Your opinion please Keep vacuum return or convert to gravity return

RayWohlfarth

I dont do much with vacuum systems and was asked about a steam system with a vacuum return. They want to change the boilers and vacuum return system I remember Dan saying that a vacuum return should stay a vacuum return. We have one system we inherited that had a vacuum return converted to gravity and it has been a nightmare. The boilers are about 5 mill btuh each and they have two
Thanks
Ray

Steamhead

In a system designed for vacuum return, the piping was sized for the increased ΔP that vacuum provides. So it's too small for gravity return. Keep the vacuum.

@Pumpguy could probably help you out here.

GBC_illinois

I am having a similar internal debate -- I have a 2-pipe steam system with a pumped return... that is, a boiler feed pump, activated on boiler water level, with the tank vented to atmosphere. However, i've studied the condensate return rate and determined the condensate tank and pump are not necessary. The system currently runs at about 11oz of pressure. Since the tank setup has caused very expensive problems in the past when it's failed, at the next opportunity I'm planning to get rid of the tank and pump, and go back to pure gravity return.

However, after discussing with @PMJ, I'm thinking that rather than just defaulting to atmospheric pressure once I re-pipe it to gravity return, that perhaps I should keep the system in vacuum while it is off cycle -- which would seem to me to allow quicker transmission of steam throughout when the boiler starts up, if it didn't have to push all the air out every time. If I'm correct, this choice of starting at atmosphere vs vacuum would be only dependent on the types of main vents I use, whether they maintain a vacuum or will vent both ways. Currently, besides the condensate tank vent, there are only two air vents in the system; one on each condensate main in the basement. There are no radiator vents anywhere in the system. When I re-pipe it to gravity return without the tank, there will also be no steam traps on the end of the mains or condensate lines. The system was built in 1920. The current boiler is only 10 years old, a Weil Mclain LGB-9 Series 2.

So my question is: Is there any downside to operating in vacuum?

ScottSecor

It's been a while since we serviced about ten accounts that had vacuum pumps on their steam systems. However, I do recall what @Steamhead mentioned that all of the system piping was "downsized" for the vacuum system. In other words, the 2.5" steel return would need to be at least 3" steel for a gravity system, that 4" supply would need to be at least 5", etc. I also recall many of the returns in this particular school district were nearly flat, as in almost no pitch at all.

I'm sure Jake will chime in with some good advice.

On most if not all of the vacuum systems we used to service, they all has little or no steam trap maintenance. As a result, the vacuum pumps would not produce vacuum, the returns would back up and many strange noises would result. Oftentimes, the vacuum overflow tank would blow out the condensate once it finally got back (tell tale sign). Another issue was the staff would often crank up the pressure to "get better heat." This would exasperate the problem and now we're trying to pump/vacuum live steam. By the way most of the original vacuum pumps we worked on were the original Nash brand, we were under the impression parts were not available (this was the 1990's).

mattmia2

@GBC_illinois you should move your question to a different thread. This is a different question, it is being asked about a large commercial system with a vacuum pump that keeps the returns in vacuum. In your case you need to understand where air is going from the mains and the returns when you make your changes. Air will eventually leak back in to the system so if you don't have the mains vented somehow the mains will have to vent that air from the mains through the radiators and out the returns. This could make for uneven heating in the shoulder seasons where the boiler might only run a couple times a day.

Pumpguy

Much depends on system conditions; returning condensate temperature, how air tight the system is, and the air removal capacity (CFM) of the vacuum pump(s).

Just a few pin hole size air leaks can really hurt the vacuum level, especially if the vacuum pump air removal capacity (CFM) is marginal.

Considering condensate temperature and achievable vacuum, the condensate temperature should be as cold as possible. As a practical matter, 140* F. is a realistic temperature. The attached file goes into detail.

A good rule of thumb in sizing vacuum pumps is to have one (1) CFM per thousand square feet EDR. Since condensate pumps use this same rule when calculating their GPM, you can have the vacuum pump's CFM equal the condensate pump's GPM.

My experience is most vacuum pump manufacturers size for 1/3 to 1/2 CFM per thousand square feet EDR. This is okay when everything is new and air tight and all the traps are working so the condensate temperature is nice and cool. Considering that today's vacuum pumps are sold for installation on existing systems, it's better to have as much air removal capacity (CFM) as practical.

Another factor is the system steam pressure. Considering the rules of EDR, the radiation is sized for 100% heat output when filled with 215* F. steam, which is just under 1 PSI. So all the pressure needed is enough to allow for pressure drops so at the point of lowest pressure we have 215* F. steam. Two inches Hg. vacuum is equal to one PSI pressure differential, so lowering the steam pressure can be made up for by increasing the vacuum (lowering the pressure) in the return lines.

Another factor in favor of vacuum is being able to heat with low temperature steam. When at vacuum, we could have steam with a temperature below 212* F. This is especially useful in mild weather. With a simple vented gravity return system, as soon as the steam temperature drops below 212*F. it condenses and no more heat is available.

Depending on what equipment is there now, it may be possible to upgrade with larger capacity, and possibly different design vacuum pumps. It is also possible to add vacuum pump(s) to an existing condensate only pump package.

If you would like to pursue this, you can contact me directly and we can discuss your particular system.

DanHolohan

The pipes, valves, and fittings will be one size too small without the vacuum pump. 

jumper

There was an engineer here,Zhadanovsky, Igor who improved existing systems with vacuum. If client pays him perhaps he can design an improved vacuum system with modern controls?
One possibility to economise with engineered vacuum is to eliminate traps and vents. Also I believe that nowadays there are dry vacuum generators that can handle water vapour.

RayWohlfarth

well that's settles it Im going to recommend keeping the vacuum. I did see the system was maintaining about 4" vacuum Thanks for sharing your expertise

Tim_D

Piping is going to be too small and the return water will not come back soon enough. Keep the vacuum pump.

Mad Dog_2

A tight and maintained Vac system and piping is as good as it gets.  Mad dog  

The Steam Whisperer

Before you say you can't go to gravity return in a system originally designed for vacuum because everything is a size smaller, consider this...

The radiation on nearly all systems installed from about 1910 to 1940 is about 60% oversized for the as built heating load. If you operate the system with supply valve orifices sized to that 60% capacity, you have now almost halved the load on the piping. Guess what... the piping is now just the right size for gravity return and regular pressure supply. We have completed this conversion on several systems and the heating is very very even and fuel usage dropped by about 40 to 50%. No more radiator traps to maintain and no vacuum pump to rebuild, install and maintain. I like the KISS principal.

Just make sure you have no lift fittings in the system. You'll need some remote condensate pumps if you do.

PMJ

I agree with @The Steam Whisperer . My experience with vacuum in 2 pipe is that the entire system remains at the same relative pressures, just lower. The maximum difference between any two points might be 2 inches of water. As such there is no effect on gravity flow. If I were to add a pump to the dry return the pressure of the whole system would go even lower, but all else would remain the same.

As he points out there is never a reason to fill radiators full and close traps. Most systems like mine (Mouat) have special return elbows that can't close completely anyway. So the entire system is just one open pipe system. Achieving significant differences in pressures anywhere would require rather extreme flow rates of steam or condensate well beyond anything I have ever observed or think can even happen.

I must be missing something and would welcome a more detailed explanation of this size thing as it relates to vacuum operation.

jumper

Published tables show less pressure drop @ lower pressure.

Another thing to consider is that mechanical vacuum generator removes more air more efficiently. Otherwise your boiler works inefficiently to expel some air. Heat transfer from burner to radiator is impaired by ANY air whatever in system.

PMJ's use of PLC control is excellent idea. Next dream improvement is to conjure up some sort of condenser trap for the vent.

>>Achieving significant differences in pressures anywhere would require rather extreme flow rates of steam or condensate well beyond anything I have ever observed or think can even happen.

I must be missing something and would welcome a more detailed explanation of this size thing as it relates to vacuum operation.<<

The Steam Whisperer

The Steam Whisperer said:

Before you say you can't go to gravity return in a system originally designed for vacuum because everything is a size smaller, consider this...

The radiation on nearly all systems installed from about 1910 to 1940 is about 60% oversized for the as built heating load. If you operate the system with supply valve orifices sized to that 60% capacity, you have now almost halved the load on the piping. Guess what... the piping is now just the right size for gravity return and regular pressure supply. We have completed this conversion on several systems and the heating is very very even and fuel usage dropped by about 40 to 50%. No more radiator traps to maintain and no vacuum pump to rebuild, install and maintain. I like the KISS principal.

Just make sure you have no lift fittings in the system. You'll need some remote condensate pumps if you do.

I should add that we downsized the boiler output accordingly... so teh boiler is supplying about 1/2 the steam of the original boiler. Also, addressing the advantage of vacuum by decreasing the amount of air in the system, installing a a check valve at the main venting device ( condensate tank or open pipe or main vents) creates a nice amount of vacuum for the next heating cycle. The larger home near me that we orificed, cut the boiler size and put check valves on the main vents has Large tube radiation and runs about 6 to 8 inches of vacuum between heating cycles.

jumper

Speaking of reduced steam requirements OP mention two boilers. Replacing them with one half the size of the other is a neat way to vary output to drag out cycle time. Begin with both firing and then shut down one according to outside temperature or time of day.

ttekushan_3

Greetings. I strongly recommend keeping the vacuum return. In my situation we’ve got two 125HP fire tube boilers serving a space heating system that’s quite convoluted owing to the fact that it’s 5 buildings stitched together over time. It was vacuum return from the beginning. 

My first couple of years gradually de-knuckleheading the system and adding to it, the vacuum return was defunct. But my instinct and imposed budgets suggested servicing the whole system, getting it to balance and behave itself without the vacuum and only then return it to vacuum. It all worked out fine. 

But don’t underestimate how much the vacuum return pump helps the speed of distribution and the evenness of it. Night and day difference if the system was designed for it. 

Tight control of the heating cycles is the result with the ultimate results of even and predictable heat with sharply reduced fuel usage. 

We obtained the liquid ring pump from. @Pumpguy and everyone couldn’t be happier. 

One thing I recommend is making sure you have a big swing check vent in parallel to the vacuum arrangement so it can exhaust should the steam rate temporarily get ahead of the vac pump capacity and strap an aquastat on the suction line to shut down the pump should the temperature rise too high. To protect the pump. Our system shuts down the pump on very cold days where nearly 100% run time is required. But at that point the system is fully saturated so there’s no air left to exhaust and our 2psi is still able to lift condensate where it’s needed. 

Also, I have it on a delay timer after boiler shut down so the pump can assist in clearing condensate for awhile. 

Anyway that’s my experience with it.  

clammy

In my book as I see it aside from a major repipe to make gravity the performance of the system will not be the same nor will distribution . Most likely a gravity system will consume more fuel and system balance will suffer and you will be working on that to get back to where it was . It was installed and designed as a vacuum return and I would not want the responsibility for screwing it up . Usually there other reasons for a vacuum return like remote building served by one or two boilers . If it vacuum keep it that way it’s the easiest and least amount of issue down the road .
As always Ray thanks for insight and keeping us interested
Peace and good luck clammy

RayWohlfarth

You guys are the best. Thanks so much Lots to think about I am going to adamantly recommend keeping the vacuum system Thanks