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PROPANE
Estimates
of fuel consumption given by the manufacturer for the
gasoline run time can be used for propane because it is
an estimate or guess. Propane only has about 10% less
BTU content than gasoline gallon-to-gallon, so the
estimate used by manufacturers for gasoline consumption
will be very close for propane.
When
figuring power output of any fuel, the size of the
engine has to be a significant factor of the total
equation. For instance, comparing a vehicle with a V8
engine to a generator with an 11 HP generator attached
can not be calculated the same way. The characteristics
of the fuels and how they mix with air, how far the
piston is in relation the gasoline float bowl effects
the power derived from the fuel among many other
things. To presume that just because the BTU content
is less, it is less powerful, is wrong.
We now
offer some basic factors, round numbers and some good
estimates to see how run time values of propane
cylinders can be worked out:
BASIC
FACTORS
-
It
requires 2 horsepower to produce 1000 watts of
energy per hour under load
-
Under
load, each horsepower consumes 10,000 BTU per
hour
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Propane
contains 92,000 BTU per gallon
-
Propane
weighs 4.2 pounds per gallon
-
Bulk
tanks are rated by gallon
Cylinder Capacities in Gallons and BTU's
|
Size |
Gallon Capacity |
Total BTU Capacity |
|
20 Lb. |
4.8 |
441,600 |
|
30 Lb. |
7.1 |
653,200 |
|
40 Lb. |
9.5 |
874,000 |
|
60 Lb. |
14.3 |
1,315,600 |
|
100 Lb. |
23.8 |
2,189,600 |
|
200 Lb. |
47.2 |
4,342,400 |
|
420 Lb. |
99.1 |
9,117,200 |
|
Using these factors we can arrive at
run times based on average load for
any generator. For instance:
How long would a 5000 Watt Generator
with a 10 HP engine at 50% load run
on a 20# propane cylinder?
10 HP at 50% load would be using 5
horse power to generate 2500 watts
of energy.
5hp x 10,000 BTU would consume
50,000 BTU per hour.
Using a 20# cylinder that produces
441,600 total BTU, the engine
consuming 50,000 BTU per hour would
run for about 8.8 hours.
BTU consumption chart based on
generator/engine size and load
|
Generator Wattage |
Engine Horsepower |
Full Load |
75% Load |
50% Load |
|
1850 |
3.5 |
35,000 |
26,250 |
17,500 |
|
4000 |
8 |
80,000 |
60,000 |
40,000 |
|
5000 |
10 |
100,000 |
75,000 |
50,000 |
|
7500 |
15.5 |
155,000 |
116,250 |
77,500 |
|
8000 |
16 |
160,000 |
120,000 |
80,000 |
|
10,000 |
20 |
200,000 |
150,000 |
100,000 |
|
12,000 |
24 |
240,000 |
180,000 |
120,000 |
Many people want to know what size
cylinders they need based on their
engine size. Here are some real
conservative estimates of the
vaporization rate of various size
cylinders based on the outside
temperature.
Vaporization Rates of
Cylinders
Output in BTU's per hour - Vertical
Cylinder 25% full - Minimum Cylinder
Pressure 10 PSI
Cylinder Size
|
Outside Temperature |
20
Lb. |
30
Lb. |
40
Lb. |
100
Lb. |
200
Lb. |
400
Lb. |
|
+60 F |
24,000 |
32,000 |
40,100 |
79,700 |
125,900 |
185,500 |
|
+50 F |
21,200 |
28,300 |
35,500 |
70,600 |
111,500 |
164,300 |
|
+40 F |
18,450 |
24,700 |
31,000 |
61,500 |
97,200 |
143,100 |
|
+30 F |
15,700 |
21,000 |
26,400 |
52,400 |
82,800 |
122,000 |
|
+20 F |
13,000 |
17,300 |
21,800 |
43,300 |
68,400 |
100,700 |
|
+10 F |
10,250 |
13,700 |
17,200 |
34,200 |
54,000 |
79,500 |
|
0 |
7,500 |
10,000 |
12,600 |
25,000 |
39,500 |
58,300 |
|
-10 F |
4,780 |
6,400 |
8,000 |
16,000 |
25,300 |
37,100 |
|
-20 F |
2,050 |
2,700 |
3,400 |
6,800 |
10,700 |
15,900 |
For the
physical properties of each
cylinder, click on the
"Cylinder Size"
above.
What does all this mean?
Well, if you went exactly by the
chart, you would need a 420 Lb.
cylinder to run a 14 HP engine
if it was 25% full and 40
degrees outside and keep a
minimum of 10 psi in the
cylinder. This is a worse case
scenario. For instance, when a
20 Lb. cylinder is full it can
run a 16 HP engine for quite
some time in 40 degree weather
before there will be any
freezing problem. But if you
wanted to use up all the gas in
a cylinder, it would have to be
sized according to the chart.
Here is why. Propane is stored
as a liquid under pressure and
boils to produce a vapor that is
drawn off at the top for the
engine to use as the fuel.
Because propane boils at -44°
(below zero), the gas will
freeze if it can not absorb
enough ambient heat to
compensate for the boiling
process. The bigger the
cylinder is compared to the
amount of load, the warmer it is
outside, the warmer the cylinder
is kept, all are a determining
factor in the likelihood of a
cylinder freezing up.
If a sweat or frost line forms
around the cylinder at the level
of the fuel, this is a telltale
sign that the cylinder over
worked and is in the process of
freeze up. If the gas does
freeze, it will stop producing
vapor and the pressure inside
the cylinder will drop to as low
as zero psi which will cause
the engine to stop running.
To compensate for an undersize
cylinder, two cylinders can be
tied together using a tee check
and pigtails. Some customers
set the cylinder near the
exhaust of the engine to help
keep the cylinder warm and have
no problem using smaller tanks
on bigger engines. This practice
needs to be carefully monitored
so that the cylinder does not
overheat and cause the relief
valve to check off.
NATURAL GAS
-
Natural gas is billed in
THERMS.
-
This represents a unit or
block of 100,000 BTU of
fuel.
-
The average price per Therm
is around $0.80 (varies
widely)
-
A generator engine running
at 3600 rpm under full load
consumes on average about
10,000 btu per horsepower
per hour.
Using these figures, we can
figure the estimated usage for
any size engine. For instance a
10hp engine used on a 5000 watt
generator running at FULL load
should use no more than 100,000
btu per hour and cost
approximately $0.80 to operate.
50% load (2500 watts output
average) should use no more than
50,000 btu per hour and cost
approximately $0.40 to operate.
To compare that to gasoline
(110,000 BTU per gallon) times
the cost by 1.1 to arrive at
$0.88 per gallon.
So if you are paying over $0.88
per gallon for gasoline, you can
save by using natural gas.
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