Cab Heating & Air Conditioning
There's more than one way to keep the cab and sleeper comfy.
ED THOMAS
CONTRIBUTING EDITOR
Stop at any truckstop or rest area along the Interstate and what do you hear? Idling engines. Some of them hum away because their drivers just do it; it's part of the culture. Other engines, however, are providing the heat and power to keep truck cabs and sleeper areas warm or cool. Is there a better way?
Yes, and no. In the winter, a fuel-burning auxiliary heater can keep a cab and sleeper toasty with no engine idling. In the summer, a “bunk cooler” can provide chilled air to the sleeper. In any season, an auxiliary generator set can run a complete heater/ventilation/air-conditioning system. But each of these cost money — sometimes thousands of dollars — which few fleet managers can justify.
Besides, today's electronically controlled diesels idle pretty cleanly, with little wear and tear to the engine itself. A few gallons of fuel burned up this way is still far cheaper than any motel room. Any driver will tell you that managers who carp about idling ought to turn off their home furnaces or air conditioners to feel what a non-idling night is like.
Fleets that plan to trade off their road tractors after only a few years shouldn't have to worry about engine life anymore. Even five years of long-haul service in most cases totals up to far fewer than a million miles for solo-driver operations, and 750,000 to 1 million miles is what today's diesels will do before needing serious repairs. If the rig is otherwise well maintained, idling its engine shouldn't hurt trade-in value.
Moreover, the basic heater-ventilating-air conditioning (HVAC) systems in modern trucks are vastly improved over those of yore. Who can blame drivers for wanting to use them to full advantage? Police enforcing anti-idling laws enacted by state or local authorities, for one. The desire for clean air and residents' demands for peace and quiet are the basis for such ordinances, which are now limited to some urban areas but could spread.
How They Work
Everyone knows that cab heat comes from engine coolant that's piped to the heater core behind the dash or under the floor of the sleeper. To get the desired amount of heat, you adjust the temperature control; this modulates the amount of hot coolant that flows through the core. You also adjust the speed of the blower, which varies the amount of air drawn across the core and into the cab.
The air conditioner is another matter. It operates like a refrigerator, using a chemical refrigerant to remove heat from the air in the cab and sleeper. Refrigerant is piped through an in-cab heat exchanger called the evaporator, then to another exchanger, the condenser, which shrugs off the heat. The refrigerant gains heat-carrying capacity when it's pressurized by a compressor, and drops heat as it's depressurized, or condensed. The high-pressure side of the system can reach 300 psi and even the low-pressure side may run in excess of 30 psi.
The heart of the A/C system is the engine-driven compressor, which includes a clutching mechanism to disconnect it when it's not needed. Other components include the receiver/dryer, which filters out moisture and impurities, and one or two blower motors.
Twenty or more years ago there were roof-mount air conditioners and "modular" units that stood somewhere inside the cab. These operated separately from the heater and were easy to work on. These have disappeared because the truck builders, working with suppliers, have greatly improved their own "integral" HVAC systems.
Today's HVACs are powerful and while they are not at all trouble-free, their components last longer and are easier to service than before. Combining the heater with the air conditioner also improves operation of the windshield defroster because the dehumidified air can absorb windshield moisture better than heated air alone.
A/C systems now run on "environmentally friendly" R-134a instead of the old R-12 (commonly called Freon, which by International Compact is no longer manufactured in western countries and has become rather expensive). R-12 from older trucks should be captured and recycled during servicing. R-134a has slightly less capacity than R-12, but systems operate in the same manner and with the same comfort levels. If you want to convert an R-12 system to R-134a, check with the truck builder; it probably has a retrofit kit with certain parts that must be changed.
The latest systems include air filters that screen out dust, pollen and other impurities so you can breathe easy. You ought to learn where these filters are in your trucks and see to it that they are cleaned and serviced regularly. Check the Driver's Manual for information on this.
It's "Pre-Spec'd," But...
With built-in components, vents and controls, the integral HVAC system is "pre-spec'd" for air volume and available temperature ranges, control style and design, and so forth. Yet, says the Technology & Maintenance Council (TMC) of ATA, there are things to check for with the integral system that can impact operation and maintenance.
• Compressor type and size— Truck builders generally make one type and brand of compressor standard; go with another type and brand if you have compelling reasons to change. Reciprocating compressors are the older of two types and are usually considered more reliable than the rotary type. The rotary is smaller, lighter and operates with less vibration, and suppliers have improved its reliability and longevity.
The compressor's displacement and operating speed must be correct or factors like low-pressure suction will be insufficient. An extra-large compressor is not a good idea unless you plan to add an extra-large sleeper box; confer closely with the sales person if you have such plans.
• Clutching— The two most likely problems with compressor clutches, says TMC, are low operating voltage and excessive air gap. Either will fail to make the clutch fully engage, causing slippage and burnout. At least 10 volts are needed to operate the clutch, and air gaps should be set according to the manufacturer's recommendations. Sufficient bearing capacity is among the other considerations.
• Mountings— Must be able to withstand the weight and operating forces, including vibration, put to the compressor. TMC recommends that the mounting bracket for the reciprocating type be made of 3/8-inch steel or low-carbon cast iron, and secured with at least four bolts of Grade 5 or better. Compressors with aluminum cases should have their pivot tabs protected with slip bushings. Refrigerant lines should be supported and secured so they do not rub on other parts.
• Belt and pulley arrangement— The belt should wrap around at least 30% or 120 degrees of the pulley's diameter. The pulley diameter must comply with the compressor maker's specs or the compressor will turn at the wrong speed, causing suction and perhaps other difficulties.
• Electrical protection and diagnostics— To protect against power surges, a fuse or circuit breaker should separate the A/C system from the rest of the truck's electrical system. Also, an electronic device is now available that diagnoses compressor problems before they become apparent to the driver or mechanic. This valuable product is standard on several truck makes and optional with others; order it if it's not standard, or arrange for the dealer to install it.
• Condenser— Usually placed ahead of the radiator, it should be securely mounted and protected from rubbing against other parts. If possible, it should be hinge-mounted on one side so it will swing away from the radiator to allow easy cleaning of both of these critical heat exchangers. OEMs have taken to buying "packaged" radiator-condenser assemblies from suppliers, so you may have little choice in this matter.
• Evaporator— Aluminum- or steel-finned units are okay. To facilitate reassembly after repairs, fasteners should be bolts rather than sheet metal screws.
• Blower motor— Can it be removed and replaced in under 30 minutes? It can in some recent truck models. If not, you're looking at extra labor expense somewhere down the road. Also, check for large oil reservoirs and vented housings to extend bearing and armature life. If your drivers have dogs or cats, be aware that their hair gets into blowers, which will therefore need servicing more often.
• Hoses— Nylon-lined hoses are preferred because they allow less refrigerant loss. But they might not be standard; check 'em out.
Auxiliary Heaters & Coolers
As we mentioned earlier, there are a number of ways around idling the engine:
• Auxiliary heaters— These burn diesel fuel and are available in many models and sizes that can keep the largest sleeper toasty warm. Correct sizing and installation are important for proper operation and safety. Incomplete combustion can be dangerous. These heaters should be checked out before each winter season.
Heaters can also be installed in the engine's cooling system so coolant stays hot. This allows easy starting of the engine after a cold night. It also allows use of the truck's HVAC for heating, though the blower must be run and this draws down the batteries. This is less a problem with a split-battery arrangement where a deep-cycle battery alone supplies power for accessories.
• "Bunk coolers"— These cool sleepers chemically: As the truck goes down the road, cold air from the HVAC blows across the cooler, chilling or freezing a special fluid. During rest breaks or on hot nights, sleeper air is blown across the cooler, which takes out heat; the cooled air then circulates in the sleeper compartment. This can reduce the amount of time the engine must be idled to run the A/C, but probably not eliminate it altogether.
Special "idle limiting" controls that shut down an engine after a pre-set time have long been available. There are also controls that restart the engine to keep coolant or cab temperatures within specified ranges, like the start-stop controls long used on trailer reefer units. The controls might be set to start the engine when coolant temp drops to, say, 100 degrees F, and/or when the sleeper temp climbs above or drops below 75 degrees. The engine runs long enough to set the temperatures right, then shuts off again.
Auxiliary Power Units
Operationally, the best all-around on-board alternative to idling is the auxiliary power unit, or APU, that supplies cooling, heating and electricity for the truck. Most APUs have small-displacement diesels that spin an alternator and, usually, an A/C compressor. The APU can power electric cab/sleeper heaters or its coolant can run the truck's HVAC. Often the APU's cooling system is plumbed into the main engine's cooling system so hot coolant keeps the main engine warm, too. Or the APU's alternator runs an electric block heater or crankcase heater in the main engine.
With electric power available, almost any number of electric appliances can be used by drivers to make their away-from-home time more homey. Inverters are catching on as a way to power 110-volt appliances off the truck's batteries, but cannot run HVAC systems. People who buy extra-large sleeper boxes with RV-like accommodations almost always equip them with APUs.
The primary drawbacks to an APU are its cost (up to $6,000) and weight (several hundred pounds). Space can also be a problem if the truck or tractor's frame is already lined with fuel tanks, air dryers and the like. Most fleets cannot justify the up-front costs, so owner-operators buy most APUs. Consumer-type gasoline gen-sets are almost certainly too weak to hold up to steady running and the harsh vibrations of a heavy truck. Lower-price commercial-type gen-sets have recently come on the market, and perhaps they will prove useful.
On the horizon are fuel cells, which will supply electrical power for heating, cooling and appliances. Chemical reactions will silently generate power, and a fuel-cell unit will probably be comparable in size and weight to a current APU. But the goal is to make them much less costly than APUs. We're probably one to two years from these. Stay tuned.
