Heat Intensity: Measured in degrees; indicative of sensible heat.
Heat Quantity: Measured in British Thermal Units (BTU); relates more to energy.
When changing water to steam, BTUs required:
970 BTUs to convert one count of water into one count of steam.
Clarification: At 212°F (boiling point), additional heat must be applied to change state.
Sensible Heat and Phase Changes
Sensible Heat:
Defined as heat that can be sensed or measured without changing the state of the substance.
Physical change in temperature without phase change.
The Relationship of Pressure, Temperature, and Airflow
Effects of Blockage in Condenser:
If a condenser gets blocked (e.g., dust), it causes:
Pressure to increase.
Temperature to rise due to lack of airflow and inability to dissipate generated or absorbed heat.
Consequences of Moisture in AC Systems
Dangers of Moisture Entry:
Rust and acid formation can occur.
Potential compressor failure or freeze-up problems.
Refrigerant Properties
Boiling Point of R-134a:
Boiling point is -15°F or approximately -26.2°C.
Moisture in an AC system can lead to rust, mold, contamination, and potential compressor damage.
Removing Moisture from AC Systems
Process:
Utilize a vacuum to eliminate moisture from the system.
It lowers the boiling point of moisture, causing it to evaporate.
Oil Levels in AC Systems
Oil Amount:
Must match as much oil as is removed during recovery operations, aligned with OEM specifications.
Typical OEM spec: Generally around three pounds of refrigerant in a vehicle, and oil types depend on the refrigerant used (mineral oil for R-134A).
Manifold Gauge Set
Usage:
Blue Gauge: Indicates low pressure (compound gauge can read negative pressure).
Red Gauge: Measures high pressure.
Gauges must remain closed when checking system pressures to prevent refrigerant release.
Typical Static Pressure:
Static pressure typically ranges between 60-70 PSI, influenced by ambient temperature.
Diagnosing AC System Issues
AC Troubleshooting:
Low refrigerant leads to compressor failure due to lack of lubrication.
If low pressure switch is open, the compressor won’t operate.
Thermostatic Control in AC Systems
Thermostatic Switch Purpose:
Senses temperature to control compressor operation to avoid freezing.
Free Switch Function:
Turns off compressor to prevent freezing at low evaporator temperatures.
Refrigerant Charge and Behavior
Air as a Condensable Gas:
Air is not considered a condensable gas in the context of AC systems.
Implications of Air Contamination:
Can lead to moisture intrusion, cooling inefficiencies, and compressor wear.
System Behavior During Operation
Pressure Equalization:
Pressure equalizes in the AC system when the engine is off.
Operating Pressure Readings:
Typically 10-35 PSI on the low side and 150-255 PSI on the high side.
Working with AC Systems
Lubricants for Seals:
Utilize mineral oil on O-rings and fittings in AC systems.
Leak Detection Methods
Electronic Leak Detectors:
Effective way to locate refrigerant leaks by sensing presence of refrigerant.
Soapy Water Tests:
Used to check for leaks in connections: bubbles indicate leaks.
Dye Injection:
Fluorescent dye can aid in tracking down hard-to-find leaks after circulating through the system.
Safety Precautions
Glove Use:
Avoid using leather gloves when working with refrigerants as they can freeze to the skin.
Effects of Ambient Temperature on AC Systems
Increased ambient temperatures lead to higher pressures in the system.
Running an AC system without refrigerant can damage the compressor.
High Pressure Situations in AC Systems
Causes of High Pressure:
Restriction in the condenser or other components can cause elevated pressures in both low and high sides.
Seasonal AC System Considerations
Wintertime Precautions:
In winter, vehicles may block airflow with cardboard or covers to retain engine warmth, which can lead to issues in summer cooling.
Reefer Trailer AC Systems
Layout and Components:
Includes various parts like compressors, condensers, and evaporators designed for high airflow and insulation to maintain appropriate temperatures for cargo.
Three-Way Valve Function:
Allows switching between refrigeration and heating modes for defrost or temperature management.
Evaporator Role:
During heating mode, the evaporator becomes a heat source rather than a cooling component, using compressor output to generate warmth.