Tuesday, April 15, 2008

Dry Air

On something as complex as a warship, you can find things that hardly anyone has ever heard of, but are critical to the ship’s being able to do its mission. In that category were the air dehydrators. This is why:

For reasons probably understood by electrical engineers and physicists (of which I am neither), you cannot send radar signals from the transmitter to the antenna by a wire or a cable. You send the actual radar wave itself up a tube called a “waveguide,” which takes the signal from the transmitter to the antenna. The return signal also comes down the waveguide to the receiver. If you had a radio transmitter in your home, the waveguide is the equivalent to the cable running from the radio set to the antenna.

Radar signals are rather powerful, with air search radars having a lot more power than surface search radars. Fire control radars are the most powerful. There is a lot of energy going up those waveguides to the antennas. If there is any moisture inside the waveguides, there will be arcing and sparking inside the waveguides, which can burn holes in the waveguides and ruin them.

Ideally, the best thing to do would be to use an inert and dry gas to fill and pressurize the waveguides. That’s impractical on a ship, for the gas canisters would be another item that would have to be supplied to the ship. So what is done instead is to pressurize the waveguides with dry air.

By “dry air”, I mean really dry. The dryness of air is measured by dewpoint, the temperature at which the water in the air will condense out. You may know that when it is summer and the dewpoint is high, the air is muggy. In the winter, when the dewpoint is low, people complain about their sinuses drying out and many people use humidifiers. Those dewpoints are positively soggy and too high for a radar waveguide.

How dry? You need a dewpoint of probably around -40degF for a generic radar. For a high-powered radar, you need a dewpoint around -70degF or even lower. The dehydrators are operated and maintained by A-Gang and the dewpoint is measured with a special dewpoint tester that takes a sample from the supply line to the waveguides. If the dehydrators are out of spec, the radars are shut down and that makes the Captain very unhappy, especially if the ship’s mission is anti-air warfare. If the dewpoint tester breaks, the radars are shut down.

The damndest things are critical items.


Eck! said...

At typical frequencies cables are very inefficient as they absorb great gobs of signal. for example A common radio cable
called RG213 (.4" diameter ance very widely used) at 400mhz will absorb 50% of the transmitter power in 100 foot length
and at 5,000w it will melt! So normal Coaxial cable for those power levels
(typically 10,000 or as much as 500,00 watts peak) would both be very large
and very inefficient. Wave guides
are RF water pipes and when properly maintained have extremely low losses which means no heating for the transmitted pulse and the weak return echo is not attenutated significantly.
To assis in maintaining it's performance it's very common to have the inside of the waveguide plated (a thick coating too) with silver because of it's conductive properties both clean and with tarnish. But like everything else shipboard water especially salt water is an aggressive enemy.


physioprof said...

Fascinating post EBM! And great comment eck!!