h:/teaching/undergrad/me227/laboratory/manual/lab …engr.usask.ca/classes/me/227/me 227 course...

ME 227.3

Thermodynamics I

Laboratory Manual

c©September 2017

Department of Mechanical Engineering

University of Saskatchewan

Laboratory Report Guidelines


Each individual student will produce two laboratory reports - one for the Reciprocating Air Compressorlaboratory and one for the Vapour-compression Refrigeration laboratory. Each laboratory report shouldcontain the following sections.

SummaryThe summary should describe the purpose for doing the laboratory and the specific, numerical resultsobtained. It should contain the information essential for a reader to understand what was done and whatthe results were. It is not necessary to describe in detail how the results were obtained. Think about someonelooking at the report and wanting to find out what the essential results were in a minimum of time. Thesummary should be alone on the first page after the title page. It does not normally include any tables orfigures.

Data and AnalysisClearly present all the data taken in the laboratory and perform any calculations required. If the data areextensive it would normally be presented in an appendix. This is not necessary for the laboratories in thiscourse but the data should be clearly presented in properly captioned tables.

Discussion of ResultsThis is the most critical part of the report. Here you should discuss any differences between theoretical andexperimental quantities in the laboratory. Critically examine any assumptions made in the theory or anyinaccuracies in the experimental procedure that may explain differences in these quantities.

ConclusionsA brief summary of what can be concluded as a result of the measurements and calculations in this laboratory.

TipsHere are some brief tips to achieve a better laboratory report.

1. Make the figure captions and table titles very descriptive. Avoid brief captions like ”Results”. Thecaption should give a reasonable description of the figure even if someone has not read the rest of thereport.

2. Avoid personal pronouns.

3. Be sure to include some narrative in the calculations. Describe the calculations that are being done.

4. Cite the source of all figures that are from other sources. This should be done in the figure caption.You have permission to use figures from this laboratory manual but the source should be cited.

5. Be certain that the report is your own writing. Plagiarism is not acceptable. Your report will becompared with others.

1 ME 227.3 Laboratory Manual


ME 227.3 Laboratory Manual 2

Reciprocating Air Compressors


LABORATORY SAFETY ALERT

1. Slippery floor hazard - Clean up all water spills immediately.

2. Rotating machinery - Be VERY aware of the shaft connecting the motor to the compressor.

3. Hot surfaces - The pipes leaving the cylinder are very hot.

4. Snagging hazard - Be very careful operating the mechanical indicators. Follow the laboratoryinstructor’s instructions very closely.

Objectives

1. Determine the heat transfer to the surroundings from the cylinder heads and heat exchangers for atwo-stage reciprocating air compressor operating at constant speed against a constant back pressure.

2. Determine the relative magnitudes of the energy transfers for each component of the compressor.

3. Measure the isothermal efficiency of the compressor.

4. Measure the efficiency of the electric motor driving the compressor.

Background InformationAn idealised indicator diagram for a single-stage, reciprocating compressor is shown in figure 1. At state 1,the inlet and exhaust valves are both closed and the gas is compressed polytropically to state 2. At state 2the exhaust valve opens, and from state 2 to state 3 the gas is discharged at constant pressure. At state 3the piston is at the top of its stroke. As it begins to move down, the exhaust valve closes and the trappedgas expands doing work on the piston until state 4 is reached. At state 4, the pressure in the cylinder is lowenough for gas to be drawn in through the intake valve until state 1 is reached and the cycle is complete.Assuming a representative value of the pressure in the cylinder can be measured, the area 1234 is equal tothe work done on the air.

If heat could be transferred from the air such that the compression process from state 1 to state 2 occurredat constant temperature, then the work of compression would be less. Compression at constant temperatureis indicated by the broken line 1-2’ in figure 1. The isothermal efficiency ηiso of a reciprocating compres-sor compares the actual work to the work that would have been required if the process had taken placeisothermally.

ηiso =Wiso

Wactual

(1)

Wiso = −mRTin ln

(

Pout

Pin

)

(2)

Although cooling the gas as it is compressed would reduce the required work input, a heat transfer ratehigh enough to achieve a significant reduction in work is difficult in practice. A practical alternative is toseparate the work and heat transfer into separate processes by having the compression take place in stageswith heat exchangers called intercoolers cooling the gas between stages. As well as reducing the work input,



Figure 1: Idealised indicator diagram for a single-stage compressor with clearance.

multiple-stage compression is also necessary to keep the gas temperature from reaching excessive values whenhigh pressures are required (greater than approximately 300 kPa). Vaporisation of the lubricating oil and itssubsequent ignition are possible if the gas temperature is too high.

Figure 2 is a schematic representation of a two-stage compressor with an intercooler between the firstand second stages. This configuration is similar to the compressor examined in this laboratory. Ideally,the intercooler reduces the temperature of the gas at the inlet of the high-pressure stage to the ambienttemperature. An idealised indicator diagram for this configuration is shown in figure 3. The line 1-2-3-4represents the overall compression process assuming a polytropic compression in the low-pressure and high-pressure stages. Line 1-2-2’ represents a single-stage polytropic compression and the line 1-3-4’ represents theideal constant temperature compression process. The effect of the intercooler in reducing the work input isrepresented by area 2-2’-4-3. Note that in figure 3, the lines 4-5 and 2-7 represent the ideal, constant-pressuredischarge of gas from the high-pressure and low-pressure cylinder respectively. Lines 6-3 and 8-1 representthe induction of gas into the high- and low-pressure cylinders.

The compressor is an open system operating under steady state, steady flow conditions. The precise locationof the control volume boundaries should be carefully considered to minimise the heat losses that cannot beaccounted for with the current instrumentation installed on the equipment. The form of the energy balanceappropriate to the compressor is given on page 180 of Moran et al. (2014). The working fluid is air, which canbe considered an ideal gas for the range of pressures and temperatures at which the compressor operates. Inthis application changes in the kinetic and potential energy of the air can be considered negligible comparedto the work and heat transfers.



Figure 2: Schematic representation of a two-stage gas compressor with intercooler. The coolant used in theintercooler is water. Three other heat exchangers are also illustrated. Both the low and highpressure cylinders are surrounded by a water jacket and an aftercooler is located downstream ofthe high-pressure stage. Note that a flow nozzle is installed downstream of the receiver to measurethe mass flow of air through the compressor.

The mechanical efficiency of the compressor is defined as the ratio of the sum of the power inputs to theair to the power input to the compressor delivered by the motor. The efficiency of the motor is defined asthe ratio of the power delivered to the compressor to the power input. The overall efficiency of motor andcompressor is the product of these two.

For the purposes of this laboratory, it is sufficient to assume that the air behaves as an ideal gas with constantspecific heats (let cp = 1.007 kJ/(kg ·K)). Enthalpy changes of the water can be calculated assuming aconstant specific heat of 4.18 kJ/(kg ·K).

ApparatusStudents should familiarise themselves with all components of the apparatus, the measurements required,and the location and type of all instruments. The following information about the compressor is requiredfor the calculations.

Laboratory Procedure

1. This laboratory is to be performed by students working in pairs.

2. The instructor will describe the main features of the compressor and outline the method of analysis.

3. The instructor will start the compressor and adjust the speed and receiver pressure to the appropriatevalues.

4. Prepare a detailed schematic diagram showing the location of all instruments and other measuringdevices between the air inlet and the outlet downstream of the flow nozzle.



Figure 3: Schematic representation of an indicator diagram for a two-stage compressor with intercooling.

Table 1: Characteristics of the two-stage compressor.

LP Stage HP Stage

Number of Cylinders 2 1Bore 10.16 cm 7.62 cmStroke 10.16 cm 10.16 cmIndicator Constant 108.6 kPa/cm 217.2 kPa/cm

5. When the compressor has reached steady state, obtain an indicator diagram and determine the powerinput to the air in both the high-pressure and low-pressure stage by measuring the area of the indicatordiagrams (see Appendix A). Each student in a group should be responsible for one stage. The indicatorconstants are given in table 1.

6. Measure the water flowrates through the heat exchangers by means of a stopwatch and bucket. Thesemeasurements take some time and groups should cooperate to ensure that two sets of readings areobtained during the session for use by the group as a whole.

7. Measure all air and water temperatures where instrumentation is available.

8. Determine the mass flowrate of the air by means of the flow nozzle (see Appendix B).

9. Record the motor voltage, the motor current, the rotational speed, and the motor force. Note that themoment arm required to calculate the motor torque is 21 in.

10. Record the receiver tank pressure and the interstage pressure.

11. While taking measurements, examine the compressor for heat losses that will not be accounted forwith the data obtained.



AnalysisThe energy equation can be applied to control volumes enclosing components of the compressor to gainan understanding of the relative magnitude of various heat and work transfers. Heat losses directly to thesurroundings can also be estimated indirectly using this approach.

1. By applying the energy equation to each compressor stage, calculate the rate at which heat is transferredwith the surroundings for each cylinder head. Discuss the magnitudes of the heat transfer, worktransfer, and enthalpy change of the air for each cylinder head.

2. By applying the energy equation to the intercooler, calculate the rate at which heat is lost to thesurroundings. How large is this relative to the enthalpy change of the air?

3. Repeat the last step for the aftercooler.

4. Calculate the isothermal efficiency of the compressor using equations 1 and 2. The actual work can beobtained from the indicator diagrams.

5. Calculate the mechanical efficiency of the compressor from the indicator diagrams and the force andspeed measurements. The radius of the torque arm is 21 in.

6. Calculate the efficiency of the motor. The electrical power delivered to the motor can be calculatedfrom the current and voltage.



Appendix A: The Mechanical IndicatorKnowledge of the variation in pressure and volume in the cylinder of reciprocating machines is fundamentalto the understanding of their operation and performance. The instrument used to measure pressure andvolume variation is called an Indicator. Electrical and optical indicators are available, and are most suitablefor high-speed machine applications because the inertia of their moving parts is very low. Figure 4 illustratesa simple mechanical indicator suitable for machines of low rotational speed.

Figure 4: The essential features and the general arrangement of an engine indicator.

The main part of the indicator is a small cylinder and piston which is connected to the cylinder of themachine by means of a short tube and valve. The volume of the indicator cylinder is small compared withthe clearance volume of the machine cylinder, to prevent the diagram from being altered appreciably whenthe valve to the indicator is opened. The indicator piston is spring loaded so that its movement is directlyproportional to the pressure in the cylinder. This movement is magnified by a link mechanism connectingthe piston and a stylus. The spring constant k is expressed as the pressure required to move the stylus pointvertically through a distance of 1 cm. The stylus point moves along the surface of a drum parallel to its axiswhile the drum is rotated by a cord connected, via a reducing linkage, to the piston of the machine. Thus,while the pencil moves along the drum a distance proportional to the pressure, the drum surface moves pastthe stylus a distance proportional to the engine piston stroke and in phase with it. In this way the P -Vdiagram can be recorded on an indicator card wrapped around the drum.

When the card is unwrapped the area of the diagram can be measured by scanning the indicator diagramand using image processing software. This area is proportional to the work done per machine cycle. Inpractice, the constant of proportionally is not evaluated and the work done is usually calculated using theconcept of mean effective pressure. The mean effective pressure Pm is defined as the height of a rectangleon the P -V diagram which has the same length and area as the measured diagram.

The mean effective pressure can be regarded as that constant pressure which, if it acted on the piston overone stroke, would do the same amount of work as is done in one machine cycle. Since the actual indicatorcard is drawn to a reduced scale (figure 5), it is first necessary to find its mean height a/l and then calculate



Figure 5: Indicator card showing the measurements needed to obtain the mean effective pressure.

Pm in kPa by multiplying the mean height by the spring constant k in kPa/cm.

Pm = ka

l

where Pm= mean effective pressure (kPa),a = area of diagram (cm2),l = length of diagram (cm), andk = spring constant (kPa/cm).

Now, the rate at which work is being done on the air can be calculated from

W = PmALN

where W = the work rate (kW),Pm = mean effective pressure (kPa),A = the piston area (m2)L = the piston stroke (m), andN = the rotational speed (rev/s).



Appendix B: The Standard Flow NozzleFigure 6 shows the arrangement of the nozzle used to measure the volume flow rate of air.

Figure 6: The flow nozzle installed downstream of the receiver.

The mass flow rate of air through the flow nozzle is given by

m = CdA

√

2∆P Pn

RTn

.

where m = mass flowrate of air (kg/s),Cd = nozzle discharge coefficient=0.98 (-),A = nozzle area (m2),∆P = pressure change across nozzle (Pa),Pn = pressure of air downstream of nozzle (Pa),Tn = temperature of air downstream of nozzle (K), andR = specific gas constant (J/(kg ·K)).

Note that Cd is a dimensionless discharge coefficient. The diameter of the nozzle is 0.625 in and R =287 J/(kg ·K) for air.


The P-v-T Surface

The P-v-T Surface


1. Slippery floor hazard - Clean up all spills immediately.

2. Potentially hazardous fluids - Do not ingest any of the test fluids.

3. Tripping Hazard - Watch for cords on the floor.

4. Explosion hazard - Do not overpressure the apparatus. Wear safety glasses.

Objectives

1. To measure pressure and volume along a line of constant temperature for sulfur hexaflouride (SF6).

2. To plot a series of isotherms on P − v coordinates for SF6.

3. To compare the data to published values for this substance.

BackgroundThis experiment will allow you to explore the P-v-T surface of a pure substance. The scope will be restrictedto the vapour, liquid, and liquid-vapour mixture regions. The shape of the surface will be explored by doinga series of constant temperature experiments. Each experiment will begin in the vapour region. The volumeof the sample will be slowly decreased and the pressure and volume will be recorded allowing isotherms tobe plotted on P − v coordinates.

ApparatusThe apparatus consists of a small, variable-volume chamber with glass walls immersed in a constant-temperature water bath. The chamber contains a small amount of sulfur hexaflouride (SF6) which hasa critical pressure of 37.6 bar and a critical temperature of 45.5 ◦C. These values are very convenient becausethey allow conditions near the critical point to be investigated at relatively low pressures and temperatures.The critical point of water, by contrast, is at 220.9 bar and 374 ◦C making a similar experiment with watermore difficult to perform. The volume of the sample chamber can be adjusted by a moving plunger which ispositioned by a hand wheel. The pressure is measured by a Bourdon tube gauge. Temperature is recordedby a platinum resistance temperature detector (RTD).

In order to get good results, accurate measurement of the volume is critical, especially at very low qualitiesand in the liquid region. There are two effects that need to be accounted for (1) the expansion of theapparatus as the pressure increases and (2) the expansion of the apparatus as the temperature increases.This is done by using a simple linear equation to correct the piston position as shown in equation 3.

x = x0 + βP (P − Pref) + βT (T − Tref) (3)

where x is the corrected piston position (mm), xo is the offset (mm), P is the pressure (bar), T is thetemperature ( ◦C), βP is the dependence of position on pressure (mm/bar), βT is the dependence of positionon temperature (mm/ ◦C), and Pref and Tref are the reference pressure and temperature. Values of xo , βP ,βT , Pref , and Tref have been determined using a calibration procedure and will be provided to you.


The P-v-T Surface

to Pressure Guage

Oil

SF6

Water Bath

Figure 7: Simplified cross section of the critical point apparatus.

Procedure

1. Record the current atmospheric pressure.

2. Adjust the constant temperature bath until the temperature assigned to your group has been achieved.

3. Set the starting piston position as indicated by the instructor.

4. Slowly reduce the volume of the chamber by screwing the hand wheel in. Move in 2 mm increments.At each point, wait for the system to reach equilibrium again. In the vapour region this will take about10-20 s. In the two-phase region this can take several minutes. Record the piston position and thepressure in the spreadsheet provided.

5. Once the conditions enter the liquid region, continue by increasing the pressure in 5 bar increments upuntil a maximum pressure of 40 bar.

Data ReportingOn the one-page data report form, record the temperature of your group’s iso-therm and the measuredsaturation pressure.


Ratio of Specific Heats of a Gas




ObjectivesTo measure the specific heat ratio of air using a piston-cylinder assembly.

BackgroundA polytropic process is a thermodynamic process in which the pressure and volume are related by theexpression

PV n = constant (1)

where the value of the exponent, n, varies depending on the nature of the process. In the special case wherethe process is adiabatic, reversible, and the gas is ideal, n = k where k is the specific heat ratio. The specificheat ratio is a thermodynamic property of the gas and is the ratio between the heat capacity of the gas atconstant pressure and constant volume.

k =cpcv

(2)

A piston-cylinder assembly can be used to measure the specific heat ratio of a gas by considering an equivalentmass on a spring as shown in Figure 1. It will be shown that the effective spring constant of the gas is afunction of its specific heat ratio.

Figure 1: Diagram showing the equivalency between a piston-cylinder assembly and a spring-mass system.

When the piston, which acts as the mass, is pressed downward, the gas within the cylinder exerts a restoringforce on the piston as it is compressed, thereby acting like a spring. If the piston is pressed downward andreleased, it will oscillate at the natural frequency of the system. The period of this oscillation T is given by

T = 2π

√

m

ks(3)

where m is the mass of the piston and ks is the effective spring constant of the gas.



Because the oscillations of the piston are small, the change in pressure of the gas is small as well. Thus,the temperature change within the cylinder is negligible and the process can be assumed to be reversibleand adiabatic. The gas can also be treated as an ideal gas. To derive an expression for the effective springconstant of the gas, begin with

PV k = constant. (4)

Because PV k is constant

d(

PV k)

= 0 =∂(PV k)

∂VdV +

∂(PV k)

∂PdP (5)

or

kPV k−1dV + V kdP = 0. (6)

Rearranging for the differential pressure gives

dP =−kPV k−1

V kdV =

−kPdV

V. (7)

Since dV = xA, where x is the piston displacement and A is the piston area

dP =−kPxA

V. (8)

Substituting equation (8) into F = dPA yields

F = −

(

kPA2

V

)

x. (9)

Equation (9) shows that the effective spring constant of the gas ks within the piston-cylinder assembly isgiven by

ks =kPA2

V. (10)

Inserting this into equation (3), the general equation for the period of the piston oscilliations, gives

T = 2π

√

mV

kPA2. (11)

Rearranging for the volume of air in the piston yields

V =kA2PT 2

4π2m= A(h+ ho) (12)

or

h =

(

kAP

4π2m

)

T 2 − ho (13)

where h is the initial height of the piston measured from the graduation marks on the side of the cylinder,and ho accounts for the offset between the beginning of the graduation marks and the bottom of the cylinder.

Equation (13) reveals that there is a linear relationship between the initial piston height and the square ofthe period of the piston oscillations. The specific heat ratio of the gas can then be determined using theslope of this curve.

k =4π2m(slope)

AP(14)



ApparatusThe apparatus consists of a small piston-cylinder assembly with graduation markings on the side of thecylinder to indicate the piston position. A pressure transducer measures changes in pressure within thecylinder as the piston oscillates up and down, thereby capturing the period of these oscillations. Thepressure transducer is connected to a data acquisition unit which processes the signal for use by a computer,which plots the signal vs. time on the screen.

Procedure

1. Raise the piston to the 90 mm graduation mark and close the clamp valve of the open port on thepiston-cylinder assembly.

2. Click ‘Record’ on the bottom-left corner of the computer screen. Carefully pluck the piston using yourfinger by pushing the piston platform down 1-2 cm and then quickly releasing. Be careful not to

permanently deform the platform or the shaft. The piston should oscillate very rapidly for abrief moment. Click ‘Stop’ to end data recording. The piston may not return to its original positiondue to air leakage; this can be ignored.

3. Zoom in to the region of the plot on the computer screen where the oscillations are present by scrollingwith the middle mouse button. Determine the period of the oscillations as follows:

(a) Click the ‘Add Coordinates Tool’ icon on the toolbar above the plot as shown in Figure 2, thenselect ‘Add Multi-Coordinates Tool’. More than one multi-coordinates tool can be added to thesame graph.

Figure 2: PASCO Capstone R© software toolbar icons.

(b) Use the tool to identify the time corresponding to two separate peaks about 5-10 oscillationsapart.

(c) Use the time difference and the number of oscillations between these two peaks to calculate theperiod.

(d) Record the initial piston height and the period in the table to the left of the graph.

4. Click ‘Delete Last Run’ to clear the plot. Release the clamp valve and repeat steps 1-3 for pistonheights of 90 to 10 mm in increments of 20 mm.

5. After collecting data for each piston height and filling out the table, click on ‘Page #2’ on the top-leftcorner of the screen to display a plot of the piston height (in m) vs. the square of the period. Determinethe slope of a linear curve fit to the data as follows:

(a) Click the arrow to the right of the ‘Curve Fit Tool’ on the toolbar above the plot. Select ‘Linear:mx+b’. Then click the ‘Curve Fit Tool’ icon itself to activate the plot.

(b) If an error message appears in the graph label box, click the ‘Highlighter Tool’ and use the shadedarea that appears to select all the data points.



(c) Record the slope of the curve-fit.

6. Use equation (14) to determine the specific heat ratio, where A is the piston cross-sectional area(D = 32.5 mm) and m is the mass of the piston (35 g). For P , use an atmospheric pressure of 96 kPa.

Data ReportingOn the one-page data report form, record the slope of the curve fit, its units, and the value you obtained fork.


Mass-Lifter Heat Engine



1. Hot surfaces - Do not touch the hot-water beaker or the hot plate surface which is used to heat it.



Objectives

1. To observe the operating principles of a basic heat engine.

2. To calculate the thermodynamic work of this cycle and compare it to the useful mechanical work oflifting the mass.

3. To calculate the thermal efficiency of this heat engine.

BackgroundA heat engine is a thermodynamic cycle that converts heat to useful mechanical work. In this experiment, theuseful work of the heat engine is to raise a mass from one point to another. Figure 1 shows the componentsof this simple heat engine, as well as a schematic of the cycle represented on a P -V diagram. The heatengine consists of a piston-cylinder assembly connected to a canister by a piece of tubing. A cold-waterand hot-water beaker serve as the cold and hot reservoirs, respectively. Heat is transferred between thecylinder-canister control mass and these reservoirs by placing the canister in the appropriate beaker.

To calculate the work and heat transfer during each of the steps of the cycle, the mass of the air containedin the cylinder-canister control mass must be calculated. This is determined using the ideal gas law withthe canister at room temperature.

mair =PVair

TroomR(1)

where

P = Patm + Pcyl

Pcyl = mpistongApiston

Vair = Vcylinder + Vcanister

Note that the volume of air within the tubing connecting the cylinder to the canister can be ignored.

At the starting point of the cycle, state 1, the canister is placed in the cold-water beaker without the mass onthe platform. With the canister remaining in the cold-water beaker, the mass is then placed on the platform,which compresses the air in the control mass to state 2. Since the cold-water reservoir maintains the airin the control mass at about constant temperature, this can be well-approximated as an isothermal process(P1V1 = P2V2). The work done to compress the gas is then given by.

1W2 =

∫ V2

V1

PdV =

∫ V2

V1

P1V1

VdV = P1V1 ln

(

V2

V1

)

(2)



Figure 1: Schematic of the mass-lifter heat engine and the P -V diagram of the cycle.

The heat transferred from the control mass to the cold-water beaker is determined using the first law for aclosed system.

1Q2 − 1W2 = U2 − U1 = 0

1Q2 = 1W2 = P1V1 ln

(

V2

V1

)

(3)

The canister is then placed in the hot-water beaker. Heat is transferred to the control mass which causes theair to expand, thereby doing useful work by raising the mass to state 3. Because the mass on the platformremains unchanged, process 2-3 is a constant-pressure process.

2W3 =

∫ V3

V2

PdV = P2(V3 − V2) (4)

2Q3 − 2W3 = U3 − U2

2Q3 = 2W3 +maircv(TH − TC) (5)

Next, the mass is removed from the platform while the canister remains in the hot-water beaker. Thisreduces the pressure in the control mass, causing the piston to rise and the volume of air to increase tostate 4. The hot reservoir maintains the air temperature in the control mass at approximately the hot-waterbeaker temperature such that process 3-4 is an isothermal process. Hence, the equations for the work andheat transfer are very similar to process 1-2.

3W4 = P3V4 ln

(

V4

V3

)

(6)

3Q4 = 3W4 (7)



Finally, with the mass removed from the platform, the canister is transferred back to the cold-water beaker,causing the air to compress at constant pressure.

4W1 = P4(V1 − V4) (8)

4Q1 = 4W1 +maircv(TC − TH) (9)

The thermodynamic work of the cycle is the net work, equal to the area of the P -V graph. The thermalefficiency of the cycle is then

η =Wthermo

Qin

=1W2 + 2W3 + 3W4 + 4W1

2Q3 + 3Q4

. (10)

The thermodynamic work can be compared to the useful mechanical work of lifting the mass, which is givenby

Wmech = mgy1−3. (11)

ApparatusThe components of the heat engine were previously shown in Figure 1. The pressure within the piston-cylinder assembly is measured with a pressure transducer. This transducer is connected to a data acquisitionunit which processes the signal so it can be displayed on a computer. Ice water is used to maintain the cold-water beaker at 0◦C, and a hot plate is used to heat the hot-water beaker.

Procedure

1. With the canister at room temperature and with the mass removed from the platform, set the heightof the piston to 50 mm and close the clamp valve of the open port on the piston-cylinder assembly.

2. Click ‘Record’ on the bottom-left corner of the computer screen.

3. With the mass removed from the platform, place the canister in the cold-water beaker until the pistonstops descending. This is state 1. Read the piston height using the graduations on the side of thecylinder, and enter the value in the table on the left of the computer screen. Read the pressure off ofthe graph on the top right of the screen, and enter the value in the table as well. To assist in readingthe pressure off of the graph, click the ‘Add Coordinates Tool’ icon on the toolbar above the plot,select ‘Add Multi-Coordinates Tool’, and drag the tool over the pressure signal to be measured.

4. While keeping the canister in the cold-water beaker, add the mass to the platform and wait for thepiston to further descend to state 2. Record the piston height and pressure in the table.

5. Insert the canister into the hot-water beaker and wait for the piston to rise to state 3. Hold the canisterby the rubber stopper, as the metallic surface of the canister may become quite hot. Record the pistonheight and pressure.

6. Remove the mass from the platform while keeping the canister in the hot-water beaker, wait for thepiston to rise further to state 4, and then record the piston height and pressure.

7. Remove the canister from the hot-water beaker and place it back in the cold-water beaker to return tostate 1. Record the final piston height and pressure. Click ‘Stop’ to end data recording.

8. Calculate the heat transfer and work for each step of the cycle. Use these values to calculate thethermodynamic work, mechanical work, and cycle efficiency. Consider how the thermodynamic workcompares to the mechanical work.

9. Click ‘Delete Last Run’ and delete the data from the table to clear the plots for use by the next group.



Note: Due to small air leakages, the piston will drop very slowly, particularly when the air is at a higherpressure with the mass on the platform. For this reason, the piston height at the end of the cycle will be afew millimetres lower compared to its original position.

Data ReportingOn the one-page data report form, record Q and W for each process, the mechanical work, and the cycleefficiency.


Vapour-Compression Refrigeration Cycle




2. Hot surfaces - Some exposed pipes behind the refrigeration units may be too hot to touch.

3. Tripping hazard - Watch cords and hoses on the floor near the refrigeration units.

4. Rotating machinery - The motor and compressor are connected by a belt drive.

Objectives

1. Measure the work and heat transfers associated with the evaporator, compressor, condenser, andexpansion valve for a simple vapour-compression refrigeration cycle.

2. Determine the coefficient of performance.

IntroductionThe basic concepts necessary to understand the operation of a vapour-compression refrigeration cycle maybe found in Chapter 10 of Moran et al. (2014). The relevant material is contained in Sections 10.1, 10.2and 10.3. The discussion includes the Coefficient of Performance (COP), refrigeration capacity, the principalwork and heat transfer in each component of the cycle, the cycle as plotted on a T − s diagram and themajor sources of departure from an ideal cycle.

A schematic diagram of the cycle is shown in figure 1. Figure 2 is a sketch of the corresponding P -h diagramfor the cycle.

Beginning at state 1, the vapour is compressed from the evaporator pressure to the condenser pressure.Vapour flow from the compressor (state 2) then enters the condenser where heat is removed by the coolant(water). As a result the refrigerant is condensed and leaves the condenser as a subcooled liquid (state 3).The refrigerant then passes through an expansion valve which results in a drop in temperature and pressurealthough the enthalpy remains constant (throttling process) (state 4). The final process of the cycle takesplace in the evaporator where heat is absorbed by the refrigerant to form a vapour at state 1. The rate atwhich heat is absorbed by the refrigerant in the evaporator is known as the Refrigeration Effect.

The Cussons Refrigeration Units investigated in this laboratory use Refrigerant-134a as the primary refrig-erant. A schematic diagram of the cycle is shown on the front panel with the instruments. The compressor isa TCCI Manufacturing twin-cylinder, single-stage unit. The bore is 47.7mm and the stroke 28.0mm givinga swept volume per revolution of 100 cm3. The compressor is driven by a swinging field motor, which isrestrained by a torque arm. This arm incorporates a strain gauge load cell in its attachment to the frameand motor torque is displayed on the panel (N-m). The compressor is coupled to the motor by a toothedbelt drive having a speed ratio reduction of 1.33 : 1. Motor speed can be varied between 8 and 22 rev/s.

The condenser is of the counterflow type. The cooling water flow is in the opposite direction to the refrigerant.The water flow rate can be controlled and is displayed on the panel. The expansion valve is mounted behindthe instrument panel and the downstream line from the valve is insulated. The valve can be adjustedmanually to maintain a selected value of the low pressure. The evaporator coil is submerged in a bathof secondary refrigerant consisting of a mixture of water and glycol. The bath also contains an electric



Figure 1: Schematic of the vapour-compression refrigeration cycle.

immersion heater that provides the refrigerating load. The resistance of the heater element is different foreach unit and is displayed on the panel.

ProcedureNOTE: CONSIDERABLE TIME IS REQUIRED FOR THE SYSTEM TO REACH EQUILIBRIUM. DONOT ADJUST THE EXPANSION VALVE, COMPRESSOR SPEED, HEATER CURRENT, OR WATERFLOW RATE.

1. The instructor will review the operation of the apparatus and the instrumentation. Take particularnote of the location of the pressure gauges and the temperature gauges as these locations affect thevalidity of the results obtained for the energy transfer on the refrigerant side of the cycle.

2. Read the temperatures and pressures at all the numbered points, the current through the heater inthe evaporator, and the mass flow of refrigerant and cooling water. The compressor speed and motortorque should also be recorded.

3. You must obtain a full set of data at two different operating conditions.



Figure 2: Principal features of the pressure-enthalpy diagram for a vapour-compression refrigeration cycle.

Analysis

1. Plot the R12 data provided to you by hand on the R12 P -h diagrams. Calculate the COP for thiscycle from the diagram. This plot should be included as an appendix in your laboratory report.

2. Use the REFPROP software to get the state point enthalpies for both operating conditions. UseREFPROP to plot the cycle on a P -h diagram for one of the operating conditions.

3. Calculate the refrigeration capacity for each operating condition using the enthalpy values obtainedfrom REFPROP and the refrigerant mass flow rate. Compare these values with the power dissipatedby the heaters. Discuss the reasons for any differences.

4. Working with your partner, calculate the work done on the vapour by the compressor using enthalpyvalues for each of the two operating conditions. Calculate the power input to the compressor fromthe motor torque and speed measurements. Calculate the mechanical efficiency and the isentropicefficiency of the compressor. Discuss the losses that each of these efficiencies are meant to measure.

5. Working with your partner, use enthalpy values from REFPROP to calculate the coefficient of perfor-mance for the cycle for each of the two operating conditions. Also, calculate coefficients of performanceusing the power input to the compressor and the heat dissipated by the immersion heater. Commenton the differences between these two values and the context in which each value is a valid indicationof cycle performance.

6. You will be provided with additional data for several other operating conditions. Using these dataand the data you collected, plot both coefficients of performance described in 5, refrigeration capacity,



mechanical compressor efficiency, and isentropic compressor efficiency all as functions of compressorspeed. Discuss the results in your laboratory report.


References

References

Moran, M.J., Shapiro, H.N., Boettner, D.D. and Bailey, M.B. (2014) Fundamentals of Engineering Thermo-

dynamics (8th ed.), John Wiley & Sons.


h:/teaching/undergrad/me227/laboratory/manual/lab …engr.usask.ca/classes/me/227/me 227 course...

Documents