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Fin-tube Radiation for Steam Heating

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Author
Noel Murdough
Published
July 16, 2009
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The basic design for a fin-tube steam system follows the same principles as other steam systems, special attention being given to the prescribed maximum limits for the lengths of elements and the slope of the fin tube.

Special care must be taken to ensure that the supply piping can deliver the rated amount of steam for the size and length of the fin tube to be installed. As an example, 30 feet of two inch steel element can be connected to a one pipe system. It is rated at 5.7 sq. ft. EDR per foot, and 30 feet is 171 sq. ft. EDR. . Multiply by 1.33 (typically) to add for piping and pick-up losses, and we need 228 sq ft of capacity to the baseboard inlet valve. A 2' steel run-out from the main to the fin tube can support only 169 sq. ft., which is below the capacity of the fin tube. A 2 ½" run-out will carry 260 sq ft. Add to this the vent that lets the air out of the fin tube at a measured rate, which can be too fast for the water that tries to travel under the steam as it returns to the boiler. The net effect of pushing the pipe size to the max, and of over sizing the vent is that the water that condenses in the cold piping gets pushed ahead of the steam. It can't drain back underneath because of the velocity of the steam coming in.

Pipe pitch is another concern with steam fin tube. If the fin tube is near the top end of the lengths allowed, a full ½" per ten feet of pitch is required. In those 30' in the example, the pitch over the whole run will be 1½" of rise. If you are at 50% of the limit, ¼" in ten feet is enough. Do NOT vent too fast, though. Figure the volume of space in the run-out and the fin tube, and the total length, and select a vent that limits the air venting and steam speed to about 10-15 feet per second MAX. Faster than that, and the water will hammer the vents to death, and spit water onto the floor.

It is easier to apply fin tube to a two-pipe system, as the inlet valve can be modulated to control the output. Thermostatic Radiator Valves (TRVs) work wonderfully with two pipe steam systems. Orifices at the fin tubes, or steam traps at the ends of the fin tubes keep steam out of the return piping. Multi tier fin tube systems can be piped as commonly manifolded units with a single trap, or can be trapped on each tier, to stretch the length limits, as long as the run out and riser are large enough to support the load.

Fin tube doesn't mix well with iron radiators in the same system. The time to heat up, and the time to give up all of it's heat is different between fin tube and standing iron radiators. The fin tube heated rooms will tend to be cooler before the radiator heated rooms are finished giving up the heat from the previous cycle. The temperature swings will be very wide in the fin tube heated rooms.

When sizing for steam systems, the chart offers different pressure drops. On a system that has 100' of pipe or less, the lowest operating pressure will be double the design pressure, as this is the way to ensure that the pressure will always be above what is required. Most systems are designed from the 1 ounce pressure drop column on the chart, and the minimum operating pressure will be 2 ounces of pressure PER HUNDRED FEET. Only measure the longest run in the system, to find this number. It is not all of the piping combined together; only the longest run. The cut-in setting will happen at this pressure, and the cutout setting will be above that pressure.

The steam pressure control should be set as low as possible, while still heating every piece of radiation. The steam air vents and some steam traps have a "drop away pressure" rating. This is the pressure that the steam must be kept below in order for the vents to reopen after closing. As the system design pressure is 2 ounces per 100 feet of piping, usually the steam system will run very well at a half pound (8 ounces) of steam pressure. The high limit pressuretrol that comes with most boilers is not a very fine-tunable control. Set the cut out very low, with a small differential, and test it. Make sure that it shuts off near the point where all of the radiators are hot, and comes back on before any of them are cool. If you set it too low, one or two rooms will be cool. If you set the cut out too high, the burner will short cycle. Sometimes, a vaporstat can be added to the system to give finer control.

If you have a two-pipe steam system, the dimension between the boiler water line, and the lowest horizontal pipe connected to the RETURN is called a "B" dimension. It determines the boiler pressure, because the water rises in the returns 28" for every one pound per square inch of steam pressure. If you only have 28 inches of height, the water in the boiler will fill the mains at 1 PSI or more, and the boiler will run out of water. Keep the cut out pressure low.

If you decide to use a setback thermostat with your steam system, get the system up and running and balanced without using any setback. Set the day and night temperature settings to the same number. Let the system become stable, balanced, and quiet when running. This way, when you introduce the setback feature, the banging, hammering, and vent spitting that occur can be considered normal for your system.

Noel Murdough

(Visit Noel's blog at Steam Heating.)