ABSTRACT

In the majority of applications where gaseous or liquid filtration is used, applying low levels of heat solves tricky issues with condensation, viscosity and flow.

Filtration system users often face difficult choices and have to make compromises when deciding how to heat their filters. Many of the current solutions provide a short operating life and could pose various health and safety risks.

In this white paper we will explore some of the choices available, the key issues to consider, and look at how users of filtration systems can solve these issues.

INTRODUCTION

For many applications, heating the filter housing will aid filtration, separation and purification. It also eliminates condensation and residual water, reduces liquid viscosity and maintains the temperature of the medium.

When vacuum breaks or after an integrity test, condensation can become a major concern within filter housings. Its presence can block filters and slow the flow. By maintaining the filter assembly (see Figure 1) at a temperature slightly above the dew point, condensation is eliminated and the life of the cartridge is prolonged.

For liquid media applications, heat reduces the viscosity of the liquids and reduces the load on the pumps and filters, allowing users to have longer intervals between maintenance.

By maintaining the medium at a constant working temperature, with no hot spots, there is no degradation or crystallisation of the medium being filtered.

OPTIONS AVAILABLE AND ISSUES TO CONSIDER

There are a number of filter heater options available, but no matter which option is chosen, the heater will be required to:

•    Provide good heating uniformity
•    Have high energy efficiency
•    Have good thermal control
•    Minimise maintenance issues
•    Be safe to use
•    Be easy to install and uninstall
•    Be available in various sizes

The choice of heat is typically between electrical and fluid heat such as steam, water or oil. Steam is often used as a by-product of the processing operation that is already being carried, therefore, it’s seen as the low cost solution. However, with steam you lack precise control, and for applications requiring higher temperature process, steam cannot reach that high temperature. The major issue with these fluid heating solutions is the capital cost and the additional maintenance burden on the plant. For more information on the issue of steam vs. electrical, please see Erickson et al, A Study of Steam vs Electrical Pipeline Heating Costs on a Typical Petro-chemical Plant Project, ft lEEE/IAS/PCIC conference Record (1990), pp. 9-18.

The main electrical heating methods used in industry today to apply heat to filter housing include; heating tape wound around the housing and wrapped in insulation (see Figure 2), thin and flexible silicone panels bolted onto the outside of the filter housing (see Figure 3) or an insulated silicone jacket (see Figure 4).

TECHNICAL ISSUES IN USE

In an ideal situation, the heater will be the exact temperature you need. It would require little maintenance and have a long working life. In reality, most heaters fall short, and some more than others. Issues such as heating uniformity, efficiency and insulation, thermal control and maintenances are all common.

Heating Uniformity

Heating tapes wrapped with layers of insulation will result in banding in the temperature — with high heat emanating from the tapes and less heat in the gaps in between (see Figure 5). Such a heater may work well initially despite that banding, but during subsequent cartridge replacements, the removal and refitting can result in a much less even thermal profile due to the difficulty of getting the tape evenly wound around the filter housing.

With silicone panels or jackets the heat comes from elements laid onto the surface and vulcanized in place so the heat is therefore more likely to be distributed evenly (see Figure 6 and Figure 7). However, proper engineering is critical in these types of heating elements.  Sometimes with thinner silicone panels where the heating circuit is only lightly vulcanised, the heating element can become detached due to process vibration and cause uneven heating.  There can also be issues with these panels warping due to internal stresses set up by the heating elements.  Figure 8 shows printing of a complex heating circuit by a machine linked to the design software.
Accurate positioning of the heating elements in silicone panels or jackets enables even the most inexperienced person to reapply the heater in a way that will provide evenly distributed heat after replacement of the filter cartridge.

Efficiency and Insulation

Thin silicone panels with well designed heating element placement may be good initially for heat uniformity, but more energy is used to maintain the required profile as they are typically used without any thermal insulation. See Figure 7 for a sample of a thin silicone panel radiating energy when used without insulation. Filter Heating System Energy Comparision, on page 4, shows the energy consumption of the three heating methods: an Isopad heating jacket, a silicone panel, and a heating tape.

With well insulated heating jackets, such as Isopad heating jackets, silicone foam is positioned and sealed inside the jacket during manufacturing thorough an injection moulding process (see Figure 9). This structure performs two major functions:

•    The insulated jacket directs heat to the surface and restricts heat loss through the back of the jacket. As a result the external temperature is kept low and is safer for the operators.
•    The insulated jacket provides a buffer giving the components inside additional protection against vibration or handling misuse and reduces damage

With Isopad heating jacket, the heater is inserted into the fabric of the silicone by CNC machines. This gives the heater additional resistance to movement and abuse and makes it almost impossible to break. The various components that make up the finished jacket are vulcanised not glued. This makes each joint a welded union and much stronger and better suited to the rough treatment that most maintenance items suffer.

THERMAL CONTROL

Ideally a heater system should contain a feedback loop to provide an alert so that once a fault occurs the users can rectify the problem. Low cost options without this feedback, depending on your process, may pose a low risk or it may become a critical issue. The same is true of thermostats without fault feedback: if the thermostat fails either high or low, the system will work in unpredictable ways and this may damage the process or media being filtered. You need to balance the superficially attractive savings of a low cost unit against the potentially high risks to your process.

Correct temperature feedback is essential. If the sensor is not properly situated on the filter, the thermostat will receive faulty information and therefore the heating control will be inaccurate.

With Isopad flexible heating jacket, the sensor is placed alongside the heater at the optimum position as determined by specialist thermal modelling software to provide the most accurate feedback to the controller. As the sensor is in the correct place for this function and it is moulded into the jacket permanently, temperature feedback will be accurate no matter how many times the jacket is removed and replaced. The sensor leads are permanently vulcanised into a predetermined position that allows flexibility, eliminating a potential weak spot and preventing vibrations or moisture ingress.

As an additional feature, each Isopad heating jacket has an integral over temperature device permanently installed at the optimum position. As silicone has a maximum withstand temperature of 200°C (392°F) this integral safety device will switch off the heating elements, protecting the jacket, when 150°C (302°F) is exceeded.

MAINTENANCE ISSUES

With filters normally needing regular replacement, the maintenance side of the design needs to be considered. If the maintenance of the heater is difficult, it will create a less than optimum balance between initial costs and maintenance overheads. A heater might be low cost, but if it needs replacement often, or takes much longer to replace in a high turnover process, then the lifecycle costs may be above what is initially seen as a higher initial cost.

In a heater tape/insulation combination the maintenance can be complex and time consuming. During cartridge changes, both the insulation and the heating tape will have to be removed each time. In many cases, heating tape becomes “set” or “formed” to the shape of the filter and may become fragile. In the worst cases the tape simply won’t go back in place. An additional problem is that the insulation can start to crumble with repeated application and removal. Its normally very friable nature will be extremely difficult to clean off. It can be a time consuming job to remove and correctly replace the heater and insulation and the end result is often an uneven application of the insulation. Add to the fact that heater tape and sensor will be difficult to get into the right positions, the heat input may become uneven and the thermal regulation not in line with the process requirements.

If a silicone panel is used other issues can still arise. Unbolting the panel from the filter housing takes time and the bolts can be difficult to remove, causing damage to the heater panel. If the panel is insulated to prevent the heat loss, then the same problems exist as described above with heater tape and insulation combination.

With the Isopad solution, the jacket is permanently moulded to the shape of the filter housing with injection moulded silicone foam, without straps or bolts to hold it to the filter housing which reduces the maintenance time drastically (see Figure 10). This foam also prevents much of the vibration damage seen in thinner silicone panels; allowing users to increase the time between periodic inspections.

Alarm Options

You may want to consider whether the controller supplied with the heater provides meaningful alarms. Better systems can give failure reporting that allows remedial action to be taken by maintenance staff.

Key sources of failures for heaters usually occur at connection points between the sensor and power lead and cables, panels and jackets. It is worth checking to see if those connection points are as sturdy as needed.

HEALTH, SAFETY AND COMPLIANCE ISSUES

Operators have to be careful when handling heating tapes and thin silicone panels as there are risks of burns when changing filter cartridges due to the higher temperatures. With Isopad heater’s thicker jackets the heat is directed to the filter by the integral insulation. Figure 11 shows a jacket with integral insulation that directs the heat inwards towards the filter rather than outwards. The results are energy savings and a safer environment.

Hygiene is another consideration, particularly with silicone panels. Dirt will accumulate around the bolts and can only be removed when the panel is cooled and separated from the filter housing. This extra time need for routine maintenance is another cost that needs to be considered. With separate insulation there are many places to catch dirt, so moulded insulation is well worth considering when there is a high risk of contamination or when the environment needs to be sterile, such as a pharmaceutical factory. If you have moisture in the environment, you may also want to consider whether a waterproof moulding is preferable to non-waterproof. Few of the heaters available today are “wipe clean”.

Some heaters are just that — a heater. The better versions come complete with dedicated temperature controllers, sensors and alarms and are available in multiple plug and electric supply options suitable for the local market.

CONCLUSION

We’ve discussed some of the key issues relating to filter heating: heating uniformity, energy efficiency, thermal control, maintenance issues and health and safety.

When you balance the issues, the cheaper options that can look superficially attractive really have flaws: poor safety, short lives and bad performance. When looking at a heater for your filters, think about the total lifecycle costs rather than simply the initial cost.

Isopad insulated silicone jacket provides even heating, higher safety, and low lifecycle cost, therefore, it is the preferred solution for filtration system users today.

By Chris Mayes, Marketing Director, Tyco Thermal Controls UK, Rob Gunning, Sales Manager, Tyco Thermal Controls UK and Daniel Nieder, Product Engineer, Tyco Thermal Controls, Germany.

Tyco Thermal Controls UK
Washington
Tyne & Wear

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