Condensate often gets overlooked and ignored. Often thought of as an inexpensive byproduct, condensate can allow you to conserve make-up water and save on expensive treatment chemicals.
Condensate is a ready-made supply of recoverable energy for a boiler system (Figure 1). Chemical plants typically should be able to recover more than 60% of the generated condensate. However, traditional system designs and installation practices often can't ensure positive condensate drainage. As a result, the condensate frequently is drained to waste when the heat exchanger stalls, diminishing performance.
Making simple changes in system designs and following practical management steps can return significant savings as well as increase heat exchanger performance and integrity.
Return of condensate to the boiler plant requires some means of motive pressure. This comes from the supply steam pressure or via a mechanical pump. Either way, the motive pressure always must exceed the condensate return backpressure to guarantee continuous drainage.
Two common piping designs for heat-exchanger condensate drainage are to incorporate a level-actuated collection pot or use steam traps. Both types of devices are direct piped to the condensate return system and can be affected by varying return line backpressure.
A level-actuated collection pot often is chosen for large, high-capacity high-pressure reboilers as well as vacuum columns and flash tanks. The heat-exchanger control scheme can be a constant-level design incorporating a modulating steam valve for temperature control, or a variable system employing a constant-pressure steam supply while changing the exposed heat-exchanger surface area. Both approaches use an actuated drain valve to maintain condensate levels.
Both designs provide process temperature control but pose potential performance and equipment-integrity problems.
The constant-level control system relies on varying steam pressure and volume to maintain desired process temperature. As the thermal requirement decreases, the modulating supply-steam control valve closes, reducing steam pressure and volume. This results in low condensate return motive pressure. If the steam pressure in the heat exchanger becomes less than the condensate backpressure, a flooded condition will arise -- decreasing thermal performance, creating a corrosive condition (carbonic acid from cooled condensate), spurring surface pitting (accelerated by trapped noncondensable gases) and potentially compromising the structural integrity of the tubes, tube sheets and other components through stress cracking and water hammer (Figure 2).
The variable-level control system relies on constant steam pressure -- the condensate drain valve is modulated to expose or flood the heat exchanger surface area. As thermal requirements decrease, the condensate drain valve closes to back up condensate into the vessel, effectively reducing the surface area exposed for heat transfer. The constant steam pressure provides a positive motive pressure for condensate return. Again, though, the downside is corrosion and decreased vessel life and structural integrity.