistat. The sensing device of a humidistat may consist of a hygroscopic (moisture absorbing) element. Controllers of this type use multiple strands of human hair for this purpose. Hair increases in length when the humidity of the air increases and decreases in length when the humidity drops. Synthetic materials are now used in place of human hair.
When a low-pressure cutoff is used on a refrigeration system, the control is usually combined with
a) a low limit thermostat.
b) the oil pressure failure switch.
c) the high pressure cutout.
d) the evaporator pressure regulating valve.
e) a thermostatic controller.
c) the high pressure cutout.
B02 CH24 Q014
Low-Pressure Cutoff control operates on the same principle as the high pressure safety cutout control. When a low pressure cutoff is used, it is usually combined with the high pressure cutout. This combination control is provided with a manual reset, so that the operator will become aware of the cause of the trouble when the compressor shuts down prematurely before temperature conditions are satisfied.
In a compression refrigeration system, the high-pressure safety cutout shuts down the compressor in the event of
a) high evaporator pressure.
b) high compressor suction pressure.
c) high refrigerant receiver pressure.
d) high compressor discharge pressure.
e) high condenser temperature.
d) high compressor discharge pressure.
B02 CH24 Q012
High-Pressure Safety Cutout:
It is possible during operation for the condenser pressure of the refrigerating system to become higher than normal due to insufficient cooling of the high-pressure vapour in the condenser or due to the presence of non-condensables in the system. To prevent the pressure from building up to a dangerous level, the CSA B52 Code requires that all refrigeration systems containing more than 9 kg (20 lbs) of refrigerant and operating above atmospheric pressure, and all water cooled systems so constructed that the compressor (or generator in absorption refrigeration systems) is capable of producing a pressure in excess of the high side design pressure shall be equipped with a pressure limiting device designed to stop the action of the compressor or generator at a pressure not more than 90% of the system high side design pressure. The Code also requires this device to be connected between the compressor and the first stop valve in the discharge line.
A refrigerant condenser cooling water regulating valve operates by sensing
a) liquid refrigerant flow entering the evaporator.
b) cooling water temperature entering the condenser.
c) refrigerant vapour flow entering the condenser.
d) cooling water temperature leaving the condenser.
e) compressor discharge pressure.
e) compressor discharge pressure.
B02 CH24 Q008
Condenser Cooling Water Regulating Valve: This valve is used to automatically regulate the flow of cooling water to the condenser when the compressor is in operation, and to shut off the flow when the compressor stops. The valve, illustrated in the figure, is operated by the vapour pressure on the discharge side of the compressor. This pressure acts on the bellows and tends to open the valve against the spring force trying to close the valve.
Using a burning sulphur candle, a refrigerant leak is detected by the appearance of
a) dark black smoke.
b) dense white smoke.
c) a strong sulphur odour.
d) bright red smoke.
e) yellow fumes.
b) dense white smoke.
B02 CH25 Q006
Litmus Paper Detector:
Leaks in ammonia systems are detected by using strips of wetted litmus paper. When moved about a suspected joint or valve spindle, any escaping ammonia vapour will change the colour of the litmus paper to a blueish hue. Ammonia leaks can also be detected by using a sulphur candle which, when lit, gives off a cloud of white smoke when exposed to ammonia fumes or vapour.
When handling refrigerants it is important to
a) wear safety goggles.
b) ensure the Inspecting Power Engineer has been advised.
c) ensure a fire extinguisher is readily available.
d) check the last inspection date on the refrigerant cylinder.
e) wear self-contained breathing apparatus.
a) wear safety goggles.
B02 CH25 Q008
When handling refrigerants, goggles must be used for eye protection since even safe refrigerants can cause serious injury by freezing the moisture in the eyes.
Additional refrigerant is needed when
a) evaporating coils are not refrigerating as well as usual.
b) all of the above.
c) a gurgling sound in the expansion valves is noticed.
d) a very low head pressure is noticed.
e) bubbles appear in the liquid flow sight glass.
b) all of the above.
B02 CH25 Q010
Flow of refrigerant should be watched through the sight glass. If bubbles appear in the flow after the system has settled down to normal, additional refrigerant is required. Also, if a very low head pressure or a gurgling sound in the expansion valves is noticed or evaporating coils are not refrigerating as well as usual additional refrigerant is required.
A refrigeration system is purged to remove
a) water vapour.
b) non-condensable gases.
c) ammonia vapours.
d) used compressor oil.
e) excess refrigerant.
b) non-condensable gases.
B02 CH25 Q012
Non-condensable gases must be removed from the system by either manual or automatic purging. Since these gases tend to collect at the highest point of the condenser and receiver, the purge connections are made at these points.
An automatic purger for a large ammonia refrigeration system returns refrigerant to the system by employing a
a) chilling coil.
b) relief valve.
c) liquid knockout drum.
d) heating element.
e) hot gas coil.
a) chilling coil.
B02 CH25 Q014
Automatic Purgers:
Most large refrigeration systems are equipped with an automatic purger. When the automatic purger is operated with the compressor running, the non-condensable gases which enter the purger may carry with them a considerable amount of refrigerant vapour. To prevent this vapour from escaping to the atmosphere with the gases, the purger is equipped with a chilling coil which causes the vapour to condense and separate from the gas mixture. The liquid refrigerant settles to the lower part of the purger where it is drained back to the receiver, usually by means of an automatic float trap. The non-condensable gases rise to the upper part of the purger where they are released to the atmosphere through an air relief valve.
When starting up a reciprocating refrigeration system that has been shut down for a short period, it is important to
a) wear self-contained breathing apparatus.
b) ensure cooling water is not turned on until the compressor oil is up to temperature.
c) manually lift all relief valves.
d) check the operation of interlocks.
e) wear safety goggles.
d) check the operation of interlocks.
B02 CH26 Q001
Before starting the compressor:
- The operator should become familiar with the entire refrigeration system and accessories before operating the equipment.
- Check that power is available to circuit breakers, water pumps, cooling tower, etc.
- Check the setting of high- and low- pressure cutout switches.
- Check the operation of any interlocks in the system. For example, the compressor should not be able to start if the fans in an air conditioning system are not operating, or the evaporative condenser is not running properly.
- Open shutoff valves in the cooling water supply and return lines of water cooled condensers, or start fan motors of air cooled or evaporative condensers if not tied in with the compressor starting system. Open the water supply valve to the evaporative condenser sump and check the water level.
- Check oil level in the compressor. It should be at or above the centre of the sight glass.
- On smaller capacity compressors open the suction and discharge valves. On larger compressors only the discharge valve should be opened; the suction valve should be left closed on starting to avoid an excessive starting torque, and the resulting high power draw. When the compressor comes up to speed, the suction valve should be opened slowly.
- All shutoff valves in the system should be open except bypass valves used for other purposes.
- Solenoid valve in the liquid line should be closed, and on magnetic coil control.
- Suction, discharge, and oil pressure gages should be connected and any valves in the connecting lines should be open.
- If the compressor is equipped with an oil sump heater, ensure it is energized and that the oil temperature is high enough to drive off any refrigerant.
When shutting down a refrigeration system, the valve closed last is the
a) compressor suction valve.
b) King valve.
c) purge valve.
d) expansion valve.
e) cooling water supply valve.
e) cooling water supply valve.
B02 CH26 Q003
To shut a system down the following procedure should be followed:
- Close the liquid line shutoff valve on the receiver to stop the flow of refrigerant to the evaporator.
- If a solenoid valve is used in the liquid line, it should be held open so that all liquid can be withdrawn from the line.
- With the entire system in operation, lower the pressure on the low side until compressor suction gage indicates 14 kPa (2 psi). It may be required to hold the low pressure cutoff in the closed position.
- As soon as pressure reaches 14 kPa (2 psi) stop the compressor and close the suction and discharge valves. Never pump down below 7 – 14 kPa (1–2 psi) since a slight positive pressure is needed to prevent air from being drawn in through minor leaks or the compressor shaft seal. Close all other valves in the system. The part of the system containing the refrigerant charge should be thoroughly checked for leaks.
- Close cooling water supply to the compressor and water cooled condenser, if so equipped. If equipment is subject to freezing temperatures, all water should be drained.
- If the system is equipped with an evaporative condenser, close the makeup water supply, drain the water and flush the condenser.
- Open the master power switch for the system and lock it in the open position.
A dual duct air conditioning system is a modified
a) multizone, CAV system.
b) two-zone VAV system.
c) single zone, CAV system.
d) single zone, high velocity system.
e) multizone, VAV system.
a) multizone, CAV system.
B02 CH22 Q028
Dual Duct System as shown in the figure, is similar in concept to the multizone system in that hot air and cold air are mixed to vary supply air temperature to each zone in order to satisfy room conditions. However, rather than mixing at the discharge of the air handling unit, the cold and hot air are ducted separately throughout the building, then blended in a mixing box located in each zone. The air handling unit includes the mixed air section, filters, humidifier, fan, hot deck, and cold deck discharge compartments. Separate supply duct mains are connected to each of these ducts by short lengths of flexible ductwork. A room thermostat in the zone modulates supply air temperature to maintain space conditions. This system is widely used for larger office, commercial, institutional, and medical buildings. It provides a constant air circulation rate in each zone and good control of space conditions.
The unitary air conditioning system installed at the exterior wall of each room of a building with an adjustable duct connection to outdoors for provision of minimum outdoor air intake is called a
a) full recirculation unit.
b) unit ventilator.
c) heat pump.
d) packaged unit.
e) fan coil unit.
b) unit ventilator.
B02 CH22 Q018
This system was developed to provide individual room temperature control in buildings such as hotels with many zones and limited available floor space. The units, as shown in the figure, are mounted on building exterior walls and include a low efficiency filter, fan, heating and/or cooling coil, and manually adjustable duct connection to outdoors for provision of minimum outdoor air. Room air is drawn into the bottom of the unit, mixed with a fixed amount of outdoor air, filtered, and blown by a fan through the coils to the room. The coils are connected to a piping system which provides chilled water from refrigeration machines for summer cooling and/or hot water from boilers for winter heating.
For ventilation, a forced air furnace for a house typically utilizes
a) 50% outdoor air and 50% recirculation.
b) 100% recirculation air.
c) a variable rate of outdoor air, depending upon time of year.
d) a fixed percentage of 10% outdoor air.
e) 100% outdoor air.
d) a fixed percentage of 10% outdoor air.
B02 CH22 Q010
In Fig.(b) the unit is operating on approximately 90% recirculation and a fixed 10% of outside air which is drawn through a connection from the return air duct to outdoors. This “minimum outdoor air” arrangement is typically utilized with a forced air furnace to provide ventilation for a house.
A small air conditioning unit with DX coils, condenser, compressor, fans and controls all located in a closed casing that only provides air filtration, cooling, ventilation and dehumidification is called a
a) central conditioning unit.
b) fan coil unit.
c) packaged air conditioner.
d) rooftop air conditioner.
e) window air conditioner.
e) window air conditioner.
B02 CH22 Q016
Window Air Conditioner is a packaged unit that utilizes a direct expansion refrigeration system to provide cooling and dehumidification in a local area. It is designed to supply conditioned air directly to the space without ductwork, and is normally installed in a window or exterior wall opening. It requires no piping connections and can usually be plugged into a standard wall outlet for electrical power supply.
In air conditioning systems, the type of air intake arrangement in which “free cooling” is achieved is
a) the mixed air arrangement.
b) the 100% outdoor air arrangement.
c) the by-pass air arrangement.
d) the 100% recirculation arrangement.
e) the fixed percentage of outdoor air arrangement.
a) the mixed air arrangement.
B02 CH22 Q012
The mixed air system is the most commonly used arrangement in central and combined systems for larger buildings. When the unit is operating on full outdoor air, all of the return air must be released from the building. The mixing dampers are usually automatically controlled to provide a mixed air temperature of approximately 12.5°C (54.5°F), which is the desired supply air temperature to the building for cooling purposes. The amount of outdoor air for a 12.5°C (54.5°F) mixed air temperature is reduced to approximately 15% during the coldest winter days, but increases as the outdoor temperature rises, and is 100% at 12.5°C . In this temperature range, the desired 12.5°C supply air temperature for building cooling can be achieved by the operation of the mixed air dampers, so mechanical cooling is not required. This is called “free cooling” and results in a significant saving in energy consumption compared to systems which do not have this feature.
The outdoor enthalpy control refers to the use of outdoor air enthalpy content to control
a) return air temperature.
b) relief air temperature.
c) exhaust air temperature.
d) mixed air temperature.
e) supply air temperature.
d) mixed air temperature.
B02 CH22 Q014
When the outdoor temperature exceeds the 24°C (75°F) return air temperature it is more energy efficient to operate on recirculation, so the dampers adjust to a 10% or 15% minimum outside air position. In some climatic locations, the dry bulb and wet bulb temperatures of the outdoor and return air are measured, and this “changeover” occurs when the outdoor air enthalpy exceeds the return air enthalpy, thus resulting in further energy savings.
The dual duct air conditioning system
a) is suitable only for a single-zone installation.
b) employs acoustic insulation in mixing boxes as a standard feature.
c) maintains a hot deck temperature at 12.5°C year-round.
d) provides variable air volume to each zone.
e) is suitable for small buildings such as schools.
b) employs acoustic insulation in mixing boxes as a standard feature.
B02 CH22 Q030
Due to the high velocity design of dual duct systems, special attention must be given during design to minimize noise transmission into occupied areas. Therefore, silencers may be installed in each duct main, and acoustic insulation is a standard feature of mixing boxes.
A high velocity terminal reheat system differs from a low velocity system in the
1. supply air temperature
2. supply air velocity.
3. size of supply air duct.
4. lack of a terminal reheat box.
5. increased fan size.
a) 1, 3 and 5 only.
b) 2, 3 and 5 only.
c) 1 and 2 only.
d) 2, 3 and 4 only.
e) 1, 2 and 4 only.
b) 2, 3 and 5 only.
B02 CH22 Q024
Terminal Reheat System - High Velocity:
This is a variation of the low velocity, terminal reheat system in which the size of supply ductwork is reduced by utilizing higher air velocities for distribution throughout the building. This reduces ductwork costs and building space requirements to accommodate the ducts. However, more powerful supply fans with larger motors are required to develop higher supply air pressures. Also, a terminal reheat box is required in the branch duct to each zone to reduce the air flow to low velocity, and to maintain a constant supply air volume in spite of air pressure fluctuations in the high velocity ducts. The boxes consist of a volume regulator, reheat coil, and acoustic insulation. Each box is rated for a maximum capacity, but actual air flow can be set by manually adjusting a volume controller on the exterior of the box.
In the single zone constant air volume system, space conditions are maintained by adjusting supply air temperature in response to a thermostat located in the
a) mixed air duct.
b) supply air duct.
c) room or return air duct.
d) fresh air duct.
e) outdoor space.
c) room or return air duct.
B02 CH22 Q022
Single Zone, Constant Air Volume System:
Space conditions are maintained by adjusting supply air temperature with a return air or room thermostat.
Evaporator capacity may be controlled using
a) a multiple fan system.
b) dual level low-side float valves.
c) sectional evaporators.
d) a staged-flow thermostatic expansion valve.
e) multiple compressors.
c) sectional evaporators.
B02 CH23 Q010
Evaporator Capacity Control:
The capacity of an evaporator may be controlled by using either sectional evaporators or evaporator dampers.
Refrigeration compressor cylinder unloaders are activated by sensing
a) evaporator pressure.
b) receiver pressure.
c) condenser pressure.
d) compressor suction pressure.
e) compressor discharge pressure.
d) compressor suction pressure.
B02 CH23 Q012
Compressor Unloader:
As the compressor suction pressure falls to a preset value, a pressure switch, sensing low-pressure on the suction side, energizes a solenoid valve to open and admit condenser pressure to the unloader piston. This pressure moves the unloader piston downward to depress the suction valves and holds them in the open position. Refrigerant vapour is drawn into the cylinder during the suction stroke and is discharged back to the suction line during compression. When the suction pressure rises to a certain value, the pressure switch de-energizes the solenoid valve causing the unloader piston to return to its normal position; allowing the suction valves to become operational again.
The amount of refrigerant charge is critical in a system equipped with a
a) hand expansion valve.
b) thermostatic expansion valve.
c) high-pressure float valve.
d) low-pressure float valve.
e) automatic expansion valve.
c) high-pressure float valve.
B02 CH23 Q008
High-Pressure Float Valve: Like the low-pressure float valve, the high-pressure float valve is also a liquid level operated control valve. However, the float is located on the high pressure side of the system and is operated by the liquid refrigerant level on that side. The figure shows a cross-sectional view of this valve.
To compensate for a large pressure drop in a dry expansion evaporator, the thermal expansion valve may be equipped with
a) a hand-operated by-pass expansion valve.
b) an auxiliary spring mechanism.
c) a solenoid-operated by-pass line.
d) a secondary bellows assembly.
e) an equalizing line.
e) an equalizing line.
B02 CH23 Q006
To compensate for a large pressure drop in an evaporator, the thermal expansion valve is equipped with an equalizing line which connects the underside of the diaphragm or bellows with the outlet of the evaporator instead of the inlet. The reduction in the downward force of the thermal element is now compensated for by a reduction in upward force exerted by evaporator pressure. The valve will now admit enough liquid to maintain the superheat at the outlet at 5°C to 6°C and to provide for maximum evaporator efficiency. A diagram of an evaporator equipped with a thermal expansion valve with an equalizing line is shown in the figure.
A refrigerant flow control that employs a hygroscopic sensing element is a
a) pressurestat.
b) humidistat.
c) thermostat.
d) purge flow regulator.
e) flooded evaporator level regulator.
b) humidistat.
B02 CH24 Q006
Humidity Actuated Control (Humidistat)
In some air conditioning systems, refrigeration is used to lower the humidity of the air by condensing the moisture on the cold surfaces of the evaporator or dehumidifier. The refrigerant flow to the evaporator is then controlled by a humidistat similar to that shown in the figure. When used to control dehumidification, the control is often referred to as a dehumidistat. The sensing device of a humidistat may consist of a hygroscopic (moisture absorbing) element. Controllers of this type use multiple strands of human hair for this purpose. Hair increases in length when the humidity of the air increases and decreases in length when the humidity drops. Synthetic materials are now used in place of human hair.
When a low-pressure cutoff is used on a refrigeration system, the control is usually combined with
a) the oil pressure failure switch.
b) a low limit thermostat.
c) a thermostatic controller.
d) the high pressure cutout.
e) the evaporator pressure regulating valve.
d) the high pressure cutout.
B02 CH24 Q014
Low-Pressure Cutoff control operates on the same principle as the high pressure safety cutout control. When a low pressure cutoff is used, it is usually combined with the high pressure cutout. This combination control is provided with a manual reset, so that the operator will become aware of the cause of the trouble when the compressor shuts down prematurely before temperature conditions are satisfied.
A refrigerant condenser cooling water regulating valve operates by sensing
a) compressor discharge pressure.
b) refrigerant vapour flow entering the condenser.
c) liquid refrigerant flow entering the evaporator.
d) cooling water temperature leaving the condenser.
e) cooling water temperature entering the condenser.
a) compressor discharge pressure.
B02 CH24 Q008
Condenser Cooling Water Regulating Valve: This valve is used to automatically regulate the flow of cooling water to the condenser when the compressor is in operation, and to shut off the flow when the compressor stops. The valve, illustrated in the figure, is operated by the vapour pressure on the discharge side of the compressor. This pressure acts on the bellows and tends to open the valve against the spring force trying to close the valve.
During periods of minimum loading, freeze-ups in liquid chillers are prevented by means of
a) an evaporator electric low-pressure float switch.
b) a chilled-liquid flow control.
c) a solenoid valve at the evaporator inlet.
d) an evaporator pressure regulating valve.
e) a hot-gas bypass from the compressor discharge.
d) an evaporator pressure regulating valve.
B02 CH24 Q010
Evaporator Pressure Regulating Valve:
In many cases the evaporator pressure must not be allowed to drop below a certain level. Very low evaporator pressure results in a correspondingly low evaporator temperature which causes frosting of coils or freeze-ups in water chillers during periods of minimum loading. An evaporator pressure regulator prevents an overly low pressure and low temperature condition from occurring in the evaporator while the compressor is running. The regulating valve is installed at the evaporator outlet.
In a compression refrigeration system, the high-pressure safety cutout shuts down the compressor in the event of
a) high compressor discharge pressure.
b) high condenser temperature.
c) high refrigerant receiver pressure.
d) high compressor suction pressure.
e) high evaporator pressure.
a) high compressor discharge pressure.
B02 CH24 Q012
High-Pressure Safety Cutout:
It is possible during operation for the condenser pressure of the refrigerating system to become higher than normal due to insufficient cooling of the high-pressure vapour in the condenser or due to the presence of non-condensables in the system. To prevent the pressure from building up to a dangerous level, the CSA B52 Code requires that all refrigeration systems containing more than 9 kg (20 lbs) of refrigerant and operating above atmospheric pressure, and all water cooled systems so constructed that the compressor (or generator in absorption refrigeration systems) is capable of producing a pressure in excess of the high side design pressure shall be equipped with a pressure limiting device designed to stop the action of the compressor or generator at a pressure not more than 90% of the system high side design pressure. The Code also requires this device to be connected between the compressor and the first stop valve in the discharge line.
When handling refrigerants it is important to
a) wear safety goggles.
b) wear self-contained breathing apparatus.
c) check the last inspection date on the refrigerant cylinder.
d) ensure the Inspecting Power Engineer has been advised.
e) ensure a fire extinguisher is readily available.
a) wear safety goggles.
B02 CH25 Q008
When handling refrigerants, goggles must be used for eye protection since even safe refrigerants can cause serious injury by freezing the moisture in the eyes.
An automatic purger for a large ammonia refrigeration system returns refrigerant to the system by employing a
a) heating element.
b) chilling coil.
c) liquid knockout drum.
d) hot gas coil.
e) relief valve.
b) chilling coil.
B02 CH25 Q014
Automatic Purgers:
Most large refrigeration systems are equipped with an automatic purger. When the automatic purger is operated with the compressor running, the non-condensable gases which enter the purger may carry with them a considerable amount of refrigerant vapour. To prevent this vapour from escaping to the atmosphere with the gases, the purger is equipped with a chilling coil which causes the vapour to condense and separate from the gas mixture. The liquid refrigerant settles to the lower part of the purger where it is drained back to the receiver, usually by means of an automatic float trap. The non-condensable gases rise to the upper part of the purger where they are released to the atmosphere through an air relief valve.
A refrigeration system is purged to remove
a) ammonia vapours.
b) water vapour.
c) used compressor oil.
d) excess refrigerant.
e) non-condensable gases.
e) non-condensable gases.
B02 CH25 Q012
Non-condensable gases must be removed from the system by either manual or automatic purging. Since these gases tend to collect at the highest point of the condenser and receiver, the purge connections are made at these points.
Additional refrigerant is needed when
a) evaporating coils are not refrigerating as well as usual.
b) all of the above.
c) a very low head pressure is noticed.
d) bubbles appear in the liquid flow sight glass.
e) a gurgling sound in the expansion valves is noticed.
b) all of the above.
B02 CH25 Q010
Flow of refrigerant should be watched through the sight glass. If bubbles appear in the flow after the system has settled down to normal, additional refrigerant is required. Also, if a very low head pressure or a gurgling sound in the expansion valves is noticed or evaporating coils are not refrigerating as well as usual additional refrigerant is required.
Using a burning sulphur candle, a refrigerant leak is detected by the appearance of
a) dark black smoke.
b) bright red smoke.
c) dense white smoke.
d) yellow fumes.
e) a strong sulphur odour.
c) dense white smoke.
B02 CH25 Q006
Litmus Paper Detector:
Leaks in ammonia systems are detected by using strips of wetted litmus paper. When moved about a suspected joint or valve spindle, any escaping ammonia vapour will change the colour of the litmus paper to a blueish hue. Ammonia leaks can also be detected by using a sulphur candle which, when lit, gives off a cloud of white smoke when exposed to ammonia fumes or vapour.
State the sequence of operations when shutting down a refrigeration system with a reciprocating compressor.
1. Stop compressor and close suction and discharge valves.
2. Open power supply switch and lock out.
3. Close cooling water supply to condenser and compressor.
4. Close receiver liquid shutoff.
5. Decrease low side pressure to 14 kPa.
a) 1, 4, 5, 2, 3.
b) 4, 5, 1, 3, 2.
c) 3, 5, 4, 1, 2
d) 5, 4, 1, 2, 3.
b) 4, 5, 1, 3, 2.
B02 CH26 Q004
To shut a system down the following procedure should be followed:
- Close the liquid line shutoff valve on the receiver to stop the flow of refrigerant to the evaporator.
- If a solenoid valve is used in the liquid line, it should be held open so that all liquid can be withdrawn from the line.
- With the entire system in operation, lower the pressure on the low side until compressor suction gage indicates 14 kPa (2 psi). It may be required to hold the low pressure cutoff in the closed position.
- As soon as pressure reaches 14 kPa (2 psi) stop the compressor and close the suction and discharge valves. Never pump down below 7 - 14 kPa (1-2 psi) since a slight positive pressure is needed to prevent air from being drawn in through minor leaks or the compressor shaft seal. Close all other valves in the system. The part of the system containing the refrigerant charge should be thoroughly checked for leaks.
- Close cooling water supply to the compressor and water cooled condenser, if so equipped. If equipment is subject to freezing temperatures, all water should be drained.
- If the system is equipped with an evaporative condenser, close the makeup water supply, drain the water and flush the condenser.
- Open the master power switch for the system and lock it in the open position.
In the event of excessively high chilled water temperature, a centrifugal chiller is protected against overload by the
a) motor demand limiter.
b) vane closed switch.
c) chilled water flow switch.
d) refrigerant low temperature cutout.
e) low speed limiting device.
a) motor demand limiter.
B02 CH26 Q005
Motor Demand Limiter:
Most refrigeration systems are controlled by a motor demand limiter which limits the maximum current flow to an electric motor. This control overrides the water temperature sensor if the motor load reaches the maximum amperage flow setting.
