[TowerTalk] Feedline question

Jim Lux jimlux at earthlink.net
Wed May 28 21:27:02 EDT 2003

Several approaches spring to mind, and some analysis as well..

Look at your flow rate and the pressure you're using and see if you can
bound the number of pinholes.  Flow through a small orifice is something you
can look up in a table.

Liquid nitrogen (LN2) is much cheaper than nitrogen in bottles, as long as
the pressure you need is <235 psi.  So called high-pressure dewars will
pressurize to 200 psi, low pressure dewars go to around 20 psi..  A standard
GP45 (that's 45 gallons) is around 2000 cu ft of nitrogen (at STP), runs
around $90 for a fill, and loses 1% a day (and that 1% would nicely
pressurize your line, eh?)  The gas connection on the dewar is the standard
CGA gas fitting for nitrogen (CGA-580?)

Using dry air is another possibility, but you have to look at the
infrastructure to dry the air adequately. W.W.Grainger and McMaster-Carr
both have the equipment in their catalogs. Several kilobucks is my
recollection, and you can buy a lot of dry nitrogen for that, and for zero
capital investment.  The other thing to think about is why they're using
N2.. Are they worried about oxidation? and if so, you'll need a molecular
sieve, which increases the operating costs.  Again, it also depends on how
dry you need it. If a dew point of 32F is good enough, then almost any
dehumidifier will work, but if you need "real dry", the cost goes up.

LN2 is probably your friend...
$90 for 2000+ cu ft in a GP45, vs $12-20 for 300 cu ft in K bottles...In the
bottles,  you're paying for the electricity to compress it to 3000
psi..since the original source of the N2 is a bulk liquid truck.

Another gas to consider: CO2 can be very cheap, although N2 is cheaper on a
cubic foot basis, usually, and CO2 combines with water to form carbonic
acid, which is muy bad from a corrosion standpoint.

Jim, W6RMK

----- Original Message -----
From: "Courtney Judd" <k4wi at earthlink.net>
To: <towertalk at contesting.com>
Sent: Wednesday, May 28, 2003 6:05 PM
Subject: [TowerTalk] Feedline question

> I have an interesting question for the gurus out there. I have been hired
> the remote site manager for a fm station. Basically, I just look after the
> site and keep the operation up and going. The question pertains to
> pressurized Nitrogen and the feedline. This is 600 feet of 4 inch
> with N tanks hooked to it supposedly to keep moisture out. This feedline
> old and has what I believe to be many pinholes thru out it. Management
> consider replacement but is complaining about the number of bottles of
> compressed N that  I am using. What about replacing N with a small air
> and keep the feedline pressurized with air taken from inside the
> house... air conditioned and de-humidified... air is 70% N anyway! It's
> running 18 KW out and SWR on the feedline is 1:1. Any comments or ideas?
> Thanks Cort
> Courtney Judd K4WI / NA4W
> 2300 County Road 61
> Uniontown, Alabama 36786
> _______________________________________________
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