Energy Efficiency: Cooling is Crucial
Smart system design and maintenance keep induction melting operations running.
Susan Hunter, Director of Development Engineering, Emsco, Massillon, Ohio
(Click here to see the story as it appears in February's Modern Casting.)
We saw how essential a cooling system is to the International Space Station in December, when one coolant pump failed. That left only one other pump to provide cooling for the station’s vital electrical and life-support systems. This required a crew member to make an emergency spacewalk to swap a spare for the pump that had failed. With its new pump in place, the station was back in the space exploration business.
While it might not be rocket science, when it comes to cooling, induction melting is as dependent on an effective cooling system as the International Space Station. Without adequate cooling, induction furnaces are not able to operate. And in the worst cases of cooling system failures, furnaces have been damaged or destroyed, endangering workers and causing significant damage to the foundry.
Metalcasting cooling systems normally operate quietly in the background and receive regular attention only from the maintenance personnel tasked with keeping them running. The goal for this article is to provide useful insights into the design and operation of effective and efficient induction melt shop cooling systems, with real-world illustrations drawn from a new system installed at Chassix Columbus Casting Operation, Columbus, Ga. Chassix is a $1.2 billion global company headquartered in Southfield, Mich., serving automotive customers from 25 locations in eight countries. Its Columbus facility melts 240,000 tons of ductile iron per year.
Cooling System Basics
Induction furnaces of all types and sizes normally are cooled by water flowing through the furnaces’ coils, which are made of heavy copper tubing. These coils generate high levels of heat, principally from the enormous electrical currents flowing through them and only to a much lesser extent from heat produced by the molten metal held in the furnace. Induction power supplies also require water cooling of their electrical components. Without an effective cooling system, induction furnaces will not operate.
At its most basic level, an induction furnace cooling system includes pumps circulating water through the furnace to absorb heat and on to a cooling tower where that heat is released. But to be safe and effective, a cooling system must incorporate a variety of vital subsystems. These include:
- Filters and other devices to keep the water clean and flowing.
- Heat exchangers, inline heaters and cold water diversion valves to maintain the optimal water temperature.
- Automatic city water makeup to keep the cooling system full.
- Flow sensors, pressure gauges, thermometers, water meters, and other monitoring and control devices needed to be sure it’s all working properly.
- An emergency backup system to maintain furnace cooling in the event of pump failure or power outage.
Because cooling systems are so essential, when the system at Chassix was no longer able to meet its needs, the management team moved quickly to repair or replace it.
According to Darold “Jack” Roop, senior project engineer, Chassix, the problems with the old cooling system had increased considerably when new furnaces were installed to support growth in the company’s casting business.
“We added three 12.5 metric tons per hour, medium frequency induction furnaces for batch melting, along with their power supplies, compressors and hydraulics,” Roop explained. “But our cooling system lacked the capacity to handle this new load. Due to inadequate cooling, the furnaces frequently overheated and tripped out. Several coils were burned up. We did not have sufficient cooling to allow us to run all of our furnaces at the same time. This reduced our metal production and limited our ability to fully benefit from the new melting capacity we had just added.”
Chassix determined repairs to the existing cooling system would not provide the cooling capacity needed, so it set up and funded a project to replace much of the system. Chassix project manager Frank Burton oversaw the creation of the new cooling system.
“Our cooling tower was old, the wood was rotting and falling apart and its three pumps had to run all the time to provide needed cooling,” he said. “There was no redundancy. If one pump failed, production had to be shut down until the pump could be replaced. Shutting down was a slow process. The only emergency backup was city water, and that outflow presented environmental concerns.”
Designing an Induction Melt Cooling System
Very small induction furnaces used in labs or for melting small quantities of precious metals may be cooled by direct connection to an incoming city water line and use a city drain for the outflow. Most other size furnaces require a pump or pumps to push cooling water through the furnace and a cooling tower of some kind to remove the heat from the water, which is then recirculated back through the furnace. This is the basis of most cooling systems.
To design a cooling system for an induction melt shop, first you must determine the heat load on the system, taking into account the size of each furnace, the power applied, the metal melted, type of melting (batch or heel), holding and pouring times and the heat loads added by non-furnace ancillary equipment.
These calculations can be complex. The new cooling system for Chassix was based on heat load calculations for the facility’s wide variety of furnace sizes, melting processes and ancillary equipment used to support them. These included:
- Three 12.5-metric-ton, medium frequency induction batch melting furnaces.
- Five 10-ton line frequency induction heel melting furnaces.
- Two 17-ton line frequency induction heel melting furnaces.
- Ancillary systems including air compressors, hydraulic pumps and air conditioners.
The calculations also had to take into account the need for backup capacity to maintain cooling during equipment maintenance or repair and to support future growth.
“I was looking for a new cooling system that would be reliable and offer the redundancy to enable it to continue running even with a pump failure,” Burton said. “I also wanted a system that would provide not just the capacity to cool all of our furnaces and equipment running at the same time, but that would have the additional capacity to support anticipated future growth.”
The next step in the overall cooling system design is to make adjustments for the desired incoming water temperature from the tower to the process, the outgoing water temperature from the process to the tower and the climatic data for the foundry location.
The three basic types of cooling towers are evaporative, dry air and hybrid (adiabatic wetted dry air coolers).
Evaporative cooling towers use water sprays and fans to reduce the temperature of the hot water returning from the furnace. They offer the advantage of being highly efficient at removing heat, even during the summer months in warm climates. Evaporative cooling towers are available as large capacity units and, when configured as vertical cross-flow units, require a relatively small footprint.
Dry air cooling towers use only air flow supplied by fans to reduce the temperature of the water. They are effective when air temperature is cool but might not function properly during the summer months in warm climates. In those cases, trim coolers, essentially added heat exchangers using city water supplies, might be needed to supplement the dry air cooling units. Dry air cooling installations require a relatively larger footprint than comparable evaporative cooling systems because air flow is less efficient for cooling than evaporation.
Hybrid units operate as dry air coolers during the cooler months and incorporate a water spray used only as needed during the hottest summer months. Essentially, evaporative cooling support is built into the hybrid unit. So hybrid cooling towers operate with equal effectiveness in hot weather and cold weather, offer a simplified design and reduce city water consumption.
Because of the large cooling capacity needed, the warm summers typical in southern Georgia and the limited space available for installation, Chassix required an evaporative cooling tower for its new system. The selected unit is a vertical cross flow cooling tower with two 75 HP fans, able to cool 5,500 gallons per minute (GPM) from 115F to 85F.
While the cooling tower’s job is to reduce the temperature of the heated water coming from the furnace, water returning to the furnace that is too cold will cause its own problems. That’s because water that is close to or below the prevailing wet bulb temperature, typically about 75F or less, will produce condensation on the furnace coil that can lead to electrical arcing. Such arcing can result in serious coil damage. To address this problem, most induction furnace cooling systems incorporate automatic cold water diversion valves. When the automatic controls detect that the temperature of the water leaving the furnace is too cold, the water is diverted from the cooling tower and flows directly back into the furnace. This continues until the desired outflow temperature is reached. Inline water heaters also may be used to bring the water up to temperature.
Open vs. Closed Systems
Two basic types of cooling systems are common in metalcasting: open and closed. In open systems, heated water travels through pipes from the furnace to the cooling tower. There it is sprayed through the air and flows down through a porous material. Evaporation, enhanced by the air flow generated by powerful fans, serves to cool the water, normally collected in a sump at the bottom of the tower and pumped back to the furnace. City water must be added to replace the water lost through evaporation and to dilute the buildup of minerals in the water. All open systems use evaporative cooling towers.
Open cooling systems are the simplest to build and operate and can achieve the lowest water temperatures. But they expose the water to dirt, debris and scale that can build up over time and clog the pumps, pipes and furnace coils, restricting water flow and substantially reducing cooling effectiveness. Open water systems, therefore, must include filters and strainers to capture solids and chemical treatments to reduce scale formation. Other treatments are needed to prevent disease-causing organisms, such as Legionella, from developing in the water. Overall, open water systems require more regular maintenance than closed systems and use more water. The sumps also require protection from freezing in cold climates.
In closed cooling systems, heated water also travels through pipes from the furnace to the cooling tower. But unlike open systems, when the water reaches the cooling tower, it continues to travel under pressure through pipes in a heat transfer coil, where the cooling takes place. The cooled water is then pumped directly from the heat transfer coil to the furnace. The heat transfer coil can be part of an evaporative cooling tower where it is cooled by water from a sump. Or, it can be housed in a dry air cooler where it is cooled directly by air flow. It also can be part of a hybrid cooling system.
Closed water systems keep water clean and free of debris and require minimal maintenance. However, closed water systems cost more to build. This is because as closed-loop systems they require heat exchange coils, expansion tanks, air vents and air scoops—all items not needed in open systems.
Whether a metalcasting facility uses an open or closed system to cool its furnaces, the water that recirculates through an induction power supply must be a closed system. Its electronic components require the water to be clean and deionized to ensure it is not electrically conductive. Typically, a heat exchanger provides cooling for the water in the power supply without intermingling with the water used to cool the furnace.
The new cooling system at Chassix is open, like its former equipment. The decision to continue to use an open system reflects the metalcaster’s desire to incorporate existing plant piping, controls and electrical network into the new cooling installation.
“Being able to reduce our costs by using our existing cooling system infrastructure within the foundry was a key requirement,” Burton said. “It probably saved about $100,000 and allowed us to stay within our capital budget. It also greatly shortened the downtime that would be needed to install the new system.”
Although Chassix’s new cooling system operates as an open system, Brian James, maintenance superintendent, reported the water remains cleaner because the strainers at the pumps are easy to clean. “With the old system, dirty water was a problem because there were no strainers at the pump. What got into the sump got into the pump,” he said. “The old system relied only on strainers downstream to catch the dirt and debris, and coils were constantly being clogged.”
Pump Selection and System Design
Pumps are the heart of the foundry cooling system and their power, flow rate and pressure must be balanced with the cooling requirements of the furnaces and other connected equipment and with the design specifications of the cooling tower. Of equal importance is the overall design of the pumping installation to provide for backup pumping capacity in the event of a pump failure or for routine system maintenance. The ability to replace a pump quickly and ease of access for maintenance also are important.
In the case of the new Chassix installation, water flow is provided by three 100-HP pumps, together able to move a total of 3,600 GPM. These are high efficiency pumps selected specifically to operate with the limited power available at the installation site.
Each of the three new pumps was incorporated into an individual module with its own piping and power/control connections. Each module also included isolation valves, making it possible to replace a pump without shutting down the cooling system. Just two of the pumps are needed to supply all of the capacity required for current operations. The third pump serves as a connected spare that can be switched in quickly if one of the operating pumps were to fail. This design proved its worth the first time Chassix’s new cooling system was turned on after installation.
“There were the normal hiccups with installation,” Roop said, “but they were handled. There was no panic, even when one of the new pumps failed at startup. Because we had planned for the cooling system to operate fully on just two of the three pumps, we quickly isolated the defective pump with the valves installed for that purpose and turned on the fully-connected backup pump, keeping the new cooling system in operation. Because we had purchased a spare pump that was stored nearby, we were able to replace the defective pump within the hour. It was a good demonstration of how well the system was designed to keep doing its job even with a pump failure.”
Backup and Emergency Cooling
Because effective cooling is so critical to the safe operation of an induction melt shop, it’s vital that a backup or emergency cooling system be in place in case an equipment or power failure shuts down the main cooling system.
A common backup covering both pump and power failures is a direct connection to a city water supply that is either automatically or manually turned on to flow through the furnaces and other equipment served by the main cooling system. This is a once-through-cooling arrangement with the heated water being routed to a drain or other outflow site.
Another common backup system for power failures is a battery (in very small systems) or generator that switches on to continue running the pumps in the main cooling system.
Many metalcasting facilities also maintain backup cooling pumps that can be switched in quickly if an operating pump in the main system fails.
Chassix is able to turn on a city water supply for backup cooling in the event of a power failure. Its new cooling system also incorporates an online backup pump that can be switched on quickly if another pump fails. A stored spare pump is ready to replace any failed pump.
Installation at Chassix
Ideally, installation of a new induction melt cooling system should take place in a large, flat area, with ample space to stage cranes and other heavy lifting equipment. There should be plenty of power available for the pumps and fans, and direct connection to city water supplies for makeup water to keep the system filled and provide emergency water in case of equipment or power failures.
This was not the case at Chassix, which wanted the new cooling tower and pumps installed adjacent to the old tower that was being taken out of service. This allowed an easy connection to the existing plant cooling water piping, electrical network and control interface, but it posed a challenge.
“The installation site was a tiny space adjacent to the old cooling tower and hemmed in by that tower and by the scrap yard with its rail spur,” explained Mark Foster, site supervisor, EMSCO. “The only access was by a narrow alleyway. Another obstacle was a drainage culvert that ran through the only spot where the new cooling tower could be placed.”
Because the old cooling system had to be completely shut down before the new system could be connected, a full plant shutdown was required. That shutdown had to be as brief as possible to minimize its effect on casting production. Installation was scheduled for a holiday weekend.
To meet the space and time challenges, every step of the installation process was carefully planned. Rail tracks were removed, and footings for the cooling tower were placed to avoid the drainage culvert. To simplify handling and speed assembly at the Chassix site, the three pumps were delivered and moved as modular units complete with their own piping and electrical connections. And for the assembly work at the site, a just-in-time delivery schedule was set up, and equipment lay-down and staging locations were cleared and prepared. Special compact material handling equipment was used to move everything as needed in the small working space. When it was time to shut down the old cooling system and the entire metalcasting operation, all that remained to be done was to make the new connections to the plant pipes, power and controls.
The project was completed on time and within budget. According to Roop, the new cooling system continues to work flawlessly. “It does what we were told it would do,” he said.
“The new system has the capacity we need and the water is staying cooler,” said James. “We have not had to shut down due to water over temperature. We can run all of our equipment all of the time, even in summer. Production is no longer impacted by cooling problems.”
As melt maintenance supervisor, Shawn J. Murray is responsible for keeping everything running, including the cooling system.
“With the old cooling system, we could not run all the furnaces at once,” he said. “And we still were having 50 to 60 high temp alarms per day. Now, with the new cooling system, we can run everything all the time. It has greatly reduced furnace trips and completely eliminated the need to shut down due to a lack of water at the right temperature. With the new cooling system, I can keep our furnace temperatures under control and all of our equipment running.”