Missed Questions Flashcards
100%
Calculate the voltage in the circuit when the total amperage is 20.4 amps and the resistors have values as follows: R1=10 ohms, R2=50 ohms, R3=20 ohms. (Parallel)
120 Volts
Volts = Amps / 1/R1 +1 ?R2 + 1/R3
Which of the following devices may be wired into the starting circuit of a single phase motor to improve the starting torque?
Start Capacitor
Another Winding
A single phase motor with an improved power factor, reduced motor current, increased efficiency and high starting torque is the
capacitor start-capacitor run (CSR) motor.
Correct. Because of the run and start capacitor, this motor offers the advantages listed above. The down side is additional cost.
In a CSR motor with a 10 microfarad run capacitor and a 110 microfarad start capacitor, the total capacitance will be
120 microfarads.
Correct. The capacitance of capacitors, when wired in parallel as in a CSR motor, are added together the same as resistors in a series circuit.
Which of the following reduced voltage starters uses a switching arrangement to connect the windings of three-phase motors in different configurations during startup and normal operation?
Star-delta starters.
Star & Wye are the same meaning
According to NEC (National Electrical Code), a general-use switch may be used as the controller for a stationary motor rated at two horsepower or less and at 300 volts or less if it is rated at not less than
twice the full-load motor current
A rather common indication of a defective starting relay in a single phase motor
is for the motor to hum but not start.
A type of current relay that uses a bimetal strip to open contacts in both the starting and running contacts is called a
hot-wire relay.
A pneumatic timing relay operates by
filling a bellows with air.
Correct. When energized, a bellows fills with air, which takes time. The time can be adjusted by a needle valve, which regulates the rate at which the bellows fills. When fully expanded, the bellows operates a snap-action switch.
The combination safety limit and fan control on a residential furnace turns the fan on and off by sensing the furnace air temperature. If the furnace temperature rises too high, the safety limit de-energizes the
gas valve.
Correct. The gas valve is shut off while the fan is allowed to continue running in order to cool the furnace.
Some codes require a manual reset device on a safety limit
so that the cause of the safety shutdown can be investigated prior to re-start.
A flow switch safety control is used to
prove that flow is occurring.
What is a disadvantage of the mercury bulb switch?
They must be level.
A thermostat is properly calibrated when
set point and control point are equal.
A pressure sensing device that relies on a fluid, such as oil, to provide a frictionless seal is the
inverted bell.
Correct. This device is suitable on only very low pressure, such as static pressure in an air duct, where pressures usually do not exceed several inches of water.
The photo cell in an infrared sensor is made of
lead sulfide.
Correct. Lead sulfide is a semiconductor having the property of decreasing in electrical resistance when exposed to radiation of certain wave lengths.
Aiming of infrared sensors is critical. Why must they be aimed directly at the flame?
Since they see heat, their sensitivity can be reduced by hot refractory.
A type of two-position control that provides for “intermediate” positions as well as the “all-on” and “all-off” positions is the
floating control.
While proportional control action has the disadvantage of control point shift but has no time-lag factor, floating action has the disadvantage of time lag influence but has no control point shift. What control action combines the advantages of both?
Proportional integral.
Checking amperage on a system during general maintenance
can indicate a problem with the system.
When changing or adding oil to a compressor
it is best to buy the smallest possible containers and use the entire contents at once.
avoid pouring the oil from a container into the compressor.
use an oil charging pump.
D. *all of the above. Correct.
As the evaporating temperature of any vapor compression system is lowered,
compressor capacity is lowered.
As a “rule of thumb”, for every 12,000 btuh of cooling capacity in an open compressor air conditioning system, the heat rejection is roughly
15,000 btuh, or 1.25 times the net refrigeration effect.
Correct. Like any rule of thumb, this number can vary. However, as a basis for estimation, it has long been considered that the heat rejection from an air conditioning system is about 15,000 Btuh for every 12,000 Btuh of cooling capacity. This assumes open-type compressors, where the refrigerant suction vapor does not receive any heat from the compressor motor as it does in hermetic motor-compressors.
The difference in temperature between condensing temperature and entering water or air temperature is known as
temperature split
Correct. The condensing temperature should be at least 15 or 20 degrees higher than the temperature of the entering water or air to avoid the need for an unreasonably large and costly condenser.
A decrease in condensing temperature will
decrease condensing pressure, decrease suction temperature and increase system capacity.
Subcooling in the condenser will
increase system capacity.
Correct. An increase in subcooling will increase the net refrigeration effect by reducing the flash gas loss.
Compressor capacity controlled by the cylinder unloading method
may result in compressor overheating
Correct. As fewer cylinders are used, less refrigerant is pumped resulting in less refrigerant vapor available for cooling the compressor.
10 in.Hg. vacuum is approximately equivalent to
10 psia.
Pressure PSIA Vaccum HG
0 \ 30 - perfect vacum
5/20
10/10
15/0 - Atmospheric pressure
A term used to rate vacuum pumps is used to describe the pump’s ability to draw a vacuum. Essentially, the pump must create a vacuum within itself that is lower than in the system, so that gases will flow from the system to the pump. The point at which a pump can no longer create a pressure difference is called
blank-off pressure.
Vacuum pump oil is a special oil because
it has a low vapor pressure to ensure that it won’t vaporize at low pressures.
Which of the following methods will not decrease the time needed to evacuate a large refrigeration system?
Triple evacuation.
Correct. Triple evacuation is a method that is no longer recommended since it vented refrigerant into the atmosphere. Further, it did not reduce evacuation time at all, but was a method used to dilute any contaminants within the system as much as possible.
A danger with leaving a vacuum pump running all night is
exceeding the pumps running time capabilities.
pump oil could be lost during a power outage.
*Both B and C from above. Correct.
Vacuum pump oil that is fresh
has a clear appearance.
A refrigerant with the ANSI/ASHRAE Standard 34-1992 safety classification of A1 has
low toxicity and no flame propagation.
All appliances containing more than 50 lbs. of refrigerant (except commercial and industrial process refrigeration) must be repaired when the annual leak rate exceeds
15%
Correct. EPA requires that all appliances containing more than 50 lbs. of refrigerant (except commercial and industrial process refrigeration) must be repaired when the annual leak rate exceeds 15%, and commercial and industrial process refrigeration must be repaired when the annual leak rate exceeds 35%.
When charging a chilled water system, charging is usually done as a liquid. However, if the machine is in a deep vacuum, the initial charge should be as a vapor. Why?
Charging as a vapor until the system temperature has risen above 32°F prevents freezing of water in the tubes.
When charging a system, several methods are available for checking the refrigerant charge. In order to charge by monitoring the subcooling, one must
monitor discharge pressure and liquid line temperature close to the metering device and charge until the liquid line temperature falls below the discharge saturation temperature by the expected amount of subcooling.
Insufficient refrigerant will cause
low suction pressure and insufficient cooling.
Most compressors are positive displacement machines. Name a compressor that is not a positive displacement machine.
Centrifugal.
The ratio of the actual volume of refrigerant vapor pumped to the theoretical displacement volume is known as
volumetric efficiency.
A compressor in which the compressor and motor are both sealed within the same pressurized housing is known as a
hermetic compressor.
Modern high-speed reciprocating compressors typically operate at
1750 rpm.
3450 rpm.
*Both A and B from above. Correct. Modern high-speed reciprocating compressors typically operate at 1750 rpm and 3450 rpm.
Compressor capacity decreases as
discharge pressure increases.
suction pressure decreases
compressor speed decreases.
D *All of the above. Correct.
An advantage of the scroll compressor is
smooth, quiet operation.
low torque variation through overlapping compression cycles.
few moving parts.
D. *All of the above. Correct.
The screw-type impellers used in helical-rotary compressors are also known as
rotors.
The component in a centrifugal compressor that actually does the compressing is known as
an impeller.
A centrifugal compressor controls capacity through the use of
variable speed.
vortex dampers.
D. *Both A and C from above. Correct.
Bearings in a centrifugal compressor can be checked by
checking the temperature of the oil returning from the bearing.
A small access valve which is similar to the air valve used on tires is known as a
Schrader valve.
A special valve used to prevent evaporator temperature from falling below a predetermined minimum temperature regardless of suction pressure is the
evaporator pressure regulator.
orrect. An example of an application for this valve is on systems with multiple evaporators operating at different temperatures.
A metering device with the characteristic of feeding liquid into the evaporator at the same rate at which the liquid flows from the condenser is the
high-side float valve.
Correct. This valve consists of a float and a needle valve, which creates the pressure drop separating the high-side from the low-side of the system. As the liquid level from the condenser raises, the float rises and opens the needle valve, forcing refrigerant into the evaporator. As the liquid level from the condenser drops, the float drops also and closes the needle valve, restricting the flow to the evaporator.
The metering device that actually measures temperature rather than pressure or liquid levels is the
thermal electric valve.
Suction line filter-driers should be installed whenever
a hermetic compressor motor burnout has occurred.
Increasing air flow over an evaporator
*increases evaporator capacity.
Correct. The more air that flows past an evaporator, the more energy is absorbed by the refrigerant. Using the formula for sensible heat transfer (below), it can be seen that if the temperature difference between the air and the refrigerant remains the same, an increase in air flow (cfm) will result in an increase of energy transfer (btuh). Btuh B. = cfm × 1 . 08 × ∆ T
Capacity control of a lithium bromide-water absorption machine is accomplished
by controlling the heat input to the generator.
Crystallization may occur in a lithium bromide-water absorption chiller
if power fails while the machine is under a heavy load.
if condenser water is allowed to get too cold.
if the machine overcools during a shut-down period.
D. *All of the above. Correct.
The heat source for adsorption liquid chillers could be
solar panels.
steam.
hot water.
D. *All of the above. Correct.
The purge units on absorption liquid chillers are designed to
*expel noncondensable gases.
A possible cause for high head pressure is
dirty condenser tubes.
insufficient water flow through the condenser.
air and noncondensables in the condenser.
D. *All of the above. Correct.
Approach temperature is the difference between
condensing temperature and leaving water temperature.
Calculate the amount of water needed per ton of refrigeration in a water-cooled condenser assuming a heat rejection rate of 15,000 btuh per ton and a 20°F water temperature rise through the condenser.
1.5 gpm.
GPM = BTUH/500xTD
What are the three ingredients necessary to start and sustain combustion?
fuel, oxygen and heat.
What are the products of perfect combustion using oxygen and a hydrocarbon fuel?
Carbon dioxide, heat and water.
Describe the appearance of a properly adjusted natural gas flame in an atmospheric burner.
Steady blue flame with a smaller lighter blue inner cone, resting on the burner port.
Radiant tubing/piping should be pressure tested to at least _______ times the expected operating pressure.
2
When filling a radiant heating system, it is recommended to fill
slowly from the low point so that the tendency for air pockets to form is reduced.
Infrared heaters use burning gas to heat a specific radiating surface instead of using the infrared energy from the open flame because
heated surfaces are better heat radiators than open flame.
High efficiency (condensing) furnaces achieve an efficiency of up 95% by
cooling the exhaust gasses to below dew point and recovering some of the latent heat of condensation.
Safety controls on a high efficiency (condensing) furnace include
high limit, flame proving, door switch, and induced draft pressure switch
The purpose of a boiler low limit control is to
start the boiler when boiler water temperature drops below a certain value.
In order for fuel oil to be burned, it must be atomized. This is done by forcing it through a nozzle under a pressure of
*100 psi.
A method of sensing flame in older style fuel oil furnaces and boilers was the stack switch, which sensed heat in the stack. This required the fuel to ignite, then heat the stack. A trial for ignition took up to 90 seconds. A newer and quicker safety control that “sees” the bright yellow flame has all but replaced the stack switch in fuel oil burners. What is this photo cell safety control?
A cad cell using cadmium sulfide as the sensor.
A compression tank is provided in a hydronic system
so that higher pressures can be built up, thereby raising the boiling point.
so that room is provided for heated water to expand.
to provide a cushion against sudden pressure changes and water hammer shock
D. *All of the above. Correct.
Expansion joints are needed in hydronic systems
*to allow for expansion and contraction of the piping.
In order to reduce water turbulence at the suction end of a pump, a suction diffuser should be used or a length of straight pipe equal to ______times the diameter of the pipe.
5
Premature bearing wear on floor mounted pumps could be caused by
suction or discharge lines that were forced into position when the pump was installed.
Correct. The torque of the pipes on the bearings causes excessive stress and could easily lead to early bearing failure.
Cavitation can destroy a centrifugal pump, is noisy and is caused by
inlet pressure that is too low.
The type of pump found in a fuel oil burner is the
gear pump.
A gear pump is a type of
rotary pump.
After the first heating season, cleaning a dirt pocket by removing the cap on the bottom of a dirt pocket
is usually not required.
Correct. The dirt pocket should be cleaned several times during the first heating season to remove heavy foreign material such as scale and pipe cuttings. Thereafter, such attention is usually not required.
The purpose of the gas regulator vent is to
allow air to enter above the diaphragm.
allow air to escape above the diaphragm.
allow gas to escape if the diaphragm ruptures.
*All of the above.
The most commonly used inspection used on fans and air handling equipment is
visual.
Which duct system delivers constant temperature air at variable volumes?
VAV
Which of the following best describes the difference between brazing and welding?
Welding is a cohesive process while brazing is adhesive.
The practice of installing flanges on pipe and fittings so that the top bolt hole in the right hand and left hand section of the flange are level with each other is known as
two-holing.
Of the valves listed below, which provides the best throttling action?
Globe valve.
Correct. The flow through a globe valve changes course through two 90° turns, causing increased resistance to flow and sometimes a considerable pressure drop.
A rating of 125 lb S-200 lb W.O.G. stamped on a valve means that the valve
has a steam rating of 125 psi and a cold water/oil/gas rating of 200 psi.
A device designed to protect pipe insulation against damage because of pipe movement is called a
pipe saddle.
Calculate the amount of weight that could be lifted in a force of 100 pounds is exerted in the direction of the arrow. (3 ropes)
300 lbs
Ropes x exerted force
The weakest portion of a chain hoist is the
lower hook.
An overload of most cable hoists (come-alongs) is prevented
by a handle that bends.
Calculate the volume in gallons of a cylindrical tank having a base diameter of 46” and a height of 70”.
*504 gallons.
rsquaredhpie/231
Current relay
ll Open
Potential relay
ll w/ / Closed
Time Delay Relay
Upside down V
A bearing used on the bottom of a motor where the shaft is positioned vertically is called a ____________ bearing.
Thrust
If resistor R3 is shorted out the current (Series)
Increases
Shorted increases, burns out goes to 0
For applications where quiet operation and light axial load characteristics are found, the type of bearing normally used is the:
Sleeve bearing
_________ occurs in a Solar Thermal System when the heated transfer fluid within the absorber piping rises within the collector and is replaced by cooler fluid downstream.
Convection
A (n) ___________ system uses a Heat Transfer Fluid and heat exchanger to transfer heat to the Domestic Hot Water without the use of mechanical means to circulate the HTF.
Passive indirect
For a voltage measurement the meter is connected:
In parallel to with load
When measuring voltage, the meter test leads are connected in _________ with the component being tested.
Parallel
Relays are made up of a __________ and one or more __________.
Load, switches
The type of refrigerant that must be used now must be ______________.
HFC
The device which prevents a vacuum from forming in a low-pressure gravity return steam heating system where automatic control is not used is a (n):
Air vent
If the water flow to a water-cooled condenser is reduced, the temperature difference across the condenser will:
Decrease
Air is eliminated from a sub-atmospheric system by a (n):
Vacuum pump
What effect will a higher than rated supply voltage have on the current draw of an electric motor?
It will be lower than it should be
