High-Temperature Heat Pump for Tunnel Oven
This project develops a new type of tunnel oven and a high-temperature heat pump for an unprecedented high temperature level above 200 °C. The heat pump will use water as refrigerant, the ultimate environmental-friendly solution. The implementation of this high-temperature heat pump in the tunnel oven reduces the supply of energy by two thirds, and the technology will contribute to an energy-efficient decarbonation process.
he energy consumption for tunnel ovens and drying units is more than 20% of the total energy consumption of the industry. Typically, such plants heat air to 150 – 250 °C, usually in terms of gas burning.
In this project, a high-temperature heat pump prototype for temperatures around 200 °C (150–250 °C) is developed and tested in a research tunnel oven. Water is used as refrigerant due to physical properties and because it is the most environmentally-friendly refrigerant.
The heat pump will be based on a compressor developed by iCooler. The compressor is a modified version of a dry running spindle vacuum pump from SIHI. The modifications cover a redesign of rotating profiles, motors, and control electronics. In addition, water injection is established in the compression process to seal the rotors and reduce the discharge temperature.
A license production agreement is expected to be signed by iCooler and CS Techcom.
The air circulated in the tunnel oven is heated by an energy unit on top of the tunnel – currently, by means of a gas-fired heat exchanger. When this unit is replaced by the heat pump, the evaporator and condenser heat exchangers are placed in the air channels leading to and from the product zone. Thus, the gas is replaced by electricity in an energy-efficient way.
Most tunnel oven applications involve a drying of the product. The heat pump uses the condensation at the evaporator as heat source. Rough calculations show that the COP will be approximately 3 (heating from 80 to 200 °C)
The heat pump will be installed at the research tunnel oven at Flexmatic, and test runs are performed to monitor functionality and performance of the compressor and the heat pump.
A compressor of the spindle type was designed for a capacity of 130 kW and a temperature lift from 105°C to 230°C with water vapor (steam) as working fluid. Moreover, the compressor was designed with a built-in pressure ratio of approximately 1:20 and water injection for sealing and reducing the discharge temperature.
The geometry of the designed spindles turned out to be too complicated for them to be immediately manufactured by turning on a lathe. It was necessary to use 5-axis machining for the first prototypes.
It also proved quite challenging to assemble the compressor with spindles and insert into an existing
housing, especially in terms of finding ways to measure clearance to ensure correct fitment.
Due to time limitations, the manufactured prototype compressor was installed on a recently established
test rig at Danish Technological Institute, where a mechanical functionality test and a performance measurement were carried out.
Mechanically, it is a very well-functioning compressor with a low vibration and noise level, and it has a mechanical loss of approx. 6 kW at full speed (8000 rpm) in the current configuration.
In terms of performance, the compressor did not perform satisfactorily without the water injection in
relation to the expectations. However, with water injection, the compressor produced a volume flow and pressure rise of several bars, limited by the test rig. The maximum pressure ratio was still lower than the design value, and is probably dependent on the selected tolerance for the distance between the spindle and the compressor housing - a relationship that will be likely to be analyzed and modified in the further development process.
The results of the measurements were very encouraging in that the compressor worked excellently mechanically. The results indicate that initially a rather large clearance was chosen between spindle and housing to be sure that the spindles did not settle or lock on the prototype. If so, a functional compressor is within reach just by varying the clearance. Otherwise, it will be a revision of the spindle design and the water injection design.
Key figures
Category
Participants
| Partner | Subsidy | Auto financing |
|---|---|---|
| Flexmatic ApS | ||
| Hamburg Vacuum | ||
| CS Techcom ApS | ||
| SANOVO Technology Process A/S |
