I am posting an operational note concerning the bottom board evaporator described by BWrangler.
Always check the delivery tube before firing the evaporator. The tube can become plugged. My evaporator became plugged when I speared a lump of propolis with a hot supply tube. This can be a problem when using a narrow, 3/8" entrance. Most resistance encountered when inserting the tubing is from the hive. And the slight resistance produced by wax or propolis lump might not be noticed.
The propolis restricted the vapor flow which caused oxalic to solidify behind the propolis. After a couple of additional firings, a very solid oxalic plug resulted as the delivery tube was not hot enough to vaporize the plug.
If the evaporator is venting fumes around the magazine threads, it's a symptom that the tube could be restricted or plugged. Immediately check the tubing for restrictions. I like to run water through mine to determine the flow rate.
When in doubt, clean it out. The propane torch can be used to heat the tubing and melt any propolis/beeswax or condensed oxalic. After removing any plug, check the flow rate with water.