Finding Energy Savings from Heat Exchanger Assessments and Tube Bundle Repairs

Energy losses – Scale build-up and leaks reduce the effectiveness of the system’s heat transfer capability. Even a thin layer of scale can reduce heat transfer by 3 to 6 percent, costing thousands of dollars in wasted energy. Open loop systems (domestic hot water, clean steam generators, steam exchange humidifiers, and some manufacturing processes) have the most scale because of the constant makeup water, which contains dissolved minerals. They should be cleaned annually. Closed loop systems (building heat systems and some manufacturing processes) should be cleaned at least every three years, or as needed.

Increased heating time – As scale develops, it takes a heat exchanger far longer to transfer heat. For example, if the facility requires reliable access to hot water, descaling the heat exchanger helps maintain the energy transfer time as designed by the manufacturer.

Leaking tube bundles – As the scale attaches to the heat exchanger, it begins to corrode the surface. Over time, the scale can eat away the metal and create leaks. Inspection, testing, and cleaning helps to avoid emergencies where a leak suddenly appears and the heat exchanger fails to work correctly.

Most heat exchangers will leak undetected until the steam system experiences water hammer, an increase in failed traps and valves, or pressure/temperature issues. In most cases, these problems can be prevented through a regularly scheduled inspection, testing, and descaling program. 

Four Key Objectives of Heat Exchanger Assessments

Regular heat exchanger assessments are critical to their efficient use. The four most important objectives of such a program—which should include testing, maintenance, and repairs—are ensuring operational integrity, guaranteeing thermal efficiency, promoting water conservation, and promoting chemical conservation.

Operational integrity – A heat exchange assessment should begin with basic identification and documentation of the heat exchangers. The documentation should include the tag #, location, type, manufacturer, model, and application. For example, APM heat exchanger surveys identify each exchanger with a unique number on a stainless steel tag, which provides a quick and easy way to reference the heat exchanger’s location and information. 

Hydrostatic pressure testing is then conducted, in which the heat exchanger is taken offline to identify and verify the functionality of isolation valves and ports for cleaning. Pressure testing will then be carried out to verify the integrity of the bundle. Fluid is run through the heat exchanger at a pressure slightly above the operating pressure to enable investigators to determine if there is a leak from one medium to the other (e.g., from the water side to the steam side). 

Heat exchanger survey reports include summary data listing the total number of exchangers, highlighting those with “function-impeding issues,” and a graphic presentation of results. Information includes total estimated annual steam losses; total estimated annual therm losses; heat exchanger valve status; and tube bundle failures. 

In the case of leaking heat exchangers, the assessment will include measurements of the water volume leaking, the temperature coming from the tap water, and the temperature desired as an output. A spreadsheet is developed to calculate how much energy is lost, which can be used by utilities to provide incentives to repair leaking heat exchangers. 

All the operational integrity information is folded into a final report with assessment details, along with information on the cost of cleaning each heat exchanger and/or any necessary repair costs.

If there are no leaks, borescope pictures can be used to capture visual assessment and prove scale build up in the bundle and an assessment of whether it can be cleaned in the future. If necessary, the report might include suggestions for installation of a port that allows for cleaning-in-place procedures. 

Thermal efficiency – Since fuel is being used to create heat, operators need to ensure that the heat goes where it is intended. In a catastrophic scenario, for example, a leaking heat exchanger, water that had been heated up to a certain required temperature is leaking from the water side into the steam side and flowing away with condensate. That water must be replenished, and, since replenishment water will be street water that usually comes in at a temperature of about 60 to 65 degrees Fahrenheit, it will take considerable energy to be heated up again to the desired output temperature of 130 Fahrenheit. This waste of water that has already been heated represents a serious loss of thermal efficiency. 

Buildup of whiteish scale on the water side of the heat exchanger also prevents the efficient transfer of heat. Perfectly clean metal transfers heat from one medium to the other way far better than metal caked with scale. In addition, the scale buildup starts to reduce the volume of medium that can go through that heat exchanger, so all things being equal, it will take more time to get the same amount of water out as the scale builds up and reduces the element’s usable volume. In this case, thermal efficiency is improved by descaling using a clean-in-place process, which is far less disruptive for facilities and operations personnel. 

Removing scale is extremely important because scale increases the differential pressure through the heat exchanger. It also reduces flow and increases energy demand on pumps and reduces the ability to transfer heat effectively and efficiently. Scale also creates an environment for under-deposit corrosion, which can shorten the life of the heat exchanging equipment.

It can be difficult to measure precisely how much energy is lost due to the scaling because that will depend upon the chemical composition of the scale, the size of the bundle, and the pressure on both ends. The assessment report may contain a picture taken by a borescope showing the bundle as installed and one with the current status to illustrate the amount of scale buildup. 

Water conservation – In the case of a leaking bundle in a heat exchanger, water must be replenished, which can result in significant additional costs, depending upon the community water costs. 

Chemical conservation – This element is especially critical for steam production applications. The makeup water going into the boiler for making steam is highly treated with chemicals, including corrosion inhibitors and other chemicals to prevent bubbling up and wet steam in the system. Without the addition of these chemicals, water from the street could destroy the boiler or corrode it faster.

Operators producing steam are looking to capture all the heat out of that steam, then return the steam that turns into condensate to the boiler. In effect, the condensate is like liquid gold—operators seek to preserve as much condensate as possible and bring it back to the boiler. A leaking heat exchanger leads to diluting water. The water ends up colder and lacking the right chemistry, so more water must be added to restore that chemistry. If not caught in time, the boiler can corrode quickly. 

Establishing Return on Investment

The heat exchanger assessment program should include energy savings calculations with return-on-investment data so operators can evaluate the business case for performing the cleaning and get an estimate of the significant overall savings that can be achieved. The information includes a calculation of approximate heat exchanger cleaning program payback time in years, based on assumptions for cost of steam, percentage transfer loss due to scale buildup, and fouled heat exchangers. 

APM Steam has found that the average steam cost for most facilities is about $15 per 1000 pounds of steam produced. Ten of those dollars (about two-thirds) of the cost is energy. Establishing the ratio of water to chemicals cost depends on the facility and how much treatment the water needs, but estimates suggest about $3 for water and $2 for chemicals to make 1000 pounds of steam. APM Steam generally provides operators with a conservative assumption that they can save about 3 percent of their energy consumption by cleaning scaling. 

The heat exchanger assessment should be undertaken at least every three years, depending upon location. In an area with poor water quality (i.e., one with hard water that creates a lot of scaling), operators might test more often, while areas with good water treatment may not require more frequent testing. Testing should ensure that heat exchangers are hitting their set points, are free of scale, and are not leaking. Regularly making sure that is the case and addressing issues promptly is the way to go. Scale build up over time degrades the system and is likely to lead to more issues. 

Evaluation on the Ground 

APM has been providing service to the 562-bed University of Vermont Medical Center since 2011, helping them run their steam system as efficiently as possible as they focus on patient care and academic research. The work included surveying several buildings with more than 500 steam traps, and over 20 different heat exchangers. Then APM built a comprehensive program to assess the most cost-effective opportunities for energy savings.