Compressor Unit Operation - Technical Information
The compressor unit simulates a single stage isentropic compression. You must use two or more compressor units connected in series to simulate multi-stage compression. An optional Aftercooler may be attached to the outlet stream to cool the products to the desired temperature.
Calculation Method
The operating specifications for a compressor unit include one of the pressure, work or head specifications, and the compressor efficiency or outlet temperature. A specific value can be entered for these parameters or a performance curve can be supplied.
PRO/II performs compressor calculations by simulating the Mollier diagram. The point corresponding to the inlet condition is determined by calculating the enthalpy and entropy at the inlet pressure and temperature. A constant entropy path is then followed until the outlet pressure is reached. The adiabatic work is determined by the enthalpy difference between the initial and final conditions. If the adiabatic efficiency is not 100%, the actual enthalpy change is computed by dividing the adiabatic enthalpy change with the adiabatic efficiency.
PRO/II predicts compressor performance at different molecular weights and RPMs with the multiple MW and RPM curves. If the inlet stream MW is with the range of MW data provided by user, PRO/II predicts the performance of the compressor by interpolating from the given data. If the inlet stream MW is 10% lower than the minimum MW or 10% higher than the maximum MW, the curve data for lowest and highest MW will be considered. If operating RPM is with the range of user given data, PRO/II predicts the performance of the compressor by interpolating from the given data. If the operating RPM is above the given data, performance prediction is not performed.
PRO/II also calculates other parameters including the isentropic and polytropic coefficients, polytropic efficiency, and polytropic work, using one of the two Compressor Calculation Methods. The default calculation method is the ASME Power Test Code 10 method, which can be changed to the GPSA Engineering Data Book method if desired. If the polytropic efficiency is supplied, the adiabatic efficiency is back calculated using these methods to determine the actual work.
The compressor unit supports both VLE and VLLE methods to determine the individual phase compositions. See VLE Model and VLLE Model for more details.
Feed and Product Streams
The compressor unit can have any number of feed streams. The inlet pressure is taken to be the lowest pressure of all the feed streams.
The compressor unit can have up to four product streams with different phases in each stream. The possible product phases are vapor, liquid, decanted water / second liquid phase, a mixture of vapor and liquid, and solids. If there are multiple product streams leaving the compressor unit, the phase condition for each stream must be specified using the Product Phases Window.
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