Heat Loss Calculations

mikeg2015

foresthillsjd said:

@kenjohnson it’s natural gas.



So, I make a lot of recipes with my pressure cooker. And I think of it like a combination boiler/radiator system. You add flame, liquid inside heats, comes up to pressure, and you reduce the flame so to keep the pressure high enough to keep the indicator up, but low enough that it is not constantly venting. And the flame required to reach that equilibrium point depends on how full the pot is.



So I still don’t understand why on a gas-fired steam boiler, it’s on or off. If you were able to modulate the flame and adjust the water level in the boiler, shouldn’t any boiler be able to dialed back? (I know the answer is no, just trying to understand why.)

An atmospheric boiler won’t maintain the same heat transfer at a lower firing rate just by reducing pressure. 3 reasons
1) Inshot burners create a Venturi effect. SO lower velocity gas through an orifice changes the fuel air ratio slightly.
2) You typically end up with excess primary air. Adjusting hte shutters will help, but these are not sealed burners, so other air leaks in at a rate not proportional to the firing rate due to stack effect.
3) Heat transfer due to radiation at the burner bow is reduced due to a shorter flame.

This is the same thing that happens when you have a car engine running at part throttle. It’s specific economy (Power delivered to fuel consumed) is actually lower than when it’s moderately loaded due to air mixing effects and less relative heat lost to the water jacket. However at very high loads they run richer to prevent preiginition and achieve max peak output and economy drops again. This is very relevant to Diesel engines which I operate most like a burner, than a spark ignition engine with modulating intake flow.

kenjohnson

Jamie Hall said:

Unless the boiler and burner system is designed for it

Does the user manual for the natural gas fired boiler indicate that flow can be restricted using a nozzle of some type, so as to reduce the burn rate? My oil-fired boiler was shipped with something like a 0.8 gph nozzle installed, but I downsized that to a 0.5 gph nozzle (the smallest the manual said could be used), and that kept the boiler from short-cycling due to being too large for the radiators (it was the smallest oil boiler I could buy...).

mikeg2015

A few boilers are sold in 2 sizes, but same burners and sections and will fire at 2 different rates using smaller orifices. But need to check there isn’t a flue restrictor of some sort. ZThe different is usually only 10% between the two sized (ie 175 vs 200, 150 v 175). 2 stage gas valve can pull it down lower, but again, you’ll lose efficiency and need ot verify any change like this with a combustion analyzer.