Liquid Ring Vacuum Pumps are limited in the vacuum they can produce. The vapor pressure of the sealant fluid is the limiting factor and vacuum is usually limited to about 25 torr. Most liquid ring vacuum pumps operate between 50-75 torr.
When deeper vacuum is required, an Atmospheric Air Ejector may be used as an extra pumping stage. The Atmospheric Air Ejector (AAE) is installed in the pump suction and can pull the vacuum down to 5-10 torr. The AAE sucks in atmospheric air and discharges it into the pump suction.
The Atmospheric Air Ejector converts the pressure energy of atmospheric air (at high pressure relative to the vacuum pump) and converts it into kinetic energy. The deeper vacuum is produced at this point, and the kinetic energy is reconverted to pressure energy in the diffuser, to match the suction pressure of the pump.
The method of operation would be to turn on the pump first. After pump vacuum is reached, the air valve can be opened.
A common application for the AAE is a two-stage cold water degasifier. Two parallel atmospheric air ejectors, one operating at 20-25 torr and the other operating at 10-15 torr, maintain different vacuum in the two different compartments of the degasifier. They both discharge to the same pressure, usually 60-75 torr. The two discharges are manifolded together and sent to the same liquid ring vacuum pump or bank of parallel pumps.
When specifying an Atmospheric Air Ejector, the following must be noted :
- Compression Ratio across the ejector is normally less than 8 times. For example if you require 10 torr at the ejector suction, your pump should pull to minimum 80 torr. Higher compression ratios are available, but these are special cases and Croll-Reynolds should be consulted before specifying. Special performance testing may be required for higher compression ratios also (available at the CR Research and Test Facility)
- LRVP flows are normally specified in m3/hr at a particular vacuum. This can be misleading when coupled to An Atmospheric Air Ejector and should be converted to mass flowrate (lbs/hr or kg/hr).The formula on the right may be used for quick conversion (for air flow only):
- The overall flow capability will decrease since air is bled via the AAE. The chart below may be used to calculate new system flows. For example:
A liquid ring vacuum pump is designed for 367 m3/hr air at 60 torr suction. User wants to increase the range to 10 torr suction.
Step 1: Convert m3/hr into mass flowrate. By the equation above Flow = 34.8 kg/hr air
Step 2: From Chart I, for use with Mass Flow (bottom group of curves), find Suction at Vacuum Pump Inlet = 60 torr, and point of intersection with 10 torr curve. The Y axis at this intersection is 0.11
Step 3: Multiply pump 34.8 kg/hr by multiplying factor 0.11 to get new Suction Flowrate with AAE coupled to vacuum pump. 3.8 kg/hr is the new system suction flowrate.
Since the pump handles 34.8 kg/hr air at its (unchanged) suction pressure, the AAE will use 31 kg/hr of atmospheric motive air and draw the new system suction of 3.8 kg/hr air
The new system suction flow is therefore 240 m3/hr (10 torr suction).
Some characteristics of the Atmospheric Air Ejector are:
Simplicity of design, operation and maintenance – no moving parts.
Availability in a wide range of materials. Standard material is cast iron with a stainless steel nozzle, but plastics, stainless steel and high alloy steels are all easily manufactured.
The Atmospheric Air Ejector is the most economical vacuum producing device available.