Staged pressure controls

JStar

What's the most efficient settings for a high/low fire steam boiler? Is there a formula or guideline for setting the exact pressures that dictate the staging, assuming that you know the proper operating/cut-out pressure.

RJ_4

controls

This may help, Lattner boilers are used alot in spas and gyms for steam rooms

Noel

low fire

The pressure that it switches to high fire FROM low fire is the lowest operating pressure during the cycle. That setting MUST be a wee bit higher than the pressure that you need to heat EVERY radiator completely. Consider THAT your actual operating pressure, so that when you drop under that pressure, the boiler goes back to high fire. All other pressure setpoints for staging the load need to be above that number.



Consequently, the other setpoints seem needlessly high, but it must be so, or the thing will sit at low fire most of the time and the end radiators will cool off every time it drops to low fire. Which will be most of the time.

JStar

Staging

So, let's set an example.



I want a boiler to operate at 12 ounces. I would just set my high fire to cut out at 12, and then the low fire will cut out at 14 or 16? The high fire cut in should be set higher than the low fire cut in, right?

Noel

Yes

Exactly, that way the pressure rises to and above the minimum nessessary. I adjusted one too low and one radiator would only see steam on high fire. Always the same radiator, too. I heard about it on a MILD day when it stayed in low fire all of the time.

Steamhead

The other thing that's in play here

is how fast the system vents air, and at what pressure.



Ideally we want to be able to vent the system with only a few ounces pressure. When the mains fill with steam, the system should drop to low fire since the pickup factor is no longer needed. Then the rads can fill with steam at the lower pressure.



We added lo-hi-lo to the system in our Find a Contractor ad. The lo-hi-lo Vaporstat trips to low fire at four ounces, dropping roughly 20% of the firing rate. If the system pressure rises to 12 ounces, which it does on occasion after a really long run, the operating Vaporstat stops the burner. But mostly it runs steadily at low fire, until the Tekmar satisfies. The radiators heat quickly and evenly



For comparison, when we first looked at this system it was running 5 pounds.

SWEI

low-hi-low

Call me clueless, but help me understand why people refer to this as a low-hi-low setup instead of just a hi-low or a 2-stage?



thanks~

Steamhead

The other variation

is "lo-hi-off" also called "low-fire start", where the burner starts at low fire, switches to high fire once the flame is established and never switches back to low fire during that cycle.

SWEI

thanks

makes sense now.



So if I were designing controls for this application and wanted to switch to low fire once the mains were filled, would I put a temp sensor at the end of the main, or is there a better indicator?

Steamhead

A Vaporstat at the boiler will do it

if you have the firing rate and main vents sized properly, the system pressure will go up a few ounces when the main vents close- this is also true for crossover traps. A Vaporstat can detect this pressure rise and drop the boiler to low fire.

SWEI

pressure, of course

Thanks.



Still evaluating pressure transducers for DDC.  Temperature is so much easier to read well...

JStar

Temp

The only problem with sensing temperature is that a pipe can stay hot long after the pressure and steam has dissappeared.