Oil Circulation-Its Effects on Compressor Capacity, Theory and Experiment

Transcription

1 Prde University Prde e-pbs International Compressor Engineering Conference School of Mechanical Engineering 1972 Oil Circlation-Its Effects on Compressor Capacity, Theory and Experiment K. W. Cooper Borg-Warner Corporation A. G. Mont York Division Follow this and additional works at: Cooper, K. W. and Mont, A. G., "Oil Circlation-Its Effects on Compressor Capacity, Theory and Experiment" (1972). International Compressor Engineering Conference. Paper 9. This docment has been made available throgh Prde e-pbs, a service of the Prde University Libraries. Please contact epbs@prde.ed for additional information. Complete proceedings may be acqired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 OIL CIRCULATION - ITS EFFECT ON COMPRESSOR CAPACITY, THEORY AND EXPERIMENT K. W. Cooper, Principal Engineer York Division, Borg-Warner Corporation York, Pennsylvania A. G. Mont, Principal Engineer York Division, Borg-Warner Corporation York, Pennsylvania INTRODUCTION This paper is an otgrowth of atomotive air conditioning system analysis done at the York Division of Borg-Warner Corporation. One of the conditions for an acceptable atomobile air conditioner is that it be able to qickly cool the car interior after a period of standing in the sn. To simlate this condition, the atomotive indstry ses a so-called "plldown test". This test consists of "soaking" a car with a simlated sn load and at an elevated temperatre ntil a given inside temperatre is reached. The car is then started and the air conditioner engaged. Selected temperatres are monitored as a fnction of time. It has been fond that the performance of the compressor with a given system is important for the initial minte or two of the plldown. If for some reason there is a deterioration of performance at this time, the system cannot seem to catch p to an initially better performing compressor even thogh the capacities of the two are the same after several mintes. Dring one of the plldown tests, this phenomenon was apparent. In attempting to analyze the test, selected calorimeter tests were performed. It was fond that the system had inadvertently been overcharged with oil casing a high oil circlation rate. When these conditions were dplicated on the calorimeter, a difference in capacity of nearly 3% was noted. In order to explain this difference, the thermodynamics of Rl2-oil mixtres was investigated. This paper presents the reslts of that stdy. THERMODYNAMICS A positive displacement compressor will indct a given volme of flid at a given speed and pressre ratio. Figre 1 is a sketch of the refrigeration system. The capacity of the evaporator will be given by; If there is only refrigerant flowing, the capacity will be; Or == fs VD(hz - h1 )"'1 r r r /Vr (2) and if there is an oil-refrigerant mixtre (3) Experience has indicated that volmetric efficiency may be somewhat affected by oil circlation de to the cooling and expansion effect. However, for the first approximation, we will assme that it is not changed. Ths; Taking the ratio of (3) to (2) and sing ( 4) ; It is known that (hz - h 1 ) is less than m r (hz - h1 ) and thatf s is greater than r r m fs ; ths it cannot be told how mch r redction will occr withot some actal calclations. When the liqid mixtre enters the evaporator, it contains circlating oil (x lbs. of oil per lb. of mixtre). As it leaves the evaporator, some of the refrigerant (w lbs. of refrigerant per lb. liqid mixtre) remains in the oil as a liqid. This redces the amont of refrigerant (4) 52

3 vapor. The amont of oil and refrigerant liqid (z lbs. oil and refrigerant liqid per lb. mixtre) is given by; z = x (6) 1 - w The amont of refrigerant vapor is (1 - z). In order to find the amont of refrigerant liqid in the oil leaving the evaporator (w), it is necessary to have an eqation of oil-refrigerant solbility. This stdy is concifned with Rl2-oil mixtres. G. Bambach has developed the following eqations to represent paraffin base Rl2- oil solbility. These eqations are written in the MKS system of nits following Eropean cstom. No attempt has been made to convert to English nits, as it is a simple matter to make the reqired conversions for pressre and temperatre. For temperatres below C. (32 F.): LoglO (P') "" w-j, T' w-l.o and for temperatres above C. (32 F.): Log 1 CP') = w-~ _ w-~ T' (7) (8) -(.2338(w -.6) ) (T' ) Where; P' pressre--kg/cm 2 =.73l(Psia) T' temperatre--kelvin For oil the enthalpy to the same base as Rl2, i.e., liqid at -4 F., is fond by considering the definition; dh = c dt P Now for a typical r,ef,rigerant oil, Ths; or; cp =.43 + o.ooos (Tl J d'h = J C dt ~ T -4 p h = JrT ( (T) ) dt -4 Integrating and sbstitting the limits: (9) (1) (11) (12) h..,.43(t) +.25 T (13) * sperscripts refer to references in the bibliography. The enthalpy of the liqid leaving the evaporator is; h 2 = ( 1 - w) h 2 + w h 2 o rl where, h2 rl liqid Rl2 enthalpy at T 2. Then the enthalpy of the total mixtre leaving the evaporator is; where, (14) h 2.., z h 2 + (1 - z) h 2 (15) m rv h2rv = enthalpy of Rl2 vapor at P 2 and T 2. Reference to Bambach 1 indicates that the heat of mixing for Rl2-oil mixtres is small and ths has been neglected in eqation (15). The enthalpy of the liqid mixtre entering the evaporator is; where, hl.., X hl + (1 - x) hl (16) m rl hlrl =enthalpy of Rl2 liqid at T 1. The mixtre density at the compressor is fond in a similar manner. For the liqid portion of the mixtre; where, 1 fz w -r;;: density of refrigerant liqid at Ts density of oil at Ts. For a common refrigeration oil; /' = ( (T- 6)) 62.4 The mixtre density is then fond by a second application of eqation (17); r where, 1 z 1 - z sm.., r;. + r srv fs '"' density of refrigerant rv vapor at Ps and Ts. Again, reference to Bambach 1 indicates that the volme contraction for Rl2-oil mixtres is small and therefore has been neglected in eqation (19). (17) Eqations (6) throgh (19) may be sed to find the nmerator of eqation (5) The (18) (19) denominator is composed of the pre refrigerant properties at the indicated conditions. It shold be noted that the density ratio calclation is based on the properties at the compressor sction while the enthalpy ratio is calclated sing 53

4 properties entering and leaving the evaporator. The next section presents some of these calclations in parametric form. RESULTS In order to obtain a feeling for the magnitde of the compressor capacity correction, Figres 2, 3 and 4 have been drawn assming no change in pressre and temperatre between the evaporator otlet and compressor sction. These crves have been calclated for a range of conditions typical of atomotive air conditioning system operation. Figre 2 indicates that the capacity correction is not a strong fnction of liqid temperatre. However, the correction factor becomes sizeable.with increased oil circlation when evaporating temperatre and sperheat are held constant. As the evaporating temperatre increases, the capacity correction factor increases. Figre 3, for constant liqid temperatre and sperheat, shows that the capacity correction does not change greatly with evaporating temperatres. The dependance on oil circlation is abot the same as exhibited by Figre 2. Figre 4 shows that sperheat has by far the greatest inflence on capacity redction for a given oil circlation rate. E.g., for a 5% oil circlation the capacity correction factor decreases from.956 to.81 as the sperheat is redced from 3 F. to 3 F. The reason for this is that the solbility of Rl2 in oil is mch greater at low sperheats. Therefore, less refrigerant is available for evaporation. In an actal system, the conditions leaving the evaporator are different than those entering the compressor. This means that the compressor capacity correction mst be broken p into a density correction and an enthalpy correction as indicated in eqation (5). The density correction is calclated sing the pressre and temperatre at the compressor sction, whereas, the enthalpy correction ses the liqid temperatre before expansion and the pressre and temperatre leaving the evaporator. The two factors shold be mltiplied to obtain the compressor capacity correction. Similarly, Figre 6 shows the enthalpy correction as a fnction of oil circlation, liqid te~peratre entering the expansion valve, sperheat and satrated temperatre leaving the evaporator. To facilitate constrction of Figres 5 and 6, some minor approximations have been made. This has been done for illstrative prposes only. For best accracy, the eqations presented above along with refrigerant property sbrotines may be easily solved sing a digital compter. The example lines in Figres 5 and 6 represent the conditions of an actal test. For 7% oil circlation, the density correction is 1.3 and enthalpy correction is.54 reslting in a compressor capacity correction of.72. When the same compressor was retested with the correct oil charge, the oil circlation rate was nearly zero. The measred capacity ratio between the tests was.715. This compares favorably with the calclated correction. A second test with an oil circlation of 2% gave a calclated capacity correction of.934 and a measred correction of.92. CONCLUSION This paper has shown that the thermodynamic effects of Rl2-oil mixtres are important when measring compressor capacity or when designing systems. This is indicated by the agreement between test and calclation. It has been seen that low sperheat has a large effect on performance when there is a significant oil circlation rate. The evaporating temperatre and liqid temperatre exhibit a lesser inflence. This points ot the need for carefl monitoring of oil circlation rates. It shold be noted that this is independent of the effect of oil circlation on heat transfer. ACKNOWLEDGMENT We wold like to express or appreciation to the York Division of Borg-Warner Corporation for the se of their facilities and data. We are gratefl for the help and cooperation of all the people involved in the testing and preparation of this paper. Figre 5 is a graph of the density correction. Enter the graph with the oil circlation, draw a line vertically ntil it intersects the compressor sperheat. From this point, draw a horizontal line to intersect with the compressor satrated temperatre and then down to determine the density correction factor. NOMENCLATURE cp Specific h Enthalpy m Mass flow P' Pressre p Pressre heat of oil rate Bt/lb-F Bt/lb lb/hr Kg/sq em lb/sq in 54

5 Q Capacity Bt/hr T Temperatre F T' Temperatre K Compressor displacement c ft/hr w x z Liqid refrigerant fraction Oil circlation rate Liqid mixtre fraction Density Volmetric Efficiency Sbscripts lb/c ft CONDENSER 1 1 entering evaporator 2 leaving evaporator L liqid 2 EVAPORATOR m mixtre oil r refrigerant s v compressor sction vapor Figre 1. Refrigeration system z liqid mixtre in total mixtre BIBLIOGRAPHY 1. G. Bambach, "Das Verhalten von Mineralol-Fl2-Gemischen in Kaltemaschinen", c. F. Mller, Karlsrhe,

6 15.8 H E-< E;J,.:; :><.7 E-o H ~..: ~.6{) "' "' gz!.5 SATURATED TE!l.PERATURE SUPERHEAT ~ 2 F PERCENT OIL CIRCULATION Figre 2. Compressor Capacity Correction. Liqid Temperatre, R12-i1 Mixtres 1..9 :>: t;. 8 H r:.l,.:; [:: tl. 7..: ~ ~ "' ~.6 "' 15 LIQUID TEHPERATURE = 13 F SUPERHEAT ~ 2 F PERCENT OIL CIRCULATION Figre 3. Compressor Capacity Correction, Satrated Temperatre, 1U2-il l1ixtres 56

7 .9 LIQUID TEMPERATURE ~ 13 F SATURATED TE~IPERATUR.E ~ 3Q F o.so PERCENT OIL CIRCULATION 9 1 Figre 4. Compressor Capacity Correction, Sperheat, Rl2-il Mixtres 57

8 6 4 2pSATURATED TEMPERATURE DENSITY CORRECTION PERCENT OIL CIRCULATION Figre 5. Density Correction, R12-i1 Mixtres 58

9 3 F SUPERHEAT LIQUID TEMPERATU~ 15 F PERCENT OIL CIRCULATION z ><H..8 ~~ E-<P::: zo r.:iu SATURATED TEMPERATURE.5 Figre 6. Enthalpy Correction, Rl2-il Mixtres 59