Page 27 - AutomationNOTEBOOK volume 41

User Solutions continued
the evaporator coil located inside the chamber where it can absorb heat, cooling the room and becoming a low-pressure gas, which is returned to the compressor to begin the cycle again.
In this application, temperature is controlled and maintained by managing the liquid line and hot gas valves. The liquid line valve is open all the time except during a defrost cycle. As a result, the temperature in the room would continue to become colder and colder, reaching temperatures below freezing, were it not for the hot gas valve injecting a portion of the gas leaving the evaporator coil inside the chamber back into the inlet of the evaporator coil.
Adding some of the hot gas into the liquid entering the evaporator coil slows the cooling process, bringing the temperature toward a desired point. Since the liquid line valve is always open, the hot gas valve is modulated at the duty cycle required to reach and maintain a specific temperature.
Humidity is controlled by heat injection to manage how much moisture in the chamber condenses on the evaporator coil.
A custom PID control method was needed to manage both the positive and the negative side of the operation curve. “Keeping temperatures within a half a degree Celsius, with researchers going in and out all day long, is not an easy thing, but Affinity Energy's integrated solution has been running flawlessly” notes Steve Hargett, Facilities Maintenance Supervisor at UNC Chapel Hill.
Although it is an advanced control function, it was relatively simple for Affinity Energy to develop the custom-control algorithm in the AutomationDirect PLC. The algorithm is essentially a PI loop, but coded explicitly rather than using the off-the-shelf PID programming module in the controller.
Even with researchers going in and out of the room throughout the day, the prototype control solution holds the temperature steady within a half a degree Celsius. With their new control system, and with PLC programming supplied by Affinity Energy, UNC Chapel Hill has the freedom to introduce future enhance- ments using their own personnel.
Go to Article
Editor's Note
New Product Focus
- WEG High-Performance AC Drives
- FREE Online PLC Training
FYI
Cut to Length Q&A: Bulk Multi-Conductor Cable
PLC Speaking
Business Notes
Cover Story
Mastering Machine Maintenance
What's New
User Solutions
Open System Provides Precise Temperature Control
Student Spotlight
Copley High School Student Project and Update
Break Room
Brain Teasers
The plan is to replace controllers on other chambers across the UNC campus using this open design based on AutomationDirect PLCs and HMIs. Program changes will be incorporated into this universal control solution as needed for the different types of chambers to maintain consistency. As the university integrates this new control solution into the rest of their environmental chambers, all they need to do is copy it and provide a few minor calibration tweaks based on each room's characteristics.
UNC Chapel Hill's future plans include full access to room temperature and alarm history using remote access. This will provide quick access to critical historical data required by various regulatory agencies.
Author Bio
Allan D. Evora is the President of Affinity Energy. He's a leading expert in con- trol systems integration with over 20 years of industry experience working
in every capacity of the automation project life cycle. With a background at Boeing Company and General Electric, Allan made the decision to establish Affinity Energy in 2002. Allan is an alumnus of Syracuse University with a B.S. in Aerospace Engineering, a graduate of the NC State Energy Management program, and qualified as a Certified Measurement & Verification Professional (CMVP).
User Solutions www.automationnotebook.com | Issue 41