How to Conduct a Thermal Audit of an Industrial Facility

The efficiency of steam-generating and distribution systems can be improved incrementally, but more substantial gains can be found using pinch analysis for better heat recovery

When it comes to improving the energy efficiency of a thermal system, most industrial organizations focus on making incremental improvements to their utilities. Typically, these incremental improvements will provide up to 5–10% in energy savings and CO₂ emission reductions, with payback times that are under three years. Chemical process industries (CPI) facilities, however, are increasingly looking at completely decarbonizing their facilities, and therefore, are looking beyond the traditional ways of utilizing thermal energy and increasingly pursuing an approach that aligns with principles of the circular economy. By applying circular principles to industrial thermal systems, plant managers can recover massive amounts of energy that have traditionally escaped the facility as waste heat. When this waste heat is put back into the process, it can result in up to 30–50% reductions in energy use and CO₂ emissions. This article describes how to conduct a thermal audit at a CPI site to explore areas ripe for energy savings.

Incremental improvements

Within most industrial facilities, steam is used as the medium to provide heating for process or building applications (Figure 1). These steam systems, however, are often very old and are likely to have been reconfigured multiple times as the facilities or processes change, resulting in the loss of efficiency over time. This is a drain on a company’s bottom line, but it also negatively affects an organization’s ability to continue operating, as regulatory bodies have been tightening their emission standards in recent years.

The stakes are high, so it is vital to understand a system’s energy consumption, in addition to its level of CO₂ emissions, safety and reliability. All of these can be evaluated via a thermal audit.

Auditing steam generation

It is possible for typical steam systems to operate at up to 80% efficiency, but very often, they operate at only 60% efficiency due to a number of factors that can be mitigated after a thermal audit.

A thermal audit begins with measuring the efficiency of a plant’s steam-generating equipment — determining how much energy is consumed (using fuel meters) in the boilers and how much energy is produced (using steam flowmeters).

In the boiler house, the first place to look is boiler stack temperatures and determine whether more energy can be recovered through the installation of an economizer, a condensing economizer, or by improving the existing economizers (Figure 2). Next, measure the percentage of condensate that is returned to the boiler. Ideally, all steam not directly used in the process should be returned as condensate. Failing to reclaim and reuse condensate contributes to energy losses.

The quality of make-up water added to the boiler should also be examined, because higher water quality leads to lower boiler blowdown losses. Next, identify the resulting boiler blowdown requirement and confirm whether a boiler blowdown heat-recovery system has been installed.

Finally, measure the quality of the combustion to identify whether the system is maintaining the correct level of excess oxygen for the fuel used, and if the air flowing into the boiler could be pre-heated. Typically, plants will have some of these improvements implemented, but rarely all. Each of these improvements can help with reducing the energy bill and consequent CO₂ emissions. The breakdown of savings includes the following:

• Boiler blowdown heat recovery — 1% energy reduction

• Stack economizer — 4% energy reduction

• Stack condensing economizer — 6% energy reduction

• O₂ trimming — up to 1.5% energy reduction

• Air preheating — Up to 1% energy savings

From a reliability standpoint, it is important to understand the dryness of the steam produced. Steam that is not dry enough will create erosion in the system and will have a negative impact on heat transfer. Steam dryness can be measured with an instrument or can be measured manually by calculating the energy contained in the steam via a bucket and stopwatch test.

Steam dryness should be above 97% (that is, the steam should contain less than 3% water droplets in suspension). If the steam quality is very poor (generally considered to be steam dryness of less than 95%), it could lead to destructive waterhammer and result in a safety hazard.

It is also important to have a sufficient number of flowmeters in the boiler house to track key performance indicators (KPIs) on a regular basis and assess deviations. This could help identify a sudden drop in boiler efficiency or a reduction in condensate return from the plant.

Steam distribution audit

A steam distribution network in a chemical process facility is usually lengthy, which means there are a lot of areas where energy can escape. Using a thermal imaging camera, it is possible to check both the state of the insulation surrounding the steam lines, and view the valve and pipe accessories that are not insulated at all (Figure 3). Another important item is ensuring the steam system is drained properly from condensate to avoid waterhammer, which could result in safety incidents (Figure 4).

And finally, ensure that the steam traps are tested at least on an annual basis and that failed steam traps are replaced. A best practice is to manage all the steam traps in a database, track their performance and maintain a failure rate below 5% (Figure 5).

Each of the following improvements can help reduce the energy bill and consequent CO ₂ emissions:

• Insulation improvements — 2.5% savings

• Steam trap management — 3.5% savings

Condensate return

Finally, at the process level, it is important to review how condensate is returned. Low-pressure steam users might have stall issues. This means that the pressure in the heat exchanger is lower than the condensate return pressure and the condensate cannot be returned by gravitational means. Very often, these applications will end up draining condensate locally through a bypass valve. In order to return that condensate, a pumping system will be necessary.

When steam is used at higher pressure, it is smart to check whether it is feasible to use flash steam (condensate re-vaporization once it reaches lower pressures) in a nearby application, which can help reduce steam use and consequent energy.

Each of the following improvements can help reduce the energy bill and consequent CO₂ emissions:

• Condensate return — up to 3% energy reduction

• Flash steam recovery — up to 1% energy reduction

Typically, when performing a full audit of a plant, 5–10% energy reduction will be found through projects with a 1–3 year payback. These projects are often referred to as quick wins (Figure 6).

A circular approach to energy

Applying a circular methodology (or “approach”) to thermal energy can help reduce up to 30–50% of energy consumption in CPI plants. However, it requires a change in perspective, moving away from following the thermal system and incrementally improving it, to understanding the process and its needs. What does the process really require?

Do you need steam? A lot of facilities use steam at higher pressures and temperatures because it is an easy medium to produce and distribute (steam requires 100 to 1,000 times less space than water and requires smaller-diameter piping to distribute). But very often, the process does not require these higher temperatures.

In the quest for net-zero emissions, the question becomes “why should we use a steam system that is 60–80% efficient in processes where temperatures below 248°F are needed?” At those temperatures, hot water can be used in systems that are >90% efficient.

Where is the energy going? In a CPI facility, most of the energy used ends up being lost into the atmosphere. Simply stated, the first law of thermodynamics tells us that energy cannot be destroyed or created — its quantity within a system remains stable. When energy is put to work, sometimes by converting it from one form to another, it is degraded to a lower quality of energy. So, if energy in an industrial plant is degraded but not destroyed, how does it leave the plant?

Currently, primary energy is brought into a plant in the form of electricity and fossil fuels. In a typical factory, less than 20% of incoming energy is used for moving things (motors converting electricity into mechanical energy) or lighting the facilities. Due to energy efficiency, part of this energy eventually ends up as waste heat that increases the building’s interior air temperature. Does that mean that the remaining 80% of the primary energy used for thermal is going into the products being manufactured? Not exactly. In most industries, only a small portion of the primary energy is converted into chemical energy contained in the final product. Furthermore, the input materials used in manufacturing are usually at the same (often ambient) temperature as output products when leaving the plant. In fact, the majority of primary energy ends up as waste heat that is frequently lost through stacks, cooling towers and sewage.

The circular approach aims to maximize heat recovery in a plant to drastically reduce the energy needed to operate it and maintain its processes. When applying this, the following methodology of pinch is used.

Pinch analysis

Pinch analysis is a methodology for minimizing energy consumption of chemical processes by calculating thermodynamically feasible energy targets (or minimum energy consumption) and achieving them by optimizing heat-recovery systems, energy supply methods and process operating conditions. It is also known as process integration, heat integration, energy integration or pinch technology.

The pinch methodology involves mapping all heat sources (processes that are cooled down and where energy is removed) and heat sinks in the facility (processes that require heat and where energy is added).

For each of these heat sinks and heat sources, we identify the temperatures available, the kilowatts needed to be added or removed, and seasonality (is there a different load in summer versus winter, and is the load stable throughout the day and night?).

Thermal mapping can be time consuming if the plant lacks meters to track the processes. In those cases, temporary ultrasonic strap-on flowmeters can be used to estimate the heat sinks and heat sources. Once the mapping is finalized, pinch methodology can be used to identify the heat recovery potential and consequently the minimum amount of energy needed for the plant if all waste heat is recovered.

An engineer can then review the model to define what is practically recoverable. Some of the specifics to be considered include the following:

• Heat sources and heat sinks that are far apart within the plant might not be practical to recover from a financial standpoint.

• If there is no time synchronization between a heat sink and a heat source, then thermal storage systems will need to be designed.

Heat can be recovered either through direct heat recovery, using heat exchangers and coils, or with heat pumps that can increase the temperature of low-grade heat to make it usable. Industrial high-temperature heat pumps have enjoyed a number of technical advancements recently, and can easily generate 248°F for hot water or even steam.

The author’s experience has shown that most of the chemical plants that use low and medium temperatures can operate with 50–70% of thermal energy if they were to apply this methodology.

Creating a pinch model for a facility provides the necessary framework for projects to follow and maximize energy heat recovery. Despite pinch rules sometimes seeming to be counterintuitive, they are effective and will be explained below.

Pinch model

As shown in Figure 7, the pinch point is the temperature at which the hot composite curve and cold composite curve are the closest. Typically, the pinch point divides the temperature in two regions: heating utility and cooling utility.

Heating utility (steam, hot water) can be used only above the pinch and cooling utility (chillers, cooling towers) below it.

Some takeaways from a pinch model include the following:

• The overlap area (gray area in Figure 7) represents the heat-recovery potential through heat exchangers. There is no need to use heating utility or cooling utility in this area.

• Never cross pinch temperature with heat exchangers. If the pinch point is crossed, a high-grade heat source will be used to heat a lower-grade source, wasting an opportunity of heat recovery at the higher temperatures. This will prevent the system from maximizing heat recovery and, in the end, use more external energy. Not crossing the pinch point is counterintuitive to a lot of engineers looking for quick wins. When the focus is on short payback times, the natural tendency is to take a very hot stream and use it to heat something that is cold. This allows for a high temperature difference on the heat exchanger, smaller heat exchange surfaces and quick payback times.

• Heat pumps have to cross the pinch. Ideally, use the lowest-temperature stream (that cannot be used for anything else) and, with a heat pump, produce the highest temperature for which external energy is needed. A high ∆T will result in a lower coefficient for performance (COP) for the heat pump (COPs will be between 2 and 3), but this is still better than an electrical boiler with a COP of 1. Having a high lift on a heat pump is counterintuitive. Typically, users who are focused on quick wins might try to find applications with lower temperature differentials to achieve higher COPS and higher payback times. But that approach does not make sense from a pinch standpoint. If that application can be completed through direct heat recovery (free heat recovery), there is no need to use a heat pump, which will consume electricity.

Once it is understood that most of the energy used in the plant is wasted, the circular approach really becomes a powerful concept, and pinch analysis helps to maximize that heat recovery.

Incremental gains or circular?

When looking at both quick incremental wins and the circular methodology, it is clear that both are viable strategies for energy reduction, and this is not an either/or situation. While quick wins can yield immediate results, the circular approach offers the potential for substantial long-term reductions in energy consumption — although it may require more significant capital expenses and longer implementation times. In the meantime, some of the quick wins identified above can start the journey toward energy reduction and can begin the process of building a roadmap toward decarbonization.

Edited by Scott Jenkins

 

Editor’s note: All photos were provided by the author, except when otherwise noted.

Reference

1. Natural Resources Candada. https://natural-resources.canada.ca/sites/nrcan/files/canmetenergy/pdf/fichier.php/codectec/En/2009-052/2009-052_PM-FAC_404-DEPLOI_e.pdf

Author

Patricia Provot is the president of the Americas region at Armstrong International Inc. (816 Maple St., Three Rivers, MI, 49093; Phone: 269-273-1415; Email: [email protected]; Website: armstronginternational.com). Provot has served in that role since 2017 after first joining Armstrong in 2000 as an energy auditor. She has served clients and assisted teams across the U.S., Belgium, India, China, the Philippines and elsewhere. Provot is heavily involved in product innovations, growing the next generation of energy auditors, standardizing Armstrong’s methodology and pursuing the company’s carbon-cutting initiatives. She received a master’s degree in chemical engineering from Haute École (de la Communauté Française) “Paul-Henri Spaak” in Brussels, Belgium.