CHAPTER 3 Basics of Heating and Air Conditioning

CHAPTER 3Basics of Heating and Air Conditioning

Temperature andPressure Fundamentals

Objectives

• Discuss the fundamentals of temperature and pressure.

• Explain the nature of atoms and molecules.

• Describe the differences between sensible, latent, and specific heat values.

• Discuss the measurement of heat energy.• Describe how heat flows.

Objectives

• Explain effects of radiation, conduction, and convection on personal comfort.

• Describe the differences between humidity and relative humidity. Explain the purpose of the heating, ventilation, and air-conditioning (HVAC) system in automobiles and other vehicles.

• Discuss temperature and humidity comfort zones.• Explain how heat is measured in both temperature

and quantity, using either metric or U.S./British units

INTRODUCTION

• For the most part, the heating, ventilation, and air-conditioning (HVAC) system of an automobile is designed to provide comfort for the driver and passengers.

• It is intended to maintain in-car temperature and humidity within a range that is comfortable for the people inside and provide fresh, clean air for ventilation.

• This temperature range also helps keep the driver alert and attentive.

INTRODUCTION

• The HVAC system in a vehicle (SEE FIGURE 3–2) can be divided into three closely related subsystems:– Air distribution, also called air

management, with the control system: The HVAC case with doors that control the air flow

– Heating: The heater core heats the air.– Refrigeration, A/C: The evaporator core cools

the air

Matter (1 of 2)

• Matter– Atoms

• All elements consist of atoms.

• Atoms are made up of– Electrons (-)– Protons (+)– Neutron (no charge)

– Molecules• Atoms of different

elements• R-134a (CF3CH2F)

Matter (2 of 2)

• Different from original element• Water molecule H2O

– Two hydrogen atoms– One oxygen atom

Temperature and Pressure (1 of 2)

• Cohesion– Attraction of molecules to each other

• Cold – Molecules tightly packed– Little movement

Temperature and Pressure (2 of 2)

• Heat– Molecules loosely

packed– Much movement of

molecules

• More heat, more movement

• More movement, more heat

HEAT

• Heat is a form of basic energy, and, like other forms of energy, heat cannot be created or destroyed. – It can, however, be converted to or from other

forms of energy.

LATENT AND SENSIBLE HEAT

• Sensible heat makes sense; it can be felt and measured on a thermometer. – If we have 1 lb of water at 40°F and add 1 Btu

of heat to it, the temperature will increase to 41°F; adding another Btu of heat will increase the temperature to 42°F; and adding another 170 Btu (212 – 42) will increase the temperature to 212°F, the boiling point.

LATENT AND SENSIBLE HEAT

• Sensible heat is fairly easy to understand, but if we add more heat, an odd thing occurs.

• If we add another Btu of heat to water at 212°F, some of the water will boil, but the temperature of both the water and the steam produced will remain at 212°F. – The added heat has caused some of the water to

change state, but it has not changed temperature.• This is an example of latent, or hidden, heat.

Types of Heat (1 of 3)

• Sensible heat– Is felt or measured– Added to raise temperature of matter– Measured with a thermometer– Increased molecule movement

Types of Heat (2 of 3)

• Latent heat– It is hidden.– It is not felt or measured.– Increasing the pressure

increases the temperature of the state of change.

– Heat is required to effect a change in state.

• Solid to a liquid [water may be a liquid or a solid at 32°F (0°C)]

• Liquid to a gas [water may be a liquid or a gas at 212°F (100°C)]

BOILING POINTS

• Boiling points can be increased or decreased by raising or lowering the pressure on the liquid.

• With water, the boiling point will rise about 3°F for each pound per square inch (psi) of pressure, or about 1°C for each 5 kPa.– CRITICAL TEMPERATURE

HEAT MEASUREMENT

• A heating and air-conditioning technician is concerned with measuring two different aspects of heat: intensity and quantity . – Intensity is what we feel; it is measured in

degrees, on either a Celsius or Fahrenheit scale.

– Quantity is the actual amount of heat; it is measured in either calories or British thermal units (Btu) .

COMFORT

• Our goal in heating and air conditioning is to maintain a comfortable in-vehicle temperature and humidity.– HUMIDITY– CLEANLINESS

1. Heat is moved into or out of the passenger compartment to obtain a good comfort level.

2. Heat intensity is measured using the Fahrenheit and Celsius scales, and heat quantity is measured using calories and Btu.

3. The comfort zone of most humans is between 65 and 80°F (21 and 27°C).

4. A/C systems reduce humidity by removing water from the air.

5. HVAC systems clean air because particles are caught by moisture on the evaporator and by filters.

Measuring Heat Energy

• Measured by what it does

• British thermal unit (Btu)– Heat energy is

required to raise one pound of water 1°F

• Calorie (metric)– 252 calories in 1 Btu

Temperature Versus Btus• Temperature is the

degree of heat.– For example, a candle

may burn at 500°F.• Btu is the volume of

heat, not its temperature.– For example, you may

need to burn four candles to boil a cup of water.

Heat and Cold (1 of 2)

• Heat is always present in matter.• Heat and cold = the amount of heat in an

object

Heat and Cold (2 of 2)

• Heat– Form of energy existing as a result of molecular

movement• Cold

– Heat removed, absence of heat• Superheat

– Temperature increased above matter’s vapor point temperature

– Heated vapor under pressure

HEAT MOVEMENT• CONDUCTION

– the transfer of heat between two parts of a stationary system, caused by a temperature difference between the parts.

• CONVECTION– the transfer of heat by the circulation or movement

 of the heated parts of a liquid or gas. • RADIATION

– the complete process in which energy is emitted by one body, transmitted through an intervening medium or space, and absorbed by another body.

Radiation

• Heat is transferred from a hot object to a colder object by heat rays.– Heat moves from a hot

surface to a surface containing less heat.

• Air movement and temperature do not effect the process.

Conduction (1 of 2)

• This is the transfer of heat through a material.

• Some of the heat is absorbed by the material, while some is given off.

• A material that transmits heat well with a small loss is a good conductor of heat.– Material that is a good conductor of electricity

is also a good conductor of heat.

Conduction (2 of 2)

• A material that does not conduct heat easily is called an insulator.

• Coolant conducts heat to the radiator and heater core.

Convection

• It only occurs in fluids: liquids, gases, or vapors.

• Hotter fluid rises, while the colder fluid sinks.– This was the principle

of early household heating systems.

Temperature

• Thermal energy moves from an area of more heat to an area of less heat (cold).

Relative Humidity

• People tend to feel most comfortable at a relative humidity of about 45 percent. Humidifiers and dehumidifiers help to keep indoor humidity at a comfortable level.

• Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapor and cannot hold any more, creatin g the possibility of rain. This doesn't mean that the relative humidity must be 100 percent in order for it to rain -- it must be 100 percent where the clouds are forming, but the relative humidity near the ground could be much less.

Comfort Levels

• Temperature• Relative humidity• Air movement• Heat movement

(dissipation)

• Convection• Radiation• Evaporation

STATES OF MATTER

• The air-conditioning process works through a fluid, called a refrigerant, that continuously changes state from liquid to gas and back to liquid.

• These changes of state are where the movement of heat needed for cooling occurs.– SOLID– LIQUID– GAS

SATURATED VAPORS AND THE PRESSURE-TEMPERATURE RELATIONSHIP

• Saturated vapor, also called a saturated liquid, is the term used to describe a liquid and gas inside a closed chamber, which is the condition in an A/C system.

• When discussing saturated vapors, we need to learn two additional terms: subcool and superheat . – Subcool refers to a liquid whose temperature is well

below its boiling point. – Superheat refers to the temperature increases of a

vapor above its boiling point.

REFRIGERANTS

• The working fluid of an A/C system is refrigerant. – Refrigerants were first developed by the

DuPont Corporation using the name Freon. • This term is used improperly by many people to

mean refrigerant.

• A new term, SUVA, refers to DuPont’s newer refrigerants: SUVA MP52 (a blend) and SUVA Trans A/C (134a). – There are many refrigerants, but the three

main ones used in automotive and other mobile systems are R-12, R-134a, and R-1234yf.

REFRIGERANTS

• R-12• R-22• R-134A• HFO-1234YF• R-152A• R-290, PROPANE, & R-600 BUTANE• R-744, CO2

• BLENDS• REFRIGERANT OILS

Gas Laws

• Boyle’s Law• Charles’ Law• Dalton’s Law

Boyle’s Law

• As pressure is applied to a volume of gas in a closed container the volume of the gas will decrease and pressure will increase.

Boyle’s Law

Charles’ Law• The volume of a fixed mass

of gas held at a constant pressure varies directly with a change in the absolute temperature.

• Conversely, when a gas is held at a constant volume its pressure varies directly with absolute temperature.

Charles’ Law

Dalton’s Law

• The total pressure of a confined mixture in a sealed container is equal to the sum of the pressures of each gas in the mixture.

Air Conditioning Saves Fuel!

• Vehicles are more aerodynamic with the windows closed.– Increased fuel consumption caused by the

A/C compressor is offset by the higher level of drag created by having windows open.