Page 17 - Volume 11 Number 8

The vent blower is located under the cockpit floor, just between and in front of the pilot’s rudder pedals. (Surprisingly, it is not shown nor labeled in the picture you have examined from the POH.) It picks up the air that is there (better wear clean, non-smelly socks, eh?) and sends it forward through a hole in the forward pressure bulkhead. It now enters the Evaporator Plenum that is located in the cavity beneath the avionics or baggage compartment floor on the left side of the nose wheel well. It passes through a filter before flowing across the coils of the evaporator. As the name indicates, this is where the air conditioning’s Freon boils, or evaporates, from liquid to vapor. Just as it takes plenty of heat energy to boil a pot of water on the kitchen stove, so also does it take energy to boil the Freon. That energy comes from the cabin air, as it sends energy, heat, from the air into the Freon, causing the air to cool. Unlike the water on the stove, however, here the pressure of the Freon is very low and hence its boiling temperature is also low enough that it evaporates without high temperature involved.
As the cabin air loses heat energy and cools, less water can be held in suspension so some condensation of water almost always takes place. That is why the evaporator plenum (plenum is just another word for chamber or space) contains a drain hole on the bottom. That puddle you see on the ramp under the left side of the nose at the end of summer flights is merely the condensed water that
exits the drain. To avoid an undesirable pressurization air leak, the drain contains a rubber seal that closes when a positive pressure differential is present, but relaxes to the open position when unpressurized. The evaporator plenum also contains a temperature-sensing switch that will shut down the AC before the plenum gets so cold as to cause the condensation to freeze, thus blocking the flow of air through the plenum. This is the “freeze switch,” set to operate when it senses the temperature approaching 32°F.
So now the air from the vent blower proceeds past the evaporator plenum to continue its circulation pattern. Keep in mind that most of the time in cruise the AC is not operating because it’s not needed. So even though the air always flows across the coils of the evaporator, no cooling may take place. During these times, the plenum merely serves as an inert component that directs the air forward so that it can continue its circulation. The diagram you examined is obviously a top-view, looking down at the environmental components. Hence, there is no way to see or visualize any vertical differences in the tubing’s location. It’s obviously not a 3-D drawing!
So, to the un-informed newcomer, there is no way to know that the dark blue and light blue crossover duct is not just a horizontal tube from left to right. That cannot be, however, because the nose wheel well gets in the way.
AUGUST 2017
KING AIR MAGAZINE • 15