Testing Boiler Output
Posted: Sun Jul 19, 2015 4:03 pm
Determining the steam flow output capability of a small steam boiler can be accomplished by several methods. Tests will generally be fairly crude with small wood or coal fired boilers that are hand fired, as the consistency of firing is generally quite variable. None the less a rough approximation of the boiler’s output can be made.
With oil or gas firing, a much more constant load can be maintained on the boiler, so better results are available.
Be aware of safe practices during testing, secure piping such that its dischadge direction is fixed, and keep people and pets away from potential danger and burns. Pets are usually smarter about this than humans.
Orifice Output Method.
In this test, a steam flow orifice, discharging to atmosphere, determines steam flow according to Napier’s Formula for saturated steam critical flow, where the outlet pressure of the orifice is less than 50% of the inlet pressure:
Steam Flow (PPH) = Knap x Ae x P1
Where: Steam Flow id expressed in Pounds per Hour (PPH)
Knap = Napier’s Flow Constant, 51.0 (PPH/PSIA/in2)
Ae = Orifice Effective Area, Square Inches
P1 = Orifice Inlet Absolute Pressure, PSIA
P2 = Orifice Outlet Absolute Pressure, PSIA, and P2 must be less than half of P1
Note that throughout this testing, all pressures are expressed as Absolute Pressures, not Gauge Pressures. Absolute pressures are generally the gauge pressure plus the local atmospheric pressure.
The Orifice Effective Area depends on the shape and length of the orifice, plus the bore diameter of the orifice. The general formula for Effective Area:
Ae = Cd x A
Where: Ae = Orifice Effective Area
Cd = Coefficient of Discharge, or how close the actual orifice or nozzle area achieves flow compared to an ideal perfect theoretical orifice.
Cd typically is 0.6 for a small short orifice with a sharp inlet bore, and ranges into the 0.9-0.98 range for orifices with well rounded and polished entrance.
A = Orifice Actual Area
For small boiler testing, use a sharp edge short orifice, around 1/8 inch diameter. Do not break the sharp inlet edge of the orifice entrance. Use a Cd of 0.6
In this test, the orifice is sized to match the boiler’s expected output at about 75% of the normal steam operating pressure. The boiler is brought up to 75% pressure, full firing commences, and all steam output is directed thru the orifice.
While maintaining water level in the boiler, full firing continues, and steam pressure at the orifice will climb or decrease until steady (constant pressure) is achieved. After holding steady pressure for a period of about 15 minutes, with steady feedwater flow maintaining boiler water level, record the orifice inlet pressure, then calculate the steam flow according to Napier’s Formula.
If pressure climbs to the maximum boiler working pressure, then you need a somewhat larger orifice. If pressure falls too low, then a smaller orifice should be setup.
Holding steady conditions is the key here, so pumping of feedwater should be fairly steady, as well as steady firing. Have a helper record the boiler steam pressure every 30 seconds during the test, as well as the boiler water level elevation.
Condensing Tank Method
In this test, the entire steam output of the boiler is valved into a cold water tank, where the steam condenses, and heats the water. The setup is the same as above, and the procedure is the same, with the following exceptions:
In place of the orifice, a small outlet throttle valve is substituted, so the flow resistance can be adjusted to match the boiler operating pressure while discharging to atmosphere.
An insulated metal water tank holding several gallons of water with an accurate submerged thermometer is provided, about 10 gallons capacity for our typical launch boilers. A typical large thin wall stainless steel cooking pot will do fo this, wrapped with an inch or so of fiberglass insulation or terry cloth layers. The tank is filled to about 90% capacity with ordinary tap water at ordinary ambient temperature, say 50F – 70F. Accurately record the mass amount of water in the tank, plus the tank metal weight.
The output throttle valve is connected to an output chamber (you can usually use a large pipe Tee for this) which has a tube connection leading into a submerged outlet within the water tank, plus a large ball valve for atmospheric steam discharge. For our typical launch boilers, the tube to the water tank can be about 1/2 to 3/4 inch diameter, about 3 to 6 feet long, and the large ball valve 3/4 to 1 inch diameter. All piping and tubing must be metal, thin wall copper or stainless steel or carbon steel is proper.
The boiler is brought up to full pressure, with full firing. Adjustment of the outlet throttle valve holds pressure, and all steam output is directed to atmosphere.
While maintaining water level in the boiler, full firing continues, and steam pressure at the boiler will be steady. After holding steady pressure for a period of about 15 minutes, with steady feedwater flow maintaining boiler water level, Quickly close the large ball valve, and start recording time with a stopwatch.
Steam will instantly flow into the tank and start condensing. When the tank water is heated to 140F to 170F, open the large ball valve and record the time. If you wait too long here, the condensing process will become less and less effective because the water will approach 212F, giving violent bubbling, with steam escaping the water tank and thus energy loss to the ambient. We want all of the steam energy to be captured in the water tank. After opening the large ball valve, immediately stir the water to assure even temperatures and record the final temperature.
Holding steady conditions is the key here, so pumping of feedwater should be fairly steady, as well as steady firing. Have a helper record the boiler steam pressure every 30 seconds during the test, as well as the boiler water level elevation.
Now a simple energy balance here will determine how much steam flow the boiler produced during the stopwatch period.
Condensation Energy = Energy Gain of Tank Water + Energy Gain of Tank Metal
Here we use “British Thermal Units” (BTU) as the typical USA energy unit. Use Metric units if you are more comfortable with them.
The energy gain of the Tank Water = Mass x Specific Heat x Temperature Rise
The energy gain of the Tank Metal = Mass x Specific Heat x Temperature Rise
For the water, the specific heat is 1 BTU per Pound (mass) per F Degree
For the Steel, the specific heat is 0.1 BTU per Pound (mass) per F Degree
The specific heat of other tank metals is usually close to 0.1, and the accuracy of this number is usually not too important, as the water mass is several times the metal mass in typical testing.
Calculate the Condensation Energy.
Steam from a launch boiler generating saturated (or very slightly superheated) steam will give up about 1000 BTU. Thus the steam generation rate for the test is:
Steam Generation (PPH) = (Condensation Energy / 1000 ) x (3600 / Seconds)
Where the time in seconds is the stopwatch reading, the time condensation was occurring.
Combined Testing
You can combine both of these tests, and get a better average output number. If the tests are conducted as described, the two test methods will be prodicing very similar results.
Repeating the tests a few times would be standard practice, to assure consistent results.
With oil or gas firing, a much more constant load can be maintained on the boiler, so better results are available.
Be aware of safe practices during testing, secure piping such that its dischadge direction is fixed, and keep people and pets away from potential danger and burns. Pets are usually smarter about this than humans.
Orifice Output Method.
In this test, a steam flow orifice, discharging to atmosphere, determines steam flow according to Napier’s Formula for saturated steam critical flow, where the outlet pressure of the orifice is less than 50% of the inlet pressure:
Steam Flow (PPH) = Knap x Ae x P1
Where: Steam Flow id expressed in Pounds per Hour (PPH)
Knap = Napier’s Flow Constant, 51.0 (PPH/PSIA/in2)
Ae = Orifice Effective Area, Square Inches
P1 = Orifice Inlet Absolute Pressure, PSIA
P2 = Orifice Outlet Absolute Pressure, PSIA, and P2 must be less than half of P1
Note that throughout this testing, all pressures are expressed as Absolute Pressures, not Gauge Pressures. Absolute pressures are generally the gauge pressure plus the local atmospheric pressure.
The Orifice Effective Area depends on the shape and length of the orifice, plus the bore diameter of the orifice. The general formula for Effective Area:
Ae = Cd x A
Where: Ae = Orifice Effective Area
Cd = Coefficient of Discharge, or how close the actual orifice or nozzle area achieves flow compared to an ideal perfect theoretical orifice.
Cd typically is 0.6 for a small short orifice with a sharp inlet bore, and ranges into the 0.9-0.98 range for orifices with well rounded and polished entrance.
A = Orifice Actual Area
For small boiler testing, use a sharp edge short orifice, around 1/8 inch diameter. Do not break the sharp inlet edge of the orifice entrance. Use a Cd of 0.6
In this test, the orifice is sized to match the boiler’s expected output at about 75% of the normal steam operating pressure. The boiler is brought up to 75% pressure, full firing commences, and all steam output is directed thru the orifice.
While maintaining water level in the boiler, full firing continues, and steam pressure at the orifice will climb or decrease until steady (constant pressure) is achieved. After holding steady pressure for a period of about 15 minutes, with steady feedwater flow maintaining boiler water level, record the orifice inlet pressure, then calculate the steam flow according to Napier’s Formula.
If pressure climbs to the maximum boiler working pressure, then you need a somewhat larger orifice. If pressure falls too low, then a smaller orifice should be setup.
Holding steady conditions is the key here, so pumping of feedwater should be fairly steady, as well as steady firing. Have a helper record the boiler steam pressure every 30 seconds during the test, as well as the boiler water level elevation.
Condensing Tank Method
In this test, the entire steam output of the boiler is valved into a cold water tank, where the steam condenses, and heats the water. The setup is the same as above, and the procedure is the same, with the following exceptions:
In place of the orifice, a small outlet throttle valve is substituted, so the flow resistance can be adjusted to match the boiler operating pressure while discharging to atmosphere.
An insulated metal water tank holding several gallons of water with an accurate submerged thermometer is provided, about 10 gallons capacity for our typical launch boilers. A typical large thin wall stainless steel cooking pot will do fo this, wrapped with an inch or so of fiberglass insulation or terry cloth layers. The tank is filled to about 90% capacity with ordinary tap water at ordinary ambient temperature, say 50F – 70F. Accurately record the mass amount of water in the tank, plus the tank metal weight.
The output throttle valve is connected to an output chamber (you can usually use a large pipe Tee for this) which has a tube connection leading into a submerged outlet within the water tank, plus a large ball valve for atmospheric steam discharge. For our typical launch boilers, the tube to the water tank can be about 1/2 to 3/4 inch diameter, about 3 to 6 feet long, and the large ball valve 3/4 to 1 inch diameter. All piping and tubing must be metal, thin wall copper or stainless steel or carbon steel is proper.
The boiler is brought up to full pressure, with full firing. Adjustment of the outlet throttle valve holds pressure, and all steam output is directed to atmosphere.
While maintaining water level in the boiler, full firing continues, and steam pressure at the boiler will be steady. After holding steady pressure for a period of about 15 minutes, with steady feedwater flow maintaining boiler water level, Quickly close the large ball valve, and start recording time with a stopwatch.
Steam will instantly flow into the tank and start condensing. When the tank water is heated to 140F to 170F, open the large ball valve and record the time. If you wait too long here, the condensing process will become less and less effective because the water will approach 212F, giving violent bubbling, with steam escaping the water tank and thus energy loss to the ambient. We want all of the steam energy to be captured in the water tank. After opening the large ball valve, immediately stir the water to assure even temperatures and record the final temperature.
Holding steady conditions is the key here, so pumping of feedwater should be fairly steady, as well as steady firing. Have a helper record the boiler steam pressure every 30 seconds during the test, as well as the boiler water level elevation.
Now a simple energy balance here will determine how much steam flow the boiler produced during the stopwatch period.
Condensation Energy = Energy Gain of Tank Water + Energy Gain of Tank Metal
Here we use “British Thermal Units” (BTU) as the typical USA energy unit. Use Metric units if you are more comfortable with them.
The energy gain of the Tank Water = Mass x Specific Heat x Temperature Rise
The energy gain of the Tank Metal = Mass x Specific Heat x Temperature Rise
For the water, the specific heat is 1 BTU per Pound (mass) per F Degree
For the Steel, the specific heat is 0.1 BTU per Pound (mass) per F Degree
The specific heat of other tank metals is usually close to 0.1, and the accuracy of this number is usually not too important, as the water mass is several times the metal mass in typical testing.
Calculate the Condensation Energy.
Steam from a launch boiler generating saturated (or very slightly superheated) steam will give up about 1000 BTU. Thus the steam generation rate for the test is:
Steam Generation (PPH) = (Condensation Energy / 1000 ) x (3600 / Seconds)
Where the time in seconds is the stopwatch reading, the time condensation was occurring.
Combined Testing
You can combine both of these tests, and get a better average output number. If the tests are conducted as described, the two test methods will be prodicing very similar results.
Repeating the tests a few times would be standard practice, to assure consistent results.