Semi-welded heat exchangers can increase the COP of industrial ammonia/NH3 (R717) heat pumps by raising the evaporation temperature of the refrigerant on the cold side and lowering the condensing temperature on the hot side.
This was the message from Tommy Ångbäck, Sales Director for Refrigeration for Swedish manufacturer Alfa Laval, in a webinar at the online ATMO World Summit on March 30. The World Summit was organized by ATMOsphere (formerly shecco).
“In general it is possible to obtain a 2K closer approach with semi-welded plate heat exchangers compared to other technologies with comparable size and cost,” Ångbäck explained, stressing that for each degree (K) higher evaporation temperature on the cold side, the ammonia heat pump can save 3–6% in power consumption.
On the hot side, heat pumps have traditionally used three heat exchangers in a series, as desuperheater, condenser and subcooler. This type of design can obtain good efficiency, Ångbäck said. However, Alfa Laval has developed the T10-EW condenser, which eliminates the need for multiple heat exchangers by integrating all three functions in the same unit, securing high COP, in a function Alfa Laval calls “subcoolcondense.”
The design lowers the condensing temperature on the hot side, thus increasing the efficiency. For every degree Kelvin of lower condensing temperature, the ammonia heat pump can save 1–3% in power consumption. The T10-EW is now Alfa Laval’s “top seller” for hot-side heat exchangers, Ångbäck revealed.
New design, lower charge
The new 3-in-1 condenser design also avoids a lot of piping and reduces the ammonia charge needed. Compared to a shell-and-tube heat exchanger, the ammonia charge is smaller by roughly a factor of 10, Ångbäck explained. Depending on the overall system design, the total charge can be reduced to less than 100g of ammonia per kW of heating (0.063 lbs/TR), he added.
The new condenser has what the company calls CurveFlow distribution area, which utilizes all available surface area and provides “perfect” distribution inside the heat exchanger channels for optimized heat transfer and cleaner surfaces. The T10-EW also has non-circular port holes that ensures the pressure drop is better utilized for heat transfer. These have been dubbed the OmegaPorts.
Case studies
Alfa Laval has supplied semi-welded heat exchangers to several district heating projects in Denmark, and Ångbäck gave a few examples in his presentation.
A district heating provider installed a 5MW (1,422TR) ammonia heat pump in April 2019. The system supplies heating for 1,100 homes using a mixture of sea- and wastewater as the heat source and green electricity from off-shore wind turbines, making the system completely fossil-fuel free. Since the inauguration, the system has delivered 80°C (176°F) hot water and has had a COP of 3.2.
Another example is a 4MW (1,137TR) ammonia heat pump for a district heating network in Broager, Denmark. It was installed in 2016 and uses 9°C (48.2°F) warm groundwater from a depth of 250m (820ft), heating it up to 70°C (158°F). The system is running with a COP of 4.1.
“I would say that today, most heat pumps try to achieve a COP of 4 or more, and with the help of our heat exchangers, this can be raised even further in the future, I hope,” Ångbäck said.
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