Well, was discussing this elsewhere, thought I'd take in a look and a few comments.

Honestly I mostly like the effect.

I'd probably provide a Reflex saving throw for them as a suggestion. It makes a certain amount of sense to me. Well, more so than "I dodge the explosion which completely engulfs my area without moving" does for a Reflex Save.

So the Saving Throw entry might read -

Saving Throw: Reflex (See Text)

With text like "Creatures that succeed on the reflex saving throw take no initial damage from the Orb spell, but take exposure damage starting on the first round, thus a creature who successfully saves against Orb of takes 1d6 cold damage for 1d4+1 rounds. Evasion prevents this spell from dealing damage to the target. Improved Evasion means the target only stakes exposure damage on a failed saving throw. If the Orb scattered due to an Anti-Magic Field, a successful Reflex saving throw by the primary target limits the damage taken to that of the area of effect damage, and successful saving throws by other targets negates the damage."

... which is a dramatic decrease in power, depending on what you're fighting I suppose. But it'd make a more interesting trade off from the higher than normal "blasting" damage. 10d6 for example at level 7 is pretty hefty for spell damage, coming online about 3 levels faster than the supposedly superior blasting school does, nevermind subsequent damage as well.

I dunno... spitballing here.

Odd suggestion of what happens to gear in these orb engulfs you scenarios? item damage is a pain in the ass. But some people are going to imagine that your Orb of Acid deals that 10d6 and 1d6 to every item they wear and hold as well... Which probably will piss off both whoever cast it and whoever it was cast at. I know things like Fireball usually mentions "Unattended items" are the only ones effected, along with combusting flammable materials. It's just one of those weird corner cases that probably won't show up 99% of the time, but when it does, it'll bring a game to a crashing halt.

Not sure if any of this is really helpful. Feverish and kinda delirious, can't sleep. But it was thoughts. Of a sort.