Use some control charts. Use ratios of temperatures. Temperature sensors are cheap.
A bigger recirculation pump would reduce the dead time without changing the point of temperature measurement.
Morton W. Reed, Ph.D, PE / Columbus Water Works
ANSWER: The control system, within its physical limitations, acts to make the temperature at the sensor equal to the set point. Thus, if it’s desired that the temperature of the bath be controlled to the set point, then the sensor needs to be in the anodizing tank at a point of high agitation, close to the outflow pipe. Moving the sensor away will make tank temperature control more difficult.
Temperature control can be improved by using derivative control, with the derivative time constant set to the characteristic response time constant of the bath. (Expect a value between 2 minutes and 30 minutes, probably closer to 8 minutes for your process.)
Cooling water flow diversion control is used to forestall the possibility of the cooling water shut-off and freeze-up within the refrigeration system. It has little or no bearing on the control response.
The process is subject to two major sources of disturbance: the anodizing load and the cooling water temperature variation. There are other, lesser sources of disturbance, such as ambient temperature, humidity, air flow near and over the anodizing bath, etc. Assuming that the set point and process temperature are at 18 ºC, and the coolant flow through the heat exchanger is at its maximum flow value (100%) and at 14 ºC, then when the cooling water temperature drops to 6 ºC, the flow must drop by two-thirds to provide the same energy extraction from the anodizing bath via the heat exchanger. This is a very large disturbance.
The cooling flow temperature variations, if objectionable as seen by the anodizing tank temperature, can be more effectively counteracted by using cascade control. A second (cascade) proportional controller would be added, as well as a temperature sensor in the cooling water flow pipe to the process. The temperature sensor would be calibrated such that 18 ºC, the process set point temperature, represents full scale. This makes the output of the temperature sensor inversely proportional to the available cooling effort. The cooling water signal is connected to the process port of the second controller, while the output of the existing temperature controller becomes the set point of the second controller. The temperature controller needs to be reverse-acting, and the second (cascade) controller direct-acting, assuming that increased second controller output will result in more cooling water flow through the heat exchanger. The scheme just outlined depends on the set point to the temperature controller is fixed at 18 ºC. If not, another equipment item is needed to generate the set point, minus cooling water temperature signal for the cascade controller.
Otto Muller-Girard, PE, ISA Fellow / Consulting Engineer (Retired) / firstname.lastname@example.org
|Ask the Experts|
Ask The Experts
“Ask the Experts,” on ControlGlobal.com, is moderated by noted process control authority Béla Lipták. Béla and his cadre of leading experts in process automation, recruited from among the co-authors of the Instrument Engineer’s Handbook 4th Edition, answer process control questions from all comers.