mp2.1 - rankine cycle

Laboratory Experiment MP 2.1

Mechanical Engineering Rankine Cycle

Objectives:

1) To apply the First Law of Thermodynamics to the power cycle to produce an energy balance and determine the cycle efficiency.

2) To compare the practical cycle with the ideal Rankine cycle and to determine the isentropic efficiency of the turbine.

Introduction

The evolution of the steam power plant from the Newcomen engines to the modern thermal power stations is a fascinating study. Despite the enormous advances made in thermal efficiency, power output, etc., there has been surprisingly little change in the cycles used and the functions of the components. However complex a modem steam power plant, these components can still be identified (Fig. 1), i.e.

(i) The steam generator or boiler in which water is converted to steam by the transfer of heat from burning fuel, a nuclear reaction, solar radiation, geothermal sources, etc.

(ii) A work producing unit, which uses the high pressure steam to produce shaft work. Formerly the power unit was a reciprocating engine, but the vast majority of modern plants use turbines.

(iii) A condenser, in which the exhausted steam is converted to water by transferring heat to cooling water.

(iv) A feed pump which pressurises and transfers the water from the low pressure in the condenser to the high pressure in the steam generator or boiler.

The ideal cycle on which it operates is known as the RANKlNE Cycle. For the discussion of the choice of fluid, theoretical Rankine cycle and practical cycle see Appendix 1.

Figure 1: Basic components of a steam power plant

Apparatus:

The Hilton F821 Vapour Turbine (Please refer to the schematic diagram - Fig. 2)

The system is charged with R141b. The vapour generator consists of a coiled seamless copper tube through which pressurised R141b flows. The coil is immersed in an insulated tank containing hot water provided by an electric resistance heater which is housed within the tank.

As the R141b flows through the vapour generator it changes from liquid to high pressure wet or superheated vapour, according to conditions. The vapour produced then flows through a convergent-divergent nozzle and impinges on blades on the rotor of a single stage impulse turbine. The turbine is mounted on the condenser and the exhaust vapour from it passes directly over the water cooled coil. The condenser, which has a high strength glass shell, is partly flooded to provide the R141b with a few degrees of sub-cooling.

A low speed reciprocating feed pump draws condensed R141b from the bottom of the condenser and delivers it, via a control valve and flow meter, to the vapour generator for re-evaporation. An accumulator fitted between the feed pump and vapour generator smooths most of the pulsation arising from the action of the pump. A small quantity of lubricating oil mixed with the R141b and is separated from the high pressure vapour line and fed to the turbine bearings.

Rankine Cycle Laboratory Experiment MP 2.1

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Figure 2. Schematic diagram of the Hilton F821 Vapour Turbine rig


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CONTROLS

Vapour Pressure and Temperature

These are determined by the setting of the R141b flow control valve and the temperature of the water in the vapour generator tank.

Turbine output

A simple band brake dynamometer applies and measures the resisting torque to the turbine shaft. A digital meter indicates the load in Newton’s being applied by the turbine to the load cell. Knowing the radius at which this load is applied the torque may be calculated. An optical sensor senses the rotational speed of the turbine and this is displayed on a digital meter.

Condenser Pressure

This is determined by the R141b flow rate and by the temperature and flow rate of the cooling water.

R141b Liquid Feed to the Generator

The feed pump runs at a constant speed. The R141b flow rate to the vapour generator is controlled by the flow control valve at the base of the R141b flowmeter. Surplus liquid from the feed pump is returned to the condenser via a spill valve within the internal accumulator.

Hot Water Temperature

When the electric resistance heater is in use, the desired water temperature in the vapour generator is set by adjustment of a water temperature control situated to the right of the vapour generator tank. Note that the advised maximum temperature is 95°C.

Cooling Water Temperature

As with all heat engines, the performance of this unit is largely dependent on the temperature range over which it works. The maximum and minimum vapour saturation temperatures are approximately 85°C, and 10K above the temperature of the cooling water, respectively. The range over which tests can be performed will be reduced as cooling water temperature increases and it is suggested that when its temperature exceeds 22°C, cooled water should be considered.

Preliminary Procedures and Calculations:

OPERATION

Assuming the unit is correctly charged with R141b and oil and the vapour generator tank has sufficient

water (half way up the level indicator), the unit may be started.

(i) Switch on the mains supply and the "Supply" switch on the panel.

(ii) Turn on the condenser cooling water to about 50 g s-l.

(iii) Check that the temperature indicator (set to station 4) and the tachometer illuminate and that the feed pump and circulating pump operate. Movement of the feed pump plunger may be observed from the front of the panel. The circulating pump usually operates quietly but a slight movement of the water in the sight glass will indicate that it is working. If the water circulating pump is noisy or if the water circulation is thought to be sluggish, it is possible that air is trapped. The air may normally be cleared by turning the main switch on and off several times.

(iv) Adjust the R141b control valve until the flowmeter shows a small flow - say 2 gs–1 (Initially, flow may be erratic).

(v) Set the water temperature controller to about 90°C.

(vi) Switch on the vapour generator “Heater" switch and set the temperature indicator to station 4 to indicate the temperature of the water in the generator tank. This temperature should be seen to steadily increase.

(vii) Ease the dynamometer belt tensioning screw so that the turbine can spin freely.

(viii) When the water temperature reaches about 50°C the turbine should start to rotate.

(ix) Gradually increase the R141b flow rate to about 5 g s-l and adjust the dynamometer so that the turbine runs at about 10,000 rev/min.

(x) Vent any air from the condenser by pulling gently on the safety valve plunger. (If the cooling water is very cold it may be necessary to reduce the cooling water flow rate so that the condenser pressure exceeds that of the atmosphere)

(xi) Adjustment of the water temperature in the vapour generator can now be made. As a general guide, the water temperature should be between 5 and l0K above the saturation temperature of R141b at the desired turbine inlet pressure. This will usually provide a few degrees of superheat at turbine in1et. Under no circumstances must the tank water temperature (t4) be allowed to exceed 98°C. If it does, boiling water may be ejected from the vent.


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(xii) By a combination of the R141b flow rate and vapour generator water temperature, the desired turbine inlet pressure and temperature can be achieved within a range to about 450 kN m-2 gauge and 90°C.

(xiii) The turbine speed may be adjusted to the desired value by turning the screw of the dynamometer load adjustment.

Notes

(a) During running, the level of R141b in the condenser will vary according to operating conditions. If there is less than 25mm for prolonged periods the R141b charge should be increased.

(b) Any changes of R141b flow rate and/or cooling water flow rate should be made gradually.

(c) The graduations on the water temperature controller must be regarded as a guide only. The knob should be set in conjunction with the value of t4.

(d) The flowmeter float will be seen to fluctuate in position (by up to 0.5 gs-1). This is due to the combined effect of the feed pump and accumulator. The actual flow rate may be taken as the mean value.

(e) At intervals, check the water level in the vapour generator tank, particularly after felling and initial start up.

(f) The vapour generator operates most smoothly when the tank water temperature (t4) is approximately 3K to l0K above the saturation temperature of the R141b. If necessary the tank water temperature (t4), or the R141b flow rate, may be adjusted to achieve the desired conditions.

Shutting Down

(i) Turn off the "Heater" switch but leave the "Supply" switch on.

(ii) Allow the unit to run for about 10 minutes - this will reduce the tank water temperature.

(iii) Close the R141b flow control valve and run for two minutes to empty the vapour generator.

(iv) Switch off the mains - this will shut off all electrical components and close the solenoid valve in the high pressure vapour line, isolating the vapour generator from other parts.

(v) Slacken the dynamometer adjusting screw.

(vi) Shut off the cooling water supply.

CAUTIONS

(i) The high pressure cut-out on the condenser is set to close the solenoid valve and shut the heater down if the condenser pressure exceeds 200 kN m~ gauge. If this operates, check that the cooling water is cool enough and flowing at the correct rate. Check also for air in the condenser by venting through the safety valve.

(ii) The safety valve on the condenser is set to discharge at 250 kN m-2 and must not be adjusted to a higher pressure.

(iii) The temperature of the water in the vapour generator tank t4 must not be allowed to exceed 98°C, otherwise steam and hot water may be discharged through the vent.

Experimental Procedures:

1. Energy balance and cycle efficiency.

(i) An energy balance can be conducted at any operating condition, but this it is advisable to use a high generator pressure (ps) and temperature (t1) and to use the highest R141b flow rate consistent with about 3-5 K of superheated at turbine inlet. The condenser cooling water flow should be 40 to 50 gs-1 in order to obtain a low condenser pressure

(ii) Run the unit on the desired conditions with dynamometer load to give a turbine speed of approximately 20,000 rpm for at least 5 minutes before making any observations.

(iii) After noting all the parameters on the standard Observation Sheet, the test may be repeated at another set of conditions.

Note that the pressures shown are Absolute Pressures.

Absolute Pressure = Gauge Pressure + Atmospheric Pressure.

Determination Of The Thermal Efficiency Of The Cycle At A Range Of Generator Pressures (optional).

(i) This test should be run with a constant condenser pressure but with varying R141b mass flows and consequent generating pressures.

(ii) The degree of superheat should be kept approximately constant by variation of the generating tank water temperature.

(iii) The first test should be run with maximum generator pressure and maximum condenser water flow rate to give minimum condenser pressure.

(iv) The turbine should be loaded so that it develops approximately maximum power. As a rough guide, maximum power is developed at about 50-60% of the no load speed.


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(v) Having allowed the unit to stabilise note the values set out on the observation sheet.

(vi) Reduce the inlet pressure by about 100 kNm-2, adjust the value of t4, the condenser pressure and the turbine speed, and when stable repeat the observations.

(vii) Repeat at similar decrements of vapour generator pressure.

(viii) It is interesting to repeat the test with a constant vapour generator pressure, but various values of the condenser pressure.

2. Practical cycle vs. the ideal Rankine cycle and the isentropic efficiency of the turbine.

(i) It is advisable to use a high generator pressure and temperature and to use the highest R141b flow rate consistent with about 3-5 K of superheat at turbine inlet. The condenser cooling water flow should be 40 to 50 gs-1.

(ii) Run the unit on the desired conditions with dynamometer load to give a turbine speed of 18,000 to 20,000 rpm for at least 5 minutes before making any observations.

(iii) After noting all the observations given on the test sheet, another set of conditions may be set up for another test.

Results and Discussion

1. Energy balance and cycle efficiency.

It is convenient to insert the observations on a simple schematic diagram. Specific Enthalpies may be obtained from the p-h diagram for R141b.

Calculate:

The turbine power output, heat transfer rate to R141b in the generator, heat transfer rate from R141b in the condenser, Energy Balance, cycle efficiency.

Feed Pump Work. In a Rankine Cycle it is normal to neglect the work input to the feed pump since it is small compared with the work output from the turbine, however, the power absorbed by the feed pump in this unit may be estimated by using either: Dynamometer Power OR by applying the Steady Flow Equation and assuming adiabatic flow, OR by measuring the power absorbed by the electric motor.

An undesirable property of R141b is that the value of enthalpy of evaporation is only about 10% of that for water. Thus, for a given energy transfer rate, the mass flow rate of R141b must be about 10 times that for water. The result is that the feed pump power input is

proportionately greater than in a corresponding steam plant.

2. Practical cycle vs. the ideal Rankine cycle and the isentropic efficiency of the turbine.

Calculate:

Turbine Power,

Find enthalpies of R141b at entry and exit from generator (p-h chart),

Heat transfer to R141b in generator,

Heat transfer from R141b in Condenser,

Energy balance for the cycle,

Assume that 50% of total heat loss in the cycle is from the turbine (all of the internal pipework is insulated and it would not be unreasonable to assume that at least 50% of the heat loss is from the turbine).

Sketch the practical cycle on a p-h diagram.

Calculate rankine cycle thermal efficiency.

Calculate practical cycle thermal efficiency.

Calculate cycle efficiency relative to isentropic.

Find tubine isentropic work in kJkg-1 from enthalpies.

Calculate actual external turbine work in kJkg-1 (as indicated by dynamometer).

Calculate turbine external isentropic efficiency.

Comments:

The differences between the ideal Rankine Cycle and the practical cycle can be clearly seen on the p-h diagram.

The principal reasons for the discrepancy in the thermal efficiencies are:

(i) The irreversibilities in the turbine,

(ii) Cooling the condensate below the saturation temperature,

(iii) Heat losses.

It should be noted that the feed pump work input are neglected in both cases.

The external isentropic efficiency of the turbine is low due to the very small scale of the unit.

The irreversibilities in the turbine include:

(i) Bearing friction (ii) Gland friction (iii) Nozzle friction (iv) Blade friction (v) Disc friction or windage (vi) Momentum changes in idle blades (vii) Kinetic energy rejected by blades.


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Useful Data

Turbine: Nozzles- Number: 1 Type: Convergent-divergent Throat Diaameter: 1.6mm Discharge Angle: 20° to plane of rotation Rotor - Blade circle diameter: 45mm Blade inlet angle: 40° Blade outlet angle: 40° Blade height: 4.25mm Brake Pulley Effecdve Mdius: 17.5mm (i.e. radius of pulley ~ half thickness of brake band) Rotating Parts - Moment of Inertia: 40 x 10-6 kg m2 Vapour Generator: Heat transfer surface area - 0.194 m2 Condenser Heat transfer surface area - 0.132m2 Feed Pump: Swept Volume - 7.6cm3. Single acting.

Standard Atmospheric Pressure = 1013 mbar = 101.3 kN/m2

Symbols and Units

Symbol Quantity Fundamental Unit

F Force N h Specific Enthalpy kJ kg-1 I Moment of Inertia kg m2

m Mass Flow Rate (Water) kg s-1

M Torque Nm n Rotational Speed rev min-1

p Pressure N/m2 P,W Power W q Heat Transfer per Unit Mass J kg-1

Q Heat Transfer Rate W

r Radius m s Specific Entropy J kg-1 K-1 t Temperature (Customary) oC T Temperature (Absolute) K

V Volume Flow Rate m3 s-1

Angular Acceleration s-2

Angular Velocity s-1

Efficiency -

Density kg m-3

Suffixes: c Condenser e The environment, i.e. the surroundings f Friction g Generator r Refrigerant 141b t Turbine p Pump

t Water ‘ (e.g. 2') denotes a state reached after an isentropic process


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Appendix 1

Choice of Working Fluid

Nearly all modern vapour power plants use steam and operate on the Rankine Cycle or the Rankine Cycle modified by the addition of reheating and/or feed heating. The use of steam as the working fluid is historical but there are many sound reasons for its continued use despite some of its disadvantages.

Among the less desirable attributes of steam are those connected with its thermodynamic properties at lower temperatures, e.g.

(i) the saturation pressure of steam at say 25°C is 3.16 kN m-2 abs. Thus the low pressure end of the turbine and condenser operate well below atmospheric pressure and great care must be taken to exclude and eject air.

(ii) the specific volume of saturated steam at 25°C is very high (43.4 m3 kg-1 compared with air at 0.83 m3 kg-1). This means that the steam passages at the low pressure end of the power unit and into the condenser must be very large and result in bulky equipment.

In the case of a vapour power plant to work over a fairly small temperature range - say from 100°C to 25°C as in the case of a simple solar power unit, the use of steam would present many practical problems. For these reasons the working fluid chosen for the F821 Vapour Turbine is R141b.

Over the temperature range likely to be encountered, the saturation pressure ranges from about 100 kN m-2 abs. (i.e. atmospheric) to about 600 kN m-2 abs. Other advantages are (i) that ester oil is miscible with R141b, and (ii) due to its large molecular mass, the nozzle jet velocity is much lower than for steam thus the turbine can run at a lower speed.

Except for the change of working fluid, the cycle on which the F821 Vapour Turbine works is identical with its corresponding steam plant and the unit displays the same characteristics.

Theoretical Rankine Cycle

The plant diagram is as sketched, Fig. 3. Both the expander and pump are reversible - adiabatic (isentropic) and there are no pressure or heat losses in pipe work.

Applying SFEE to Vapour Generator and then to the condenser,

Heat transfer from heat source qz-w = hw - hz

Heat transfer to cold sink qx-y = hy - hz

By lst Law qnet = wnet

wnet = (hw - hz) + (hy - hz)

= (hw - hx) - (hz - hy)

= Turbine Work - Feed Pump Work

The Cycle Thermal Efficiency is defined as (Net Work/Heat from Source)

hw hx hz hy

hwh

z

In most cases the feed pump work (hz - hy) is small compared with the turbine work (hw - hx) and may be ignored, thus

h

w h

x h

wh

z

The specific enthalpies and other properties are most readily obtained from charts of properties for the fluid used, although tabulated properties may also be used.

For steam, h/s diagrams are usually used, but for R141b as used in the F821 Vapour Turbine it is more convenient to use a p/h diagram.


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Figure 3

Rankine Cycle on p/h diagram

Figure 4

The therrnal efficiency is readily seen as

h

w h

x '

hw h

z

Practical Cycle

In a practical plant, the expansion is not reversible and some over-cooling of the condensate is likely. The resulting p/h diagram is as shown in Fig.5.


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Figure 5

Isentropic Efficiency

Due to irreverslbilides in the expander, the actual work transfer is less than that indicated during an isentropic expansion.

The ratio (actual work/isentropic work) is called the isentropic efficiency.

Neglecting heat losses from the expander, the actual work is hw - hx and the isentropic effciency

is e nt r h

w h

x

hw h

x '

Other detrimental effects in practice are heat losses and pressure drop in the pipework and elsewhere, and as a result the thermal efficiency of a practical plant is significantly less than that of the Theoretical Rankine Cycle.

Scale Effects

In order to keep the operating and maintenance costs to a minimum the F821 unit has been reduced in scale to the minimum practical size. At small size the losses normally associated with the turbine comprise a greater proportion of the absolute work output. These losses typically are associated with:

Bearing friction. Shaft seal friction. Windage friction. Back pressure associated with non-condensible gases.

For these reasons the cycle thermal efficiency and turbine isentropic efficiency will be lower than that associated with a full size plant. However the trends and graphical form of turbine data will be similar to that of full size plant.


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Appendix 2

Demonstration of A Thermodynamic Cycle (Rankine Cycle).

The First and Second Laws of Thermodynamics are fully discussed in most Thermodynamics text books and among others the following conclusions are drawn:

(a) When a closed system is taken through a cycle, the net work delivered to the surroundings is equal to the net heat taken from the surroundings. (First Law)

(b) If a system operates in a cycle and produces work, it must exchange heat with at least two heat "reservoirs" at different temperatures. (Second Law)

The F821 introduces students to the concept of a Thermodynamic Cycle and to the truth of the above conclusions.

With the unit operating normally, students should identify:

(i) The system (i.e. the R141b).

(ii) The "hot source", i.e. the hot water in the vapour generator tank, from which heat is transferred to the system.

(iii) The "cold sink", i.e. the cooling water flowing through the condenser coils, to which the system rejects heat.

(iv) The places where work is transferred - identified by a shaft turning (at the turbine), or by displacement (at the feed pump).

(v) That the cycle executed by the system is made up from a series of processes at the end of which the system returns to its initial state.

These processes are:

Compression - the cold low pressure liquid from the condenser is pressurised and pumped into the generator. This requires a Work Input ( -ve by convention).

Warming and Evaporation - the high pressure cold liquid is raised to its saturation temperature and evaporated. This requires a Heat Transfer from the Hot Source (+ve by convention).

Expansion - the hot, high pressure vapour expands to the condenser pressure and in doing so produces a Work

Output at the turbine, (+ve by convention).

Condensation - the low pressure vapour is condensed to forrn low pressure co1d liquid, thus completing the cycle. This requires a Heat Transfer to the Cold Sink (-ve by convention).

Note: There is a third heat transfer i.e. the unintentional heat transfer from the R141b to the metal surfaces and thence to the environment, ( -ve by convention).

(vi) The student can then make the algebraic statement,

WQ

pumpturbinelosseskcoldsourcehotWWQQQ

sin__

By reducing the vapour generator tank water temperature and/or reducing the cooling water flow rate, the effect of reducing the difference between the source and sink temperatures can be demonstrated. The reduction of tulbine power output is obvious due to speed reduction without taking measurements. Note also that if the cooling water temperature is high(above 22-C.) then the turbine power output will be reduced due to the high sink temperature.


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mp2.1 - rankine cycle

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