Performance of Centrifugal Compressors with Variable Vaned Diffuser

Transcription

1 Purdue University Purdue e-pubs nternational Compressor Engineering Conference School of Mechanical Engineering 1996 Performance of Centrifugal Compressors with Variable Vaned Diffuser V. Sishtla Carrier Corporation Follow this and additional works at: Sishtla, V., "Performance of Centrifugal Compressors with Variable Vaned Diffuser" (1996). 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 Herrick/Events/orderlit.html

2 Performance Of Centrifugal Compressors with Variable Vaned Diffuser Abstract Vishnu Sishtla Carrier Corporation Syracuse, NY n a water cooled chiller system the centrifugal compressor operates over a wide range of inlet flow and pressure ratios. Centrifugal compressors with vaned diffuser are used for high efficiency at full load. t is well known that a compressor with vaned diffuser has low operating range and the pressure ratio at surge decreases rapidly as inlet guide vanes are closed. Rotatable diffuser vanes are used in some cases to improve surge margin at lower guide vanes. n this paper performance results with variable diffuser throat area are presented. ntroduction A Centrifugal refrigeration machine basically consists of a centrifugal compressor, evaporator or cooler, condenser and an expansion device. The cooler is a shell and tube heat exchanger with refrigerant on shell side. The condenser is also a shell and tube heat exchanger utiliing water for condensing refrigerant. The expansion device is either a throttle valve or a two phase turbine. n some applications an aircooled condenser with refrigerant on tube side is used. The compressor, for cooling capacities below 15 btu/hr, is usually a single stage with inlet guide vanes for capacity control For moderate efficiencies, a vaneless diffuser is used downstream of the impeller to convert the kinetic energy to pressure. n installations where efficiency is of utmost importance, a vaned diffuser downstream of impeller is used. An efficiency gain of 3-5 percentage points can be obtained by using a vaned diffuser. t should be pointed out that the cooling load on a refrigeration machine varies from 1% down to 1%. n a typical installation in U.S, the compressor head drops to 5% of full load value at 1% cooling load. n Asia, the head drops to 8% of full load value at 1% cooling load. Hence, the compressor should be capable of handling the wide range of inlet flows and pressure ratios without getting into unstable operation, such as surge. t is well known that a vaneless diffuser compressor has a wider operating range than a vaned diffuser. However, the higher efficiency of a vaned diffuser makes it a more attractive choice and at the same time offers a challenge to come up with ways of increasing the operating range. The study in this paper is aimed at investigating methods of obtaining wide stable operating range with a vaned diffuser compressor. Casey et al( 1 l and Colin Rodgers( 2 l, have given an excellent account of Centrifugal compressor performance at design and off~design conditions. A brief mention of variable vaned diffusers and their effect on performance is presented in these papers. Simon et al( 3 l have presented compressor performance with adjustable inlet guide vanes and diffuser vanes. n all the papers mentioned above, diffuser vanes were adjusted by rotating in the circumferential direction. The variable vaned diffuser methods investigated in this paper are based on varying throat area alone. Test Arrangement Testing was performed on a 4 ton low pressure refrigerant water cooled chiller consisting of a evaporator, compressor, condenser and a throttle valve. Evaporator and condenser are flooded shell and tube type heat exchangers. The hermetically sealed compressor (Figure 1) is a single stage geared machine driven by a fixed speed motor. t has a set of inlet guide vanes and a curved vaned diffuser downstream of impeller. Some of the main characteristics of the aerodynamic components are given in below: 767

3 mpeller: Speed = 77 RPM Outside Diameter = N Number Of Blades= 17 Discharge Blade Angle = 3 (measured from radial) Tip Speed Mach Number =1.4 Diffuser nside Diameter= 2.3 N Number Of Vanes = 2 Diffuser Width =.675!N The chiller is placed in a test cell (Figure 2) consisting of two separate water loops one on evaporator side and the other on condenser side. The water flow rates on both the loops can be varied from 2 to 16 gpm. A steam heat exchanger in the cooler loop is used to simulate building load on evaporator. The flow of steam to the heat exchanger is controlled by a pneumatic control valve that is adjusted to provide a given water temperature at the inlet of the test machine. The condenser water circuit in addition to a closed loop through the test machine condenser includes supply and return water piping to the cooling tower. nlet water temperature control is accomplished by a water exchange that occurs when an automatically controlled valve releases water back to the tower. An equivalent amount of cold water through the supply valve replaces the hot water and is mixed with the remaining closed loop water to obtain the desired inlet temperature. Water inlet and outlet temperatures are measured by thermocouples and the temperature difference is measured by a thermopile. Water flows on cooler and condenser side are measured by ASME PTC calibrated orifices. Refrigerant mass flow through the compressor is calculated by conducting heat balance on the condenser. n order to carryout the calculations refrigerant state points are measured at inlet and outlet of the condenser. Refrigerant side pressures in the cooler, condenser and compressor discharge are measured by a scanivalve system. Compressor inlet temperature is measured in the pipe upstream of inlet guide vanes by four equally spaced thermocouples around the circumference. Discharge temperature is measured by two thermocouples placed in the discharge pipe. Thermocouples are of copper constantan type with an accuracy of.25 F. The accuracy of scanivalve pressure transducer is less than 1%. Compressor head and polytropic efficiency are calculated using the temperature and pressure measurements at inlet and discharge. The equations for calculating the head and efficiency are given below: where: H = ln (Pal P ) * (Pa Va - P V ) ln (Pa Val P V ) F = mcna. ll. Tend CP Vo (ho-h4) TJ = H h = Refrigerant enthalpy at compressor suction h 2 = Refrigerant enthalpy at compressor discharge h 4 = Refrigerant liquid enthalpy at condenser mend = Condenser water mass flow rate Cp = Specific Heat of Water a Tend= Condenser water side temperature rise 768

4 P 2 = Compressor discharge pressure P = Compressor suction pressure V = Specific volume at compressor suction conditions V 2 = Specific volume at compressor discharge conditions H = Polytropic Head F = nlet Flow 'T1 = Polytropic efficiency Test Results Testing was aimed at investigating compressor performance with the following throat area varying devices: 1) Cylinders (Figure 3a) 2) Serrated ring (Figure 3b) 3) Slotted ring (Figure 3c) Throat area is varied by stacking different device heights. Data was gathered for each guide vane position from surge to choke conditions. Base line testing was done without any throat varying mechanisms. Performance data was gathered with three different cylinder heights corresponding to 8, 6 and 4% diffuser opening. nlet guide vane position was varied at each diffuser opening. The cylinders were fixed on diffuser wall on impeller shroud side. Performance results, at surge, are shown in Figure 4a & 4b. Also shown in Figure 4a & 4b is surge line with fixed diffuser. Results are normalied with respect to maximum inlet flow (F ) and maximum polytropic head (H ) Performance results for slotted and serrated ring are shown in Figure Sa & Sb and Figures 6a & 6b. Combined surge line plots for fixed diffuser, cylinders, slotted and serrated ring are shown in figures?a & 7b. The results clearly show improvement in surge margin and part load performance with all the devices over fixed diffuser. Performance of serrated ring and slotted ring is better than cylinders. Due to its simple shape, slotted ring is better suited for manufacturing. Conclusions Range extension of a vaned diffuser centrifugal compressor was studied with variable area throat devices. Tests performed with 3 different variable area throat devices have shown improvement in surge margin and efficiency at part load over fixed diffuser. Of the 3 devices, slotted ring offers the best performance and is easy to manufacture. Acknowledgements The author wishes to thank United Technologies Carrier Corporation for their permission to publish this paper. References 1) Casey, M. V., Marty, F., " Centrifugal Compressors - Performance at Design and Off-Design". Proceedings nstitute Of Refrigeration, ) Rodgers, C., "Variable Geometry Gas Turbine Radial Compressors". ASME 68-GT-63 3) Simon, H., Wallman, T., Monk, T., "mprovements in Performance Characteristics Of Single-Stage and Multistage Centrifugal Compressors by Simultaneous Adjustments of nlet Guide Vanes and Diffuser Vanes". Transactions Of the ASME Journal Of Turbomachinery, January 1987, Vol19, PP

5 Fig 1. Compressor cross section Fig 2. Chiller Test Loop 77

6 Fig 3a. Cylinders At Diffuser Throat :J~tt-., aa ~s -- - Fig 3b. Serrated Ring Fig 3c Slotted Ring 771

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8 ng a. VARABLE DFFUSER SERRATED RNG - SURGE,_,., 1.11 ~,_., ' t.. c e: - c ;:s :Z: JS.1.U o :-r-2~--o:-r ---o:-r.,: f,,... FLOW FRACTON - F,/F nc 7o. VARABLE DFFUSER VS nxed DFFUSER - -r SURGE UNE COP4RSON - HEAD FAttiON Q AXED fi.!!!.j)pen + U!2.~'!. COX 2._PEN_ AD lb. VAR41LE DFFUSER '-"-r ,..;;. ~ o.u : -~ o:;: u t.,7 ~.. ;;: t; o.... o.ll SE1ti4TED ling - AT SURGE i / /,' t'. t #5 /4/.! l '/ eft ; /... lda~--~---~---~--~--p--~ / FLOW FRACTON - F/F nc 7b. YAR1411LE DFFUSER VS nxm DFFUSE -r SURGE UNE CO'o\RSON - EFFCENCT FACtiON l.z AXm :, aoxo~ +!!!!!!.!'P!.!2.X OPEN_ ~-U ri '. t. i! - / c ::,_,. 1.1,.,. -.U.1 -~ Sr',/ Z FlOW FRACTON - F,/F,. f}.~.m.p.!ffir:!p.! l:l. SERRATED RNg + SlDTTEO RNG X CYUNDERS 1'1' o. s ~ ;i o.u fi i! ll t e.... ;;. 1.$ (5 f.....&; 1.1,, FLOW FRACTON -ii'.a.. ~ ;::~=~ + SlDTTEORNd X CYUNDERS 773