I want to mimic a steam boiler in a gravity return system

Jim_158

I have been tasked with making a two pipe gravity return heating system in a two story house work with an eternal steam supply. The steam is to be supplied from the building next door at 8-10psi. The house is about 1,500 sq. ft. I have not calculated the radiator output yet.

My thought was to make a generously sized vertical manifold where the original boiler would have been. To this I would connect the steam supply at the top, the return pipes from the house near the bottom. I would also connect the external steam supply somewhere in the middle (with the 10psi reduced to ~1psi) and near the bottom, but above the returns, I would connect an F&T steam trap to remove the condensate. The F&T would drain to a condensate pump.

I believe this would mimic the original boiler. The thermostat would control the steam supply valve(s) as if it were the oil/gas burner.

I have a few questions and concerns:

1. I assume I should maintain a waterline in the “manifold” as would be with a boiler, and the dry returns should be tied in below the waterline.

2. Looking for comments and ideas on reducing the steam pressure from 8-10psi to ~1psi or less. Would a vaporstat(s) with motorized ball valve(s), solenoid valves(s) on the steam supply work? Would it be too noisy? Should I use two vaporstats set at slightly different pressures? How about a pilot operated PRV? Pneumatic control valves are not an option at this time.

3. Finally, what works well as a timer to cycle the “boiler” when using thermostatic control valves on the radiator air vents?

4. Any other thoughts and input on this idea?

Pumpguy

I would use.....

a pressure reducing valve at the incoming steam line.  Fisher is a well known maker of PRV's and I'm sure they could help with this. 



Do you really need such a high supply pressure?  Unless there is a very long distance between the 2 buildings, 2-3 psi should work just fine, and then there would then be no need for a PRV. 



Not an expert on these matters, but I don't see a need for the "waterline" you speak of.  There are probably several piping arrangements that would work, but it would depend on how this building is piped now.  It would be helpful if you could post a drawing of the piping arrangement you are considering. 



It sounds like you are considering a "Master Trap" at the inlet of the condensate pump, and this is a NO NO.  This Master Trap will cancel the pressure differential across the radiator traps, and with no pressure differential, there will be no flow, and with no flow, there will be no heat. 



Do some searching on Master Traps here on this site for more information. 



The arrangement you envision is very common on campus type applications where a central power plant sends steam out to the campus buildings, and the condensate pump in each building  sends it back to the boiler's feed tank in the central power plant.



Just be sure you have enough condensate pump capacity to send the condensate back to the boiler building. 

Jim_158

Thank you for the reply.

I am familiar with the "mis-application" of a "master trap" on the return and the problems it causes. My intent is to keep the supply and return at the same pressure as was the original system. The F&T would be an overflow.

I am considering a Spence valve (E2D5?) for its ability to handle small pressure drops.

Due to space restraints at the 10psi steam main I expect the PRV will be in the building using the steam. There should not be any concern with pressure drop for the short run between buildings at 10psi.

My intent was to have the returns connect below the waterline to provide a water seal. I am not sure if that is required, but I also assume it won't cause a problem either.

I have sized many condensate pumps for large steam distribution systems (college, military, hospitals, etc...). I try to find a pressure gauge on the return system that works to get an accurate back pressure, but admit I have selected a discharge pressure that I know is higher than needed, just to be sure my pump will get the condensate out of the building.

I know how gravity return steam systems work, and don't work, but I have not done this before. I am hoping to learn from the experiences of others.

nicholas bonham-carter

Giant orifice system

Couldn't this whole system be treated as a giant orifice system, with a gate valve sized for the EDR of the whole, regulating the steam supply so that the local steam pressure was maintained at 6 ounces?--NBC

jumper

how much do want to spend?

Steam to steam generator has advantages over steam PRV.