Boeing 737CL Gen Drive Oil Temp

[Aviaservice]

Hi! I've recently discussed with pilots about one item in FCOM.

RISE scale (outer) - Displays the temperature rise within the generator drive.
•Higher than normal temperature rise indicates excessive generator load or
poor condition of the generator drive

•Lack of adequate cooling will generally cause the temperature RISE to
decrease.

Why does the temperature different decrease??? I mean, if IN oil temp (due to lack of cooling) increases, the OUT temp has to do the same, therefore RISE must remain fixed (in case of normal GEN DRIVE condition and normal GEN loading of course).

[RotaryWingB2]

Oil IN temp is after cooling, before entering CSD.

Oil OUT is after leaving the CSD, so the rise will be less due to the oil IN temp being higher in the first place (Due to weak cooling).

Probably.

[Jetdoc]

The rise temp is the difference between the OIL IN and OIL OUT. If the cooling becomes less efficient, the Rise temp will decrease.

[Aviaservice]

If the cooling becomes less efficient, the Rise temp will decrease

WHY?

[RotaryWingB2]

Because the oil isn't being cooled, therefore it starts off hotter, and the difference between cold and hot is smaller, hence less of a rise?

If the oil is 90 degrees Celsius after cooling, and 120 Celsius before cooling this is a rise of 30 degrees.

However if the oil is 110 degrees after cooling (due to weak cooling) and 120 degrees before the rise is only 10 degrees....

Therefore the rise is less due to poor cooling.

Does this make sense?

[Aviaservice]

Really? I know this. Ok. I'll try again. Lets except cooler and problem with it. Oil goes from tube to CSD IN temp = 110(for example), CSD works and heats up (because gears and shafts rotate etc) OUT temp = 120 (for example) RISE = 10. Then IN Oil heated up to 130, what will happen with OUT temp and RISE respectively? Moreover, as I can judge this is closed cycle!

[Fargoo]

The increase in temp isn't linear. The rise wont always be your hypothetical 10 degrees.

Hence cooled oil will have a greater rise than oil coming in thats had poor cooling.

[Jetdoc]

The OIL OUT temp bulb looks at the oil scavenged from the CSD and going to the Oil Cooler while the OIL IN temp bulb looks at the return oil from the cooler.
Normal inlet temps are around 120 degrees C and 10 degrees C is a normal Rise temp.

[Aviaservice]

I guess that CSD has maximum heating capacity.

[HAWK21M]

Because its a comparison reading.......Rise = OUT-IN,so less cooling will reduce the difference.