Intermediate oven with CO2 heat pump: cost-efficient and environment-friendly
Bietigheim-Bissingen, December 2016 – Dürr offers an environment-friendly process for drying automotive bodies, in which the process air in the paint shop’s intermediate oven is dehumidified and heated in a highly efficient manner. The process involves the use of a high-temperature heat pump from Dürr thermea’s proven thermeco2 HHR series. This works with carbon dioxide – R744 – as a climate-friendly refrigerant and ensures significant savings in operating costs.
Once the base coat has been applied to the body, the water contained in this coat has to be removed. This evaporative process in the intermediate oven requires dry air, which is usually generated via condensation dehumidification or adsorption.
The operating costs are a crucial aspect of both technologies. There is a high energy demand for heating the very cold dry air after condensation dehumidification and for regenerating the adsorber system. The energy required has to be provided via fossil fuels (natural gas) or via the central heat supply system.
Direct evaporative chilling systems represent the state of the art among condensation dehumidification systems, which are in widespread operation. To date, these have used the climate-damaging refrigerant R134a or R404c and then reheated the very cold dry air using some of the recooling heat generated in the operation of chillers.
However, there is an upper limit to the temperature level that can be achieved with recooling, i.e. the condensation temperature of the refrigerant. Only some of the waste heat generated by the chillers is usable for heat recovery and is actually used to reheat the dry air. Most of the waste heat has to be discharged to the atmosphere at roof level – a major energy loss.
Despite heat recovery, significant amounts of fuel have to be used to achieve the target temperature for the dry air. Natural gas-fired line burners are generally used.
Not so in the case of the proven CO2 high-temperature heat pump technology which Dürr thermea is now offering for the intermediate oven: The thermeco2 HHR system from the Dürr subsidiary allows the line burner to be largely or even completely dispensed with. That cuts operating costs and ensures a significantly better carbon footprint. One car manufacturer is already using this innovative concept in Mexico and another plant is being built in China
The natural refrigerant CO2 offers a future-proof, environment-friendly solution. It does not contribute to ozone depletion, and its greenhouse gas potential is many times lower than conventional refrigerants. On October 15, 2016 200 countries agreed to phase out climate-damaging refrigerants with a major greenhouse effect. The European Union introduced a regulation back in 2015 specifying the phase-out of climate-damaging fluorinated gases.
The high-temperature heat pump in the new intermediate oven concept works as follows: The dehumidifier unit contains the refrigerant evaporators of the thermeco2 heat pump system. Using direct evaporation, they cool the moist, warm air from the intermediate oven to the dewpoint temperature required by the system.
The water contained in the air condenses and separates out. Consequently, the air is dehumidified. Of necessity, cooling of the air for the purposes of dehumidification takes place at a temperature level which requires subsequent reheating.
This is where the thermeco2 really shows its strength. Thanks to the advantageous thermodynamic properties of the CO2 refrigerant, it is possible to reheat the dry air stream to the required level in one process step. The hot gas exiting the high-temperature heat pump transfers its energy directly to the dry air stream in the refrigerant-driven air heater, which ensures an extremely high efficiency.
The outlet temperatures for the dry air can be set between 50°C and 110°C – which no other system can match. Depending on the application, fossil fuels are only required at most for fine adjustment of the dry air temperature.
Dürr thermea has many years of experience in the field of CO2 high-temperature heat pump technology. With its product brand thermeco2, the Dürr subsidiary is the technology leader for large-scale heat pumps using CO2 as their refrigerant.