*4.3. Screw Expanders*

The screw expander is a displacement type device. It consists of an interlocking pair of spiral rotors placed in a housing, which together form the working chamber. In addition to the pair of screw rotor and cylinder, the structure consists of bearings, synchronous gears, a seal assembly, etc.

The expander has the opposite operating principle to the screw compressor, which has a similar basic structure. The production process and control system are much more complex for an expander than for a screw compressor.

A cross-sectional view of the screw expander is shown in Figure 34.

**Figure 34.** View of the screw expander made by Green Secure Power Systems [164].

Screw expanders can work with superheated steam and wet steam. Currently available units are powered by a working medium with a pressure of 3 to 25 bar. The power range of screw expanders ranges from a few kilowatts to 630 kW.

The world leader in the production of systems with a screw expander is the British company Heliex Power. One of the Heliex devices was installed in Poland in K ˛edzierzyn-Ko´zle at Grupa Azoty in 2016. The installation was performed together with the Polish partner of Heliex-the Zamkon company. The device installed in Poland is 160 kW. Figure 35 shows a container system with a screw expander installed at Zakłady Azotowe K ˛edzierzyn.

**Figure 35.** View of the screw expander made by Heliex Power installed at Zakłady Azotowe K ˛edzierzyn [164].

The layout of the example system based on screw is presented in Figure 36.

**Figure 36.** View of the system with screw expander.

The American company ELECTRA THERM [167] developed ORC system using screw expander that can be fed by different heat sources (i.e., geothermal water, biomass combustion products, waste steam or hot water from industrial processes. The minimum temperature of these sources should be 90 ◦C. The system can be also supplied with waste gases featuring different temperature (205–540 ◦C). The power output of this system ranges between 50 and 500 kW, depending on the thermal power of the heat source.

Screw expander is also applied in the ORC system developed by the German company KÖHLER-ZIEGLER [167]. This system can utilize waste heat carriers featuring a minimum temperature of 90 ◦C. The power output of this system ranges from 50 to 200 kW, depending on the heat source thermal parameters.The ORC systems with screw expanders are also manufactured by the Swedish company OPCON [167]. These ORC systems are featuring a modular design and a power output of 350, 500 and 800 kW. These systems can utilize waste heat sources featuring a temperature of 55–120 ◦C. Substances featuring a temperature of 0–30 ◦C can be used as the condenser coolant. The efficiency of these systems is ranging between 5 and 10%.

The other types of screw expanders that are applied in ORCs are single-screw expanders. The results of research on these expanders are presented in [168–177]. Single-screw expanders consist of three movable rotors, one of which is the main rotor, located centrally. The main rotor has helical teeth cut around the circumference. The other two rotors are having outer teeth of a rectangular cross-section. The teeth of the cooperating rotors mesh with the teeth of the main helical rotor. In this way, working chambers are formed on both sides of the central rotor.

Single-screw expanders can operate with different working fluids, including natural gas, superheated and saturated steam and liquid-gas mixtures. The power output of these expanders is in the range of 1–100 kW. Single-screw expanders feature a simple design, good performance at part load, high volumetric efficiency and low leakage rate. They are operating quietly and do not generate vibrations. Experimental research on application of single-screw expanders in ORC systems were proceeded in different research centers. In [173] studies on the ORC system utilizing a single-screw expander and using Solkatherm SES36 as a working fluid were described. The working fluid temperature at the inlet to the expander was varied in the range of 119.3–125 ◦C, the inlet pressure was varied in the range 4.5–10.2 bar, the expansion ratio in the range 3.12–10.97 and the expander rotational speed in the range 2000–3000 rpm. For these experimental conditions, the internal efficiency of the expander was ranging between 51% and 64.78% and expander rotational speed was ranging between 2000 rpm and 3000 rpm. The maximum power output of the expander of 7.8 kW and the efficiency of the ORC system of 9.8% was achieved during experiments.

Preliminary tests on single-screw expander prototype, which was designed for application in ORC systems, were also carried out with the use of air [171,176]. The total expander efficiency of 55%, the maximum torque of 100 Nm and the power output of 22 kW were achieved experimentally [176]. In [171] the results of research on a singlescrew expander, which was supplied with compressed air (pre-heated to temperature of 80 ◦C) were reported. The experimental results showed that depending on the expander rotational speed (which was varied between 500 and 3500 rpm) the expander power output varied in the range of 1–5 kW. Expansion ratio was kept in the range 5–6.75, gas temperature drop was observed in the range 32.5–62.5 ◦C and the expander efficiency was varying in the range 10–60%. Single-screw expanders were also tested using R123. The results of these experiments are reported in [175]. The expander power output of 8.35 kW, internal efficiency of 56% and the maximum efficiency of the ORC system of 7.98 % were achieved. In [168] the results of experiments on a single-screw expander using R245fa as a working fluid were presented. In [173] the results of experimental studies on a single-screw expander applied in an ORC system utilizing SES36 as a working fluid were presented. The experimental tests were proceeded for varied thermodynamic parameters of the gas at the inlet to the expander. The temperature of gas at the inlet to the expander was varied in the range of 100–125 ◦C, the inlet pressure was varied in the range of 3–7.5 bar and the outlet pressure was kept in the range of 0.6–0.8 bar. The expander rotational speed was 3000 rpm. For these experimental conditions, the achieved expander output power was ranging between 1.5 and 6 kW and the achieved internal efficiency was ranging between 51 and 66%. In [170] the results of experimental studies which were proceeded on six prototypes of single-screw expanders featuring a power output between 5 and 172 kW were reported. The applied working fluid was R123. For varied experimental conditions the expanders efficiency varied between 52% and 73% and the efficiency of the ORC system reached 9.3%.
