Growing up as a kid I suffered through several long vacation rides to Florida in the back of my father’s station wagon with all the windows rolled down. I was always jealous of the kids whose parents had the “fancy” cars with power windows and air conditioning. Air conditioning for fire trucks started when early fire truck cab designers began bolting in an evaporator unit borrowed from the construction industry and calling it good. Since then, both Fire Apparatus Manufacturers’ Association (FAMA) members and our customers have become more sophisticated in our air conditioning designs and expectations. That said, there are many nuances in air conditioning system design that an apparatus purchasing committee can and should consider.
There are four main parts to any mobile air conditioning system: the condenser, the pump, the evaporator, and the expansion valve. We will not review the science that makes it work, but suffice it to say each of these components must be sized properly and the connecting plumbing properly insulated. That is the job of the apparatus designer. The system will only perform as well as the weakest link, so more important than individual component capacity is that they all be well matched as a system.
As noted, specifying the size or capacity of individual components will not ensure the performance of the system. Even claims of system capacity (usually stated in British Thermal Units (Btus) should be taken with a grain of salt. There is no accepted standard method of testing Btu ratings, and when they are tested, the values are usually obtained in laboratory conditions on individual components. The only system performance test applicable to large fire apparatus is SAE J2646 Cab Air-Conditioning Test Procedure—Heavy Trucks with and without Sleepers. This test looks at the time it takes to drop the average cab temperature a given number of degrees in a given time from a given starting temperature and given conditions. It is a good indication of system performance, but the pass/fail criteria may or may not meet your department’s expectation.
KEY COMFORT FACTORS
While average cab temperature is important, customer feedback and experience has proven that it is much more important for comfort that cool air can be directed on the occupant’s faces. Even if the cab is significantly cooler than the outside temperature, a hot and sweaty occupant will feel much better if he can get cool air blowing on his skin. This is equally important for front and rear occupants.
Temperature and air velocity out of the vents are also both important. Lots of vents may do a fine job of dropping the average temperature, but if there are too many, each vent may not have as much force to its flow. Gentle flow may be fine for the trip to the scene, but strong flow is crucial for faster cool-down of occupants attempting to recover at the scene or during the ride back to the station.
STARTING OUT COOL
The best performance out of a system comes when you can spin the compressor at higher speeds. This happens naturally while driving, but the compressor RPMs drop while the truck is idling. A high idle switch can help with this when the cab is being used for firefighter recovery, and the engine is not pumping. Another factor to consider is the size of your protection area. If trips to the scene are very short, the system will have little time to drop the cab’s temperature before arriving on scene. Departments in very hot and humid climates may consider a secondary 110-V-powered unit that runs on shore power to keep the cab cool while it is in the station. A cab that starts out cool will stay cooler in operation.
OTHER DESIGN FACTORS
A great air conditioning system by itself is not sufficient to provide comfort in hot and sunny climates. The cab walls and roof should be insulated as much as possible to reduce the amount of heat that is passed through to the interior. To minimize the impact of solar loading, be sure to specify aggressive window tinting, or glass with reflective coatings. A white cab roof is another key feature that will reflect as much of the sun’s rays as possible, reducing the load on the air conditioning.
PUTTING IT ALL TOGETHER
Apparatus manufacturers have come up with many ways to cool a cab, and components and systems often vary between cab models from the same manufacturer. The best way to be sure the system you will be getting is to experience it for yourself. Schedule a demo on a hot (and preferably humid) day and see how the system works. Think about how your cab seating will be arranged and be sure you will be satisfied with the location and power of the vents. Your planned configuration may be different from the demo truck in cab size or seating configuration, so be sure to talk through the details with the salesperson.
KEEPING YOUR COOL
When I started designing fire trucks in 1992, air conditioning was being ordered in perhaps 20 percent of the apparatus. Today it is uncommon for a cab not to include air conditioning, and occupants expect it to work well. I speak from experience when I say that fire departments do not always have their expectations met. Sometimes it has been the fault of errors in system design, but more commonly there just was not enough thought given to these factors during the specification process either by the department or by the sales organization. If you are a member of a truck committee based in a hot climate, work with your FAMA member company representative to ensure that the apparatus you specify will help everyone involved keep their cool.
For a more in-depth review of fire apparatus air conditioning systems download FAMA’s HVAC Buyers Guide at https://www.fama.org/fire_service/hvac/
FAMA is committed to the manufacture and sale of safe, efficient emergency response vehicles and equipment. FAMA urges fire departments to evaluate the full range of safety features offered by its member companies.