Steve.
Christmas trees were common on the VIICs, below is an image of the blowing panel, as you probably know the VIICs used HP air for blowing the ballast tanks, the blowing christmas tree consisted of a common blowing valve for all the ballasttanks, each tank could be individually adjusted as nessesary by a separate valve after the main blowing valve, which of course resulted in a lot of valves. In order to save HP air you only blew the main ballast tank partly by hp air and continued blowing the residue by exhaust from the main diesels. The system required a constant adjusting of the exhaustblowing as the resistance varied because the ballasttanks were situated at different depth and thus the counterpressure varied, if not adjusted the exhaust would only work on the tank having the lowest counterpressure. Thus you must be able to distribute the exhaust to the right tanks. This panel or christmastree if you like, is situated outside the pressurehull having the adjusting valve wheels on top of the blowing panel as can be seen on the image.You started to exhaust blow the the MBT 2 & 4 being closest to the surface (least resistance ), then shut the valves to MBT2&4, continued with 1 and 5 now having the lowest resistance and as the submarine ascended shut the MBT 1 and 5 and finishing up with the MBT 3 the (deepest tank highest resistance ) now having an acceptable resistance, until you was completely surfaced. This required a constant surveillance by the operator and a frequent adjustment of the valves in the exhaustblowing panel.
Tore