Process heating accounts for a large portion of the energy demand in industrial plants, and the fuel that is required to run a steam plant represents a costly and necessary reoccurring expense. For this reason, many plants stand to gain from efficiency measures that target process heating systems. One of the most viable areas for realizing cost-efficient improvements is the steam boiler. Efforts that seek to optimize steam-generation processes can have a tremendous financial impact on facility operations.
Steam boilers present a range of opportunities for process heating enhancements, but perhaps the easiest way to impact fuel consumption and expenditures is through maximizing the cycles of concentration. This change can bring significant improvements in terms of economies of water, chemistry and fuel. This article will explain four approaches for increasing cycles of concentration. These strategies include optimizing the cycles of concentration, enabling for a higher condensate return, producing higher quality makeup water and implementing remote continuous operation and monitoring.
Optimize Cycles of Concentration
Every steam boiler has a “sweet spot” in terms of its most optimal ratio — that of the total dissolved solids (TDS) concentration of the blowdown water to TDS concentration of the feedwater. This ratio is known as the cycles of concentration. Boiler manufacturers and professional associations issue recommendations in terms of the allowable quality of boiler feedwater and boiler water. These recommendations are based on the operating pressure and the type of use intended for the steam. Those operational targets always should be applied to ensure protection of equipment, and they should be set to run within a safe distance of the boiler’s limits without being overly cautious.
Plants operating steam boilers below the recommended limits are underutilizing their systems and wasting valuable heat and water. These losses that must be made up for with freshwater additions that need heating as well as mechanical and chemical treatment to remove dissolved solids. Likewise, plants that exceed the recommended limits are putting their systems at risk from deposition, overheating, wear and tear and, ultimately, failure. Ignoring these risks can lead to shutdowns, lost production and severe financial impacts.
If plants are unaware of the maximum recommended boiler water limits for their system, they should not assume that the current limits applied are optimized. Instead, they should inquire with the boiler manufacturer or water treatment supplier.
Return More Condensate
Despite potentially being viewed as a low quality process waste stream, condensate often is the most valuable source of water in a facility. With little to no dissolved solids, condensate is normally very clean. In addition, it often contains residual heat that was not transferred to the process. Condensate’s high purity and heat content are two strong reasons to value this source water higher than makeup to the boiler feedwater, which requires treatment and heating.
In many existing plant configurations, however, condensate is not reused as feedwater for a number of reasons — most commonly, due to contamination or a lack of piping infrastructure. Condensate contamination can originate from the process and may indicate a process equipment failure such as a heat exchanger leak.
Plants considering projects to enable condensate reuse need to weigh the projected benefits against the costs associated with treatment or installing new infrastructure. Depending on the site and the amount of water involved, condensate reuse may not always represent the best option. In each site-specific case, a cost-benefit analysis should be completed to better inform decisions.
Condensate’s high value is mostly due to the purity of the water. In the case of low purity boiler systems such as those that use softened makeup, condensate reuse implies a significant opportunity to increase cycles of concentration. Recycling condensate to the feedwater supply allows plants to reduce their freshwater makeup by an equal amount — which may bring significant pumping and treatment cost savings. Moreover, because condensate is generally purer than even treated makeup, plants can reduce their blowdown ratio, saving water and heat. Combined, these benefits can provide significant fuel-cost reductions.
Produce Higher Quality Makeup
As a steam boiler’s cycles of concentration are increased, plants can make more steam with the same amount of feedwater. The limit on the number of cycles of concentration determined by the feedwater quality: The purer the feedwater, the more cycles that can be achieved, and the more steam that can be produced without having to inject supplemental water. With this in mind, plants can evaluate the economic feasibility of incorporating more robust treatment for generating a higher quality makeup. Depending on the type of pretreatment in use, adding purification capacity may offer a significant benefit.
At a minimum, every boiler system is equipped with softeners designed to remove more than 99 percent of the calcium and magnesium hardness ions. Compared to all other dissolved solids, hardness ions will form deposits in a boiler at any concentration, hence the importance of removing as much as possible.
The limit of cycles of concentration is determined mainly by the boiler feedwater’s alkalinity and TDS. If alkalinity is the limiting factor, a dealkalization process may be added after softening. Although this will not reduce dissolved solids because the alkalinity will be replaced by other ions, dealkalization may still allow a boiler to operate at higher cycles, saving on fuel and water costs.
Reverse osmosis (RO) is another option to consider as an alternative to dealkalization. In addition to addressing alkalinity, RO also will drastically reduce TDS (conductivity) from the makeup, leading to a more substantial increase in cycles of concentration. Obviously, cost differences between RO and dealkalizers specific to the size and type of operation must be carefully weighed against the potential of each option for increasing cycles. In comparing options, a thorough evaluation of goals and methods must be conducted as the engineering, operation and maintenance associated with each respective technology differs.
Other water purification technologies such as demineralization also are available for generating a higher quality makeup. For the purpose of improving cycles of concentration, however, dealkalization and RO are the processes most frequently used.
Remote Continuous Operation and Monitoring
Even when condensate recycling is maximized and boiler cycles are optimized, the benefits of achieving a higher cycling average can be severely impacted by variability in daily boiler operations. It can be challenging for manual blowdown operations to keep pace with steam-rate fluctuations that may be impacted by outside temperatures or production rates. Often, blowdown becomes a steady flow with steam production varying, implying fluctuating cycles of concentration.
To overcome these challenges and better control blowdown variability, plants can use online meters to continuously monitor blowdown conductivity and ensure for continuous cycles control. These instruments are configured to automated control valves for flushing blowdown lines if conductivity readings hit predetermined set points. Conductivity data and valve signals also can be linked directly to computer systems, enabling operators to have continuous access to readings and alarms.
Continuous-monitoring systems are available in all shapes and forms and for all budgets, but quite often, price is not a guarantee of quality of control. This is where an effective installation is vital for ensuring that instruments can operate successfully and deliver the expected savings. It also is important to follow instructions relative to each unit and carefully consider the orientation of flow, positioning of probe and the pipe distances between different components.
Caveat on Blowdown Heat Recovery
While the four approaches discussed in this article are generally applicable for many steam boilers, the capacity for these strategies to deliver beneficial cost savings from higher cycles of concentration can become limited in cases where heat recovery is already employed.
The purpose of a blowdown heat exchanger is to transfer a portion of the energy contained in the blowdown water to the colder feedwater. Instead of most of that energy going down the drain, the heat exchanger allows it to be contained within the boiler system.
When the cycles of concentration are increased, a similar fuel savings is achieved. However, higher cycles of concentration also will result in less blowdown flow — thus reducing the amount of heat that can be recuperated by a blowdown heat exchanger. As such, the opportunity for a heat exchanger to lower the amount of fuel required for heating feedwater becomes lost when cycles of concentration are increased.
The conclusion is that energy savings cannot be multiplied when heat recovery and strategies to increase cycles are simultaneously employed. Both target the same heat. The advantage that maximizing the cycles of concentration have over heat recuperation is the ability to also provide savings related to water and chemical treatment conservation.
In conclusion, maximizing the boiler cycles of concentration is a complex activity that considers many different aspect of the system, including chemical, mechanical and operational processes. When considering these approaches, plants should consult with their water treatment representative. The technical expertise and predictive tools offered by a water treatment specialist can help quantify the potential economic and environmental savings for determining the best path forward.