Unit 9
What is a k factor?
A measure of thermal conductivity in BTU/hr/ft squared/degree F/in of thickness
Three basic methods of heat transfer are:
Conduction, convection, and radiation
Natural convection occurs because of:
Differences in fluid density due to temperature difference
What process causes dew to form on grass?
condensation
The air circulating inside a refrigerator is an example of
convection
As the distance between two objects increases, the radiant energy transferred between them:
decreases
According to Joe's pressure-temperature chart, the saturation temperature for 12 lb of pressure is 36.9 degrees Fahrenheit. The actual refrigerant temperature measured was 35.2 degrees Fahrenheit. What does this tell you about the refrigerant?
it is saturated
An air conditioner with refrigerant R410a has a high-side pressure of 340 psig and a temperature of 95 degrees Fahrenheit. According to the pressure-temperature chart:
it is sub-cooled
An air conditioner with refrigerant R-22 has a low-side pressure of 70 psig and a temperature of 55 degrees Fahrenheit. According to the pressure-temperature chart:
it is super heated
What is an absolute temperature scale?
A temperature scale with no negative temperatures and 0 degrees is equal to absolute zero
What is the operating principle of glass stem thermometers? A. Expansion and contraction of the fluid makes the liquid level rise and fall with temperature B. The fluid inside the thermometer boils as it is heated, causing the liquid level to rise C. The fluid climbs the walls of the tube through capillary attraction, which works better at higher temperatures
A. Expansion and contraction of the fluid makes the liquid level rise and fall with temperature
A metal rod is heated on one end and heat travels from that end to the cooler end through the rod. This is an example of:
Conduction
Which of the following statements is true of specific heat capacity? A. All substances in a given state have the same specific heat capacity B. Specific heat cannot be calculated for every substance C. Specific heat capacity is only relevant to substances in the solid state D. Each substance has its own specific heat capacity regardless of state
D. Each substance has its own specific heat capacity regardless of state
Which of the following changes occur in a melting ice cube? A. The water increases in temperature as it melts B. The increase in water temperature causes it to melt C. The water evaporates as it changes state D. The water changes from a solid to a liquid
D. The water changes from a solid to a liquid
The first law of thermodynamics states that
Energy cannot be created or destroyed
What temperature scale uses 32 degrees as the freezing point of water and 212 degrees as the boiling point of water?
Fahrenheit
How many steps are required to calculate the heat necessary to change 10 lbs of ice at -20 degrees Fahrenheit to steam at 250 degrees Fahrenheit?
Six
The amount of heat required to change 1 pound of a substance 1 degree Fahrenheit is called
Specific heat capacity
What process occurs when dry ice disappears?
Sublimation
If molecules in Substance A are moving at a higher rate than the molecules in substance B, which substance has the higher temperature?
Substance A
How do the freezing point of water and absolute zero compare?
The freezing point of water is a higher temperature than the absolute zero
A heat process that causes a change in a substance's temperature is called:
sensible heat
The Kelvin temperature scale is:
the absolute temperature scale for Celsius
The Rankine temperature scale is:
the absolute temperature scale of Fahrenheit
The absolute temperature scale for Celsius is:
Kelvin
Convert -100 degrees Celsius to Kelvin
-100 + 273 = 173 degrees Kelvin
Convert -100 degrees Fahrenheit to Rankine
-100 + 460 = 360 degrees Rankine
Calculate the amount of heat in BTU required to change the temperature of 10 lbs of copper from 100 degrees Fahrenheit to 150 degrees Fahrenheit. The specific heat of copper is 0.09 BTU/lb
10 lb x 50 degrees F x 0.09 BTU/lb = 45 BTU
Convert 100 degrees Rankine to Fahrenheit
100 - 460 = -360 degrees Fahrenheit
Convert 100 degrees Kelvin to Celsius
100-273= -173 degrees Celsius
How much heat is required to increase the temperature of 20 lbs of water from 32 degrees Fahrenheit to 212 degrees Fahrenheit?
20 lb x 180 degrees Fahrenheit = 3,600 BTU
Convert 200 degrees Kelvin to Rankine
200 x 1.8 = 360 degrees Rankine
Convert 200 degrees Rankine to Kelvin
200/1.8= 111.11 degrees Kelvin
What is the temperature of 125 Kelvin converted to Rankine?
225 Rankine
Convert 40 degrees Fahrenheit to Celsius
40 - 32 = 8; 8/ 1.8 = 4.4 degrees Celsius
Convert 40 degrees Celsius to Fahrenheit
40 x 1.8 = 72; 72 + 32 = 104 degrees Fahrenheit
Calculate the amount of heat required to change 5 lb of 32 degree Fahrenheit ice to water
5 lb x 144 BTU/lb = 720 BTU
Calculate the amount of heat required to change 5 lbs of 212 degree Fahrenheit water to steam
5 lb x 970 BTU/lb= 4,850 BTU
What measurements are needed to determine if refrigerant in a refrigeration system is saturated, superheated of subcooled?
Accurate pressure readings and temperature readings
Which is true of absolute temperature? A. The reference points in traditional scales were useful in absolute temperature calculations B. Discovering the starting point of temperature required mathematical calculations C. There is no start or end point to temperature D. Discovering the end point of temperature required complex calculations
B. Discovering the starting point of temperature required mathematical calculations
The pointer on a dial-type thermometer is moved by a:
Bimetal element
Which is the largest similarity between the Kelvin and the Rankine temperature scales?
Both begin at absolute zero
Which of the following is an example of forced convection? A. A radiant heater placed in a room B. Air moving in a refrigerator C. A ceiling fan forcing warm air down D. Heat traveling along a metal rod
C. A ceiling fan forcing warm air down
Most materials that are good electrical conductors: A. Will transfer heat by radiation, but not by conduction B. Are poor thermal conductors C. Also tend to make good thermal conductors D. Will transfer heat by convection, but not by conduction
C. Also tend to make good thermal conductors
Two materials that are considered good thermal conductors are: A. Paper and wood B. Glass and ceramic C. Copper and aluminum D. Thermoplastic and rubber
C. Copper and aluminum
One disadvantage of infrared thermometers is that they:
Can only read the temperature of objects
What temperature scale uses 0 degrees as the freezing point of water and 100 degrees as the boiling point of water?
Celsius
The two metals used in a type K thermocouple are:
Chromel and alumel
What is the purpose of adding fins on the tubing used for heat transfer?
Fins increase heat transfer by increasing the surface area
According to the second law of thermodynamics, how does heat travel?
Heat energy travels from a warmer body to a cooler body
If water in a cup is 10 degrees Celsius and the cup is 20 degrees Celsius, which way will the heat travel according to the second law of thermodynamics?
Heat will travel from the cup to the water
Which type of thermometer can measure the temperature of a surface from a distance?
Infrared
An air conditioner using Refrigerant R-134a has a low-side pressure of 35 psig and a temperature of 40 degrees Fahrenheit. According to the pressure-temperature chart:
It is saturated
Refrigerant in the evaporator coil of an air conditioner changes from a liquid to a gas when hot air passes over it. The temperature of the refrigerant does not change. What concept does this demonstrate?
Latent Heat
A heat process that causes a substance to change state is called:
Latent heat
Which of the following forms of heat travels in waves?
Radiant heat
Campers being heated by a fire outside are an example of:
Radiation
The absolute temperature scale for Fahrenheit is:
Rankine
The temperature scales that can be used to work gas law problems?
Rankine and Kelvin
What is the first color of visible light that is given off by heated objects as they increase in temperature?
Red
Temperature is the measure of:
The average kinetic energy in a substance
What is absolute zero?
The coldest temperature possible where all molecular motion stops
What happens to the temperature of water once it starts to boil?
The water temperature stays the same once the water begins to boil
The relationship between Celsius and Fahrenheit is that:
There are 1.8 Fahrenheit degrees for every Celsius degree
The rate of heat transfer is faster if:
There is a large temperature difference between the two objects
Electronic temperature sensors can be:
thermocouple, thermistor, or wire-wound
