One of the most common pieces of equipment in many industries is a heat exchanger. As its name implies, it is designed to move heat from a process fluid to another fluid — which might be liquid or air. The process fluid is heated or cooled as the application demands. The transfer fluid might be air or a liquid, also as the application demands.
In a previous article, I looked at air-cooled heat exchangers, where a hot process fluid typically is cooled by ambient air. In this article, I’ll examine liquid-to-liquid designs, discuss how they work and explain how to monitor their performance to gain higher efficiency and reduce operating costs. Some may be liquid-to-gas (steam, for example), but they differ from air-cooled designs in that both sides are closed systems. For the balance of this article, I’ll simply call these heat exchangers.
While there are many possible configurations, two of the most common are shell-and-tube and plate heat exchangers. Shell-and-tube designs send the process fluid (usually a liquid) through a group of parallel tubes that are enclosed by a shell (figure 1). The transfer fluid flows around inside the shell as directed by baffles, and heat is transferred through the tube walls.
Plate heat exchangers use a stack of alternating-shaped plates that seal around the outside edges to form liquid passages (figure 2). There are additional passages in the corners that allow the process and transfer fluids to flow between alternate plates. This design provides a great deal of surface area for heat transfer and can support a large or small number of plates, depending on the application. Operators can open the frame and add or remove plates to the stack as needed. This also permits inspection for internal deposits and makes cleaning easier than with most shell-and-tube designs.
Monitoring the critical temperature, flow and pressure variables in a heat exchanger with WirelessHART instruments provides the data required for analysis to optimize operation and maintenance. Using preconfigured apps to perform the analysis greatly simplifies this task. When combined, these two technologies substantially simplify the task of improving heat exchanger performance.
Maintaining Effective Transfer
Creating conditions for effective heat transfer with any equipment configuration depends on two things: fully distributed fluid flow and unimpeded heat transfer. Any heat exchanger has a theoretical maximum transfer capability based on the amount of surface area and the heat conductivity of the metal. These factors relate to the hardware itself, but operational effectiveness also depends on flow rate/residence time of the fluids and temperature differentials. These variables have to be considered in light of the limits of the equipment.
Fluid flows must be continuous and spread out over the entire surface area. Movement creates turbulence, avoiding the formation of thermal barriers and boundary layers where much of the liquid is kept away from the actual transfer surface. Internal flows are determined by the equipment’s physical configuration, so there is no way to improve it without changing the structure. Internal obstructions can block off sections and cause dead areas, however, which reduces the available effective surface. These dead zones need to be avoided.
The heat transfer surface must be free of any deposits that can serve as insulation. Sometimes, these are carried and deposited by the liquids, or they may be the result of corrosion. Any solids on the metal on either side can reduce the heat transfer effectiveness. If the deposits are thick enough, they also can impede flow, compounding the problem.
FIGURE 1. Shell-and-tube heat exchangers typically have a large, open, free passage for the transfer of liquid, so changes in differential pressure can be difficult to capture.
Efficiency considers how much effort it takes to get the temperature change desired. For example, if the application calls for raising the temperature of a feedstock from 212 to 302°F (100 to 150°C) at a flow of 20 gal/min, the transfer liquid temperature will have to be hotter than 302°F (150°C). But, how much hotter, and how high a flow rate is needed? This is determined by the efficiency of the heat exchanger. Because heating the transfer liquid costs money, less heat is better.
No heat exchanger can achieve 100 percent of the theoretical maximum transfer. But, operators normally want to be as close as possible, and controlling these flows and temperatures has a major effect. A given heat exchanger will have a sweet spot: the ideal flow and temperature range where it performs best. Operating above or below this range will result in a loss of efficiency.
Where the process calls for highly variable flows and temperature changes, the facility may have multiple heat exchangers of different capacities, often operable as trains. For example, a thermal power plant might have a conventional boiler feeding steam to turbines to generate electricity.
Exhaust steam is fed to air-cooled heat exchangers to act as condensers, but then liquid condensate flows to shell-and-tube heat exchangers heated by steam and acting as feedwater heaters. Because electric power demands fluctuate, the amount of steam generated also fluctuates. Power plants typically have multiple heat exchangers for condensers and feedwater heaters. This allows the generating unit to operate with the amount of cooling and heating capacity needed at any moment, with every heat exchanger operating in its efficiency sweet spot.
FIGURE 2. Plate heat exchangers can be configured with a different number of plates to optimize the unit for a specific flow rate or temperature change.
Getting the most efficiency out of a common shell-and-tube or plate heat exchanger requires knowing much detail about what is happening inside. Because the main operating parameters relate to flow rate and temperature differential for the two respective fluids, it is important for them to be monitored.
It is possible to calculate the maximum theoretical heat transfer for the installation at a specific set of temperature differentials and flow rates. The equipment manufacturer usually can provide some supporting data on its ideal operating range. Any deviation from the calculated maximum indicates a loss of efficiency, which raises the operating cost.
Efficiency losses typically are caused by fouling or other material deposited on the surface of the tubes, which acts as insulation. Such deposits can be on either side of the tube or plate wall, or both, depending on the conditions. The challenge becomes determining how far below maximum efficiency the unit is running and where the fouling might be occurring. Data from instruments can help plant personnel make these critical determinations.
Whether using a shell-and-tube, plate or other heat exchanger configuration, it is important to have a basic set of instruments to determine what is happening and what kind of efficiency performance the exchanger is delivering. Things to consider regarding basic instrumentation include (figure 3):
- The process fluid inlet and outlet must have temperature sensors so any change can be measured.
- The transfer fluid inlet and outlet also must have temperature sensors.
- The process fluid flow is likely controlled by the automation system controlling the process unit, but such is not always the case. If the flow rate value is not already being measured or the data is impossible to access, a flowmeter should be added.
- The transfer fluid flow may be measured somewhere in the system, but just in case it is not, or if the value may not be easily accessible, it is helpful to have a flowmeter on the outlet.
- The pressure drop across the process fluid side should be measured using a differential pressure (DP) transmitter, with the high side tied to the inlet and the low side to the outlet.
- The pressure drop across the transfer fluid side is a valuable measurement for plate heat exchangers, but it is not as critical for shell-and-tube units because the internal passages are usually wide open.
FIGURE 3. Strategically placed WirelessHART instruments can measure all the operational variables for a heat exchanger.
Where heat exchangers are arranged in a train, it is valuable to have temperature sensors on each to measure the performance of the intermediate units. This allows easier visibility to see which specific exchanger may be experiencing problems, which is not possible if only monitoring the temperature at the beginning and end of the train.
If every instrument listed above is deployed, the result will be four temperature instruments, two differential pressure instruments and two flowmeters. It can be difficult to add the required wiring infrastructure for eight points of measurement for a single heat exchanger and incorporate each into the existing process automation system. The wiring and system integration costs alone decrease the overall savings potential.
An easier way to capture these readings using a minimum amount of hardware is a wireless network using a protocol such as WirelessHART. Being wireless, it eliminates most wiring and I/O costs.
Here is a typical setup using WirelessHART instrumentation:
- The temperature readings can all be handled by a single WirelessHART temperature transmitter capable of sending data from four sensors on one wireless signal, reporting each reading in turn. The host system sorts out the data and updates each sensor reading individually. This setup also coordinates all temperature readings, eliminating the impact of reporting lag in the temperatures.
- The differential pressure readings can be provided using a native WirelessHART differential pressure instrument.
- The flowmeters for the transfer and process fluids can use self-contained WirelessHART differential pressure-based instruments.
The cost and possible downtime required for installing WirelessHART instruments must be balanced against expected benefits. WirelessHART reduces installation costs and time, providing a quick return on investment from even smaller capacity heat exchangers.
Understanding Performance from Data
When a heat exchanger is fully instrumented, how can the data help improve performance and reduce costs?
When the temperature change and flow rates of both fluids are known, it is possible to determine how close the heat exchanger is running to its theoretical limit. Naturally, in the real world, 100 percent efficiency is not practical, so a company must determine how much deviation it is willing to tolerate before it takes the heat exchanger out of service for cleaning. This practice allows for more predictive and proactive maintenance compared to reactive and preventive approaches common today.
If there is degradation due to fouling, the instruments help determine where the deposits are forming. If there is a rise in the differential pressure reading of the process fluid without a corresponding change in flow and no change in the transfer fluid, the fouling is on the process side. Inexplicable changes in pressure or flows could indicate internal leakage where one liquid is mixing with the other.
FIGURE 4. Preconfigured dashboards within modern process-monitoring apps make it easy to recognize abnormal situations.
Analytical Mechanisms and Tools
Efficiency calculations based on temperature differentials and flow rates can be performed in real time if the right tools are available. Historically, methods for performing such analysis had to be created via programming and unique user interfaces or manual calculations and spreadsheets.
Today, pre-configured apps using drawn from the consumer electronics industry simplify user interactions. For instance, a heat exchanger app can perform complex analysis to determine and display the overall health of the heat exchangers using data from the instrumentation. When provided with some basic values related to the equipment configuration and data from the wireless (or sometimes wired) sensors, such apps can monitor, record and analyze how the heat exchanger is performing. App dashboards that operate independently from the larger distributed control system allow basic process control functions to be accessed independently. The necessary functionality and intelligence are built in, so the user needs only add basic configuration details.
Apps like these often are dedicated to a specific type of plant asset or subsystem, so they are preprogrammed to perform whatever specialized analytics may be necessary. These analytics vary from failure-mode analysis to leveraging machine-learning techniques. They connect with the monitoring instrumentation directly through the WirelessHART network and do not have to work through the larger process automation system, although this may be an option.
The ability to monitor and evaluate critical plant assets such as heat exchangers, pressure-relief valves, steam traps and centrifugal pumps helps an industrial plant or facility reduce operating costs and plan maintenance efforts more effectively. WirelessHART instruments combined with easy-to-use apps is one way instrumentation is creating new possibilities for improvements.