Last week my lab partner and I finished exploring the controls system on the Dow Heat Transfer unit at the Unit Operations Lab. This lab was very interesting because it allowed me to better understand what we had learned in Process Controls class last semester. Being able to physically adjust valve positions had made what I learned on paper came to life. It was a quite satisfying experience when I generated some of the same plots that I had seen in a textbook. Such unique ChE lab experiment has really add different dimensions towards my full ChE experience at Lehigh.
The objective of this experiment was to determine the transfer function of the system by using an open-loop step test. This transfer function will relate the controller output in terms of percent steam valve open to the temperature of the process water exiting the heat exchanger. In addition, the ultimate frequency of this closed-loop system was also studied. Then, the ultimate gain and ultimate period was calculated in order to to tune the temperature controller with Tyreus-Luyben method. The controller response was then studied. The experiment was then repeated for different steady state process and cooling water flow rates.
Side story here: Tyreus was a student of Luyben’s. Professor Luyben is one of our Senior Design Project Professor. It must be pretty amazing to have a tuning method named after yourself!
Basically, this system consists of a process water holding tank, a pump, and two shell-and-tube heat exchangers. Process water is first pumped from the holding tank into the first heat exchanger, which heats the process water with steam. The process water is then cooled in the second heat exchanger with cooling water before being returned to the holding tank. The first heat exchanger has process water on the tube side. It is a 2-pass heat exchanger with 54 total brass tubes 14.1” per pass, ID 0.187”, and OD 0.250”. The shell ID is 4.125”. The second heat exchanger has process water tube side. It is a 4-pass heat exchanger with 26 stainless steel tubes per pass. Tubes are 48.3” long, OD 0.375” with wall thickness 0.0375”. The process water and cooling water are flow controlled with air-to-close valves. The flow of cooling water is controlled based on the temperature of the process water leaving the second heat exchanger. The flow of steam is controlled based upon the temperature of the process water exiting from the heat exchanger with an air to open valve.
Here’s a bonus picture of the actual set up: