MAINTENANCE MATTERS
COLD COMFORT:

 

Understanding Freon® regrigerant and Freon Levels


Air conditioning systems do more than just cool the air: They lower humidity and also remove dust, dirt and pollen by moving the air through filters and over the wet surface of the evaporator coils. The easiest, and often most effective, maintenance step is to regularly change or clean your air filter. When filters become clogged with dirt, the system must work harder to do its job of keeping your passengers comfortable on the most sweltering of summer days.

Depending on the amount of dust in the air, filters can become clogged in just a short period of time. Most late-model coaches have disposable filters that should be checked weekly and replaced when necessary. Earlier-model coaches have a cleanable screen that should be checked at routine intervals. A coach should never be operated without filters, which could lead to decreased system efficiency and a need for more frequent cleaning of the heat-exchanger coils.

Likewise, keeping the air-conditioning unit free of debris and blockage is vital to maintaining proper airflow and efficient heat removal. Clean air can also reduce load and wear on the blower motors, extending their service life.

But filters and debris-removal aren't the only story. One of the most overlooked aspects of the air conditioning system is the freon itself and what it means to the air-conditioning system. A refrigerant can be any substance that will transfer heat from one location to another. In liquid form, a refrigerant will absorb heat when it evaporates; it is this conditional change that produces the cooling effect in the refrigeration process. When the vapor is changed back into a liquid through the process of condensation, heat is again moved, this time out of the freon. The word 'freon' is actually a trademarked name, but is often used to refer to any refrigerant gas used in a refrigeration process system. R134a and R22 are the two most common refrigerants used today, due to their respective boiling points and heat-carrying properties.

The compressor compresses cool freon gas, causing it to become hot, high-pressure gaseous freon. This hot gas runs through a set of coils so it can dissipate its heat load into the surrounding air, and it condenses into a liquid. The liquid freon then runs through an expansion valve that reduces the pressure suddenly, and in the process it becomes cold, low-pressure freon liquid. This cold liquid runs through coils that allow the freon to absorb heat from the air supplied by the blowers and evaporate back into a gaseous state. The now cooler air is then blown into the passenger compartment.

The freon level is a major factor in maintaining not only the temperatures of the HVAC cycle, but also in maintaining system pressures. These operating pressures become one of our most valuable diagnostic tools for an AC system. Most coaches use receiver tanks as storage devices to separate the gaseous and liquid freon. The gaseous freon stays at the top, and the liquid freon settles to the bottom, where it is delivered to the thermal expansion valve. The thermal expansion valve is designed to meter liquid freon, and must receive only liquid freon. If the expansion valve doesn't meter correctly, it will flood or starve the evaporator.

If the expansion valve receives vapor instead of liquid from the receiver tank, as it would from a low refrigerant level, the evaporator will starve. Since there can be no vaporizing and heat transfer in the evaporator if that condition occurs, pressure levels are also affected.

It will also cause the evaporator to work harder because the freon, already in a gaseous state, can't change states and absorb heat from the air. This puts a larger load on the system and impairs its capacity to cool.

If the evaporator is flooded with liquid, the evaporator can't vaporize all of the refrigerant; in turn, liquid freon can escape into the compressor, where it can saturate the compressor and make it work harder. As the compressor attempts to compress the liquid freon (an impossibility), the buildup of heat and pressure can damage the compressor.

Having too much freon in an air-conditioning system also creates problems. It increases the pressure and temperature, inside the system because the freon has less space to change states, which also reduces cooling efficiency. The increased pressures prevents the freon from boiling, just as the increased temperature prevents the removal of heat from the air across the evaporator coil.

In conclusion, it's important to get a handle on the heat-transfer and air-conditioning cycle to truly understand freon and freon levels. Should you have additional questions that aren't addressed in the MCI Maintenance Manual, please contact your MCI service professional or by calling the MCI Technical Call Center at 800-241-2947.

Refrigerant Capacity Chart
MODEL SYSTEM CAPACITY
All MCI coaches thru MC-9 5F30 Compressor 24 lbs
MC-9 to MC-12 O5G Compressor 26 lbs
96/102 A, B, C R12 / R22 24 lbs
102D3 R134a 35 lbs
102D3 R22 24 lbs
102DL3 – w/ Parcel Rack AC R134a 37 lbs
102DL3 – w/ Parcel Rack AC R22 26 lbs
D4000 / D4500 R134a 28 lbs
D4000 / D4500 R22 28 lbs
102EL3, E4500, J4500 Dual Evaporators R134a 22 lbs
E4500, J4500 w/o receiver tank Single Evaporator R134a 17.5 lbs
E4500, J4500 w/ receiver tank Single Evaporator R134a 22.5 lbs
F3500 Seated coach Full system R134a 22 lbs
F3500 Conversion Shell Drivers unit only R134a 6.6 lbs
G4100, G4500 Thermo-King 24 lbs
G4500 Carrier R134a 27 lbs
DINA Carrier R134a 24 lbs

The FYI from MCI editorial staff values your feedback. Please e-mail any suggestions, comments, or ideas for future articles to fyi@mcicoach.com.

Freon is a registered trademark of DuPont.

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