[D2L] Midterm Flashcards
What are 2 important things to remember with Solenoids? MULTI-SELECT
1) Never power the coil when it is off the stem
2) Coils are polarity sensitive
3) Solenoids must be installed in the correct direction with piping flow
4) Solenoids must be installed level
Never power the coil when it is off the stem
Solenoids must be installed in the correct direction with piping flow
Why is the use of proper control terminology important?
1) Can result in unnecessary costs
2) Miscommunication or inaccurate uses of terms causes frustration
3) It can result in injury to equipment users
4) All of the above
All of the above
What is an analog signal?
A control action that is continuously variable
What is an example of digital signal?
A light switch
What is a bimetal element?
two metals bonded together with different coefficients of thermal expansion
An internally powered controller would most likely be used in which type of system?
HWT (Hot water tank)
What is a closed loop system?
A control system that allows system feedback.
Open loop system does not allow system feedback
What normal state should a high limit control be in?
normally closed
What is a low limit control used for?
interrupt the system operation if the condition drops below the desired value
What does it mean to modulate?
Any controlled movement that changes through tiny increments and decrements
What is the Differential?
The difference in the values of a controllers on and off set points
What is an example of an open loop?
a kitchen faucet
What is a pressure controller?
Converts a pressure signal to electronic
What is a flow (sail) switch used to control?
1) air
2) water
3) fluid flow
4) all of the above
All of the above
What type of control system are relays and contactors considered?
electromechanical
Relays are used when the electrical loads are rated 15 amps and over
False
Relays -> 15 amps and under
Contactors -> 15 amps and over
On a call for cooling in a furnace, R to Y closes in the thermostat which also sends power to G
True
Relays typically have both N.O. (normally open) and N.C. (normally closed) contacts
True
What type is a timer?
1) electromechanical
2) electronic
3) thermostat
4) both A & B
both A & B
electromechanical
eletronic
What device is used to protect end use devices only?
Overload device
What is the most important factor with regards to piping design in low temperature applications?
compressor oil return
When is an OFC (Oil Failure Control)used?
when a reciprocating compressor has a oil pump
A crankcase heater helps reduce refrigerant migration and oil frothing on start up
True
How is compressor short cycling reduced?
Delay on break timer
What cycles in low temperature refrigeration?
1) defrost cycle
2) freezing cycle
3) warming cycle
4) both A & B
both A & B
defrost cycle
freezing cycle
What does DTC stand for?
defrost time clock
A timer motor in a time clock is powered at all times
True
If you want a pressure controller to turn on a compressor at 20psi and off at 2psi what should you set it to?
High event: 20psi
Differential: 18psi
Refrigerant boiling out of the oil in a compressor crankcase can cause a system to short cycle
True
Why are electrical thermostats preferred over electromechanical?
1) more reliable
2) auto-changeover functions
3) more accurate
4) all of the above
All of the above
What must be done to a mercury thermostat to maintain set point accuracy?
Be level
What is a high limit control used for in a residential furnace?
shut off gas valve if heat exchanger becomes too hot
You have been asked to deal with a co-workers call back, the customer is complaining about his old furnace that was just fixed and now the blower is short cycling. What should you expect to find?
1) The differential is set too small on a fan/limit controller
2) The differential is set too great on a fan/limit controller
The differential is set too small on a fan/limit controller
What is a fan cut in cut out switch 120F-140F used for in a residential furnace?
Cuts in(activate) the fan at 140F
Cuts out(deactivate) the fan at 120F
What is a manual reset on a high pressure switch?
If the unit trips on high pressure it will need to be physically reset
You just did a great job replacing a failed high pressure control. Before you leave the job what Saturated discharge temperature should you set the controller to?
135F
You are working on a job and finds a large contactor with a failed coil, what should you do?
Call the supplier and replace just the coil
What does an OFC protect?
Compressor running without oil
What is a heat exchanger an example of?
1) controller
2) controlled device
3) control agent
4) process plant
process plant
What is the actuator in a closed loop furnace system?
gas valve solenoid
If the thermostat is calling for cooling and you read 24v between R and Y which component is running?
Nothing, because they are not connected
If 0v is measured between R and Y what components should be running?
1) Indoor blower
2) Air conditioning
3) Heating
4) A & B
A & B
Furnace will be heating if 24v is measured across R and W
False, it should measure 0v to turn on the heat
What kind of control(s) do most RTU (roof top unit) use?
1) Electronic
2) Electromechanical
3) Both
Both Electronic & Electromechanical
What type of sensor can be formed in a Helix, disk or spiral shape?
bimetal strip
Where would you find an averaging element?
Across the face of a heating coil
Your boss asked you to pick up and replace a constant cut-in thermostat on a cooler at a Boston Pizza. Where will you mount the fast response element?
attached to the evaporator
What sensors react to pressure?
1) diaphragm
2) bellows
3) bourdon tube
4) all of the above
All of the above
Where is a bourdon tube most commonly used?
Refrigeration gauge
What does a transmitter do?
transmits signal to the actuator in the controlled device
What position should a heating zone valve be in the event of a power outage?
depends on the most recent signal from B.A.S.
You are in a mech room and see an oil failure control cover on the ground and reinstall it, Your apprentice asks why you bothered to, what do you say?
“It prevents the heat generated by the internal differential heater from escaping”
What is the purpose of a Refrigeration Fan Delay thermostat?
Prevent evaporator fans from spraying water after defrost
What do fuses protect?
loads
The motor on top of a zone valve is referred to as the
Actuator
You are doing a startup on a brand new WIC (walk-in cooler) and notice the compressor short cycling, which component do you probably suspect?
Solenoid
