When dealing with a material that is melted to a specified temperature, there are several options in the asphalt industry to choose from. These methods carry over into other industries such as the roofing industry, thermoplastic industry as well as a variety of other industrial applications but what we'll focus on here is how it applies to the asphalt crack filling and thermoplastic line marking industries.
The three most common types of melters are Direct Fire, Oil-Jacketed, and Air-Jacketed. These types of melters have been around for decades and have evolved over the years to become efficient and cost-effective portable melting systems.
Direct Fire melters are the least expensive way to melt a material. They work by firing a torch style flame beneath the melter which heats the bottom of the kettle. Just like air-jacketed melters, the surrounding air is heated and rises along the sides of the double wall to heat the sides of the kettle. Some of these types of melters use a burner style flame instead of a torch, but the main point is to heat the bottom of the melter and surrounding air. The biggest drawback to direct-fire is their typical design creates "hot-spots" that can ruin the material if it is not frequently agitated. Some material manufacturers have designed rubbers that can handle the spot temps, giving a lot more freedom from messing things up.
Oil-Jacketed melters are probably the most known type of melting kettles that are sealed with a heating oil in-between double walls of the melter kettle. The oil is usually heated with either a flame source or an electric heat source. Oil-Jacketed melters take a little more time to heat up from our experience, however, they stay hot longer than air-jacketed melters which can either be good or bad depending on your needs. This makes oil-jacketed a bit more fuel-efficient at the cost of maintaining the oil or dealing with potential leaks caused by age or poor overall craftsmanship. Many oil-jacketed manufacturers refuse to sell-direct to customers and only sell through distributors because: "Customers need to know where to go if it breaks". Oil-jacketed melters are generally more complex and require someone with extensive knowledge on troubleshooting and fixing these types of units. In comparison, Air-jacketed propane melters are by far the easiest to operate and maintain, making them the best overall option if you need a system that "just works".
Air-Jacketed melters have had a bad wrap over the years but with the advancement of better agitation, heating designs, and automatic temperature controls, air-jacketed melters have become a fantastic alternative to the costly oil-jacketed melters. Air-Jacketed melters heat the surrounding air which moves up through the double walls of the kettle or through flame pipes positioned below the kettle. As the air heats up, it rises through the double wall and escapes out vent holes or a vent chimney. The hot air as it passes through the double walls, heats up the metal which in turn, heats the rubber. Most air-jacketed melters are heated from a fire type of heat source that is usually fueled by either propane or diesel though there are also electric style melters available on the market. Air-jacketed melters are super simple to maintain and rarely have problems with any of the heating system, outside of the burners or fuel regulators. One thing to look for when choosing an air-jacketed melter is the ability to maintain a consistent temperature. If a unit isn't designed well and doesn't have a really good thermostat system, keeping the temp locked in can be difficult. This is usually only a problem with inexpensive direct-fire push melters or inexpensive stationary melters. That said, high-end melters will come with a thermostat.
When it comes to the crack rubber materials used, the rubber manufacturers usually have two types of rubbers they sell. One is designed for direct fire melters and the other for oil-jacketed and air-jacketed melters. The less expensive Oil-Jacketed / Air-Jacketed material flows at a slightly lower temperature, but that temperature must be maintained constantly in order to keep from damaging the rubber. Excessive heat makes the rubber go bad which means a melter needs to have good agitation to prevent hotspots from damaging the rubber. On the flip-side, direct fire rubber is more expensive, requires a higher temperature to flow, but is extremely forgiving with temperatures. While direct-fire rubbers do have a max temp that will damage it, the range between flow and ruining it is generally a lot higher.