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The challenges posed by dirty vacuum pumps

In processes where vacuums have to be used, mechanically driven vacuum pumps and jet ejectors compete with one another. Each type of pump has pros and cons. The process concerned determines which pump is the best choice. Processes prone to cause a lot of soiling in particularly highlight the importance of comparing both types of pumps.

The challenges posed by dirty vacuum pumps

Trend towards switching pumps

Recently there has been a noticeable trend towards replacing jet ejectors with mechanical vacuum pumps. There are many reasons that explain this change. The emergence of energy-efficient vacuum pumps and their manufacturers' robust market position are just two. Mechanical vacuum pumps are primarily powered by electricity. As a result, they are cheaper to operate than jet ejectors if no less expensive motive steam, e.g. low-pressure steam, is available.

A new generation of many machine operators plays another role. This fact also meant that the alternative to mechanical vacuum pumps was neglected. The experience with jet ejectors instead of mechanical pumps in processes liable to cause a lot of dirt was forgotten in favour of energy efficiency. Other solutions aren’t considered until pumps have been in operation for ten to 15 years and huge pressure on costs for repairs and maintenance is felt. By this point at the latest attention is refocused on the jet ejector as an alternative.

Soiling problems

The soiling problems that vacuum pumps cause have a major influence on operational reliability. During the compression process in mechanical pumps, vaporous substances turn into liquids because the steam condenses. When the pump starts, the air in the system is expelled by process steam. Volatile components generated from the condensing process steam during the compression process can evaporate more easily on the surfaces of the pump. As a result, the remaining substances become concentrated and can crystallise.

Similar situations occur with insoluble substances like grease which clogs the ring gap on the mechanical pump. If suction-flow components in the process fluid aren’t soluble, it’s almost impossible to use a mechanical pump. This usually occurs with long-chain hydrocarbons (oligomers, polygomers) during the manufacture of plastics, evaporation processes when separating substances and in crystallisation processes.

Dust and sludge as by-products of steel manufacturing for example are another problem. When a substance in a process has a high viscosity, i.e. in petrochemical processes, or in the production of edible oil, mechanical vacuum pumps reach their limits. In some cases in the chemical and pharmaceuticals industry, mechanical pumps are virtually useless.

Maintenance – replacing dirty pumps

Maintaining the vacuum is absolutely vital in a wide range of processes. In these processes, replacing dirty mechanical pumps that create vacuums entails associated costs because of downtime. Steps can be taken to minimise downtime however. Mechanical pumps can be kept in stock on site for example. However, if stored over longer periods of time, the pumps’ movable parts must be activated at regular intervals. This is the only way of guaranteeing that these aren’t damaged during storage already.

Developing alternatives

Machinery operators can save substantial costs by switching from mechanical pumps to jet ejectors. When processes cause a lot of soiling, jet ejectors are much more robust. They do require steam as a propellant which can be more expensive than an electric, mechanical pump. On the other hand, downtime and costly replacements for dirty pumps can be avoided.

Advantages of jet ejectors

The advantages of jet ejectors over mechanical vacuum pumps lie in their simple designs, large cross-sections and the fact that they require no movable parts at all. They are ideal for a whole range of applications. Using heating or fitting spray nozzles can effectively combat soiling of the jet ejector. Should a jet ejector get dirty during a process, this can be detected at an early stage by reducing the power. As a result, the operator can tackle soiling directly long before the machinery breaks down. Furthermore, jet ejectors are easy to store and are therefore quickly available should the need arise.

Corrosive media are also no problem. Jet ejectors can be produced from materials best suited to the process. Options include various metallic models (ranging from traditional steel to Hastelloy jet ejectors) and non-metallic models made of graphite, plastics (PTFE), or ceramics. This variety of resistance to corrosive media is a significant advantage vis à vis mechanical pumps. In some corrosive processes, mechanical vacuum pumps have literally proved to be disposable items with service lives of just a few weeks.

A combination of mechanical liquid ring pumps and jet ejectors is also possible. These hybrid systems are utilised in many processes and have good track records. Custom-enhancing the system as a whole can combine the advantages of both types of pump.

The requirement for motive steam is often cited as a disadvantage of steam jets. If steam is generated solely to drive the pump, operating steam jets can be very costly. If low tension steam or even waste steam is available from other processes, this disadvantage can be compensated for. Where these sources function as propellants for jet ejectors, energy usage is much more efficient. Therefore, massive cost savings are achievable.

How a jet ejector works

In contrast to mechanical pumps, jet ejectors require no movable parts. A propellant as the source of energy (steam, liquid or gas) generates the pumping action. The basic principle is the same whatever the design. The applications the jet ejector is destined for determine its shape and flow cross-section.

The propellant introduced via the motive nozzle flows through the jet ejector. During the process, some of the extreme motive energy is transferred to the suction flow. The motive flow pulls the process medium. As a mixture, the motive flow and suction flow run through the diffuser in a decelerated from and with a gain in pressure. In other words, in the case of a jet ejector the static pressure energy of the motive medium (which cannot be transferred directly) is turned into kinetic energy. This can be passed to the suction flow by transferring pulses in the mixture. The diffuser then transforms the energy from the mixture of the motive and suction flow back to static pressure energy.

Cost-benefit analyses determine solutions

Operators of machinery therefore have two types of pumps to generate vacuums. But each process needs to be examined individually. Based on the experience gained by operators with their processes, the right pumps are selected and integrated superbly into processes.

Many of the above-mentioned processes quote the higher energy requirements of jet ejectors as opposed to regularly replacing mechanical vacuum pumps. What’s required is an in-depth cost-benefit analysis to identify which pump will operate reliably and satisfactorily in the long term.

A hybrid system consisting of a steam jet vacuum ejector, liquid ring vacuum pump and surface condenser
A graphite, multi-stage steam jet vacuum system with surface condenser for corrosive media

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