Steam Design Question

Steve_35

We do very little steam so I'm digging back into "The Lost Art" again before I quote this job. The existing boiler is a coal converted to gas steamer. It's got an automatic damper on it that's driven by gas pressure. Natural gas pressure that is. There's a note on it from a previous owner dated in the early 50s saying the gas company suggested he replace it but since the sheet metal shop quoted him $10 in parts and $3-$5 in labor he felt he'd just keep the old damper till it failed since the worst case would be gas going up the flue if the diaphragm failed.

In any case, I'm trying to size this boiler. We have 232 sq ft EDR connected. Dan mentions in one spot multiplying the connected load by 50% for pickup losses but in the single pipe section there's a mention of 33% if the piping is uninsulated and 20% if insulated.

What should I allow for pickup losses on a single pipe system? I think this is a counterflow, but I'm going back to double check.

One other thing. If it is a counter flow system, it seems to me I want to limit the maximum velocity to allow the condensate to return. That shouldn't be a problem in the old piping but what maximum velocity should I design to? 15fps or use the chart on page 43 with 23fps for 2", 26fps for 2-1/2", etc?

Anything else I should be on the watch for?

Thanks.

Steve_35

EBEBRATT-Ed

if parallel flow 2 ' is good for 386 sq ft, 2 1/2 is good for 635 so a 2" main will do the job. If counter flow you may want to go 2 1/2 ". What size are the mains now? Most counterflow comes off the boiler with two takeoffs. If you have 2 2" on counterflow you should be ok if piped correctly.


ed

Steve_35

Steam Velocity

The boiler has a single 3" outlet. The manufacturer's instructions says, for this model, to reduce it to 2". My concern is the steam velocity should be a tick over 29fps with 2" pipe. If I'm reading "The Lost Art" right the max velocity in 2" pipe that condensate can flow against is 23fps.

At 2-1/2" pipe the velocity should drop to 20-1/2fps and, again if I'm understanding thsi correctly, the max velocity for counter flow condensate is 26fps so that should be okay.

What about the pickup losses? 50%, 33% or 20%?

Steamhead (in transit)

There's nothing wrong

with increasing the pipe sizes in the boiler's header. We do it all the time. It slows the steam velocity and dries it out better.

With 232 square feet EDR, a 2-inch parallel-flow steam main connected to that larger header should be fine. Counterflow might be a bit iffy though.

Make sure the main is well insulated. And if this is the only main in the building, vent it well since all the boiler's output goes into it.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Steve_35

Well, the connected load is 232 EDR

I'm also trying to figger out just how much to add for pickup loss. Dan mention 50% early in his book, but in the single pipe section he mentions 33% for uninsulated pipes and 20% for insulated. What would you use for pickup loss on a counter flow, single pipe system, Frank?

At 50% the EDR is 348 and the boiler DOE is 98,000btuh.

We've already talked about venting the main and changing the rad vents.

Thanks for the input.

Steamhead (in transit)

If everything's insulated

33% is fine. That's already figured into the boiler's "Net" or "IBR" rating, so no more calculations are needed.

Which boiler are you going to use here?

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

Steve_35

Peerless 63 Series

is about the only thing in our area now for steam.

Thanks for the info, Frank.

clammy

velocity

I have found that most steam boiler espically older systems work the best when you can get your velocity down to about 15 fps ,how do i know this besides dans book (which are the bible )Every job i have done and has worked outstanding my header was sized to bring the exiting velocity down to 15 fps and theses systems work excellent .I believe the lost art has the formula for figuring out steam exiting volicity .As for peerless steam boiler since they have increased there tapping sizes i have used them and found them to be excellent steam boilers i would though use both taps and over size your headed to one size large then your riser it will truely slow the exitind velocity down quite a bit and a drop header will also help dry it out also .If this is a counter flow system then definetly go with the oversized header it will make that system shine counter flow really need dry steam other wise you will end up with promblems also try using a vaporstat they really work well on counterfl;o0w systems peace and good luck clammy

Patrick Linhardt, \"Linhardt's Field Guide to Ste

minimum pipe sizes

Hi pitman9,

The recommended pipe sizes in the manufacturer's instructions are the minimum size required, not the only size to use. Using a larger than minimum size helps slow the steam velocity, which helps dry the steam before it enters the supply main.

The oldtimers were quite concerned with dry steam. The manufacturers used large steam chests (called steam domes on a round boiler) and lots of large supply outlet tappings to keep the steam velocity down. Our modern boiler manufacturers now rely on the near boiler piping to help dry out the steam. In their defense, they keep the boiler steam chest size smaller for better efficiency.

The result is installers have to be more careful of near boiler pipe sizing and orientations. Pay attention to the required 24" minimum distance between the waterline and the header. This is one of the techniques of the manufacturers to dry the steam.

As steamhead and clammy confirm in this post, using larger than minimum pipe sizes slows the steam velocity, which assures dry steam, which makes for a better job, especially if it is counterflow. Use the same pipe size as the steam outlet if possible. Use all supply tappings at full size if economically possible. Some extra money spent on the installation details of near boiler piping initially will make for a better job for the customer and for you.

On the issue of sizing, the calculated radiation load has to be equaled by the boiler output. The gross outlet is the estimated loss from combustion. The net output on steam boilers takes into consideration the normal piping and pick-up losses in a typical system. The percentage varies by boiler size and may vary by boiler manufacturer. If you use the net output, you normally don't have to calculate any more loss.

However, if you have uninsulated steam supply piping, that should be calculated and added to your radiation load. If you have unusual piping situations, you may want to add an appropriate amount to the radiation load.

Good luck with the job. I will be at the NAOHSM convention in May to teach two classes on steam heating system troubleshooting. Hope to see some wallies there.

Best regards, Pat