Boeing use lots of terms from boats. The Keel beam is at the bottom of the fuselage bridgeing the gap in the airframe where the wing structure passes thru a cut out of the cabin tube. At the forward end it attaches to a baulkhead frame casting, goes under the wing centre section fuel tank, thru the main gear bays, and attaches to another frame casting baulkend at the rear of wheel wells. Best places to view keel beam is in the wheel well, as the air condition packs bay doors are harder to open.
The keel beam ties the fuselage together, is about 24 inches by 12 inches cross section.
Over heat sensor to detect leaks from bleed ducts and air conditioning pack.