What is a Surface Steam Condenser?

A surface steam condenser removes heat from turbine exhaust steam and returns it to liquid form. Condensing at a low pressure reduces turbine backpressure and allows the turbine to extract more energy. Because the steam and cooling water remain separated by tube walls, suitable condensate can return to the steam cycle.

In a typical water-cooled design, cooling water flows inside many tubes while steam condenses on their outer surfaces. Condensate drains to a hotwell and a condensate pump returns it to the cycle. The warmed cooling water then goes to a cooling tower, another heat-rejection system or a permitted once-through discharge. A leaking tube can contaminate the condensate, so tube integrity and water chemistry matter.

Steam condensation creates most of the low pressure in the shell. Steam-jet air ejectors or mechanical vacuum pumps remove air and other non-condensable gases that enter with the steam or through leaks. If these gases accumulate, they impede heat transfer and raise turbine backpressure.

Condenser descriptions can refer to the number of cooling-water passes, the direction of shell-side steam flow or the heat-rejection method. These labels describe different design features, so they are not a single set of mutually exclusive types.

Two-Flow Surface Condenser

This arrangement is more commonly called a two-pass surface condenser. Cooling water travels through one group of tubes, turns in a partitioned water box and returns through another group. Steam condenses on the shell side. The water-pass count does not by itself specify the direction of steam flow.

The advantages of a two-flow surface condenser are:

• A two-pass arrangement can provide the required tube velocity and cooling-water temperature rise in a compact layout.
• The inlet and outlet can be placed at the same end when the water-box arrangement permits it.
• Performance can be predicted from the actual tube geometry, flow rate, fouling allowance and steam-side conditions.

The disadvantages of a two-flow surface condenser are:

• For otherwise comparable geometry and total flow, the return path can increase waterside pressure drop and pumping power.
• The partitioned water box adds seals and internal surfaces that need inspection and maintenance.
• It is not inherently better or worse for condenser pressure. Cooling-water inlet temperature, flow, cleanliness and heat-transfer area determine the operating result.

Multi-Flow Surface Condenser

The term multi-flow is ambiguous. A multipass condenser sends cooling water through more than two tube passes in series. A condenser with separate water circuits may instead operate those sections in parallel. The drawing and water-box piping must be checked before using either description.

The advantages of a multi-flow surface condenser are:

• Pass and circuit arrangements can maintain a suitable water velocity across the expected operating range.
• Separate circuits can suit a wide shell or more than one cooling-water pump train.
• A suitably valved design may allow part of the waterside to be isolated, although this is not available on every condenser.

The disadvantages of a multi-flow surface condenser are:

• Additional water-box partitions and piping make the hydraulic design and maintenance more involved.
• More passes in series can increase pressure drop; parallel circuits distribute the total flow differently. Pump selection must use the actual arrangement.
• Fouling risk depends mainly on water chemistry, biological activity, material, velocity and treatment rather than the pass count alone.

Downflow Surface Condenser

In a downflow surface condenser, turbine exhaust enters above the tube bundle and moves downward as it condenses. Liquid drains naturally to the hotwell. Cooling-water direction and pass count are separate design choices. Non-condensable gases are drawn from a designated air-cooling section near the low-pressure region, not simply from the lowest point of the shell.

Central Flow Surface Condenser

In a central-flow, or radial-flow, arrangement, steam moves inward through the tube bundle towards a central air-cooling zone. Air-removal equipment connects to that zone. The water-side direction and number of passes depend on the water-box design.

The advantages of a central flow surface condenser are:

• Radial steam lanes can expose a large part of the tube-bank perimeter to incoming steam.
• A defined central air-cooling zone can collect non-condensable gases for removal.
• The arrangement can provide a compact steam path when it matches the turbine exhaust and shell geometry.

The disadvantages of a central flow surface condenser are:

• Steam lanes and the central air-removal zone require careful tube-bundle layout.
• Poor distribution can leave inactive surface or let steam reach the air-removal section before it condenses.
• No universal fouling or pressure-drop advantage follows from central steam flow. Those results depend on the separate waterside design and operating conditions.

Inverted Flow Surface Condenser

In an inverted-flow arrangement, exhaust steam enters near the lower part of the shell and moves upward through the tube bundle. Condensate still drains to the hotwell, while non-condensable gases are removed from an upper air-cooling zone. Cooling-water direction and pass count remain separate design choices.

The advantages of an inverted flow surface condenser are:

• The upward steam path can suit a turbine exhaust or plant layout that cannot use a conventional downflow shell.
• An upper air-cooling section gives non-condensable gases a defined route to the air-removal equipment.
• The arrangement can keep the air off-take away from the hotwell when the internal steam lanes work as designed.

The disadvantages of an inverted flow surface condenser are:

• Lower steam admission and upward flow need internal paths that avoid impeding condensate drainage.
• Steam-side pressure loss and distribution must be checked for the actual shell and tube-bundle geometry.
• It has no inherent waterside fouling penalty. Water quality, treatment, tube material and velocity govern that risk.

Evaporative Surface Condenser

An evaporatively cooled surface condenser keeps the condensing steam separate from a recirculating spray-water and air stream. Steam condenses inside the process tubes while water sprayed outside the tubes removes heat; part of that spray evaporates into the passing air. Unevaporated water returns to a basin. The steam condensate leaves through its own closed path, while moist air leaves through the air side. The system needs make-up for evaporation, drift and blowdown, not evaporation alone. Suitability depends on climate, water quality, plume, hygiene, maintenance and plant scale.

The advantages of an evaporative surface condenser are:

• It can use less water than a once-through water-cooled system, although the comparison depends on the source boundary and operating conditions.
• Evaporation can cool towards the outdoor wet-bulb temperature, which can be useful in hot, dry conditions.
• A packaged arrangement can combine condensation and heat rejection without a separate cooling tower loop.

The disadvantages of an evaporative surface condenser are:

• Fans, spray pumps, water distribution, drift eliminators and treatment equipment add capital and auxiliary-power requirements.
• Evaporation concentrates dissolved solids. Poor water treatment can cause scale, corrosion, fouling or biological growth.
• Design and operation must control drift, blowdown, plume, noise and aerosol exposure. Water vapour itself is not evidence of pollutant release.

Conclusion

A surface steam condenser rejects turbine exhaust heat through a barrier between the steam and cooling medium. Condensation at low pressure reduces turbine backpressure, and the recovered condensate can return to the steam cycle if its quality is acceptable. Air-removal equipment clears non-condensable gases; it does not create the vacuum independently of condensation.

Cooling-water pass count, steam-flow pattern and heat-rejection method describe different parts of a design. Selection therefore depends on turbine backpressure, heat load, cooling-water temperature and quality, site limits, pumping power, maintainability and condensate-purity requirements.

Steam-turbine power plants and industrial condensing-turbine systems are common applications. Reliable performance depends on clean tubes, low air in-leakage, effective air removal and adequate cooling. Water use, cost and emissions depend on the complete plant and cooling system rather than the condenser label alone.