Step 1: Purchase smoke bombs
There are a number of sources for smoke bombs, some of which include:
1. Superior Signal Company (www.superiorsignal.com)
Note: Order Type W3C
2. McMaster-Carr Supply House (www.mcmaster.com)
Note: These typically come in multiple burning-time ranges (Table 1). Purchase the type that will burn for at least three minutes.
Step 2: Prepare the furnace for testing
The furnace must be cold. When the test is under way, all openings (e.g., eductors, front end, access doors) must be positively sealed or closed off. Kaowool can be used as a temporary seal material. If your furnace has a quench chute, do not seal it off. Turn off all fans and pumps. Hook up a compressed (dry) airline through a fitting on the furnace (be sure it is positively sealed) so as to have the ability to create a positive pressure inside the unit. Try to regulate the compressed pressure to around 30-60 psig if possible. In extremely large furnaces, two airlines should be hooked up (one near the front end, one near the back end).
Note: Be sure to figure out a way to locate the smoke bomb to the proper spot within the furnace before igniting them.
Step 3: Ignite the smoke bombs
Light the smoke bombs. You will see an instant billowing of gray-white smoke. The smoke media is zinc-chloride mist. Again, be sure the furnace is positively sealed. After about two minutes, turn the compressed air on. Within a matter of a minute or two, smoke will begin to exit the furnace through any furnace opening (leak).
For checking radiant tubes, shine a flashlight on the exhaust leg. If the tube is leaking, you will see small traces of smoke in the light beam.
There are other techniques used for testing furnaces with smoke bombs. I encourage you to post a reply on ways and techniques in which you test your furnaces.