Note: Descriptions are shown in the official language in which they were submitted.
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Expander Lubrication in Vapour Power Systems
This invention relates to the lubrication of expanders used in closed-circuit
vapour
power generating systems in which lubricant is soluble in, or miscible with,
the
working fluid. The invention is particularly, but not exclusively, concerned
with
systems for generating power from moderate or low grade heat sources such as
geothermal brines, industrial waste heat sources and internal combustion
engine
waste heat streams where the maximum temperature for the working fluid of the
system is rarely in excess of 150 C. Such systems typically use organic
working
fluids such as tetrafluroethane, chlorotetrafluoroethane 1.1.1.3.3 -
Pentafluoropropane or light hydrocarbons such as isoButane, n-Butane,
isoPentane, and n-Pentane and operate on the Rankine cycle or some variant of
it.
According to one aspect of the invention there is provided a vapour power
generating system for generating power by using heat from a source of moderate
or low grade heat, comprising a closed circuit for a working fluid, the system
including heating means for heating the fluid under pressure at a temperature
not
usually more than 200 C with heat from the source, a separator for separating
the
vapour phase of the fluid from the liquid phase thereof, an expander for
expanding the vapour to generate power, a condenser for condensing the outlet
fluid from the expander, feed pump means for returning condensed fluid from
the
condenser to the heater and a return path for returning liquid phase from the
separator to the heater, wherein the liquid phase contains a lubricant for the
bearing which lubricant is soluble or miscible in the liquid phase and a
bearing
supply path is arranged to deliver liquid phase pressurised by the feed pump
means to at least one bearing for a rotary element of the expander. The
condenser may also initially desuperheat the vapour from the expander.
With this system the lubricant is dissolved or emulsified with the liquid
phase of
the working fluid and a proportion of the liquid phase leaving the separator
is fed
along the bearing supply path to the bearing where heat generated in the
bearing
evaporates the working fluid, leaving sufficiently concentrated lubricant in
the
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bearing to provide adequate lubrication of the bearing. Preferably, collection
spaces are provided around and below the bearing. Lubricant leaving the
bearing and entering the expander travels to the condenser with the working
fluid
exhaust from the expander. The lubricant again mixes with, or dissolves in,
the
liquid phase formed in the condenser and returns, via the feed pump, to the
heater. Build-up or deposit of lubricant in the evaporator section of the
heater,
which would reduce its efficiency, is prevented by its retention in the liquid
recirculating through the evaporator section and partially drawn off to flow
through the expander, condenser and feed pump. Advantageously, each bearing
supporting the rotary element or elements of the expander is lubricated in
this
manner. The total mass of lubricant required is not more than 5% of the mass
of
working fluid. Typically 0.5% to 2% is sufficient.
The expander may be a rotary expander. The expander may for example be a
turbine of the radial-inflow or axial flow type. Particularly where power
outputs up
to about 3MW are required, the expander may be of the twin-screw type. Where
the twin-screw type expander is of the lubricated rotor type, the lubricant
will be
an appropriate oil and some of the mixture of oil and liquid from the
separator will
be fed into the expander, typically through the normal lubrication port
provided for
lubricated rotor twin-screw machines or a similar port nearer the high
pressure
port.
According to another aspect of the invention there is provided a vapour power
generating system for generating power by using heat from a source of heat,
comprising a closed circuit for a working fluid, the system including heating
means for heating the fluid under pressure with heat from the source to
generate
vapour, a plural screw expander for expanding the vapour to generate power, a
condenser for condensing the outlet fluid from the expander and feed pump
means for returning condensed fluid from the condenser to the heater wherein a
bearing supply path is arranged to deliver liquid phase pressurised by the
feed
pump means to at least one bearing for a rotary element of the expander, and
the
liquid phase delivered to the at least one bearing contains a lubricant for
the
expander which lubricant is soluble or miscible in the liquid phase.
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In embodiments of the invention the liquid phase may be delivered from an
intermediate point of the heater,
The invention will now be further described by way of example with reference
to
the drawings in which:
Figure 1 is a circuit diagram of a vapour power generating system
according to the invention,
Figure 2 is a circuit diagram similar to Figure 1 but incorporating a
modification,
Figure 3 is a sectional view through the rotor axes of a twin screw
expander suitable for use in the circuit of Figure 1 or 2,
Figure 4 is a longitudinal section on the line IV - IV of Figure 3,
Figure 5 is a diagram showing the vertical disposition of components of a
system similar to those shown in Figures 1 and 2, and
Figure 6 is a circuit diagram of an alternative embodiment of the invention
using a single pass boiler.
The Organic Rankine Cycle system shown in Figure 1 defines a closed circuit
for
an organic working fluid having a boiling point at atmospheric pressure below
100 C. Up to 5% (usually between 0.5 and 2%) by weight of a compatible natural
or synthetic lubricating oil is added to the fluid.
The circuit comprises a heat exchanger assembly 1 for heating the working
fluid
in counterflow heat exchange with a hot liquid such as geothermal brine or
waste
from an industrial source at a temperature up to about 150 C.
The heat exchanger assembly 1 defines a path 2 for the hot fluid from the
source,
the path 2 extending from an inlet 3 to an outlet 4. The assembly also defines
a
path, extending in counterflow heat exchange with the path 2, through a heater
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section 5, for heating liquid working fluid, and an evaporator section 6 for
evaporating at least some of the working fluid.
A line 7 leads from the outlet of the evaporator 6 to a separator 8, at a
higher
level than the heater section 5, for separating the vapour component of the
evaporator output from the liquid component. Lines 9 and 10 serve to return
the
hot liquid component to the junction 11 between the heater and evaporator
sections 5 and 6.
A line 12 connects the vapour output of the separator 8 to the inlet 13 of a
twin-
screw expander 14 for expanding the vapour to a lower pressure and thereby
generating power to drive an external load such as an electrical generator G.
A line 15 leads from the exhaust outlet 16 of the expander to a condenser 17
for
condensing the expanded vapour in heat exchange with a cooling fluid flowing
through a circuit 18.
A line 19 connects the liquid outlet of the condenser to a feed pump F for
returning the liquid to the heater under pressure through a line 20. To
lubricate
and cool the bearings of the expander 14, a line 21 leads from the junction 22
of
the lines 9 and 10 to inlets 27, 28 in bearing housings 23, 24 containing
bearings
for the rotating elements of the expander.
The bearing housings 23, 24 provide sufficient space around the bearings for
the
oil content of the liquid working fluid to be concentrated as the working
liquid
evaporates into the expander as a result of heat generated in the bearings.
Since much of the working fluid leaves the separator 8 as vapour, and thus
free
of this oil, the oil content in the lines 9, 10 and 21 will already be
increased. As
oil leaves the bearings and flows into the expander, it is constantly replaced
by
further oil from the line 21. The oil leaves the expander outlet 16 with the
vapour
and dissolves into the liquid condensed in the condenser 17.
Since the separator 8 is higher than the heater section 5 (and preferably
higher
than the evaporator 6), and since the column of liquid in the line 9 is denser
than
the column of fluid in the evaporator 6 and line 7, there will be continuous
circulation through the evaporator section.
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Similarly, the feed pump F ensures continuous circulation through the heater
section 5. By tapping off the flow from the junction 22 to the bearings, a
continuous circulation occurs through the heater section, bearings, condenser
and feed pump so that an accumulation of oil on the surfaces of the heater and
evaporator sections, which would lower their efficiencies, is prevented.
Where the expander is of the lubricated-rotor type, the line 21 may also be
connected, by a line 25, to the normal oil-supply port 26 of the expander.
The circuit shown in Figure 2 differs from that shown in Figure 1 in that the
lubricant-containing liquid tapped off from the junction 11 is cooled, for
example
from 80 C to 35 C, in a heat exchanger 30, in counterflow with the liquid
delivered by the feed pump F to the inlet of the heater section 5. Thus, the
outlet
of the feed pump F is connected by a line 31 to the inlet of a pre-heater
section
32 of the heat exchanger 30. The outlet of the pre-heater section 32 is
connected by a line 33 to the inlet of the main heater section 5.
Instead of feeding the lubricating flow directly from the junction 22 to the
bearings, this flow is taken by a line 34 to the inlet of a cooler section 35
of the
heat exchanger to flow therethrough in cooling heat exchange with the liquid
in
the pre-heater section 32 before being fed by a line 36 to the expander
bearings
23, 24. Where the expander is a twin-screw expander, the lubricating flow may
also be taken to the rotor surface lubrication inlet 37.
By cooling the lubrication flow, for example from 90 C to 35 C, the risk of
the
working liquid flashing into vapour, and thus interrupting the supply of
lubricant, is
avoided. Further, the flow can be controlled by means of restrictors or
control
valves, again without vaporisation. By this means also heat that would
otherwise
be wasted in the bearings is recovered and used to increase the power output
of
the expander. The flow rate delivered to the inlet 37 depends on the working
fluid
and the operating conditions of the cycle but typically is of the order of two
to four
times the total flow delivered to the rotor bearings.
Figures 3 and 4 show a twin-screw expander suitable for use in the circuits of
Figures 1 and 2. The expander has a housing 40 containing a helically lobed
rotor 41 meshing with a helically grooved rotor 42. The rotor profiles, as
seen in
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cross section are of the low friction type having helical involute bands in
the
region of their pitch circles, being preferably of the type disclosed in EP
0,898,655. The rotors 41 and 42 are supported in rolling bearings 43, 44 in
the
bearing housings 23, 24. The rotor 41 has an extension 45 projecting through
the bearing housing 24, with a sealing assembly 46, to drive the external load
such as the generator G.
The housing is formed with the rotor surface lubrication inlet 37 in a
position just
downstream of the vapour inlet 13 to ensure a sufficient pressure drop to
provide
an adequate lubrication flow.
The working liquid portion of this flow forms the major part of this flow and
is free
to vaporise and provide work as it flows through the expander while depositing
lubricant on the rotor surfaces. The resulting surplus lubricant is carried by
the
flow of vapour leaving the expander to the condenser and is thus recirculated.
It may be found advantageous to provide collecting spaces (47, 48) adjacent to
the rotor bearings.
Where the source of heat is formed by the exhaust gases and cooling jacket of
an internal combustion engine, chlorotetrafluoroethane is a particularly
suitable
working fluid.
As shown in figure 5, the condenser 17 is positioned at the highest point in
the
system and the heater I and feed pump are positioned low down. Since the
expander 14 is of the positive displacement type (e.g. twin screw expander)
which can tolerate the possible presence of liquid droplets in the vapour
flow, the
separator 8 and liquid return line 9 can be omitted. Instead, the vapour from
the
evaporator section 6 is supplied by a line 51 to the inlet 13 of the expander
14.
The expander inlet 13 is at the bottom at one end and the low pressure vapour
outlet 16 is at the top of the expander (in contrast to the orientation shown
in
figure 4). Although excess oil will tend to be expelled with the vapour into
the line
15, residual oil may remain in the expander 14. This will ensure adequate
lubrication of the rotor surfaces under all working conditions, and also
improve
the sealing of the working fluid by filling up the leakage gaps formed by the
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inevitable clearances between the rotors and between the rotors and the casing
with oil.
As shown, the liquid condensed in the condenser 17 is conveyed by a line 19A
to
a liquid receiver 52 which holds a reservoir of working liquid. Liquid from
the
receiver 52 is conveyed by a line 19B to the inlet of the feed pump F. The
hydrostatic head between the condenser 17 and the feed pump reduces or
avoids the risk of cavitation in the inlet to the feed pump.
If it is found that the build of up oil in the expander is too great, an oil
return line
53, of very small bore, connects an outlet 54 in the bottom of the casing of
the
expander to the return path from the condenser to the feed pump, in this case
being connected to the liquid receiver 52. The outlet 54 is positioned just up
stream of the main outlet 16 of the screw expander in a position where the
pressure is just sufficiently higher than that in the receiver 52 to enable
the
excess oil to leave the expander.
The heater 1, preferably a plate-type heat exchanger and the liquid flow to
the
bearings of the expander may be accumulated in a storage vessel 55 before or
after cooling in the heat exchanger 30 and being supplied to the bearing
housings
23 and 24 and if necessary to the rotor surface lubricating inlet 26.
As shown in figure 6, in an alternative embodiment the working fluid is heated
in
a single pass boiler 60 in which cold liquid enters at the inlet 61 and
slightly wet
vapour leaves at the exit 62, without internal recirculation through a
separator. In
this case, the lubricant e.g. oil contained in the working fluid cannot
accumulate in
the boiler but is transported by the vapour to enter the expander 14. However,
the presence of oil in the working fluid has the effect of raising the
saturation
temperature of the vapour for a given pressure and this effect can be used to
advantage in this embodiment.
At oil concentrations of 5% or less, by mass, this temperature displacement
is, in
most cases, negligible and the working fluid thermodynamic properties are
virtually identical with those of the pure working fluid. In the case of a
boiler in
which the working fluid recirculates through the evaporator, the recirculation
flow
rate is normally at least 5 times the bulk flow of fluid through the boiler.
Thus, if
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the oil concentration is initially, say 2% by mass, the increase in
concentration of
oil as a result of evaporation of about 20% of the fluid, has a negligible
effect on
the fluid behaviour.
However, in a single pass boiler, with the same initial concentration of oil,
the
presence of oil has an increasing effect on the fluid behaviour as evaporation
proceeds. Thus, initially, as evaporation proceeds, the working fluid. behaves
as
a pure fluid. However, when 80-90% of the evaporation is complete, the oil
concentration in the remaining liquid will become significant and further heat
transfer to it, from the external heat source to the boiler, will result in
the
remaining liquid becoming superheated while retaining most of the oil. This
means that the working fluid will enter the expander 14, as a wet vapour, with
some 5-10% liquid containing a high percentage of oil. In a screw or any other
type of positive displacement expander, the presence of liquid can be
beneficial
since
i) It may help to seal the gaps and lubricate the machine.
ii) It evaporates during the expansion process and thereby decreases
the superheat with which organic working fluids normally leave the
expander 14.
Thus, the superheated liquid effectively carries the oil to the rotating parts
of the
expander and leaves an oil deposit there as expansion proceeds in exactly the
same manner as it would, if drawn from the recirculated liquid of a
conventional
boiler.
The oil build up in the expander will eventually drain or be transported into
the
condenser 17 where it will be redissolved or entrained. Thus, the cold working
fluid leaving the feed pump will contain oil. Cold liquid can therefore be
drawn
from downstream of the pump and delivered directly to the bearings without
preheating and the consequent need of a regenerative heat exchanger. Thus,
the use of a single pass boiler leads to further simplification to the
lubrication
system, as shown.
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Although it is not shown in figure 6, the arrangement of that figure could
also
include a liquid receiver arrangement of the type shown in figure 5 to collect
and
hold liquid condensed in the condenser 17 and/or excess oil from the expander.
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