Experimental Compression – Resorption Heat Pump For Industrial Applications

Wet compression resorption heat pumps are a promising type of heat pumps that operates with nonazeotropic refrigerant mixtures. The main differences, when compared to the classical Rankine cycle, are the nonisothermal phase transition of the mixture in the heat exchangers and the compression of the two-phase mixture in the compressor, which works as a gas compressor and at the same time as a liquid pump. Wet compression results in reduction of the consumed power, excludes the vapour superheating and is especially attractive for high temperature applications. For such applications estimated gain in COP is up to 20% when compared to conventional dry compression heat pumps. A 50 kW experimental ammonia water heat pump of this type has recently been taken in operation. It has been designed to upgrade a water flow from 110 to 130°C, using a water waste flow with 80°C. The resorber and desorber are of the falling film vertical shell-and-tube type, with the ammonia water film inside the tubes and a distribution system in the top header. The intermediate heat exchanger is of the shell-and-plate type. Oil-free wet compression of ammonia water was obtained with an especially designed twin-screw compressor. Details and performance results of this compressor are discussed in a parallel paper. This paper discusses the experimental set-up design and the experimental data of resorber, desorber and intermediate heat exchanger. INTRODUCTION Compression resorption heat pumps have the potential to give significant contributions to the improvement of the energy performance of heating processes. Specifically for industrial heating processes they allow for energy performance gains of more than 20% when compared with vapor compression heat pumps. Ammonia-water high temperature heat pumps that are used to upgrade industrial waste heat show a number of advantages: ♦ High temperature operation is possible at relatively low operating pressures ♦ The cycle can be designed to show a temperature glide in the resorber that corresponds to the temperature glide of the industrial waste flow that has to be heated. ♦ For specific operating conditions the cycle performance is significantly beter than for the vapor compression cycle. There are two types of compression resorption heat pumps: with solution circuit (liquid pump and saturated vapor compressor are used to overcome the pressure differential) and with two-phase (wet) compression. Here the compressor simultaneously compresses the vapor and increases the liquid pressure. Itard [1998] has shown that, specially for high temperatures, wet compression leads to higher energy efficiencies than the solution circulation alternative. Next to this energy advantage, wet compression also allows for higher operating temperatures. Dry compression in the solution circulation variant leads to large superheating temperatures of the vapor and associated problems and exergy losses. A large number of theoretical and experimental studies have been dedicated to the solution circulation variant of the compression resorption heat pump cycle (reviewed by Groll [1997]). On the opposite only a few studies / experimental set-ups have been dedicated to wet compression cycles (Malewski [1988], Bergmann [1990], Torstensson [1991], Sixt [1995] and Itard [1998]). The main problem of the cycle is the compressor that has to be suitable for oil-free wet compression and still show acceptable isentropic efficiencies. Torstensson and Sixt used oil-lubricated compressors without oil recovery. Torstensson used a scroll compressor while Sixt used a wankel compressor. In both cases the lubricant circulated together with the solution through the whole system reducing the efficiency of resorber and desorber. Malewski used a monoscrew compressor with closed grease bearing lubrication and separated liquid injection at intermediate pressure. Itard used a liquid ring compressor. Bergmann used an oil-free twin-screw compressor with timing gears with separated liquid injection at suction and intermediate pressure conditions. In the present study also an ammonia-water twin-screw compressor with liquid injection has been used but no timing gears are used. The gaps between rotors and between rotors and housing are smaller and the male rotor drives the female rotor. The bearings are oil lubricated and separated from the process side by radial lip seals. EXPERIMENTAL SET-UP Making use of Itard’s [1998] experience, her experimental set-up has been rebuilt to allow for high temperature operating conditions to upgrade industrial waste heat. The design operating conditions have been selected to allow for application as heat pump for heating of waste water flows from 110 to 130°C in food processing plants. The design conditions are listed in Table 1. Table 1: Design conditions for experimental set-up. inj4 inj3 w2 2 V2 suc 2e w1 1