Modifications to air conditioning systems, which include the use of new refrigerants to comply with emission standards, lighter weight materials, and electric and roof-mount system designs, show the changes that are taking place in the industry to meet the needs of transportation providers and their fleets. We asked manufacturers to explain the new systems, trends and factors to consider when spec'ing an A/C system.
Spec'ing a System
While transit agencies in the past specified BTU (British Thermal Unit) ratings for A/C systems depending on what capacity system they needed, transit agencies are now more often sending in performance specifications, says Rick Stephens, sales manager for Elkhart, Ind.-based ACC Climate Control.
While the BTU rating system is a good way to get a general idea of system sizes for pricing comparisons, BTU ratings may range for different manufacturers and not provide the desired cooling in different climates. Cheyne Rauber, VP, sales and marketing, for Rifled Air Conditioning in High Point, N.C., says, "The best way to get the best performance system for your bus is to spec it on a performance basis. The manufacturer has to come up with the most economical answer to that performance spec."
System size may depend on the region that the vehicle functions in, with more cooling capacity needed in hotter climates such as the far South and Southwest, says Stephens.
According to an inspector at the University of South Florida Center for Urban Transportation and Research, Florida's pull-down test is the most stringent in the nation. Factoring in the heat and humidity, the fact that it can be 110 degrees on the pavement where passengers are loaded and unloaded and opening of lift doors for extended periods, the system must be able to pull about 93 degrees down to 72 degrees in 30 minutes.
Tech, design improvements
There is an increasing importance being placed on the horsepower draw of the A/C compressor, according to Matt Lish, director of transport sales, North America, for Bitzer US, an Oakwood, Ga.-based company that manufactures compressors. The compressor is a belt-driven pump that determines the system's power and capacity. Technology is available that unloads the compressor, cutting its capacity when the vehicle has reached its ideal temperature, which saves almost 50 percent of horsepower draw. Unloading is triggered by a pressure switch, and Lish estimates that in motorcoach applications, the unloader could be used over 50 percent of the time.
Less horsepower draw results in greater fuel efficiency and more power to drive the bus and for other amenities. Thermo King, based in Minneapolis, Minn., offers a unique screw compressor that has unloading capabilities. The benefits of a screw compressor include fuel savings, 80 percent fewer moving parts, and a reduction in noise and vibration.
Compressors are normally driven by a belt from the diesel engine, called an open-drive piston compressor. As more and more agencies are requesting electric compressors, various manufacturers have begun making products to fill the demand. Electric compressors are normally hermetically sealed and airtight, usually with an orbiting scroll rather than piston technology.
Carrier Corp., based in Syracuse, N.Y., offers its AvantAC all-electric bus air conditioning system, currently in use in California-based transit agencies in Montebello, Torrance, at Napa County Transit, Norwalk and Fresno.
According to Joe Giacona, Carrier's North America bus product manager, the roof-mount unit has its own compressors and evaporator and condenser motors. A dedicated generator supplies power, eliminating the need to worry about engine RPM. "The AvantAC system operates at whatever capacity is required at the time. The system is designed to operate at near-peak performance even when the bus is idling," he explains.
While electric systems have both operational and environmental benefits, Giacona estimates the cost difference can be 30- to 40-percent more than a traditional system. "When federal monies are available, that helps lean the scale toward an all-electric solution," Giacona adds.
In partnership with Germany-based Konvekta Thermosystems, Trans/Air Manufacturing, headquartered in Dallastown, Pa., offers the UltraLight Series rooftop air conditioning units for hybrid and electric heavy-duty transit applications. Lightweight aluminum and foam and recyclable materials used in the construction of the system result in an overall weight reduction of 30 percent, which leads to reduced fuel consumption. According to the company, the system also has reduced noise emission.
In addition to an electric system for hybrid vehicles, Thermo King also offers an all-electric HVAC system for diesel-engine buses. According to the company, this option leads to fuel savings and dramatically improves the reliability of the system. Steve Johnson, product manager for large bus and rail HVAC, explains that this option allows the same all-electric benefits to diesel-engine buses, and the only difference between the systems is the power source. "On a conventional diesel-engine driven bus, the electric power for the HVAC unit comes from the alternator or generator driven by the bus engine and the power electronics needed to convert the alternator-generator output are in the unit," he says. On a hybrid bus, this would come from the hybrid drive system.
Compressors are also available in lightweight options. Bitzer's recently-released, third-generation design F-series compressors for heavy-duty transit vehicles offer a 50-pound weight reduction from traditional compressors and weigh between 51 pounds and 59 pounds.