Mains Vent Sizing

IflyHG

have read Gerry Gill's and Steve Pajek's "Balancing Steam Systems" paper. The section on how to size radiator vents makes sense and I like that math can be applied to get it right or at least close before buying parts. I'm less satisfied with how to size the main vents, it requires testing the existing system and testing solutions. I wondered if a more analytical solution were possible, I believe there is, this is how I got there.

I used Dan Holohan's procedure for calculating the steam velocity in the mains and the CFM of steam generation by the boiler. My 127 MBH boiler produces 55 CFM of steam at 1 psi, there is no way it makes sense to have to vent that volume of air that fast. Fortunately, the system doesn't have to. In addition to pushing the air out of the mains, the steam also has to heat the pipes. This slows down the speed at which the mains will vent considerably.

The EngineeringToolbox.com has a table for Condensate Generated in Cold Steam Pipes - Sizing of Steam Traps. Use the imperial table, at 0 psi, and the column for your main pipe size. My 2" mains generate 6.2 lbs of condensate per 100 ft raising the pipe temperatures from 70 F to 212 F. I converted my boiler's rating of 132 lbs/hr to 2.2 lbs per minute. I took the total length of the mains (including all the near boiler piping, 91') divided by 100' multiplied by 6.2 lbs/100ft divided by 2.2 lbs/minute = 2.6 minutes or 154 seconds to heat the entire mains up to the vents. This is for the case starting with cold pipes, which does happen when there has not been a call for heat for a long time, but is not the worst case.

The worst case is when there is a call for heat just after the pipes (and entrapped air) cools to below the main vent temperature trip point, I used 180 F but you need to use the trip temperature of your main vents. If there is a call for heat above this temperature the hot air will close the vents before the steam fills the mains. This is a thermostat problem, not a vent problem, the thermostat needs to hold off calling for heat until the air in the pipes cools to below the vent trip temperature. The hotter mains will generate less condensate than cold mains so there will be more steam to push the air out faster. In the above equation multiply the 6.2 lbs/100ft by the ratio in temperature differentials (212-180)/(212-70). Now the time to fill the mains is 0.6 minutes or 35 seconds. This is the fastest the mains can be vented, it is limited by the boiler output and the amount of pipes to be heated to 212 F, from the hottest hot start that will keep the vents open until the steam gets to them. There is no point in putting on more vent capacity than this, it just costs more money for no benefit. You can vent more slowly but, you do want to have enough vent capacity so that the boiler doesn't shut off on high pressure before the main vents close, and you want the total mains vent capacity to be considerably greater than (5-10X-ish) the total radiated vent capacity so that the mains vents close before the radiators start filling with steam.

From here I was curious what the pressure in the boiler had to be to get 1, 2, or 3 oz of pressure at the vent input. Since the air is moving Bernoulli's equation applies. I set up an excel spreadsheet so I could play with number of vents, vent rating (CFM), and pressure. The short answer is that if I used 3 oz of pressure I could get the vent rate I wanted with 1 Gorton #2 per main but the boiler pressure had to be 1.17 psi. If I used 2 Gorton #2s per main at 1 oz of pressure I achieved the same CFM venting, so same vent timing, but now with a boiler pressure of 0.3 psi.

So far this has all been just a mathematical exercise, but I'm about to have the system worked on so it will be interesting to see what my contractor says and ultimately how the system ends up performing.

Big Ed_4

Where that all matters is on a main with many branches, you would need to balance the main ..

Steamhead

How long are your steam mains, and what pipe size?