Much like throwing a stone into a pond creates a series of ripples in the water, heat transfer fluid deterioration can have knock-on effects on the components in the system. Here, Dr Chris Wright, head of research and development (R&D) at thermal fluid specialist, Global Heat Transfer, explains why heat exchanger maintenance is a whole system issue.
When considering how to maintain a heat exchanger, it might seem logical to start with the component itself. However, the plant manager should consider the whole system and in fact, the whole plant, to keep the heat exchanger in good condition.
So, what makes heat exchanger maintenance a whole system issue? The heat transfer fluid circulating through it. A typical system includes a heater, end user, pump, expansion tank, temperature control valve and a filter, all of which are influenced by the circulating fluid.
A stone in the pond
All heat transfer fluids degrade over time, generating degradation products that have a knock-on effect on other parts of the system, just as dropping a stone in the pond generates ripples. As the process is unavoidable, maintenance should focus on maintaining the life span of the oil.
In an open loop system, where the fluid comes into contact with air, oxidation can occur. Heat transfer fluid oxidation can lead to elevated carbon levels and the formation of polymers or solids on the internal coating of the pipework, altering the rate of heat transfer.
In a closed loop system, degradation most commonly occurs because of overheating a heat transfer fluid. Heating a fluid above its boiling point leads to the production of light and heavy hydrocarbon chains. Light chains reduce the fluid’s flash point and autoignition temperatures, creating a potential fire risk should there be a thermal oil leak. Operators can use a Light Ends Removal Kit to remove volatile components in the oil and ensure the system is safe, clean and cost effective.
At extremely high temperatures, heavy chain hydrocarbons are produced, leading to fouling of the system and pipework and insulation on the internal pipe surfaces. The build up decreases the thermal efficiency of the plant, so fluid must be heated to a higher temperature, which further stresses the pipes, accelerates breakdown and increases running costs.
It is possible to maintain the long-term health of the fluid and system by having knowledgeable and trained staff conduct regular maintenance. Operations staff can assess fluid health to determine the health of the system and its components.
Plant managers should consider several approaches when maintaining a heat transfer system. The first step to maintenance should be sampling to detect the by-products of degradation. Percentage carbon residue is a direct representation of fluid health, but also an indicator of system health, as carbon presence will lead to accumulations in the system. Maintenance staff can either take multiple samples from one part of the system, or samples from across the system, for example from the heater outlet or heater return.
To reduce oxygen exposure in an open system and prevent accelerated degradation, operators should seal the system from air. Operators can use a nitrogen blanket, which can help prevent acid formation, the production of corrosive sludge in the expansion tank and fouling of internal surfaces. Reducing oxidation can also be achieved using an anti-oxidant pack, which depletes the oxygen content of the fluid.
Operators can prolong fluid life in a closed system by reducing the operating temperature of the fluid – as higher temperatures can accelerate degradation and the formation of by-products. Global Heat Transfer recommends sampling your heat transfer fluid at least twice per year if is operating near its upper temperature, or four times a year if it is 20 degrees Celsius below its upper operating temperature.
The best way to ensure your heat transfer system is healthy is to use a proactive, condition based thermal fluid lifecycle maintenance programme, such as Global Heat Transfer’s Thermocare. By taking a proactive approach, plant managers reduce the risk of downtime, health and safety issues or regulatory non-compliance, at a fixed and predictable cost.
Limiting degradation will improve the performance of the system, which is why heat exchanger maintenance is a whole system issue. To prevent problems with your heat transfer system, remember how the effects of heat transfer fluid degradation can ripple through the system and be proactive in your approach.