Precise vibration control of the centrifugal chillers in rooftop

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Precise vibration control of the centrifugal chillers in rooftop

 

Author: Chan, Shui-yan
Title: Precise vibration control of the centrifugal chillers in rooftop
Degree: M.Sc.
Year: 1997
Subject: Air conditioning
Noise control
Vibration
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Pages: iii, 84, [22] leaves : ill. ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b1403354
URI: http://theses.lib.polyu.edu.hk/handle/200/4392
Abstract: Vibration and Vibration-induced noise, which are major sources of occupants' complaints are increasing rapidly in modern buildings, especially in hospital. In Hong Kong, it is very common to allocate the chillers plant on the rooftop of the building in order to fully utilise the space, fulfill building height restriction and achieve better heat rejection etc. Unfortunately, this rigidly mounted machine transmits in the same time all vibration forces to the supporting structure and increases the structure-borne vibration transmission in the result of their operation. This regenerated noise from structural movement can be heard also in remote sections of the building. It will disturb the occupants seriously. Furthermore, some energy required to operate the chillers is wasted in converting to mechanical energy through vibration transmission. In this case, a set of identical air-cooled centrifugal chillers has been installed at the rooftop of a hospital, which is just above the patient ward. As a result of chillers operation, the patients always hear an undesired noise which is a low-frequency rumble in combinating by unbalanced operation of condenser fan and compressor noise. This dissertation is going to shoot off this vibration and vibration-induced noise problems by using "Dynamics of Rigid Bodies" techniques and other acoustic engineering approach. Firstly, it uses the "Dynamics of Rigid Bodies" techniques to analyse the rooftop chillers system. It involves system definition, mathematical system modelling, development of equation of motion and calculation of various parameters of the vibrating system. Based on the calculated system parameters and the comparison of possible isolation materials, an appropriate metal spring vibration isolator has been selected for breaking through the vibration problem of system. Afterwards, the "Floating Floor on Cast-in-place Cannisters System" has been developed for solving the problem of low-frequency rumble. It is an integrated construction in combinating by reinforcement concreting floors and vibration isolators with significant air gap separation and sound absorption material. This floating floor system effectively partition the low-frequency rumble into the patient ward through rooftop structural slab provided that the natural frequency of the design floating floor is within 7 Hz to 15 Hz by 50mm thick air gap. Actually, the natural frequency of this applied floating floor system is 14 Hz by approximate approach and 13.29 Hz by calculation and both of them are within the recommended range. With this development, the noise transmission level from rooftop to patient ward has been improved and is far below the statutory required noise level. Finally, there are still 3 recommendations as follows for further improvement in reducing noise at rooftop plant areas. - add acoustic silencers to condensing fans, - add acoustic enclosure to compressor units, and - add acoustic barriers in front of the chiller units. Generally, this dissertation is a successful study. It has given a certain effect in the human comfort. The engineers can use the result of this study as a design guide when they face similar problems in their career.

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