WD Catalog 2018

Chapter 1

– Air Conditioning / Heating Function

1-6

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Table of Contents

Index

Heat Relationships

Relative humidity plays an important part in our comfort. So does air move-

ment. The relative humidity is a measure of the moisture in the air. Air

movement is a measure of the speed or velocity of air as it moves. We use CFM’s

or Cubic Feet per Minute as a measure of air movement. As they operate, truck

HVAC systems remove humidity or moisture and circulate the air around us to

keep the cab occupants comfortable.

Changes of State

We talked about “change of state” when we mentioned earlier that 970 BTU’s of

heat energy were needed to change a pound of water into vapor. We use water

in our example because it is familiar to all of us. In AC systems the refrigerant

is used instead of water. It evaporates and condenses (changes state) in the

system almost continuously. It is this action that makes an air conditioner cool

the cab and its occupants. A “change of state” works for us by moving large

amounts of heat energy fast, under the right conditions.

Heat Movement During Changes of State

Evaporation and condensation both take place inside the air conditioning

system. When either of these conditions occur, a lot of heat moves. We use

refrigerant because its temperatures of vaporization and condensation are

nearly optimum for this application. Using the familiar 20 pound refrigerant

cans, Figure 1-9 shows the effect of a “change of state” on R-12. You can see how

fast a change in pressure in the can changes the temperature of the refrigerant.

WARNING

Refrigerant can be dangerous if released as shown

below. These drawings are for illustration purposes

only—to show “change of state”.

VALVE CLOSED

VALVE CRACKED OPEN

VALVE OPEN WIDE

70.1 PSI

46.6 PSI

10.0 PSI

70° F

50° F

2° F

VAPOR

VAPOR AT -21.6° F (0 PSI)

AIR TEMPERATURE

70° F OUTSIDE

OF CAN

Figure 1-9

In these drawings the dis-

pensing valve is used to

release pressure inside the

cans. Note that R-12 boils at

a temperature of -2

degrees Fahrenheit, com-

pared to water at 212 de-

grees Fahrenheit.