Descripiion of the bench ond its operotion

Legend to schematic view of compressor testing bench

1. Dffirential micromanometer

2. Surge tank

3. Pneumatic uncottpler

4. Max. pressure switch

5. Safety valve

6. Reciprocating compressor

7. Intermediate aír/water cooler

8. Final air/water cooler

9. Water flowmeter (íntermediate cooler)r l'1 0.1 lY ate r flow m e t e r' (fin al c o ole r)

11. Water flow control valve (intermediate cooler)

12. Water flow control valve (final cooler)

13. Check valve

14. Air tank

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15. Condensate discharge valve

16. Samplíng vatve

17. Control valve

18. Aírflowmeter

19. Calibrated diaphragm 0= 4,628 mm

20. Calibrated dínphragm Q- 5,329 mm

21. Calibrated dinphragm Q= 5,989 mm

22. 23-24-25-26. Pipes of dffirent shapes

27. D íffe r e ntial m an o m e t e r

2 8-29 - 3 0. Shut - off v alv e s

3 1 -32- 3 3 -34- 3 5. Shut-off valve s

36.Compressed aír sampling tap

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\- Pressure measurement points

Mt Delívery pressure, I't stage

r-, M2 Delívery pressure, 2"d stage

iv M3 Delívery tank pressure

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!_ Mt Pressure upstream of diaphragms

\- M5 Pressure upstream of fashioned pipes

!-

\, Temperature measuring points

t. T1 Incoming aír temperature

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\_ T2 Delivery air temperature, 7" stage (inlet to intermediate cooler)

Y- T3 Intake air temperature,2"d stage (outlet of íntermediate cooler)\ l

I' ) 1 , T4 Delivery aír temperature,2"d stage (inlet to fínal cooler)

\-. T5 Aír temperature at outlet of ftnal cooler

T6 Water temperature at ínlet to intermediate cooler

(- Tz Water temperature at inlet to final cooler

v T6 Water temperature at outlet of intermediate cooler

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Tg Water temperature at outlet of final cooler

Tto Air temperature upstream of diaphragms

Ty Air temperature upstream of fashíoned pípes

Tn Aír temperature in delivery tank

\' PN30D -,U$er's Monuol

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Description of the bench ond its operotion

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Fig 2.2 Schematíc víew of electrical control and monitoríng board of PN30D compressortesting bench

I*gend to electrícal control and moníforing panel

38. Main switch n

39. Voltmetríc swítch

40. Start and stop switch

41. Lumínous telltales

42. Wattmeter _. .:

43. Ammeter

44. Voltmeter

45. Dígital temperature display

46. Swítch for thermometric probes

47. Thermal protection reset switch

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2.2 Preporing the testing bench to storl work

Proceed as follows:

a) rnake sure that the power supply voltage corresponds to the value stated in theshipping documents.

b) connect the bench to the power mains and make sure that the voltage values in thechain correspond to the specific voltage (perform this operation by means ofvoltmeter 44 and voltmetric switch 39; permissible deviation: * l}Vo).

c) make sure that the direction of rotation of the electric motor corresponds to thevalue shown on the motor, by starting the motor and stopping it immediately withthe starlstop switch 40.

d) make sure that the oil level in the compressor case reaches ca half the distancevisible in the inspection window.

e) connect the cooling circuit to the water network and open shut-off valves 11 and12; make sure that the flow of coolant through flowmeters 9 and 10 is ca 150 lt./h.

f) make sure that pneumatic uncoupler 3 is set ̂ t ca 9 bar, that pressure switch 4 isset at ca 10 bar, and the valves linking the uncoupler and the pressure switch totank 14 are both open.

g) start the motorcompressor by means of starting switch 40.

h) make sure there are no compressed air leaks from the joints, the valves, thestuffing boxes.

i) by means of manometer M3, make sure that the pressure in the tank increasesprogressively; if it does not, check for leaks again.

j) operate the system until the pressure in the tank is ca 9 bar, and make sure thatpneumatic uncoupler 3 steps in at the specified pressure.

k) cut out the uncoupler 3 by closing the relative shut-off valve, and make sure thatthe maximum pressure switch steps in at the calibration pressure of ca 10 bar andturns off the electric motor.

l) re-open the uncoupler cut-out shut-off valve closed previously.

l 9PN30D - User'sMonuol

Description of the bench ond its operol ion

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Chopter 3.

3. Loborotory lests

3.1 Meosuring the power obsorbed by ihe compressor

3.l . l Theoret icol bockground

The power absorbed by the compressor can be measured by means of the

asynchronous motor that operates the compressor.

The electrical power P.r drained from the power mains by the motor, in fact, can be

measured with a Wattmeter (no. 42 in fig. 2.2); tts should be taken into account,

however, that some of this power will be dissipated inside the motor itself and along the

belt drive connecting the motor to the compressor: as a result, the latter will actually

receive only a fraction Pn'".. of the value P¿¡ ITIe?SUfed by means of the Wattmeter.

The mechanical power received by the compfessor is therefore given by:

P.... = P"¡4o

where:

P"r electrical power drained from the mains (kW)

P*... mechanical power actually received by the compressor (kW)

Ir overall efficiency of the electrical machine and the drive (in this case it may

be assumed It' = 0.9).

Having determined the mechanical power actually received by the compressor it is

necessary to take into account the fact that only a fraction of it, P¡, will be transferred to

the fluid, since a portion of it will be dissipated by mechanical losses inside the

compressor.

The power supplied to the fluid therefore will be:

P, = P*.sgIm

where:

Pi power supplied to the fluid (kW)

P*.". mechanical power actually received by the compressor (kW)

Im mechanical efficiency of the compressor (in this case it may be assumed

In' = 0'9) '

.:

a

PN30D - User's Monuolr ) l

Loborotory tests

The power to be supplied to the fluid, and hence the mechanical power required bythe compressor, are a function of the head specified: the purpose of this test is todetermine the evolution of power required as a function of pressure delivery measuredby means of manometer M3, i.e., the manometric ratio B between said delivery pressureand the intake pressure.

3.1.2 Test execution method

Worning

Before starting the compressor, make sure that the testing bench has been preparedfrom start-up according to the procedures described above in 9 2.2.In particular, checkthe settings of the pneumatic uncoupler (ca 9 bar) and the maximum pressure switch (ca10 bar), and make sure that the delivery pressure inside tank 14, as measured by meansof manometer M3, is sufficiently below these settings (e.g. delivery pressure = 2 - 3 bar)so that the uncoupler and the pressure switch will not be triggered during the tests.

' ,a) measure the pressure and the temperature of the air in the laboratory.

b) start the compressor by means of the start/stop switch (no. 40 in fig.2.2).

c) open fully the delivery valve 16 and any of the shut-off valves 31-32-33-34-35;then open adjustment valve 17: this will simulate the presence of a unit requiringthe compressed air flow produced by the compressor.

_--d) wait for-+he+-irne-neeessary-for+hepressure,inside-tankl4-to st+bilise-.then read-the values of the electrical power P.r by means of Wattmeter (no. 42 in fig.2.2)and delivery pressure by means of manometer M3 and enter them in Table 3.1.

e) work on adjustment valve l7: close it partly to obtain an increase in deliverypressure in tank 14; wait for the time necessary for the pressure in the tank tostabilise and repeat the readings of the pressure and electric power absorbed.Enter the values in Table 3.1.

f) repeat the process until the triggering pressure of the pneumatic uncoupler or themaximum pressure switch is reached, and enter the pressure and power drainedvalues in Table 3.3.

g) using the equations given in $ 3.1.1, complete Table 3.1, calculating the value ofthe mechanical power required by the compressor Pr"". and the value of thepower supplied to the fluid Pi,; then plot them in the chart shown in fig. 3.1.

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3.1 .3 Test results

Deliverypressure

(bar)

Manornetricratio

B

Pet(kw)

p¡ mecc

(kw)Pi

(kw)

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Ambient pressure: .. . . . . . . Ambient temperature:

Table 3.1 - Measuring the power required by the compressor

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Fig. 3.1 - Power supplíed to the fluid P¡ and mechanícal powef reqaíred by the compressor

P^"", as a function of the manometric compression ratío p

PN30D,- Useris'Mdnuol 23

Loborotory tests

3.1.4 Anolysis of the results .

The power that has to be supplied to the fluid, and hence'the mechanical powerrequired by the compressor, must display a monotone evolution increasing withincreasing required delivery pressure and hence with increasing manometric ratio p.

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3.2 Meqsuring the mqnometric chqrqcter¡siic of ihecompressof

3.2.1 Theorel icol bockground

To assess the suitabi l i ty of a compressor to a specif ic use it is necessary to know itmanomelric characteris_tic,i.e., the evolr,rtion of the manometric ratio, p, as a function ofdelivery flow-rat. This characteristic curve, in fact, makes it possible to determine, foreach value of the flow-rate required by the equipment, the head that the compressor isable to supply (or, vice versa, for each value of the head required, determine the flow-rate that the compressor can supply). The purposes of this test therefore consists ofplotting this characteristic curve by measuring the delivery flow-rat from the compressorwith the aid of a flowmeterno. 18 in fig. 2.1) for varying delivery pressure, as measuredwith manometer Mr.

Since the flowmeter supplies a reading of the gas flow expressed in Nm3/h, i.e., withI reference to specific fluid pressure and temperature conditions (called normal conditions

and corresponding to p - 760 mmHg,T = 20" C), it is necessary to measure the pressureand the temperature of the fluid in the proximity of the flowmeter during theexperimental measurements, by means of manometer M5 and thermometric probe T11,and to calculate the actual flow-rate with this formula:

rn = l.2rrr, tr-T"-' 1 ip , I r

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wnere:

úr is the actual flow-rate (kg/h)

rhr is the flow-rate measured during the tests (Nm3¡n)

L2 is the density of the air in the reference conditions (kglrn')

Po is the pressure of the air in the reference conditions (corresponding to 760mmHg, i.e. 1.013 bar)

ps is the pressure of the air measured during the tests

To is the temperature of the air in the reference conditions (i.e. 20' C)

Tt I is the temperature of the air measured during the tests

Finally, since in addition to being affected by the manometric ratio, p, the delivery airflow from the compressor also depends on the intake conditions, in the study ofcompressors it is common practice to refer to a corrected flow-rate, rh *, which standsfor the delivery flow-rate in reference conditions, using the following relationship:

PN30D - User's Mcnuol 25

Loborotory tests

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where:

rh * is the corrected flow-rate (kg/h)

rn is the effective flow-rate measured during the tests (kg/h)

po is the reference atmospheric pressure (usually 760 mmHg)

. pamb is the atmospheric pressure measured during the tests

To is the reference ambient temperature (usually 20' C)

Tu,n6 is the ambient temperature measured during the tests

N'oté

In the formulas given above, the atmospheric pressure can be expressed in any unit ofmeasure, but temperatures have to be expressed in degrees K.

3.2.2 Test execution mefhods

Worning

Before starting the compressor, make sure that the testing bench has been preparedfor start-up according to the procedures described above in $ 2.2.In particular, check thesettings of the pneumatic uncoupler (ca 9 bar) and the maximum pressure switch (ca 10bar), and make sure that the delivery pressure inside tank 14, as measured by means of

manometsr M3, is sufficiently below these settings (e.9. delivery pressure = 2 - 3 bar) sothat the uncoupler and the pressure switch will not be triggered during the tests.

a) measure the pressure and the temperature of the air in the laboratory.

b) start the compressor by means of the starlstop switch (no. 40 in fig.2.2).

c) open fully the delivery valve 16 and any of the shut-off valves 3I-32-33-34-35;then open adjustment valve 17: this will simulate the presence of a unit requiringthe compressed air flow produced by the compressor.

d) wait for the time necessary for the pressure inside tank 14 to stabilise, then readthe value of the delivery pressure by means of manometer M3, the value ofdelivery flow-rate by means of flowmeter 18 and thérmometric prove Trr andenter them in Table 3.2.

e) work on adjustment valve 17: close it partly to obtain an increase in the value ofdelivery pressure in tank 14; then wait for the time necessary for the. pressureinside the tank to stabilise and take again the readings of delivery pressure with

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Chopter 3,

t- manometer M3, delivery flow-rate by means of flowmeter 18, and the values of

!- fluid pressure and temperature in the proxirnity of the flowmeter by means of- manometer M5 and thermometric T¡ l; enter the values obtained in Table 3.2.\.-

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a g) using the equations given in $ 3.2.1, complete Table 3.2, determining the values of

manometric compression ratio, the effective flow-rate and the conected flow-rate,

- then plot the characteristic manometric curve of the compressor in the chart shownw in fts..3.2. /

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PN30D - Useris Monuol 27

Loborofory tests

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Manom.ratiop

rhr

flowratemeasured(Nm3/h)

Pressure atflowmeter

M5(bar)

Temperature a t

flowmeterT r r('c)

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Effectiveflowrate(kslh)

Correctedflowrate(ks/h)

I

3.2.3 Test results

Ambient pressure: .. . . . . . . Ambient temperature: . . . . . . . . . . . . . . . .

Table 3.2 - Measurement of the manometric characterístic of the compressor

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10 15 20 25 30m. (kg/h)

Fíg.3.2 - Manometric characteristic: manometric compression ratío p as afunction ofcorrected flowrate rn*

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\- 3.2.4 Anolysis of lhe resulls\-

C The characteristic manometric of a reciprocating compressor has a virtually vertical

r i evolution: with increasing required delivery pressure, and hence with increasing

.- manometric ratio, p, the delivery flow-rate remains approximately constant, showingv appreciable troughs only for high values of p, as the normal utilisation range includes\\-' values of the manometric ratio well below the limit B¡¡6 coffesponding to zero flow (see\- g 1.4.3) .

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3.3 Meosuring the intoke oir flow ond the delivery flow

3.3.'l Theorelicol bockground

In the study of reciprocating compressors, it might prove useful to determine, as afunction of p, not only the evolution of the delivery flow-rate, as described in theprevious test, but also the evolution of the flow of air taken in by the compressor: acomparison between these two flow-rates, in fact, might enable us to assess the leakstaking place during the fluid compression stage. The purpose of this test is to obtainsuch data, by measuring the delivery flow-rate with a flowmeter (no. 18 in fig. 2.1) andthe intake air flow-rate by means of a nozzle connected to a differential micro-manometer (no. I in fig. 2.I), for varying delivery pressure, as measured withmanometer IVI3.

For the determination of the delivery flow-rate, see the indications provided in $3.2.I: the intake air flow, instead, should be determined by keeping in mind thefollorying considerations.

The nozzle, situated at the inlet of a surge tank (no. 2 in fig.2.1) placed before thecompressor, creates a narrowing point in the fluid passage section, resulting in anincrease in the velocity of the fluid and hence a reduction in pressure in the narrowsection. By applying Bernoulli's theorem, we can demonstrate that the specific flow-raterrr,of the fluid current is directly proportional to the square root of the pressure differentAp between the narrow section and the environment upstream: r,.

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rrr" = X^/lp

K being a proportionality factor depending on nozzle geometry and the air pressureand temperature conditions in the intake environment. The flow-rate may therefore bemeasured in a relatively easy manner, as a difference in pressure.

The pressure difference being very small, it is determined with the aid of a micro-manometer.

The flow-rate can be worked out easily, without having to rely on formulas, by meansof charts of the type shown in fig. 3.3; with these diagrams, the flow-rate can becalculated at once, based on the measured value of np.

In actual fact, the values read in the diagram should be corrected further as a functionof the ambient conditions (atmospheric pressure and temperature): however, if thedifferences in pressure and temperature from the standard values are small enough, thiscorrection can be neglected.

The presence of the surge tank upstream of the nozzle is indispensable to dampen theoscillations in Ap which would be produced by the pulsing evolution of the air flowtaken in by the compressor.

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NOZZLEBoccaglio d=19.05 mm (-75'l)

p=760 mm Hg t=15'C

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PN30D - User,'s Monuol 3 t

3.3.2 Tesf execution method

Worning

Before starting the compressor, make sure that the testing bench has been preparedfor start-up according to the procedures described above in $ 2.2.In particular, check thesettings of the pneumatic uncoupler (ca 9 bar) and the maximum pressure sr.vitch (ca i0bar), and make sure that the delivery pressure inside tank 14, as measured by means ofmanometer M3, is sufficiently below these settings (e.g. delivery pressure = 2 - 3 bar) sothat the uncoupler and the pressure switch will not be triggered during the tests.

a) measure the pressure and the temperature of the air in the laboratory.

b) start the compressor by means of the start/stop switch (no. 40 in fig.2.2).

c) open fully the delivery valve 16 and any of the shut-off valves 31-32-33-34-35;then open adjustment valve 17: this will simulate the presence of a unit requiring

, , , ,' ,h" compressed air flow produced by the compressor.

d) wait for the time necessary for the pressure inside tank 14 to stabilise, then readthe values of delivery pressure by means of manometer M3, the pressure drop atthe nozzle by means of micro-manometer 1, the delivery flow-rate by means offlowmeter 18, the pressure and the temperature of the fluid in the proximity ofthe flowmeter by means of manometer M5 and thermometric probe T11 and enterthe values obtained in Table 3.3.

pressure in tank 14; wait for the time necessary for the pressure in the tank tostabilise and repeat the readings of the delivery pressure by means of manometerM3, the pressure drop at the nozzle by means of micro-manometer 1, the deliveryflow by means of flowmeter 18, the pressure and the temperature of the fluid inthe proximity of the flowmeter by means of manometer Ms and thermometricprobe T¡1, then enter the values obtained in Table 3.3.

repeat the process until the triggering pressure of the pneumatic uncoupler or themaximum pressure switch is reached, and enter the relative values in Table 3.3.

using the equations given in $ 3.2.1, complete Table 3.4, calculating the values ofthe manometric compression ratio and the effective delivery flow; using the chartshown in fig. 3.3 determine the value of intake flow; finally enter in the chartshown in fig. 3.4 the evolution of the effective delivery flow and the intake flowas a function of manometric ratio B.

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Loborotorv tests

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32 Dídocto ltolio

Chopier 3,

3.3.3 Tesl results

Ambient pressure: .. . . . . . . Ambient temperature: . . . . . . . . . . . . . . . .

Table 3.3 - Measurement of the delivery flow and the compressor intake flow (test datameasured)

Deliverypressure

M3(bar)

m l

Deliverypressuremeasured(Nm3/h)

Pressure atflowmeter

M5(bar)

Temperatureat flowmeter

T l t

("c)

Ap

Pressure drop atthe nozzle(mmHzO)

Table 3.4 - Measurement of the delívery flow and the compressor intake flow (calculatedvalue)

Deliverypressure

M¡(bar)

Manometricratio

op

m l

DeliverypressuremeasuredlNm3/h)

fn

Actual deliverypressure(ks/h)

m ^

Intake flowrate(ks/h)

PN30D - User's Monuol ? a

Loborotory tests

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m(kg/h)

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Fig. 3.4 - Evolution of intake flow rhaand delivery flow rn ds afunction of the manometríc

compressíon ratio, p

3.3.4 Anolysis of the results

Rased on the,{revious disclrssion of the manometric characterisfic crrrve, theevolution of the delivery flow and the intake flow is generally nearly constant forvarying p, displaying a progressive reduction only for high values of P. A comparisonbetween the delivery flow and the intake flow can also make it possible to determine theentity of the leaks that take place at the fluid compression stage; however, the fluid flowlost through the léaks is generally modest (mostly smaller than 5Vo of the delivery flow-rate)and hence its determination by means of the measuring units habitually employed atindustrial level, such as flowmeters, diaphragms and nozzles, whose accuracy is of theorder of 2-3Vo, will often prove difficult.

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