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How to achieve optimal temperature control

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How do I achieve optimal temperature control?
How to Achieve Desired Temperature Control in Process Cooling
John DuBois PhD, Thermalogic Corporation

 

Preliminary Considerations
Successful control of your cooling process and selection of a suitable
control requires a good understanding of the flow of heat in that
process.
No matter how intelligent or elaborate a control, it cannot overcome
the limits of basic thermodynamics in a system or process.
Regardless of any control system, heat applied to a process or
generated within by exothermic reaction will cause a steady rise of
temperature unless a path exists for removal of the heat energy.
Usually, provision for suitable flow of heat out of a process is not a
difficult task and requires little more than balancing heat applied (in
BTU/hour or similar units) with heat removal by passive conduction,
convection, radiation, or active cooling over a suitable period of time.
Only after this balance is designed should a control system (electronic
or mechanical) be considered to keep process temperature within
desired limits during operation.


Selection of a suitable control system is then subject to another set of
limitations. Accuracy, repeatability and speed of temperature control
cannot be arbitrary. These parameters must also respect
thermodynamic and practical limitations as well as cost.


Basic Heat Flow:
How fast can heat accumulate in your process? A common measure
is BTU per hour (or equivalent units) and is available from heater
specifications or knowledge of exothermic heat generation in the
process.


How fast can heat be removed from the process? This may be a more
difficult value to obtain but no less important. It can be obtained from
conduction, convection or radiation charts, manufacturer specifications
or calculated with knowledge of system dimensions, materials and
environment.
Is the rate of heat removal greater than its generation? If so, then it
will be possible to control the process temperature, otherwise even the
most elaborate control system available will be unable to help.


Selecting a Sensor:
Sensor type will have a direct bearing on cost and performance.
Select a sensor that meets your expectations for performance rather
than specifying a particular type (thermistor, RTD, or thermocouple)
simply because it is familiar.


The most energy efficient types are thermistor and semiconductor
because they offer the best system stability at lowest cost. If these
types do not offer sufficient accuracy or range, then lower sensitivity
but higher accuracy RTDs might be appropriate. Thermocouples offer
extended temperature ranges and a fast response time, but they have
the lowest stability and accuracy. 

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