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Combo Systems - Direct Contact Water Heaters

Initially developed in the 70s through a grant from the DOE's Invention and Innovation Program, the direct contact water heater is promoted primarily as an alternative to a boiler for providing industrial process hot water and space heating. Some models also produce potable hot water. As a boiler alternative, direct contact water heaters offer several advantages. Unlike pressurized boilers, direct contact water heaters operate at atmospheric pressure eliminating the need for a full-time boiler engineer, boiler permits and annual inspections, and the higher insurance rates associated with a pressure system. The water heaters also do not require the chemicals and frequent maintenance of a boiler. The higher efficiencies delivered by direct contact water heaters far surpass average boiler efficiency of 60 to 75%.

Direct contact water heaters boast a superior heat transfer system that delivers significantly higher efficiency and lower emissions compared to conventional heat exchanger technology. In conventional water heaters, heat is transferred from the hot combustion gases to the incoming water indirectly via one or more metal walled heat exchangers. The direct contact water heater achieves efficiencies approaching 100% due to the direct, rapid transfer of heat from the hot combustion gases to the incoming water. Efficiency is further enhanced by capturing heat that would normally be lost into the atmosphere with the exhaust gases. Low NOx burners and special firing chambers reduce carbon dioxide (CO2) and NOx emissions to levels low enough to meet the most stringent environmental requirements.

How It Works
The direct contact water heater consists of a cylindrical water vessel fitted with a low NOx burner that fires directly into the lower heating chamber and a heat transfer zone made up of stainless steel packing material located in the upper section. As water flows into the unit, it cascades over the packing material and is instantly heated as it comes into contact with the rising hot combustion gas and the burner flame. In models that assure potable hot water, a water-jacketed combustion chamber prevents impingement of the water on the flame and water contamination. The stainless steel packing material increases the contact area and slows the water flow, providing improved heat exchange. The combustion gases cool quickly as they give up their heat to the incoming water and exit the exhaust stack at near ambient temperatures. The heated water collects at the bottom of the tank where it is available for on demand use.

Capacity/Energy Input
Depending on brand and model, direct water heaters can deliver 20 to 7000 gallons per minute with energy inputs of 1.2 to 70 million Btus/hr.

Efficiency
96% to 99.7%

Energy Types
Depending on brand and model, units can operate on natural gas, propane or oil.