Compressor
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Contents
General Description
The compressor model can be used to increase the pressure of streams which consist mostly of gases. The user will receive a warning if the fraction of vapours in the feed is less than 99%.
The compressor model can be inserted in a steam line that is part of a Flash Train.
Diagram
The diagram shows the default drawing of the Compressor, with the required connecting streams. The unit will not operate unless all of the above streams are connected.
The physical location of the connections is not important, the user may connect the streams to any position on the drawing.
Inputs and Outputs
Input /Output  Required / Optional  Number of Connections  Description  
Min  Max.  
In  Required  1  1  Input stream to compressor. 
Out  Required  1  1  Output stream from compressor. 
Behaviour when Model is OFF
If the user disables the unit, by unticking the On tick box, then the material will flow straight through the Compressor with NO change to either Temperature or Pressure.
So basically, the unit will be 'bypassed' without the user having to change any connections.
Model Theory
Equations
For an ideal gas:
 (1) [math]\mathrm{C_p = C_v + R}[/math]
 where:
 [math]\mathrm{C_p}[/math] = heat capacity at constant pressure on a per mole basis
 [math]\mathrm{C_v}[/math] = heat capacity at constant volume on a per mole basis
 R = universal gas constant
The k value, the ratio of specific heats, can be calculated as follows:
 (2) [math] k = \cfrac{C_p}{C_p  R} [/math]
For an ideal gas with constant heat capacities, which undergoes a mechanically reversible, adiabatic (or isentropic) process, the following equation applies:
 (3) [math]\cfrac{T_2}{T_1}=\left (\cfrac{P_2}{P_1}\right)^{((k1)/k)}[/math]
This can be rewritten as:
 (4) [math] T_{out} = \left (\cfrac{P_{out}}{P_{in}}\right)^{((k1)/k)} * T_{in}[/math]
 where:
 [math]\mathrm T_{out} [/math] = outlet temperature
 [math]\mathrm P_{out} [/math] = outlet pressure
 [math]\mathrm P_{in} [/math] = inlet pressure
 [math]\mathrm T_{in} [/math] = inlet temperature
Thus given the inlet temperature and pressure and the outlet pressure, the outlet temperature can be calculated.
The adiabatic efficiency is used for the Isentropic method and is defined as:
 (5) [math]\text{Adiabatic}\,\text{Efficiency}=\cfrac{d{{T}_{s}}}{d{{T}_{a}}}[/math]
 where:
 [math]\mathrm dT_i [/math] = calculated temperature change for isentropic process
 [math]\mathrm dT_a [/math] = actual temperature change
The polytropic exponent has the following form:
 (6) [math]\cfrac{n}{n1} = \cfrac{k}{k1} * \mathrm {Polytropic\ Efficiency}[/math]
then
 (7) [math]T_{out} = \left (\cfrac{P_{out}}{P_{in}}\right)^{((n1)/n)} * T_{in}[/math]
For all cases,
 (8) Ideal Power = Rate of Enthalpy Out  Rate of Enthalpy In
 (9) [math]\text{Compressor}\,\text{Efficiency}=\cfrac{\text{Ideal}\,\text{Power}}{\text{Actual}\,\text{Power}}[/math]
Calculation Steps
Step 1) Determine the outlet pressure based on user specified value, boost or ratio.
Step 2) If the user has specified a value for k, use this value in the calculations, otherwise calculate k as the ratio of Cp to Cv using equation (2).
Step 3) Calculate the outlet temperature based on the outlet pressure:
 a) for the Isentropic method: use equation (4).
 b) for Polytropic method: use equations (6) and (7).
Step 4) Calculate the outlet enthalpy based on the outlet temperature and pressure.
Step 5) Calculate the Power using equations (8) and (9)
Assumptions, Limitations and comments
 This model assumes that the gases behave as ideal gases.
 The feed stream should contain only gases. Small amounts of liquids and solids will have little affect. The outlet temperature and pressure are determined by the gases present in the stream only. Any liquids or solids in the inlet will be assumed to exit the compressor at this new temperature and pressure. When the power is calculated using the enthalpy difference between the inlet and outlet, this will include any solids or liquids in the stream. Thus the presence of solids and liquids will usually lead to an increase in power requirements.
 Note: If the stream contains no gases, then the model will produce unrealistic results.
References
1. Bloch H.P. A Practical guide to Compressor Technology, McGrawHill 1996
2. Perry et al Perry's Chemical Engineers' Handbook 6^{th} Edition, McGrawHill 1984
Data Sections
Summary of Data Sections
 Compressor tab  contains the main configuration information relating to the unit.
 Info tab  contains general settings for the unit and allows the user to include documentation about the unit and create Hyperlinks to external documents.
 Links tab, contains a summary table for all the input and output streams.
 Audit tab  contains summary information required for Mass and Energy balance. See Model Examples for enthalpy calculation Examples.
Compressor Page
Unit Type: Compressor  The first tab page in the access window will have this name.
Tag (Long/Short)  Input / Calc  Description/Calculated Variables / Options  
Tag  Display  This name tag may be modified with the change tag option.  
Condition  Display  OK if no errors/warnings, otherwise lists errors/warnings.  
ConditionCount  Display  The current number of errors/warnings. If condition is OK, returns 0.  
GeneralDescription / GenDesc  Display  This is an automatically generated description for the unit. If the user has entered text in the 'EqpDesc' field on the Info tab (see below), this will be displayed here. If this field is blank, then:
 
Requirements  
On  Tickbox  If the unit is disabled, by unticking this box, then material flows straight through the unit, with no change to temperature or pressure.  
PressMethod  Fixed  The user specifies the outlet pressure from the Compressor.  
Boost  The user specifies the pressure boost of the compressor.  
Ratio  The user specifies the ratio of the outlet pressure to the inlet pressure.  
PressureReqd / P_Reqd  Input  Only visible if Fixed is chosen for the Pressure method. This sets the required pressure of the outlet (discharge) stream.  
PressBoostReqd / PBoostReqd  Input  Only visible if Boost is chosen for the Pressure method. The difference in pressure between the inlet and outlet streams.  
PressRatioReqd / PRatioReqd  Input  Only visible if Ratio is chosen for the Pressure method. The ratio of the outlet pressure to the inlet pressure.  
MaxPressRatio / MaxPRatio  Input  The maximum compression ratio that the Compressor can provide. If the required final pressure / inlet pressure > than this value, then SysCAD will restrict the final pressure so that the maximum pressure ratio is not exceeded. The unit will give the user an error message if this occurs.  
CalculationMethod / Method  Isentropic  The isentropic outlet temperature will be calculated using the pressure ratio, inlet temperature and the ratio of heat capacities (K).  
Polytropic  The outlet temperature will be calculated using the pressure ratio, inlet temperature, ratio of heat capacities (K) and the polytropic efficiency.  
SpecifyK  Tickbox  This allows the user to specify the value of K, the ratio of heat capacities. If this is left unchecked, then the model will assume ideality and use equation (1) to calculate K based on Cp values in the species database.  
K  Input  Only visible if SpecifyK option is selected. This is the user specified value of k, the ratio of specific heats of the gas in the feed stream.  
Adiabatic.Efficiency / AdiabaticEff  Input  Only visible if Isentropic is chosen for the Calculation method. This efficiency is used to alter the calculated isentropic temperature change. A lower efficiency will lead to a larger temperature change.  
Polytropic.Efficiency / PolytropicEff  Input  Only visible if Polytropic is chosen for the Calculation method.  
Compressor.Efficiency / CompressorEff  Input  This is the mechanical efficiency of the compressor drive and gear box (if present) and is used to calculate the drive motor power required from the IdealPower. A lower efficiency will lead to a larger Power and a larger difference between Power and IdealPower.  
Results  
MassFlow / Qm  Calc  The total mass flow through the unit.  
TemperatureIn / Ti  Calc  The inlet temperature.  
TemperatureOut / To  Calc  The outlet temperature.  
PressureIn / Pi  Calc  The inlet pressure.  
PressChange / dP  Calc  The pressure change across the Compressor.  
PressureOut / Po  Calc  The outlet pressure.  
PressRatio / PRatio  Calc  The pressure ratio of the Compressor = Pressure Out / Pressure In.  
VapourFracIn / Vfi  Calc  The fraction of vapours in the inlet stream.  
VapourFracOut / Vfo  Calc  The fraction of vapours in the outlet stream.  
IdealPower  Calc  This is the amount of power put into the fluid being compressed and is the enthalpy difference between the inlet and outlet streams per unit time. The lower the efficiency the more power required to achieve the same pressure.  
Power  Calc  The required compressor drive power for the given adiabatic and drive efficiencies.  
Gas.MWT  Calc  The weighted average of the molecular weights of the vapours in the inlet stream.  
Gas.Cp  Calc  The weighted average of the heat capacities at constant pressure (Cp) of the vapours in the inlet stream.  
Gas.Cv  Calc  The heat capacity at constant volume (Cv) of the vapours in the inlet stream. This is calculated using the heat capacity at constant pressure (Cp).  
Gas.K  Calc  The ratio of the Gas Cp to the Gas Cv, i.e. the K value. 
Adding this Model to a Project
Insert into Configuration file
Sort either by DLL or Group.

DLL: 
Piping2.dll 
→ 
Units/Links 
→ 
Piping: Compressor 
or 
Group: 
General 
→ 
Units/Links 
→ 
Piping: Compressor 
See Project Configuration for more information on adding models to the configuration file.
Insert into Project

Insert Unit 
→ 
Piping 
→ 
Compressor 
See Insert Unit for general information on inserting units.