[HOW TO] Select Required Vacuum Pump Capacity

Today i wanna Demonstrate HOW TO SELECT VACUUM PUMP CAPACITY for a operation in pharma operations, especially pharma because the use of vacuum can make tremendous change in field of Pharma, one right selection of Vacuum pump can reduce huge efforts in specific operations like crystallisation, drying, saturation, Separations etc.
So the its the main target of a process engineer or project engineer to provide with the right vacuum pump to manufacturing team for achieving high yields in small span of time, but before going to discuss about vacuum pump selection, i want all to know about the vacuum some more briefly,

What is Vacuum? Vacuum is simply a pressure below atmosphere. To create vacuum in a system, a pump is required to remove mass (gas/vapor) from the system. The more mass is removed, lower is the pressure that exists inside the system. Various vacuum levels are defined depending upon the ultimate vacuum as:

                           Range                                Absolute pressure range ‰
                           Coarse Vacuum                          10 – 760 Torr ‰
                           Medium Vacuum                        0.001 – 10 Torr ‰
                           Fine Vacuum                              10^-3 – 10 ^-7 Torr ‰
                           Ultra High Vacuum                     < 10^-7

Pumping speed: It is the volumetric rate of exhausting, generally expressed in Lts/min., m3 /hr or cfm. It is the rate at which the inlet of the pump actually removes the gas / vapor load. It should not be confused with Displacement of the pump. Displacement of a pump is the geometric volume swept by the pump per unit time at rated operating speed. For most of the pumps, pumping speed is close to displacement value at no load conditions (FAD-Free air delivery) and changes with inlet pressure, reaching to zero where the pressure attained is said be pumps ultimate pressure.







The below data gives an approximate idea of how booster combination can yield lower pressures while maintaining high pumping speeds.
                               Vacuum Pump              Pressure Range   Pressure Range with Booster

                            Single Stage Ejector             150 Torr                  15 – 30 Torr
                           Water Ejector                         100 Torr                  10 – 20 Torr
                           Water Ring Pump                  40 – 60 Torr             5 – 10 Torr
                           Liquid Ring Pump                 20 – 30 Torr             2 – 5 Torr
                          Piston Pumps                          20 – 30 Torr             2 – 5 Torr
                          Rotary Piston Pumps             0.1 Torr                     0.01 Torr
                         Rotary Vane Oil Pump          0.01 – 0.001 Torr       0.001 – 0.0001 Torr.
Vacuum Booster being a very versatile vacuum machine is used in a wide range of processes, some of them being;
Coarse vacuum applications
• Vacuum Drying Application.                                • Tray dryer
• Rotary vacuum dryers                                           • Flash Drying
• Vacuum Distillation processes.                             • Solvent recovery.
• Vacuum Filtration                                                  • Replacement of Steam Ejectors.
• Enhancing the performance of Water Ring Pumps /Water ejectors
• Vacuum Flash cooling / Evaporative Cooling        • Vacuum Crystallization
Medium Vacuum applications
• Efficient backup for Diffusion Pump Systems.     • Thin Film Deposition /Coating
• Short path/ Molecular distillation.                         • Solvent Recovery.
• Vacuum Heat Treatment and Degassing / Vacuum Furnaces
• Freeze Drying                                                        • Vacuum Impregnation
• De-humidification                                                 • De-gassing




And Now i think you gained some brief knowledge regarding vacuum, and now i'll get into point directly of selecting a pump capacity,

Also Read:

Batch Failure Analysis by Fault Tree Analysis 

How to Handle an Out Of Specification[OOS] 

Practical Way to Determine Order of Reaction

For an installed system air leakage load can be estimated by “Drop Test / Pressure rise test” method. Based on the fact that air leaks into the system at a constant rate as long as the pressures in the system is below 400 Torr because of critical flow conditions, the above test is effectively used to determine the leak rate of assembled system.
The system is evacuated to pressures between 10-100 Torr and isolated. The pressure is allowed to rise, but not exceeding about 300 Torr and the time lapsed is noted.




* The Leak Rate “QL” is calculated as, 

QL = ∆PxVs/ t

where QL leak tare in Torr Ltrs/sec 

∆P pressure rise in Torr

VS system volume in Litres 

t is elapsed time in seconds 

* For the known leak rate, the capacity of vacuum pumping system can be evaluated by the expression, 

Savg = 3.6xQL / P

Where Savg = Average pump speed in m3/hr 

QL = Leak rate Torr ltr/sec

P = System Pressure in Torr 

[ You might get a doubt that, how the formula for pump capacity is derived, but there is nothing but just a unit conversion by incorporating the required end vacuum(i.e., ultimate pressure).

Let's get into units now, air leakage rate holds units of Torr. Lt/Sec.

Ultimate pressure holds units of Torr.

So, (Torr. Lt/Sec) / Torr = Lt/Sec, which can be derived by dividing Air leakage rate with Ultimate pressure.

by doing so, we'll get in Lt/Sec, now simply convert it into m3/hr, 

for that we need to multiply with 3600/1000 = 3.6,

Now i think you got an idea, how i used that 3.6 in the main formula.]

* As a thumb rule for pressures in the range of 10-100 Torr pipeline “D” may be selected as

D = 2.4 (Q)^0.5

where D= diameter of pipe in mm

Q= Pumping speed in M3/hr .

Case Study was requested by Mr. Srikanth, 

Pl find below case:

Let's suppose, i need to select a vacuum pump for a 10 KL distillation system, where i need to attain 750 mmHg of vacuum atleast.

So, lets start the show,

I've taken a vacuum leak test for the system, 

applied vacuum, attained 756 mmHg of vacuum.

Hold the vacuum for 10 mins, so my evacuation time is 10 mins,

Observed vacuum leak is 30 torr. Let's suppose the air load is 3 Kg/hr at that particular area.

Now using above formulas,

Air leakage rate = 500 Torr.Lt/Sec,

Pump capacity required = 180 Cu.m / hr,

Considering safety i'll prefer 30% excess, i.e., 234 Cu.m / hr ~ 250 Cu.m /hr.

Line size from pump to system required is 1.27" ~ 2".

That's it

That's it Cheers, comments are appreciated,

Many of the readers found this somewhat difficult to calculate, and for them i've simulated these in an excel sheet, and download this excel sheet below.

DOWNLOAD HERE

Also Read:

How to Design a Condenser?  

How to Select a Pump , Motor, Line Sizings ? 

How To Select a Condenser? 

Determine the Power Required for an Operation?  

Industrial Distillation Column Design Steps

About The Author

---

Hi! I am Ajay Kumar Kalva, Currently serving as the CEO of this site, a tech geek by passion, and a chemical process engineer by profession, i'm interested in writing articles regarding technology, hacking and pharma technology.
Follow Me on Twitter AjaySpectator & Computer Innovations