chapter 6 moving heat: heating and air conditioning principles

CHAPTER 6Moving Heat: Heating and Air

Conditioning Principles

Objectives

After studying Chapter 6, the reader will be able to:

1. Understand heating and cooling loads.

2. Explain the ways to handle a heating load.

3. Explain the ways to handle a cooling load.

4. Describe the effect that compression and expansion have on the temperature of a gas.

Objectives

• Discuss heat transfer in the refrigerant system.• Describe the effect humidity has on the air

conditioning system.• Explain the pressure and vacuum relationship in

the air conditioning system.• Understand the basic function of the various air

conditioning components.• Discuss the role of refrigerant in the system.

Objectives

• Explain the pressure verses temperature relationship.

• Know the physical state, pressure, and temperature of the refrigerant in different areas of the refrigerant system.

• Describe the nomenclature and function of the manifold and gauge set.

• Identify the scaling of the low- and high-side gauges in English and metric units.

Objectives

• Calibrate a gauge.• Connect a manifold and gauge set into an

automotive air conditioning system.• Perform a performance test on an

automotive air conditioning system.

INTRODUCTION

• Heating and air conditioning must follow the basic rules of heat transfer.

• An understanding of these rules helps greatly in understanding the systems:– Heat always flows toward cold.– To warm a person or item, heat must be added.– To cool a person or item, heat must be removed.– Fuels can be burned to generate heat.– A large amount of heat is absorbed when a liquid changes

state to a vapor.– A large amount of heat is released when a vapor changes

state to a liquid.– Compressing a gas concentrates the heat and increases

the temperature.

HEATING LOAD

• Heating load is the term used when we need to add heat.– SEAT HEATERS

COOLING LOAD

• Cooling load – describes the removal of heat, which is the

purpose of air conditioners and evaporative coolers.

– We need to move heat to a cooler location in order to handle a cooling load.

COMPRESSION HEATING

• When we compress a gas into a higher pressure, we also increase the temperature of the gas.– CO2 SYSTEMS

EXPANSION COOLING

• If we can raise the temperature of a gas by compressing it, we can lower the temperature if we allow it to expand. – Expanding a volume of gas spreads out the

heat energy over a larger area and lowers its temperature.

Air Conditioning System Components

• Compressor• Condenser• Receiver dryer/Accumulator• Metering device• Evaporator• Hoses and lines• Service valves

Heat Transfer in A/C Systems• The basic function of the

automotive air conditioning system is to remove heat and humidity from the passenger compartment and transfer this heat to the outside air.

• Heat is removed from the airflow by an evaporator.

• Refrigerant is pressurized by the compressor and set onto the condenser.

• Condenser acts like a radiator, removing heat from refrigerant.

• The system uses two different operating pressures (high and low).

Air Conditioning and Humidity

• Humidity increases the heat load on the refrigerant system.

• Maximum refrigerant system performance and passenger compartment cooling is achieved by selecting Recirculation mode.

• Moisture removed from the air steam passing through the evaporator will drain out of the duct system.

How is Cooling Achieved in the Air Conditioning System?

• Cooling in the refrigerant system is achieved by the rapid expansion of refrigerant gas.

Basic Air Conditioning System Components

• Compressor– Heart of the air

conditioning system– Pumps refrigerant

through the system– Contains a suction

side (low-pressure vapor, cold) and a compression side (high-pressure vapor, hot)

Basic Air Conditioning System Components

• Condenser– Heat exchanger– Change of state takes place– High-pressure vapor (hot) changes into high-

pressure liquid (hot)

Basic Air Conditioning System Components

• System uses either– Receiver dryer –

high-pressure liquid side

• Placed between condenser and metering device

• Used with thermostatic expansion valve

– Accumulator – low-pressure vapor side

• Placed between evaporator and compressor

• Used with fixed orifice tube

Receiver Dryers & Accumulators

To play, click the video screen.

Basic Air Conditioning System Components

• Metering Device– High-pressure liquid

changed into a low- pressure liquid

– Hot on the high-pressure side, cold on the low-pressure side

– Two basic types• Thermostatic expansion

valve• Fixed orifice tube

Two Basic Air Conditioning System Designs

• The Expansion Valve designs

Two Basic Air Conditioning System Designs

• The Orifice Tube design.

Basic Air Conditioning System Components

• Evaporator– Heat exchanger

• removes heat from the air entering the passenger compartment (cold)

– Change of state takes place

– Low-pressure liquid changes into low-pressure vapor

Basic Air Conditioning System Components

• Hoses and lines– Suction line

• Low-pressure, cool vapor

– Discharge line• High-pressure, hot

vapor

– Liquid line• High-pressure, hot

liquid

Basic Air Conditioning System Components

• Service valves– R-134a (HFC- 134a)– R-12 (CFC-12)

Refrigerant• The substance that flows through

the refrigeration system is referred to as refrigerant.

• The most important properties of a refrigerant are:1. A refrigerant must not be

explosive or flammable.2. A refrigerant must not be

hazardous and a leak should be easily detectable.

3. The refrigerant must be highly stable and allow for repeated use without decomposing or changing its properties.

4. The refrigerant must not cause damage to parts or materials used in the compressor or other components.

5. The refrigerant must vaporize easily in the evaporator.

6. The larger the latent heat value at the vaporization point of the refrigerant, the smaller the volume of refrigerant that will be required for circulation and the smaller the total size of the refrigeration system.

7. The critical temperature of the refrigerant must be higher than the condensation temperature of the system.

8. Evaporator pressure must be higher than atmospheric pressure.

Refrigerant R-134a

• If the pressure of R-134a is high the temperature will also be high.

• If the pressure is low the temperature will also be low.

Refrigerant R-134a• The following graph

represents the relationship of pressure and temperature of R-134a.

• The curved line on the graph is the change of state point of R-134a between a liquid and a gas under various pressures and temperatures.

Pressure verses Temperature Relationship

• Temperature at which refrigerant vaporizes or condenses is the saturation temperature.

Pressures

• High Side– Prevents refrigerant from boiling– Moves heat to the condenser– Pressurizes refrigerant back to the evaporator

• Low Side– Sudden loss of pressure at the metering device– Allows refrigerant to absorb heat in the evaporator– Moves heat to the compressor for refrigerant

pressurization

State of Refrigerant in the Air Conditioning Circuit

• Thermostatic Expansion Valve (TXV) System.A – Low pressure vapor

B – Low pressure liquid

C – Low pressure liquid and vapor

D – High pressure vapor

E – High pressure liquid

F – High pressure liquid and vapor

State of Refrigerant in the Air Conditioning Circuit

• Fixed Orifice Tube (FOT) System.A – Low pressure vapor

B – Low pressure liquid

C – Low pressure liquid and vapor

D – High pressure vapor

E – High pressure liquid

F – High pressure liquid and vapor

Touch and Feel Test

To play, click the video screen.

Refrigerant Temperature Pressure Chart

Manifold and Gauge Set

• Is the primary diagnostic tool

• Used to measures high- and low-side air conditioning system pressures

• Also used to connect A/C system to recovery, recycling, or charging stations

Gauge Scales (1 of 2)

• Low Side– Compound gauge– Vacuum down to 30

in.Hg– Pressure to 120 psig– Metric– Absolute kiloPascal

(kPa)– 1 psi = 6.895 kPa

Gauge Scales (2 of 2)

• High Side– Pressure only– Up to 500 psig– kPa gauge or absolute – Must be calibrated

periodically

Connecting Gauges

• R-134a (HFC-134a)– Close all valves on Gauge Manifold.– Remove caps.– Pull up on quick-release collar and connect

hose to A/C line fitting.

Connecting Gauges

• Safety glasses, gloves, fender cover• Schrader valve – R-12 (CFC-12)

– Close all valves.– Remove caps.– Connect hoses firmly.

Performance Test

• Protect engine with external fan• Connect manifold and gauge set• Set engine speed 1500-1700 rpm• Set A/C on MAX (Recirculation mode)• Check gauges for typical pressures• Check high- and low-side lines• Test control devices per manufacturer

Performance Test

To play, click the video screen.

Summary

1. A heating load occurs when heat is desired; a cooling load calls for more cooling.

2. Heat is added by moving hot coolant to the heater core and blowing in-car air across the heater core.

3. Cooling can occur through evaporative cooling.

4. Most vehicle cooling uses mechanical refrigeration to move latent heat from the evaporator to the condenser.

5. Gases heat when they are compressed and cool when they expand.