Pressure-based control strategy for two stage firing

SWEI

We have a client with a beautiful old two pipe vacuum/vapor system so I'm starting to look at ways we might make it work better. Tiny little Scotch Marine boiler that the owner partly tore down and cleaned. Currently controlled by two mercury-based Pressuretrols, one with a manual reset. I'll do a combustion analysis across a range of manifold pressures to see what the prospects are for retrofitting it to two-stage firing. Assuming that all works out, and barring a full DDC implementation, I'm planning to propose using a pair of Photohelics to stage the burner based on steam pressure.

I've heard low-hi-lo and low-hi-off and various other terms employed, but it would seem to me that the optimal strategy might depend on whether the thermostat is employed as a high limit or as an active operating control.

Any guidance here on the sequence we want to see? As pressure builds and falls are the responses or setpoints different?


As usual, thanks in advance...

Jamie Hall

@vaporvac has done some work on this, only with two boilers. My own inclination, should @Charlie from wmass and I find one someday and feel ambitious enough to get a two stage oil burner, would be to start on high fire up to a couple of ounces, then drop to low to maintain, bringing the high back up if the pressure dropped again. The Photohelics will be nice, but I'm lazy and would probably just add another vapourstat -- if I could find another mercury one. Keep the manual pressurestat as a safety backup.

I can see an advantage to starting low, as that would reduce the demand on the venting, but with adequate venting that shouldn't be a factor -- and the control strategy for a high start low maintain is a lot easier to implement!

Come to think of it, if the boiler is really matched well to the system, you might not need the second vapourstat at all, as the maintain level of fire might be adjusted to exactly fill the system... I think I'd have it anyway, though.

Dave in QCA

As Jamie has said, VaporVac has a two boiler setup intended to run as two stage. My understanding is that she intends to employ a 2 stage thermostat but has not gotten it installed yet. Very similar, perhaps exact, Dave Bunnell has also installed systems using to boilers running in a staged scenario controlled by a 2 stage thermostat.

I have a two stage setup, but mine operates a little different. First, I have orifice plates on every radiator inlet, sized at 8 oz, from Tunstall. I am not using the orifices as a means to keep steam out of the returns, but rather, as a device to provide for even distribution of steam, as described by C.A. Dunham. The system was originally set up as a Dunham Vacuo Vapor system and was pictured in their 1911 booklet on The Dunham Vacuo Vapor System. In the system, as it exists now, when operating, it is desirable to maintain the pressure at 8 oz, plus or minus a couple ounces. Given that it is a large building and that the connected load may vary with adjustment of radiators, tuning the boiler into one set firing rate is not appropriate. A two stage burner that can respond to the varying connect load is advantageous.

My system utilizes a Weil-McLain 680 boiler. While a 580 was sufficient in size, it was not available with a 2-stage burner. The burner is a Riello 2-stage burner. It is set for low fire at 400,000 BTU/Hr and High Fire at 690,000 BTU/Hr. Firing rate is controlled by a Hi/Lo Vaporstat, set at 10 oz. It cycles up to hi fire at 6 oz and down to low fire at 10 oz. Additionally, the controlling Vaporstat shuts the burner off at 16 oz. This is within the pressure limits that allow for gravity return without backing condensate into the return piping. Also, there is a manual reset Pressuretroll set for 2 Psi.

Temperature control is via Tekmar 279 with 2 controlling sensors. Three points in the building that are routinely checked for temperature status are routinely within 1 degree of the set point. One of the monitor points occasionally will run higher because of southern solar gain.

My setup is probably very simple, so much so that I don't know if will be of any help to you. I can say, that upon switching from a very oversized Pennco atmospheric boiler, that operating cost and energy consumption immediately dropped 30%.

Main venting is through 3 mepco 1E crossover traps. System venting is through a 1/2" swing check valve. The system falls into vacuum between firing cycles. Venting in subsequent cycles is minimal.

SWEI

Thank you both.

Sounds like I could use two Photohelics -- one for on/off with a wide spread and one for high/low with a narrower spread. For an arbitrary 8 ounce-inch target pressure:

On/off set at 2 and 14
High/low set at 6 and 10