GB2282852A - Single screw expander for the recovery of power from flashing fluids. - Google Patents

Abstract

A single Screw machine is used as an expander of a volatile fluid to develop mechanical power. The fluid enters the machine at high pressure as either a saturated or subcooled liquid or as a two-phase mixture with saturated liquid and vapour in thermal equilibrium. The expander acts as a prime mover and can be used for driving a gas compressor, an electric generator or any machine which would normally utilise a prime mover. The expander may be used with an economiser (16, fig 6) in a refrigeration or heat pump system. <IMAGE>

Description

USE OF THE SINGLE SCREW EXPANDER FOR THE RECOVERY OF POWER FROM FLASHING FLUIDS Background Both Twin and Single Screw mechanisms have been used for gas compression and are well known for this function in the Refrigeration, Air Conditioning and Industrial Gas compression industry. Twin screw gas compressors based on the Lysholm design were introduced into the refrigeration market in 1958.

Development of Single Screw compressors began in 1960 and was introduced into the refrigeration market in 1970. As long ago as 1946, A Lysholm, the original inventor of the twin-screw machine, recognized its potential as a gas expander. However, its potential to recover work from flash expansion of vapours was not realised until much later.

This was first proposed by R Sprankle who was granted US patent 3,751,673 for its use as a direct expander of hot geothermal brines in August 1973. Latterly, UK patent 2,114,671 was granted to I. Smith for its use as an expander of hot organic fluids in a closed cycle described as a Trilateral Flash Cycle system.

Additionally, numerous attempts have been made to use twin-screw machines to replace throttle valves in refrigeration systems and utilise the power to either generate electricity or reduce the power input to the compressor by some form of direct coupling.

Patents have also been awarded for specific forms of this application such as UK patent 1,593,521 in July 1981 issued to D N Shaw.

The use of the Single Screw machine as a means of compressing gases is well known in the prior art and is adequately covered by a number of patents. This application is concerned with the use of Single Screw machines as expanders in the following applications.

i) The direct expansion of hot geothermal brines, as described by R Sprankle.

ii) As an organic fluid expander in the Trilateral Flash Cycle system, as described by Smith.

iii) As a throttle valve replacement in Chemical Process Plant and vapour compression refrigeration and heat pump systems.

Configuration of a Single Screw Expander Referring to Fig 1, the expander uses a single screw rotor, 1, which meshes with two rotary seals, 2. The rotor is metallic and has six flutes machined around its periphery. The rotor seals are flat plastic star shaped discs normally having eleven teeth which mesh in turn with the flutes of the screw rotor,3. The stars are spaced symmetrically about the rotor and are supported by metallic backing plates fixed to their respective supporting shafts, 4. The star teeth protrude through the cylindrical housing which contains slots and allows the teeth to mesh with the rotor. It is an important characteristic of the machine that the expansion process takes place in the upper half of the machine at exactly the same time as an identical process occurs in the lower half using the second star.This results in zero radial gas load on the screw rotor bearings. The axial loads are also zero because the flutes terminate on the periphery of the rotor. The only bearing loads in the machine, apart from the weight of the parts, are the small loads on the star shaft bearings due to high pressure gas acting on one side of each tooth in mesh. Angular contact ball bearings, 5, are used at the high pressure end of the expander to accommodate the small radial and axial loads and rolling element bearings, 6, at the low pressure end, to allow expansion and contraction of the rotor and any small radial loads. There are no contacting seal grids to trap the fluid in the expansion chambers. Instead, the machine is constructed with very small clearances between the rotor and casing and star wheels and rotor flutes. These are filled with the liquid component of the two-phase mixture.The only moving parts of the machine are the main rotor and the stars. Each flute is used twice per revolution i.e, once by each star, to give a very compact design. The single screw expander uses a fixed inlet port, 6, Figs 1 and 2, and the inherent expansion ratio can be optimised to match the external conditions by selecting the appropriate inlet port size. A sliding valve, 7, Figs 1 and 2, incorporates the inlet port and this also provides a means for varying the output of the machine by exposing a bypass slot, 8, Fig 2, and vent to the outlet, 9. The slide valve capacity control mechanism, 7, is actuated by a piston in a cylinder, 10, Fig 1, which is controlled by liquid through tubes 11 and 12. Fig 3 shows a diagrammatic view of this arrangement.

The Expansion Process in a Single Screw Expander The expansion process in a single screw expander can be conveniently described in three stages; Fig 4, namely filling, 1, expansion, 2 and discharge, 3.

i) In the filling process high pressure liquid or liquid and its associated vapour, enter the machine at one end through the inlet port, 6, Fig 1, and fill the available flutes, 3, causing rotation of the rotor, 1, until the inlet port is covered. The filling chamber, 1, fig 4, is therefore created by the space contained within the flute walls, the casing and the star teeth.

Fig 5.

In the case of dry gases, this process occurs at approximately constant pressure. There is therefore no associated change in gas density during filling and, neglecting leakage, the mass flow induced is roughly proportional to the rotor speed.

In the case being considered, the process is more complex. The liquid, or two-phase mixture, is much more dense then compressed gases. The loyal acceleration associated with its passage through the inlet port therefore produces a more significant pressure drop. This in turn leads to flashing off of vapour from the mixture and hence changes the fluid density. The greater the rotor speed, the larger the acceleration and thus the more the density changes. Accordingly, the mass flow rate does not increase proportionally to the speed and account must be taken of the pressure and volume changes associated with filling in the design of the machine to achieve a specified overall expansion ratio. Such changes cannot be inferred from dry gas expansion analyses.

ii) In the expansion process, further rotation of the rotor leads to an increase in volume of the chamber containing the trapped fluid, 2, Fig 4. During this process the star teeth act like stationary pistons in the moving flutes or cylinders. And the gas expands until the outlet chamber is exposed.

In the case of dry gases, the energy change associated with expansion is relatively large. The gain in kinetic energy of the gas due to its rotation in the chamber is then only a small percentage of the total. It may therefore be conveniently neglected and the analogy between expansion in the rotor chamber and in the expansion process in a reciprocating engine cylinder is close.

In the case of two-phase expansion, the decrease in energy per unit mass of fluid is much less. This is because the liquid component of the mixture, which can be of the order of 90% of the total mass, makes a negligible contribution to the work of expansion. The increase in kinetic energy of the fluid due to its rotation is therefore a much larger percentage of the total energy change. The pressure change associated with the expansion must include allowance for this or estimates of mass flow through the machine may be twice the value of those actually attainable.

iii) The discharge process, 3, Fig 4, begins when the flute becomes exposed to the outlet chamber, 13, due to further rotation of the rotor. The liquid-vapour mixture then passes into the outlet chamber. Ideally the pressure within the flute should be substantially equal to that in the outlet chamber at the instant discharge begins. If this is not achieved then substantial losses in efficiency can occur. If the pressure within the flutes is too great then further wasteful flash expansion will take place during discharge. If the pressure is too small, fluid will flow back into the flutes at the initial stage of the port exposure. Further rotation of the rotor then expels the residual fluid displaced by the flutes. It is desirable that the exit port of the two-phase expander is directed vertically downwards to prevent the accumulation of liquid within the body of the machine.

An important feature of the two-phase expansion process is that the overall expansion ratio must be determined by external conditions such as the evaporation and condensation processes in vapour compression systems. It is therefore an essential feature of this inventio: that the expansion process begins by passing the fluid through a variable control valve, 7, Fig 2, which may be opened or closed in response to control signals in order to maintain the volume ratio required for total expansion.

A further feature in the case of the single screw expander installation in vapour compression systems is the need to balance the capacity of the expander with that of the compressor during part load or transient operation. Under these conditions, the slide valve within the screw expander supplies the necessary modulation in conjunction with the variable liquid control valve immediately preceding the expander, 23, Fig 6.

The Single Screw Expander offers a number of advantages over the Twin Screw machine in two-phase applications. The most significant of these is that timing gears and additional seals associated with them, are not required. It may therefore be manufactured at a much lower cost. In the case of twin screw machines using dry gases it is possible to eliminate the timing gear by injecting relatively large masses of oil with the gas.

This is not possible with two-phase expansion, especially when the working fluid is a refrigerant. This is because the oil affects the vapour-liquid equilibrium point and leads to greatly reduced flash off of vapour from the liquid as expansion proceeds and the expulsion of supersaturated liquid together with the saturated vapour.

The Use of the Expander with an Economizer It is necessary to consider the implications of using a two-phase expander in either an existing refrigeration system, as a retrofit, or in a new system. Many large refrigeration and heat pump systems use an economizer and it is important when fitting an expander to them that its advantages are maintained.

Referring to Fig 6. As all the liquid from the condenser, 18, and the receiver, 21, passes through the two-phase expander, 14, an economizer, 16, can be used. The liquid and gas from the outlet of the expander pass into the economizer vessel. The pressure in the economizer vessel is held slightly higher than the pressure in the evaporator, 17, with liquid passing through a control valve, 22, into the evaporator and flash gas from the expander, via the economizer, is taken into the main compressor, 15, after the main compression process has started. This process provides the facility of a substantial increase in the cooling effect per mass of refrigerant passing through the evaporator while simultaneously reducing the required size and work input of the compressor. Where an economizer is not included in the system, liquid and gas passing from the expander outlet will be taken directly into the evaporator.

Claims (8)

1. Claims

   i) The use of a Single Screw Expander for the direct expansion of hot geothermal brines.

   ii) The use of a Single Screw Expander as the prime mover in the Trilateral Flash Cycle system.

   iii) The use of a Single Screw Expander as a throttle valve replacement in chemical process plant, air conditioning, refrigeration and heat pump systems.

   iv) The use of a Single Screw Expander as a throttle valve replacement whereby it drives a compressor in a sealed unit to augment the performance of existing refrigeration, air conditioning or heat pump systems.

   v) The use of a Single Screw Expander with an economizer in Refrigeration, air conditioning or heat pump systems.

   Amendments to the claims have been filed as follows 1) The use of a Single Screw Expander for the direct expansion of hot geothermal brines.
2. 2) The use/of a Singe Screw Expander as the prime mover in the Trilateral Flash Cycle System.
3. 3) The use of a Single Screw Expander as a throttle valve replacement in chemical plant process plant, air conditioning, refrigeration and heat pump systems.
4. 4) The use of a Single Screw Expander as a throttle valve replacement whereby it drives a compressor in a sealed unit to augment the performance of existing refrigeration, air conditioning, or heat pump systems.
5. 5) The use of a Single Screw Expander with an economiser in refrigeration, air conditioning or heat pump systems.
6. 6) The use of a Single Screw Expander in an oil free mode.
7. 7) The use of a Single Screw Expander with no oil control seals between rotors and bearings.
8. 8) The operation of a Single Screw Expander with large quantities of liquid refrigerant, compatible with the TFC system, passing through the machine.