Generally, the roof-mount systems are thought of as easier to work on as you can work on the components inside the bus by dropping down the AC filter to work on the system. Some don’t like the position where the blower fans are on the roof as you have to climb up there to work on them.
The other disadvantage is that sometimes the roof height is higher and agencies have to watch their garage clearance.
Johnson adds, “Traditional roof- and rear-mount systems are not the only options — there can be integrated solutions where the components are placed in various parts of the bus and connected.”
“For years, we have been using various combinations of integrated systems based on powertrains and customer requirements,” says MacNeil.
Looking at Compressors
The compressor’s purpose is to withdraw refrigerant vapor from the evaporator and compress the gas to a pressure that will liquefy in the condenser. There are several methods used to seal the compressors to prevent the escape of refrigerant. An open-type compressor is simply bolted together and can be driven by an electric motor. A leakproof seal is required where the crankshaft extends out of the crankcase.
A hermetically sealed compressor has both the electric motor and the compressor in the same airtight housing and they share the same shaft. After assembly, the two halves are welded together to form the airtight seal. This type of configuration eliminates a source of leaks, but it is inaccessible for repair.
“Hermetically sealed scroll compressors are typically used in electric HVAC systems,” says David Knezic, HVAC Technical Lead for New Flyer. “These reduce the amount of maintenance, reduce the refrigerant requirements in other parts of the bus and allow for variable speeds, which provides a more efficient operation.”
Systems with electric and hybrid buses are moving more and more to hermetically sealed systems as opposed to open drive compressors. More recently, this technology has shifted to conventional diesel engine buses as well.
“Creating electric power from an alternator or generator is not a new concept,” says Steve Johnson, bus HVAC product manager with Thermo King.
“The primary obstacle when your alternator or generator is driven by the bus engine is maintaining clean, stable voltage throughout the speed range of the engine.”
Thermo King began operation with this technology in June 2004, in Prague, Czech Republic. The vehicle runs on average, 18 hours a day from the city of Kladno, about 37 miles outside of Prague, into the city. The route operates with both highway and stop-and-go conditions.
Having a compressor that relies on engine speed, a bus that spends a lot of time idling could create hot conditions. One alternative is a large air conditioner with a large compressor to provide enough cooling. Another is an air-conditioning unit that does not rely on the engine for capacity. Electric compressors can provide constant capacity. In Mass Transit magazine’s December 2006/January 2007 issue, Tony Bryant, director, bus maintenance at the Tri-County Metropolitan Transportation District of Oregon (TriMet), talks about the prototype system it was running for an all-electric cooling system.
As he explains it, the system uses a variable-speed electric coolant pump, cooling fans and mixing valves. The system has proven simple, reliable and effective.
With its screw compressor, Thermo King has attained an 80 percent reduction in moving parts as compared to a reciprocating compressor. The reduction in moving parts means a reduction in the chance of breakdowns.
Screw compressors can save gallons of fuel for agencies using reheat. According to Don Nielsen, engineering manager for Thermo King, reheat is a mode of operation that introduces heat into the refrigeration cycle to stabilize the temperature as it reaches set point. It is used to balance the cooling load during light-load conditions and also to keep windows from fogging.
The screw compressor stabilizes the load by unloading some of its pumping ability during these conditions. Reducing the amount of energy used during reheat results in less horsepower required from the engine to drive the compressor.