Numerical Investigation And Performance Analysis On Centrifugal Steam Compressor For Mechanical Vapor Recompression(MVR) Applications

In this study,Krain impeller is numerically investigated for steam compression,which was originally designed by H.Krain[40]for air compression purpose.It is a high pressure ratio transonic centrifugal compressor impeller with vaneless diffuser.It has a highly efficient design which enables it to have a smooth internal flow field with no jet-wake flow at the impeller exit,which is a highly undesirable flow phenomenon often observed at impeller exits.The reason for selecting this impeller model for steam compression is that steam compressor models are not available on open access to study on.So,this impeller model is chosen and investigated to determine its applicability for steam compression in Mechanical Vapor Recompression(hereafter MVR).MVR is one of the most efficient energy recovery methods used in evaporation systems taking places in industries such as distillation,desalination,petrochemical and food and beverage.In MVR vapor are compressed by a steam compressor,thereby increasing vapor saturation temperature.These high temperature compressed vapor are used as heat source for the evaporator,thus the energy contained in the vapor is recycled.Compressor is a core component of a MVR system.Different types of compressors are used for recompression of steam,however centrifugal compressor is most commonly employed in medium and large scale MVR units for their high stage pressure ratio,large flow rate capacity and high efficiency.Many researchers have studied MVR system.Most of that work is related to the system characteristics such as energy saving and cost estimation and effects of different parameters such as temperature difference,operating temperature,evaporator area on the overall systems performance.But,an in-depth study on a steam compressor is lacking.With the increasing scope of MVR,consequently increasing use of steam compressor,it is necessary to study steam compressor deeply to have better understanding of it,which could help in design and development of steam compressor for MVR specific applications with better performance.Krain impeller is numerically investigated and a thorough analysis is performed on its performance and internal flow field at different operating conditions,i-e:65 ~0C,72 ~0C,80~0C and 88 ~0C saturation temperature.First,a numerical model is developed for the compressor and the model is validated by available experimental data.Then,the compressor is simulated for steam compression and results are obtained.Overall compressor performance is analyzed at design condition firstly,then its internal flow field is investigated at design,stall and choke condition.Transient analysis is also performed to examine the stability of the compressor.In the end,compressor parameters relevant to MVR are studies.The investigation showed that the compressor has lower pressure ratio and mass flow rate when it is used for steam compression as compared to that of air,which is due to the fact that the steam density is much low as compared to that of air at atmospheric conditions.Consequently,it has relatively much smoother flow field structure compared to that of air,which results in its higher efficiency.Nevertheless,its pressure ratio and mass flow rates fall in a range required in typical MVR systems.