1、488 2010 ASHRAEABSTRACTAir-Conditioning Components and Systems Leak Tight-ness is defined by numerous specifications/standards in termsof g/yr of refrigerant loss. Implementation of these standardsto a production leak tightness specification has been based ontheoretical models that have resulted in
2、potentially biasedspecifications that do not consider all micro-fluidic phenom-ena that can cause a given micro leak-path to be self-plugged.A generic method for leak tightness specifications knownas Equivalent Micro-Geometry (EMG) is presented. The EMGis the maximum size of a micro-channel or pin-h
3、ole that willlikely self plug during normal operation and therefore meetscurrent environmental and functional specifications. The testmethod and apparatus used to empirically develop the properEMG sizes and a correlation to refrigerant leakage isdescribed along with examples of test results and reco
4、m-mended implementation steps.INTRODUCTIONThe maximum allowed refrigerant emission or loss spec-ifications are typically specified in g/yr of refrigerant leak rate,derived from a few functional requirements. One is emissionlimits of AC systems, which are defined by numerous inter-national standards
5、such as the European Commission (EC)directive relating to emissions from AC Systems in motorvehicles (EC 2006) and others. These requirements for overallsystems and components are in refrigerant emission (such asHFC-134a) in g/yr, or “functional emission tightness specifi-cations”. Another common fu
6、nctional requirement (especiallyfor small Refrigeration systems) is the amount of refrigerantallowed to leak during the normal life cycle of a given refrig-eration system without performance loss.The total emission rate or refrigerant loss (sometimescalled leakage) is measured typically during desig
7、n andsystem validation as well as in an ongoing quality assuranceprograms using common practices such as a standard SHEDmethod (EC 2007).Two common mechanisms exist for refrigerant emissionor loss from a given refrigeration system. The first is perme-ation, which results from material selections (or
8、 usage) andcomponents or system design, and is typically validatedduring early system design qualification. The second mecha-nism is refrigerant loss due to LEAK, which is the topic of thisresearch work.Leak is defined (ASTM 2007) as “a hole, or void in thewall of an enclosure, capable of passing li
9、quid or gas from oneside of the wall to the other under action of pressure or concen-tration difference existing across the wall, independent of thequantity of fluid flowing”. Therefore leakage flow will occurdue to one or more production process defects or micro-geom-etries. The purpose of any leak
10、 tightness specification forproduction quality control is to enable detection of suchmicro-geometries (defects) which may cause any air-condi-tioning component and/or system to exceed its allowed refrig-erant loss.Due to the tight refrigerant emissions/loss allowances, theLeak Tightness Specificatio
11、n for in-line quality assurancepurposes should aim to eliminate refrigerant loss due to leaks.In other words, the required leak tightness is “no refrigerantloss due to leaks”. This leaves all or most of refrigerant lossbudgets for a given component or system to permeation orother design limitations.
12、Production leak tests are done for most of the productionprocesses with fluids other than refrigerants (air and tracerMethod to Specify and Empirically Develop Air-Conditioning Components and System Leak Tightness for In-Line Leak TestingRanajit Ghosh Hemi SagiRanajit Ghosh is a standard product and
13、 laboratory manager and Hemi Sagi is technical director of Advanced Test Concepts (ATC), Inc., Indi-anapolis, IN, USA.OR-10-051 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For persona
14、l use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 489gases such as helium or hydrogen). Some production leak testsystems are “correcting” the measured tracer gases (Helium)
15、leak rates to refrigerant loss (g/yr) using a transport modelsuch as the Hagen-Poiseuille equation (Gorti and Sagi 2006;Clodic 1997). According to this viscous flow model, flow ratesthrough circular channels for low Reynolds number flows arecalculated using:(1)where= mass flow rate = average fluid d
16、ensityd = diameter of the channelP = pressure differentialL = length of the channel (or wall thickness) = dynamic viscosity of the fluidThis model, as stated in numerous publications (Gorti andSagi 2006; ASTM R2000), is quite limited and is only appli-cable to lower gas pressure ranges. It also assu
17、mes viscousflow while slip or molecular flow regimes may be more appro-priate, also ignoring the complexity of real life behavior wheredual phase flow and mixed flow (mixture of lubricant andrefrigerant) exist. The ASTM standard (R2000) uses the sameEquation (1) for such conversion and recommends a
18、“safetyfactor of 10 or more”.Existing production types of leak tightness specificationsare commonly based on converting maximum allowed refrig-erant loss in g/yr (due to emission budgets or functional/warranty considerations) into tracer gases leak flow rates. Thechallenge is to theoretically conver
19、t these “functional”requirements into realistic production LEAK tightness speci-fications (such as helium or air maximum allowed leak rate)taking in consideration micro-fluidic phenomena and fluidmixtures properties.Currently, the literature describes theoretical calculationsfor this conversion (Clo
20、dic 1997). First, the user calculates themaximum diameter and length of a micro-channel that yieldsthe maximum allowed refrigerant for a given component orsystem (Clodic 1997). In the second step, by using this calcu-lated micro-channel geometry the user now calculates themaximum allowed leak rate a
21、t a given production test condi-tion (gas type, inlet and outlet pressure).This approach uses underlying assumptions of mathemat-ical models that are sometimes quite different and do notreflect all aspects of the actual transport mechanisms occur-ring at the micro-level (Gorti and Sagi 2006). Extens
22、ive workdone in automotive applications (Gorti and Sagi 2006) forHydro-Carbon emissions (SAE International 2005) haspointed out the limitation of such theoretical models asoutlined in Equation (1) and the complexity of the transportmechanisms at micro-levels. Air-conditioning system fluidspresent ad
23、ditional complexity due to the dual phase flow thatis frequently occurring in parts of the AC loop, and the fact thatthe fluid is a mixture of refrigerant with lubricant and additives(e.g., HFC-134a and PAG oil). Some of the transport phenomena such as micro-flow slipeffects (Gorti and Sagi 2006; Ka
24、rniadakis and Beskok 2002)when it occurs will increase gas leakage rate compared to theanalytical models (Clodic 1997) as well as the capillary forces(de Gennes et al. 2004) causing liquid to rise to the surface andevaporate/boil due to the lower ambient pressure. However,other transport mechanisms
25、will reduce the leakage rate to apoint of actually plugging certain leak paths. Surface tensioneffects (Gorti and Sagi 2006; de Gennes et al. 2004), andmicro-electrostatic Electric Double Layer (EDL) boundarylayers (Karniadakis and Beskok 2002) are some of the prom-inent phenomena observed at the mi
26、cro scale.Micro-electrostatic boundary layers are formed due to theflow of a polar fluid through a microchannel. When a polarfluid flows through a microchannel, the surfaces of the chan-nel walls acquire an electric charge, which then influence themigration of charges within the liquid. A boundary l
27、ayer offluid ions, known as the Stern layer (Karniadakis and Beskok2002), adheres strongly to the channel wall. This layer influ-ences the fluid and causes a thicker layer of excess chargescalled the Diffuse or Gouy-Chapman layer. The two layers,combined, are called the Electric Double Layer (Karnia
28、dakisand Beskok 2002). The effect of these layers are to reduce theeffective microchannel diameter that can transport the fluid,and an increase in the drag forces opposing the flow, therebyreducing or inhibiting leakage. The EDL thickness is a strongfunction of the fluid and wall electrical properti
29、es (Zeta poten-tial, dielectric constants, wall electric potential) and tempera-ture (Karniadakis and Beskok 2002). Real life fluids in ACsystems include lubricants, contaminants and additives, whichmay be as large as a few microns. During refrigerant fluid cir-culation and cold start up, the lubric
30、ant will build a boundarylayer across the refrigerant flow path walls. The EDL phenom-ena and larger particulates can cause certain micro-geometriesto get plugged after a relatively short period of time. Further-more, as the leaking refrigerant is exposed to atmospheric con-ditions, the refrigerant
31、will become vapor, generating a skin ofnon-homogeneous fluid. The effect of liquid non-homogene-ity and contamination will further reduce leak rates by “plug-ging” the leak paths. A mathematical expression of this“plugging” effect does not exist, but can be assumed to be afunction of the refrigerant
32、 type, lubricant content (% ofweight), contaminants, additive sizes, bi-polar forces, fluidviscosity, microchannel diameter, microchannel length,microchannel wall electrostatic properties and the drivingpressure and temperature.The nature of leak paths and especially ones due toproduction process fa
33、ilures can be random in shape andfrequency. Therefore, simulating such defects is practicallyimpossible. However, two boundary conditions for suchdefects exist which can be reliably reproduced using one of thefollowing two methods (Gorti and Sagi 2006). The first one isa micro-channel, a circular di
34、ameter (d) with known length (L)md4P128L-=m 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print o
35、r digital form is not permitted without ASHRAEs prior written permission. 490 ASHRAE Transactionswhere L/d 100 (see example Figure 1). The second one is asharp edge orifice where L/d is small ( 1.67 (equivalent to R SAE Interna-tional 2005) mentioned in the literature for automotive appli-cations ha
36、ve indicated similar plugging effect results.Initial tests were 96 hours long in compliance with thesoaking time per the EC 706/2007 standard test protocol (EC2007). These tests indicated that EMGs that got pluggedwithin the first 24 hours stayed plugged throughout the test atthe same test condition
37、s. For that reason all tests were short-ened to only 24 hours. Furthermore, the air flow tests ran aftercompletion of the refrigerant tests (Table 3) and repeated testsa week later showed that the plugged EMGs stayed plugged.The same EMGs were then exposed to the refrigerant in thePVTt system and th
38、e test was repeated per the test proceduredescribed in section C, proving that these EMGs remainedplugged. These earlier experiments indicated that the 2 micronEMG that plugged early on will likely stay plugged through-out the life of the refrigerant system.IMPLEMENTATION STEPS OF EMG SPECIFICA-TION
39、S FOR IN-LINE QUALITY CONTROL TESTINGThe following steps are recommended to implement theEMG concept for production leak tightness:Step 1: Conduct a larger sample of EC and/or ED testsusing the actual refrigerant and lubricant to be used for a giventype of refrigeration system using an apparatus as
40、capable asthe one mentioned above. Based on this test method, select theED or EC that meets the component refrigerant loss require-ment, and plugs within a reasonable amount of time. As shownin the above examples, 2 micron Equivalent Diameter (or 2micron Equivalent Channel) were selected.Step 2: Eac
41、h production leak test system used for in-linequality control should include the selected EMG. Periodically,challenge the test system (Figure 5) by running an acceptedpart or “master” part with this EMG attached to it (or as apermanent part of the test circuit). The test method selected,Figure 4 (A)
42、 10 micron orifice inlet before test, the centerhole open (top picture). (B) 10 micron orifice inletafter test the center hole is plugged (bottompicture).Figure 5 Implementation of 2 micron ED during in-lineleak test system periodic challenge/verificationwith Air-Micro-Flow Leak Test System. A known
43、“master” part is tested (bottom graph) and samemaster part tested again with an EMG attachedto it. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reprod
44、uction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 494 ASHRAE Transactionsgas used (helium, hydrogen or air) and test pressure parame-ters are all application/user dependent. However, the testcircuit or “master” non-leakin
45、g parts with the EMG attachedto it should demonstrate consistent test rejection above theacceptance limit when the EMG challenge test is periodicallyperformed. As an example, Figure 5 shows an intentionallylong test cycle using a 2 micron ED and a micro-flow sensor,type: intelligent molecular flow s
46、ensor (Sagi et al. 2004) usingair as the test fluid. Enhancement of throughput can beachieved by using the selected EMG along with a dynamic testsignature concept to reduce production cycle time and achievegood separation as well as acceptable repeatability and repro-ducability.Note the clear differ
47、ence in Figure 5 between measuredleak rate of AIR in both cases, clearly indicating that themaster part leak tightness is better than 2 micron ED. A known“master” part is tested (bottom graph) and the same masterpart is tested again with an EMG attached to it. Maximumaccept limits and optimized cycl
48、e time can be set using themaster with 2 micron limits.SUMMARY AND DISCUSSIONThe complexity of transferring refrigerant loss relatedleak tightness requirements of various refrigerants and lubri-cants into realistic production leak tightness specifications canbe overcome by using an empirical concept
49、 described as theEquivalent Micro Geometry. The test method and apparatus isa PVTt inside an environmental chamber designed for lowlevel and accurate refrigerant leakage measurements. Theuncertainty of measurement is defined using common uncer-tainty analysis. The PVTt inside an environmental chambersimplifies the development of refrigeration system leak tight-ness requirements. It enables one to find the correlationbetween a given refrigerant fluid mixture at operating condi-tions and Equivalent Channel (EC) or Equivalent Diameter(ED) leakage and their plugging
