Page 24 - Volume 14 Number 2
P. 24
Allow me to run through a simple AC system review. A rule of nature is that thermal energy always flows from hotter to colder. If we drop an ice cube into a glass of water, the energy in the warmer water flows into the cooler ice cube and melts it. We obviously never see the ice cube losing thermal energy and getting cooler while the water is receiving that energy and getting warmer! So how do we take energy from our 70°F cabin air and send it out into the 110°F outside air?
We do so by utilizing the prop- erties of a refrigerant that goes by the DuPont tradename of Freon. (R-12 and R-134A versions are the most common refrigerants in King Airs. The first is a CHC – Chloro- fluorocarbon – while the second is an HFC – Hydrofluorocarbon.) Like water, this substance can be in solid, liquid or gaseous states of matter. Also, like water, the temperature at which the transition from liquid to vapor (evaporation) and from vapor
to liquid (condensation) takes place changes as pressure changes. As you all know, water on the stove boils at 212°F with standard sea level pres- sure, but its boiling temperature is less in Denver at a high altitude under less pressure.
What the AC system does is use the Freon as a mechanism to transport thermal energy from colder to hotter – the cool cabin to the warm outside. It does this by making the Freon’s pressure very low, hence making its evaporation temperature also low. The expansion valve is the mechanism that causes this big drop in pressure and temperature. As the Freon flows through the coils of the evaporator, perhaps now with a boiling point of 30°F, the 70°F cabin air can lose energy into the Freon as it boils from liquid to gas. Next, the compressor squeezes the gas, making it hotter, say 180°F. The compressed Freon now flows through the coils of the condenser. The outside air flowing across the
other side of the coils, perhaps at 110°F, receives energy from the hot Freon and blows harmlessly out the left side louvers on the nose. (Ever hold your hand near those louvers with the AC running on the ramp? Isn’t it hot air coming out?) To summarize, we make the Freon cold so that cabin air energy flows into it then we make it hot so that it may expel that energy into the outside air ... the Freon is our energy transport mechanism.
There have been three versions of the underpressure/overpressure reset mechanism installed in the 200-series. The very early 200s had a fuse holder easily accessible in the right wing’s center section. Beneath the battery box, on the bottom of the wing, do you see that small hinged door with the three camlock fasteners? The fuses are in there. On these early models it contains a fuse and a spare fuse mounted beside it. If the fuse is blown (take it out and look at the element in the glass tube) merely replace it with the spare and maybe you will be lucky enough not to experience another pressure trip. If the spare fuse also blows, it’s time for maintenance help.
The second version – which began with BB-345 and could be retrofitted to earlier models – replaced the fuses with a PCB. There are two things of interest here. First, the PCB contains two small red lights. With the battery switch turned on, the illumination of one indicated that the pressure trip was caused by low pressure; the other indicated high pressure was the reason for the shutdown. The second item of interest is a reset button. Just push it once, the pressure-indication light should extinguish and we’re back in business for another try. (Make a record of what caused the trip – low or high. That information will help if further troubleshooting is required.)
The third and last of the BE-200 reset mechanisms – and the one most serial numbers now have – began with BB-729 and after, plus BB-688. (All of the 300-series also has this system.) With this version ›
DEANBENEDICTA&P,AI,CONSULTAN T TEL:702-773-1800DR.DEAN@BEECHMEDIC.COM
22 • KING AIR MAGAZINE
FEBRUARY 2020
