The primary factors in how an ejector system is configured is going to be driven by the capital cost of the system and its efficiency. There is a trade-off between the capital cost of a system and the system’s efficiency. The more stages of compression a system has and the more intercondensers the system has, the more efficient the system design can be, up to a point. This is because ejectors require more motive steam as the compression range of each ejector is increased. Intercondensers help remove load from the process stream meaning less load needs to be compressed and less motive steam is needed for the downstream ejectors. More optimal compression ranges and lower ejector loads due to the installation of condensers, allow a system to use less motive steam, which makes the system more efficient. This results in a lower operating cost but a higher up front capital cost.

This graph shows the different configurations that can be designed for given conditions. For example, at a suction pressure of 10 torr (10 mmHgA) the most efficient system will be 3 ejector stages with condensers, next most efficient will be 2 ejector stages with intercondensers, followed by 3-stage or 2-stage without intercondensers.

It’s often a good idea to understand if the process is going to be a batch process or a continual process. Understanding the type of process and how often it is running can provide insight into how a system might be best laid out.

Continual Process – A process that is run around the clock often for weeks, months, years at a time. With a continual process it is often best to design a more elaborate system with a high operating efficiency as the increased capital cost of the system will often be negated quickly by a more efficient operation. This type of operation is very common in many refining and chemical processes.

Batch Process - A process that is run intermittently with frequent outages and downtime. If a system is not run frequently or only run for short periods of time, the cost to operate the system will be much lower. This type of system will not benefit as much from improved efficiencies because it will have less operating hours. This type of operation is common in many start up applications like hogging or certain production campaigns.

If a system is only going to be run once a month for an hour or two, it might not make sense to install a more elaborate system as the steam savings will not offset the additional costs up front. If a system is going to be run for months or years on end, then it almost always makes sense to optimize the system’s efficiently because the additional up front cost will be negated by the savings in operating cost.

The cooling water supply temperature also plays a role in how an ejector system is set up. If the system has intercondensers, the condensing vacuum can only be as deep as the cooling water temperature will support. In this way the cooling water limits how deep of a vacuum the first condenser in a system can operate at, which directly impacts how the system can be laid out. The colder the cooling water temperature is, the deeper the vacuum a condenser can operate at. The deeper the vacuum at which the load can be condensed, the less motive steam the system will need. This is also why systems that operate at very deep vacuum will often have ejectors in series on the front end without condensers. The cooling water is not cold enough to do any work at very deep vacuum. In systems that operate at a vacuum roughly 40-100 mmHgA and with sufficiently cold cooling water, a precondenser could be used. This type of system is ideal as the precondenser removes a large amount of the load before the ejectors leading to a more efficient system.

Limitations in steam and cooling water supply also impact how a system can be configured. If a boiler has a limited amount of steam available, the system needs to be optimized to use less steam. Using a liquid ring vacuum pump in conjunction with ejectors to further reduce steam consumption may also be an option. Sometimes the condenser cooling water flow will be limited due to pump capacity or cooling tower capacity and a system needs to be optimized to use less cooling water. Optimizing a system to use less steam or less cooling water often results in compromises being made elsewhere in the system either in efficiency and/or capital cost.

As you can see there are many factors that come into play when laying out a new system design. This is one of the reasons each system designed by Graham is unique and tailored to each installation’s needs. Understanding the site’s needs is paramount to providing the right layout for a system. If you are considering a new system or a revamp to an existing system, please contract Graham for engineering support as our engineers are experienced with system design and are happy to assist.