GB2459326A - Saturated vapour turbine system - Google Patents

Abstract

A device for converting the latent heat stored in a flowing stream of saturated vapour into useful work, comprising a linear array of sets of turbine blades or separate turbines, characterised by the vapour processing capacity of each successive set of turbine blades or turbine units decreasing in volume, in a proportionate manner that motivates the saturated vapour pressure to remain constant as the stream passes through the array, and vapour condenses out as latent heat is released. The device may have adjustable baffles 1,2,3 to create extra viscous drag at low vapour rates. The drag causes an increase in evaporation which is observed as an increase in vapour volume. The baffles only come into operation at low saturated vapour flow rates.

Description

Improved saturated vapour driven turbine system

Technical Field

According to the invention, there is provided a device for converting the latent heat stored in a flowing stream of saturated vapour into useful work, comprising a linear array of sets of turbine blades or separate turbines, characterised by the vapour processing capacity of each successive set of turbine blades or turbine units decreasing in volume, in a proportionate manner that motivates the saturated vapour pressure to remain constant as the stream passes through the array and vapour condenses out as latent heat is released.

Brief description of the drawings

Figure 1 relates to a thought experiment and depicts the passage of gas or unsaturated vapour through a constriction in a length of conduit.

Figure 2 is a sketch graph showing temperature changes on passing though a constriction.

Figure 3 is an isothermal for a pure substance on a pressure-volume graph.

Figure 4 illustrates the passage of initially dry saturated vapour/gas through a real constriction.

Figure5 depicts the passage of wet saturated vapour through a real constriction.

Figure 6 depicts the same thought experiment system as in Figure 1, but with a turbine added.

Figure 7 depicts a section of a turbine chain with three turbines illustrated.

Figure 8 depicts the pressure plateau section of a pure substance isothermal.

Figure 9 is a graph for one kilogram of saturated steam entering the first turbine unit.

Figure 10 depicts a section of a turbine chain with external work at constant pressure being done and the system also including a slight increase in the volume of each turbine wilt, to cater for the evaporation caused by viscous drag.

Figure 11 depicts part of a turbine chain including adjustable baffles. These are added to create enhanced viscous drag at low vapour throughput rates.

Figure 12 illustrates a typical thermodynamic cycle for the invention on a pressure vs. volume graph.

Figure 13 illustrates a thermodynamic cycle in which the saturated steam pressure gradually drops as the steam passes through the chain of turbines.

The prior art

Improvements to Newcomen type steam engines have previously been revealed by the present inventor in patent application CT/GB2007/OO 1380 (Courtney). In the earlier Courtney invention, saturated steam was drawn through a chain of turbo-generators in order to generate electricity when the turbines rotated. When a vapour does external work, for example driving a turbine under load, it cools. In the case of the engine revealed by Courtney, the vapour is saturated. Consequently any cooling that drives it towards super saturation causes condensation and the release of latent heat. This rejuvenates the vapour by increasing its temperature, with the final temperature after passing through any of the turbines only being a fraction of a degree below that on entering the turbine. However, if not correctly managed, there is also a drop in saturated vapour pressure and density. This pressure drop has the tendency to cause the vapour to accelerate after passing through a turbine, so that its volume expands, causing the vapour to move away from saturation conditions.

Dlsclosuie of the invention This invention employs some form of saturated vapour, for example, saturated water vapour throughout the novel parts of the system.

The temperature and pressure variations as saturated vapour passes through a constriction differ markedly from the passage of unsaturated vapour or a gas. These differences will be discussed as a series of thought experiments before revealing the invention in detail.

Figure 1 relates to a thought experiment and depicts the passage of gas or unsaturated vapour through a constriction in a length of conduit, 1. It will be assume that all conduit sections are well lagged, so that processes are adiabatic.

In all of the following thought experiments, the conduit will be assumed to exist inside a large reservoir of the working gas/vapour fluid. With reference to diagrams ito 4, it will also be assumed that viscous drag is negligible.

In Figure 1, the forward velocity increases from A to 13 as the nozzle 2 tapers. Inside the taper, the kinetic energy of the fluid increases but the total energy remain must remain unchanged. Consequently, the gas cools to offset the increase in kinetic energy. At a molecular level the drop in temperature is explained as a change from random to ordered molecular movement. The reduction in random movement of the molecules is manifested at a macroscopic level as a drop in static pressure. In the flared section 3, there is a corresponding increase in pressure and temperature as the forward velocity of the fluid decreases. A notional impellor pump 4 is added to the thought experiment, to maintain forward motion. In this idealised thought experiment where drag is absent, the pump only has to do work creating a small pressure drop at the mouth of the conduit and an excess pressure at the exit, to pump the working fluid back into the reservoir.

The working fluid is compressible; nevertheless, changes in pressure can be estimated with a fair degree of accuracy using Bernoulli's equation. This states that for an incompressible, non-viscous fluid undergoing steady flow, the pressure (p) plus the kinetic energy per unit volume (112x density,p x velocity, v squared) plus the potential energy per unit volume (density,p x acceleration due to gravity, g x height h) is constant at all points on a streamline. Thus,

p + l/2pv2 + pgb A constant If the gas is replaced with a saturated vapour then Bernoulli's equation breaks down. Any tendency to cool on passing through the nozzle taper will result in the production of small condensation droplets and the release of latent heat. Consequently, the temperature and pressure drops will be minimal, even though the saturated vapour has acquired kinetic energy. On passing through the flared section, the latent heat processes are reversed, with heat being absorbed as the water droplets evaporate. The rate of mass flow remains constant through all sections of the conduit perpendicular to the streamlines. In the case of a saturated vapour, the rate of volume flow drops as a consequence of condensation in the nozzle, then increases as a consequence of evaporation in the flared section.

This argument assumes that that the condensation droplets continue to move forward as an aerosol and do not come to rest as pools of liquid inside the conduit.

Figure 2 is a sketch graph showing temperature changes for two different types of fluid on passing though a constriction. Item 1 is the graph for gas or a vapour that remains unsaturated at all points along the conduit. Item 2 is a graph for a vapour that remains saturated at all points along the conduit.

A small drop in temperature is likely when saturated vapour passes rapidly through a constriction because of the finite time required for he evaporation and condensation processes.

Figure 3 is an isothermal for a pure substance on a pressure-volume graph. If the fluid passing through the constriction remains in the region between 1 and 2, the substance is in the unsaturated vapour state. So, assuming drag is negligible, Bernoulli's equation is obeyed. If the fluid passing through the constriction remains in the region between 2 and 3, the substance is in the saturated vapour state and Bernoulli's equation, in its basic form breaks down.

In order to produce an equation that is useful at all points between 1 and 3 an additional term dQ /dV needs to be added to the equation. The term dQL IdV represents the latent heat lostlgained per unit volume.

Thus the generalised form of Bernoulli's equation is p-f-1/2pv2+pgb-dQL/dVAconsbnt When condensation occurs and latent heat is liberated, the minus sign is retained in front of the latent heat term. A positive sign is used if evaporation occurs and latent beat is absorbed.

Nozzles that funnel saturated vapours and operate in the manner described above are an important part of the present invention. In order to ensure that condensation occurs reasonably uniformly throughout the volumes of the nozzles, condensation seeding mechanisms may be introduced. Experienced engineers will be aware that a range of mechanisms are available. These include; (i) introducing negative ions discharged from an array of high voltage point electrodes, (ii) adding spiked, grounded metal surthces which possesses an induced positive charge, because of their proximity to the negatively charged electrodes, (iii) building weak alpha particle sources into the conduits, (iv) injecting dust particles from a coal fired boiler furnace, (v) adding coarse wire meshes at right angles to the lines of streamline flow.

Any obstacles such as wire mesh add to the viscous drag naturally present in any flowing fluid system. Viscous drag converts some of the forward kinetic energy of the fluid passing through a constriction into heat This is an irreversible process that takes place at all cross sections along the constriction.

At a molecular level heating is explained as an increase in the random movement of the molecules. At a macroscopic level viscous drag induced heating is manifested as an increase in pressure.

Figure 4 illustrates the passage of initially dry saturated vapour/gas through a real constriction.

Viscous drag increases the fluid temperature. The hachuring of the conduit walls symbolically represents the heating effect of viscous drag. If v and v2 are the flow velocities before and after entering the constriction, then, in order to maintain forward motion with V2 v1, an external pressure drop -p has to be applied. This is required to neutraiise the pressure increase and a result of the fluid temperature increase induced by drag.

Figures depicts the passage of wet saturated vapour through a constriction where drag is present. The temperature of the vapour cannot increase in the manner of an unsaturated vapour/gas until all of its moisture content has evaporated. In place of a pressure increase an increase in volume is observed. In order to maintain forward motion with v2 = v1, the cross sectional area of the wide part of the conduit has to increase so that the cross sectional area at 2 is greater than at 1. Additional work has to be done by the notional pump 3 is order to cope with the increased volume. If the cross sectional area of the wide parts of the conduit are not allowed to expand to cope with the increased volume of saturated vapour caused by evaporation then an additional external pressure drop -p needs to be applied, to increase the flow velocity.

In order for engineers to design turbine systems according to the present invention, a further correction to Bernoulli's equation is required. This version of the equation can be written as p+1/2pv2+pgh+dQ/dV -dQL/dVAconstant The additional term dQ/dV takes into account the conversion of kinetic energy into heat as a consequence of drag.

In the next two sections we will analyse a zero drag turbine design, then modify the design to allow for drag effects.

Figure 6 depicts the same thought experiment system as in Figure 1, but with a turbine I added. In the following analysis, the working fluid is assumed to be a gas or unsaturated vapour. The rate of mass flow on entering the turbine at B and exiting at C must be identical. But momentum is transferred from the gas to the turbine blades such as 2, so the forward velocity of the gas is reduced. To accommodate this, the exit port cross sectional area A2 has to be greater the than the entrance port cross sectional area A1.

If 75% of the kinetic energy of the gas is transferred to the turbine, then the forward velocity of the gas is reduced by 50% because the kinetic energy is a function of velocity2. In this case, A2 = 2x A1 approximately.

External work has been done by the gas, so the Law of Conservation of Energy requires the temperature of the gas/unsaturated vapour to fall across the whole turbine unit, that is, between A and D in Figure 6.

(The relationship A2 2x A1 is only an approximation because the gas contracts on cooling.) At a molecular level, the temperature drop is explained as follows: Gas molecules impacting on the turbine blades cause the blades to move forward. The molecules lose forward momentum, but due to inter-molecular collisions, this loss is rapidly redistributed between the local gas molecules. That is, the local value for the root mean square velocity and kinetic energy of the gas molecules is reduced. At a macroscopic level, this is observed as a drop in local gas temperature.

If the gas is replaced by a saturated vapour, then condensation occurs whenever there is a tendency for the temperature to fill, so the temperature drop is minimal, thanks to the release of latent heat. The mass and volume of saturated steam leaving the turbine unit per unit time is less than that entering because some fluid exits in the form of an aerosol of condensation droplets.

According to the invention, a turbine chain arrangement can be used to extract latent heat from the steam at constant pressure, provided that each turbine along the chain has a progressively smaller volume, to cope with the progressively decreasing volume flow rate.

Figure 7 depicts a section of a turbine chain with the turbines 1,2,3, and related apertures A1 to A6 decreasing in capacity in a calculated way such that the pressures inside the first and final turbines are the same.

FigureS depicts the pure substance isothermal for a turbine chain according to the invention with the gaps between the markers on the pressure plateau such as 1, 2 and 3 representing the volume shift along the plateau as the saturated steam passes through each successive turbine unit. The pressure, P remains constant, but the product of pressure, P times volume V passing through each turbine unit decreases steadily along the turbine chain.

Figure 9 is a graph for one kilogram of saturated steam entering the first turbine unit. It shows how the rate of saturated vapour mass flow drops off along the chain. If viscous drag is negligible, there is a corresponding drop in turbine chamber volume along the chain.

The following sample calculation explains how the data points were obtained.

Assumptions: (i) Temperature of saturated steam = 100°C, (ii) Mass of saturated steam entering first turbine = 1 kg, (iii) Turbine converts 75% of kinetic energy of steam into work, (iv) Nozzle velocity, v 450 rn/s.

(The velocity must remain below the speed of sound to avoid Laval nozzle type expansion. For steam at 100°C, this is 473 m/s.) Values used: (i) Specific latent heat 2.26 x 106 J/kg, (ii) Density of steam = 0.6 kg/rn3 Calculation: Work done on turbine � x V2 x mass x (velocity)2 3/4xV2x 1x450x450 =76x 103J Mass of steam condensing = 76 x.1 / Specific latent heat of steam = 76 x l0 J 2,26 x 106 Jñcg =33.6x 103kg Mass of saturated steam entering second turbine (l -33.6 x l0) kg = 0.966 kg Volume of steam/second entering first turbine volume of 1 m3/450 mis = l/(O.6x450) =3.7xlO3m3 The graph presented as Figure 9 assumes ideal conditions in which viscous drag is negligible. It has been explained with the assistance of Figure 5 that in order to maintain the steady flow of a wet saturated vapour, the cross section area of the conduit must increase to cope with the increase in the rate of volume flow caused by evaporation.

The novel features of the invention that allow it to operate at constant pressure, in spite of the problems caused by viscous drag will now be revealed.

Figure 10 depicts a section of a turbine chain with external work at constant pressure being done and the system also including a slight increase in the volume of each turbine unit, to cater for the evaporation caused by viscous drag. Line 1 is the trend line in volume size for ideal, zero drag conditions; line 2 is the trend line in volume size for real conditions where drag is present.

The invention has to include an additional novel design feature in order to allow it to cope with different saturated vapour flow rates. Viscous drag increases as the rate of steam flow through the chain increases. Consequently a system that maintains uniform pressure at maximum flow rates will overcompensate by providing too much volume at lower flow rates.

To offset the excessive increase in volume, the pressure will drop unless remedial measures are taken. A pressure drop would create problems because it would cause the vapour to expand and move away from saturation conditions. In the extreme case, the pressure drop could cause the vapour to accelerate to supersonic speeds, with the nozzles acting in the manner of Laval nozzles.

(Laval nozzles are used in conventional steam turbines. They cause permanent expansion as the price for high nozzle velocities and hence efficient production of kinetic energy. Permanent expansion is compatible with conventional turbine designs, but unsatisflictory for the present invention.) In order to prevent a pressure drop at low saturated vapour flow rates, additional viscous damping can be added to that naturally present.

Figure 11 depicts part of a turbine chain with adjustable baffles 1, 2 and 3. These are added to create enhanced viscous drag at low vapour throughput rates. The extra drag causes an increase in the evaporation rate. This is observed as an increase in volume. It is emphasised that the baffles only come into operation at low saturated vapour flow rates. At the maximum flow rate, corresponding to maximum power output the baffles are designed to offer a profile that creates a minimum of drag. Expert steam turbine designers will recognise that conventional flow rate monitors will need to be added after each turbine unit to measure the vapour volume flow rate. They will also recognise that the monitors will need to be electronically or mechanically coupled to the baffles, so the baffles can alter their drag profile. Alternatively, the baffles may be spring mounted such that they offer an increased drag profile as the rate of saturated vapour flow decreases and unloads the springs.

It is emphasised that the illustrations of turbines presented in this application are for the guidance of experts in turbine design. They are relied upon to use their existing knowledge to create working versions of the invention. For example, experts will be able to construct power generation units according to the invention in which a chain of Pelton wheel type turbines are constructed helically around a single power transmission shaft. Alternatively, experts will be able to incorporate their prior knowledge to construct constant pressure integrated axial flow turbine systems, with successive sets of turbine nozzles and blades tapering in size according to the invention, with adjustable baffles and flow rate sensors being added after each set of blades to ensure that the flow is maintained at constant pressure.

The adjustable baffles are depicted In Figure 11 as being similar to quarter turn valves. This is for illustrative purposes only and is not intended in any way to limit the shape or relative size of the baffles compared with the enclosing conduit. Skilled engineers will be able to use exiting devices such as velocity sensors, governors, worm and screw drives, levers, eccentrics and motors to create feedback loops that enable the baffles to automatically vary their profile, in order to regulate vapour velocities and prevent net movement away from saturation conditions.

In another version of the invention, the variable damping obstructions take the form of variable pitch blades, analogous to small wind turbine blades, but carrying out a different function. In stead of mimicking a wind turbine by converting kinetic energy into electricity and tending to cool the vapour with the risk of condensation, the rotating blades are allowed to rotate unloaded, so the vapour loses kinetic energy and tends to warm up. A merit of using a rotating blade damper is that the blades provide a centrifuge action, helping to strip water droplets out of the vapour by flinging them towards the side walls. If the rotating shaft the blades are mounted on is coupled to a small open circuit generator, the electromotive force (EMF) it generates will be a function of the rate of saturated vapour flow. This EMF value can be used as a feedback signal to electronically adjust the effective profile of the damping baffles Figure 12 ilLustrates a typical thermodynamic cycle for the invention on a pressure vs. volume graph. This example, which incorporates a superheated fluid phase is not intended in any way to limit the scope of the invention.

There are some similarities with the Rankine cycle, but there are also crucial differences. Item 1 is the saturated/unsaturated vapour boundary for the unit mass isotherm for substance used in the cycle. This will probably be water, but the invention is extended to cover other fluids such as pentane, hexane, ammonia, carbon, sulphur and nitrogen dioxides. Without in any way limiting the scope of the invention, in what follows, the fluid will be referred to as water or steam.

The following items refer to different parts of the cycle curve.

Item 2 is the section where water in its liquid phase is compressed and heated to its boiling point in a boiler.

Along the plateau 3 additional heat is supplied so that the hot water gradually evaporates to saturated steam. In section 4 further beat is added so that the steam becomes superheated and unsaturated. Along section 5 the steam then passes through a first dry part of the condensing turbine or through a separate turbine. In this section, work is done wider Rankine cycle type conditions. Along the plateau 6, work is being done by condensing turbines according to the invention. Item 7 represents the condensation that is returned to the boiler. A stage is reached where the volume flow rate is so low that it is no longer practical to use it to participate in driving the condensing turbines.

A design choice now follows. The residual steam can be compressed back to the boiling point. This is represented by section 8. Alternatively, the residual steam can be condensed out. This is represented by section 9. Condensation is mechanically simpler but compression but is more energy efficient.

Figure 13 illustrates a thermodynamic cycle in which the saturated steam pressure gradually drops along the line 1 as the steam passes through the chain of turbines. The pressure can drop for a number of reasons including a time lag in condensation as discussed with reference to Figure 2 and a time lag in the adjustable baffles responding to a change in the rate of steam flow.

The present invention preferably operates under thermodynamic cycle conditions as discussed with reference to Figure 12 but is extended to operate under the conditions discussed with reference to Figure 13.