bricks) inside the firebox to direct the flow of hot flue gasses up into the top of the firebox before it flowed into the fire tubes increased efficiency by equalizing the heat between upper and lower fire tubes. To hold these in place, a metal bracket was used, but to prevent these brackets from burning and eroding away they were built as water tubes, with cool water from the bottom of the boiler moving upwards by convection as it heated, and carrying the heat away before the metal reached its failure temperature. Another technique for increasing the heating surface is to include internal rifling inside the boiler tubes (also known as Serve tubes). Not all shell boilers raise steam; some are designed specifically for heating pressurized water. Reverse flame In homage to the Lancashire design, modern shell boilers can come with a twin furnace design. A more recent development has been the reverse flame design where the burner fires into a blind furnace and the combustion gasses double back on themselves. This results in a more compact design and less pipework. Package boiler The term "package" boiler evolved in the early- to mid-20th century; it is used to describe residential heating boilers delivered to the installation site with all insulation, electrical panels, valves, gauges, and fuel burners already assembled by the manufacturer. Other delivery methods more closely resemble prior practice from the coal burning era, when other components were added on-site to either a pre-assembled pressure vessel, or to a "knock-down" boiler, where the pressure vessel is delivered as a set of castings to be assembled on-site. As a general rule, factory assembly is much more cost-effective and the packaged boiler is the preferred option for domestic use. Part-assembled deliveries are only used when necessary because of access limitations - e.g. when the only access to a basement installation site is down a narrow flight of stairs. Safety considerations Because the fire-flume boiler itself is the pressure vessel, it requires a number of safety features to prevent mechanical failure. Boiler explosion, which is a type of BLEVE (Boiling Liquid Expanding Vapor Explosion), can be devastating. Safety valves release steam before a dangerous pressure can be built up Fusible plugs over the firebox melt at a temperature lower than that of the firebox plates, thereby warning the operators by the noisy escape of steam if the water level is too low