- 05 March 2008 -

Preventive maintenance for heat transfer systems

Today's heat transfer fluids are designed to operate over a long period with high production efficiency - if the fluid and system are maintained properly. Gaston Arseneault of Petro-Canada shares some of the industry's best practices for keeping heat transfer systems in top shape.

Heat transfer fluids are vital to ensuring proper system performance. Factors such as contamination, oxidative or thermal degradation can decrease a fluid's heat transfer capability, shorten its life and possibly cause damage to the system itself. Along with initiating a regular schedule of fluid testing, there are a number of functions that end-users can perform on a regular basis, as well as some important information to keep in mind to ensure peak system performance and maximum fluid life.

System Start-up and Shutdown

It is imperative that, in all cases, the users follow the original equipment manufacturer (OEM) instructions for start-up and shutdown. The following procedures should be part of your routine:

At start-up, ensure that any moisture in the system is drained off and any air or steam is vented off.

After the system has reached operating temperature, ensure that the fluid temperature in the expansion reservoir is below 65 °C for longer fluid.

At shutdown, continue to circulate the fluid until the temperature in the system is 65°C or lower before turning the system off.

Daily Performance

The system operator should establish a monitoring schedule that includes the following procedures. Keep a daily record so performance can be tracked over time.

Check pressure values across the filters-an increase can indicate a plugged filter. Debris in the filter decreases fluid flow and heating performance. Solids formation in the heat transfer fluid can be an indication of excessive heat flux, which causes thermal degradation.

Check fluid flow rate and pump performance. Look out for noise, pressure, etc.

Check pressure at the expansion tank. Increased pressure can indicate the presence of water vapour.

Compare the temperature at the heater with the process temperature. If sufficient solids are present in the fluid, flow will be reduced and the process will not run at the correct temperature. Alternatively, if the heater needs to operate at a higher temperature to maintain the process temperature, it may indicate fluid degradation. It is better to investigate sooner rather than later, because this vicious cycle will eventually lead to extensive downtime.

Check the fluid level in the sight glass at the expansion tank for fluid loss.

Be sure to follow manufacturers' instructions and procedures on system operation.

Preventing Fluid Degradation

The key to maintaining an efficient operating system and to prolonging the life of the heat transfer fluid is to prevent solids from forming. Solids can be formed in the fluid through thermal degradation, oxidative degradation, and the incompatibility of fluids if the system is not flushed out properly when changing out from one fluid type to another.

Thermal degradation can be minimized by keeping the fluid within the temperature range recommended by the fluid manufacturer. Thermal degradation is not normally a problem with electrical immersion heaters, as long as the flow rate and heat flux are carefully balanced.

Another common form of fluid degradation in an electrical immersion system is oxidative degradation. Oxidative degradation results from the reaction of the oxygen from air in the expansion tank vapour space with the fluid. This reaction mechanism forms carboxylic acids that undergo polymerization and solids, resulting in an increase in fluid viscosity. A more viscous fluid will be more difficult to pump, have poorer heat transfer characteristics and increase the chance of coking and sludging.

To slow down the process of oxidation, it is essential to position the expansion tank so that the heat transfer fluid is kept relatively cool. As a rule, the expansion tank should be cool enough that one can comfortably touch the bare metal (although this practice is not recommended for obvious safety reasons). The fluid temperature in the expansion tank should be 'warm' and measured below 65°C. For the best results, choose a heat transfer fluid containing additives that resist oxidation. Many synthetic chemical aromatics do not contain such performance enhancing additives.

Water

If there is water in a system, two things can happen:

1. The water can lead to premature fluid degradation through hydrolysis. Water may remove some additives from the oil, causing precipitation and obviously leaving the oil unprotected, and;

2. Pumps may be damaged by cavitation as the water expands to steam, which can also cause rust.

If you suspect that water is in a system, run the fluid through the expansion tank at a temperature just above the boiling point of water (100°C) and vent off the steam.

Fluid Analysis

Operators of small, multiple unit systems such as those found in the plastics and die-casting industries can easily establish the optimum fluid change-out schedule. Select a few test machines from each type of application and analyze the heat transfer fluid after one hour and then again every 500 hours. Based on the results, the useful service life of the fluid in a given environment can be determined. The system operator can then set up a schedule for change-out of the fluid on all the machines to keep the process running smoothly.

The optimal oil change interval is one where analysis reveals the fluid starts to degrade without actually showing the physical signs that affect heat transfer efficiency or the safety of the operation, such as viscosity increase, rise in acidity or reduction in flash point. This way, downtime is minimized because the unit can simply be drained and refilled. Extending the fluid further may cause the formation of sludge or carbon solids, which will require manual removal or the use of an expensive chemical cleaner to circulate for several hours. Operating with an overly degraded fluid will cause significant downtime later. Using a robust heat transfer fluid can extend time between oil changes and maintain high productivity longer without jeopardizing the long-term condition of the system.

Several heat transfer fluid manufacturers offer fluid analysis service. Petro-Canada, for example, offers this service to determine the condition and suitability of our Calflo heat transfer fluids. Tests include those for volatility, thermal degradation, water content, viscosity, acid number (AN), elemental analysis and solids content. Any fluid contamination or problems with the heat transfer system can also be identified, even process leaks sometimes.

Calflo Heat Transfer Fluids

Calflo is a family of heat transfer fluids that offers synthetic-like heat transfer efficiency benefits without raising the same objectionable odours, workplace health and safety or environmental concerns caused by chemical aromatic compounds. In fact, a few years back, a Calflo fluid was chosen to fill a 100,000 gallon system pumping crude oil through the sensitive rain forests of Ecuador . An increasing number of users realize Calflo fluids offer no trade-off in efficiency if used within their specified temperature limits, with added benefits of lower volatility, longer life in systems with no inert gas blanketing and substantially fewer health and safety concerns for users.

Calflo HTF is a premium high temperature heat transfer fluid recommended for systems operating with bulk temperatures up to 326°C and skin tube temperature up to 343°C. Typical applications include power generation and metal processing plants.

Calflo AF is a highly efficient heat transfer fluid recommended for systems requiring a greater resistance to oxidation operating with bulk temperatures up to 316°C. Calflo AF's outstanding oxidation stability maintains fresh fluid properties over a longer period. This is extremely critical in smaller systems with no inert gas blanketing such as die casters, plastics manufacturing and temperature controlled units (TCUs) of all kinds.

Calflo LT is a synthetic heat transfer fluid suitable over a wide temperature operating range from 5°C to 288°C. Low temperature pumpability enables cold start-up in ambient temperatures as low as -40°C. The product's low viscosity offers a substantial increase in the overall heat transfer coefficient, enabling Calflo to improve productivity in normal systems operating below 288°C or in processes where the oil is expected to play a dual heating/cooling role.

Gaston Arseneault
E-mail: lubecsr@petro-canada.ca

Web: www.calflo.com

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