E is relatively high. Each successive stage is required to handle the load plus the motive steam from the previous stage. MNEs are frequently used when low first cost is more important than long-range economy. They are also used for intermittent service or when condensing water is not available. MNEs are usually two-stage, although six-stage units have been used successfully.
Multistage condensing ejectors (MCEs) are available in two through six stages. Intercondensers (surface or direct-contact) between stages condense steam from the preceding stage, reducing the load to be compressed in the succeeding stage. A multistage system is shown in Figure 1.
Four-, five- and six-stage ejectors are used to achieve suction pressures as low as 5 Âµm Hg abs. Under such vacuum conditions, pressure between the preliminary stages is too low to permit condensation of ejector steam, and only the final two stages are fitted with condensers. MCEs remove condensable vapor ahead of a given ejector stage. They also permit use of a smaller ejector, and a reduction in the amount of steam required. Condenser nomenclature is determined by the corresponding operating conditions and functions.
Precondensers are used when the absolute pressure of the process is sufficiently high to allow condensation at the temperature of the available water supply. Noncondensables are removed from the precondenser by one or more ejector stages. Condensers or inter-condensers liquefy process vapor and motive steam from one or more preceding booster ejectors. Aftercondensers condense steam discharging from the last-stage ejector, generally at atmospheric pressures.
There are two basic types of condensers — direct-contact and surface-contact. In direct-contact (countercurrent, barometric design) condensers, cooling water is mixed directly with the vapor to be condensed, then discharged to atmosphere through a barometric leg or tailpipe of sufficient length to overcome the atmospheric pressure. A means of cleaning up or otherwise disposing the water that has become contaminated by process material is often required. The surface-contact condenser permits main-condenser cooling water to be used as cooling water through inter- and aftercondensers, for energy and process-water conservation.
Where To Start: When a vacuum problem arises there are several preliminary checks that should be made on an ejector system before components are disassembled. First, is the system design data readily available? Most ejector systems are custom-designed to operate at a specified vacuum, given process loads, minimum available steam pressure, maximum steam temperature, maximum discharge pressure and maximum water temperature. It is impossible to evaluate the complete system unless the manufacturer’s design parameters are known.
A data sheet can be obtained from the manufacturer of the ejector system and should list the design vacuum, capacity, interstage ejector vacuums, motive steam pressure and temperature, condenser water-inlet and -outlet temperatures, and discharge pressure. Critical dimensions such as the diameters of the ejector nozzle orifice and the diffuser bore should also be know.
Once the design data are located, there is an ordered series of steps to take in isolating ejector problems. One should check:
- Process loads
- Field report and process logs
Pay Attention To Instruments: When troubleshooting the ejector system, accurate pressure and temperature measurements are needed to quickly locate the source of the problem. Therefore, a check of the system’s instrumentation is a necessity. All vacuum, pressure, and temperature gages should be calibrated or replaced. A malfunctioning vacuum gage may be found to be the entire vacuum “problem.”
If the vacuum gage is several feet or more from the ejector system, check the line running from the system to the gage for air leakage. It would take a very small amount of air leakage in an instrument vacuum line to throw off the measurement. If leakage is suspected, connect the test gage directly to the ejector and compare vacuum measureme