A coil is a coil is a coil ... well, not exactly
“The performance of coils varies from one company to the next,” stated Doug Simmons, product planner for the Heat Transfer Division of Heatcraft, Inc., in Grenada, MS. Moreover, “the fin surface of the coil can vary from one company to the next.”
Even the seemingly rudimentary fin is not so rudimentary; its design can change from job to job. For example, Simmons’ division has over 30 fin surfaces that it uses depending on the application. And these can be unique to the manufacturer. Heatcraft holds patents for fin surfaces as well as various heat transfer components.
Also, different types of coils, such as evaporator, condenser, or steam coils, “are different not only on the performance side, but also on the construction side,” he noted. The steam coil requires a floating core, to allow for movement due to quick temperature changes.
Each application as well — an ice machine, air handler, or dehumidifier — has different customer criteria “as far as the size of the coil, thickness of the sheet metal, od of the tubes, and things of that nature,” said Simmons. “There are tons of different ice machines out there, and they all kind of look the same, but everybody designs their machine a little different.”
For a replacement job, the keys to success are “made to fit” and performance.
The majority of the coils that his division produces are copper tube and aluminum fin. Copper and stainless steel fins are also used. Precoated fins are used but not very often. It’s “kind of expensive,” Simmons said, and there are other alternatives.
Regarding working with the new refrigerants, he remarked, “Right now, under the current UL standards, we could see some change in materials. However, with the proposed changes, we probably will not see any change in construction.
“The new refrigerants are one of the reasons we are looking at alternative construction for heat exchangers. Alternative construction involves better manufacturing methods, different materials, new coil concepts, and performance enhancements for our current product.”
Where are we going?“I’m always looking at the future …what’s coming up new, what are the opportunities,” said Ira Richter, research manager of Heatcraft’s Refrigeration Products Division in Stone Mountain, GA.
Before looking ahead, Richter explained where refrigeration is at. Commonly, his group uses copper tubes and aluminum fins for its coils. For refrigeration as opposed to air conditioning applications, “We typically see wider fins per inch so that our coils are tolerant of frost buildup.”
Edge enhancements on fins, which are used for air conditioning, are not normally applied for refrigeration. They “are not common in lower temperature applications because they tend to be sources of frost formation,” said Richter.
He also noted that his division’s customers are more concerned about coil blockage from dirt and debris, again calling for fewer fins per inch.
Changes to a coil are very carefully considered, Richter related. From a fabrication standpoint, there’s a great deal of capital invested in equipment, so it’s a big decision to go from one kind of construction to another.
To handle the new refrigerants, though, you do have to examine coil design, he said. For evaporator coils, you check nozzles, distributors, the circuit, and pressure drops to make sure they’re correctly designed for the refrigerant. But there haven’t been “any dramatic changes in the construction.”
Regarding recent trends, Richter remarked that “There’s been a significant shift from smooth tube to tubes that have internal enhancements,” in order to improve heat transfer. “We’re seeing more and more of that happening.
“If you look at the patents, there’s a lot of work being done on the air side in terms of enhancing the fins — their ability to transfer the heat.”
While edge enhancements are not normally used for refrigeration, corrugation enhancements are beneficial. Richter’s group looks at what styles of corrugation would work best.
A new technology that looks promising for the future, he stated, is the vortex generator, “a device design that causes the air to swirl and be turbulent as it flows through the fins.” Microchannel technology, which is used in the automotive industry, may also see greater use.
Richter also believes there are opportunities in changing the shape of the tubes. “There’s nothing that says we have to stay with circular tubes.”