Another Look at Vapor Vacuum

Dave in QCA

Not the Boiler Equalizer

Vaporvac, I don't recall how your boiler equalizer is set up, but the return pipe equalizer that Dunham shows on virtually all of its combinations and types of vapor and vacuum heating is not in any way, a boiler equalizer.  I am attaching one of the Dunham drawings with a label and arrow added.  This helps to illustrate exactly what I am referring to.

nz

Vacuum Vent vs. Check Valve

Dave,



Yes, I did post that pic before, but on another thread.



For the check valve - are you suggesting I install it in front of the vents on the antler? Should I get a 3/4" since that's the size of the pipe feeding the antler?



Would the check valve be sufficient instead of buying the Hoffman 76s?



Also - I checked, it doesn't look like I have a return equalizer, nor stubs where one might have been...however, the pic you posted below seems very similar to my near boiler piping...so its possible it was removed when the boiler was replaced several decades ago.

Dave in QCA

check valve in lieu of

Yes. I am suggesting using a check valve in lieu of a couple of Hoffman 76 vents.  

If your traps are working, you will not have steam in the return piping.

If you are controlling with a vaporstat you can control your dimension B to a level lower than the vent location, thus you won't have water in the return piping.

So, a check valve would work.  I'd use a 1/2" size from the rational, that there is less mating surface in the check and thus more likely to seal tightly.  Also, 1/2" should give you adequate venting.



EnReynolds has told me of a very tight spring check valve designed for vacuum service that has a cracking pressure of 1.2 oz.  Kinda pricey, but perhaps worth it if you determine that your system is tight enough but the check valve doesn't hold tight enough.  



On the other hand, I have seen a bronze 1/2" swing check valve used as a vacuum breaker on a shell and tube steam to hot water converter.  Steam pressure operated at 5 psi.   It never leaked at all!  Remained totally sealed against steam and condensate yet opened freely when needed.  Based on that, it seems like it would work for the purposes we are talking about here.

PMJ

More Details

I posted earlier about having converted my home two pipe system to vacuum some years back before I came across this site. Now that I have and have learned more (read the LAOSH book) I have added some gauges around to see what was going on a little better.

I have a 1957 Bryant steam boiler model #7-446. It is a replacement for the unknown 1926 original. It has a rated output of 369,000btu/hr which from what I read matches pretty well with the about 1000sqft total EDR I measure from 23 radiators. I am guessing the header is original because it is larger than what the boiler manual calls for. It is 4" tapped from both sides - 90 up to about 28" over the waterline then 90 forward then 90 and 90 again to the center and tees up in a single 4"( I read this is a no-no but I have no problems) then up another 6" and 90 to the back where the 4" pipe runs about 5 feet( we are now about 32" over the waterline) into the 2-1/2" main. That main runs in a loop around the entire basement in a rectangle which is about 30' x 50'. A 1-1/2" dry return runs right next to the main all the way around too. In a far corner from the boiler both supply and return drop straight down the wall and join together about 6" below the boiler waterline and from there the wet return runs all the way back to the boiler. Two stories of rads tap off these runs all around the perimeter of the loop.

There is no other piping at the boiler - no equalizer or anything else. There is a check valve in the wet return at the boiler but after all these years I suspect it does nothing. Now when I decided to try letting the system fall into vacuum between cycles I removed the vent from the supply main in that far corner where it dropped down and meets the return. I also removed all vents on rads wherever there were any. I also removed the vent on the far end of the dry return and replaced it with a solenoid valve I run with the PLC. My logic went this way - if the rads were sized to only fill all the way on the very coldest days (with windows open) then basically at least the big ones would never close their traps. That being the case then the supply main would always be open to the return and so venting would only be required at the end of the return. I use a pressure switch right next to that solenoid valve which trips at .05" of water pressure and signals the PLC to open the valve. That valve stays open until the burner cycles off, the valve closes, and the system sinks back into vacuum - typically 20" of water up to 40" in cold weather.

Now to pressure. Recently I installed several compound gages to see how much steam pressure I was getting as well as the vacuum at various points. I put one next to the vaporstat on the boiler(which doesn't do anything because the pressure is so low) one at the end of the main where I removed the original vent, and one at a point in the dry return in the corner opposite the solenoid valve/vent. By the way, I never hear any air coming out of the solenoid valve which is the only place in the whole system that gets opened to the atmosphere. At the end of the steam main the most pressure I have ever seen is 1" of water. In the dry return during fire it is always 0. My only surprise is at the boiler. The vaporstat tap is about 2" above the waterline. There during full fire I am showing 1-2" of vacuum still. I know the gauge is working because if I do leave the solenoid valve closed I will see pressure build there. Can anyone explain why I see vacuum there?

This system works flawlessly. All radiators hot, no noise anywhere of any kind except perhaps hearing an expanding pipe sliding through a wall somewhere. Originally all traps were of the small orifice elbow type - I have installed a few thermostatic ones on smaller rads. I guess I am just wondering how I am able to get away without some of the controls and piping I read is essential. I am a believer that you basically need no pressure at all to deliver the steam if the pipes are big enough. Seems I really lucked out.

Any comments?

jumper

equalize from main?

From your description the equalizer runs from main instead of from header ? Why shouldn't that work ? Do you know if joints are painted or otherwise sealed underneath insulation ? If air can't get in then pressure depends on boiler temperature.



Buildings with long horizontal runs have been built with one pipe distribution but two connections to radiators so that there's parallel flow. As long as condensate can return steam continues to heat.

PMJ

Equalize from the main

Not sure I understand your question. Everything is insulated but I'm sure I can see all the piping. I know there is no connection at the boiler to the header down to the wet return. At the far end of the main it simply 90's down a wall below the waterline where it tee's with the same drop from the dry return. There is no trap or any other device on the main or the dry return. The lone vent for the whole system is at the end of the dry return before that drop below the waterline.

jumper

equalizer ?

As I understand your description, I figure pressure in header = pressure in main = pressure in dry return (because they're connected) . So that's an equalizer.

Dave in QCA

steaming under vacuum

After reading your discription of the operation of your system and then the question at the end, I think I understand you to ask why the boiler is still showing a vacuum when it is making steam.  I thought that perhaps you were saying that the boiler is showing a vacuum while the end of the steam main is not, and this happens at the same time.  If that is the case, you probably have a gauge that is slightly off.

But, to the first assumption... when your boiler shuts off and the steam condenses, and it falls into a strong vacuum.  There is vapor continuing to flow at that time.  Vacuum creates a lower boiler temperature of the water. When it gets to a certain temperature and vacuum combination, flow of vapor will cease.  When the fire comes on again, the water will begin to boil and vapor will flow at a temperature below the normal boiling point of water.  As the subatmospheric vapor flows through the system, warming the pipes and radiators, the temperature of the whole system is raised, thus causing decrease in the amount of vacuum and a gradual increase in the boiling temperture.  The longer the boiler runs, the higher the temperature/pressure will become.  If it fires long enough, the vacuum at the boiler will cease.  If it continues to fire longer, you will begin to build pressure.  However, it sounds like the firing rate or heating capacity of your boiler is closely matched to your system, and so, it may take a long time to build pressure, and may not do it at all.

SteamCoffee

Any further updates?

This was an awesome thread, I am sure many would like to hear anything new for the upcoming heating season, good, bad or ugly! Thanks!

Dave in QCA

Other Project taking Priority

Thanks for bringing this thread back to the top.



My attention has been focused and stabilizing a foundation of the old garage and chauffer's apartment and putting a new roof on the main house building.  I recently sent out requests for proposals for a new boiler.  I will plan on completing my plan to install the crossover traps and a check valve in the common vent at that time.

nz

Check Valve

Dave,



I'm tightening up my system this year - replacing valves all over the place - I have 10 or 11 that leak now (small leaks, those 82 year-old packless valves are failing), those are all being replaced, some with TRVs (for the bedrooms.)



I added a vacuum gauge to my pigtail as well.



So - for the check valve, would this be a suitable option? I was thinking about installing it before my main vent(s) at the end of the return line (two pipe Dunham here.) However, if would need to go with a 3/4" check valve - but the same model.



http://www.pexsupply.com/Hydrovalve-SC050T-1-2-Threaded-Swing-Check-Valve



Nick

Dave in QCA

1/2 inch I think

I would opt for the half inch size.  My thinking is that it is adequate for the venting and with the smaller size, I would think it is more likely to seal tight enough to hold a vacuum.

I would install it at the end of the first horizontal nipple coming our of the tee, that is located on the vertical 1/2" line that runs between the upper and lower connections on your return piping.

All the reasons why you don't need a vent valve are stated in the posts above this one.   Neither steam no water will ever get to this area of the piping unless something else has failed, in which case, it is good to know it so it can be corrected.  I have seen Hoffman vents on the return line vent locations and they got hot and closed because of presence of steam.  The homeowner didn't have a clue that there was something wrong.  But, one or more traps had failed and needed to be serviced.  No way to know if the symptom is shielded from view.



There have been some other ideas posted as to the use of super duper vacuum checks that have a very low cracking pressure.  But, a swing check will open very easy.  As long as it seals tightly, it will work fine.  You will be able to see how it works.

PMJ

Check Valve

I found it difficult to get a valve with low enough cracking pressure (less than 1oz) and seal well enough to not be the biggest leak when in vacuum. I tried several and would always hear the air coming out when it did finally open. I have a 1/2" pipe and figure if I can hear the air coming out of it I have more pressure in the dry return than I want (which is zero). I ended up with a 1/2 inch 110volt solenoid valve which seals completely. Just before it I installed a pressure switch that trips at about 1/10th of one oz which signals the PLC to open the valve. Never hear anything coming out - never any pressure in the dry return. When the burner goes off the PLC closes the valve and we go back into vacuum. Really happy with it.

nz

Took the plunge...and a bonus too!

So, about six weeks ago, I decided to convert my Dunham two pipe system to vacuum. I took off my Gorton #2 and Hoffman #75 return vents, and put in a single 3/4" check valve.



I had previously put a 0-30 inches of water vacuum gauge in, which never really showed a vacuum since the main vents let the air back in as quickly as the vacuum formed. However, after installing the check valve, I quickly found that a 0-30 inches of water vacuum gauge was not going to cut it, as the gauge wound around 1.5 times. I fixed two leaks, and the gauge wound itself around twice.



I recently installed a new 30-0-30 gauge where the vacuum is measured in Hg., which seems to work better. I currently am running between 6 and 7 inches of mercury vacuum, which is fairly decent I believe. To give you an idea, if I open a trap somewhere in the system, air whooshes in to fill the vacuum for about 20 seconds.



I've noticed that the steam does move quicker through the system. It should be boiling quicker too, since in a 6-7 in. Hg vacuum, I should be boiling water at 199-200F...however I haven't really timed it. This was all expected based on what I have read and it seems to be true in case there are any non-believers :-P



One potential side effect I have thought about, but not actually calculated, is as I should be making 199-200F steam during the upswing of the burn cycle, there should be a reduction in effective BTUs/EDR as the steam is not as hot (initially). Towards the end of the cycle, there is a very small vacuum, or very little pressure.



This makes me wonder two things:

1) Should I even bother with my vaporstat? Maybe set it at the lowest pressure possible (2oz I believe)

2) Do they even make a vacuum vaporstat, so the burner cuts out at...let's say...-1in Hg.



This is the best vacuum/boil table I have found yet:

http://www.accontrols.com/assets/docs/eng-resources/Water%20Boiling%20Points%20Under%20Pressure%20Tables.pdf



I've attached a pic of the new gauge, in case anyone is curious.



On a side note... One of the radiators in my garage has had a leak in it since I bought the house 2.5 years ago. I put a little JB weld on their last year, and reduced the leaking.



However, since its a wall-mount unit that is 20 sections wide, it would be difficult to remove...so I was forced to put my fingers in and reach to the 4th tube in the back to get to the leak. This proved difficult because the steam would leak and the surface would be covered in water. So, I would shut off the valve and wait a day for the radiator to dry off, and then put some more JB Weld on, and wait a day, and then turn the heat back on. After 3-4 times of doing this, it was a little better - but still leaked.



Converting to vacuum made this easy...while the steam was condensing in the rest of the system, a decent vacuum (as noted above) was produced. This caused the small leak in the radiator to make noise almost indefinitely, as the air was rushing back in through the trap. While this was happening I could apply the JB Weld with the radiator dry and cold, and actually HEAR when the hole was plugged. I'm not sure if the vacuum was enough to hold it in place, but this time I had the leak completely fixed after two attempts! Now I have (what I believe to be) a completely sealed system, and the garage heats better now too!

Dave in QCA

FANTASTIC NEWS!!!!

Nick, thanks for the update and for the very good work!  I am in the process of putting in a new boiler and within a month I hope to be able to report results that are similar to yours.



Everything you report makes sense and is consistent to what I have read in the various Dunham publications.  Regarding your vaporstat, it is fine.  It only comes to play when you hit a maximum pressure, which 8 oz is still fine.  As you have noticed, you begin to make steam while there is still a vacuum.  Steam circulates quicker, and it is at a cooler temperature.  If the call for heat is short the boiler stops before the pressure even rises out of vacuum.  If the call for heat is longer, the pressure (and temperature) will climb higher.  If it runs long enough, the pressure will climb to the cutout on the vaporstat of 8 oz., so that is still just where you want it.



A side benefit is that once you get the air out of your system, you're not constantly sucking all of the O2 and CO2 back into the system and therefore, oxygen corrosion and formation of carbonic acid should be minimized, lengthening the life of your boiler.



You should take the time to measure the timing on a couple of things.  First, when the boiler has been off since its last cycle, say one hour, time how long it takes to begin to make steam.  Second, once you begin to make steam, time how long it takes to reach your crossover traps.



Both of these times should be greatly reduced, especially the time that it takes for steam to reach the crossovers.  



Did you ever fix the piping over your garage?  If not, has the changes to your system improved it any.



You also had commented about the clinking sounds that your in-wall convectors on your first floor made.  Since the vapor is now coming up at a lower temperature, has the improved at all.



Are you able to tell that vapor continues to flow in the system after the fire has shut off?  This would coincide with the cooling off of the boiler water, the result of boiling under vacuum.



GREAT WORK!!!!

PMJ

You'll love it

Sounds great. I am sure you will love the performance.

I don't bother with the vaporstat anymore. There is never any measurable pressure to talk about anywhere in the system. With the system in vacuum between firings your time to steam is shortened and your steam delivery to the rads is faster. Between those two things you will notice quite an improvement. 



I have found that the real issue with steam systems is the time it takes to get steam from the call for heat. This time obviously varies with the length of time the boiler has been off since the last cycle. Anything you can do to shorten this time helps dramatically as the longer it is the more the system overshoots. Then it sits off longer and the problem repeats. Anyone who tries vacuum will find that it really does cut down time to steam from initial fire and that the vacuum speeds that steam to the rads - especially the remote ones. In sum it smooths things out considerably.



Also about overshooting - generally speaking with a simple thermostat approach by the time the stat is satisfied the rads are already hotter than was really necessary and hence the overshoot. Built in anticipators help some, but I have really enjoyed introducing a PLC between the boiler and the stat. I monitor how long the boiler has been off and vary the next fire time accordingly. The actual fire time over the long haul has been about 1/3 of the total time the stat is actually calling for heat. I fire for a while (longer if boiler has been off longer) and then wait a while (even if the stat is still calling) to give the steam in the rads a little time to heat the rooms. Really smooths things out too. Rads end up just sort of warm all the time instead of cycling so much.

nz

Overshoots

I don't have much of a (noticeable) overshoot, as I installed TRVs in all the cast iron rads on the second floor. The first floor is all in-wall fin-tube convectors, which cool off substantially quicker.



The first floor rads do heat up quicker though, and give off quite a bit of heat so some rooms are warmer that others.

nz

Update - 8 in. Hg now!

It's been a few days since I took that pic. I'm up to now 8 in. Hg. vacuum, so I'm boiling water around 196-197F.

Dave in QCA

would be enteresting if you....

Run the boiler until all of the rads are fully heated.  In this condition, you will have driven out the maximum amount of the air that you can.  When it cools off, it will be pulling the maximum vacuum possible via self induced vacuum.  It would be enteresting to know just how deep of a vacuum you can achieve and how low the boiling temperature will be.

nz

Other items

I did not get the pipes above the garage fixed - yet. Still have a slight hammer there. The vacuum did not seem to have any effect on this. I had a contractor who said he would come out but now he's dodging my calls.



The fin tube rads - the expansion noises seem to be closer together now, but no other effects. I will be experimenting with some milk jug pieces, I already started playing with one the other day.



On a side note - I did fix some of the "ticking" of the pipes against the wood floors with pieces of a cut-up milk jug as someone suggested. I had one of the three rads in my master bedroom that ticked very loudly...no matter how much I moved the rad, it would find its way back. This helped greatly, now there is only one tick that happens (I believe its below the floor (maybe between the joists) and its much quieter, instead of 20 ticks about 1 second apart.

PMJ

Questions

8" hg is really good - I haven't seen that much but then all of the valves on my rads are original and I'm guessing leaking some. How long will it take in your system if the boiler stays off for the 8" to go back to room pressure?



Also, the idea of making steam at lower temp sounds more efficient but it seems to me that the heat loss of the structure for a given outside temp is what it is and lower temp steam contains less heat so more of it will be required to heat the same space than 212F steam. Also, the vacuum is killed very quickly after the burner comes on so we are mostly at 212F steam anyway. Isn't the real advantage in the fact that the steam delivery from the call for heat is so much faster. Boiling starts a minute or so earlier in the deep vacuum so steam (though at a slightly lower temp) is moving to the radiators significantly sooner after the burner comes on. And, it continues to move several minutes after the burner goes off as the steam in the system collapses and the vacuum develops again. Basically, the total amount of time the boiler spends at full fire while delivering no heat at all to where you want it is significantly reduced. Minutes may not sound like a lot but when considered against  run cycle of 15-20 minutes it is significant.



Maybe I miss something here - comments?

nz

Answers

The vacuum holds - until the burner kicks back on. I have my t-stat set to one cycle per hour, and depending on the temp outside, it may or may not cycle every hour. Either way, I do not appear to be losing the vacuum.



The vacuum is eliminated once the boiler runs for a little while, however I have seen the boiler cut off (from the t-stat) while still in a vacuum.



Your comments around the steam temperature dovetail into my comments about regarding concerns of EDR/BTU. You're correct in that once the boiler has been making steam for a few minutes, the pressure raises and so does the temperature of the steam.



In my case...in an 8 in. Hg vacuum, going to 1 in. Hg. towards the end of the burn cycle, this isn't a noticeable issue. However I can hypothesize that in cases of an extreme vacuum, the effects could be significant (e.g. 100F steam)



FYI I checked the gauge last night a while after a burn, and I was at around 9 in. Hg, so with each burn, it appears that I am gaining a little vacuum.

Ban

Need a new Dunham Air check

For an Air Eliminator 220. I have been working on my Dunham Air Eliminator and when I took off the "air check" I noticed that it doesn't hold a vacuum. My question is, can I use a piece of metal to repair the air check? Can I add a new style one-way valve to operate the way Dunham advised or is there a way to purchase a NOS air-check somewhere?



Thanks!



Photos below are of me cleaning the Air Eliminator with Hydrogen Chloride.

Dave in QCA

222 air check

I have been told by a reliable source, but have not verified with MEPCO, that there is a new version of the #222 air check.  If you unthread the old one the new one should thread right in.  I understant that it may have some nylon and/or other synthetic materials in it.



Also, for cleaning the metal parts, I would suggest that you used a lime scale remover such as LSR or Limeaway.  These are citric acid based and do a great job removing lime scale and at the same time are MUCH LESS aggressive with the metals.

Ban

Please Delete

Ban

Hydrocloric Acid

It is interesting you mention Citric acid. I agree with you and have used it on my potable hot water galvanized plumbing system. Regarding the steam system, I found a few years back that if I soak the thermostatic traps in Hydrocloric acid the copper acordeon diaphragm retracts almost back to its original position. I believe this to be due to the mineral deposit build-up between the fins dissolves and causes the diaphragm to act normally again. Years of build-up and no maintenance takes its toll and some 80 years later they barely work. It's amazing to see such an antique device rejuvenated just from cleaning. I will look into the new air-check. In the meantime do you think I could use a small piece of copper or aluminum?

jumper

thread makes me happy

I'm happy to see continuing interest in vapor heating. There's an article on this website about steam heat improvements in Stuyvesant Village. I've been teasing my memory to remember details a dead man explained years ago about the design of systems he installed in Toronto fifty years ago.

Ban

Fatal Error

It said there was a fatal error when I entered this post but it did go through. Please delete. Also, Is there a way to delete posts yourself?

Ban

Dunham air check disk; Please delete

It said there was a fatal error when I entered this post but it did go through. Please delete. Also, Is there a way to delete posts yourself?

Ban

Dunham air check disk

Here is the "air check" diagram. If you look closely you can see the disk. Any Ideas on replacing this disk? Does anyone know what the original disk looked like or could possibly take a photo of one of theirs? I looked at the MEPCO website and only found vacuum breakers. Here is the link: http://www.mepcollc.com/steamspec/vbreakers.htm

gerry gill

This is a very fascinating thread,

my head is spinning with its possibilities. I see more experiments taking place!

PMJ

Possibilities

I would think as a contractor it would be tough to implement vacuum in customers homes - especially older ones where many leaks have to be found and old valves replaced. I would think it would be tough to talk people into giving you the time required.

But as an owner able to continuously monitor and fiddle with the system I can tell you that  vacuum has transformed how I think about my heating system. The heat is dramatically more even and I am convinced 10-20% more efficient than without it. I have been in my house 20 years now.

I have no measureable pressure anywhere in the system at any time when the burner is on and vacuum when it is off. I have no vent devices at all in the entire system except a 3/4" solenoid valve on the main dry return.

Dave in QCA

Disc for Dunham type 222 air check

I would estimate the disc to be roughly 14-12 gauge metal.  It is constructed of either copper or brass.  It must be perfectly flat and smooth.  The diameter should be as large as possible and still be able to fit into the screw off cap.



Do I understand your post that the disc is missing from your air eliminiator? 



In my system the air eliminator is missing completely. I have found a Trane float vent that has a good sized vent port and the same type of check on top as the Dunham Air Eliminiator.  However, in my Trane vents, the disc is missing as well and I am working on finding a way to replace it.   I will let you know how that turns out. 



Regarding the new 222 air check from Mepco, you are correct, it is not shown on their website.  The website is woefully inadequate.  But, a reliable source at RC Equipment in Marshalltown, IA, who used to work at Mepco for many years tells me that it is currently available.

ChrisJ

1 pipe

Anyone sucessfully running a vacuum 1 pipe system?



I feel left out of the party here.

PMJ

One Pipe Vacuum

Just guessing but I would think each rad would need its own check valve. Obviously that makes it a considerably tougher project. Also, you want check valves that seal well but also have a very low cracking pressure. Not sure how easy that is to find.

nz

Almost to 9 in. Hg vacuum now

In case any of you were wondering, i'm making steam around 195F now

Dave in QCA

One pipe vacuum

There were essentially two types of one pipe vacuum. 



One was the self induced type with vacuum checks on the vents, both the main vents and the radiator vents.  There is no longer anybody making a vacuum radiator vent, so that option is out.



The second type of system was the Paul system.  On this, instead of normal radiator vents, they was an air service trap installed instead.  Paul type vents are still made by Hoffman Specialty.  The outlet of the Paul vent is connected to a 1/4 pipe and all of these run back to the boiler room.  Originally, the Paul System air lines were connected to a vacuum pump and the system was evacuated of air and allowed to boil at lower temperatures than atmospheric conditions. 



I think the Paul information is in the Library, but I just looked and couldn't find it.   I have an old Dunham book with their "Air Line" system described quite well.  I could scan it an post it here as a pdf, if that would help.



Also, I would suggest that the subject of 1-pipe vacuum be started in its own thread.  It deserves to be discussed and I think it will be better if the two-pipe and one-pipe discussions are not mixed together just because of the confusion that it may create.  Vacuum is confusing enough by it self I think. 

Dave in QCA

EXCELLENT!!!

I am hoping I have the same results!!!!



Also, regarding your earlier comment/question regarding the btu/edr decreasing because of the lower temperature, you are absolutly right.  That is what they were striving for, especially in pumped systems.  The goal was to be able to produce a softer modulated heat in mild weather in the same manner that a how water system does. 



According to my tables, you're at about 195 BTU/EDR, if you were actually steaming at that vacuum, which I'd guess you're not.   However, I bet your boiling begins very quickly when the boiler starts and I bet that steam hits the rads VERY quickly!

jumper

one pipe vacuum

It depends how well terminals and connexions drain. Ideally you evacuate the air and don't let anymore in. In one pipe everything is connected so why do you have to evacuate at the radiator ? I think Lost Art mentions buildings of yore that used steam ejectors to induce vacuum. The book also describes a way to use two connections on a fin tube with a one pipe system. There are other ways as well to make a terminal parallel flow with one pipe. See the Wilson patent on this site. The key is adequate drainage. Even in a two pipe system inadequate drainage causes bang and sizzle.

vaporvac

One-pipe vacuum:Dave Bunnell?

I believe Dave Bunnell had a thread or article on this subject. I didn't find it under resources and he has 32+ pages of posts, so I'll leave it to someone with it at hand. Since I have 2-pipe I didn't pay too much  attention to it, as Dave so rightly states one's vacuum is confusing enough as it is!