Heat transfer units that use steam to pro- duce hot water are known as indirect heaters . They are often shell and tube type heat exchangers and are generally referred to as converters, hot water gen- erators, and instantaneous heaters . The ASME Code for Unfired Pressure Vessels is the nationally recognized authority pre- scribing their construction for given tem- peratures and pressures. The term used varies with the heating medium and the manner of application. When these heaters use steam as the heat source they are usually called steam to water converters. In steam heated converters, the water to be heated circulates through the tubes and steam circulates in the shell sur- rounding the outside of the tubes. This results in condensate draining to the bottom of the heat exchanger shell as the steam gives up its latent heat.