Jeff Brett on sun 1 oct 00
Does anyone have formulas to figure out the cost of firing electric
and gas fired kilns? The kilns I wish to determine firing costs for
are in a college art department and none of the kilns are seperately
metered. I only know amperage, voltage and watts for each of the
electric kilns and BTUs and psi per burner on our gas fired kilns.
Any help would be greatly appreciated. Please respond to my email
address and i'll post a summary of responses to ClayArt.
......Jeff
John Baymore on tue 3 oct 00
Does anyone have formulas to figure out the cost of firing electric and g=
as
fired kilns? The kilns I wish to determine firing costs for
are in a college art department and none of the kilns are seperately
metered. I only know amperage, voltage and watts for each of the electric=
kilns and BTUs and psi per burner on our gas fired kilns.
Jeff,
Hi. Thought I'd post direct to the list..... because there are probably
many others who would be interested in all the feedback on this question.=
=
This is actually probably a pretty "broad interest" question. =
Some thoughts........
There are lot of variables involved here that might make this job a bit
difficult.... so I won't say this is easy in EVERY case.....but it is not=
too hard most of the time. For example....... if the electric kilns are =
on
computerized controllers...then you have a bit more complicated problem
that involves duty cycle information (on/off time for each pulse of the
controller). Or if you have "infinite" switches on the electrics it take=
s
a little more work to set up what you need to know to do the calculations=
...
Othewise, for the electrics it is pretty easy.
Electricity is usually (see below) charged for by the realtionship of wat=
ts
(amps x volts) over a unit of time. Typically this "charging unit" is
the kilowatt hour...... which is 1000 watts consumed for one hour.
If you have watt values for the electric kilns at each of the element
settings you use, then you simply have to record the amount of time spent=
on each setting, multiply them, and you have a value in watt hours. Add
all the watt hours used for the various settings together. Find out what=
you are billed for electricity. Usually this is a certain amount per
kilowatt hour. A kilowatt is 1000 watts. Take the total watt hours you
calculated....and divide by 1000 to convert the number to killowat hours.=
=
Multiply that value by the cost per killowat hour. =
Bingo!
Said another way............
Low setting wattage figure X hours on low =3D Low watt hours used
Medium setting wattage figure X hours on medium =3D Medium watt hours use=
d
High seting wattage figure x hours on high =3D High watt hours used
Electricity Used (in watt hours) =3D Low setting watt hours + Medium set=
ting
watt hours + High setting watt hours
Electricity Used (in kilowatt hours) =3D Electricity Used (in watt hours)=
/
1000
Electric Firing Cost =3D Electricity used (in Kilowatt Hours) x cost per
kilowatt hour =
If you don't have the watt values for the kiln settings, you can get an
ammeter (Radio Shack or Graingers) that can be temporarily clamped around=
the cable going to the kiln that measures amperage of the draw in the lin=
e =
(with reasonable accuracy). Turn the kiln on low and measure the amps on=
the meter. Write it down. Repeat for medium and high. Check the supply
voltage for the kiln....either by looking at the plate on the kiln (might=
not be exactly correct), using a meter (be careful....lethal voltages), o=
r
asking your electrician. Watts =3D amps x volts. So now you have watt=
age
figures for each switch setting.
If you have the "infinite" type switches..... then you could use this
approach to get watt values for each of the finer numbered settings.... a=
nd
then record the time at each setting as if it were the high / medium / lo=
w
type switches. More complicated but certainly doable.
If you have a computerized controller then it gets a little harder. The=
controller does not use a "low...medium..... high" approach to controllin=
g
the heat input....wattage does not vary. The controller simply pulses th=
e
kiln to full on and full off.... so it is then a "digital" kiln . (I'=
m
surprised someone didn't market a "digital ready" kiln .) The durati=
on
of and the time between the on and off pulses determines the heat input p=
er
hour and hence the climb rate. If you know the amperage (and voltage) at=
full "on", then you also know the wattage. So to get kilowatt hours used=
,
you just have to know the total hours of time that the full "on" pulses a=
dd
up to. Simple.
To actually get this hour figure is a bit of a pain. One way is to
purchase a little accurate clock / timer that is powered by the same
control voltage that closes the contactor that supplies the 220VAC to the=
kiln. See the Grainger catalog again. You'll have to do some wiring. =
Best to power the timer off the lower rated contactor control voltage
......not the 220/240VAC side of the circuit.....but that could be done to=
o.
Once installed in the circuit, set the clock to zero at the beginning of=
a
firing. Every time the controller pulses the kiln full on....the clock
runs. When the kiln is not powered....the clock is not either. By the e=
nd
of the firing the clock shows total elapsed time that the kiln was actual=
ly
"on" (electricity running). Multiply the "full on" setting wattage figur=
e
of the kiln by the amount of time "on" as recorded by the timer, and you
have watt hours once again. Divide by 1000 and you have kilowatt hours. =
Multiply by the cost per kilowatt hour....and you have the firing cost.
(BTW.... this simple "elapsed ON time" timekeeping function could be an
added capacity that the computerized controllers could EASILY be set up t=
o
track by a manufacturer. Anyone know if any of them already do this now?=
=
They didn't the last time I checked. Anyone "on-list" from one of the kil=
n
companies got a comment here?) =
Some places have different rates per kilowatt hour depending on time of
day, total usage, and a sliding scale for different total killowat hours
used. Another issue with some electric suppliers, particularly for
businesses, is "peak demand" charges..... which an electric kiln could ha=
ve
a significant impact on. For all that stuff ........... talk to your
electric company and/or your accounting department. If it is
complicated..... just use a simple rate figure that is as "representative=
"
as possible. Close enough for government work .
The gas kiln is a little more difficult. The only way to have REALLY
accurate gas usage figures is to install a flow meter on the gas line.
These can be had from places like Ward Burner in TN...... and typically
cost something in the $350-$500 range....depending on peak flow capacity.=
=
Great tools. (See my other post recently on them.) They measure gas flow=
in cubic feet per minute/hour.....and are quite accurate. If you record
the flow per hour off the flowmeter and the duration for each flow
rate....you'll get total gas consumption by multiplying the cubic feet pe=
r
hour by the number of hours. You'll get the total usage by adding all th=
e
individual timed figures together. Multiply that total by what you pay p=
er
cubic foot, and you are there.
Otherwise, you'll have to get an answer that is a little more of a
"ballpark" answer which can be obtained form more typical "potter"
instrumentation. =
If you know the orifice size in the burners then you are off to a good
start. Without that, you are getting a little further away from an
accurate figure. You also need to have good pressure guages. The less
accurate they are ......... the further you get from an accurate answer. =
Keep in mind that many typical potter-type guages are pretty inaccurate, =
or
are used in a range of measurement that makes them effectively inaccurate=
(ex.: -measuring a range of 0 to 1 lb. usage on a 0 to 10 lb. indicating
guage).
The flow of gas in cfm through a specific orfice is based on the pressure=
drop across that orfice. The gage on the gas line, located as most potte=
rs
place it after the control valve but before the orifice, sort of measures=
this pressure drop. Close enough. If you record the pressure settings
from your guages and the time spent at a specific setting once again....y=
ou
can calculate roughly the cubic feet of gas going through each burner in =
a
unit of time. Add the figures for each burner together for each "step" u=
p.
It is too complicated and extensive to list here....but there are "gas
flow through orfice at certain pressure" tables available from which you
can then calculate gas flow in cubic feet. There is a limited set of the=
se
tables in Olsen's Kiln Book. North American Mfg. and Pyronics, also have=
handy charts available. Your gas company should be able to get a copy of=
one of them to you for your specific needs.
Lacking that chart information, orifice flow can be calculated. Hard to
write this correctly here without the square root symbol available .
Flow (in CFH) is 1658.5 times the orifice area (in Sq. In) times an
efficiency factor (called "K"....you can safely use a value of .8) times
the square root of the result of dividing the pressure drop (in In. Water=
Column) by the specific gravity of the fuel gas. =
Q=3D flow in CFH
K=3D efficency factor
A=3D area of hole in Sq. In.
P=3D pressure drop across orifice in In. W.C.
d=3D Specific Gravity of the gas flowing
Q=3D 1658.5 A x K x Square Root (Pdrop / d)
Natural gas has a S.G of about 0.65 and Propane is about 1.52
Usage is rate of flow per hour at a given pressure setting multiplied by
the amount of time at that pressure. Add all the flows you come up with
for the indvidual pressure settings....and you'll have total cubic feet
consumed. Multiply by your cost pert cubic foot, and you have the answer=
.. =
Don't forget to add the consumption of ALL the burners together if it has=
multiple burners .
If you don't know the orifice size installed in the burner...then you'll
need to know the supposed BTU rating of the burner for the specific fuel =
at
a specific pressure (ex.: 250,000 BTU /hr at 1 lb, propane) ....and wor=
k
backward to a bit more of a "guestimate" of which orfice is actually
installed. Probably close enough.....but sometimes people have changed
orifices and you don't know it. This reverse calculation is also done fr=
om
the gas flow charts for orfices which I mentioned above. Or you can chan=
ge
the formula above to solve for the area value (A) instead. (But then
you'll still need a table of orfice "numbers" related to square inches.) =
Then from the "assumed" orfice size you come up with either way, calculat=
e
usage as indicated above.
If you don't have any of this more technical information.......... turn o=
ff
all other kilns and devices that use gas on the main supply. Read the ma=
in
meter or tank guage. Fire the kiln. Read the meter or guage again. =
Subtract end number from start number. Close enough. If it is a natural=
gas meter...... you are reading the cubic footage directly. Multiply by
the cost per cubic foot. If it is a propane tank guage, you have to do
some math because it'll be in a "percentage of full tank" format. Find o=
ut
the full capacity of the tank you have (typical is 500 gal, 1000 gal, 200=
0
gal, and so on). Take the start reading and subtract the end reading. =
Multiply the total liquid capacity by the difference you just found. Tha=
t
is the number of gallons of propane used. Multiply by the price per
gallon. If you have multiple propane storage tanks manifolded
together....the ideas is the same...just a little more math .
A little aside here....but a VERY important one at times......
When calculating the "COST" to fire electric and gas kilns.......... it i=
s
often forgotten to add in the amortization of the cost of replacing the
unit itself, the cost of consumables like kiln wash, cones, elements,
shelves, stilts, and the like, the labor to load, unload, and fire, and t=
he
cost of routine maintenence on the kiln. These are REAL costs to fire. =
If this figure you need happens to be related to a "what to charge for
firing" bit of research ...... then make sure you take this into account
also. I have worked with many clients that are in coop or school
situatiuons that fail to deal with this stuff and then when the kiln need=
s
replacing....... or the shelves are shot...or repairs need doing.........=
..
all of which WILL occur.......... they have no funds in place to cover su=
ch
issues.
Hope all this is of help. I think I covered a few bases. You'll probabl=
y
get more from others.
Best,
.......................john
PS: Kilns are my favorite technical topic ....... I've been doing kil=
n
consulting / design/ installation/ workshop work for about 25 years now. =
Never get tired of them. Fun stuff.
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
"Earth, Water, and Fire Noborigama Woodfiring Workshop August 18-27,
2000"
Dave Murphy on tue 3 oct 00
Hi all:
John has explained as usual with his excellent technical abilities. I will
give you my rendition (sp?) . I was lucky enough to have a meter between
the kiln and the electrical box for the space I was renting so could
actually get a kilowatt/hour reading for bisques as well as ^6 and ^10
electric glaze firings as follows:
normal bisque of approx 12 hours (slow early ramp) of 60 kilowatts
^6 glaze firing of 90-95 kilowatts
^10 glaze firing of 115 kilowatts
Naturally when your elements age these will get longer and thus more
expensive but it does give one a ball park for budgeting.
As far as my gas kiln (60 cu/ft) it takes a little less than 4% of my tank
(1000 gal)they only ever fill it to 80%. My kiln is newish and that figure
is getting smaller as I learn to make it more efficient. Also it took some
time to dry out. I figure it takes about 180-200 litres of propane to fire
to ^10-11.
Hope this helps.
Barbara Murphy
Waterloo County Pottery
Waterloo, Ontario
Canada
----- Original Message -----
From: "John Baymore"
To:
Sent: Tuesday, October 03, 2000 11:31 AM
Subject: Calculating electric and gas firing costs
Does anyone have formulas to figure out the cost of firing electric and gas
fired kilns? The kilns I wish to determine firing costs for
are in a college art department and none of the kilns are seperately
metered. I only know amperage, voltage and watts for each of the electric
kilns and BTUs and psi per burner on our gas fired kilns.
Jeff,
Hi. Thought I'd post direct to the list..... because there are probably
many others who would be interested in all the feedback on this question.
This is actually probably a pretty "broad interest" question.
Some thoughts........
There are lot of variables involved here that might make this job a bit
difficult.... so I won't say this is easy in EVERY case.....but it is not
too hard most of the time. For example....... if the electric kilns are on
computerized controllers...then you have a bit more complicated problem
that involves duty cycle information (on/off time for each pulse of the
controller). Or if you have "infinite" switches on the electrics it takes
a little more work to set up what you need to know to do the calculations..
Othewise, for the electrics it is pretty easy.
Electricity is usually (see below) charged for by the realtionship of watts
(amps x volts) over a unit of time. Typically this "charging unit" is
the kilowatt hour...... which is 1000 watts consumed for one hour.
If you have watt values for the electric kilns at each of the element
settings you use, then you simply have to record the amount of time spent
on each setting, multiply them, and you have a value in watt hours. Add
all the watt hours used for the various settings together. Find out what
you are billed for electricity. Usually this is a certain amount per
kilowatt hour. A kilowatt is 1000 watts. Take the total watt hours you
calculated....and divide by 1000 to convert the number to killowat hours.
Multiply that value by the cost per killowat hour.
Bingo!
Said another way............
Low setting wattage figure X hours on low = Low watt hours used
Medium setting wattage figure X hours on medium = Medium watt hours used
High seting wattage figure x hours on high = High watt hours used
Electricity Used (in watt hours) = Low setting watt hours + Medium setting
watt hours + High setting watt hours
Electricity Used (in kilowatt hours) = Electricity Used (in watt hours) /
1000
Electric Firing Cost = Electricity used (in Kilowatt Hours) x cost per
kilowatt hour
If you don't have the watt values for the kiln settings, you can get an
ammeter (Radio Shack or Graingers) that can be temporarily clamped around
the cable going to the kiln that measures amperage of the draw in the line
(with reasonable accuracy). Turn the kiln on low and measure the amps on
the meter. Write it down. Repeat for medium and high. Check the supply
voltage for the kiln....either by looking at the plate on the kiln (might
not be exactly correct), using a meter (be careful....lethal voltages), or
asking your electrician. Watts = amps x volts. So now you have wattage
figures for each switch setting.
If you have the "infinite" type switches..... then you could use this
approach to get watt values for each of the finer numbered settings.... and
then record the time at each setting as if it were the high / medium / low
type switches. More complicated but certainly doable.
If you have a computerized controller then it gets a little harder. The
controller does not use a "low...medium..... high" approach to controlling
the heat input....wattage does not vary. The controller simply pulses the
kiln to full on and full off.... so it is then a "digital" kiln . (I'm
surprised someone didn't market a "digital ready" kiln .) The duration
of and the time between the on and off pulses determines the heat input per
hour and hence the climb rate. If you know the amperage (and voltage) at
full "on", then you also know the wattage. So to get kilowatt hours used,
you just have to know the total hours of time that the full "on" pulses add
up to. Simple.
To actually get this hour figure is a bit of a pain. One way is to
purchase a little accurate clock / timer that is powered by the same
control voltage that closes the contactor that supplies the 220VAC to the
kiln. See the Grainger catalog again. You'll have to do some wiring.
Best to power the timer off the lower rated contactor control voltage
......not the 220/240VAC side of the circuit.....but that could be done too.
Once installed in the circuit, set the clock to zero at the beginning of a
firing. Every time the controller pulses the kiln full on....the clock
runs. When the kiln is not powered....the clock is not either. By the end
of the firing the clock shows total elapsed time that the kiln was actually
"on" (electricity running). Multiply the "full on" setting wattage figure
of the kiln by the amount of time "on" as recorded by the timer, and you
have watt hours once again. Divide by 1000 and you have kilowatt hours.
Multiply by the cost per kilowatt hour....and you have the firing cost.
(BTW.... this simple "elapsed ON time" timekeeping function could be an
added capacity that the computerized controllers could EASILY be set up to
track by a manufacturer. Anyone know if any of them already do this now?
They didn't the last time I checked. Anyone "on-list" from one of the kiln
companies got a comment here?)
Some places have different rates per kilowatt hour depending on time of
day, total usage, and a sliding scale for different total killowat hours
used. Another issue with some electric suppliers, particularly for
businesses, is "peak demand" charges..... which an electric kiln could have
a significant impact on. For all that stuff ........... talk to your
electric company and/or your accounting department. If it is
complicated..... just use a simple rate figure that is as "representative"
as possible. Close enough for government work .
The gas kiln is a little more difficult. The only way to have REALLY
accurate gas usage figures is to install a flow meter on the gas line.
These can be had from places like Ward Burner in TN...... and typically
cost something in the $350-$500 range....depending on peak flow capacity.
Great tools. (See my other post recently on them.) They measure gas flow
in cubic feet per minute/hour.....and are quite accurate. If you record
the flow per hour off the flowmeter and the duration for each flow
rate....you'll get total gas consumption by multiplying the cubic feet per
hour by the number of hours. You'll get the total usage by adding all the
individual timed figures together. Multiply that total by what you pay per
cubic foot, and you are there.
Otherwise, you'll have to get an answer that is a little more of a
"ballpark" answer which can be obtained form more typical "potter"
instrumentation.
If you know the orifice size in the burners then you are off to a good
start. Without that, you are getting a little further away from an
accurate figure. You also need to have good pressure guages. The less
accurate they are ......... the further you get from an accurate answer.
Keep in mind that many typical potter-type guages are pretty inaccurate, or
are used in a range of measurement that makes them effectively inaccurate
(ex.: -measuring a range of 0 to 1 lb. usage on a 0 to 10 lb. indicating
guage).
The flow of gas in cfm through a specific orfice is based on the pressure
drop across that orfice. The gage on the gas line, located as most potters
place it after the control valve but before the orifice, sort of measures
this pressure drop. Close enough. If you record the pressure settings
from your guages and the time spent at a specific setting once again....you
can calculate roughly the cubic feet of gas going through each burner in a
unit of time. Add the figures for each burner together for each "step" up.
It is too complicated and extensive to list here....but there are "gas
flow through orfice at certain pressure" tables available from which you
can then calculate gas flow in cubic feet. There is a limited set of these
tables in Olsen's Kiln Book. North American Mfg. and Pyronics, also have
handy charts available. Your gas company should be able to get a copy of
one of them to you for your specific needs.
Lacking that chart information, orifice flow can be calculated. Hard to
write this correctly here without the square root symbol available .
Flow (in CFH) is 1658.5 times the orifice area (in Sq. In) times an
efficiency factor (called "K"....you can safely use a value of .8) times
the square root of the result of dividing the pressure drop (in In. Water
Column) by the specific gravity of the fuel gas.
Q= flow in CFH
K= efficency factor
A= area of hole in Sq. In.
P= pressure drop across orifice in In. W.C.
d= Specific Gravity of the gas flowing
Q= 1658.5 A x K x Square Root (Pdrop / d)
Natural gas has a S.G of about 0.65 and Propane is about 1.52
Usage is rate of flow per hour at a given pressure setting multiplied by
the amount of time at that pressure. Add all the flows you come up with
for the indvidual pressure settings....and you'll have total cubic feet
consumed. Multiply by your cost pert cubic foot, and you have the answer.
Don't forget to add the consumption of ALL the burners together if it has
multiple burners .
If you don't know the orifice size installed in the burner...then you'll
need to know the supposed BTU rating of the burner for the specific fuel at
a specific pressure (ex.: 250,000 BTU /hr at 1 lb, propane) ....and work
backward to a bit more of a "guestimate" of which orfice is actually
installed. Probably close enough.....but sometimes people have changed
orifices and you don't know it. This reverse calculation is also done from
the gas flow charts for orfices which I mentioned above. Or you can change
the formula above to solve for the area value (A) instead. (But then
you'll still need a table of orfice "numbers" related to square inches.)
Then from the "assumed" orfice size you come up with either way, calculate
usage as indicated above.
If you don't have any of this more technical information.......... turn off
all other kilns and devices that use gas on the main supply. Read the main
meter or tank guage. Fire the kiln. Read the meter or guage again.
Subtract end number from start number. Close enough. If it is a natural
gas meter...... you are reading the cubic footage directly. Multiply by
the cost per cubic foot. If it is a propane tank guage, you have to do
some math because it'll be in a "percentage of full tank" format. Find out
the full capacity of the tank you have (typical is 500 gal, 1000 gal, 2000
gal, and so on). Take the start reading and subtract the end reading.
Multiply the total liquid capacity by the difference you just found. That
is the number of gallons of propane used. Multiply by the price per
gallon. If you have multiple propane storage tanks manifolded
together....the ideas is the same...just a little more math .
A little aside here....but a VERY important one at times......
When calculating the "COST" to fire electric and gas kilns.......... it is
often forgotten to add in the amortization of the cost of replacing the
unit itself, the cost of consumables like kiln wash, cones, elements,
shelves, stilts, and the like, the labor to load, unload, and fire, and the
cost of routine maintenence on the kiln. These are REAL costs to fire.
If this figure you need happens to be related to a "what to charge for
firing" bit of research ...... then make sure you take this into account
also. I have worked with many clients that are in coop or school
situatiuons that fail to deal with this stuff and then when the kiln needs
replacing....... or the shelves are shot...or repairs need doing..........
all of which WILL occur.......... they have no funds in place to cover such
issues.
Hope all this is of help. I think I covered a few bases. You'll probably
get more from others.
Best,
.......................john
PS: Kilns are my favorite technical topic ....... I've been doing kiln
consulting / design/ installation/ workshop work for about 25 years now.
Never get tired of them. Fun stuff.
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
"Earth, Water, and Fire Noborigama Woodfiring Workshop August 18-27,
2000"
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