A tube-in-shell heat exchanger is the energy source of a domestic hot-water system at many commercial facilities. Steam is piped into the shell. It condenses on the tubes, and heat is transferred to the water circulating in the tubes. The steam flow is modulated by a control valve that opens on a signal from the bulb thermometer in the hot-water outlet. The steam is vented from the heat exchanger by a line at the bottom. A steam trap in this line prevents live steam from escaping and allows only liquid condensate to flow down-line.
This type of instantaneous water heater is often located in a mechanical room near the boiler. If so, the condensate can be used as boiler feedwater. Then the thermal energy of the water is not wasted.
Some facilities use two temperatures of hot water. The hotter water, at 180°F, is used in the kitchen or laundry, or to sterilize equipment in an autoclave. To create this high temperature, a portion of the 140°F water leaving one steam converter is diverted into a second steam converter, with the steam control valve set to maintain a 180°F leaving water temperature. In this way, hot water is supplied to each space at the lowest practical temperature.
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Excerpt from Energy Conservation Projects.