WO2012052777A2

WO2012052777A2 - Improvements in or relating to screw expanders

Info

Publication number: WO2012052777A2
Application number: PCT/GB2011/052047
Authority: WO
Inventors: Ian Kenneth Smith; Nikola Rudi Stosic
Original assignee: The City University
Priority date: 2010-10-21
Filing date: 2011-10-21
Publication date: 2012-04-26
Also published as: WO2012052777A3; GB2484718A; GB201017791D0

Abstract

A method of reducing the pressure of a working fluid comprises expanding the working fluid in a screw expander from an initial pressure and bleeding a portion of the working fluid from the screw expander at an intermediate pressure lower than the initial pressure. The remaining working fluid is expanded in the same screw expander and discharged at a discharge pressure lower than the intermediate pressure. A screw expander for performing this method comprises: a casing having an inlet for admitting working fluid to the casing at a relatively high initial pressure; an outlet for discharging working fluid from the casing at a relatively low discharge pressure; and at least one bleed port located downstream of the inlet and upstream of the outlet for extracting working fluid from the casing at an intermediate pressure between the initial pressure and the discharge pressure.

Description

Improvements in or relating to screw expanders

This invention relates to screw expanders, particularly screw expanders that may be used to recover power from the expansion of steam as the working fluid.

Positive-displacement expanders such as screw expanders are increasingly popular for use in power generation. Examples of screw expanders are disclosed in UK Patent Nos.

1 197432, 1503488 and 2092676, all to Svenska Rotor Maskiner (SRM), and in International Patent Application No. PCT/GB97/01333 published as WO 97/43550, to the Applicant.

A screw expander comprises a casing having at least two overlapping bores defining a cavity. The bores accommodate respective meshing helical lobed rotors, which contra-rotate within the fixed casing. The meshing action of the lobes is similar to that of helical gears. The casing encloses the rotors totally, in an extremely close fit. The central longitudinal axes of the bores are coplanar in pairs and are usually parallel. A male (or 'main') rotor and a female (or 'gate') rotor are mounted to the casing on bearings for rotation about their respective axes, each of which coincides with a respective one of the bore axes in the casing.

The rotors each have helical lands, which mesh with helical grooves between the lands of at least one other rotor. The meshing rotors effectively form one or more pairs of helical gear wheels, with their lobes acting as teeth. Viewed in cross-section, the or each male rotor has a set of lobes corresponding to the lands and projecting outwardly from its pitch circle. Similarly viewed in cross-section, the or each female rotor has a set of depressions extending inwardly from its pitch circle and corresponding to the grooves of the female rotor(s). The number of lands and grooves of the male rotor(s) is different to the number of lands and grooves of the female rotor(s).

The principle of operation of a screw expander is based on volumetric changes in three dimensions. The space between any two successive lobes of each rotor and the surrounding casing forms a separate working chamber. The volume of this chamber varies as rotation proceeds due to displacement of the line of contact between the two rotors. The volume of the chamber is a maximum where the entire length between the lobes is unobstructed by meshing contact between the rotors. Conversely the volume of the chamber is a minimum, with a value of nearly zero, where there is full meshing contact between the rotors at the end face. The shape of the lobes must be such that at any contact position, a sealing line is formed between the rotors and between the rotors and the casing in order to prevent internal leakage between successive chambers. In prior art screw expanders, fluid to be expanded enters the casing through a high-pressure or inlet port on an upstream side of the casing. After expansion, the fluid leaves the casing through a low-pressure or discharge port on a downstream side of the casing.

Typically the high-pressure or inlet port is defined by an opening situated mainly in a front plane of the casing. The fluid thus admitted fills the chambers defined between the lobes. The trapped volume in each chamber increases as rotation proceeds and the contact line between the rotors recedes. At the point where the inlet port is cut off, the filling or admission process terminates and further rotation causes the fluid to expand as it moves downstream through the screw expander.

Further downstream, at the point where the male and female rotor lobes start to re-engage, the low-pressure or discharge port in the casing is exposed. That port opens further as further rotation reduces the volume of fluid trapped between the lobes and the casing. This causes the fluid to be discharged through the discharge port at approximately constant pressure. The process continues until the trapped volume is reduced to virtually zero and substantially all of the fluid trapped between the lobes has been expelled.

The process is then repeated for each chamber. Thus, there is a succession of filling, expansion and discharge processes achieved in each rotation, dependent on the number of lobes in the male and female rotors and hence the number of chambers between the lobes.

The working fluid for expansion may be obtained from various sources, such as steam from a geothermal source. In this respect, a key advantage of a screw expander over a turbine expander is the ability to handle a wet working fluid (i.e. a fluid containing both gaseous and liquid phases) with little risk of damage. Another advantage is that screw expanders are potentially more cost-effective than turbines for relatively small power outputs. In this respect, industrial steam systems represent a major potential application of screw expanders. Many industrial processes require a supply of steam, examples being food preparation, paper-making and chemical processes. Typically, a central boiler generates steam at a moderately high pressure and that steam is distributed around a factory, plant or other industrial installation via a pipe system. Steam is drawn off through a branch pipe at each location where it is required.

As different processes may require different steam pressures, each branch pipe typically has a control valve that throttles the steam to whatever lower pressure may be required for the process in question. For example, as shown in the schematic diagram of Figure 1 , control valves may be disposed in downstream succession to provide progressively stepped-down steam pressures. Here, incoming steam from a supply pipe 10 at a high initial pressure Pi is reduced in pressure by a first control valve 12 to a first intermediate pressure P₂, at which some steam is bled off through a first branch pipe 14. The remaining steam is reduced again in pressure by a second control valve 16 to a second intermediate pressure P₃, at which more steam is bled off through a second branch pipe 18. The remaining steam is reduced again in pressure by a third control valve 20 to a final low pressure P₄ at which that steam is supplied through a third branch pipe 22.

It is known to use a screw expander instead of a throttle valve to reduce steam pressure. This makes it possible to recover power from the expansion process while still supplying steam at the required lower pressure. This arrangement works well when only a single throttling process is required and there is a large-enough pressure ratio between the incoming and outgoing flows of steam for an efficient screw expander to be designed. However, if one or more intermediate steam pressures are required, additional screw expanders will be needed - one for each required pressure, analogously to the valves of Figure 1. Also, it is possible that the pressure ratio between stages will be too small for a screw expander to recover power efficiently.

It is against this background that the present invention has been devised.

From one aspect, the invention resides in a screw expander comprising two or more rotors having meshed helical formations and being supported for rotation in respective bores inside a casing, wherein the casing comprises: an inlet for admitting working fluid to the casing at a relatively high initial pressure; an outlet for discharging working fluid from the casing at a relatively low discharge pressure; and at least one bleed port located downstream of the inlet and upstream of the outlet for extracting working fluid from the casing at an intermediate pressure between the initial pressure and the discharge pressure. Preferably, the bleed port presents an elongate slot to a bore. That slot may be

generally helically disposed with respect to the bore, advantageously to match or follow the helical formations of a rotor in that bore. The bleed port suitably has a smaller cross-sectional area than the outlet. In a preferred embodiment to be described, the expander has at least two bleed ports comprising an upstream bleed port located downstream of the inlet and a downstream bleed port located downstream of the upstream bleed port, the upstream bleed port being for extracting a portion of the working fluid from the casing at a first intermediate pressure and the downstream bleed port being for extracting a further portion of the working fluid from the casing at a second intermediate pressure below the first intermediate pressure.

The inventive concept also embraces a method of reducing the pressure of a working fluid, comprising expanding the working fluid in a screw expander from an initial pressure, bleeding a portion of the working fluid from the screw expander at an intermediate pressure lower than the initial pressure, expanding the remaining working fluid in the same screw expander and discharging the remaining working fluid from the screw expander at a discharge pressure lower than the intermediate pressure.

The working fluid is preferably steam. In that case, the inventive concept extends to a steam supply system comprising at least one expander in accordance with the invention or operating in accordance with the method of the invention. The same inventive concept also covers a power generator comprising at least one expander in accordance with the invention or operating in accordance with the method of the invention. Thus, the invention involves bleeding-off some working fluid from the screw expander at one or more intermediate pressures and then continuing to expand the remaining fluid in the screw expander to one or more lower pressures. The advantages of this are twofold. Firstly, the greater overall resulting pressure ratio in the expander enables it to operate more efficiently. Secondly, the entire power recovery from the successive pressure drops is achieved by a single machine.

During expansion in a screw expander, the actual volume ratio of expansion of the working fluid may differ significantly from the inherent volume ratio of the machine. If this is the case, then under-expansion of the fluid will lead to low expander efficiencies, or the need for a very much larger machine. Conversely, over-expansion of the fluid will lead to catastrophic losses in performance due to the need to recompress the fluid as it is expelled from the expander.

To keep the expander as small as possible while maintaining good expansion efficiency, the invention proposes extracting or bleeding some of the working fluid at an appropriate intermediate point so that the working fluid remaining in the expander can perform a full expansion within the existing expander volume. This may decrease the expander power. However if the extracted working fluid is used for process or regenerative heating, the utilisation efficiency of the fluid may be higher.

The expander volume at which the working fluid is extracted can be determined in order to obtain the pressures and temperatures required for further processing using the extracted fluid. It is possible to have more than one extraction point or bleed port in the expander, depending on the inherent volume ratio of the machine, or angular shaft movement during expansion and the number of lobes of the main rotor, or the inter-lobe shaft angle. The inter- lobe shaft angle is defined as the angle of one full rotation divided by the number of rotor lobes.

The angular movement of the shaft between any two extractions must be more than one inter-lobe shaft angle plus the angular width of the extraction slot. Thus, as it is desirable to keep the volume of the expander small and the number of lobes in the main rotor is relatively small, it will not be possible in most cases to have more than two fluid extraction points or bleed ports. Reference has already been made to Figure 1 of the accompanying drawings to illustrate prior art employing control valves. In order that the invention may be more readily

understood, reference will now be made, by way of example, to the remaining drawings in which:

Figure 2a is schematic view of a screw expander in accordance with the invention, arranged to supply three branch pipes with working fluid at successively stepped- down pressures; Figure 2b is a part-sectional side view of the expander shown schematically in Figure

2a; and

Figure 3 is a part-sectional side view of an expander casing showing a preferred port arrangement of the invention.

Referring then to Figures 2a and 2b of the drawings, Figure 2b shows key components of a screw expander 24 shown schematically in Figure 2a. The expander 24 comprises a fixed casing 26 containing two meshing helical lobed rotors 28 that contra-rotate in respective bores within the casing 26 about parallel axes. Only one rotor 28 is visible in Figure 2b as the other rotor is hidden behind it in that view. The rotors 28 are of any suitable material such as mild steel, high-speed steel or ceramics. Normally, if of metal, the rotors 28 are machined but alternatively they can be ground or cast. Preferably, the rotors 28 have an 'N' rotor profile as disclosed by the Applicant in WO 97/43550. Various coatings may be applied to the rotors 28 for wear resistance, and/or various lubrication systems may be employed; these may include lubrication by liquid phase of the working fluid itself.

Each of the rotors 28 is mounted to a respective shaft 30 that is mounted, in turn, to the casing 26 by bearings (not shown) supporting each end of each shaft 30. At least one of the shafts 30 extends out of the casing 26 as shown to drive a generator (not shown) for producing electricity. The working fluid to be expanded (in this example, steam) enters the casing 26 at a high initial pressure (P-i) through an inlet port 32 in a front plane of the casing 26. The steam flows and expands through the interior of the casing 26, causing the rotors 28 to turn at high speed, and exits the casing 26 at a low final pressure (P₄) through a discharge port 34 extending along a side of the casing 26.

In accordance with the invention, steam is bled off at intermediate pressure(s) from one or more intermediate locations on the casing between the inlet port 32 and the discharge port 34. In this example, there are two intermediate locations, each having a respective bleed port 36, 38. Steam is bled off at a first intermediate pressure P₂, through the first or upstream bleed port 36 and at a second intermediate pressure P3 through the second or downstream bleed port 38.

In the example shown, the first bleed port 36 is narrower than the second bleed port 38, which in turn is narrower than the discharge port 34. However, the port sizes must be calculated for each application and the order of sizes may be reversed or otherwise varied.

Moving on now to Figure 3, this shows an embodiment of the invention in the form of an expander 40 in which a bleed port is in the form of a helicoid slot 42 through the casing 44, communicating with the bore 46 within. The slot 42 is characterised by being narrow in width - which helps to accommodate more than one bleed port in the casing 44 of the expander 40 if desired - but the slot 42 also has a large flow area by virtue of its considerable length.

Consequently, the slot 42 is arranged helically with respect to the bore 46 of the expander 46 to follow the rotor helix along the full length of the slot 42.

Whilst only one slot 42 is shown in Figure 3, there may be at least one more slot 42 extending generally parallel to the first slot 42. It will be appreciated that the helical arrangement helps to fit more than one slot 42 in the casing 44 while maximising the length and hence the flow area of each slot 42.

Claims

1. A screw expander comprising two or more rotors having meshed helical formations and being supported for rotation in respective bores inside a casing, wherein the casing comprises: an inlet for admitting working fluid to the casing at a relatively high initial pressure; an outlet for discharging working fluid from the casing at a relatively low discharge pressure; and at least one bleed port located downstream of the inlet and upstream of the outlet for extracting working fluid from the casing at an intermediate pressure between the initial pressure and the discharge pressure.

2. The expander of Claim 1 , wherein the bleed port presents an elongate slot to a bore.

3. The expander of Claim 2, wherein the slot is generally helical with respect to the bore.

4. The expander of Claim 3, wherein the slot is shaped to follow the helical formations of a rotor in that bore.

5. The expander of any preceding claim, wherein the bleed port has a smaller cross-sectional area than the outlet.

6. The expander of any preceding claim, wherein the working fluid is steam.

7. The expander of any preceding claim and having at least two bleed ports comprising an upstream bleed port located downstream of the inlet and a downstream bleed port located downstream of the upstream bleed port, the upstream bleed port being for extracting a portion of the working fluid from the casing at a first intermediate pressure and the

downstream bleed port being for extracting a further portion of the working fluid from the casing at a second intermediate pressure below the first intermediate pressure.

8. A method of reducing the pressure of a working fluid, comprising expanding the working fluid in a screw expander from an initial pressure, bleeding a portion of the working fluid from the screw expander at an intermediate pressure lower than the initial pressure, expanding the remaining working fluid in the same screw expander and discharging the remaining working fluid from the screw expander at a discharge pressure lower than the intermediate pressure.

9. The method of Claim 8, wherein the working fluid is steam.

10. A power generator comprising the expander of any of Claims 1 to 7 or operating in accordance with the method of Claim 8 or Claim 9.

1 1. A steam supply system comprising at least one expander as defined in any of Claims 1 to 7 or operating in accordance with the method of Claim 8 or Claim 9.