a simple technique for refrigerant mass measurement

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Applied Thermal Engineering 25 (2005) 1115–1125

A simple technique for refrigerant mass measurement

Erik Bjork *

Department of Energy Technology, Royal Institute of Technology, SE 100 44 Stockholm, Sweden

Received 12 June 2004; accepted 19 September 2004

Available online 2 November 2004

Abstract

A simple technique for refrigerant mass measurement is described and evaluated. First, quick-closing

valves trap the refrigerant in the section under consideration. Then, the refrigerant is expanded into a tank,

thus reaching a superheated state. Finally the mass is calculated by p–v–T relationship. The technique was

implemented on a domestic refrigerator and was computer automated (no need for manual intervention).

Preliminary 1 data are reported of the charge distribution during an on–off cycle.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Charge inventory; Void fraction; Domestic refrigerator

1. Introduction

Knowledge of the quantity of charge has a great importance for heat exchanger design and heatpump system efficiency. Recently, there has been a growing interest in hydrocarbons as refriger-ants, which creates a demand for reduced charges due to their flammability. Many studies havefound that an efficiency-maximum exists for a certain quantity of charge in a heat pump system.

1359-4311/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.applthermaleng.2004.09.008

* Corresponding author. Tel.: +46 87908602; fax: +46 8203007.

E-mail address: [email protected] A second article is intended that will focus on the experimental results rather than the experimental technique.

mailto:[email protected]

Nomenclature

R measurement resultdR uncertainty in RT temperature (�C)V volume (m3)X variabledX uncertainty in Xm mass (kg)p pressure (bar)v specific volume (m3/kg)q density (kg/m3)

Subscripti index

1116 E. Bjork / Applied Thermal Engineering 25 (2005) 1115–1125

The development of correlations for two-phase flow heat transfer and pressure-drop is a researcharea where information of the local void fraction is essential.

Hewitt [1] discussed different techniques for void measurements; including methods based onradioactive absorption and scattering, impedance, sound velocity, electro-magnetic flow, micro-wave absorption, light scattering, and volume of the liquid or vapour phase within the channel.He noted that the volume measurement or ‘‘quick-closing valves’’ technique, as being intrusive,needed repeated re-establishments of the steady-state conditions compared to the different on-linemethods. Mulroy and Didion [2] used quick-closing valves to isolate sections of a split-unit aircondition, from which the refrigerant was removed and weighed. Janssen [3] used on-line weighingof the heat exchangers to find the mean void fraction at steady-state condition. Whalley [4]pointed out that the quick-closing valves technique is a ‘‘simple if rather brutal method of meas-uring void fraction’’, but that ‘‘the method has the great advantage that the results are not open tointerpretation’’. The technique to measure mass of a superheated gas by the p–v–T relationshipwas for instance used by Philipp et al. [5].

Several of the techniques discussed by Hewitt [1] are today implemented in commercially avail-able products. However, in general all the different on-line methods are sophisticated and expen-sive, and demand special calibration and handling. In contrast, the quick-closing valves techniqueis time-consuming and needs hand-operation. Moreover, if the refrigerant is removed from thesystem and weighed, there is a risk of errors from refrigerant losses.

It is the purpose of the present paper to present an improvement to the quick-closingvalves technique. By this improvement, the technique can be made autonomous, and with-out the errors introduced from the remove-and-weigh procedure. The experiments from whichpreliminary results are reported were conducted on a domestic refrigerator during on–offcycling conditions. The results, and the proposed technique itself, are evaluated throughan error analysis and through a comparison to the conventional remove-and-weigh technique.

E. Bjork / Applied Thermal Engineering 25 (2005) 1115–1125 1117

2. Methods

2.1. The mass measurement technique

The cooling system is subdivided into control volumes (all with known volumes) by quick-clos-ing valves, hereby trapping the refrigerant. Each control volume is then expanded into a tank (alsowith known volume) large enough to ensure a superheated state. As thermodynamic equilibriumis reached (i.e. at the temperature of the ambient), the temperature and pressure are measured.Finally, the density is obtained from the equations of state (p–v–T relationship), and the massis calculated according to

m ¼ q � V : ð1Þ
From the equation it is evident that the combined volume (control volume and tank) must beknown.
2.2. Sizing of the tank

The required tank volume can be calculated (also using Eq. 1) from the maximum quantity ofcharge and the refrigerant density (as saturated vapour at the ambient temperature). The experi-ence from the present work is that the tank should be selected as large as possible to reduce thetime to reach thermodynamic equilibrium, to reduce uncertainties, and to expand the possibleparameter space (for instance a lower ambient temperature).

2.3. Experimental arrangement

The technique was implemented on a domestic refrigerator in such way that the experimentalarrangement was made automatic (no need of manual intervention during an experiment). Thiswas obtained by computer control and by letting the compressor evacuate the tank after a test(thus bringing the refrigerant back to the cooling system). Four components were investigatedfor their charges: the condenser, the filter drier, the evaporator and the compressor. In order tolimit the number of components, three of the control volumes were stepwise expanded into onecommon tank. However, the compressor was kept as a separate control volume, as being largeenough to ensure a superheated state. The system is displayed in Fig. 1. Information about thecontrol volumes is provided in Table 2.

The test object was an Electrolux ER8893C refrigerator with free convection heat exchangersand a piston compressor (ZEM HQY70AA) with low-pressure oil sump (265 ml mineral oilcharge). The nominal refrigerant charge was 34 g of Isobutane (R600a). The capacity controlwas by intermittent run (on–off cycling) with self-defrosting in every off-cycle.

The following modifications were made to the cooling system: five valves and a tank wereadded. The charge was increased by 1.3 g (total 35.3 g) in order to compensate for the charge thatcould not be evacuated from the external tank. An electrical heater with a small fan, positioned inthe cabinet, was used to accelerate the test series. Pressure taps were fitted on the tank, on thesuction line, and on the discharge line. The valves were servo actuated ball valves (Swagelok s-ser-ies) with internal diameters close to the original system. These valves give a low pressure-drop and

tank

capillary tube

condenser

evaporator Ptank

Pcond

Pcomp

cabinet

filter drier

suction valve

discharge valve

condenser valve

capillary valve

tank valve

1

2

3

4

5

electrical heater

evap in evap mid acc in

accu

mul

ator

evap out

discharge

cond highcond midcond out

filter-drier

compressor low

compressor high

suction

Co

ntr

ol V

olu

me

1C

on

tro

l Vo

lum

e2

Co

ntr

ol V

olu

me

3

+

+

Co

mp

ress

or

Co

ntr

ol V

olu

me

Fig. 1. Experimental apparatus.


are bi-stable, thus generating no heat when passive. The tank was positioned elevated, comparedto the compressor, in such way that any oil would be drawn back to the cooling system by gravity.

Overall, small modifications were made to the original system (all original components were atoriginal positions, connection pipes were kept short and with internal diameters close to the orig-inal system, thin tubes were used for external piping). As a result, the behaviour of the modifiedsystem was close to the behaviour of the original system. The experiments were conducted in closeto ISO 7371 conditions. The refrigerator air temperature was controlled to 5 ± 1 �C for a test, witha cut-in of 6 �C and a cut-out of 4 �C. The climate chamber temperature was 25 ± 1 �C with arelative humidity of 65± 10%.

A computer (with software developed in HP-VEE) controlled the compressor, the electricalheater, and the valves via a relay-box (Utronix SwitchBox44) and a servo-controller (Basic StampMini SSC II). The signal from the temperature (T-type thermocouple) and pressure transducers(Druck PDCR 4011) were AD-converted in a logger (HP 34970A) before transferred to the com-puter where it was recorded for later analysis. Positions for thermocouples and pressure transduc-ers are shown in Fig. 1.

2.4. Experimental procedure

In all, 91 tests were performed, each after re-established steady cycling conditions. By this,‘‘snap-shots’’ of the charge distribution were collected at unique times throughout the on–offcycle, together displaying the charge variation with time.


Prior to the test series the cooling system was carefully evacuated and charged so that the exactquantity of charge in the system was known. The desired points of time to be tested in the on–offcycle were entered into the computer program. After this, the experimental apparatus wasautonomous.

Each test started with pressure equalization to give the compressor an unloaded start. Then,the compressor was started with the valves positioned in such way that the tank was evacuated(thus bringing the refrigerant back to the cooling system). After this, the tank was isolated fromthe system by the tank valve. Next, four on–off cycles were completed, allowing the refrigeratorto approach steady cycling conditions. During the fifth cycle, a timer determined the momentfor the valves to close around the control volumes and to open the tank valve so that the evap-orator charge (control volume 1) was expanded. Time was then given for the system to ther-mally equalize with the tank and the ambient, a process that was accelerated by the electricalheater in the cabinet. Finally, control volumes 2 and 3 were expanded stepwise, each step withtime given for thermal equalisation. After this, a new test started beginning with pressure equal-ization and evacuation of the tank. During the entire process pressures and temperatures wererecorded for later analysis. Table 1 provides a timeline for the experimental procedure of onetest.

2.5. Calculation procedure

Calculations were made in a spreadsheet program (Excel) coupled to a refrigerant propertyadd-in program (Refprop). First, the residual charge in the tank was calculated (the pressure inthe tank was about 0.1 bar after the evacuation). Then, control volume 1, 2 and 3 by subtractingone mass from another. Next, the compressor vapour charge. Finally, the charge dissolved intothe compressor oil by adding all vapour masses and subtracting them from the total charge.

2.6. Experimental comparison with remove-and-weigh technique

The proposed technique was compared to a conventional mass measurement technique bywhich the refrigerant is extracted into a tank and weighed. First, the refrigerator compressorwas powered and run for 1 h, thus causing the refrigerant to distribute over the cooling system.Then, the valves were closed, the compressor switched off, and a tank (which was kept in a bath oflow temperature liquid nitrogen) connected to the cooling system. The refrigerant was then ex-tracted, control volume by control volume, into the low vacuum tank. At each step of the emp-tying process the tank was carefully weighed. In parallel, the mass was measured by the proposedtechnique.

2.7. Influence of oil

In the examined cooling system inevitably some oil will circulate outside the compressor. SinceIsobutane is soluble in mineral oil, any oil present will reduce the pressure in the control vol-umes, thus giving an error with the proposed technique. It is not likely, however, that oil accu-mulates in the condenser (which basically is a 14 m long, 3.5 mm internal diameter steel pipewith downward return bends), the vertically mounted cylindrical shaped filter drier, the capillary

Table 1

Experimental procedure for one test

Experimental step Time Compressor Valves

open

Valves

closed

Heater Scan rate

(scans/s)

Data used

for calculation

(1) Pressure equalizing 0 Off 1,2,3,4,5 Off 1

(2) Evacuation starts 300 s On 1,2,4,5 3 Off 1

(3) Evacuation ends 650 s On 1,2,3,4 5 Off 1

(4) Normal operationa Four

complete

cycles

On/off

cycling

1,2,3,4 5 Off 1

(5) Timing the moment

to close valvese0–17 min On or off 1,2,3,4 5 Off 1 Last scanf

(6) Measurement start,

control volume 1b0 Off 5 1,2,3,4 On 1 15 s after valves 1–4

closingg

(7) Heater off 2400 s Off 5 1,2,3,4 Off 1 5 last scans before

valve 3 opensh

(8) Control volume 2c 3000 s Off 3,5 1,2,4 Off 0.1 5 last scans before

valve 2 opensh

(9) Control volume 3d 3600 s Off 2,3,5 1,4 Off 0.1 5 last scans before

pressure equalizationh

(10) Pressure equalization 5400 s Off 1,2,3,4,5 Off 1

a Based on a separate experiment, showing that the on–off cycle was almost unchanged after the fourth cycle.b Evaporator + expansion tank + part of suction line + capillary tube.c Filter drier + note 2.d Condenser + note 3.e Controlled by computer program by elapsed time from compressor start or stop.f Expansion tank pressure and temperature.g Compressor pressure and temperature. Point of time based upon a separate experiment, in which the temperature

difference between the internal parts of the compressor (electrical winding) and the compressor surface temperature was

measured.h Average expansion tank pressure and temperature.


tube, or the suction line. On the other hand, it is possible that oil accumulates in the evaporatoraccumulator. For this reason, a separate experiment was conducted to estimate this quantity (seeAppendix A for details). It was found that only a small quantity of oil was present and a moredetailed analyse, using the Raoult�s mixing rule (cited by [6]), showed that the error introducedwas small.

2.8. Uncertainty analysis

The uncertainty estimations are based on the computerized method described by Moffat [7].Hence, input values are sequentially perturbed with its estimated uncertainty, and the individualuncertainty contributions accumulated and combined through root-sum-square. Thus, it is as-sumed that: (a) each variable is independent, (b) each variable is Gaussian distributed if repeatedlymeasured, (c) all uncertainties are expressed with the same odds, and (d) the result is a linear func-tion of the input variables (which is true for small perturbations). The estimated uncertainties areprovided in Table 2.

Table 2

Experimental quantities

Quantity Best estimate/range Uncertainty

Expansion tank volume 2.894 l ±2%

Control volume 1 3.0194 l ±2%



Compressor volume 1.766 l ±2%

Temperature expansion tank 24–26 �C ±1 �CTemperature compressora 41–50 �C ±2 �CPressureb 0.1–3 bar ±1%

Density-conversionc ±0%

Charge 35.3 g ±0.2 g

Oil in evaporatord 0.2 g ±0.2 g

a Combined uncertainty from (A) temperature measurement and (B) vapour temperature measured indirectly at

compressor surface.b Pressure in expansion tank (0.1–3 bar) and compressor case (0.8–1.7 bar).c Uncertainty in density-conversion assumed embedded in pressure and temperature uncertainties.d Based on a separate experiment. See Appendix A for details.


As a starting point, the experimental result, R, is calculated from several variables, Xi:

R ¼ RðX 1;X 2;X 3; � � � ;XNÞ:
The uncertainties are estimated for each variable so that the true value is covered with 20–1 odds(95% confidence):
X i ¼ X i ðMeasured or best estimateÞ � dX i ð20 : 1Þ:
Next, the input variables is perturbed with each estimated uncertainty resulting in the individualuncertainty contributions
dRXi ¼oRoX i

dX i:

Finally, the overall uncertainty is calculated by the root-sum-square of the individualcontributions

dR ¼XNi¼1

oRoX i

dX i

� �2( )1=2

ð20 : 1Þ:

3. Results and discussion

3.1. Charge distribution during one on–off cycle

The experimental result is shown in Fig. 2. As can be seen, the largest charge-displacementsoccur immediately after a compressor start and stop. At the start, the quantity of charge in the

0

5

10

15

20

25

0 200 400 600 800 1000 1200 1400 1600

time (s)

char

ge

(g)

evaporator

condenser

compressor oil

compressor vapour

filter drier

ON OFF

Fig. 2. Cooling system charge distribution during one on–off cycle.


evaporator is large due to off-cycle migration. Immediately after the start it decreases rapidly to aminimum of less than 4 g in half a minute. During this part of the cycle the system is stronglyunbalanced with a high mass flow through the compressor—as a consequence of a high evapora-tion temperature, and a low mass flow through the capillary tube—as a consequence of vapour inthe capillary tube. For the rest of the on-cycle, the evaporator charge increases as the filter drier,condenser, and compressor charges decreases.

At the compressor stop (after about 600 s), the system undergoes a pressure equalisation. Thiscan be clearly seen on the evaporator and condenser masses. The refrigerant moves from the con-denser to the evaporator in two and a half minutes. It is interesting to note that, for the remainingoff-cycle, the evaporator charge decreases from a peak of 22 g to a low 19 g. Apparently, therefrigerant is evaporating in the evaporator during the off-cycle, and as can be seen, dissolves intothe compressor oil and increases the mass in the compressor vapour.

Fig. 2 also provides the uncertainties, given at 95% confidence. In absolute numbers the uncer-tainties are low. The largest uncertainties are associated with the quantity of charge dissolved intothe compressor oil. This is a consequence of the calculation procedure in which this charge is cal-culated as all other charges subtracted from the total charge. Thus, several uncertainties are influ-encing this quantity. In total, 17 input variables were perturbed. The largest uncertaintycontributions (averaged over all measurement points) were caused by the uncertainties in the pres-sure (adding 0.25 g for the condenser charge uncertainty) and the uncertainty in the quantity oftotal charge (adding 0.2 g for the uncertainty of the charge dissolved into the compressor oil). Theuncertainty contribution due to the oil in the evaporator was less than 0.06 g.

Table 3

Experimental results of comparison between techniques

Volume Remove-and-weigh technique (g) New technique (g) Difference (g) Difference (%)

Evaporator + tank 12.2 12.12 �0.08 �0.7

Filter drier 2.1 2.21 +0.11 +5.2

Condenser 11.9 11.67 �0.23 �1.9

Compressor 8.8 9.0a +0.2 +2.3

Total 35.0 –b – –

a Of which 2.9 g as vapour.b By definition equal to the experimental 35.0 g that was taken as the true mass.


3.2. Comparison between the proposed technique and the remove-and-weigh technique

The quantities of refrigerant, as measured by the remove-and-weigh technique and the newtechnique, are shown in Table 3. As can be seen, the differences are small ranging from less than1% for the evaporator to 5% for the filter drier. Due to the interaction between the charge in thecompressor vapour and the compressor oil, it was only possible to measure the total compressorcharge (i.e. not explicitly the charge as vapour and the charge dissolved into the oil) with the re-move-and-weigh technique.

4. Summary and concluding remarks

The objective of this work was to describe and evaluate a refrigerant mass measurement tech-nique, based on the quick-closing valves technique and the equations of state. The results indicate,overall, that the proposed technique should make a useful contribution to future charge/voidmeasurements. Timesavings are possible through autonomous operation. Errors caused by leak-ages in the remove-and-weigh technique are eliminated. Experiments showed that the technique isaccurate.

The technique was implemented on a domestic refrigerator cooling system. A test series wascarried out from which preliminary results are reported of the charge distribution during oneon–off cycle. The results showed off-cycle migration and on-cycle redistribution of the refrigerant.In general, the error analysis resulted in low uncertainties for the experimental results. It wasfound that the amount of oil in the evaporator was small, and that its influence on the measuredcharge was small.

The proposed technique, implemented for autonomous operation, is restricted to compressorsystems in which re-established running conditions, after a measurement, is possible by pumpdown of the tank. Another (but general) limitation is if a large and unknown quantity of oil, sol-uble into the refrigerant, exists within the control volume. In this case, the errors may becomeconsiderable.

It is recommended for future work to use a tank as large as possible. By this, uncertainties aredecreased and measurement conditions are established in a shorter time. In fact, by using a tankmuch larger than the control volume, the volume and temperature of the control volume itself


becomes less important. Another possible improvement would be to electrically heat the compres-sor, thus driving the refrigerant out of the oil so that it could be directly measured.

It is also recommended for future work to use the experimental set-up itself for volume deter-mination of the control volumes. First, the tank volume is determined from measurements orgiven data. Then, the tank is charged with a suitable gas while the other parts of the systemare evacuated. Finally, the gas is expanded stepwise from the tank into the control volumes whilepressure and temperatures are observed so that the volumes can be calculated.

Acknowledgement

The SwedishNational EnergyAdministration andElectroluxABprovided support for this work.

Appendix A

A separate experiment was conducted in order to estimate the quantity of oil in the evaporator.A refrigerator of the same type was modified so that the evaporator refrigerant-line (and accumu-lator) was visualised through an inspection glass.

First, the refrigerator was run for four consecutive on–off cycles to stage a base-condition.Then, the compressor was powered for a certain time (that was varied in the experiment between0 and 40 h) before switching it off. Since the refrigerant is soluble into the oil, the refrigerant dis-solves into the relatively large compressor oil tray. After 20 h the remaining liquid in the evapo-rator was observed (using a digital camera) and interpreted for its volume. To ease theobservation the refrigerator was slightly tilted, so that liquid in the evaporator accumulated inthe corners. The result is shown in Fig. 3.

0

0.5

1

1.5

2

2.5

3

0 5 10 15 20 25 30 35 40 45

continuous compressor-on time (h)

liqu

id (m

l)

Fig. 3. Observed quantity of liquid in evaporator.


Bearing in mind that the liquid observed also includes some dissolved refrigerant (in fact sup-ported by observations at the compressor start-up), it is seen that the oil accumulates with com-pressor on time to a quantity of about 2.5 ml. At this level it remains steady even for longercompressor on times. It is also seen that the oil quantities at shorter compressor on times (repre-sentative for the normal on–off cycle) is very small. At the compressor start-up, in a regular on–offcycle, it was observed that liquid refrigerant was pumped out from the evaporator as liquid slugs.It appears that oil is carried back to the compressor at this time in the on–off cycle. On the limiteddata available and taking into account that a small quantity of oil also could be accumulated as athin oil-layer covering the refrigerant-line it was estimated that 0.2 (±0.2) g of oil was accumulatedin the evaporator at regular on–off cycling mode.

References

[1] G.F. Hewitt, Measurement of Two Phase Flow Parameters, Academic Press Inc., 1978, ISBN 0-12-346260-6.

[2] W.J. Mulroy, D.A. Didion, Refrigerant migration in a split-unit air conditioner, ASHRAE Transactions 91 (Part

1A) (1985) 193–206.

[3] M.J.P. Janssen, Cycling losses in cooling circuits, MSc Thesis, WOP-WET 89.002, Eindhoven University of

Technology, 1989.

[4] P.B. Whalley, Boiling, Condensation and Gas–Liquid Flow, Oxford University Press, 1987, ISBN 0-19-856181-4.

[5] J. Philipp, W.E. Kraus, H. Quack, Numerical simulation of refrigeration cycles with capillary tubes in on/off cycling

mode, in: Proceedings of the 4th IIR-Gustav Lorentzen Conference on Natural Working Fluids, Purdue University,

USA, 2000.

[6] J.J. Grebner, R.R. Crawford, Measurement of temperature–pressure–concentration relations for mixtures of R-12/

mineral oil and R-134a/synthetic oil, ASHRAE Transactions 99 (1) (1993) 387–396.

[7] R.J. Moffat, Describing the uncertainties in experimental results, Experimental Thermal and Fluid Science 1 (1988)

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a simple technique for refrigerant mass measurement

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