96% Ultra-high Efficiency Low NOx Direct Contact Water Heater
Direct contact heat exchanger technology is typically more efficient than conventional counter-current heat exchange technology because it eliminates the performance reductions caused by fouling of the heat exchange surfaces and the associated energy losses. However, efficiency can be greatly reduced by high return fluid temperatures. The Water Vapour Pump (WVP) compensates for this by removing the energy still in the outlet gases and returning it to the system, thus increasing efficiency.
In order to demonstrate a full-scale application of the system, a 4.4 mmBtu/hr unit was installed. The building was selected as a good example of a high-temperature, closed-loop space heating application for testing the technology.
The Direct Contact Heat Exchanger used relies on the direct contact heat and mass transfer between the hot combustion gases and the water to be heated. The combustion gases rise along the vertical column from the natural gas burner as the water descends, picking up heat energy from the gases. Stainless-steel packing in the counter-current heat exchanger increases the contact area and slows the flow, providing improved exchange of energy.
Schematic diagram of the direct contact heat exchanger system.
Most Valuable Player: The WVP
A solution in the form of a Water Vapour Pump (WVP) was developed which effectively maintains a constant efficiency over a range of operating temperatures. The WVP consists of a condenser and an evaporator, both of which are direct contact vertical column designs like the water heater. The condenser removes sensible and latent heat from the hot exhaust gases and sends it to the evaporator, which returns it to the system by preheating and humidifying the combustion air.
It All Boils Down to Savings
In the early 2000s, the installed cost of a 4.4 mmBtu/hr system was estimated at $70,000 USD. When replacing a seasonal efficiency boiler system, which has an estimated installed cost of $40,000 USD, the simple payback period is about four years. This compares favourably with the alternative of installing a high-efficiency hot water boiler with a cost of $85,000 USD and a simple payback period of seven years.