Explain why limiting steam flow into radiators prevents steam from going into return?

wilder0m

I've learned from this forum that in 2-pipe systems it's often recommended to use orifice plates, metering valves, or just plain old trial and error with regular valves to limit the flow of steam into a radiator, particularly if you don't have steam traps at the outlet.

I understand the principle that we don't want steam to flow past the radiator into the return.

What I'm trying to wrap my head around is how limiting the flow actually accomplishes this. Is it by:

1. simply slowing down the rate at which steam gets into the radiator, so that you can control how much of the radiator fills with steam in a given time interval, e.g. by the time the boiler turns off, or
   
2. (something like) once the radiator fills with steam up to a certain pressure, the pressure differential is reduced, such that for a given orifice diameter, the steam flow will nearly stop altogether?
   
3. something else?
   

Mad Dog_2

Only if there is enough cold cast Iron to cool down and condense the steam BEFORE it gets to the outlet. Mad Dog 🐕 

Pumpguy

Agree with @Mad Dog_2.

So long as the amount (volume) of steam admitted to the radiator is less than the amount the radiator can condense, there won't be any steam left in the radiator to get into the return line.

Steamhead

This article explains it- note that this was invented right here in America:

https://www.heatinghelp.com/systems-help-center/european-heating-systems-circa-1907/

wilder0m

@Mad Dog_2 @Pumpguy @Steamhead Ok, thanks, but I think my confusion is about how controlling the flow *rate* translates into controlling the total *volume* of steam getting into the radiator. Say we restrict the flow with a smaller orifice. Wouldn't that just mean it takes longer for the total volume of steam to exceed the desired volume, but it would still happen?

As an analogy: I can fill up a water glass by turning the tap to barely drip or by turning it on full blast, the only difference is how much time it takes.

mattmia2

The radiator condenses the steam in to water at a given rate by taking the heat from that steam and connecting and radiating it in to the surrounding air. If the rate at which the radiator condenses the steam exceeds the rate at which the steam is entering the radiator, all of the steam entering the radiator will condense to water and no steam will pass to the outlet of the radiator.

Your confusion is that this is a continuing dynamic process. As steam condenses on the inner surfaces of the radiator, more steam enters from the main so the radiator is always partially or completely filled with steam depending on that rate at which the steam is entering the radiator.

The radiator doesn't just fill with steam once, condense it, and it is done heating. More steam enters the radiator as the steam that is currently in the radiator condenses and it will produce more heat as long as the boiler is producing steam.

wilder0m

mattmia2 said:

The radiator condenses the steam in to water at a given rate by taking the heat from that steam and connecting and radiating it in to the surrounding air. If the rate at which the radiator condenses the steam exceeds the rate at which the steam is entering the radiator, all of the steam entering the radiator will condense to water and no steam will pass to the outlet of the radiator.

Your confusion is that this is a continuing dynamic process. As steam condenses on the inner surfaces of the radiator, more steam enters from the main so the radiator is always partially or completely filled with steam depending on that rate at which the steam is entering the radiator.

The radiator doesn't just fill with steam once, condense it, and it is done heating. More steam enters the radiator as the steam that is currently in the radiator condenses and it will produce more heat as long as the boiler is producing steam.

Thanks! That makes it much clearer. So then taking it a step further, is there a flow rate into the radiator for which only part of the radiator (say half) will get hot and remain in sort of equilibrium in that state (as long as the boiler is producing steam)? That is what I understood from some comments in another thread where I was asking about replacing valves. With your clarification I think I understand better how that is possible.

mattmia2

Exactly. You usually aim for admitting about 3/4 of the EDR through the metering valve. Most metering valves will have instructions telling you how to set them based on the EDR of the emitter they are connected to.

DanHolohan

It can be confusing if you try to compare water to steam. The big differences is that steam will condense and water cannot. Typically, they would size the radiator to be larger than what was needed to heat the room, and then size the orifice to allow into the steam radiator about 80% of what the radiator was capable of condensing

wilder0m

DanHolohan said:

It can be confusing if you try to compare water to steam. The big differences is that steam will condense and water cannot. Typically, they would size the radiator to be larger than what was needed to heat the room, and then size the orifice to allow into the steam radiator about 80% of what the radiator was capable of condensing

Thanks, it makes sense now!