Performance of Centrifugal Compressors with Variable Vaned Diffuser (1996)

Vishnu Sishtla Carrier Corporation Syracuse, NY 13221 In 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. It 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. In this paper performance results with variable diffuser throat area are presented. Introduction 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 utilizing water for condensing refrigerant. The expansion device is either a throttle valve or a two phase turbine. In some applications an aircooled condenser with refrigerant on tube side is used. The compressor, for cooling capacities below 150 000 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. In 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. It should be pointed out that the cooling load on a refrigeration machine varies from 100% down to 10%. In a typical installation in U.S, the compressor head drops to 50% of full load value at 10% cooling load. In Asia, the head drops to 80% of full load value at 10% 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. It 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(l and Colin Rodgers(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(l have presented compressor performance with adjustable inlet guide vanes and diffuser vanes. In 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 400 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. It 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: