From a theoretical point of view, it is well known that co-current evaporation is the preferred operation mode. The higher temperature difference at the heat exchanger entry stimulates the evaporation more than it would with a counter-current flow resulting in advantages such as:

• Improved refrigerant distribution
• Improved freeze resistance
• Possibility to optimize both heating and cooling performance in the reversible system

The issue that arises with co-current flow is to achieve a proper and stable level of superheat. Since the primary and secondary side temperatures will approach each other quickly at the heat exchanger outlet, there is a risk for pinch point, which means that high performance and reasonable superheat cannot be reached.

Adding another heat exchanger to the suction line (suction gas heat exchanger, SGHX) removes the superheat from the evaporator. The suction gas heat exchanger generates the necessary superheat of the evaporated refrigerant by using the subcooled refrigerant in the liquid line. It enables the evaporator to operate efficiently with less risk of a pinch point as the SGHX generates the superheat. Adding an SGHX can increase footprint, cost and additional pressure drop. Hence it is not always an appreciated solution.

With Hypertwain, SWEP introduces an innovative heat exchanger that combines an SGHX and an evaporator. It gives you all the benefits of an SGHX but none of the disadvantages. Unlike a conventional brazed plate heat exchanger, the plate has a dedicated area purely for the evaporation process and just a small area close to the outlet port which is optimized for superheating the refrigerant. The small superheating area works as an integrated suction gas heat exchanger connected to the warmer refrigerant liquid line and it uses this liquid to superheat the evaporated refrigerant. With the SGHX integrated in the plate, there is no physical distinction between the SGHX and the evaporator part.

The great density difference of liquid and vapor tends to generate a maldistribution with higher concentration of vapor in one part of the suction gas heat exchanger and more wet refrigerant in another part, and consequently the external suction gas heat exchanger becomes either much oversized or undersized, and since distribution is unstable it becomes difficult to control during variable load. Most solutions to this problem tend to increase the pressure drop after the evaporator and negatively affect system performance. An integrated suction gas heat exchanger makes this problem is much smaller since the distribution is mostly solved with same distribution device as for the evaporator. By only using one heat exchanger, the port pressure drop only occurs in one instead of in two heat exchangers.

For a conventional chiller/heat pump that operates in reverse (provide cooling and heating), the efficiency gain from increased evaporation/decreased condensation temperatures is several degrees thanks to the improved temperature approach. Besides efficiency gain in the operating modes, the Hypertwain heat exchanger possesses high thermal performance and low secondary side pressure drop utilizing an asymmetric plate design. As evaluation means, a demo chiller (water to water) with two Hypertwain units was constructed at SWEP early on to demonstrate the concept. The design point of 100kW cooling capacity (evaporator heat flux 12.7kW/m2 at 100kW, and condenser HF 15.7kW/m2) resulted in established SEER values over 6.0 and SCOP values over 6.5 in test according to EN14825 with variable water flow.

Figure 1. Typical performance with R410a and chiller condition Heatflux 10kW/m2, water 7 / 12°C and a liquid condensing temperature of 30°C.

As described with co-current evaporators the superheat will generate a pinch point, meaning that an increase in superheat will give equal decrease in thermal performance (temperature approach). Hypertwain is designed so that the high thermal performance of the evaporator can be reached using superheat levels that are in an area that the market is used to handle. Above chart shows a demonstration of how the combo of evaporator and integrated suction gas heat exchanger is behaving. As with all heat exchangers a higher load gives a higher temperature approach and consequently the curve will be moved down or upwards dependent on the load. The slope might also look different depending on the thermal case and refrigerant but for most of the relevant full load a part load cases a stable superheat can be found around 4-5K.

Over the product lifetime a major cost associated with a heat pump or chiller is the operational cost. Operational cost is directly connected to the system efficiency. A poor system efficiency will increase the electricity consumption and thereby the total cost of ownership.

Compared to a system using conventional BPHEs, Hypertwain can improve the seasonal COP and EER greatly. For a system optimized for cooling* the heating SCOP can be improved 10-15% and the cooling SEER can be improved around 5%. These improvements decreases the electricity consumption and cost of operation. On a yearly basis the electricity savings for a reversible heat pump/chiller can be considerable.

Over the product lifetime a major cost associated with a heat pump or chiller is the operational cost. Operational cost is directly connected to the system efficiency. A poor system efficiency will increase the electricity consumption and thereby the total cost of ownership.