HVACR UNIT 9

Convert -100° Celsius to Kelvin.

-100 + 273 = 173°K

Convert -100° Fahrenheit to Rankine.

-100 + 460 = 360°R

Calculate the amount of heat in BTU required to change the temperature of 10 lb of copper from 100°F to 150°F. The specific heat of copper is 0.09 BTU/lb.

10 lb × 50°F × 0.09 BTU/lb = 45 BTU

Convert 100° Kelvin to Celsius.

100 - 273 = -173°C

Convert 100° Rankine to Fahrenheit.

100 - 460 = -360°F

How much heat is required to increase the temperature of 20 lb of water from 32°F to 212°F?

20 lb × 180°F = 3,600 BTU

Convert 200° Kelvin to Rankine.

200 × 1.8 = 360°R

Convert 200° Rankine to Kelvin.

200/1.8 = 111.11°K

What is the temperature of 125 Kelvin converted to Rankine?

225 Rankine

Which of the following is an example of forced convection?

A ceiling fan forcing warm air down

What is a k factor?

A measure of thermal conductivity in BTU/hr/ft 2/°F/in of thickness

What is an absolute temperature scale?

A temperature scale with no negative temperatures and 0° equal to absolute zero

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.

One disadvantage of infrared thermometers is that they:

Can only read the temperature of objects.

What temperature scale uses 0° as the freezing point of water and 100° as the boiling point of water?

Celsius

The two metals used in a type K thermocouple are:

Chromel and alumel.

What process causes dew to form on grass?

Condensation

The three basic methods of heat transfer are:

Conduction, convection, and radiation.

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.

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.

Natural convection occurs because of:

Differences in fluid density due to temperature difference.

Which is TRUE of absolute temperature?

Discovering the starting point of temperature required mathematical calculations.

Which of the following statements is TRUE of specific heat capacity?

Each substance has its own specific heat capacity regardless of state.

The first law of thermodynamics states that:

Energy cannot be created or destroyed.

What is the operating principle of glass stem thermometers?

Expansion and contraction of the fluid makes the liquid level rise and fall with temperature.

What temperature scale uses 32° as the freezing point of water and 212° as the boiling point of water?

Fahrenheit

What is the purpose of adding fins on the tubing used for heat transfer?

Fins increase heat transfer by increasing the surface area.

If water in a cup is 10°C and the cup is 20°C, 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

According to Joe's pressure-temperature chart, the saturation temperature for 12 lb of pressure is 36.9°F. The actual refrigerant temperature measured was 35.2°F. What does this tell you about the refrigerant?

It is saturated.

An air conditioner using refrigerant R-134a has a low-side pressure of 35 psig and a temperature of 40°F. According to the pressure-temperature chart:

It is saturated.

An air conditioner with refrigerant R-410a has a high-side pressure of 340 psig and a temperature of 95°F. According to the pressure-temperature chart:

It is subcooled.

An air conditioner with refrigerant R-22 has a low-side pressure of 70 psig and a temperature of 55°F. According to the pressure-temperature chart:

It is superheated.

Select the temperature scales that can be used to work gas law problems. You may select more than one.

Kelvin and Rankine

The absolute temperature scale for Celsius is:

Kelvin.

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.

What is the first color of visible light that is given off by heated objects as they increase in temperature?

Red

A heat process that causes a change in a substance's temperature is called:

Sensible heat.

How many steps are required to calculate the heat necessary to change 10 lb of ice at -20°F to steam at 250°F?

Six

The amount of heat required to change 1 pound of a substance 1°F 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

The Kelvin temperature scale is:

The absolute temperature scale for Celsius.

The Rankine temperature scale is:

The absolute temperature scale for Fahrenheit.

Temperature is the measure of:

The average kinetic energy in a substance.

How do the freezing point of water and absolute zero compare?

The freezing point of water is a higher temperature than absolute zero.

Which of the following changes occur in a melting ice cube?

The water changes from a solid to a liquid.

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.

Calculate the amount of heat required to change 5 lb of 212°F water to steam.

5 lb × 970 BTU/lb = 4,850 BTU

Convert 40° Fahrenheit to Celsius.

40 - 32 = 8; 8/1.8 = 4.4°C

Convert 40° Celsius to Fahrenheit.

40 × 1.8 = 72; 72 + 32 = 104°F

Calculate the amount of heat required to change 5 lb of 32°F ice to water.

5 lb × 144 BTU/lb = 720 BTU

What measurements are needed to determine if refrigerant in a refrigeration system is saturated, superheated, or subcooled?

Accurate pressure and temperature readings