by Cameron Harman, Jr.

Q. What are the relative merits of high velocity, low velocity and aspirator type burners?

A. The important point to consider is that heat is transferred by the radiation
of the products of combustion (flue gases).
To get uniform heating and fast, safe transfer of heat,
you must be able to direct the flue gases of each burner uniformly
to the areas surrounding the individual pieces of ware.
A high velocity burner provides this benefit in larger kilns
where there are large dimensions to penetrate;, medium velocity burners
are effective in kilns with smaller cross sections (two feet to six feet),
and low velocity burners are not really up to the task.

High and Medium Velocity Burners Generally speaking, high velocity burners should be used only in conditions
where the heat has to be pushed over a long distance or if there is a strong cross draft.
If the kiln is only six feet across, a high velocity burner may not be necessary.
A medium velocity burner, such as many of the standard burners popular a decade ago,
would be quite satisfactory in that case.
On the other hand, in the preheat zone of a tunnel kiln,
there may be a strong flow of air moving towards the exhaust ports.
In this case, a stronger burner is required to cause the heat from the burner
to reach across the width of the kiln. This is particularly helpful in a tunnel kin with no doors.
The action of a strong jet across the front can stop cold air infiltration into the front end of the kiln.

Low Velocity and Aspirator Burners
A very low velocity burner will not allow good mixing of the flue gases even
in a medium sized periodic kiln. The heat tends to leave the burner and tilt upwards almost immediately.
Therefore, a very low velocity burner is of limited value in a modern kiln. Aspirator style burners are nearly always low velocity burners.
They are used in very small periodic kilns,
especially where the burners are placed underneath the kiln and fire straight upwards.
In these kilns, the heat rises slowly and eventually heats the kiln to temperature.
However, it does not carry enough velocity to make the temperature uniform inside kiln.
The areas exposed to the rising flue gases will heat up faster.
Uniformity is achieved by opening the setting plates in such a way as to allow the rising heat
to move all around the load. Sometimes one section of burners is regulated to burn harder to bring
the temperature in that area of the kiln up a little quicker. The difficulty with an aspirator burner is that it will not burn effectively into a high kiln pressure,
such as the pressure usually found near the top of most kilns.
The effect is that aspirator burners do not work well in kilns having vertically placed zones of control
with horizontally firing burners. As a result, almost all hobby ceramic gas kilns and
many of the less expensive gas firing kilns are fired without the benefit of vertical zoning.
Temperature uniformity is achieved in those kilns only with extended firing cycles,
often many times longer than necessary.
Aspirator burners are generally chosen because the initial capital cost is significantly lower,
but temperature uniformity, repeatability and control are usually sacrificed.
Kilns constructed with any of these burners will work.
The choice is one of initial vs. long haul economics,
and the situation should be thoroughly analyzed before a decision is made.