Ejector systems can be divided into condensing units and non-condensing units. The usual method of staging is to use a vapor condenser between the stages. Condensers are used to condense out motivating steam plus the suction vapor from the first stage, allowing only saturated non-condensables to pass on to the following stages. The size and type of condenser used is a function of the air-vapor ratios, cooling water temperatures available, steam and water costs, and contaminants in the first stage suction vapor.

The term non-condensing is used where a stage discharges directly into the following stage. Steam consumption in this type of unit is higher because the second stage must handle the motivating steam plus the suction capacity from the first stage. Non-condensing units are used where the interstage pressure is lower than could be obtained with the temperature of cooling water available.

Two and three stage non-condensing units are used where purchase and installation cost out-weigh the consideration of steam consumption. Two-stage non-condensing units used as primers are more efficient than those used for single point operation. A two-stage non-condensing unit uses approximately 100% more steam than a two-stage condensing unit, but when used as an evacuator, its priming time is only approximately 20% longer. This is due to the oversizing of the final or atmospheric stage.

Condensers

A condenser is an apparatus used to reduce a vapor to its liquid state by removal of latent heat from the vapor. Its function as part of a steam jet vacuum system is to remove condensable vapor ahead of a given ejector stage, thus reducing the size of the ejector and the amount of steam required.

Condenser function may be defined as follows:

1. Precondensers: Used for direct condensing of vapors from the process. Non-condensables are removed from the precondenser by one or more ejector stages. The absolute pressure of process must be sufficiently high to allow condensation with the available water supply.
2. Condenser or booster condenser: Used to condense process vapor and motive steam form one or more preceding booster ejectors which compress process vapors from required low absolute pressure to the higher absolute pressure necessary for condensation by the available condenser water.
3. Intercondenser: Used between the ejector stages where two or more stages are required to compress non-condensables from a process or condenser pressure to atmospheric pressure, eliminating the handling of motive steam from a preceding stage.
4. Aftercondenser: Used to condense steam discharging from a “Z” or last stage ejector at atmospheric pressure. Non-condensables are vented into atmosphere.
5. There are two basic types of condensers: Direct Contact and Surface. Here are the advantages and chief characteristics of each type.

1. Lower purchase cost.
2. Lower installation cost.
3. Less water needed for a given vacuum condition.
4. Smaller terminal difference allows operation at lower absolute pressure.
5. Less floor area required.
6. Scale or solids build-up has little effect on condenser performance, therefore little or no maintenance is generally required.
7. Can be fabricated readily with corrosion resistant materials or supplied economically with rubber lining.
8. Open barometric discharge provides safe operation without an atmospheric relief valve.

1. Steam condensate may be recovered.
2. Process product may be recovered as condensate, or gas at a higher pressure.
3. No contamination of condenser water can occur.
4. Vacuum surges will be less likely to carry water back to the process.
5. Less head room required.

• Jacketed Ejectors