Measure and Improve Air-Cooled Heat Exchanger Performance
Combining a few strategically placed instruments with analytical tools can provide insight into the condition and improve the performance of critical plant assets.
When heat must be dissipated from a process fluid — either liquid or gas — it is usually transferred to another fluid — either liquid or gas — using a heat exchanger. The two most common types of heat exchangers are liquid-to-liquid and liquid-to-air. The latter, which is more often called an air-cooled heat exchanger, is the focus of this article (figure 1).
Air-cooled heat exchangers in process plants transfer heat from process fluid to the atmosphere. Normally, the process fluid is a liquid although two-phase gas and liquid streams are possible. (A few examples might be when condensing steam or dissipating heat from mechanical refrigeration.) The hot fluid passes through a long section of folded or coiled pipe, usually fitted with fins to create more surface area to transfer heat. Both the air and process fluid must be kept in constant motion to maximize heat transfer; otherwise, warm boundary layers form and act as insulation. Typically, the process fluid is pumped through the pipe, and the heat exchanger must have a mechanism to keep the air moving. This can be convection via a fan. Ideally, however, the method should offer a means of control and not require excess energy consumption.
In the past, monitoring air-cooled heat exchanger performance was often too expensive. Wireless instruments and pre-built analytics have greatly simplify the required effort. But before we examine the solutions, we will look at some of the monitoring challenges.
Heat Transfer Effectiveness
The best an air-cooled heat exchanger can do is to cool the process fluid to the ambient temperature. If the temperature needs to be lower, a spray of water on the pipes and fins adds an evaporative element. In such cases, the process liquid can approach the wet-bulb temperature. This can increase cooling capacity significantly where the climate is dry, but it has far less effect in a humid environment where the wet- and dry-bulb temperatures are very close.
In many applications, the desire is to cool the process liquid to a specific temperature rather than maximizing the amount of cooling. In other applications, the desire may be to dissipate as much heat as possible. This usually means pulling the maximum amount of air through, but again, such an approach can waste energy. Once the process fluid has reached the ambient temperature, any additional air movement, if driven by fans, simply wastes energy. Ideally, the fans should not run any harder than needed for the liquid to reach the ambient temperature (or very close to it).
Figure 2. It takes only a handful of instruments to measure and characterize the performance of an air-cooled heat exchanger installation.
In either case, determining when ideal cooling conditions have been reached requires some effort. The size of the installation and the potential for energy savings will determine the return on investment possible compared against the cost necessary to realize such savings. The cost of running a fan at full speed may seem high, and implementing a solution will likely involve purchasing and installing a variable-frequency drive and all the mechanisms to capture the data and perform closed-loop control.
Heat Transfer Efficiency
Any type of heat exchanger has its limits based on the amount of cooling surface area available and the heat conductivity of the pipe and fins. With those fixed factors in mind, it is possible to determine the theoretical maximum transfer possible for a given set of flows under specific fluid and air temperature conditions. No application is going to achieve 100 percent efficiency, so each must be evaluated to determine what degree of deviance between actual and ideal is acceptable.
The great enemy of efficiency for every kind of heat exchanger is fouling: Particulates carried by the fluid streams deposit on the heat exchanger surfaces, eventually building up a layer that acts as insulation. Heat conductivity declines in proportion to the thickness of the boundary layer. If left unchecked, the material will eventually build up and constrict the pipe diameter, thereby decreasing flow and degrading efficiency.
With air-cooled heat exchangers, fouling usually occurs inside the pipes, but air passages also can be clogged. Anything carried by the wind — dust, leaves and the like — can be pulled into the air passages.
Fouling from within and without quickly reduces heat transfer and, therefore, efficiency. The pump must work harder to move liquid through constricted pipes, increasing velocity and further reducing residence time and cooling. Similarly, the fans cannot move as much air. Once the downward spiral of fouling begins, if left unchecked, it only gets worse.
Figure 3. Surface-reading sensors can read temperature through a pipe wall, avoiding the need for a process penetration.
Characterizing Performance and Condition
The ability to determine how efficiently a heat exchanger is operating depends on measuring what it is doing. Ideally, any installation should be equipped with a full complement of instrumentation to monitor process fluid inlet and outlet temperatures along with process fluid flows (figure 2).
The cooling air temperature also can be useful for efficiency calculations, but because this is the ambient temperature, it may not need its own sensor. If the cooling pipes are wetted with a water spray, the wet-bulb temperature also may be desirable.
With the data from these instruments, it is possible to determine the heat exchanger’s efficiency. A sophisticated installation will likely have these instruments — or at least some of them — in place. The actual instruments may not be attached directly to the heat exchanger, but the same temperatures may be measured at other points upstream or downstream. By contrast, a more sparse installation may have none of these measurements, leaving the heat exchanger to operate at one speed and the process to deteriorate without warning.
Adding instrumentation with all the required wiring to interface with the distributed control system (DCS) or other process automation platform is often viewed as too expensive. In addition, creating the code to perform the necessary calculations and HMI graphics to display the information is beyond what many companies are willing to spend.
Figure 4. Temperature transmitters designed to gather data from up to four sensors can send multiple readings via single WirelessHART data transmission.
Easier Equipment Monitoring
Just as WiFi, Bluetooth and LTE signals have transformed home and personal networking, technology advances have made industrial process monitoring much easier.
Developed as a multi-vendor, interoperable wireless standard for industrial facilities, WirelessHART has simplified process monitoring by facilitating industrial wireless sensor networks. WirelessHART sensors ease installation and eliminate the cost of wiring. The cost to add to existing WirelessHART networks is especially low.
Advantages of using WirelessHART sensor technologies to monitor air-cooled heat exchangers include:
- Temperature instruments can be added to the process fluid pipes without any penetrations. Surface-reading sensors (figure 3) can accurately read the fluid temperature regardless of ambient conditions.
- If it is practical to use conventional temperature sensors, a single transmitter (figure 4) can send data from up to four sensors on one wireless signal. This often allows the addition of air or ambient temperature data.
- Vibration and bearing temperature sensors can be added to fan motors to warn of developing mechanical problems.
- Reading differential pressure across the process fluid inlet and outlet can determine when fouling is beginning to accumulate.
- Measuring airflow and air passage fouling is the most difficult, but a negative pressure reading inside the heat exchanger structure provides the required data.
As mentioned, if any of these instruments are not already included with the heat exchanger installation, they can be added using WirelessHART transmitters. Because these all are used for monitoring, they do not need to be integrated with the DCS. Instead, the sensor data can be sent directly to a specialized application designed for evaluating the condition and performance of heat exchangers.
Figure 5. Preconfigured dashboards display information about a heat exchanger without the need for any custom code writing.
Evaluating Condition and Performance
The data generated by these instruments is interesting, but without proper analysis, it may go unused. Analysis apps designed to perform specific asset monitoring and evaluation functions augment and complement the wireless sensor network. These apps are purpose-built to capture data from instruments attached to plant assets. They include algorithms to look for fault conditions such as fouling by calculating changes in differential pressure across the process fluid line and temperature implications.
Because the app only has to deal with the processes related to its purpose, it can perform sophisticated functions with low overhead. Using preconfigured HMI graphics and information drawn from a large population of real-world installations, such apps can present performance and condition data analysis via intuitive dashboards (figure 5), without the need to write any custom code.
An app can send alerts when certain conditions —a change in vibration levels, or when fouling crosses a threshold, for instance — are met. Different individuals in different departments can access the same information in real time from any device capable of hosting a web browser.
Information from a group of heat exchangers and other types of assets —liquid-cooled heat exchangers, centrifugal pumps, steam traps and others — using similar apps can be compared and analyzed across a plant or company. These industrial internet of things-based solutions are quickly gaining ground with industrial users.