Keeping Your Cool When Specifying Apparatus

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.

MAIN COMPONENTS

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.

SPECIFYING PERFORMANCE

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 occupants’ 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 revolutions per minute dr