The average long-haul tractor-trailer will have its engine running for 21 hours a day, including six to seven hours at idle so the driver can heat or cool the cab during a rest period.
With most heavy trucks logging between 100,000 and 125,000 miles a year, the warranty period on heavy-duty A/C components is longer than the life of most automobiles.

"The big difference between a car and a commercial vehicle is that the owner is counting on that truck or piece of equipment to help him earn a living," says Gary Hansen, vice-president and chief engineer at Red Dot Corp. in Seattle, which designs and manufactures heavy-duty mobile HVAC systems and components. Downtime isn't an inconvenience, it's an expensive loss of productivity.

Which is why suppliers to heavy-duty trucks and off-highway equipment have been vocal in the debate over what should replace R-134a even before regulations for commercial vehicles have been drafted.

Driving Toward a Common Alternative to R-134a

The European Union's prohibition on fluorinated greenhouse gases with a global warming potential (GWP) greater than 150 applies only to cars and light commercial vehicles starting in 2011. Currently, there are no regulatory timetables in North America or abroad to require low-GWP refrigerants in on- or off-highway commercial vehicles.

"The auto market is where the production volumes are," Hansen says. "But what works well in a car in Europe may not provide the best combination of performance, cost, and reliability for our segment of the industry. Practically speaking there should be one low-GWP refrigerant that can be used all over the world in many different types of vehicles, including those in heavy-duty and severe-service environments."

In the United States, heavy trucks alone account for 21% of transportation-related greenhouse gas emissions, behind passenger cars (33%) and light-duty vehicles (28%). These figures include direct emissions from fossil fuel combustion as well as HFC emissions from mobile air conditioners.

Red Dot has been a leading voice on low-GWP alternative refrigerants for heavy-duty mobile HVAC systems. In 2007, the company received the prestigious Climate Protection Award from the U.S. Environmental Protection Agency for its research into the viability of HFC-152a.

"It makes sense for us to be proactive on the issue and have a voice in the process," Hansen says. "Otherwise, we may face a mandate to use a refrigerant that doesn't meet the specific needs of the heavy-duty market."

CO2: Heavy-Duty Concerns

An example of such a refrigerant is carbon dioxide (CO2), or R-744, which has been promoted as a potential alternative to R-134a. While CO2 refrigerant has the benefits of being non-flammable, non-toxic, and readily available, the drawbacks of CO2 are amplified in heavy-duty vehicles.

A CO2 system is transcritical: the refrigerant operates in one physical state instead of transitioning between a liquid to a gas. High operating pressures are necessary in order for CO2 refrigerant to pick up and give off heat. Head pressures can exceed 2,000 psi compared to 275 or 300 psi on a typical R134a system. To handle the pressure and contain the relatively small CO2 molecule within the system, metal hoses and seals would be required.

"Unlike rubber, metal seals don't tolerate foreign material or slight imperfections. A spec of dirt, a human hair, a scratch-it doesn't take much to compromise a metal seal," Hansen says. Because CO2 molecules are relatively small (less than half the molar mass of R134a), and in a highly pressurized system, leaking seals could be a problem.

Metal lines present another challenge. On a commercial truck, most cabs use a supplemental air suspension to cushion the ride. This isolates the cab from the vibrations and bumps endured by the rest of the truck. Metal lines are vulnerable to these differential movements between the engine and the cab.

Furthermore, the physical differences between a Mercedes-Benz and a BMW car-or a minivan and a sedan-aren't particularly great, which makes it easier and more cost-effective for mobile A/C system suppliers to design their products.

Heavy-duty vehicles come in a wide range of configurations. In some cases, the cab and engine may not be in close proximity.

"Think about a big off-highway excavator, or a truck-mounted telescopic crane, where you have a separate cab that's dozens of feet away from the main engine," Hansen says. "You'd have to snake long runs of metal lines, which would make it difficult to keep up the operating pressures in a CO2 system, or complicate things with additional hydraulics or a second A/C system running off another engine that's closer to the cab."

Certainly, a CO2 system would require longer operating periods in high ambient conditions. The higher the ambient, the less efficient a CO2 system becomes compared to R-134a systems. For a heavy truck, the increased power demand would compromise fuel economy on the highway and require higher engine rpm at idle. That would be hard for truck owners-who consume more than 50 billion gallons of fuel a year and measure consumption in tenths of a mile per gallon-to accept.

Finally, there's an issue of engineering and development costs. Compressors, heat exchangers-virtually every component would have to be redesigned to handle the higher pressures.

"Today, the low-side pressure in our system is 50 psi in the evaporator. Well, you're at 500 psi in the evaporator on a CO2 system," Hansen says. "If something lets go at 500 psi, that's not a trivial problem."

He says there's a chasm between what is technically feasible versus what a factory can mass produce at a reasonable cost.

"We've developed components that we know will last a long time in hot, humid, and dusty environments. To go back to ground zero and engineer new components that would deliver the exact same results as what our customers expect now would be a monumental task," Hansen says.

The Emergence of HFO-1234yf

Ideally, a substitute for 134a would deliver similar performance without requiring a radical redesign of components. HFO-1234yf, developed jointly by Dupont and Honeywell, shows the most promise.

"Tests show that 1234yf has 5% less cooling capacity than 134a in a drop-in state, but with some simple modifications we can optimize the A/C system," says Hansen. "We may need to fine-tune the TXV, or use XH7 desiccant instead of XH9, but these are minor changes compared to what CO2 or 152a would require. A 1234yf system would look and perform very much like today's 134a systems."

Red Dot says it is not tied to any one refrigerant as much as it is to technology and performance.

"If you ask a customer what he wants from his air conditioner, greenhouse gas reduction is probably not going to be on the list," Hansen says. "It has to be effective, reliable, easy to service, and not raise the cost to operate his vehicle. As we transition to a new refrigerant, the best change we can make to the A/C system is one the customer won't even notice."

Courtesy Red Dot Corp.