Controlco Blog

“A while back a local dairyman came to the Bakersfield branch with a problem. It seems the controls on the fans that move air through his cooling condenser were causing problems and jeopardizing large batches of milk. This condenser is the heart of a system that cools milk from cow temperature to a safe storage temperature.
The system works by blowing air over a heat exchanger that is part of a loop filled with anhydrous ammonia. The ammonia gives up its heat to the air, and then a second closed loop of brine gives up its heat to the ammonia in another heat exchanger. This chilled brine is then circulated through a third heat exchanger which cools the milk. By monitoring the pressure of the ammonia in the primary heat exchanger the amount of airflow needed to cool it can be determined.

The control scheme in use involved two pressure switches and two across-the-line motor starters. When a certain pressure was reached, the first pressure switch tripped, starting the first fan motor. When a second (higher) pressure was reached, the second pressure switch tripped, starting the second fan motor. The problem with this system came in while Fan A was running and Fan B was idle. Because they are both in the same cabinet, Fan A would draw air in that would be pushed out through Fan B, causing Fan B to spin backwards. When fan B tried to start up while spinning backwards it required a much greater current to reverse direction than it would have if starting from a standing start. This made the overload protection for Fan B trip to protect the motor. In effect, Fan B was now useless.

Luckily for our intrepid dairyman, Controlco had a solution that would not only control his fans in a better way, but also save him money on energy. We designed and built a panel that used a Square D Altivar 21 VFD, a Red Lion P-16 loop controller, and a Wika S-10 pressure transducer to modulate the speed of both fans at the same time. In order to insure that product would not be lost in the event of a VFD failure, we included a bypass circuit that will put line power to both motors if the VFD faults out. We also included a means by which each motor was individually protected from overload and could be “locked out” and “tagged out”. How’s that for a feature rich panel!?!

This panel is now in use and performing exactly as intended. No longer does this dairy farmer have to worry about losing an entire day’s production to a control failure. Also, instead of the head pressure on the ammonia loop bouncing up and down within an acceptable range, it now stays at a constant point determined by the loop controller. Because of the way a VFD modulates motor speed, he is also using less energy to run these fans.”