Bus and Motor Coach Library

Keeping Your Cool

Author – Ian Balfour (2005)

Keeping your riders cool and comfortable is one of the basic tenants of professional transportation services.  In most cases, riders have other choices when it comes to transportation, not only other bus companies but other modes of transit as well. If riders are uncomfortable with your service or have an unpleasant experience, chances are good that they may seek out alternate transportation that better suits their expectations. 

Often times we take things for granted, the air we breathe, the water we drink or the cool air that seems to magically spew forth from our air conditioning systems.  Not cold enough?  Just turn the knob and we have the instant gratification of a cooler breeze. But when that cool breeze fails, usually in the middle of July with a full load of passengers, we suddenly realize with the help of a few colorful metaphors, that all the knob turning in the world will not bring that cool breeze back.
For the most part, air conditioners operate on a closed system making it difficult to monitor exactly what is going on without the use of specialized equipment.  However, there are some things that you can do to increase the life and the efficiency of your air conditioning equipment. Proper service checks, as well as following the manufacturers suggested maintenance programs can help save you from costly repairs and even more expensive down time. 

How Does A/C work?
Whether you are using internal air systems, roof top air units, or in-dash systems, the basic theory behind air conditioning systems remain the same. Modern air conditioning systems work by means of heat transfer, similar to engine coolant systems.  The system takes heat from one place and moves it to another. In the case of vehicle air conditioning systems, the heat is taken from inside the cabin and transferred to the exterior of the vehicle via a four-step process that includes pressurization, condensation, expansion and vaporization.

This process is based on a basic principle of physics stating that a fluid absorbs heat as it changes from a liquid to a vapor, and the vapor releases that heat as it changes back into a liquid. A good example of this is a pot of boiling water. The water absorbs the heat until it reaches its boiling point, creating steam. As the steam cools, releasing the heat, it reverts back into its liquid state.
In order to transfer the heat that is inside the vehicle to the air outside, a chemical refrigerant is circulated throughout the various hoses and components that make up the A/C system. Chemical refrigerant is unique in that it alternates repeatedly between a liquid and a vapor at desired temperatures as it travels through the air conditioning system.  The cooling process is achieved through the repeated evaporation and condensation of the refrigerant by constantly changing the pressure and temperature within the system.
The compressor is the heart of the air conditioning system; this is the component that circulates the refrigerant throughout the system. The refrigerant enters the compressor through the suction port as a low-pressure vapor. The vapor is then compressed within the compressor and released through the discharge port at a higher pressure. The system is designed such that the pressurization also causes the refrigerant to become hotter than the outside air, preparing it for the next step of the cooling process, the condenser. 
Following this first process, the pressurized and heated vapor travels from the compressor, through the high-pressure hose and enters the condenser. The condenser looks much like a radiator, and its job is very similar. Air is passed through the condenser by means of electric fans or the ram-air effect produced by a moving vehicle. As the air passes through the condenser, it quickly takes the heat from the refrigerant vapor causing it to condense and change back into liquid form. This completes the second process, condensation.
This high-pressure liquid is then expanded through a metering device, either an orifice tube or a thermal expansion valve, depending on the system manufacturer. This metering device limits the amount of refrigerant passing through, dropping its pressure and it's boiling point. The boiling point is now about 34 degrees. This process prepares the refrigerant to absorb the heat from the air being blown through the evaporator.

The evaporator, also resembling a small, deep radiator, is paired with a blower that forces the warm cabin air through the evaporator fins. As the refrigerant travels through the evaporator, the heat from the air is absorbed by the refrigerant, thus cooling the air as it passes through the fins. This cooled air then continues through the air ducts and vents to create what we all know as air conditioning. The refrigerant, now a low pressure vapor then continues back to the compressor, completing the fourth and final process.     
Refrigerants Now and in the Future

Although there are many refrigerants on the market today, there are only two that are approved for use by vehicle manufacturers.  The first is R12, better known as Freon, which has been widely used in the past.  However due to its damaging effects to the ozone layer, it has been phased out in accordance with environmental legislation. As a consequence there is only R12 now available for older vehicles, and it has become extremely costly. 

The second and most widely used refrigerant today is R134a and has been developed as an environmentally friendly alternative to the R12 refrigerant, retaining the same basic characteristics as R12, but with no impact to the ozone layer. One of the biggest differences between R12 and R134a is the discharge pressure; the discharge pressure of R134a is higher than that of R12. This means that some vehicles using R134a may not cool as well when idling for long periods of time as compared with an R12 system.

Although R134a is a more environmentally friendly alternative than R12, the environmental hazard of chemical refrigerants is still there due to global warming.  However, a proposed new refrigerant is being considered for introduction into the market that is truly environmentally friendly.  This refrigerant is CO2.  Modine Manufacturing Co. is a leading company in CO2 air conditioning systems.  By utilizing this new technology, air conditioning systems could offer improved performance with faster cool-down rate than R134a, while being neutral to the environment. "We are working on CO2 air conditioning systems for a variety of vehicles, including cars, trucks, buses, even HMMWVs for the U.S. Army. We are testing these systems in the toughest of conditions, like the desert, and they are performing very well," said Dr. Steve Memory, Modine's Global Manager of HVAC Technology.

"There is legislation pending in Europe which proposes to ban R134a in automobiles. It is also planned to include buses and trucks into this legislation, but at a later date, so CO2 is truly the next generation of products," added Memory. "We believe you'll see CO2 air conditioning systems in cars in four or five years and in buses in the next decade."

Accumulator and Receiver Drier
If water should contaminate the refrigerant, corrosive acid forms, which can eventually cause leaks resulting in diminished performance that can ultimately destroy the A/C system. There are two component types designed to remove moisture from A/C systems, the accumulator and the receiver drier.
These apparatus use silicate in desiccant bags to remove any moisture in the system that could potentially damage the system. The drier also acts in a storage capacity, holding excess refrigerant.  The accumulator ensures no liquid refrigerant returns to the compressor while allowing the oil to remain circulating, letting oil return. The accumulator can be found attached to the evaporator on the low-pressure side of the system. The receiver drier is found on the high side of systems that utilize thermal expansion valves: today, they are often integrated with the condenser.

In order to ensure a long and productive life for your A/C system, there are some maintenance items that require attention. One of the most important factors in the air conditioning system is lubrication. The compressor interior operates at high speeds, with components spinning in very close proximity to the outer casing and other moving parts.  Without the proper lubrication, your compressor can literally destroy itself from the inside out. The correct amount and type of lubrication needs to be maintained in order to avoid catastrophic and costly part failure.

Running a system that is low on refrigerant can also harm your air conditioning system. Without the correct level of refrigerant your compressor will have to work much harder to keep the interior cool and this can lead to a breakdown of the oil and cause seals and O-rings to fail prematurely.
If the compressor does happen to fail, the entire system needs to be completely flushed out or you run the risk of destroying the new compressor with the particulate matter left in the system.
Because the oil travels along the system with the refrigerant, a visual inspection of the fittings and components can sometimes alert you to possible leaks. The refrigerant is colorless and odorless and evaporates when exposed to the atmosphere. But the oil that accompanies it often leaves traces at the leak site. Look for any residue around joints and fittings, the compressor and within the coils and fins of the condenser and evaporator.

Keeping the condenser coils clean and free of debris is also important if you want to keep your air conditioning unit functioning at peak performance. Particle build up can interrupt the air flow and cause a reduction in the condensers ability to dissipate the heat. The fins of the condenser as well as the evaporator are easily damaged so proper care is needed when cleaning these components.
Also, one of the most neglected items in the entire A/C system is the compressor clutch. Routine preventative maintenance of the clutch will save you problems down the road.  Ian Ridout, of Coach Refrigeration of Southern California, advises, "Many small fleet operators will tell you that replacing the sealed bearing and re-setting the air gap on an annual basis is an inexpensive but effective way to prolong the life of your clutch."

Another maintenance item on modern A/C systems are filters. Most of today's systems employ cabin filters that remove debris and particles that are picked up as the air makes its journey from the blower and into the interior vents. These filters need to be periodically monitored and either cleaned or replaced.
With proper care and maintenance these A/C systems can last the life of the vehicle, keeping your customers, and you, cool and happy.