5 Hacks to Optimize Your Process Vacuum Using Mechanical Vacuum Boosters

Optimization of Process Vacuum using Mechanical Vacuum Booster based on Roots Vacuum Pump

Vacuum Pumps and Systems are widely used in the Chemical Process industry for various applications such as Drying, Solvent recovery, Distillation, Short Path Distillation (Molecular Distillation), Concentration etc. It is therefore essential that the vacuum principles are understood which can be employed to maximize process throughputs, product purity and quality & minimize power consumption.

Success has been achieved in many industries such as food products, essential oils, aromatics, solvent recovery and steam jet replacements. Wide range of pumps and vacuum equipment is being used in the industry to achieve the desired vacuum and pumping speeds. The understanding of their advantages and limitations can result in optimizing their performance.

TIP-1: What is a Vacuum?

Vacuum is simply the pressure below the 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:

Generally, the chemical industry operates in the Coarse and Medium Vacuum range. In this range the vacuum is generally measured in mm Hg gauge or Torr (absolute pressure). Measurements from datum as atmosphere are gauge reading, whereas the measurements referred to absolute zero are expressed in Torr. For example at sea level (atmospheric pressure 760mmHg), a system maintained under vacuum of 700mmHg, as indicated by vacuum Bourdon gauge, is said to have absolute pressure of 60 Torr. Vacuum gauges, Mercury Manometers, Transducers etc. indicate Gauge Pressure and their reading when subtracted from atmospheric pressure gives Absolute Pressure. It is important to understand the above since all vacuum principles and calculations are based on absolute pressure units.

It is evident from the curve that Pumping Speed drops with drop in pressure. This must be taken into consideration while selecting a pump.

The inlet pressure at which the pump’s speed falls to zero is termed as “Ultimate pressure or Blank-off Pressure” of the pump. It is a pump characteristic, dependent on the type of pump/ pump construction. The ultimate pressure/Blank off pressure of a pump can be easily checked by measuring the inlet pressure, with the inlet of the pump Blanked off. At Blank-off pressures, the effective pumping speed of the pump is ZERO. This means that a process can never achieve vacuum better than the blank off vacuum of the pump. While selecting a pump, the desired process vacuum and that achievable by a pump must be verified. Ultimate vacuum is the pump type characteristic and the general conception that using a bigger Pump (of the same type) would yield a better vacuum is false. The process engineer’s should establish the desired process vacuum and the selection of the pump should be made accordingly. To get a better working vacuum and higher pumping speed, Boosters are invariably used in combination. In most cases much higher speed and lower pressure can be achieved with a fraction of extra power, when Booster combination is used.

The table below gives an approximate idea of how booster combinations can yield lower pressures while maintaining high pumping speeds.

The most important parameters affecting the vacuum pump selection are the Suction Pressure that must be reached or maintained and the throughput that the pump must handle. Pumping characteristics for the pump selected are of prime importance as most of the pumps have a working pressure range where they are most efficient and below which their performance drops considerably. The process working pressure and load are the major factors governing the pump selection.

Invariably, the process demand of higher working vacuums ends up in installation of higher capacity pumps adding to considerable capital & working costs with little or no gain in vacuum. For example if a process demands system pressures to be maintained at 50 Torr (710mmHg) with a non condensable load of 10 Kg/hr at 30OC, an ideal pump should have a capacity of 130 m3/hr at 50 Torr. Use of a water ring pump which has it’s ultimate at 710mmHg would be a wrong choice. A Booster and Water ring combination would be the most energy efficient choice.

TIP-4 : Pipe Sizing:

The piping that connects the vacuum vessel to the vacuum pumping system plays a vital role in the overall performance of the system. Sizing of the pipe requires relatively complex calculations based on various factors like Flow conditions – Turbulent, Steady state, Molecular, Friction Coefficient, Reynolds’s No etc. Too small a pipe would have low conductance (High Resistance) restricting the flow rates due to higher pressure drops across it and too large a piping would increase the capital cost.

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

For a flow rate of 900 m3/hr the suitable pipe calculated is 72 mm, as per above and 80 NB line, the nearest standard size should be selected.

Tip-5 : Backup Vacuum Pump Choice

In order to ensure satisfactory operation of any Vacuum process it is essential that a suitable pumping system be used. There is generally no single pump that meets all the requirements of the process and so combinations of pumps are increasingly being used to optimize the process performance. Process condensable & non-condensable loads, air leakage loads, out-gassing loads and the working process pressures are the important parameters that influence the pump selection. Some of the widely used pumps for the vacuum process are described below along with their limitations.

5.1 Rotary Vane and Piston Pumps

These types of pumps have high power to capacity ratios and are therefore not available in large volumetric capacities. They are effective for pumping non-condensable loads but have limitations of not being able to pump large & regular quantities of water vapor (condensable loads) released in low-pressure vacuum processes. Various precautions have to be taken if they are used for food grade applications to avoid contamination of process material by the pump oil or back streaming of oil vapors.