Heat – A Balancing Act

We all know that temperature is an important parameter in cleaning and cleaning-related processes.  Too little and cleaning will be ineffective.  Too much can lead to possible chemical separation, part degradation and loss of ultrasonic cavitation.  So providing the proper temperature is mandatory for the best cleaning result.  Sounds easy?  Well, yes and no – consider the following.

A process tank in simple terms is like a pan on a stove burner.   A vessel containing a liquid is heated by a contact heater mounted to its exterior or by an immersion heater immersed in the liquid.  It all seems very simple – – if the temperature is too cool, you turn the heat source on or, if it is at the required temperature you turn the heat source off.  But wait!  There’s more.  First, let’s consider insulation.  It is common for heated tanks to be insulated to prevent the loss of heat to the surrounding environment.  More effective insulation results in less heat loss and a reduced need for heat input.  Similarly, a covered tank loses less heat to the environment than one that is not covered.  This can be confirmed by a simple kitchen test heating a pan of water to the boiling point with the cover off and then again with the cover on.  Cover on will win every time.

The whole temperature and heat thing is further complicated by the fact that many processes, eg. ultrasonics and pumping, can add a considerable amount of unanticipated heat to a closed system.  The unfortunate result can be uncontrolled temperature rise.

Ultrasonic Heating –

Ultrasonic energy in a liquid creates heat.  Consider a typical 15 gallon tank with 1,000 watts of ultrasonic power being supplied by an immersible ultrasonic transducer.  Nearly all of the 1,000 watts of energy supplied by the ultrasonic generator ends up as heat in the liquid.  There are two components to the energy release.  The first is that resulting from the slight inefficiency of the transducer itself which results in transducer heating.  Since the transducer is submerged in the liquid, this heat is transferred directly to the surrounding liquid.  Secondly, the radiated sound energy resulting in cavitation and implosion ultimately ends up as heat in the liquid produced due to cavitation implosions and internal friction.  The purest may point out that some sound energy is radiated to the environment but, in fact, the energy radiated in this way is very very small.  The following link to a previous blog provides some more understanding of this phenomenon.

Ultrasonic Power vs. Tank Size

One watt hour of energy converts to approximately 3.4 BTU’s of heat.  The 1,000 watt immersible ultrasonic transducer, then, becomes a heater capable of delivering nearly 3,400 BTU’s of heat energy.

Note – A BTU (British Thermal Unit) is defined as the amount of heat that is required to raise the temperature of one pound of liquid water one degree Fahrenheit at standard atmospheric pressure.

This is sufficient to heat water contained in a well-insulated, covered tank by approximately 27 degrees Fahrenheit in one hour of ultrasonic operation!  The heat delivered by an externally mounted ultrasonic transducer is somewhat less but not by much considering today’s highly efficient ultrasonic transducer designs.

Heating Due to Pumping –

Similarly, most of the energy consumed in pumping liquids in a closed system ends up as heat.  I was recently surprised when I discovered that a pump recirculating 10 gallons of water at a rate of approximately 5 gallons per minute (a relatively high pumping rate)  increased the temperature of the liquid by approximately 10 degrees in just thirty minutes of operation.  Just how much of the energy delivered to the pump ends up as heat may be a little more complex to compute than the example for ultrasonics above but the end result in all cases is that pumping adds heat at least to some degree.

Controlling the temperature of a cleaning tank, then, becomes a delicate balancing act between BTU’s in and BTU’s out.  We’ll consider some ways to strike this balance in the next blog.

–  FJF  –