The Development of High Efficiency Compressors by Reducing Suction Gas Temperature

Transcription

1 Purdue University Purdue e-pubs nternational Compressor Engineering Conference School of Mechanical Engineering 1982 The Development of High Efficiency Compressors by Reducing Suction Gas Temperature H. Kawai H. Nishihara K. Hamada S. Nakaoka Follow this and additional works at: Kawai, H.; Nishihara, H.; Hamada, K.; and Nakaoka, S., "The Development of High Efficiency Compressors by Reducing Suction Gas Temperature" (1982). nternational Compressor Engineering Conference. Paper This document has been made available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at HerrickEventsorderlit.html

2 THE DEVELOPMENT OF HGH EFFCENCY COMPRESSORS BY REDUCNG SUCTON GAS TEMPERATURE Hideki Kawai, Hidetoshi Nishihara, Koshi Hamada, and Seishi Nakaoka Compressor Engineering Division Matsushita Reiki Company, Ltd. Fujisawa-shi Kanagawa, Japan 251 ABSTRACT Higher efficiencies (reduced KWH consumption) and lower noise levels are the two most sought after features of household refrigerator and freezer compressors the world over. This paper discusses a method by which the suction muffler system of a reciprocating compressor is modified to improve the compressor's efficiency and noise level by reducing suction gas temperature. With low side shell, hermetic compressors, it is very difficult to move the suction gas into the cylinder from outside the compressor shell without a very significant increase in suction gas temperature. Further, when we think about the problems of mass production of high efficiency compressors with suction gas heat separation, there are many other problems to be solved involving reliability, noise levels and manufacturing techniques. This new suction gas muffler system increases compressor efficiency by reducing suction gas temperature. ts operating characteristics offer reliability, lower noise levels,_ and the design lends itself to mass production manufacturing techniques. w :: :::> r.n en w ::.. 1. Suction Gas Superheat and ts nfluence on the Compressor. nitially, we will explain the effect of the suction gas superheat with R-12. Figure l shows the pressure-enthalpy diagram at suction gas temperature T 1 and T 2 (T 1 < T 2 ). The following. three relationships are well known: l Pd f's vl< v2 h2 - h < T3< T4 T1. l T4 T3 h3 - h4 Vl V2 ENTHRLPT '' hl h4 h2 h3 Fig. 1. Pressure-Enthalpy Diagram 1-1. Specific Volume The theoretical value "G" of mass flow (KgH) is expressed with the following equation: G "" 6 X N X VD v ( 1) 222

3 Therefore, if the mass flow G1, is at T 1, then G 2 is at T 2. Refrigerant capacity Ql and Q2 (2) (K calhr) is in direct proportion to G. Using the adiabatic exponent curv1 at a high-side pressure of kgem~ and a low-side pressure of 1.33 kgem~.. The adiabatic exponent "K" varies 3.9% in a process of constant entro~y cornpression (from Ps = 1.33 kgem, T1 = 363K to Pd = kgcm2, T 2 = 413 K). Then the refrigerant capacity is inversely proportional to the specific volume (m3kg). ( 3) K varies.18% with a reduction of about l C suction gas temperature at a Ps = 1.33 kgcm2 and constant pressure. ll N The specific volume can be obtained by using the R.C. Downing formula. v - bo = 5 RT + V-bo ~ P -p~l i=2 Ai + BiT + Cie -KT,i C(V-bo)2 TC (4) N f,!3.73 Kgc1 f,!.33 Kga,..18 X... N N >" lfl The relationship between specific volumes and temperatures T at a suction pressure of 1.33 Kgcm2 is shown in Figure 2. We can see an improvement of about 3% in the specific volume with a 1o c reduction of the suction gas temperature. P::1.33 Kgem o.bs - " :!! l! i 3X ~--+---r--- ~L--+l-L+--~--~--~--~ TEMPERATURE C "Kl Fig. 2. Specific Volume versus Suction Gas Temperature 1-2. Work of Compression The work of compression W is usually expressed using the polytropic exponent "n". n-1 W (KW)!! X Ps D X l(pd) --n n Ps We can see that the polytropic exponent "n" is significantly affected by temperature variations. f we consider the case when the adiabatic exponent "K" is used as the polytropic exponent, then the relationship between adiabatic exponent "K", temperature and pressure is shown in Figure 3. Fig TEMPERATURE ( "KJ Adiabatic Exponent with Gas Temperature n addition, the polytropic exponent "n" and the adiabatic exponent "K" are generally expressed as: 1 ~ n< K K's variation (fluctuation) by temperature is similar to the variation of the polytropic exponent. Using Formula 5, we can consider the work.of compression Wl and w 2 at suct1on gas temperatures T1 and T2. The ratio of W1W2 compared to the adiabatic exponent "K" (which is similar to the polytropic exponent's variation) is shown in Figure 4. n Figure 4, w1;w 2 closely approximates K's variation (wh1ch is approximately n's variation). "n" has a variation of about.18% with a 1 C decrease in the suction gas temperature. Therefore, we can estimate that w 1 ;w2 is about.2%. As mentioned earlier, this confirms that the theoretical work of compressor is fixed even if the suction gas temperature varies. 223

4 tn ~------~ ~~----~ a ADABATC EXP~NENT.. k (.J Fig. 4. Work of Compression Ratio with Adiabatic Exponent. (Ko = 1.119) 2. New Suction System Therefore, a decrease in the suction gas temperature will increase com Pressor capacity and will also increase compressor efficiencies. However, with a low side shell, hermetic refrigeration compressor, there are many other important factors to be considered including high volume production manufacturing techniaues, reliability improvement, noise reduction, etc. Figures 6 and 7 illustrate our new plastic muffler system which is separated from the cylinder. l-3. Efficiency Compressor efficiency varies in direct proportion to specific volume value improvements as long as changes in motor winding temperature and oil temperature (caused by decreasing suction gas temperature) are not significant. f efficiencies corresponding to T1 and T2 are El and E2: We must, however, be very careful when the specific volume value is improved. While Figure 5 shows that as the (6) specific volume value improves, (the unit figure actually decreases) there is actually a reduction in the capacity ratio which is accelerated as the piston-bore clearance increases. Therefore, unless we pay particular attention to th~ mechanical design in the shape of port, amount of valve lift and the clearance between the piston and cylinder wall, the benefits gained, will be lost. suer 1 an ---~ SHELL TUBE 2. SUCTWN TUBE --"i':::j"-,"---- BAFFLE. SUCTON MUFFLER. DSCHARGE MUFFLER. SUCTON <++-~-1-1 CHAMBER.!fi''<;;"":#- DSCHARGE *SUCTON MUFFLER SYSTEMS CHAMBER. Fig. 6. Component Placement of "D" Type Compressor > - Uo cr:oo () cr: u 2 4 s PST~N-B~RE CLEARANCE!MCR~ MMl Fig. 5. Capacity (%) With Piston-'lb-Bore Clearance Fig. 7. Appearance of New "D" Type Compressor Showing Plastic Muffler With this new design, we have obtained a 14 C reduction in decreasing the suction gas temperatures and a 2% reduction of suction pressur~ loss. 224

5 The resulting improvements in compressor performance are: Displacement (cc) BTUHr EER BOO We will now describe this new suction system in more detail. While the motor winding temperature increases about 2 C in the D type, it does not affect motor performance Liquid Refrigerant and Oil Return Careful attention must be paid to avoid the return of liquid refrigerant and refrigerant oil directly into the cylinder. This new suction system has the following features: l Modification By adopting a plastic suction muffler. Dual chamber muffler. Snap fitted baffle Suction gas route (suction tube, flexible tube connectionsmuffler suction tube). control the clearance between the suction-tube and the muffler inlet. Two (2) diagholes on the baffle. Benefit mproves the suction gas temperature by reducing heat transfer. mproves the productivity. Ease of assembly. Direct suction intake reduces suction gas temperature. Reduces sound level. Acts as an oil liquid separatol 2-4. Experimental test results showing liquid refrigerant, oil, and gas flow back to the compressor are shown in Figures ll, 12, and 13. Refrigerant liquid and oil from the suction tube cannot enter directly into the cylinder because of the muffler design. The baffle acts as an oil separator and the muffler itself ac'ts as an oil and lubricant accumulator. Particles or impurities are collected at the bottom (right side) of the muffler because of the gas flow path and the muffler then traps these foreign particles. These design features have been verified by life test. Noise The flashing back of gas into this new suction muffler system can increase sound levels at low frequencies especially the resonance frequency of 5 Hz. However, the sound level in this area is controlled by varying the clearance between the suction muffler inlet and the suction tube, which is shown in Figure Temperature Characteristics Figures 8 and 9 show the temperature characteristics under standard conditions for both our current "F" type compressor and our newly developed "D" type compressor with the improvements mentioned in this papel Figure 1 shows the temperature characteristics during cycle operations for both the F type and D type (which have the same refrigerant capacity at 32 C ambient). We see a l6 C decrease in the suction gas temperature and 7 C decrease in the discharge gas temperature with the D type as compared to the F type. Also, since this baffle increases the suction resistance, it tends to reduce suction gas noise Pulsation of the Suction Line The pulsation of the suction line is shown in Figure 15. t increases slightly using the new suction muffler (D type). This pulsation difference is so small, however, that it can be ignored. f it were to be a problem, it could be solved by a redesign of the suction line. 225

6 3. Conclusion We have achieved a 6-1% improvement in compressor efficiency with t.his new suc'l:ion muffler system. 363"K SUCTON GAS 35"K F TYPE (5. 1 G C) TYPE (5. 1 cc J Fig. 8. Suction Gas Path of the "F" Type Compressor Versus New "D" Type Unit... c > :.::c c w ::: :::;) 1- a: 1-c w.,...1<> X: w 1- c TEMPERATURE F type 5.7ee F t ;y p e 5. lee,.,.,.,.,.,.,.,. ~,,. f!j, _. ",.,. type 5. 1 e e --' ::: ::: :z ::: ::: --' w w ld w w ww --' m m --' ;x::x:: u.. :J: (f) ::E u.. (f) 1- (f). u.. a: (f) a: LL. a: :::;) :X: w :X: ~... ::E ~ w :J: u ::: u r t:l... u (f) u u!:j (f) (f) ~ :z: ~ :::J u... (f) (f) (f) Cl " :-.:; llj ::: ;:J 1- r:r: ::: llj n.. :::c llj DS. & sue. CHAMBER TEMPERATURE ' (f & D TP E. 55 BTU RANGEl... -! ;-' f'"",..j : DS. sue. F hpe (5.7Gcl D type (5. 1 c c) ON ('ff B 16 2 TME!MN. l Fig. of "D" 9. Temperature Measurements Type Versus "F" Type Compressor Fig. 1. Time Versus Temperature Curves of "D" Type Versus "F" Type 226

7 SUCTCJN BAFFLE. [CJJL SEPARRTCJRJ.. REFRGERANT & rjl DRCJPLETS. rjl ('JUTLET t- HEAD. Figures 12 (~bove) ~nd 13 (below), Fig. 11. Detail of New Suction Muffler Used in "D" Type Compressor Cross Section Views of New Suction Muffler Fig. 14. Noise Versus nlet Clearance ll..l ::: m --.:1 ::1... V) ll..l n > ll..l ll..l :::..... Cl z ::1 D n "" "" CLEARANCE & NOSE CSOOHzl (SUC. MUFFLER-NLET TUBEJ loo NLET CLEARANCE (mm ) Fig. 15. Ti~e Versus Pulsation Level _... of "D" Type Versus "F" Ty9e fulsrtirjn rjf SUCTON LNE 2," ' : '...,.., t,. " ' F TYPE TYPE " 1,.. ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' TME (mm sec. J 227

8 Key to Symbols T: V: h: G: VD: N: Q: n: K: Pd: Ps: D: W: E: bo Ai Bi Ci Temperature ( K) Specific Volume (m kg) Enthalpy (kcalkg) Mass Flow (kgh~ Displacement (m rev) Revolution per minute (RPM) Refrigerant Capacity (kcalh) Polytropic Exponent Adiabatic Exponent Discharge Pressure (kg2m abs) Suction Pressure (kgm abs) 3 Suction Volume per minute (m min) Work of,compression (KW) Efficiency (BTUWH) Constants 3 2 References 1. "Thermophysical Properties of Refrigerants," Japan Association of Refrigeration 2. Shengemi Nagatomo, "nternal Temperatures, nfluence on Reciprocating Compressor Performance," Japan Association of Refrigeration, Vol. 57 No. 651, p