Noise test and control for household refrigerator compressor
Sound comfort becomes more and more important in residential houses. Home appliance is one of the main sources of indoor noise. Noise emission is a distinctive feature for the selection of the home appliances. Noise levels in household appliances are increasingly attracting attention from manufacturers and customers. Refrigerators as common appliances used by most families run continuously all day. It produces a lot of noises. National standards have published severe rules on the acceptable noise levels, and noise-free operation becomes the preferred choice of customers in the refrigerators choosing. Therefore, it is necessary to reduce the noise level of refrigerator. In this paper, the noise control study has been carried on a type of air-cooled household refrigerator. The noise level of the refrigerator was tested under different operation conditions at first. Then the maximum sound pressure level and noise characters were obtained by analyse the spectra of the noises. Test data shown that the refrigerator noises are mainly composed of compressor and fan noises. According to the characteristics of the compressor and fan noise, a method of using sound insulation board was proposed to reduce the compressor noise. Considering the actual installation restrictions and the compressor cooling demand, a kind of sound insulation board with holes was applied. Experimental studies indicated that the proposed noise control method can suppress the noise of the household refrigerator effectively.
Evaluation model of refrigerator cooling performance
In order to comprehensively and quantitatively evaluate the cooling performance of the refrigerator, a model for evaluating refrigerator cooling performance was designed. The model selected several key parameters of the refrigerator cooling process as the evaluation index, and assigned its set value, base deviation and weight coefficient, calculated the deviation proportion of each evaluation index, and its individual scores, finally got a percentage score that reflect the refrigerator cooling performance with summing the weighted scores. Application results showed that the model can comprehensively and quantitatively evaluates the refrigerator cooling performance and the differences between each other's, by calculating the data collected form the cooling process with the same evaluation scale and testing error.
Vibration Isolation for Noise Control in Residential Condensing Units
Resonances in the compressor mounting feet and the base pan of the condensing unit can reduce the effectiveness of simple elastomeric isolators resulting in unacceptable noise levels. For this case, the simple single-degree-of-freedom transmissibility model is no longer valid and a multi-degree-of-freedom model should be used. In this paper a case study is presented which illustrates how a three-degree-of-freedom transmissibility model can be used to make design decisions for reducing noise levels in a residential condensing unit when the higher frequency system resonances are important. INTRODUCTION The condensing unit represents one of the major sources of noise in a residential HV AC system. A typical condensing unit consists of a set of condensing coils and associated tubing, a sheet metal cabinet and base pan, a fan, and a compressor which is mounted to the base pan using a set of simple elastomeric isolators. The primary emphasis of many programs for reducing condensing unit noise is to reduce the airborne noise from the fan and the compressor. Large reductions in fan and compressor noise can, however, expose structure borne noise problems which might otherwise be masked by the higher background noise levels. The structure borne noise problems can then become the dominant sources in the system and further noise reduction can only be achieved by addressing these sources. In a typical condensing unit, one of the major sources of structure borne noise is the mechanical vibration transmitted from the compressor to the base pan. The mechanical transmission problem is typically addressed using vibration isolation techniques based on a transmissibility model for a single-degree-of-freedom (SDOF) system. The SDOF model and the associated transmissibility curve are shown in Figure 1. For this case the mass represents the mass of the compressor and the stiffness and damping are associated with the isolator. This method is generally effective for dealing with the low frequency components associated with the running speed but may not be sufficient for dealing with higher frequency components which can be important from a noise control point of view. When using vibration isolation for noise control in a typical condensing unit, there are two complicating factors which should be considered: internal machine resonances and base flexibility. Internal machine resonances refer to the flexural motion of the compressor mounting feet as opposed to the overall rigid body motion of the compressor and base flexibility refers to the fact that the base pan of the condensing unit is not a rigid structure and is considerably less massive than the compressor itself. For this case, the transmissibility model should be developed using a multi-degreeof-freedom (MDOF) model. The transmissibility curve for a simple three-degree-of-freedom system is shown in Figure 1. In this simple model two additional degrees-of-freedom are added; one to represent the dynamics of the compressor mounting feet an one to represent the dynamics of the base pan. As illustrated in Figure 1, the added degrees of freedom introduce additional peaks in the transmissibility curve which correspond to the higher resonant frequencies of the combined system. At these frequencies, the transmissibility will be much higher than predicted using a simple SDOF model. Although the transmissibility is less than one, the peaks are still important because the sheet metal base pan is, in general, a very good radiator of noise and only a small amount of mechanical power is required to generate a large amount of sound power. The decrease in performance of the isolation system and the subsequent increase in noise levels will be more pronounced when the dominant excitation frequencies from the compressor, the internal machine resonances, and the resonances associated with the base flexibility are all close to one another. From a quantitative point of view, the use of a MDOF model for the analysis and design of an isolation system can be difficult because there are usually a large number of resonances associated with the mounting feet and base pan and so the transmissibility curve will be more complicated than the one shown in Figure 1. The simple MDOF