Aside from simple blending processes, most chemical processes involve a temperature change in one way or another. For example, products may need to be heated to facilitate a reaction, or cooled to control reaction rate. Many of these applications involve using a heat exchanger to add or remove heat from the process fluid.
Heat exchangers come in every imaginable size and configuration, but they all have a common function: transferring heat in or out of a process fluid. The transfer medium can be air (cooling), steam (heating) or another liquid for either purpose. However they are configured, it is important to keep the process fluid contained.
Similarities exist from an operational standpoint as well. Heat exchangers depend on complex internal geometry to provide a large amount of surface area. This can create convoluted internal flows, small passages prone to plugging from debris or fouling due to solid deposit accumulations. Such plugging and fouling reduce heat transfer efficiency and can limit throughput when severe enough. These problems drive up operating costs while capacity goes down, which can reduce the overall process performance. If cooling capacity is critical for a potentially hazardous exothermic process, process safety could be threatened.
Equipment performance studies in oil refineries have produced critical findings on operational costs. For instance, fouling capable of restricting throughput by just 2 percent effectively wastes 10 percent of total energy input. Naturally a typical refinery has a large amount of heat exchanger capacity, and when such losses are multiplied across all U.S. refineries, fouling and plugging cause a total of $2 billion in losses, industry-wide.
Despite the scale of the problem, many refineries and chemical plants depend heavily on manual inspections or schedule-based cleanings to maintain their heat exchangers. As a result, few heat exchangers have more than the most rudimentary instrumentation. This can at least partly be explained by the fact that process heat exchangers usually are purchased as complete units from an original equipment manufacturer in a competitive bidding situation. Reducing the amount of instrumentation reduces the price, and if the buyer does not ask for the instrumentation to begin with, it will not be provided. If the plant engineers stay with manual inspections and time-based cleaning cycles, such instrumentation will not be missed. For automated monitoring, however, instrumentation is key.
FIGURE 1. It takes only a handful of instruments to measure and characterize the performance of a heat exchanger installation, but they must be placed strategically.
How to Evaluate Heat Exchanger Performance
What should inspectors be looking for anyway? How can heat exchanger health be evaluated in the first place? How can instrumentation provide useful insights? Let’s unpack the relationships.
Any type of heat exchanger, whether liquid-to-air or liquid-to-liquid, has a transfer capacity limit based on the amount of cooling surface area available and the heat conductivity of the internal pipes and fins. With those fixed factors in mind, it is possible to determine the theoretical maximum heat transfer available for a given set of flows under specific fluid and air temperature conditions.
As mentioned previously, fouling is a universal problem. Particulates carried by the fluid streams deposit on the heat exchanger surfaces, and heat conductivity declines in proportion to the thickness. Fouling from either side of the exchanger — and potentially both — quickly reduces heat transfer rates and, therefore, efficiency. Once the downward spiral of fouling begins, if left unchecked, it only gets worse.
Determining how efficiently a heat exchanger is operating depends on measuring what it is doing (figure 1). This can be accomplished with instrumentation that includes, at a minimum, devices measuring:
- The process fluid temperature differential (inlet compared with outlet) and flow.
- For liquid-to-liquid designs, the transfer fluid temperature differential and flow.
- For air-cooled designs, the cooling air temperature and flow.
FIGURE 2. Surface-reading sensors can read temperature through the pipe wall, avoiding the need for a process penetration.
With the values from these instruments, it is possible to determine how much heat is actually being transferred. When this is compared with the theoretical maximum, it provides the heat exchanger’s efficiency. The process fluid temperature change is the first step, and it is examined in light of its flow rate.
Having these three basic measurements via instruments is the bare minimum. If they are not already part of the installation, they should be the first to be added. Keep in mind that it is possible that these measurements are already being taken at points upstream or downstream. In such cases, they can be extracted from the distributed control system (DCS). If they are not available or practical to access, however, it will be necessary to add instruments. There are many options for instruments that can mitigate the costs and complexity.
FIGURE 3. Temperature transmitters designed to gather data from up to four sensors can send multiple readings on a single WirelessHART signal.
Equipment Monitoring with Wireless Instruments
Technology advances have simplified heat exchanger monitoring. One option for all heat exchanger measurement applications is WirelessHART-based sensors. The wireless devices ease installation and eliminate the cost of wiring. In plants with existing WirelessHART networks, adding devices for heat exchanger measurement applications has a low cost.
Benefits of using WirelessHART devices for heat exchanger monitoring include:
- Temperature instruments can be added to the process fluid and transfer fluid pipes without any penetrations. These sensors read through the pipe wall (figure 2) and can measure the interior fluid temperature accurately regardless of ambient conditions.
- If it is practical to use conventional temperature sensors, a single WirelessHART transmitter (figure 3) can send data from up to four sensors on one wireless signal. This can cover all four required measurements for process and transfer fluids while eliminating any lag between the four measurements.
- Reading differential pressure (DP) across the process fluid inlet and outlet can determine when fouling is beginning to accumulate or if there is a leak in any heat exchanger tubes.
If working with an air-cooled heat exchanger, additional instruments may be needed:
- Vibration, position and bearing temperature sensors can be added to fan motors to warn of mechanical problems developing.
- Sensors to measuring airflow can help determine possible air passage fouling.
All of these instruments are available with WirelessHART transmitters. Because they are used only for monitoring, they do not need to be integrated with the distributed control system (DCS). Readings can be integrated if desired, however, for other purposes.
FIGURE 4. Pre-configured dashboards display information about the process heat exchangers. Should the plant operator wish to view detailed data, it can be accessed from links on the dashboard.
Evaluating Condition and Performance
The data generated by these instruments is interesting, but without proper analysis, it generates no benefit. Fortunately, modern wireless transmitters can send data to analytics applications designed to perform asset monitoring and evaluation functions. Some applications are purpose-built for heat exchangers.
These types of applications are equipped with algorithms to look for conditions such as fouling by watching changes in the aforementioned measurement points. Such indications can warn of developing problems before they cause outages.
For instance, with one heat exchanger monitoring application — because it deals only with the processes related to its purpose — it can perform sophisticated functions with low IT overhead. It uses preconfigured human-machine interface (HMI) graphics and information drawn from real-world installations to present performance and condition data analysis via dashboards (figure 4).
The software uses the data from the basic instruments (temperature differentials, flows, DP, etc.) combined with design data related to an individual heat exchanger (area, coefficient, heat capacities, etc.) and process fluid characteristics (heat of vaporization, inlet/outlet vapor fraction, etc.). From those inputs, a list of actionable information can be derived, including:
- Overall heat exchanger health.
- Fouling factor.
- Fouling rate.
- Heat duty.
- Duty error
- Lost energy costs.
- Heat transfer coefficient.
- Cleaning recommendations.
The application also can be used for business planning and scheduling purposes. It identifies abnormal situations and responds by sending alarms when certain conditions — when fouling crosses a threshold, for instance — are met. While these findings can be presented using the preconfigured dashboards, the detailed data can be reviewed easily.
Providing the plant engineers allow it, different individuals in various departments can access the same information in real-time via a web-based user interface. Information from a group of heat exchangers as well as from other types of assets such as centrifugal pumps, steam traps and others using similar applications can be compared and analyzed across a plant or company.
These Industrial Internet of Things (IIoT)-based concepts are quickly gaining ground with industrial users. In plants using such technologies, every monitored asset is available from a centralized analytics platform. Such systems offer features such as:
- An accessible dashboard.
- The ability to display actionable information a tabular summary for each asset.
- Filterable and searchable data to allow users to identify trouble equipment.
- Sortable assets for project prioritization.
- Exportable summaries for custom reports.
- Raw data and analysis for each specific asset.
In conclusion, these types of applications — combined with new WirelessHART instrumentation options — provide capabilities that previously have not been available. With IIoT and wireless technologies, continuous monitoring is practical and less expensive to install and operate. Such pervasive sensing combined with analytics can reduce unplanned production losses, cut maintenance costs related to heat exchanger cleaning and reduce energy use.