The fluid ripple effect
Published: 06 November, 2018
Dr Chris Wright, head of R&D at Global Heat Transfer, explains why heat exchanger maintenance is a whole system issue.
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.
Put simply, a heat exchanger is used to heat or cool something, either a base, intermediate or final product. Most commonly, the heat exchanger acts as a heater, raising the temperature of a heat transfer fluid which will then circulate through a system, transferring heat.
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 fluid degrades 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. According to Fourier’s Law, because carbon build up reduces the cross-sectional area of the pipes and alters the surface heat is transferred across, the rate of heat transfer is altered. As well as this, sludge can form in the expansion tank and the fluid can thicken, reducing pump efficiency and increasing operating costs.
In addition to carbon formation, oxidation can result in acidification, which is corrosive to the internal workings of the system. Even worse, it can lead to scaling of the pipes and the heat exchanger.
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 are a concern as they reduce the fluid’s flash point, fire point and autoignition temperatures and this creates a potential fire risk should there be a thermal oil leak. To address this issue, 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.
System maintenance
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. Research published in the Journal of Applied Engineering suggests a relationship between sampling and fluid condition – with more frequent sampling associated with improved condition.
In an open system, oxygen exposure commonly leads to fluid degradation. This means that sealing the system from air can be a simple way to prevent accelerated degradation. Operators can achieve this using 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.
In a closed system, degradation is often a result of excessive temperature. It is possible to prolong fluid life 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 20oC below its upper operating temperature.
Reducing contaminants
Plant managers can reduce the levels of carbon and other degradation by-products in their systems by part-draining the fluid and refilling it with a virgin heat transfer fluid. This can also be achieved by adding a filter to remove contaminants and wear particles. Reducing the level of carbon present will reduce build up in the system.
In more extreme cases, the plant manager can drain the entire system before using a flushing agent and refilling the system with virgin heat transfer fluid. If degradation is serious, it may be worth considering switching to a synthetic heat transfer fluid, as they are more resistant to degradation and can operate at a higher temperature.
The best way to ensure your heat transfer system is healthy is to use a proactive, condition based thermal fluid lifecycle maintenance programme. 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.
