Best Of

"The fact that I struggle to find good info about a very very specific question (how/do dynamic orifice traps work in low pressure steam vacuum systems as opposed to non-vacuum systems?) just so happens to also slot in nicely here!"

Ok, let's get rid of one thing here. Orifices do not sacrifice efficiency. What they do do is limit the volume of flow at a given pressure differential between the feed side and the downstream side. If the orifice diameter is correct for the pressure differential, just enough steam will enter the radiator to condense fully in the radiator, leaving only condensate to pass. If the pressure differential is less, the steam flow will be less, of course, and the heat output somewhat reduced. If it's greater, not all the steam will condense and you will get steam in the returns.

Now. You mention vacuum vs. non-vacuum systems. This is where things get interesting. First off here, if you are talking about vacuum assisted returns, but with "normal" supply pressures, the pressure differential up there is thrown way off and, without traps, the system will work poorly if at all. There are a few ways to create such systems, but the most obvious -- and most efficient -- is to have some of the condensing, at least, taking place at relatively low temperatures. This can, of course, be arranged in various ways, such as preheating the feed for domestic hot water. In power boilers it is done by air or water cooled condensers, and the efficiency loss there is more than offset by the power gain across the turbine or engine.

However, if you are talking about systems where the entire system is operating at less than atmospheric pressure, the system will work and work well, but the pressure differential between the supply side and the radiator side must be less. Why? Because at the lower radiator pressure the radiator condensing capability is reduced dues to the lower boiling point, and thus less steam can be allowed to enter the radiator. Many times when people talk about operating a residential or other heating system in a vacuum, they are dealing with a system which operates at sub atmospheric pressure at a low firing rate at the boiler. This was actually quite common in the days of coal fired boilers, and it worked well -- many vapour systems were equipped with vents which closed against a vacuum, and thus when the firing rate dropped the system as a whole would drop into a vacuum -- in principle extracting more heat from the low (or dying) fire..

The problem with running any vacuum assisted or sub-atmospheric system is, of course, the same as running low pressures in any heat exchange system which depends on evaporation and condensation cycles to transfer heat: elimination of non-condensable gas in the system and keeping it out. In a normal low pressure heating system, this is done by means of the vents. No problem. In sub-atmospheric systems, using a vacuum pump, water vapour must be kept out of the pump (or the water loss accepted) -- which means a low temperature condenser or that the system is vacuum tight.