Cooling a high-powered inductive output tube carries its own set of concerns, but there is a whole new set when the IOT has multiple collectors, all of which are at different voltages. The multistage depressed collector IOT uses a collector that is segmented into three to five separate sections, each at a different voltage or potential to ground. This allows the electrons in the beam to strike a part of the collector that’s closer to its own energy level, thus expending less energy and using less electricity. The key to using less energy is the number of collectors and their voltages, so it becomes important that the different collectors never make electrical contact. The design of the IOT itself is the first step in this process, but the collectors must somehow be cooled. Early MSDC IOTs were air-cooled, but with the power requirements of TV broadcasting, liquid cooling is necessary. (See Figure 1.)
Water cooling was first used on klystron tubes in the form of vapor cooling; this is where the klystron collector sits inside a boiler where pure water is introduced, and the heat of the collector would change the pure water into steam. This steam would then be delivered to the heat exchanger via steam pipes, cooled back to liquid water and then pumped back to the boiler. (See Figure 2.)
The first MSDC klystrons could not use vapor cooling because of the electrical potential between the different sections of the multistage collector. Just as in electrolysis, a voltage difference across water will cause a migration of metal from one side to the other. If the water was allowed to flow over all the collectors at once, corrosion would result because metal would be removed from one set of collector cooling fins and deposited on another. This electrolysis would also contaminate the water and provide a conduction path through the water.
To overcome this problem, different collector sections are provided with their own water-cooling channels and fed in series with water hoses connecting them together. In this way, the water flows from one collector section to the next, as long as the water hoses are long enough to present a high resistance between the collectors. (See Figure 3.)
Today’s MSDC IOTs use the same methods for cooling as the early MSDC klystrons, although they don’t have to hold off as much high voltage as the MSDC klystrons did — only about half of the beam voltage in an IOT. They also use a two-stage cooling system where the water that is circulated through the IOT is cooled by a plate heat exchanger in a closed-loop system. The IOT side uses only high-quality demineralized water, and the other side of the plate is cooled by a glycol water mixture that is pumped through a traditional outdoor, fan-cooled heat exchanger. (See Figure 4.) In this way, the pure water sent through the IOT is never exposed to contamination or freezing weather. These MSDC IOTs use demineralized water and incorporate deionizing filters, because ions are what carry out the electrolysis process that leads to corrosion. Due to the required water hose lengths, the collector assembly tends to be fairly complex and is sealed at the factory. The conductivity of the water must be monitored to prevent electrolysis because the stainless steel fittings will turn into sacrificial anodes if there are too many ions in the water, making it conductive. (See Figure 5.)
As with any cooling system, the fluid must be checked on a regular basis. When checking the pure water of a water-cooled MSDC IOT, testing should be done for chemicals as well as silicon and silicone, which can come from hoses used by the cooling system.
Algae and bacteria should also be tested for because they tend to grow in warm, moist places like an IOT cooling system. The recommended way to keep the water free from such contaminates is to employ a UV light source in the same bypass used to run the cooling water through the filters. This will kill off any living contaminates in the water.
Cooling a broadcast tube using oil is relatively new to broadcasters but has been used by the military for several decades. Oil-cooled tubes are in used in guided missiles, fighter jets, ships at sea and land-based radar and have proven to be reliable and safe.
Oil cooling an MSDC IOT allows for a simpler design of the collector in that it’s much more like a traditional IOT collector. The oil is allowed to circulate around the collector coming in contact with all the electrical potentials at once. This is because the oil used is an excellent dielectric, which means it will hold off the high voltages used on the MSDC collector. The oil used also contains properties that help prevent corrosion and has very good heat transferring ability, which makes for a lower flow rate to achieve the required cooling. (See Figure 6.)
Oil cooling an MSDC IOT also uses a two-stage cooling system using a plate heat exchanger to transfer the heat collected to a glycol water mixture, which, in turn, uses a standard air-cooled heat exchanger that is mounted outside. It is interesting to note that the oil used is less toxic than glycol and, in one case, a future transmitter site in a protected wooded area is planning to use an oil-only cooled transmitter because if the oil leaks onto the ground, there will be less danger than if glycol spilled.
There have been concerns about the oil catching on fire around the high voltages used, but when the specifications are compared, the flash point for the oil used is much higher than that for pure glycol and well above the temperatures found in a MSDC transmitter.
One problem with the type of oil used, in the words of one installer, is that “it will leak through a glass jar.” Fittings and seals are a potential problem with oil-cooled systems and need to be checked and tightened on a regular basis after installation. Without a dye in the oil, it is almost colorless and hard to detect. A good solution is to lay down paper towels at certain areas and wrap them around pipes. Any leak will easily be detected. From experience, after a break-in period of tightening the fittings and the oil filter, the leaks will stop.
Both water- and oil-cooled MSDC IOTs work very well. The choice of which one is better is more a matter of opinion than there being true advantages of one over the other.
Buzz Miklos of L-3 Communications and Vijay Patel of e2v contributed to this tutorial.
In the last “Transition to Digital” tutorial, it was stated that tap water could be used for flushing the cooling system. This is true, but it is highly recommended to only use pure, high-quality demineralized water for all flushing to keep any contaminates from the local water supply out of the system and to never use well water in a transmitter cooling system at any stage.