In the final part of this series, we'll cover steps three and four of steam system optimization.
Step 3: Review steam utilization by various steam users. The most useful part of steam's heat content is its latent heat, rather than its sensible heat. To fully utilize the latent heat in steam, two critical factors apply:
1. continuous removal of condensate from the heat exchanger and
2. maintaining the lowest possible backpressure.
If the condensate removal is reduced, condensate could flood heat exchanger surface, limiting the area available for heat transfer. If the backpressure at the outlet of an exchanger increases, the latent heat available from steam would be reduced gradually. When the backpressure equals the supply steam pressure, the heat exchanger potentially could stall. Hence, it is better to check the existing heating control systems to ensure the steam-heated exchanger is neither flooded nor stalled.
For optimized steam use consider other options to do the same job. Switching to motor drives, instead of steam turbines, is a common alternative when excessive low-pressure steam is vented.
Another option is to use mechanical vacuum pumps in place of steam jet ejectors. At low ranges of vacuum creation, steam jet ejectors need at least ten times more input energy than mechanical vacuum pumps. Also, whenever steam jet ejectors are used the condensate must be drained to the sewer, adding to the wastewater treatment plant load.
Many process plants use steam strippers, where steam directly contacts the process streams to raise the temperature of the incoming stream and then strip out the intended component. It would be more efficient to indirectly heat the incoming stream in a separate (external) exchanger, then supply steam only for the purpose of stripping. This would recover part of the supplied steam as condensate and reduce the amount of wastewater generation.
It is very common to see many air-cooled and cooling-water exchangers. Thus, one option to optimize steam use is to preheat the incoming stream with a process stream that goes to cooling. It may be possible to identify a suitable heat source nearby. In some plants, the wasted heat from boiler or furnace exhausts also could be utilized to preheat the incoming stream. Keep in mind the concept of "pinch" technology — matching suitable heat sinks and heat sources.
[Read Steps 1 & 2]
Step 4: Recover and reuse the condensate to the maximum possible extent. Steam systems are designed to work on 100% make-up boiler feed water. In the most efficient steam systems, the make-up water addition is only about 20%.
At present, most process plants must treat and dispose wastewater they generate. The steam condensate, if not collected and reused, would end up in the wastewater stream. So first, have your energy or utility engineer calculate the value of the steam condensate. It could be surprisingly high, justifying many condensate recovery actions.
Reasons for not recovering and reusing the condensate include:
1. collection pipes and pumps weren't provided in the initial design,
2. fear of contamination in the condensate, or
3. concerns about backpressure/water hammer in the return system.
Modern online analytical instruments can obviate fear of contamination, eliminating that excuse for draining the condensate. Specialists can easily address backpressure and water hammer in the return system. In addition, reengineering the existing system with necessary piping changes and additions could eliminate the water hammer problem. Cases exist where prolonged water hammer caused catastrophic damages to the integrity of steam systems — with a few ending in fatal accidents. Eliminating water hammer not only optimizes the steam system, but improves the system's integrity.
Ven V. Venkatesan is Chemical Processing's Energy Columnist. You can e-mail him at VVenkatesan@Putman.net.