There are now only 4 types of gas fryers and how they transfer their heat. There was a fifth type where the gas burner heated a plate at the bottom of the fryer and this plate heated the oil, these are no longer in production, leaving 4 basic types. All 4 aim to heat the oil from the middle of the vat to create a cold zone. The oil in the middle is usually kept at 180c but the oil at the very bottom is colder in comparison, so cool in fact you can put you hand on the bottom of the vat even when fully up to temperature. The idea is that stray product falls into the cold zone and stops cooking hence saving your oil. Lets look at the 4 types.
These fryers have stainless steel tubes, usually 2 or 3 running through the middle of the fryer. Burners then direct the flame through these tubes and the heat is transferred into the oil. This type is very common and you will know if you have one as you will see the tubes as you try clean around them. Dean, Annett, Pitco Zanussi and Goldstien are common brands using this method. The downside can be, food can land on the tubes and continue to cook damaging your oil, harder to clean and a lot of your heat is wasted going up the flu.
These fryer usually have a u shaped manifold with gas jets spaced around it. Gas comes out the injector it is mixed with the air and ignites to produce a stream of flame upwards, above the jets are metal targets that the flame hits the targets then deflects or aims the heat 90 degrees into the side of the fryer vat. Fryers using this method are frymaster, garland, fastfri, cobra and 18 litre version of Waldorf FN8118G and Blue Seal. These are easy to clean as there are no obstructions in the vat but the heat can weaken the vat over time so expect only 5 years life, also a lot of heat is lost up the flu. Eattucker by Commercial Catering Services supply the Fastfri FF18 well under $2000.00. https://www.eattucker.com/fastfri-400mm-gas-fryer-ff18-only-1-892-00/
These fryers use an infra red gas burner going along each side of the vat. These burners work by having a gas air mixture flow through a metal mesh and ignite at the other side of the mesh. This produces a red glowing heat that radiates into the vat. It is a very clean burn, it is gentle on the vat and extremely efficient at transferring the heat to the oil. These models are common on the Blue Seal and Waldorf FN8120G models. Eattucker by Commercial Catering Services supply the Blue Seal GT45 for well under $4000.00. https://www.eattucker.com/blue-seal-gt45-450mm-gas-deep-fryer-only-3833-00/
The high performance operation. Mainly designed for very busy sites where there is no time for any heat recover, typically football stadiums and busy fish n chip outlets. Normal fryers run on 1 kpa gas pressure and can mix with surrounding air to produce a good burn and heat source. To get more heat these HPO fryers run at 1.5 kpa where the gas air mixture is pushed thru holes of a ceramic plate at each side of the vat to produce an intense heat. In order to get more air getting into the mixture air fans are used, these gets more air into the gas mix and produces a steady forced gas stream thru the burner. These fryers are very reliable and can be seen in the Blue seal GT60-HPO model and the Waldorf FN8160HPO. https://www.eattucker.com/commercial-catering-equipment-cafe-restaurant/fryer-gas/blue-seal-gt60hpo-hi-powered-gas-fryer/