Effect of Mechanical Design – UA

The product of the overall heat transfer coefficient and area (UA) determines the controllability of temperature processes. The deterioration of the UA is a particular concern for crystallization and polymerization processes. Frosting and fouling can decrease process efficiency and production rate even if the controllers are retuned. For exothermic reactors, a decrease in the UA can lead to a runaway.

The secondary thermal time constant is inversely proportional to the UA. An increase in this lag from frosting and fouling increases the loop period necessitating an increase in the reset time and rate time. The detrimental effect increases as the process self-regulation decreases. Batch operations, which have an integrating response, are more adversely affected than continuous operations, which have a steady state. Exothermic reactors with low activation energies, which have a runaway response, suffer the most from a decrease in UA. If the secondary thermal lag approaches the positive feedback time constant of the process, the window of allowable controller gains closes and the reactor is unstable for all tuning settings. The maximum allowable controller gain drops to and crosses the minimum allowable controller gain. For details on how UA affects process dynamics take a look at the application note “First Principle Process Deadtimes, Gains, and Time Constants

Fortunately, there are ways of computing or inferring the UA online. The UA can be computed from an online heat transfer rate calculation using 4 measurements. The UA can also be inferred for a given production rate or time in a batch cycle from the following:

(1) Approach temperature (minimum delta between process and utility temperature)
(2) Utility use rate with supply temperature correction
(3) Process time constant identified online by an adaptive controller

For more information on online heat transfer calculations see “Identifying Equipment Performance Problems