In many industrial processes, heating and cooling play essential roles in the implementation or achievement of special quality characteristics. More specifically, processes in which heat transfer fluids supply or remove energy serve this purpose.
The ideal heat transfer medium is water. Unfortunately, the vapor pressure of water rises sharply with increasing temperature, so water-based systems must be designed for high pressures. This makes them expensive. Alternatively, mineral oil-based and synthetic heat transfer fluids as well as silicone oils have been developed that allow operation at high temperatures under moderate pressure.
Mineral oils can be used for temperatures up to about 570°F (299°C); they have dominated the market for decades. Application temperatures up to 750°F (399°C) can be covered with synthetic heat transfer fluids. For a long time, 750°F marked the feasibility limit for heat transfer systems.
Technical progress demands higher temperatures, however. Silicone oils now offer the possibility of significantly exceeding the 750°F limit.
The Need for High Temperature Pumps
In concert with the development of higher temperature heat transfer fluids, pumps must be designed to meet the ever-increasing requirements. The critical component in the pump is its shaft seal. The components used for this purpose often are only suitable for temperatures below the temperature of the heat transfer oils being pumped.
Initially, attempts were made to bring the installation space of the shaft seal to a compatible temperature level by cooling. This method causes high operating costs and finds its limits relatively quickly. Seals are never hermetically sealed. The escaping heat transfer oil oxidizes on contact with air, and the residue impedes the sealing effect.
In a next step, a heat barrier was built between the pump and the seal chamber. This provided for cooling of the seal chamber by convection. These systems quickly gained acceptance and are still considered state of the art. Even these machines, however, cannot eliminate the disadvantage of basic seal leakage.
Depending on the application temperature, the vapor pressure of modern heat transfer fluids also may need to be taken into account. In many high temperature applications, to prevent oxidation of the oil, it is overlaid with nitrogen. The feed pump must, therefore, be designed to withstand the sum of all pressures in the system. In the case of one commonly used heat transfer fluid, Therminol VP1, for example, this means a vapor pressure of 12.9 barg at 788°F (420°C) — in addition to the delivery pressure generated by the pump. Normally, these pumps have to be designed for approximately 20 barg.
In addition, with the application of synthetic heat transfer fluids, the shaft-sealing systems had to be questioned. As an alternative, magnetic-drive pumps were used increasingly. These, too, contained materials that were not suitable — or suitable only to a limited extent — for the operating temperatures. Special attention must be paid to the installed magnetic materials and the drive-shaft bearings.
To protect the sensitive components of the magnetic-drive pump from overheating, the principle of the heat barrier was adopted from the pumps with shaft seals. A component is placed between the pump and the magnetic drive, which also prevents heat transfer from the hot pump to the magnetic drive (figure 1).
Due to this design, the magnetic coupling operates in the “dead-end” and must be kept operational by various measures:
- The rotation of the magnetic coupling generates both frictional heat and induction heat when metallic cans are used. This heat energy must be dissipated by convection (smaller pumps) or heat exchangers (large pumps).
- For cooling of the magnetic coupling and lubrication of the sliding bearing, the coupling must be filled with heat transfer fluid. This is done by venting valves. The venting must be repeated at regular intervals because gaseous components separate from the heat transfer fluid and displace the oil from the installation space.
Simultaneously, the heat barrier protects the roller bearings of the pump drive from overheating. The desired temperature level prevents the full utilization of the technical limits of the magnetic material, however.
Pumps designs such as these are very safe with regard to operation, handling and the environment. Practical experience shows, however, that the required maintenance effort for such pump designs often cannot be performed adequately by the operator. As a result, the pumps fail prematurely and lead to plant shutdowns.
For instance, it has been found on several occasions that the drive-chamber venting has not been carried out at all or has been carried out incorrectly. In these cases, gaseous components separate from the heat transfer fluid and concentrate around the pump shaft. The gas displaces the fluid in the coupling and hinders the dissipation of the frictional heat loss, as well as the energy, due to the eddy-current losses induced by the magnetic coupling in the containment can.
In the worst case, this results in extreme overheating of the magnetic coupling and, ultimately, pump damage. Depending on the design of the pump, insufficient lubrication of the plain bearings also can occur.
At cold temperatures, heat transfer fluids become viscous or solidify. The more complex the pump design, the more problematic this can be for its safe operation. Flow through auxiliary holes in the pump, as well as through any coolers that may be present, becomes restricted, limiting the pump’s range of applications.
Material Development Improves High Temperature Pump Design
So, how can the design of a magnetically coupled heat transfer pump be improved to increase the availability of the machine? Samarium-cobalt magnetic materials have been available for years and can be used reliably at temperatures up to 660°F (349°C).
It should be noted that permanent magnets for high temperatures have been available for many years; however, they had a lower energy density and were less resistant to demagnetization. System malfunctions easily could lead to destruction of the magnetic coupling. Today, these materials are hardly ever used in magnetic couplings for pumps.
Recent developments have produced samarium-cobalt (SmCo) magnets that push the temperature limit significantly to higher temperatures. With couplings made of samarium-cobalt high temperature magnets, their advantages such as high remanence (energy density) and high coercivity (resistance to demagnetization) also can be used for temperatures above 660°F. In fact, temperatures above 840°F (449°C) are feasible currently.
Magnetic couplings made of the new material can be used at temperatures up to 840°F without a thermal barrier. This eliminates the need for venting. This offers an advantage when operating such machines in processes that do not allow regular pump maintenance. A typical example is the manufacture of high quality printed circuit boards. There, every hour of downtime costs money.
One maintenance-free pump design for application temperatures up to 840°F offers features tailored to high temperature applications:
- A magnetic coupling that seals the pump shaft, so leakage of the heat transfer oil to the atmosphere is not possible.
- A magnetic coupling that can be operated with a flow temperature of 840°F.
- A design that incorporates a wear-resistant material for all pump components.
- A design that does not require bleeding the pump. (Gas separations are discharged automatically with the pumped oil to the system’s expansion tank.)
- A pump drive bearing design that is arranged in such a way that the operating temperatures are below 200°F (93°C).
The magnets used in the magnetic coupling are made of Sm2Co17, which is designed for a service temperature greater than 840°F (figure 2). The heat loss in the magnetic coupling is absorbed and dissipated by the heat transfer oil.
The pump’s plain bearings are fixed in special components to ensure that the stationary components are held securely over the temperature range. The components are designed so that the differing thermal expansion between the steel parts and the ceramic components are fully compensated.
Heat transfer fluids have poor lubricating properties at high temperatures. Magnetic-drive pumps must use plain bearings that are lubricated by the heat transfer fluid. The axial bearings are particularly sensitive. Ideally, the axial thrust of the pump is controlled in such a way that it is balanced under all load conditions or only reaches a low value. This measure means that the axial bearings are only subjected to low loads and do not require intensive lubrication.
A fan is installed between the magnetic coupling and the pump’s roller bearing to ensure that the bearing remains below critical temperature. A temperature distribution over the entire pump is shown in figure 3.
Between the magnetic coupling and the fan, a packing ring provides an additional barrier. To use this safety element sensibly, it is advisable to equip the installation space of the magnetic rotor with a leakage-monitoring system. Also, due to the direct flushing of the magnetic chamber, all common options for magnetic-drive pumps such as internal or external filters can be integrated.
With the pump design, the only maintenance task is to change the oil in the pump drive every 20,000 hours. The internal components of the pump are wear-free.
In conclusion, while for many years, the application limit for pumps and heat transfer fluids was 750°F, recent developments have opened up application ranges above this temperature. It will now have to be seen whether there are applications for temperatures above 840°F in industry.
Rainer Landowski is head of sales with Dickow Pump Co. Inc. The Marietta, Ga.-based company can be reached at 770-952-7903 or visit dickow.com.