John Baymore on thu 19 oct 00
She is a bit worried about not having enough gas pressure during the
winter. (10 F - 20 F)
The kiln is a 50 cu. ft. sprung arch powered by 4 B-4 Ranson burners. She=
has a 500 gallon tank that is above ground.
Should she bury the tank? How close should the tank be to the kiln?
Should the line be buried? Are there tank warmers to keep up the
pressure?
She has talked to the gas company but didn't think they had a lot of
experience with potters. That is why we are asking here for advise.
John and friend ,
Michigan only gets down to 10-20F ABOVE zero in the winter? Maybe I have=
a
complete mis-impression of that area of the country...... but I thought i=
t
got COLD. Or is she thinking that she would not FIRE on days that were
colder than that? Unless she waits a long period after colder than 10-2=
0F
above weather for the store of liquid gas to warm up, the colder weather
from the days before she fires still can be a significant factor. =
A 50 cubic foot kiln using vaporous propane withdrawal on a 500 gal. tank=
in a climate that I THINK is like Michigan is just a tad iffy for
mid-winter firing. Right on the cusp. As long as the tank is at 50
percent full or above (400 gal) when you start firing ....... it'll likel=
y
be OK. So typically she'll probably have to refill the tank in winter
about every 3 firings to keep it above 50 percent. That can be annoying =
or
difficult to arrange with the propane company if she fires a lot of wares=
..
If ANYTHING else is on the same propane tank feed and is running when the=
kiln is firing ..... I'd say "no go"....very likely problems.
Some of the tank sizing issue when it is "iffy" depends on exactly HOW yo=
u
fire the kiln, since different firepersons can consume pretty different
amounts of gas in getting the same job done . (See mel's recent post=
alluding to "heavy hands on the throttle".) Using a 10,000 BTU / cu ft
design figure for a 50 cubic foot IFB kiln..... says that you'll use a ma=
x.
of 500,000 BTU's per hour at peak draw. It'll probably be less than
that....but use that for a "worst case" scenario in sizing the tank for
vaporization issues. You'll only draw at that level for few hours of the=
firing..... but will have a long period of steadily increasing draw rate
preceeding the hours of peak draw. Describe the firing in these terms to=
the propane company to help them to help you in your specific climate. =
They will have very specific climate data....since they likely supply hom=
e
heating appliances and probably are calculating "degree days" for automat=
ic
delivery schedules.
If firings tend to get very "back to back".... the thermal mass of liguid=
left in the tank after the first firing could still be quite "cold" (heat=
energy depleted from evaporation). This could affect a second firing
adversely even if the first one was OK and the tank is relatively full.
If you can afford it, my personal recomendation from building lots of
propane fired kilns in cold climates is to go for a larger storage like a=
1000 gal. for peace of mind, reliability, and fillup convenience. Or yok=
e
two 500's and only fill the second during the winter months. That way yo=
u
know you'll not have any problems and won't have to adjust your firing
schedule if the temperature suddenly heads for the sub-basement. The
biggest downside is the initial fillup . 800 gallons kinda' hits you =
in
the wallet.
The vaporization rate of propane from liquid to gas is directly related t=
o
the surface area of the liquid from which it can evaporate, and also the
heat energy available in the liquid gas store to "drive" the evaportation=
.. =
Heat energy is used to "cause" the evaporation...and the evaporation
lowers the temperature of the liquid. Also, at any temperature of the
liquid gas storage.... the larger the surface area the more gas will
evaporate. To maximize vaporization........ have the liguid gas at as hi=
gh
a temperature as possible (within reason )..... and have the largest
evaporation area possible. =
It is important to note that a small amount of a material at a high
temperature can have the same heat energy stored in it as a large amount =
of
material at a lower temperature. Hence the need for a large store of
liquid propane when the liquid is stored at a low grade temperature
level...... the heat used in evaporation lowers the temperature of a larg=
e
mass less than it would lower the temperature of a small mass. =
As a "submarine style" horizontal storage tank gets below 50 percent full=
,
the surface area open to evaporation as well as the thermal mass of liqui=
d
starts decreasing. This means that there is something to be said for
possibly yoking a series of large vertical tanks together in certain
situations...the surface area remains pretty constant under draw. As a
horizontal tank goes from 80 to 50 percent, the exposed evaporative surfa=
ce
area is increasing.
(In nothern Japan -Tochigi-ken- I saw that many propane fired kilns
were supplied by large banks of their equiv. of 100lb. cylinders all yoke=
d
together.) =
Problems with propane fired kilns are caused when the temperature of the
liquid propane lowers to a point where for a given exposed surface area,
the needed amount of BTU's of gas is not being evaporated in a unit of
time. The kiln "needs" more gas than the liquid can evaporate from its
open surface. So the tank pressure starts to drop..... eventually gettin=
g
down to the minimum that the inlet side of the primary stage regulator ca=
n
deal with. Then the regulated output pressure drops too...... then BTU
input at the kiln falls, the kiln stalls, atmosphere changes, and so on. =
The tank can get so cold that any water vapor in the surrounding air can
freeze on the exterior of the propane storage tank..... hence some of the=
common concept of "freezing up the tank". =
(As an aside..... any water present IN the tank can also freeze.....
anywhere in the system...and at withdrawal rates lower than where you are=
experienceing any tank icing symptoms. Have the propane company inject
"dryer" into your storage tank. The propane equavilent of "drygas" .)=
The surface area of the exterior of the tank(s) relative to their liquid
volume also affects the rate at which they heat up or cool off based on t=
he
surrounding air temperature and any radiant cooling to the environment. =
Sometimes manifolding multiple tanks can help (to warm things) ........ a=
nd
sometimes it can hurt (cools off more quickly) ...depending on the exact=
details of the installation. If you have a sunny spot..... it can help. =
If not....... and maybe it is a windy site or you tend to fire at
night...... it can also hurt.
Burying the tank may be a little helpful... since the ground temperature
even near the ground surface tends to be far warmer than the air
temperature on really cold days....particularly if there is snow pack. B=
ut
burried tanks NEVER get sun . If you put the tank below the frost lin=
e
for your area....that would be very helpful...... but in some areas that'=
s
completely impractical or illegal. The downside of burried tanks is that=
tank maintenence and inspection is more difficult. Also the ground
chemistry in some areas can cause tanks and hardware to deteriorate more
quickly........ and requires some expensive protection. Also that you
have to keep the filling area for the deliveries and the overpressure ven=
t
clear of snow for refilling. And if you DO have a suspected leak relatin=
g
to the tank end of things...... digging it up can significantly add cost =
to
repairs.
If you stay above ground with the tank, try siting it so that it gets ful=
l
sun for as much of the day as possible. You can also make a temporary
black plastic "solar heater" tent over it to help warm the tank in winter=
.. =
Don't block the vent! Another old "potter trick" is to use a hose and ru=
n
luke warm (not hot) water over the tank exterior to supply the heat
necessary for continued gas evaporation. This emergency solution can mak=
e
a real mess! Water .....and then ice....... everywhere. Best to size
correctly for your needs to start with .
The line should be burried simply for safety reasons so that no one
inadvertantly punctures it. Amazing how when something like a shovel, ax=
e,
or hardbrick falls it finds the exposed gas line to land on . Local
code will tell you if you HAVE to do this and how deep it needs to be and=
if it needs to be encased in any protective materials before burrying and=
all that kind of stuff.
Pipe length doesn't matter much in the vaporization issue as long as you
get the diameter correct for the maximum draw to start with. The propane=
company should be quite competent in this regard. Tell them the design
pressure you need to see at the burner manifold and the total design BTU
requirement. They can calculate the drop for the distance to the tank an=
d
the type of line and the number of twists and turns and get the right siz=
e
line for you. Very basic stuff for them.
Code may say where you HAVE to site the tank. If not..... I'd keep it =
a
good 25 feet away or so at the minimum. Preferably downwind so that if t=
he
automatic tank vent happens to blow off some gas..... it tends to go away=
from the kiln area. Not really a major issue..... pretty unlikely to cau=
se
a problem. Tank size usually affects code clearances to buildings, doors=
,
lot lines and setbacks, and such..... the bigger the farther.
There are vaporizing units available that are sometimes used where high
withdrawal rates are necessary. They are mostly the province of industry=
.. =
They are somewhat expensive...and they add a level of complexity to the
installation that most potters are not prepared to deal with. These unit=
s
take liquid propane directly off the storage tank and convert inside
themselves to gaseous propane to supply to the combustion system. They t=
oo
are sized according to necessary draw rate. Talk to your propane company=
....... if they have any industrial clients I would bet that they know all=
about them and can give you a price quote based on the kiln draw rate. =
Personally this is not a route I'd take unless other options would not
work.
There are also some liquid withdrawal burners available. There are
industrial units..... but they are probably way too costly for your need=
s.
Nils Lou recommends some lower tech ones in his "Art of Firing" book. =
Without exact figures on possible air entrainment in front of me, I'd say=
that when compared to the Ransome burners you mention ..... the primary a=
ir
entrainment figures for those ones would be enough similar to say that
they'd give you a roughly equavilent level of P air control. Not the sa=
me
when compared to good forced air burners though. But the liquid withdraw=
al
solution would require a complete new burner system....and so would add
some significant cost. =
As I have said in a lot of my posts lately........ check the archives too=
.. =
Propane storage went around a lot not all that long ago. There was also=
a
long discussion that revolved around the "running water over the tank"
concept.
Hope all this is of help to your friend.
Best,
.........................john
John Baymore
River Bend Pottery
22 Riverbend Way
Wilton, NH 03086 USA
603-654-2752 (s)
800-900-1110 (s)
JBaymore@compuserve.com
John.Baymore@GSD-CO.com
"DATES SET: Earth, Water, and Fire Noborigama Woodfiring Workshop =
August 17-26, 2001"
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