Technical ContributionFrom Atmospheric Pressure to Vacuum
Whether in foodstuff or pharmaceutical industry, in the chemistry or process engineering, metall processing or in automotive industry: in many branches of industry most different manufacturing processes require gas conditions with a pressure considerably below atmospheric pressure. A pressure of -700 mbar (300 mbar abs.) is considered as negative pressure. Below 300 mbar absolute the vacuum range begins, subdivided in rough, fine, high and ultra high vacuum (see chart).
In plants, rough, fine and high vacuum can economically be achieved, if so-called pumping sets are used. These vacuum pumping sets are working at least double-stage. Backing pumps and positive displacement blowers are working together. Aerzener Maschinenfabrik GmbH, manufacturer of positive displacement blowers since 1868, has also been manufacturing special positive displacement blowers for vacuum generation since 1940. Thus, AERZEN is not only one of the pioneers of this technology. Today, the company is a worldwide leading manufacturer with a wide range of negative pressure and vacuum blowers. This success is based on technical competence, high-precision manufacturing, permanent developments, experienced staff and a continuous, intensive dialogue with the customers. For generation of negative pressure up to 500 mbar abs. AERZEN supplies positive displacement blowers series Delta Blower G5. The newly developed rotary lobe compressors series Delta Hybrid even generate negative pressures up to 300 mbar abs. Negative pressures up to 500 mbar abs. or up to 300 mbar abs. are each achieved in one stage already.
Interaction of backing pump and vacuum positive displacement blower
However, negative pressures below 300 mbar abs. can only be achieved double-stage in a so-called pumping set by the combined use of a backing pump together with a vacuum positive displacement blower. So, the volume flow required by the operator, the so-called operating point, is safely achieved. In the first stage, the backing pump reduces the pressure of the medium available in a reservoir or a room to a pre-vacuum of e.g. 200 mbar abs. Then, as second stage the vacuum positive displacement blower starts and achieves due to the continued common operation with the backing pump the required final vacuum, or the required volume flow. The future operator of the vacuum plant (e.g. a steelworks in China) indicates to the manufacturer of the pumping set (e.g. a German plant manufacturer) all essential parameters for the required plant:
- The size of the room or reservoir to be drained,
- The maximum vacuum required (the so-called operating point) or the required volume flow and
- The maximum possible pumping time.
Thereupon, the manufacturer of the pumping set selects the suitable backing pump and the suitable vacuum blower together with AERZEN.
As possible backing pump system, depending on the application, water ring vacuum pumps, oil-lubricated rotary vane pumps or controllable claw pumps for neutral gases are used. For particularly high-quality applications in chemical industry for evacuation of process gases expensive screw vacuum pumps may become necessary. As specialist with experience for many years AERZEN has extensive performance documents of all backing pump systems, advises the manufacturer of the pumping set concerning the selection of the optimal backing pump system and selects in close cooperation with him the optimal AERZEN vacuum positive displacement blower. To meet the parameters required by the operator of the pumping set, backing pump and AERZEN vacuum blowers are energetically and thermally optimal matched to each other.
Picture 1 shows in a theoretical calculation the interaction of backing pump (orange line) and an AERZEN vacuum blower series GMa (green line) as double-stage solution. However, for reducing the pumping time multi-stage solutions are also possible with one backing pump and several successively working vacuum blowers. On the x-axis the pressure ranges of backing pump and vacuum blower, on the y-axis the volume flows are shown. In this example, first of all, the backing pump starts alone. When a vacuum of 200 mbar abs has been achieved, the AERZEN vacuum blower starts. Up to the operating point at 1 mbar the green curve rises considerably. At the operating point, the volume flow rate of the combination is approx. 1,750 m³/h. The two thermal critical pressure ranges appearing first of all in this theoretical calculation can be corrected by modified parameters in the design programme, so that the combination of backing pump and AERZEN vacuum blower selected for this example ensures the required operating point, in this example 1 mbar, and can be operated successfully in this combination.
Based on this procedure, observing the thermal limits and using the best possible combination of backing pump and vacuum blower, the manufacturer of the pumping set and AERZEN meet the parameters of the pumping set stipulated by the operator. AERZEN offers...
for the vacuum range from 300 to 10 mbar
- Vacuum Blowers with pre-inlet cooling series ... mHV,
for the vacuum range from 200 to 10-3 mbar (0,001 mbar)
- Vacuum blowers series HV,
for the vacuum range from 200 to 10-5 mbar (0,00001 mbar)
- Vacuum-blowers with canned motor drive (so-called canned motor blowers) series CM and HM.
The optimal selection of the correct combination of backing pump and vacuum blower guarantees a cost-effective pumping set and a long-term and economical operation with highest energy efficiency.
Vacuum blowers with pre-inlet cooling (vacuum range 300 to 10 mbar)
AERZEN supplies vacuum blowers with pre-inlet cooling (so called pre-inlet blowers) series mHV in 11 sizes for theoretical nominal intake volume flows from 250 to 61,000 m³/h. Their maximum admissible differential pressure depends on the corresponding thermal load. Pre-inlet blowers are mainly used in rough vacuum and negative pressure range as backing pump or negative pressure stage against atmosphere, in order to achieve high differential pressures in one stage, and for high compression ratios in the rough vacuum range up to p2/p1 = 5. Pre-inlet blowers of series mHV are used preferably to achieve continuous operation without overheating problems. For this purpose, atmospheric air or already recooled gas is fed into the packaged unit on the discharge side without additional valve, regulators etc. via a third intake channel. When using cooled gas this is recooled before in a gas/air cooler or gas/water cooler installed between backing pump and pre-inlet blower. The housing flanges of the pre-inlet blowers are equipped with O-ring seals, a splash lubrication arranges for a supply of the vacuum blowers with pre-inlet cooling with lube oil. The blowers are driven via a directly coupled motor or a spur gear, in case of limited differential pressure via narrow V-belt. The conveying chamber is sealed by combined splash ring-piston ring labyrinth seals, the drive shaft is sealed by double radial seal rings with oil barrier.
Vacuum blowers for the fine vacuum range from 200 to 10-3 mbar
The air-cooled blowers of series HV for the vacuum range of 200 to 10-3 mbar are available in 12 sizes for theoretical nominal intake volume flows from 180 to 97,000 m³/h (speeds from 3,000 to 3,600 rpm). In type of construction GMa they work with vertical direction of flow, type of construction GLa with horizontal direction of flow makes a particularly compact design possible. Both types of construction are used among others in the coating process, in chemical and process engineering, in the metallurgy and packing industries, in central vacuum plants, in helium compression and helium leak detection systems, in the manufacturing of lamps, tubes, solar equipment and in the automotive industry. For particular applications, special seals can be used for air-cooled, splash lubricated blowers and special material variants e.g. for cast parts and
Due to their standard drive with motors type of construction IE 3 the blowers work with utmost energy efficiency and can be used in many markets - among others also in the USA, in Canada and Russia. Furthermore, they are suitable for frequency converter operation. The motors are flanged directly to the blowers. A special splash ring-piston ring labyrinth seal avoids penetrating of oil from the bearing chambers into the conveying chamber. In addition, a largely dimensioned neutral chamber with condensate channels is installed. To achieve an increased purging effect the neutral chamber can be purged with seal gas. As a unique feature, vacuum blowers series HV can optionally be manufactured according to ATEX approval 94/9/EG. They offer an explosion pressure spike resistance up to 13 bar, work without bypass regulation and are the only vacuum blowers approved for zone 0 (internally) and externally for temperature class T4. For increased process safety shutting down the monitoring option below 50 mbar is possible.
Canned motor blowers for the high vacuum range from 200 to 10-5 mbar
The AERZEN canned motor blowers series CM (for aggressive gases) and HM (for neutral gases) suitable for permanent operation make particularly short pumping times possible and are used in industrial high vacuum technology for a vacuum range of 200 bis 10-5 mbar. These blowers work with a hermetic drive, as the drive shaft is sealed by an integrated canned motor without connecting passage to atmosphere. Due to the almost doubling of the speed to 6,000 up to 7,200 rpm with the same size, very short pumping cycles in the seconds range result, which leads to a considerable acceleration of the production processes for the operator. Even if, for increasing the performance further, two backing pumps and one canned motor blower are integrated into a pumping set, the entire unit still distinguishes itself by a particular compact design - a special advantage concerning multi-systems with numerous pumping sets. Canned motor blowers made by AERZEN are available
in type of construction CM for corrosive gases
- in 14 sizes for theoretical nominal intake volume flows from 110 to 15,340 m³/h.
in type of construction HM for neutral gases
- in 9 sizes for theoretical nominal intake volume flows from 406 to 15,570 m³/h.
These systems are used for the generation of industrial vacuum, for example in chemical and process engineering, in foil and glass coating, evacuation of hydrogen, in helium leak detection systems and in any situations where leakages must absolutely be avoided. In addition, these blowers are working in the semiconductor industry, in microelectronics, in the manufacture of flat screens, in laser technology and in solar technology. The blowers can deliver in both directions - vertical and horizontal. Due to the standard water cooling they are appropriate for application under clean room conditions. An excellent mechanical resistance (up to 230 mbar) reduces the pumping times. The application of a frequency converter makes a high control range (1:5) possible, and therefore, the use of smaller blower sizes. Thanks to different motor variants for network, cyclical and continuous operations, an individual solution can always be found, even for special applications.
For the optimal design of a pumping set for vacuum generation there are no ‘off the shelf’ solutions, as the performance parameters of backing pump and vacuum positive displacement blower must be optimally matched to each other. Only then the pumping set will achieve the parameters required by the operator and the stipulated operating point. Therefore, the optimal solution can only be achieved by intensive advice by AERZEN as supplier of the vacuum blower required together with the manufacturer of the pumping set, who purchases the backing pump as well as the vacuum blower from external suppliers. With extensive software AERZEN inspects the pump combination consisting of backing pump and AERZEN vacuum blower planned or stipulated by the manufacturer of the pumping set. “We pay particular attention to avoiding thermal critical pressure ranges and to providing the energetically best gradation ratios. For AERZEN applies “we do not only advise the manufacturer of the pumping set concerning the process vacuum application, but also concerning the design of the entire pumping set combination.”
Author: Norbert Barlmeyer, specialised journalist for compressed-air technology, Bielefeld