The plant was operating at less than 30% production, but the refrigeration system still was at capacity and barely keeping up. What was going on?
When product demand dropped, the East Plant was shut down and the West Plant began operating at reduced rates. Yet, the massive refrigeration plant was having a hard time meeting the lower refrigeration demand, partially due to equipment failures; however, there was more to it than that. Jake had been called to the plant to help with troubleshooting the situation.
Jake met with Joe, the staff engineer responsible for the refrigeration plant. Its available capacity was more than required but still the plant couldn’t keep up with production demands. The refrigeration system consisted of a batch process used to supercool the -18°C brine in the off-batch times but it was getting close. The three largest units were available but something seemed to be holding their capacity back.
Looking at the round charts from the day before, Jake noted that No. 26 was producing only about 40 % of its nameplate rating. In addition, the outlet brine temperature seemed to be close to the maximum. Jake found fouling in many of the 26 other refrigeration machines previously, and so thought that might be a good place to start. He also observed the power consumption per ton was about double design requirements.
Jake and Joe headed to the operations control room to review with the lead operator what might be happening. Doug appeared very tired and confirmed it with Joe and Jake. “We are just barely keeping our heads above water and spending lots of time just getting units back online as they seem to be tripping out every hour or so. I hope demand does not pick up as we would not be able to meet their refrigeration loads.”
No. 26 was a 2,500-ton -18°C brine machine with a 6,000-hp motor. While steam turbines drove most of the plant, the last three units contained motor drives to help the site’s steam balance.
Joe and Jake headed out to take a look at No. 26. The first observation: the motor amps were already at the maximum. Checking the pneumatic controls showed the motor override had activated and the suction vanes were restricting flow to the compressor. Jake calculated the load on the machine to be only about 1,200 tons. Fouling could not explain this very poor performance.
No. 26 was fitted with a split-range capacity control, meaning the pneumatic controller would try to open up the inlet vanes when the outlet brine temperature increased, putting more load on the machine. If the temperature decreased, it would pinch back on the vanes to restrict flow to match the load. If the load dropped below a minimum, the hot gas bypass would open, causing the flow to keep the compressor from going into surge. Careful testing and data logging determined this point. Excessive surge could damage the machine.
Joe and Jake tested the control. They attempted to override temperature control to put more load on the machine. Nothing happened; something was wrong with the controls. They put the machine in manual and attempted to cut back on the hot gas bypass. Still nothing happened. An operator wandered by and said they were having trouble with the controls, so had put the hot gas bypass in manual at the valve controller. The valve had been set nearly wide open.
The team placed the main controller in manual and set in a safe load position. Then they switched the hot gas bypass controller to auto and returned to master control by the main system. The valve immediately cut back on the flow, the motor unloaded and the outlet brine temperature returned to the set point. They then set the main load controller; other machines in the system began unloading and overall brine temperature returned to standard operating conditions.
Joe and Jake sat down with operations to reinforce standard operating procedures and emphasized the need to report to engineering when a machine was having problems. They determined the cost of the off-standard operation to be about $150,000 per year on just that machine. Other units were checked with similar results but not nearly as large an impact on costs.
So, many of you out there may still have pneumatic controls on old process refrigeration machines. Or you may have upgraded to single loop electronic controllers. Do you know how those controls are operating? Do you know if off-standard operations are costing you energy dollars? It might be time to check. Happy energy hunting!