Performance monitoring of heat exchangers – Not as simple as it seems

When a heat exchanger generates little heat loss, it may seem that the device works in every way as intended; the heat received by the cold fluid corresponds very well to the heat delivered by the hot fluid. The efficiency approaches unity and the customer can be informed that the heat exchanger works in the desired manner. A closer look, however, shows that the performance evaluation of heat exchangers is not quite such a simplistic matter.   

Often, heat exchanger functioning is monitored only for the rate of heat transfer. Furthermore, the heat transfer rate can be controlled by altering flow rates. An experienced process operator can see when a heat exchanger no longer transfers its typical thermal output under certain flow conditions. In such cases, the operator has three options available to handle the situation:

1. Increase the flow rate to increase the thermal output.
2. Reduce the flow rate to achieve the required outlet temperatures of the fluids.
3. Do nothing.

In the first alternative, the heat transfer rate of the heat exchanger can be raised to its former level, but the outlet temperatures of the fluids do not attain their former levels. In the second alternative, the outlet temperatures of the fluids can be returned to approximately the starting position, at the expense of the fluid flow rates, which may slow down production. In the third alternative, both the outlet temperatures of the fluids and the heat transfer rate are driven further away from their original operating points. It is clear, nevertheless, that in such a situation, the heat exchanger’s performance has deteriorated and the previous operating level is no longer possible.

The most significant reasons for the reduced performance of heat exchangers are the running methods and fouling. Substances dissolved or mixed with fluids may accumulate on heat transfer surfaces and slow down the flow of heat from one fluid to another. However, fouling does not increase heat loss and, therefore, does not affect efficiency at all. The performance reduction is only indirectly reflected in the heat transfer rate and must always be compared with the conditions prevailing in the process.

Parameter tracking in relation to flow conditions

The explanation for the reduction of the heat exchanger’s performance must, therefore, be found in other parameters than the heat transfer rate or efficiency. Many parameters vary greatly depending on fluid flow rates. In order for the parameters to be comparable they must, therefore, be proportioned to the prevailing flow conditions. Appropriate efficiency monitoring methods are limited by the available process measurements. However, diverse analyses can also be conducted with very few measurements. The efficiency of a heat exchanger can be examined, inter alia, with the help of the following measurements:

1. Temperature
2. Pressure
3. Flow
4. Concentration

The simplest indicator of the performance of a heat exchanger is the temperature change of the fluid under consideration. From our high school lessons on thermodynamics, we can recall that the rate of heat transfer is the product of the rate of heat capacity and the temperature change. Thus, the temperature change can be interpreted as the ratio of the heat transfer rate to the heat capacity rate, i.e. when the temperature change increases, more heat is transferred from one fluid to another in relation to the heat capacity rate. The mere change of temperature can therefore be interpreted as an indicator of the performance of the heat exchanger.

When the temperatures of both fluids on both sides of the heat exchanger are known, the following can, for example be determined for it:

• Effectiveness
• the degree of approach for outlet temperatures of the fluids and
• specific heat flux.

The effectiveness is the ratio of the transferred heat output to the theoretical maximum heat output of the heat exchanger. The approaching degree of the fluid temperatures, on the other hand, shows how much the outlet temperatures of the fluids approach each other in the heat exchanger and can be used to evaluate how the heat exchanger is used. The specific heat flux indicates the overall thermal conductance of the heat exchanger relative to the heat capacity rate of the fluid.

An elevated fluid pressure differential across the heat exchanger, i.e. a pressure drop, is a sign of fouling of the heat exchanger, which generally degrades the performance of a heat exchanger. The fluid flow rate in a heat exchanger is directly connected through pumping to the pressure drop therein. For filtering the effects of the changes in flow rate, the pressure loss can be used for calculating, for example, the specific pressure drop per mass flow, whereby the cleanliness of the heat exchanger can be estimated irrespective of the flow conditions.

The fluid flowing through the heat exchanger may include components with distinct specific heat capacities. In this case, fluctuations in the concentrations of the components lead to fluctuations in the resulting total specific heat capacity of the fluid. Increased specific heat capacity, however, results in a decrease in fluid temperature changes, which can be mistakenly interpreted as a decrease in the performance of the heat exchanger. In this case, concentration measurement can be utilised to filter the effects of changes in concentration, whereby the performance calculations are comparable even with different input stream compositions.

Relative performance monitoring in other parts of the process

Performance calculations such as described above can also be applied to other processes. For example, the specific energy consumption of a stripping process can be seen as the ratio of the vapour consumption of the stripper and the feed stream of the stripped fluid. In this case, changes in energy consumption rate can be detected regardless of the amount of feed stream.

For larger process entities, it is also possible to determine their own separate performances as a ratio of production to resource use, but this is often a complex problem. The process can consume and produce both raw materials and energy in various forms, so the overall performance is not unambiguous. Performances of the individual parts of a process can be combined to make up the efficiency of larger entities by scaling the performances across the same value range. It is then also easier to focus performance monitoring on the most interesting targets.

Conclusion

Typically, the performance of heat exchangers is monitored only for the heat transfer rate and the operational efficiency is generally not considered. However, the performance of a heat exchanger can be evaluated flexibly with a small number of measurements. As the number of measurements increases, the number of possible analytical methods also increases. This information can be considered as a justification for increasing the number of measurements in some places. Performance monitoring can also be extended to other processes, whereby the performance of separate processes can be combined to make up the performance monitoring of larger entities.