The two general categories of boilers are hot water and steam boilers. Most small commercial buildings, manufacturers, and even some food processors use hot water boilers. Water is heated to appropriate distribution temperatures, typically 140-180°F, and usually returned about 20°F lower temperature for reheat. These systems are often "closed" with virtually no fresh water make-up. Where applicable, hot water boilers are preferred because they normally do not need operators or special water chemistry. Further, since they operate at lower temperatures, hot water boilers can operate at higher fuel conversion efficiencies than steam boilers.
Steam boilers are found in many different configurations, but all serve one purpose: to contain water and transform it into steam by the application of heat. Some boilers heat the steam even hotter than the boiling point temperature. This is referred to as superheated steam.
Clean firing - no emissions of products of combustion
No stack - with no combustion there is no venting or stack required
Highly efficient with minimum losse
Compact size - smaller volume and footprint than fossil fired boilers
Available in wide range of sizes as boiler only or in tank-type models
Demand and energy charges can result in higher operating cost
Electric boilers can be applied wherever there is a need for hot water at 30 to 250 psig pressure, and steam at 15 to 250 psig pressure and in capacities up to about 3,000 kW (10 million BTUH) in a single unit with tank models up to 10,000 gallons.
Installations requiring steam or hot water relatively few hours in the year; or where a supply of gas or oil is not readily available.
Technology Types (Resource)
Electric boilers typically use resistance heating elements to convert electric energy to heat energy. Heavy duty medium watt density elements with low surface temperature provide excellent protection against oxidation and scaling. Elements are operated by heavy duty UL rated magnetic contactors. Safety controls are provided including low water cutoff and temperature and pressure relief valves. A wide range of options are available including step controls, circulating packages, voltages, circuit breakers, peak load controllers, and safety interlocks.
Electric boilers are relatively 100% efficient in converting power to heat (3,412 BTUH per kW or 293 kWh per million Btu) with only a minimal percentage of radiation loss from the exposed surfaces of the boiler.
This contrasts to fossil fuel boilers that have combustion efficiencies in the 60 to 80% range, depending on the boiler age and condition, plus other losses.
Combustion efficiency simply indicates the flue gas loss. Boiler efficiency also includes the blowdown and standby losses. Whether comparing new or existing boilers, their most efficient operating point is usually somewhere between 60-90% load.
However, when you are evaluating a change in steam production due to a new steam use (which would increase steam generation) or steam conservation (which would obviously reduce steam production), be very careful to consider incremental boiler efficiency.