CHEs in heat pumps

In its Annex 33, "Compact Heat Exchangers in Heat Pumping Equipment" issued in September 2010, the Heat Pump Programme details the benefits of using compact heat exchangers (CHEs): . Improved heat exchanger thermal effectiveness: thermal effectiveness values in excess of 0.95 are economically possible with CHEs - up to 0.98 for printed circuit heat exchanger -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 - i.e. an approach temperature of 22.5°C - A compact heat exchanger with an effectiveness of 0.95, for the same application, would give a cold stream outlet temperature of 95.5°C - i.e. 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. . Smaller size: compared to most shell-and-tube heat exchangers, CHEs have smaller physical characteristics for a given heat transfer duty. When their total installed cost is considered, CHEs tend to be significantly cheaper than their conventional counterparts. This benefit becomes more apparent when the heat exchanger is 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 components 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 such as 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. Particular aspects that it is considered worth highlighting in the Annex 33 conclusions are: - 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 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 materialize. - The vast portfolio of research on heat transfer and fluid dynamics in narrow channels in CHEs. - The role heat pumps could play in industry, where reduced payback times could be aided by CHEs. The UK study highlights the market possibilities. - 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 could reveal new opportunities.!.aspx