Pre-Test #2 Flashcards
A cubic foot of water weighs _____lbs
a) 8.333
b) 10
c) 62.4
d) 64.2
62.4
A US gallon of water weighs ____lbs
a) 8.33
b) 10
c) 62.4
d) 64.2
a
The specific heat of water is?
1
The latent heat of fusion of water (ice) is _______btu/lb
144
The latent heat of vaporization of water (steam) is _____btu/lb
970
The opening force on a TXV is?
a) spring pressure
b) bulb pressure
c) suction pressure
d) head pressure
b) bulb pressure
The TXV regulates refrigerant flow to the evaporator based on?
a) heat of compression
b) superheat
c) external equalization pressure
superheat
An external equalizer is used when?
a) the pressure drop across the evaporator is too great
b) the liquid line pressure becomes too great
c) the receiver is too full of refrigerant
d) pressure drop at the axe is too great
a) pressure drop across the evaporator is too great
When the load on the evaporator increases, the txv?
a) increases the flow of refrigerant
To determine superheat with a blended refrigerant the correct formula is superheat is equal to
a) measured vapour temp minus dew point temp
b) measured vapour temperature minus bubble point temp
a)
To determine superheat with an azeotropic refrigerant (no glide) the correct formula is superheat is equal to:
a) measured liquid temperature minus saturation temperature
b) measured vapour temperature minus saturation temperature
b)
To determine sub-cooling with a blended refrigerant the correct formula is
a) measured liquid temp minus dew point temp
b) bubble point temp minus measured liquid
b)
In the b52 code a refrigerant of A3 classification indicates
a) a toxic refrigerant that burns easily
b) a non toxic refrigerant that burns easily
c) a toxic refrigerant that cannot be burned
b)
The term toxic as used in b52 refers to:
a) causes damage to surfaces
b) dangerous to use in a hermetic system
c) hazardous to human health
d) causes ozone layer damage
c)
When converting R12 to R134, one must always;
a) install new filter drier
b) check txv is compatible
c) check oil
d) all of the above
d)
the term glide when referring to blended refrigerants means
a) alkyl benzene oil must be used
b) temp difference between dew point and bubble point at constant temperature
c) amount of txv calibration
b)
3412 btus is equal to
a) 1 watt
b) 3.4 watts
c) 1 kw
d) 3.4 kw
c) 1kw
A typical heat pump will have a high pressure switch to shut the unit off if head pressure becomes high. The control action of this switch closes the contacts on a pressure:
a) rise at comp discharge
b) fall at comp discharge
c) difference between high and low side
b)
Which of the following is the correct formula for determining wattage in a resistive electrical circuit?
a) W=E x I
b) W=E/I
c) W=IR
a
The RPM of a motor is determined by
a) motors voltage
b) amperage
c) number of poles
c)
An ohmmeter reading of infinity across switch contacts means?
a) open
b) closed
c) good
d) bad switch
a)
Contractors are rated according to max amperage through the
a) coil
b) contacts
c) thermostat
d) fuse
b) contacts
Lowering airflow over a coil will
a) increase temp drop and increase moisture removal
b) decrease temp drop and increase moisture removed
a)
On which would the following most likely be powered by 240v single phase
a) natural gas furnace
b) residential condensing unit
c) roof top unit
d) residential humidifier
b)
Which of the following would be the best place to mount a thermostat
a) kitchen
b) bathroom
c) exterior wall
d) near a return air grille
d)
Condensate piping must be properly trapped to prevent
a) ice formation
b) condensate running off evaporator coil
c) overflowing
d) excessive air noise
d)
Which of the following would only be used on an induced draft condensing furnace?
a) electronic fan control
b) hot surface ingnition
c) induced draft blower motor
d) secondary heat exchanger
d)
Which precaution should an installer perform to keep copper from oxidizing internally while brazing?
a) hold torch flame steady
b) purge tube with nitrogen
c) increase heat
b)
Charles law states
a) at constant pressure, volume of gas varies directly with absolute temp
b) total pressure of a confined mixture of gasses is the sum of the pressures of each gas
a)
How is compressor compression ratio determined?
a) discharge psia/suction psia
b) discharge pig/suction psig
a)
Where in the system is the excess superheat added that must be removed before condensation in the condenser can take place?
a) start of evaporator and end of receiver
b) end of receiver and start of txv
c) end of evaporator and in comp
c)
Fresh air intakes ducts, used to route outdoor air to inside, connect to the
a) vent piping
b) combustion air duct
c) supply side of cut
d) return side of duct
d)
Daltons law states
a) constant pressure, the volume of gas varies with absolute temp
b) total pressure of a confined mixture of gasses is the sum of the pressures of each of the gasses in the mixture
b)
What are three commonly used metering devices for residential and light commercial air conditioning?
a) TXV, AXV, Low side float
b) high side float, low side float, cap tube
c) cap tube, txv, piston type
c)
The external equalizer connection to the suction line, should be located
a) next to the txv sensing bulb
b) 6 to 8 inches upstream from the txv sensing bulb
c) 6 to 8 inches downstream from the txv sensing bulb
d) none of the above
c)
Which of the following refrigerants is a HCFC?
a) R12
b) R22
c) R-502
d) R-134a
b)
Technician arrives on a call for an air to air heat pump during heating season. He notes the system operates until defrost energizes. System stays in defrost for approx 10 min when there is no ice?
a) defrost timer is defective
b) defrost termination switch stuck closed
c) defrost termination switch stuck open
c)
What are the symptoms of an air conditioning system that has a low charge?
a) evaporator coil will be partially frosted and discharge pressure will increase
b) suction line will sweat and low side pressure will decrease
c) low side pressure will decrease and the evaporator coil will be partially frosted
c)
An AC has an inoperable blower. When the stat is set below room temperature, there is 24v across R and G. What is the problem?
a) bad blower motor
b) bad blower relay
c) bad thermostat
d) any of the above
c)
With belt driven equipment, alignment of the motor and driven equipment pulleys:
a) is not critical because belts flex
b) accomplished using straight edge
c) when belts are parallel
d) using dial indicator
b)
Compressor capacity controlled by cylinder unloading
a) may result in compressor overheating
b) used only on hermetic comp
c) does not save energy
a)
Most compressors are positive displacement. Name a negative
Centrifugal
The ratio of actual volume of vapour pumped to the theoretical displacement volume is known as
a) volumetric efficiency
b) volumetric ratio
c) compression efficiency
a)
Compressor in which the compressor and motor are both sealed is known as?
hermetic
Compressor capacity decreases as:
a) discharge pressure increases
b) suction pressure decreases
c) compressor speed decreases
d) all of the above
d)
Scroll compression does what?
a) vapour enters from outside and exits centre
b) enters from centre and exits outside
a)
Capacity control on a screw compressor is obtained using
a) inlet vanes
b) hot gas
c) variable speed motors
d) a sliding valve
d)
Bearings in an operating centrifugal compressor can be checked by
a) sliding the impeller back and forth to check for slop
b) trying to move the shaft at right angles to bearing
c) checking the temperature of the oil returning from the bearing
c)
As a rule of thumb, for every 12,000btuh of cooling capacity in an open drive compressor air conditioning system, the heat rejection is roughly
a) 12,000btuh or 1 times the NRE
b) 9,000btuh, or .75 NRE
c) 15,000btuh or 1.25 NRE
c)
A decrease in condensing temperature will
a) increase condensing pressure, increase suction temperature and increase capacity
b) decrease condensing pressure, decrease suction temperature and increase system capacity
b)
Subcooling in the condenser will
a) increase system capacity
b) decrease system capacity
c) not affect system capacity
a) increase
A possible cause of high head pressure is
a) dirty condenser
b) insufficient water flow
c) air and non condensibles
d) all
d)
In order to protect a load, safety limit devices are most often wired
a) parallel
b) series
c) upstream
b)
A flow switch safety control is for
a) control of amount of flow
b) turn on a pump or fan
c) divert fluid flow
d) prove flow
d)
What is a disadvantage of a mercury bulb switch
a) they are unreliable
b) they must be level
c) they tend to burn contacts
b)
A thermostat is properly calibrated when
a) set point and control point are equal
b) room temp is comfortable
c) no dead band
d) control point and thermometer are equal
a)
With a change in temperature, a thermistor will change
a) resistance
b) voltage
c) current
d) impedance
a)
A gauge where an increase in pressure straightens the tube
a) whiskey tube
b) bimetal tube
c) bourdon tube
c)
In a TXV, three forces act on the diaphragm in the power head, which is connected to the valve. Evaporator pressure acts to _____ the valve, spring pressure acts to ____ the valve, and the bulb pressure _____ the valve
close, close, open
A common indication of a defective potential starting relay on a single phase motor is
a) motor hums but does not start
b) motor starts, runs normal, then shuts off
c) motor runs at half of speed
a)
Level of voltage created by the back EMF that opens contacts on a potential relay is called
a) lift voltage
b) open voltage
c) run voltage
d) pick-up voltage
d)
As the evaporator temperature of any vapour compression system is lowered
a) compressor suction vapour density increases
b) volume of suction vapour decreases
c) compressor capacity is lowered
c)
When charging a chilled water system, charge as a liquid. However, the machine is deep in vacuum, initial charge shall be vapour. why?
a) faster
b) allows pressure to stabilize and prevent shock
c) prevents condensation
d) charge until system temp above freezing prevents freezing of water in tubes
d)
A special valve used to prevent evaporator temperature from falling below a predetermined minimum temperature regardless of suction pressure is the
a) EPR
b) pressure reducing valve
c) suction pressure regulator
a)
In general, a greater number of rows in an evaporator
a) air leaving closer to refrigerant
b) more dehumidification
c) colder the leaving air will be
d) all of the above
d)
Increasing air flow over an evaporator
a) increases capacity
b) decrease evaporator capacity
c) does not affect evaporator capacity
d) none of the above
a)
A hermetic motor should never be energized or tested with a merger while in a deep vacuum because
a) electric arcs can contaminate freon
b) motor could draw too much amp
c) vacuum pump could be damaged
d) dielectric strength of the motors insulation would be greatly reduced
d)
Term used to describe a pumps ability to draw a vacuum. Pump must create a vacuum in itself lower than the system. The point at which a pump can no longer create a pressure difference is
a) dead head
b) blank off
c) blocked tight static
d) point of no pressure difference
b)
The oil pressure safety is operated by
a) oil pressure alone
b) sum of oil pump discharge and crankcase pressure
c) difference in crankcase pressure and oil pump discharge pressure
c)
In order for an oil failure control to operate correctly, it must be wired so that it gets power to the heater circuit
a) all times
b) only when comp running
c) only when good oil pressure
d) after oil change
b)
When the compressor has adequate oil pressure, the contacts between L and M are?
closed
Most common cause of low oil pressure is?
failed pump
The coil of a current relay is open. The compressor has a start capacitor but no run capacitor. The problem noted is
a) overload trips due to high current in start winding
b) compressor draws no current and fails to start
c) compressor starts when pressure equalizes
b)
If the temperature of the air being handled by a forward curved fan is reduced substantially
a) cfm increases
b) static pressure increases
c) cfm decreases
d) motor amperage increases
d) amps up because the cold air is more dense causing the motor to work harder
