Process cooling systems design

July 14, 2017

Centrifugal compressors deliver variable cooling capacity that is precise, reliable and efficient for a plastics manufacturer

Millions of cell phone cases, laundry baskets, plastic totes and storage bins are produced by a process known as plastic injection molding, which has spawned a lucrative, multibillion-dollar plastics injection molding industry in the U.S. This domestic industry faces stiff competition from overseas producers. However, Intertech Plastics, a manufacturer based in Denver, Colorado, illustrates the new trend in “reshoring” by re-energizing domestic plastics manufacturing with the help of innovative, energy-efficient Danfoss Turbocor compressors.

“In a plastic injection molding machine, it is very important to cool the mold properly [for] maximum productivity,” said Mike Clavelli, West Coast regional manager for Thermal Care (based in Niles, Illinois), Intertech’s process cooling system provider. As a manufacturer of process cooling equipment and systems, Thermal Care recently installed two centrifugal chillers using variable speed compressors.

Clavelli explains, “When Intertech added two new Husky 1,100-ton, high-speed molding presses, we implemented the chiller installation using an array of variable speed technologies. This solution delivers variable cooling capacity that is precise, reliable and efficient, which gives Intertech a huge domestic and global competitive advantage.”

Handling the heat

An injection molding machine uses two basic components: A clamp unit to hold the mold and an injection unit in which the plastic is heated and mixed before injection into the mold. Time, speed, pressure and temperature must be controlled to ensure proper operation and maximum throughput.

Depending on the resins, mold temperatures reach up to 180ºF. At the beginning of each production cycle, the clamp unit closes the mold, and a screw in the injection unit pushes forward to shoot molten material into the mold. Continual pressure is applied to hold the plastic in place until it solidifies. While the material in the mold cools, the screw rotates and retracts to draw in new material for the next shot. When the part reaches the precise temperature setpoint, the clamp opens to eject the finished part and start the next cycle. Depending on the part’s size and the type of plastic, the entire cycle may take from a few seconds to more than a minute.

“Productivity depends on cooling that mold properly,” said Clavelli. “The difference in throughput can be huge. For example, with good temperature control you might be able to make 20 parts per hour instead of just three.”

Energy-efficient cooling for injection molding machines

For efficient cooling, a plant-wide, central chiller cooling system was designed and manufactured to handle the unique requirements of Intertech Plastics’ two injection molding presses. The system has two centrifugal chillers, a cooling tower, and condenser and evaporator circuits that use pumps operated by variable frequency drives (VFDs). The piping was designed to scale up so that another chiller can be added if needed. To maximize the chillers’ efficiency, Clavelli used centrifugal compressors with an oil-free, magnetic-bearing design instead of rotary screw compressors commonly used in the industry.

“We’ve been successful using these centrifugal compressors in injection molding cooling for nearly a decade,” said Clavelli. “In this case, a 140-ton chiller employs two compressors, and a 70-ton chiller uses one compressor.”

In this installation, the chillers supply a 1,000- gallon thermal storage tank with chilled water at 50ºF. From there, a liquid cooler pump control circulates water to the injection process to cool the molds as needed. The tool and mold sections can range from 150 to 180ºF. Depending on the temperature requirements for the part, a mold temperature control unit may be used to mix warmer water with the 50ºF supply water to provide the exact degree of cooling required. The temperature of the water that returns from the process is approximately 60ºF.

Because the amount of supplied chilled water varies, this application proved ideal for the new compressor’s variable speed capability.

“A rotary screw compressor may use stepless speed control or slide valve control to vary capacity, but those will be inefficient in certain conditions,” said Clavelli. “The new centrifugal compressors simply vary their speed to deliver the needed cooling capacity.”

When full cooling capacity is not needed, which is about 95 percent of the time, the compressor reduces its rpm to produce just the right amount of compressed refrigerant gas needed to match the cooling load. In a centrifugal compressor, electrical consumption is directly proportional to motor shaft speed. This slowing of the shaft speed when cooling requirements are reduced minimizes the amount of electricity consumed linearly: Running at 50 percent speed reduces the compressor’s electricity consumption by about 50 percent. At the same time, the coefficient of performance (COP) – a measure of energy efficiency calculated by dividing the amount of cooling by the amount of power supplied to the compressor – is much better because the cooling load is reduced. For example, at 40 percent capacity the COP of the compressor chosen is more than double that of a rotary screw compressor.

Easily integrates with a free-cooling configuration

Clavelli appreciates the inherent energy efficiency of the centrifugal compressor, but he also said he values its ability to use colder cooling tower water.

“Denver is the Mile-High City, so outdoor ambient temperatures let Intertech use cooling tower water to drop the condenser water temperature,” said Clavelli. “Whenever you can do that, it means less work for the compressor.”

However, conventional centrifugal chillers cannot always operate with cooling tower water below a 65ºF temperature limit. This is because temperatures below 65ºF do not allow oil in the refrigerant to circulate properly to ensure compressor lubrication.

“We don’t have oil circulation or oil management issues with these centrifugal compressors,” Clavelli said. “That is because the design uses magnetic bearings that do not need oil. The impeller shaft spins in a friction-free magnetic field. Because there is no oil to clog the system, a condenser water reset option can be used to enhance chiller efficiency to reduce energy consumption.

“That makes it is easy to use a free-cool circuit with VFD pumps for both the cooling tower and chilled water loops. This configuration lets us take full advantage of off-design outdoor temperatures to avoid mechanical compression as much as possible, while variable speed capacity control lets us scale cooling capacity up and down as needed.”

The result: compared to the chillers it replaced at Intertech’s facility, the Thermal Care system with centrifugal compressors has decreased energy costs dramatically. Instead of paying $38 per hour for cooling, costs are now down to $3.40 per hour – a remarkable order of magnitude in energy savings.

Size, silence, reliability & controllability

Clavelli also notes that the new chillers have a 50 percent smaller footprint than the chillers they replaced, maximizing available space in the mechanical area. The magnetic bearing design eliminates the bearing race support structures and oil system, reducing the actual operating weight to 265 pounds per compressor, while a conventional screw compressor can weigh more than 1,000 pounds.

Plus, with a peak acoustic level of 71 A-weighted decibels, the new chillers are much quieter than a typical screw compressor. “Low sound was critical to this project, and it is nice to be able to stand beside the chiller while the compressors are running and hold a conversation without ear protection,” Clavelli said.

Another critical benefit is controllability and reliability. “We implemented an on-board programmable logic controller to provide a user interface. With inherent digital operation, each compressor provides nearly 80 points of diagnostic information: temperatures, pressures, volts, amp draws, log and save data, adjust set points or control parameters, alarm history, faults, and demand profile, including kilowatt usage,” he said. “Access to that level of information assures us and the customer that things are running right, and we can take prompt corrective action if needed, with the option to access that information remotely over the internet.”

Total cost of chiller ownership makes company more competitive

“We’ve been using these compressors for years,” said Clavelli. “Having a reliable, oil-free centrifugal compressor design that runs efficiently at part load and that integrates smoothly with a free-cooling configuration is a big plus.”

By cutting cooling costs from $38 to $3.40 per hour, the new chillers will save about $42,000 in annual energy costs at the current process load when compared to rotary screw chillers. Just as important, the new cooling system has improved the manufacturing operation by optimizing cycle time efficiency by 5 percent, increasing plant yield by 9 percent and reducing scrap by $194,000. Finally, the installation qualified for a sizeable five-figure rebate from the local utility.

On an energy-efficiency side note, the customer recently commented to Clavelli, “I stopped by the chiller system this morning. The total compressor demand was at 3 percent, which means we’re taking advantage of 97 percent free cooling plant-wide. Those power reductions, plus our operating efficiency gains, are a constant source of conversation here at the plant. Everyone remembers how the old chiller ran full out, all of the time.”

Clavelli added that the oil-free compressors lower maintenance costs. “The compressor’s magnetic-bearing design dramatically cuts Intertech’s total cost of ownership,” he said, “and by enabling a chiller solution that enhances manufacturing operation, we are giving Intertech a significant competitive advantage. In fact, it attracted a large customer to reshore 70 percent of its business, which added 25 percent more jobs. When you can use innovative technology to save energy and reshore American jobs, it’s something to be proud of.”

Ken Koehler serves as a key account manager for Danfoss Turbocor Compressors Inc., a manufacturer of high-efficiency, oil-free centrifugal compressors. He has 30 years of experience in global HVAC and aerospace/defense, including sales, project management, quality control, and sustainable engineering. He may be reached at [email protected] or 850-504-4821.

Danfoss Turbocor Compressors Inc.

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