GB2294294A

GB2294294A - Orbital scroll expander for recovering power from flashing fluids

Info

Publication number: GB2294294A
Application number: GB9524384A
Authority: GB
Inventors: Ian Smith; Leonard Charles Constant
Original assignee: TFC POWER SYST Ltd; City University of London
Current assignee: TFC POWER SYST Ltd; City University of London
Priority date: 1995-11-29
Filing date: 1995-11-29
Publication date: 1996-04-24
Also published as: GB9524384D0

Abstract

An orbiting scroll machine is used as an expander of a volatile fluid to develop mechanical power. Fluid enters the machine through port 3 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 is particularly suited for applications where the required swept volume is in the order of 10 - 30 M.cu/Hr. <IMAGE>

Description

USE OF THE ORBITING SCROLL EXPANDER FOR THE RECOVERY OF POWER FROM FLASHING LIQUIDS.

POWER FROM FLASHING LIQUIDS.

Background.

The Orbiting Scroll mechanism was introduced into the refrigeration industry in the early 1980s as a gas compressor for use mainly on small air conditioning and heat pump applications. Larger sizes were subsequently developed although the fundamental geometry of the mating scrolls will probably limit the swept volume of the largest machine to about 30 M.cu/hr. in the forseeable future. The use of the Orbiting Scroll machine as a means of compressing gas is well known in the prior art and many units have been sold and are operating in the industrial, commercial and air conditioning markets throughout the World. It is noted for its low noise level, smooth operation and reliability, particularly in applications where substantial quantities ci liquid refrigerant enter the machine.The capability of the scroll mechanism to accept entrained liquid with the gas being compressed is largely due to the semi- rolling action between the mating scrolls rather than the sliding action normally associated with piston, rotary and screw compressors. The low frictional contact between the two scroll discs allows scroll machines to operate without the need for oil injection into the scroll mechanism. It is this feature which makes the scroll machine particularly suited for the Trilateral Flash Cycle.

This application is concerned with the use of the orbiting scroll machine as an expander in the following; i) The direct expansion of hot geotnermal brines as describedby Sprankle in Aug 1973.

ii) As an organic fluid expander in the Trilateral Flash Cycle System described by I.K.Smith.

iii) As a throttle valve replacement in chemical process plant and vapour compression refrigeration and heat pump systems.

Configuration of an Orbital Scroll Expander.

Referring to Fig.l. two parallel flat discs 1. and 2 each having a raised spirally shaped strip on their flat surfaces are brought together to form an enclosed volume between them. The engagement is such that high pressure gas and liquid introduced between them through port 3.is trapped in the crescent EnaF;ec; cnamt,erc ;. and fr use of the discs is free to orbit while the other remains stationary. The fluid is introduced into two diametrically opposite chambers simultaneously and expansion occurs evenly either side of the central driven shaft, thus ensuring a completely dynamically balanced assembly with very low loads on the driven shaft bearings.The expanding fluid is eventually discharged from the crescent shaped chambers when the orbiting scroll exposes the chambers to the low pressure outlet of the expander casing.There are no contacting seal grids to trap the fluid in the expansion chambers. Instead the machine is constructed with very small clearancesSbetween tie two scroll discs and these are filled with the liquid components of the two phase mixture.

Expansion process in a Orbiting Scroll Expander.

The expansion process in an Orbiting Scroll Expander can be described in four stages with reference to Fig.2.

i) In the filling process stage 1. high pressure liquid or liquid and its associated vapour enter the machine at the centre through the inlet port. 3. and fill the available crescent shaped chambers.

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 speed of the orbiting scroll.

In the case under consideration the process is more complex. The liquid or two phase mixture is more dense than compressed gases. The local 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 speed of the orbiting scroll the larger the acceleration and thus the more the density changes. Accordingly the mass flow rate does not increase proportionally to the speed of the orbiting scroll 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 analysis.

ii) In the expansion process stage 2. further orbiting of the moving scroll leads to an increase in volume of the chambers 4. and 5 containing the trapped fluid. This expansion continues until the chambers containing the gas are exposed to the outlet of the expander casing.

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 scroll chambers and 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 in the order of 90% of the total mass, makes a negligible contribution to the work of exparision.The increase in kinetic energy of the fluid due to its rotation is therefore a much larger percentage or the ttal energY change The pressure change associatpd with tne inc u expanslon must lnclude aIlowance for thus or estimates of mass flow through the machine may be twice the value of those actually attainable.

iii) The optimum expansion process stage 8. is reached immediately prior to the chambers 6. and 7. being exposed to the expander outlet gallery due to further orbiting of the moving scroll.

iv) Completion of the expansion process stage 4. occurs when chambers 6. and 7. are exposed to the outle galleryat points 8. and 9. The liquid- vapour mixture passes into the outlet gallery of the expander and through the outlet connection. Ideally the pressure in the outlet gallery should be substantially equal to that in the scroll chambers at the instant that discharge begins. if this is not achieved then a loss in efficiency can occur. If the pressure in the scroll chambers is greater than the outlet pressure when the chambers are exposed then wasteful flash expansion will take place during discharge. If the pressure in the scroll chambers is lower than the outlet pressure, fluid will flow back into the expansion chambers at the athe pointlof exposure, resulting in inefficiency.Further orbiting of the scroll expels the residual fluid displaced by the engaging scrolls at points t3. and 9. Ideally the exit port from the casing of the expander should be 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 aid condensation processes in vapour compression systems.

It is therefore an essential feature of this invention that the expansion process begins by passing the fluid through a variable control valve which may be opentd or closed in response to control signals in order to maintain the inbuilt volume ratio required for total expansion.

The main feature oi the Orbiting Scroll Expander which makes it ideally suited for small two phase applications is that it is not necessary to inject oil into the scroll chambers for lubrication or sealing of the engaging scroll discs. A small amount of oil migrates into the expansion chambers from te bearings but this is less than 3 < which is insufficient to affect the vapour pressure of the fluid and hence the expansion efficiency. Liquid in the expansion chambers fills the the small clearances and ensures that leakage is minimal.

Claims

1. The use of an Orbiting Scroll Expander for the direct expansion of hot geothermal brines

2. The use of an Orbiting Scroll Expander as the prime mover in the Trilateral Flash Cycle system.

3. The use of an Orbiting Scroll Expander as a throttle valve replacement in chemical process plant, air conditioning, refrigeration and heat pump systems.

4. The use of an Orbiting Scroll 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. The use of an Orbiting Scroll Expander in an oil free mode.

6. The use of an Orbiting Scroll Expander with no oil control seals between rotors and bearings.

7. The operation of an Orbiting Scroll Expander with large quantities of liquid, compatible with the TFC system, passing through the machine.

8. Use of an Orbiting Scroll Expander over the capacity range 10-30 M cu/hr. This size cannot be covered by the larger Twin and Single Screw Expanders.