Heat – A Balancing Act (cont.)

In the preceding blog I indicated that liquid in process tanks can be heated by the effects of ultrasonics and pumping as well as by surface-mounted or immersion heaters.  The fact is that some process tanks may require cooling, not heating, to remain at the desired temperature.  Since we have looked at the possible sources of BTU’s that go into a process tank, let’s now look at where those BTU’s go (or don’t go).

Parts being processed consume BTU’s if they enter the tank at a temperature lower than the liquid.  How much heat a part requires to raise its temperature to that of the liquid depends on its mass, conductivity and heat capacity.  This was discussed in my earlier blogs on drying.  Water has a very high heat capacity compared to most metals while metals have a much higher heat capacity than something like plastic or glass.  In most cases, the BTU’s consumed to heat a part to the same temperature as the liquid in the tank is relatively small compared to the total number of BTU’s stored in the liquid.  However, in cases where the mass of the parts being processed is large compared to the volume of the tank, heating the parts to temperature can require a significant number of BTU’s and may lower the temperature of the tank by a considerable amount.  The BTU’s consumed from the liquid must then be replaced using heaters.  BTU’s are also lost from the tank due to radiation, conduction and evaporative cooling.  An uninsulated or uncovered tank will lose more BTU’s than one that is insulated and/or covered.

In a well-insulated and covered tank with a low thermal load (due to the introduction of cold parts) the heat added by ultrasonics and pumping can easily exceed the normal heat losses of the tank in which case overheating (or “run-away” temperature) occurs.  It is, in fact, very easy to raise water to its boiling temperature with ultrasonic activity alone.  In a typically-powered tank with insulation and a cover this can happen in a matter of one to several hours.

A simple and obvious solution to tank overheating when it occurs is to reduce the temperature setting of heaters on or in the tank.  In some cases, setting the heat control at a temperature below that desired may be required to compensate for temperature increases that are inevitable due to the cleaning process itself.  If a temperature balance can not be achieved in this way, one must consider other ways to overcome the effects of excess temperature not related to heaters.  First, ultrasonic sources and pumps should be turned off when they are not needed or when cleaning is not being done.  Continuous unnecessary ultrasonic operation not only raises tank temperature but also shortens the life of the ultrasonic components.  Removing the cover or removing insulation are other ways to reduce chronic tank overheating.  A final consideration is cooling of the tank by using an external water jacket or submerged heat exchanger supplied with cold water for cooling.  Although this seems like an extreme move, it is appropriate and especially so in cases where lower temperatures must be maintained over long process cycles.

In summary, heaters may not be the only source of heat in a system.  Other sources of heat must be considered when attempting to maintain temperature within a narrow range.  Cooling, not simply the removal of conventional heating, may be the only means to prevent run-away temperature in some processes.

–  FJF  –