cargo compartment questions
With respect to commonly used medium and long-haul passenger aircraft (any type you are comfortable talking about):
Can someone outline how heating and cooling of lower deck cargo compartments is accomplished? Is there a direct air exchange (air vents removing air from and/or supplying air to the compartment) or is heated/cooled cabin air just passed along the outside of the cargo compartment walls to facilitate a temperature change?
How do cargo compartment temperatures compare to cabin temperatures?
Is there a technical difference with regards to heating/cooling between class C and class D compartments?
How does one verify by looking into a cargo compartment whether this is a class C or a class D compartment?
I hate double-posting, but I had no luck in the engineers' forum with those questions.