[D2L] Midterm Flashcards
The refrigerant in the center of the evaporator is in what state?
Saturated
What pressures do Refrigeration Gauges read? MULTI-SELECT (Select 2)
Inches of Mercury in a vacuum
Psia
Psig
Inches of water column
volume in cubic inches
Inches of Mercury in a vacuum
Psig
When a liquid is lowered beyond its boiling point while at the boiling pressure is called
Subcooling
1 micron =
0.001 millimeters
What order does refrigerant flow? (start with the compressor)
- Compressor
- Condenser
- Metering device
- Evaporator
All matter is composed of molecules that are in constant motion.
T or F
True
Which of the following statements is true?
1) An increase in work equals an decrease in heat.
2) A decrease in work equals a decrease in heat
3) A decrease in work equals an increase in heat
A decrease in work equals a decrease in heat
What two heat concepts affect refrigeration significantly?
Superheating vapours and subcooling liquids
What principle is needed between two objects in order to transfer heat?
There must be a temperature difference
Which method of heat transfer uses liquids and gases and involves fast vibrating molecules take up more space than slower vibrating molecules?
Convection
You finish brazing and decide to grab the pipe, what type of exciting heat transfer do you experience?
Conduction
Radiation can travel through space.
T or F
True
What kind of mechanical energy do we use to transfer from a low zone to a high zone?
1) Pumps
2) Compressors
3) Fans
4) All of the above
All of the above
What is a Btu?
The amount of heat required to raise the temperature of 1 lb of water by one Fahrenheit degree.
The unit for specific heat is Btu/lb./Fahrenheit degree
T or F
True
Latent heat is considered hidden heat because it does not register on a thermometer.
T or F
True
What is the freezing temperature for water?
1) 32F
2) 32C
3) 0C
4) Both A & C
Both A & C
What makes up the sum of total heat?
1) Sensible heat
2) Latent heat
3) Both
Both
What is dry bulb temperature?
the sensible temperature of the air determined with a dry thermometer
Which has the fastest moving molecules?
1) A hot cup of water
2) a cold cup of water
3) both hot and cold cups of water have the same rate of molecular motion
A hot cup of water
What will a vapour changing to a liquid do to heat?
Release heat
Specific heat is the amount of heat required to changed one pound of substance by one Fahrenheit degree.
T or F
True
kinetic energy is energy based on position
T or F
False
What is positive displacement?
The process of moving a piston to squeeze vapour in a cylinder
How does a centrifugal compressor increase refrigerant pressure?
Forces the acceleration of refrigerant
Where does the refrigerant change pressure? (MULTI-SELECT)
1) Metering device
2) Condenser
3) Compressor
4) Evaporator
1) Metering device
3) Compressor
What is the order of the state change that occurs in the condenser?
- Desuperheat
- Condensing
- Subcooling
What should the state of the refrigerant be at the outlet of the evaporator?
1) High pressure vapour
2) High pressure liquid
3) Low pressure liquid
4) Low pressure vapour
4) Low pressure vapour
What is superheating?
The process of adding sensible heat to a vapour while it remains at a constant pressure.
List the order of state the refrigerant is in the evaporator.
- Mostly liquid
- 1/2 liquid and 1/2 vapour
- Complete vapour
Absolute zero is when molecular motion stops.
T or F
True
What temperature is absolute zero?
0R
Which law is the sum of each individual gas equals the total pressure?
Daltons law
There are 4 gauges attached to this closed vapour filled system. What pressure will each gauge read? you see 200psi
200 psi each (Pascal’s Law)
It’s a cold winters day, your journeyman tells you to grab a bottle of R22 from his van outside, to a 85F degree mechanical room, what will happen to the pressure inside the cylinder?
Increase
What will happen to the pressure inside a cylinder with a pressure of 30 psig and a volume of 10 cubic inches, if the piston moves upstroke and reduces the volume to 2 cubic inches?
Increase
What will cause the most turbulent flow?
1) Deburred pipe
2) Bends made with pipe benders
3) Short radius 90’s
4) Couplings
Short radius 90’s
You are working on a boiler in the basement of a 100’ tall hotel, if the pressure gauge in the basement shows 40psi, how much pressure should you expect on the top floor?
0psi
What is the velocity of the refrigerant flowing through the refrigeration system?
Mass flow rate
What is the amount of heat added by the compressor and the compression process during the refrigeration cycle?
Heat of Compression
What is the amount of heat gained in the evaporator during the refrigeration cycle?
Net Refrigeration Effect
What is the amount of heat rejected by the condensing process?
Heat of Rejection
Lower CoP (Coefficient of Performance) values result in an effective performance
T or F
False
Lower CR (Compression Ratio) values result in a greater system performance
True
You grab a bottle labelled r410a from a 65F room and hook up you gauges to it. It reads 185.1psig. What is in the bottle?
saturated refrigerant
You grab a bottle labelled r134a from the 65F storage room and hook up your gauges. It reads 140.5psig. What is in the bottle?
unknown/contaminated mixture
A change of state from a solid directly to a vapour without having a liquid state
Sublimation
Convert 5psia to “hg
19.8 “hg
Convert 150kpaa to psig
7.07
Convert 460F to K
510K
Its a cold -40F day. you take a big birthday balloon from your car at -40F to inside at 70F, and it started at 4 cuft, what should the current volume of the balloon be?
5 cuft
How much heat would be required to turn 1 lbs of 40F water into 412F steam @ 0.46
1234
a compressed gas cylinder of 3cu/ft is filled to 150psig. Trent dropped the bottle from the building and dented it, reducing the volume to 2cu/ft. as the ambient temperature has remained constant, what is the new pressure in the bottle?
232psig
Fluid pressure based on the height of a column of liquid
Static/Head Pressure
Your boss can’t figure out what’s wrong with a cooler so he’s asked you to plot the system out on a PE chart for him. Plot the system below on a PE chart (provided) so your boss can figure out what’s wrong
Use the 134a PE Chart
SDT 140F
SST 20F
Superheat 60F
Subcooling 20F
What is H1 in Btu/Lbs? (H1 = TXV inlet)
(Answer to nearest whole number, no decimals, no units) Example: 74
Answer: 40
Use the 134a PE Chart
SDT 140F
SST 20F
Superheat 60F
Subcooling 20F
What is H2 in Btu/Lbs? (H2 = TXV outlet)
(Answer to nearest whole number, no decimals, no units)
Answer: 40
Use the 134a PE Chart
SDT 140F
SST 20F
Superheat 60F
Subcooling 20F
What is H3/H4 in Btu/Lbs? (H3 = Outlet of compressor, H4 = inlet of compressor, for this they are the same number)
(Answer to nearest whole number, no decimals, no units)
Answer: 110
Use the 134a PE Chart
SDT 140F
SST 20F
Superheat 60F
Subcooling 20F
What is H5 in Btu/Lbs? (H5 = Outlet of Compressor)
(Answer to nearest whole number, no decimals, no units)
Answer: 130
On a PE chart what component is on the far left?
TXV
If you increase subcooling will it increase your NRE?
Yes
Hooking up your gauges to the 134a system, you read 30psi and 230psi. What is the Compression ratio?
5.47:1
Which type of compressor is a non-positive displacement compressor?
Centrifugal
SST = 35F
SDT = 100F
Evap Supply temp = 50F
Evap Return temp = 70F
Condenser Supply Temp = 125F
Condenser Return Temp = 95F
What is the Temp Drop for this system?
20F
What type of compressor did we take apart in class?
Semi-hermetic
Energy travels from High energy position to Low energy position, AKA: Heat travels from Hot to Cold
T or F
True
Leaving the metering device, what state is the refrigerant in?
80% Liquid, 20% Vapour
You are working on a cooler and find the evaporator fan has died. What should you be worried about? (Hint: Evaporators fans push warm air across a colder coil)
Too little superheat
You show up to a service call and the unit has tripped on high head pressure. What could the issue be? (MULTI-SELECT) (SELECT 6 of the Answers)
Undercharge (Low on refrigerant)
Overcharge (too much refrigerant)
Restricted/Dirty condenser
Restricted/Dirty Evaporator
Heavy Load (high return air temp)
Light Load (low return air temp)
Non-condensables in the system (moisture/bad gas in the system)
Low outdoor ambient
High outdoor ambient
Dead condenser fan motor
Dead evaporator fan motor
Overcharge (too much refrigerant)
Restricted/Dirty condenser
Non-condensables in the system (moisture/bad gas in the system)
High outdoor ambient
Dead condenser fan motor
Heavy Load (high return air temp)
