API Heat Transfer SIGMADUAL semi-welded plate heat exchangers feature plates that are welded using advanced laser welding techniques. Two plates are welded together and can be fitted with a Viton® O-Ring and/or Teflon® gasket liner. The resulting plate pack has every other plate fully serviceable, while maintaining integrity of the welded plate pair. This is especially suitable for critical fluids and gases, such as ammonia or caustic process chemicals where fluid loss is not acceptable. In the case of ammonia refrigeration, the reduced volume results in significant savings.