This operation models the transport of gaseous materials using a vacuum pump. A vacuum pump is a device whose main function is to remove gas molecules from a sealed volume by suction and leave partial vacuum behind.
● Pumping by a Vacuum Pump Procedure
The suction rate, suction temperature and suction pressure are assumed equal to the volumetric flow rate, temperature and pressure of the inlet stream, respectively.
The exit pressure is assumed equal to the ambient pressure (which is specified at the flowsheet level).
The power consumption can be either set by the user or calculated by the program. Three options are available for specifying the power consumption: the user may specify the total power consumption or the power consumption per unit or the specific power consumption (i.e., the total power consumption divided by the suction rate). To calculate the power consumption, the user may choose to select either the isothermal compression model (which is suitable for liquid ring vacuum pumps) or the isochoric compression model (which is suitable for roots vacuum pumps). For isothermal compression, the compression power can be estimated as:
Compression Power = Suction Rate * Suction Pressure * ln(Exit Pressure/Suction Pressure).
For isochoric compression, the compression power can be estimated as:
Compression Power = Suction Rate * (Exit Pressure - Suction Pressure).
The actual power consumption must account for mechanical losses. This can be calculated as:
Power = Compression Power / Efficiency
where the efficiency factor takes into consideration the energy losses in the pump (e.g., wall friction losses, turbulent exchange losses, thermodynamic losses). Usually, the efficiency is 0.25-0.4 for a liquid ring vacuum pump. A suitable efficiency for a roots vacuum pump is 83%.
An adiabatic energy balance is first performed to calculate the adiabatic outlet enthalpy, and from that, the adiabatic outlet temperature. The adiabatic energy balance is simply:
Outlet Enthalpy = Inlet Enthalpy + Power Dissipation
The power dissipation is calculated by multiplying the power consumption by the power dissipation %.
If the adiabatic outlet temperature is lower than or equal to the specified maximum exit temperature, then no cooling is required, and the cooling duty is zero, and the outlet enthalpy is set equal to the adiabatic outlet enthalpy.
If the adiabatic outlet temperature is higher than the maximum exit temperature, then the outlet temperature is set equal to the maximum exit temperature and the outlet enthalpy is calculated at that temperature. Then, the cooling duty is calculated based on the following energy balance:
Cooling Duty = Inlet Enthalpy + Power Dissipation - Outlet Enthalpy.
In Calculate (Design Mode), the user sets the maximum power of the equipment, and the program calculates the operation’s suction rate and power consumption (if not set by user). If the operation’s power consumption exceeds the maximum power of the equipment, then the program assumes that there are N parallel equipment units, each having a power consumption equal to the total power consumption divided by N. In addition, the program calculates the suction capacity of the equipment by dividing the total suction rate by N.
In User-Defined (Rating Mode), the user specifies the number of units, rated power and suction capacity. In this case, too, the program calculates the operation’s suction rate and power consumption (if not set by user). The program simply displays a warning if the operation’s suction rate per unit exceeds the specified suction capacity of the equipment or if the power consumption per unit exceeds the specified (rated) power of the equipment.
1. http://www.ppipumps.com/all_about_vacuum_pump.html.
2. Bannwarth H. (2005). Liquid Ring Vacuum Pumps, Compressors and Systems: Conventional and Hermetic Design, Wiley-VCH.
3. Umrath W. (1998). Fundamentals of Vacuum Technology (https://www3.nd.edu/~nsl/Lectures/urls/LEYBOLD_FUNDAMENTALS.pdf).
4. Hucknall D. J. (1991). Vacuum Technology and Applications, Butterworth-Hennemann.
The interface of this operation has the following tabs:
● Oper. Cond’s, see Pumping (by Vacuum Pump): Oper. Conds Tab
● Labor, etc, see Operations Dialog: Labor etc. Tab
● Description, see Operations Dialog: Description Tab
● Batch Sheet, see Operations Dialog: Batch Sheet Tab
● Scheduling, see Operations Dialog: Scheduling Tab