Experimental and numerical analysis of two different bottom pumps of a hermetic reciprocating compressor at low speeds

Abstract

Operating at low speeds (< 2000 rpm) is critical in achieving optimal energy efficiency for inverter-type hermetic reciprocating compressors. However, ensuring the efficient delivery of lubrication oil to the compressor components at such low speeds is challenging. The hermetic reciprocating compressor crankshaft utilizes a machined helical channel on the outer surface to provide lubrication oil to the upper bearings. The lubrication oil is provided to this channel's inlet from the compressor's oil sump by an eccentric internal channel in the bottom of the crankshaft. However, this design cannot deliver the oil to the helical channel under low-speed conditions. This research investigates a screw pump, the smooth inner crankshaft wall with threaded pin inside (SC-TP) configuration, and compares its performance numerically and experimentally with a conventional double eccentric internal channel pump (DEC) design. For this purpose, computational fluid dynamics (CFD) is used. For the simulation validation, the pumps are fabricated from transparent plexiglass. Also, two sets of solvers for two-phase flow, i.e., the implicit volume of fluid model (VOF) with Compressive scheme and explicit VOF model with Geo-Reconstruct scheme, are used to examine the methods' accuracy. When running above 1400 rpm, the DEC pump delivers faster and more oil than the screw pump. However, while operating under 1400, the screw pump provides oil until 800 rpm, which the DEC pump cannot do. This research shows how shear force dominates centrifugal force during low-speed pumping situations and also investigates how different viscosities impact performance.