Page 19 - Volume 14 Number 9

a large hole in the wing’s center section leading edge. Similar to an automobile’s radiator – where the engine’s hot water transfers energy to the colder ambient air flowing across its fins – the air- to-air heat exchanger allows the bleed air’s thermal energy to be transferred into the ambient air which heats up and then blows harmlessly overboard through slots in the bottom of the wing’s center section.
The Y-shaped bypass valve controls the temperature of the air feeding into the cabin during the majority of flight time.
but it’s a good place to start. To drive both left and right bypass valves from one extreme to the other – full hot to full cold bleed air or vice versa – requires one full minute, 60 seconds. Since going to either extreme is likely unnecessary, that’s why I suggest starting with one-fourth of the full travel: 15 seconds. If we use less than that, the change may be so little that we cannot tell if our attempt achieved anything. If we use much more we may overshoot and find that we have a very warm cabin.
Well, hallelujah! It is indeed getting a little warmer! So, we wait 15 minutes or so to see what final cabin temperature we have. Still a little chilly? Then let’s hold the Man Temp switch up to “Incr” for, oh, 10 seconds. A half-an-hour later, the cabin is a little too toasty? Then hold down to “Decr” for 5 seconds. Ah ... perfect!
Each bypass valve requires 30 seconds of continuous motor running time to drive the butterfly valves from one extreme to the other. The two bypass valves, left and right, run sequentially not simultaneously. For heating – getting more bypassed air and less air from the heat exchanger – the left bypass valve heats first and the right bypass valve heats second. For cooling, it’s the opposite: Right side goes full cold before the left side leaves the full hot position. As a pilot there is little need to know which valve heats or cools first. Yet, it explains
The bypass valve is a Y-shaped device, shaped like a slingshot. One ofthetwotopsoftheYisfedbythe air from the flow pack that followed the path that did not contain the heat exchanger. Of course, this air is relatively hot since it never transferred any of its energy into ambient air. The second of the Y’s two tops is fed by the air that flowed across the heat exchanger’s fins, lost energy to the ambient air flowing across the other side of the fins and is therefore relatively cold.
Mounted inside each of the two tops of the Y is a “butterfly valve.” The dictionary defines butterfly valve as: A valve consisting of a disk rotating on an axis across the diameter of a pipe to regulate the flow, as in the throttles of many engines. The two butterfly valves in the bypass valve assembly are operated through 90 degrees of travel by a single electric motor and a gearbox assembly mounted on the top of the bypass valve, the Y-shaped assembly. As one of the two butterfly valves is rotating to open, the other is rotating closed. The motor can stop both valves’ travel at any place. Therefore, the flow that leaves the bypass valve assembly – flowing out the handle of the slingshot, you might say, and then on into the cabin – can consist of 100% hotter air that bypassed the heat exchanger and 0% of the cooler air that flowed across the heat exchanger’s fins. Or it could be 83% hot and 17% cold; or 50/50; or 25/75; or full cold: 0% hot and 100% cold.
I often teach that a good analogy of a bypass valve is a modern single
SEPTEMBER 2020
faucet feeding a sink. By moving the handle, we can control how much water from the hot water line under the sink and how much water from the cold water line mix to feed the faucet. That’s what the two – left wing and right wing – bypass valves accomplish in the airplane: They control the temperature of the air feeding into the cabin. At cruise altitude we never need an electric heater nor a refrigerant air conditioning system since the heat we want is readily available from the proper amount of bleed air heat. Do you see why the bypass valves are our “Mighty Little Giants?” They alone control cabin temperature during the majority of our flight time.
Say we are in cruise using our normal procedure of being in Auto mode, the cabin is colder than we desire, and turning up the Cabin Temp rheostat control has had no effect. Time to try Man Heat mode.
We rotate the mode selector to Man Heat and then hold the Man Temp switch up to the “Incr” position for 15 seconds. Why 15? The exact number is not critical
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KING AIR MAGAZINE • 17