Upgrading large facility pool boilers for energy-efficient and consistent heating

by arslan_ahmed | December 15, 2023 2:00 pm

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By Tom Soukup

When it comes to heating water in large pools, one thing is clear: a substantial amount of heat is required. Consequently, the water heating systems employed are robust. In many cases, these systems use boilers and heat exchangers as the heat source, opposed to direct-fire pool heating units commonly found in smaller pool facilities.

This choice is driven by two primary factors: boilers and heat exchangers offer extended system life cycles, and boilers are available in much larger capacities than direct-fire heaters. At many of these installations, boiler systems serve not only for natatorium space heating but also dehumidification loads in addition to heating the bodies of water.

The author’s company specializes in designing, installing, and servicing pool heating systems, both large and small. The most prevalent method for transferring heat from the boiler to the pool water is using large shell-and-tube heat exchangers, although plate-and-frame heat exchangers are occasionally used as well. The author’s company prefers shell-and-tube heat exchangers but adds a unique twist to the design. By implementing minor adjustments to large pool heating systems, the company has found energy savings in the range of 10 to 20 per cent can be achieved. With more substantial modifications to the system, gas savings can exceed 40 per cent.

If heat exchangers are not regularly maintained, efficiencies will decline, and premature component failure can occur. Often, by the time the company is contacted for service, efficiency has decreased by more than 25 per cent due to factors such as metal degradation, deposits, clogging, and water bypass.

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Beyond routine maintenance, one of the most effective practices to reduce fuel costs in large pools is to replace aging heat exchangers. Surprisingly, an in-kind replacement is seldom the best solution. Recalculating the heating load and choosing from a range of heat exchanger options can yield great results. The key lies in choosing the right heat exchanger.

Shell-and-tube versus plate-and-frame

Shell-and-tube heat exchangers are preferable to plate-and-frame heat exchangers due to their resistance to deposits, ease of servicing, and their notable reduction in head loss (resistance to pumping). While some plate-and-frame heat exchangers may offer slightly superior efficiency when compared to their shell-and-tube counterparts, they often require the addition of a booster pump package to operate within the system as intended.

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The disparity lies in the size of water passages within the heat exchangers. Plate-and-frame heat exchangers feature significantly smaller passages compared to their shell-and-tube counterparts. The larger passages in shell-and-tube units not only facilitate water to flow through the unit more freely, but also mitigate concerns regarding the accumulation of deposits, considering pool water contains elements such as calcium and additives.

In practice, it is a common sight to encounter large steel shell-and-tube heat exchangers with cupronickel tube bundles when servicing existing equipment at pool complexes or water parks. Many existing heating systems are sized to raise the water temperature from startup conditions, assuming the entire body of water is at the utility supply temperature. Sizing for the startup load means the system will be oversized for the vast majority of time the system is in operation. There exists room for improvement.

There are three primary areas where pool owners stand to benefit from a modified heat exchanger system.

1. Reducing the overall heat exchanger capacity

To achieve an immediate reduction in energy consumption when updating the heat exchanger design of a facility, the most effective approach is to size the heat exchangers according to the actual operating load rather than startup conditions. Typically, large pool heating systems are initially sized to accommodate peak load conditions, which should ideally occur only once throughout the pool’s entire lifespan, typically when it is new. Otherwise, the body of water is extremely unlikely to be completely drained and refilled, as the facility’s highest heat demand takes place at this point. Maintaining the desired temperature thereafter requires considerably lower heating input.

Sizing the heat exchangers to match the maintenance load, opposed to the startup load, results in a reduction in initial costs, energy consumption, and maintenance expenses. When tasked with replacing heat exchangers, a determination is made regarding the extent to which the system’s heat transfer capacity can be reduced. This involves calculating the input required to sustain the desired temperature while considering factors such as the ambient air temperature, incoming water temperature, and the speed at which the customer wishes to attain the setpoint temperature following backwashes, among other considerations.

It is important to note that while this approach may lengthen the time required to reach the setpoint temperature during initial startup, this becomes inconsequential when the facility is already in operation. As long as the pool reaches the desired temperature overnight following a backwash, downsizing the heat exchangers carries no drawbacks.

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   Reducing the size of individual heat exchangers

In addition to diminishing the overall heat transfer capacity of the heating system, resizing individual heat exchangers yields several advantages. This may necessitate an increase in the number of heat exchangers, but this change is insignificant.

Smaller heat exchangers offer simplified installation and maintenance. For instance, during the last replacement of large heat exchangers with smaller units, the required servicing personnel decreased from four individuals to just one.
In addition, the process no longer requires rigging and heavy equipment to extract the tube bundle from the shell, presenting a safety advantage. The size and weight of the heat exchanger components are far less likely to cause substantial damage if accidentally dropped.

Another benefit of employing smaller heat exchangers is models with a capacity of less than 400,000 BTUh do not mandate American Society of Mechanical Engineers (ASME) inspections every other year, which typically cost around $200 per unit. This cost factor becomes especially significant for large facilities that encompass a substantial number of large heat exchangers, such as the 27 units maintained by the author’s company in a single facility.

Finally, the installation of numerous smaller heat exchangers introduces an element of redundancy, safeguarding against potential maintenance issues or the failure of a single unit.

3. Selecting the highest-performance heat exchanger material

When the author’s company undertakes the replacement of shell-and-tube heat exchangers, they offer customers two material options: steel construction with cupronickel coil bundles or a fully titanium unit. Based on their experience, the steel units typically have a lifespan of five to seven years. In contrast, a properly maintained titanium heat exchanger is expected to last a lifetime.

In terms of cost, a titanium model is approximately 50 per cent more expensive than a steel model. However, the steel units are accompanied by a two-year warranty, while the titanium units come with a 10-year warranty. Essentially, customers pay a 50 per cent premium for an 80 per cent extension in warranty coverage when they opt for the titanium model.

At larger facilities, the company strives to employ the same model of heat exchangers wherever possible, which allows for the stocking of common parts. This practice is instrumental in minimizing downtime in case of a failure.

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Energy savings: A compelling case for boiler upgrades

In this example, the author’s company performed the repair work at a large facility with a heating requirement of 48 million BTUh, and their staff manages the maintenance.

Earlier in the year, one of the existing heat exchangers at this facility experienced a failure. After evaluating the loads, the company replaced it with a unit of lower capacity. Post-replacement testing revealed the new unit, although 250,000 BTUh smaller in capacity, heated water more effectively. This improvement can be attributed to the degradation and deposits present in the original unit.

When replacing a heat exchanger, it is critical to select the unit with serviceability in mind. Units that are easier to service tend to receive more regular maintenance, an insight rooted in the company’s strong service background. Ensuring the service-friendliness of heat exchangers involves the use of isolation valves, the installation of easily disassembled joints, and providing ample clearance for tube bundle removal.

Upon completing calculations and creating a piping diagram, the replacement of a heat exchanger should generally be accomplished in less than a day, allowing the continued use of the pool. The exact duration depends on factors such as the amount of piping required and the mechanical room layout.

Through enhancements to heat exchangers in pool facilities, achieving a conservative expectation of 15 per cent efficiency improvement is feasible. If this improvement falls short of expectations, facility owners should contemplate upgrading their boilers. By replacing standard efficiency boilers with high-efficiency models, and ensuring proper sizing, design, and installation, a minimum energy savings of 35 per cent can be anticipated.

 Author

Tom Soukup is the principal of Patriot Water Works Co. with more than 20 years as a hydronic designer and installer. He specializes in high-efficiency and green technology and brings his expertise to custom hydronic work, pool heating, and agricultural projects. He can be reached via email at twsoukup@patriotwaterworks.com.

Source URL: https://www.poolspamarketing.com/trade/features/upgrading-large-facility-pool-boilers-for-energy-efficient-and-consistent-heating/

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