Heat Exchangers for the Green Heat Pump

Jun. 15, 2015 | Andreas Strehlow, AKG

Introduction

Heat pumps are an efficient alternative to conventional methods for heating. For their ease of installation especially air-water heat pumps have become more and more popular especially for retrofit applications. To further improve the performance of air-water heat pumps leading research institutes and manufacturers have joined for the Green Heat Pump project.

Using the natural refrigerant propane (R290), the consortium aims for a heat pump system with a high operating efficiency to provide a heating capacity of 30 kW. In this system the evaporator and the condenser are designed in aluminium. Using multi-port extruded tubes (also referenced to as MPE-tubes or micro-channel tubes) in both heat exchangers allows to increase efficiency and to minimize the refrigerant charge. Features and advantages of the two heat exchangers are presented in this article.

Evaporator – Air Side

Brazed aluminium heat exchangers using MPE-tubes are the dominating technology for air condition condensers in mobile applications. When transferring this technology to an evaporator in a domestic heat pump several challenges have to be solved:

·       Maximum Performance         
Especially for high heating powers and retrofit applications the footprint required for the evaporator is often not in line with the available space for the unit. So very effective surface structures with high heat transfer coefficients need to be used.

·       Low Noise Emissions           
High heat transfer coefficients typically come along with high pressure losses, which require more fan power to make enough air pass through the evaporator. And more fan power means more noise emission.

·       Ice-Formation
While operating at temperatures near the freezing point humidity from the air will collect on the surfaces and form a layer of ice on the surfaces of the evaporator. This reduces the heat transfer efficiency, blocks off the air flow and consequently requires more fan power (and noise emission).

·       Corrosion
Especially in maritime environments evaporators may be exposed to rather corrosive atmospheres, which can be significantly more aggressive than typical automotive specifications.

Obviously the first three challenges require opposing solutions. So a careful balance has to be found between thermal performance, pressure loss and handling of ice formation. While balancing heat transfer and pressure loss is daily business of a heat exchanger manufacturer, the impact of icing on the air fins required extensive testing.

Together with our partners at AIT various air fin designs were evaluated in physical and numerical icing and de-icing tests. A detailed description of the investigations was given by AIT in the EHPA newsletter XXX. In the end it was found that wavy fin geometries showed to be least sensitive to ice formation while still providing a significant thermal performance. Also during defrost cycles they showed good performance. This result is well in line with AKG’s experience with heat exchangers for dusty operating environments like in construction machinery and the like.

In the end it was decided to proceed with two layers of wavy fins between the MPE-tubes as shown in Figure 1. The low pressure loss of the wavy structure and its good heat transfer coefficient allow to build a rather deep core providing enough heat transfer surface with a minimised footprint. The horizontal position of the evaporator allows easy draining through the vertical air passages especially during defrost cycles. Other than round tubes the flat tubes hardly block the flow and allow low air side pressure loss and improved draining.

 

Fig 1. Detail view of evaporator with wavy fins and micro-channel tubes

State of the art automotive heat exchangers with MPE-tubes can easily handle corrosive atmospheres without any protective coatings. Usage of modern corrosion resistant aluminium alloys for the tubes and fins also allow application in typical heat pump environments.

 

Evaporator – Refrigerant Side

MPE-tubes are a good device to reduce the refrigerant charge in the evaporator. But nevertheless some obstacles have to be overcome:

  • Pressure loss 

The small holes in the tubes do not allow to form long meanders as with conventional round tubes: the pressure loss would increase dramatically. Therefore multiple tubes need to be used in parallel.

  • Operating pressure    

MPE-tubes themselves can handle very high internal pressures. But to better connect them to the rest of the heat pump system transitional sections are required that are not as pressure resistant.

  • Distribution of 2-phase refrigerant flow         

The need to equally distribute the refrigerant to a large number of parallel tubes requires a novel fluid distribution concept.

While AKG has long experience in manufacturing large heat exchangers with hundreds of tubes, the new system to connect many tubes while maintaining an equal flow distribution of the refrigerant was challenging.

Based on a development of Fraunhofer ISE a set of subsequent bifurcations is used to distribute the two-phase refrigerant equally to all MPE- tubes. A prototype of ISE’s bionic distributor used for the evaporator is shown in Fig. 2. The system is designed such that the straight section upstream allows development of a symmetric flow regime for the two-phase refrigerant flow before the bifurcation. The formed plate from Fig. 2 is brazed on a plate to form the channels distributing the flow. This provides a sufficient mechanical stability to handle the pressures on the refrigerant side.

The connection between the bionic distributor and the pairs of MPE-tubes is made from two concentric tubes as shown in Fig. 3. The connectors are brazed on the MPE-tubes while brazing the heat exchanger core. The round tubes can easily withstand the typical system pressures.

Fig 2. Bionic Distributor

Fig 3. Connection between bionic distributor and Micro-channel tubes

Condenser

State-of-the-art condensers for heat pumps are made from stainless steeI. Especially for large heat pumps they can become quite heavy and may contain a lot of refrigerant.

In this project aluminium MPE-tubes are also used on the refrigerant side of the condenser. For ease of production the MPE-tubes were integrated in a bar-plate-style heat exchanger design as shown in Fig. 4.

Fig.4: Condenser:detail view of core

The cold water enters the condenser from the side (w. in) and then flows in the passages opposite to the refrigerant direction. Multiple MPE-tubes form a separate passage with the condensed refrigerant exiting the core at its face (ref. out). Header tanks for water and refrigerant are then fitted on the inlets and outlets of the core to form the condenser. The 30kW-condenser with counter-flow required for this project is shown inFig. 5. Especially the tanks on the refrigerant side are minimised to reduce the refrigerant load to a minimum. Cast parts would allow a further reduction of the refrigerant load.

Fig.5: Aluminium condenser

Another topic to be observed when using aluminium in contact with water is the chemical stability of the aluminium. While water itself is not harmful to aluminium dissolved oxygen or traces of more noble metals can attack the aluminium.

To evaluate the influence of these factors extensive tests were carried out by Gränges. The results showed that the corrosion is not critical when the water is circulating in a closed loop. This keeps oxygen out of the system and also contamination with copper ions etc. can be controlled. Usage of advanced aluminium alloys may further optimise the corrosion performance.

Next steps

After performance testing of the condenser and tests of the evaporator with the fan all components will be integrated in the heat pump system.