Andrew XC100 and XT300 Dryline Dehydrator Installation for WKCN – October 12, 2012

Rigid metallic feedlines / transmission lines used in broadcasting need positive pressurization with dehumidified air (or with dry nitrogen.) This is done because any moisture or other contamination between the inner and outer metal conductors of the transmission line can cause arcing and severe damage to the transmission line, which runs from the transmitter’s output all the way to the antenna. The existing dehydrator system had failed, so I installed a replacement system which I had recently inspected, tested, and modified the “plumbing” on specifically for that purpose.

Failed Andrew dehydrator system

This is the failed Andrew MT-300 dehydrator I pulled out during this service call. It had totally failed and would not even power up.
The failed dehydrator had been put in service on February 25, 2021. At that time it had 8,717.2 hours of compressor runtime on its meter.
This was the compressor runtime meter reading at the time it failed: 9,233.7 hours.

My calculations indicate that the failed dehydrator had run 516.5 hours over the course of around 221 days (just “approximating” that the dehydrator failed at the end of September 2021.) That’s roughly 5,304 hours of time (221 days x 24 hours.) That equates to a “duty cycle” of roughly 9.7%. That’s really pushing the envelope, especially for a dehydrator of this size and operating capacity. What was more disconcerting, however, was the fact that when it was first installed back on 2/25/21, it was running with a duty cycle of approximately 5%. This meant that over time it had started working harder and harder, having to run more often and for a longer time each time it switched on to rebuild the positive pressure in the feedline. I had actually noted on more recent site inspection and P/M visits that the dehydrator was running almost continuously, so in reality the duty cycle had reached roughly 100% before it failed.

Plans to do a full servicing of the rigid transmission line were already in the works. We plan to have the entire transmission feedline extensively serviced, possibly in the Spring of next year. It’s a major job (and a very expensive one) requiring the coordination of the station management, consulting broadcast engineer, and a tower rigging crew with the expertise and equipment needed to carefully disassemble each piece of the transmission line (being approximately 20′ in length each), lower each one to the ground, remove the inner conductor from the outer conductor, remove the “bullet” connectors at each end of the inner conductors, clean each piece (or replace if necessary), reassemble with new O-rings and silicone between each section, install new “hanger” hardware and springs for support and attachment to the tower… you get the idea. It’s a HUGE job and one which takes a lot of time and effort. That’s why it has to be carefully planned and isn’t something which can be done in a hurry — at least not if you want to do it right in order to minimize the risk of a future catastrophic transmission line failure. Most listeners to great stations like WKCN-FM really aren’t aware of the extensive amount of work which goes on behind the scenes to keep great programming flowing over the airwaves for them to enjoy. Nonetheless, it’s part of the major, ongoing work which is done to keep the magic of radio (or television) happening. Anyway, back to our little story of the day…

In with the new…

The replacement Andrew XC300 / XT300 Dryline Dehydrator system, seen here in the lab recently while it was being fully tested and prepared for installation at the site.
At the time of installation, the compressor runtime meter on the new system read 6,759 hours. That might sound like a lot of hours, bur these systems will typically run for thousands upon thousands of hours, as long as routine servicing and maintenance is kept up on them. This one has been well maintained and serviced, and was ready for action.
The newly installed system keeps the positive pressure on the feedline between 1.5 PSI and 5.0 PSI, automatically cycling on when the pressure drops to 1.5 PSI, and off when it reaches 5.0 PSI. It is intentionally designed to allow a slight back flow of pressure during the “off” portion of the cycle, at which time any moisture which might be in the air in the system is allowed to flow back through the multiple types of filters built into the unit. That helps it continuously keep the humidity down in the air that is in the system and transmission line. In this photo, the system had successfully pressurized the line to 5.0 PSI, cycled off, and the pressure in the transmission line had been allowed to slowly drop. It had taken a considerable amount of time for it to do so, which is a good sign — it means the transmission line, dehydrator system, and the air “plumbing” between them did not have any huge leaks which would have been a sign that emergency attention was needed somewhere in the system. It also indicated that — based on the high duty cycles that the old system had reached prior to complete failure — the old dehydrator had serious internal leakage. That leakage was most likely in the compressor’s gaskets and other internal components. Those do wear out over time from heat, vibration, and just aging in general. This new dehydrator should provide us with years of good service — as long as the maintenance kits with all the gaskets and other routine service parts are available for the particular compressor assembly in the dehydrator system.
After a couple of cycles, we determined that the newly installed dehydrator system is running around a 3% duty cycle. Much better! Another successful service job taken care of for an awesome client. Now we can move on to other things… and there’s always more on the list when it comes to keeping stations on the air.