Keeping Thermal Options Open
By Mike Brezonick06 June 2016
Every engine that burns fuel to generate power also generates heat. Whether it’s a reciprocating engine or gas turbine operating on diesel, natural gas or heavy fuel, heat is a natural byproduct of combustion. And no matter the application — generating electricity, propelling a ship, pumping oil or moving a train down the rails — it’s important to keep the heat where it belongs in the engine system. That’s where thermal insulation comes in.
“There are many factors that play into the selection of the correct insulation for an application,” said Bruce Kaplan, product development manager at Thermal Structures, Inc. Headquartered in Corona, California, U.S.A, Thermal Structures is a global supplier of insulation and composite thermal barrier products used in an array of marine, power generation, off-highway, transportation and military markets.
“Some engines use insulation that is encapsulated in fabric while others require sheet metal or metal foil,” Kaplan said. “There are a number of decisions that go into selecting the proper thermal insulation.”
There are four primary types of thermal insulation configurations used in engine systems, Kaplan said. Fabric insulation consists of blankets that are made from insulation material sandwiched between two layers of flexible fabric. For lower temperature applications, the fabric is often fiberglass or silicone-coated fiberglass for fluid resistance. In higher temperature applications, silica, quartz or ceramic cloths are used to retain strength at elevated temperatures.
Fabric insulation blankets are typically sewn in custom shapes, and attached with lacing wire, springs, rope ties or, most often, straps.
Metal foil encapsulated products incorporate insulation material sandwiched between two thin metal skins. The skins are commonly made from stainless steel, Inconel (a nickel alloy) or other alloys with high heat resistance.
Metal foil insulation requires tooling to form custom shapes, welded together rather than sewn, and are typically fastened with lacing wire, hooks and springs or mechanical fasteners.
Sheet metal insulation structures are the most heavy-duty encapsulation option available, as well as the heaviest, Kaplan said. “They are made from insulation material that’s sandwiched between two layers of sheet metal,” he noted. “Sometimes an outer layer of sheet metal and an inner layer of thin metal foil or fabric insulation can be fastened to it.”
Sheet metal insulation is formed into custom shapes and typically mechanically fastened with bolts and joints built into the insulation itself. These components can also provide structural strength, Kaplan said.
Composite insulation blankets are not commonly used in power generation, oil and gas, or marine applications, as they’re more typically an option for low- to medium-heat applications where weight is a key consideration, such as aerospace. Composite insulation consists of insulation sandwiched between layers of resin-infused cloth, with the assembly is molded into shape under heat and pressure.
Each type of insulation has key characteristics that make them suitable for specific applications, Kaplan said.
Tooling costs — the investment required to make the molds and forms used in the production of a part — are an important consideration, he said. The same molds and forms can be reused through the entire production run, meaning that the larger the quantity, the lower per-part tooling costs.
Fabric insulation has the lowest tooling costs because patterns are programmed and cut on CNC machines for each production run. Initial orders can be produced quickly because the process requires little to no tooling.
“That’s a big part of what makes fabric insulation popular on large engines with low production runs,” Kaplan said. “With a limited run of generators built each year, a company may never see the type of production volume that would justify the tooling costs.”
During the life of any engine, it’s likely that at least some of its insulation will need to be removed for an inspection, service, maintenance or a repair task. When this happens some insulation may be re-installed when the task is done, while others will be replaced. Kaplan said that whether the insulation is treated as a reusable or consumable item depends on the insulation used, and how long it has been in place.
“Fabric insulation starts out flexible, but in the presence of high temperatures it will tend to ‘cure’ over time and become brittle,” Kaplan said. “When a fabric insulation blanket is unwrapped after it has been exposed to high temperatures, it will likely crack and need to be replaced.”
In contrast, because of their ability to resist higher temperatures, Kaplan said that foil insulation and sheet metal fabrications can be removed and reused after exposure to high temperatures over a long period of time.
Kaplan added that the insulation typically used in engine systems — fabric blankets, metal foil insulation and sheet metal insulation structures — can be engineered to resist absorption or attachment from flammable liquids from chemicals. “The metal skins of foil and sheet metal encapsulated insulation are impervious to common engine fluids and chemicals,” he said. “Fabric insulation blankets are typically coated on their outer surface to resist fluids as well.
“However, if a flammable liquid spills between a protected surface and the inner surface of a fabric insulation blanket, there’s a chance that at least some of the fluid will be absorbed into the fabric and into the insulation material itself. In case of a flammable chemical spill, both sheet metal and foil encapsulated insulation can be pressure-washed clean. Fabric insulation may require a more careful assessment to see if a flammable chemical has been absorbed that could pose a fire danger. “
When it comes to toughness, it’s hard to beat metal, Kaplan said. “Sheet metal insulation is one of the toughest, most durable insulation materials available,” he said. “That’s what makes it so popular with large, off-highway construction and mining equipment where impacts with flying rocks, generally rough conditions and the occasional dropped wrench are all in a day’s work.”
Foil insulation is durable and can last for decades and is commonly found on stationary engines, as well as on-highway trucks.
Because of its flexible nature, fabric insulation can withstand high-force or high-speed blunt impacts, but it can be torn by the impact of sharp objects — the aforementioned dropped wrench or a screwdriver — and it doesn’t offer as much mechanical protection as sheet metal. Also because of the curing factor, it usually becomes brittle over time and therefore doesn’t have the same long-term durability as metal encapsulated options, Kaplan said.
Interestingly, heat resistance is an area where Kaplan said “many people have the wrong impression.”
“It seems intuitive that thick sheet metal would be able to operate in much higher temperature environments than either foil insulation or fabric insulation blankets,” he said. “The truth is that while sheet metal encapsulated insulation can be a good option for the highest heat environments, so can fabric and foil encapsulated insulation.”
When selecting the proper materials for the application, Kaplan said that all three types of encapsulation can operate at temperatures up to 1204° C (2200°F) and sometimes higher. Even with that, Kaplan said that Thermal Structures typically recommends either metal foil or sheet metal for temperatures above 648° C (1200°F).
Field application and service is another area where there isn’t a clear “good, better, best,” Kaplan said. In some ways, fabric insulation is the easiest form of insulation to apply and service, since it often uses straps or other quick attachment methods and simply wraps around the surface to be insulated.
Because of their high design tolerances, foil insulation and sheet metal are also often easily assembled and their fasteners can be quickly secured and removed. Factory settings typically prefer rigid options such as foil encapsulated insulation and sheet metal, Kaplan said, while field installers tend to prefer the more forgiving nature of fabric installation. “But,” he added, “there’s no set rule.”
Weight is an area where Kaplan said there is a clear good, better and best. “Foil insulation is extremely lightweight, unequaled in this aspect when it comes to high-heat, durable insulation options,” Kaplan said. Industrial fabric insulation is somewhere in the middle — it’s heavier than foil insulation, but significantly lighter than sheet metal. “There are some exceptionally lightweight aerospace fabrics that can be used to create lightweight fabric insulation blankets,” Kaplan added, “but these are the exception.”
Sheet metal insulation offers strength and toughness, but are not known for being especially lightweight. Methods of reducing the weight of sheet metal encapsulated insulation includes the use of thinner and lighter gauges of sheet metal and hybrid configurations — sheet metal on the outer side for protection and either fabric or foil encapsulated insulation on the inside — are also options.
“There’s no one-size-fits-all in high performance thermal insulation,” Kaplan said. “Every option has its own advantages and disadvantages. Every application has its own ‘best option.’ Once engineers get into the specific needs of an application, in terms of weight requirements, order of assembly, available space and tolerance requirements, it should become clear what the best option is.”