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Determining the Air Content in Small Refrigeration Systems V Purdue University Purdue e-Pubs International Compressor Engineering Conference School of Mechanical Engineering 1976 Determining the Air Content in Small Refrigeration Systems V. Gustafsson Follow this and additional works at: https://docs.lib.purdue.edu/icec Gustafsson, V., "Determining the Air Content in Small Refrigeration Systems" (1976). International Compressor Engineering Conference. Paper 232. https://docs.lib.purdue.edu/icec/232 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html DETERMINING THE AIR CONTENT IN SMALL REFRIGERATION SYSTEMS Vibeke Gustafsson Danfoss, Nordberg, Denmark INTRODUCTION DEFINITIONS For many reasons the presence of air in In the following, references to refrigera­ a refrigeration system is unfortunate. tion systems are taken as meaning complete Not only does it reduce the capacity of units in domestic use: the refrigeration system, but more impor­ tant, it destroys the valve system of the Refrigerators compressor, since it promotes the formati­ Deepfreezers on of carbon i.e. during the oxidative bre­ Air conditioning units ak down of the refrigerator oil. In a her­ etc. metically sealed Freon refrigeration cir­ cuit using a compressor lubricated with mi­ Described here is the way in which the NC­ neral oil there exists - other things being gas content in such complete systems is de­ means N nitrogen equal - a simple correlation between air termined. The term NC-gas 2 life time. oxygen and Ar Argon, and it is short for content and compressor o2 It is therefore in the interests of both non condensable gas. the compressor manufacturer and the appli­ ance manufacturer to minimize the air con­ tent in a refrigeration system. NC-GASSES IN REFRIGERATION SYSTEMS To do this effectively, it is necessary to have a measuring procedure for determi­ As an introduction a resume of where the ning the air content. Gas chromatographic NC-gasses in a refrigeration system come analysis of refrigerants in connection from and what damage they produce is given. with life tests on compressors and the ma­ The description is a little brief but ser­ terials used in their manufacture have been ves only to indicate the necessity of gas known and used for several years by com­ analysis. pressor manufacturers, but so far this technique has not been applied to any great In our opinion the most important source of extent for rutine control of air in finis­ failure is the evacuation of the assembled hed refrigeration systems. We use gas system before charging with Freon. chromatography for such measurements, and In a refrigerant system the free gas volume find it is a suitable method in connection typically amounts to 2-3 1, and the air with modern refrigeration system assembly content in this volume has to be removed. line: It is quick and requires only little With plainly deficient or just deficient mannpower. The equipment ne·cessary is in­ evacuation the complete system will contain expensive to procure and easy to maintain. air; NC-gas. The technique is generally known and re­ But this can also be found in a well evacu­ cognized. ated, finished refrigeration system partly This article describes how the absolute that which is not evacuated from the oil in a refrige­ partly that which is charged together with air quantity (N 2 , 0 2 and Ar) ration system is deEermined by first de­ the refrigerant. As is well known oil can termining the vol % of NC-gas (by means of contain air and water which is difficult to gas chromatography) in a small sample of remove (1), but these amounts are so small the gas charge in the system and then, on that they will hardly cause failure. the basis of this and knowledge of other If the refrigeration system is charged with system data, calculating the total air con­ Freon liquid direct from supplyers' tanks tent. Finally, an error estimation is made -via a charging hoard - any NC-gas which for the method. can enter the system will be very small. On the other hand it is our experience that 480 it is just these charging processes which Gas chromatography is an other way of ma­ are often critical, because of leaks, line­ king such determinations. connections etc. This method is a normal and reliable one As a final source of failure we can consi- for whole or part separations of gas mixtu­ der how well the refrigeration system is res in individual components. The more dif­ sealed. A system which sooner or later de­ ferent these components are the easier it velops leaks on the pressure side will is to separate them one from another by loose its charge. If leaks appear on the gas chromatography. suction side the compressor will periodi­ In the case of NC-gas in Freon it is fairly cally suck in air. Air diffusion due to easy to separate the NC-gasses from Freon, partial pressure differences must also be because the individual gas molecules horned are in mind. very different in size and polarity. The principle of gas chromatography is as Hhen the compressor is not running the air follows: in the system 3 will not cause damage. The A small sample - one or more em - is pla­ oil and the other materials used are com­ ced on a column, which has been filled pletely with stable in air at room temperature. a suitably porous material. A carrier gas flows constantly through the column carry­ When the refrigeration system is put into ing the sample with it. Each sort of operation mole­ and the compressor begins to run cules in the sample will move through the the presence of air will produce two speci­ column at a different speed and separation al effects: will occur. The first will influence system capacity. Each component of the sample is lead by the The compressor and condenser are not effi­ carrier gas into a detector. ciently utilized. The degree of influence The most suitably is a hot Wire detector or will depend very much on how much of it similar, i.e. an electric bridge concisting there is in the system in relation to sy­ of two platinium wires havj_ng the same e­ stem dimensions and load. With a rising lectrical resistence, but each placed in content of NC-gas, discharge pressure rises its own chamber (~ borings in a metal as wll as the compression end temperature. block) . The hot wires form part of a wheat­ An example of this is shown in fig. 1. If stone bridge circuit and are kept hot by the NC-gasses are removed from the system the bridge current flow. When this is a it will operate as it was meant to. In o­ constant the temperature of the wires (~ ther words the effect is reversible. the resistence) will be determined by the The second effect may damage the valve conductivity of the gas in the chambers i.e. function. carrier gas and carriergas + sample compo­ If the NC-gasses are allowed to remain in nent. Since the carrier gas is choosen so the system for long periods it will result that its conductivity is different to the in irreversible changes like thermal break gasses under analysis even small amounts of down of the oil around the valves and oxi­ sample in carrier gas will change its con­ dative break down because of oxygen in the ductivity resulting in a difference of vol­ NC-gasses. tage between the hot wires. This voltage This problem is probably by far the worst difference is used as measuring reference arising from NC-gasses in the system. It and with the help of electronics the chan­ will show up mainly as leaky valves there­ ge is registered as a function of time on a by reducing compressor capacity, increasing normal recorder. valve temperature etc. The detector signal is also sent to an in­ Even though the system is evacuated and re­ tegrator which convert it to diqital form charged the compressor will have been dama­ enabling a calculation to be made of the ged to the extend where replacement is ne­ component concentration. cessary. As will be seen, the sensitivity of such a detector varies partly with the material measured and partly with the amount of the GASANALYSIS IN GENERAL material being measured. For this reason the detector must be calibrated. To be able to control the evacuation and This is done by analyzing a standard gas charging processes mentioned it is necessa­ mixture, the composition of which must of ry to have a method for determining the to­ course be known qualitativly and quantita­ tal content of NC-gas. tivly. For example air can be used with a The NC-gas content in refrigerants is tra­ volume lo-loo times smaller than the volume ditionally determined by suitably freezing used for the analysis itself. out or absorbing the refrigerant. Then the For each of the components in the standard amount of gas residue can be determined by gas a factor is calculated corresponding to a pressure - volume measurement. concentration divided by integrator output. For taking these types of measurements the­ This factor is an expression for the sensi­ re has over the years been developed speci­ tivity of the detector to the component and al glass equipment, measuring procedures concentration range concerned. etc. ( 2, 3, 4,) • For years gas chromatography has been used 481 by compressor producers in connection with sturb the system.
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