What size is the evaporator? If it is smaller than 2x6 that stack should be good, on a 2x6 it could be marginal. As tcross said, get rid of the cap and add more stack. Above the roof you can just use single wall pipe. If you add more than an additional 2' you may want to brace the stack.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.