Don’t run your heat exchangers blind foldedValue Paper Authors: Jeffrey Vasel, Richard Vesel, Edgar Jellum, Alexander Horch, Conni Hinkel

2 Don’t run your heat exchangers blind folded | ABB Value Paper

Don’t run your heat exchangers blind folded

AbstractThis article discusses the basic concepts and types of heat exchangers (HX), along with their problems and resulting consequences if these problems occur. ABB’s approach and solution to dealing with heat exchanger fouling and general degradation of heat exchanger efficiency is described. The article concludes with future trends of heat exchanger monitoring.IntroductionThe purpose of heat exchangers is to warm, evaporate, cool, or condense process fluids. Typically, a heat exchanger is used to cool and/or condense a hot fluid down to a desired temperature or to heat and/or evaporate a cold fluid up to a desired temperature. Simply stated, heat exchangers exchange heat energy between two fluids. The two fluids, however, never mix. Instead heat energy is transferred between the fluids through a barrier wall. Heat exchangers are part of our everyday life. They can be found in our automobiles as well as our refrigerators. Heat exchangers are used throughout the process industry including chemical plants, oil and gas sites, utilities plants and mining applications. Some chemical plants consider heat exchangers to be the most important asset next to distillation columns.

In most heat exchanger applications, a hot fluid must be cooled to a certain temperature. The hot fluid flows through the primary side and a cold fluid flows through the secondary side. The secondary side cool fluid absorbs the heat energy from the primary fluid through the barrier wall. The flow rate of the fluids will have a direct affect on the heat transfer.

Types of Heat ExchangersThere is a variety of heat exchanger types including Shell and Tube, Spiral Plate and Tube, Double Pipe, Air Cooled and several others. Each has a unique geometry that allows heat to be exchanged efficiently in specific applications. The selection of heat exchangers depends on the situation. For high pressure applications, a Shell and Tube type is typically used. A Shell and Tube heat exchanger is comprised of tubes encased in a pressure vessel. One fluid flows through the tube bundle while the other flows over the outside of the tubes1. A Spiral Plate heat exchanger is used where space is limited while Air Cooled heat exchangers are usually found in HVAC systems.

Problems with Heat Exchangers and their ConsequencesThe biggest problem with heat exchangers is fouling. Fouling occurs when the fluids flowing through the heat exchangers leave deposits behind. There are various types of fouling including chemical, biological, deposition and corrosion. In each case, deposits left behind either plug the fluid flow or build up on the walls of the heat exchanger, changing the HX’s thermo conductivity properties.

Regardless of the type of fouling, the consequences are the same: a lower rate of heat transfer between the process fluids which results in operational inefficiencies. Secondary consequences include the following:

− Plant processes may have to be slowed down to allow the primary fluid more time in the heat exchanger

− Greater thermal differentials may need to be applied. For example, by increasing the flow on the secondary side of exchanger or by changing the secondary side temperature

− Higher energy costs may be associated with decreased efficiency

− Process yield/quality could decrease as HX efficiency declines

According to Mueller-Steinhagan et al, “Fouling is ubiquitous and generates tremendous operational losses, not unlike corrosion. For example, one estimate puts the losses due to fouling of heat exchangers in industrialized nations to be about 0.25% of their GDP (Gross Domestic Product, BIP).” 2 So it is economically important to have well maintained heat exchangers. Maintenance of heat exchangers, however, can be difficult and time consuming. In some cases, the heat exchanger must be sandblasted to remove fouling as power washing will not do. Moreover, cleaning a heat exchanger often requires partial or full plant shutdown. The end goals are to know when the heat exchanger is not running efficiently and to avoid unnecessary cleaning of heat exchangers as both of these issues will be costly.

ABB Value Paper | Don’t run your heat exchangers blind folded 3

ABB’s Solution – System 800xA Heat Exchanger Asset MonitorThere are many critical issues that effect efficiency and performance of a heat exchanger. These include fouling, inefficient operating points, leakage between hot and cold side process fluids and out of design range thermal and pressure stresses. It is critical that the maintenance department has a tool that provides visibility into the HX so that the end user can make informed decisions.

ABB’s Heat Exchanger Asset Monitor or HXAM is a condition-based monitoring tool that identifies performance changes and operational degradation of heat exchanger operations and reports them back to the end user. It is geometry independent, so it works with all types of heat exchangers. It requires only four temperature measurements: Hot In and Hot Out on the hot fluid input side, and Cold In and Cold Out on the cold fluid output side. Fluid mass flow and pressure are helpful measurements but are not required. Unlike other heat exchanger monitoring tools, no other physically invasive measurements are needed to detect operational degradation including fouling or moving away from optimal flow. HXAM is an integral part of System 800xA Asset Optimization, which monitors heat exchanger performance 24/7.

The HXAM calculates effectiveness using a proprietary algorithm3 while the heat exchanger is running satisfactorily. In this training phase, we calculate effectiveness and create a Base Operating Point Set or BOPS. The training phase lasts about 1% of the heat exchanger’s typical maintenance interval. The HXAM can work with multiple BOPS and it can train on live or historical data. See Figure 1 below. The Variations plot is comprised of % Relative Effectiveness versus Time. There are four distinct BOPS in the plot. After the training phase is complete, the HXAM will determine which BOPS the heat exchanger is running in. Should the heat exchanger deviate from the BOPS, alerts will be generated.

Figure 1 – Analysis and identification of Base Operating Point Sets (BOPS)Configuration and operation are simplified with the help of the HXAM faceplate. See Figure 2. Each step in the configuration of the HXAM is identified with a tab. The end user simply goes through each tab and enters in the requested information. Once completed, the HXAM monitors the heat exchanger and reports back to the end user performance changes or operation degradation making the heat exchanger health visible to the maintenance department.

Figure 2 – A faceplate of a HXAMOnce trained, the HXAM periodically reads the four temperatures (Hot In, Hot Out, Cold In and Cold Out) and optional flow and pressure data. This can be done on a user-defined schedule. Using ABB’s proprietary algorithm, the HXAM then assesses the current thermal performance of the heat exchanger relative to performances measured during the training phase. Freshness of training data is checked prior to performing each assessment. If performance or heat transfer effectiveness declines or improves to a preset degree, an alert is generated. The alert is then sent out to the appropriate persons. These contacts can be pre-determined by the end user, who can indicate one list, lists that vary by recipe or shift, etc. Alarms can be directed to notify via email to client PCs, data automatically entered into Excel Reports, paging, text messaging or via CMMS (Computerized Maintenance Management System). Figure 3 shows the workflow of the HXAM.

Year 0 Year 5Year 4Year 3Year 2Year 1

Variations20.0%

10.0%

0.0%

-10.0%

-20.0%

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1 10 19 28 37 46 55 64 73 82 91 100 109 118 127 136 145 154 163

BOP1

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BOP3

BOP4

BO

BOP2

BOP3

BOP4

OEE Improvement

Maintenance cost

Plot areaLoad E2

4 Don’t run your heat exchangers blind folded | ABB Value Paper

Figure 3 – Workflow of a HXAM

HX Thermal Efficiency vs. TimeUnder conditions where there is a known tendency to foul, maintenance procedures are implemented to clean the HX to restore thermal effectiveness. Cleaning procedures are scheduled based on time, or on (typical) manual analysis procedures which monitor the HX.

ABB has Heat Exchanger Asset Monitors installed at several oil and gas installation sites including StatoilHydro (Tjeldbergodden, Norway site) and Shell (Ormen Lange, Norway site).

Future TrendsFuture trends in the process control industry indicate the need to move the HXAM from the field to the home office4. From the home office, a centralized maintenance department will monitor the effectiveness and performance of all their heat exchangers and then dispatch local maintenance personnel to tend to the equipment. With a centralized maintenance department, performance data of the same heat exchanger equipment can be compared among all of the sites. As the maintenance engineers gain knowledge of their heat exchanger operations, this knowledge can be incorporated into the HXAM and maintenance procedures, thus, capturing workforce knowledge. In many instances this knowledge can be compromised, lost altogether or used less effectively when an asset monitor is not implemented.

References

1. http://www.secshellandtube.com/

2. H. Mueller-Steinhagen, M.R. Malayeri and A.P. Watkinson,

“Fouling of Heat Exchanger – New Approaches to Solve Old Problem”,

Heat Transfer Engineering, 26(2), 2005

3. US Patent US2006020420, Richard Vesel, ABB Inc.

4. Chemical Engineering Magazine Making the Most of What You’ve Got,

Joy LePree, February 2008

Author Bios – Jeffrey Vasel Richard Vesel Edgar Jellum Alexander Horch Conni Hinkel

Data Sample

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Maintenance cost

Heat Exchanger Unit

T Hot InT Hot OutT Cold InT Cold OutMass Flow Patented

AssetMonitor

Self Training

BOPs

EfficiencyComparison

CMMS

Email Excel

Paging

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