How a rotary regenerative heat exchanger works?
Rotary regenerative heat exchangers are used for heat recovery, and are designed to transfer heat (non-hygroscopic version), or to transfer humidity (hygroscopic version) while simultaneously maintaining the ability to transfer heat from the outlet air to the inlet air. The heat or humidity transfer takes place in the rotor, one half of which reaches into the hot outlet air flow and the other half into the cold inlet air flow. As the rotor turns, the heat-exchange surface of the heat exchanger passes in turn through the outlet and inlet air flow, and thus heat or heat and humidity transfer is enabled.
Rotary regenerative heat exchangers are, due to their design and mode of operation, among the most efficient pieces of equipment. The non-hygroscopic version achieves up to 80% heat transfer efficiency while the hygroscopic version achieves up to 80% heat transfer efficiency and up to 70% humidity transfer efficiency. Rotary regenerative heat exchangers are mainly used as components of air-handling unit assemblies for inlet and outlet ventilation air. These heat exchangers can be delivered in versions for both vertical and horizontal parallel arrangement. Rotary regenerative heat exchangers can also be used as individual components integrated into air-handling ducts or into the brick walls of air-handling plant rooms. It is advisable to place filters in front of the rotary regenerative heat exchangers to avoid fouling.
Rotary regenerative heat exchangers can be used for air flow rates from 300 m3/hour to 80,000 m3/hour. Optimal nominal air flow rates are determined depending on the rotor diameter. Recommended air flow velocity for standard operational conditions ranges from 2 m/s to 4 m/s. If higher air flow velocity is required, it is necessary to consult the manufacturer; the manufacturer will then propose the necessary changes in the heat exchanger's rotor design. Standard temperature of the transported air ranges from -20°C to +55°C.
Reduced heating costs are the most important advantage of rotary regenerative heat exchangers, which results in reduced room requirements and reduced initial costs for the boiler room. Furthermore, the sizes and initial costs for air heaters, air-handling ducting and pumps are also reduced. Lower thermal energy consumption results in reduced operating costs. Especially with humidity transferring heat exchangers, the reduced energy consumption for air humidifying is a significant advantage – the system can work with air humidity. The requirements for cooling energy are also reduced, which results in smaller sizes and reduced initial costs for the cooling system (compressor …), chillers, pumps and piping as well as lower energy consumption. The reduced impact on the environment is another big advantage of rotary regenerative heat exchangers.
The designation of rotary regenerative heat exchangers is based on the rotors' diameter range. The heat exchanger's size is determined by the air discharge in relation to the rotor's pressure loss and efficiency. The main dimensions of regenerative heat exchangers can be found in the catalogue sheet. Other modifications of rotary regenerative heat exchangers, including technical specifications, precise calculation and dimensional design, can be agreed with the manufacturer in accordance with specific demands.
The hygroscopic and non-hygroscopic versions of rotary regenerative exchangers can be used in a normal indoor environment (standard versions), outdoor environment or normal environment with Zone1 and Zone 2 motors. For aggressive environments, it is necessary to select a heat exchanger finished with epoxy coating.
The heat exchanger's rotor can be made of heat conductive aluminium foil, specially finished aluminium foil treated to transfer both heat and humidity, or aluminium foil finished with epoxy coating for heat transfer in aggressive environments. The rotor shaft is supported by ball bearings, respectively taper roller bearings. Rotors up to a size of 2,920mm are delivered as one piece; larger rotors are always delivered divided. The heat exchanger's frame can be made of galvanized sheet steel, sectional aluminium or galvanized hollow sectional steel. Galvanized sectional steel or sectional aluminium is connected by corner-irons. The sections are pressed together and secured with rivets or screws. Galvanized hollow steel sections are welded together. The casing consists of sandwich heat-insulating panels. The panel casing is mostly made of galvanized steel sheets which can optionally be colour coated. The frames and panels are sealed with sealing tape. The rotary regenerative heat exchanger can be equipped with a flushing chamber which prevents outlet air being mixed with inlet air. The flushing chamber is a wedge-shaped chamber fitted in the parting plane of the heat exchanger. The flushing chamber carries out its function only if the directions of the inlet and outlet air flow oppose each other. The rotor is sealed to the flushing chamber by special sealing. As standard, the rotary regenerative exchanger is equipped with a driving mechanism which consists of a motor with worm-gearbox, pulley and belt. The supply voltage of the delivered motor can be 3 x 400V or 1 x 230V. The motor power can be from 90W to 750W, depending on the rotor size. The transmission between the motor and the exchanger's rotor is ensured by a rubber belt which can be connected either by welding, or optionally a mechanical coupling.
The rotary regenerative heat exchanger can be operated at constant speed (without control), or it can be equipped with a frequency inverter to enable external control or its own speed control (autonomous control). The rotary regenerative heat exchanger with a frequency inverter for external control is operated at variable speed; the speed is controlled by the frequency inverter using an analogue signal of 0-10V or 4-20mA. The recommended speed is 11-13 RPM. The speed of rotary regenerative heat exchangers equipped with their own speed control is automatically set by the controller and passed to the frequency inverter. The controller and frequency inverter are delivered in a compact unit.
During operation, it is necessary to check the heat-exchange surfaces for fouling. For cleaning, we recommend using pressurised water, best using WAP cleaning equipment. Be careful when cleaning the exchanger because of the structural material used (aluminium). Further, it is necessary to check the temperature and the flow direction of the transported air, both filters in front of and behind the heat exchanger for fouling and the oil level in the gearbox.
- The highest heat transfer efficiency (enthalpy) – up to 80%.
- Ability to transfer humidity (reduced need for additional air humidification, especially during the winter season).
- Small size, approx. 450mm (compared to plate heat exchangers, which are significantly larger).
- Lower risk of ice build-up (unlike plate heat exchangers, they do not freeze in common winter conditions characteristic of Central Europe – temperatures approx. -12°C and relative humidity 40-60%).
- There is no need for a bypass damper.
- The quickest return on investment.
- Rotary heat exchanger leakage
- Return of small amount of outlet air with new inlet air.
- Need for rotor drive.
- Return of limited amount of humidity (10-15%).