Tosser tube.
We tried these last year as an inline heater by soldering one into a 22mm T fitting and keeping the flow running throught the other two connections.It worked well until the heater melted the solder and the flow of oil pushed it out of the fitting!! It would probably work fine using compression fittings though.Is there a reason that you're not buying the 15mm ones that wouldn't need a big spanner?http://www.ebay.co.uk/itm/Stainless-Steel-Tube-Cartridge-Heater-Heating-Element-220V-600W-15mm-x-180mm-/290750228465?pt=LH_DefaultDomain_3&hash=item43b21067f1
Anyone done anything with there cartridge heaters?