Redesigning heat exchangers to achieve reduced charges in heat pumps

By Sabine Lobnig, Sep 15, 2010, 13:16 4 minute reading

As the trend towards reduced refrigerant charges continues, a project within the IEA Heat Pump Programme has put together a report on possible options for compact heat exchangers, used as evaporators, condensers and in other roles in heat pumps. The report encompasses guidelines for selecting and using compact heat exchangers and a number of novel heat exchanger concepts.

The work of Annex 33 “Compact Heat Exchangers in Heat Pumping Equipment” within the International Energy Agency Heat Pump Programme has been completed. The principal goal of the Annex was to identify compact heat exchangers (CHEs), either existing or under development, that may be applied in heat pumping equipment. The aim has been to reduce the possible direct and indirect effects of the systems on the global and local environments by decreasing the working fluid inventory, minimising the environmental impact of system manufacture and disposal, and/or increasing the system performance during the equipment life.

Moreover, to promote and simplify the commercial use of CHEs by heat pump manufacturers, a second goal was to identify, propose and document methods of predicting heat transfer, pressure drop and void fractions in CHEs, while a third goal was to present listings of operating limits etc. for the different types of CHEs.

Main benefits of using compact heat exchangers in heat pumps

The main benefits of using CHEs in heat pumping equipment are:
  • Improved heat exchanger thermal effectiveness: Thermal effectiveness (E) values in excess of 0.95 are economically possible with compact heat exchangers - up to 0.98 for printed circuit heat exchanger (PCHEs). This compares to typical values of 0.75 for shell and tube heat exchangers.
  • Closer approach temperatures: Approach temperature is an alternative measure of heat exchanger performance. A shell and tube heat exchanger with an effectiveness of 0.75, heating a single phase fluid from 10°C with a hot source stream at l00°C, will give a cold stream outlet temperature of 77.5°C - that is, an approach temperature of 22.5°C. A compact heat exchanger with an effectiveness of 0.95, (achievable with current designs of plate heat exchangers), used for the same application, would give a cold stream outlet temperature of 95.5°C - that is, an approach temperature of only 4.5°C.
  • High heat transfer coefficients and transfer areas per exchanger volume: Due to the small hydraulic diameter of the flow passages CHEs have high heat transfer coefficients. In many instances the high heat transfer coefficient is frequently achieved without excessive pressure drop. Moreover, CHEs have high heat transfer surface areas for a given volume of heat exchanger. Typically, compact exchanger area/volume ratios may be up to an order of magnitude greater than those of shell and tube exchangers depending on the exchanger type, with the new generation of ‘micro’ heat exchangers being able to approach a reduction of two orders of magnitude. Although an arrangement of shell and tube heat exchangers in series could provide high heat transfer areas, the greater area/volume ratio and high heat transfer coefficients of compact heat exchangers give a high effectiveness with a much smaller overall volume.
  • Smaller size: Compared to most shell and tube heat exchangers, CHEs encompass smaller physical characteristics for a given heat transfer duty. This entails benefits in terms of reduced support structure and more convenient location. When their total installed cost is considered, CHEs tend to be significantly cheaper than their conventional counterparts. This benefit become more apparent when the heat exchanger has to be made from an expensive material such as nickel or titanium
  • Energy savings: Thanks to the ability of CHEs to operate with smaller driving temperature differences between streams, it is possible to reduce the power requirements of plant items such as refrigeration compressors that were previously sized for the greater temperature differences required with shell and tube heat exchangers.
  • Reduced fluid inventory: CHEs operate with much lower fluid inventories compared to many conventional heat exchangers. This enables safer operating conditions when handling fluids like ammonia, for example and reduced refrigerant costs as the price of working fluids rises.
  • Process intensification: Process intensification is generally associated with active (and in the case of many CHEs, passive) heat and mass transfer enhancement that allows for one or two orders of magnitude reduction in the size of equipment.

Report conclusions

Some of the highlights from the Annex conclusions include:

  • The significant role heat pumps could play in industry, where reduced payback times could be aided by CHEs. The report section from the UK highlights the market possibilities. The vast portfolio of research on heat transfer and fluid dynamics in narrow channels in CHEs. The research highlighted in Sweden, Japan and the USA is of particular note.
  • There is a need to educate the heat pump industry in the use of CHEs, their merits and limitations, and the types that are available. The use of new materials, as indicated in some of the research in the USA, could reveal new opportunities.
  • The growing market for domestic heat pumps, where efficiency, arising in part out of the increased use of CHEs, is critical to further sustained market growth, particularly in countries where heat pump use has been slow to materialise.
  • The increasing interest in and use of CO2 as a working fluid. This has interesting implications in terms of the equipment used and the concepts for heat pumping that might be applied –The inputs from Austria and Japan highlight this aspect. 

MORE INFORMATION

By Sabine Lobnig

Sep 15, 2010, 13:16




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