Condensate

Bob Lojac

Hi Brad,

A couple of problems I have with a vertical condensate line being 10” in diameter. The condensate will be returning at about 35 gallons a minute. Having that volume with a possible free fall of 60 feet might make quite an impressive sound when striking the elbow at the bottom of that drop.

I don’t believe that a free fall of 60 feet will have too much to do with pressure drop. It has 60 feet of potential energy. I was told that it would reach terminal velocity after it falls about three stories. If it was just water a 1-1/2” diameter pipe should do the job. However its condensate and if the condensate slugs on its way down it may pull a vacuum on the condensate above it and flash it. That would make for interesting sounds.

The condensate return line that is running underground is pitched at ½” for every 10’ of horizontal run. That’s quite a pitch. A 6” line should be large enough to handle the condensate and vapor that accompanies it (at least according to ASHRAE).

Also I don’t believe that a vacuum system would pull the condensate down any faster in a vertical pipe with a free fall of 60 feet. If that liquid slugged on its way down the vertical drop it would quickly pressurize the line ahead of it and quickly reduce that vacuum to a positive pressure and over run the vacuum pumps. Its accelerating down at 32 feet per second squared. That’s cranking. That’s gravity.

Bob Lojac

Vertical Condensate Return Piping

I have a question relevant to vertical condensate pipe sizing and design.

I have a vertical condensate return line that drops 5 stories from a mechanical machine room in a 5th floor penthouse to a basement. The flow rate can reach 20,000 lbs/hr. This is a low pressure installation (12 psig operating pressure at the boilers, and no vacuum pumps on the existing system,plus I'm expanding off an existing steam system).

I have not found what I feel is an appropriate answer for sizing of this return riser. ASHRAE indicates a 4” line, Carrier a 10” line, Hoffman a 5” line, Sarco, Amstrong, etc., they all disagree. Plus there are no examples for this type of installation (anchoring, acoustics’, etc.).

I have not found any reference that directly addresses the sizing, design, or installation of a solo vertical condensate line for a 65 foot drop that elbows outdoor to a buried return line back to the ground floor MER (210 feet away with its slab 15 feet below grade).

I am currently leaning towards using a 4” diameter vertical pipe offset at each floor (every 14 feet) with two 45’s thus jogging it back and forth from floor to floor, schedule 80, with sound attenuation at the basement and connecting to a 6” horizontal return line (buried outdoors) pitched at 0.5”/10 of horizontal run.

So Dan, what do you recommend?

Very truly yours
Bob

Brad White_9

Depends on your assumed pressure drop, Bob

At least you have gravity working for you. I use the Carrier tables almost exclusively and they are based on tried and true ASHVE tables compiled prior to the Wilson Administration.

[Question: Are you sure of that load? I mean, how can the system take that much capacity locally off of an existing system yet have a 6" main return? (See below). I would double then triple check my load profile.]

With risers the flow rate is fairly fixed because it is gravity doing the work. It is not so dependent on pressure differentials. For this reason the assumption is a dry return, not wet. An 8" line will conduct 18,900 PPH and a 10" will conduct 29,800 PPH. If vacuum this will more than double in capacity.

I would use a 10 inch for the riser or break it up into smaller express risers.

Your 6" lateral main could handle (at 0.5 psig per 100'), only 12,800 PPH.

Brad White_38

Condensate free fall

Bob, Hi.

We do agree that pressure drop is not an issue. The tables for risers read across without changing in the pressure drop categories. I think the sizing has to do with the fact that the pipe will not be full and is not supposed to be. It runs down the sides leaving the center open to act as a vent, like any good drain.

The notion of condensate free-fall (or any liquid for that matter falling in a pipe) does not approach the velocities you may think. Reason is the fluid does not fall so much as cling by cohesion to the outside walls of the pipe.

Natural surface tension holds it together and the core remains open to act as a vent (much as you suggest it should). When high rises were first constructed, sanitary engineers thought much the same thing, the idea of flushing from the 62nd floor and woe to the superinendent whose office was squarely below. Their initial concerns were unfounded. One conundrum was just what you suggested, that pulling a vacuum would remove the one limit of air resistance that allowed a terminal velocity. Sure there is an impact loss at the base but the XH fittings were more for erosion than impact.

You are right in that, if indeed 20,000 PPH the flow rate will be 40 GPM by my calculation and a 10 inch pipe does seem awfully large...but the rational of flashing was and is a concern.

One way to find out though and my guess is you will use the ASHRAE table (6") and see what happens. 6-inch, for a gravity drain at that flow rate does make sense. I will have to crack open my alternate sources and re-think a few things now.

Bob Lojac

Condensate Free Fall

Hello Brad,

Since you mentioned drainage I decided to talk to the plumbers today. They gave me a copy of “Table P-803.2 SIZE OF VERTICAL CONDUCTORS AND LEADERS” from “THE BOCA NATIONAL PLUMBING CODE”. At 41 gpm the size of the leader or conductor is 2 ½”.

If I use a back-pressure regulator valve in the ground floor mechanical equipment room to flood the condensate return line to 50 feet in height would it be considered a wet condensate return. If so, would it be sized based on the Darcy-Weisbach equation thus providing a 3” IPS line capable of receiving 22,300 lbs/hr flow rate at 4 ft/100ft pressure drop? How about that for thinking out of the box, or is that just out of my mind? "ASHRAE 2001 Fundamentals Handbook - Table 20 Vented Wet Condensate Return for Gravity Flow Based on Darcy-Weisbach Equation"

If I use a dry closed return system, ASHRAE 2001 Fundamentals Handbook “Table 21 Flow Rate for Dry-Closed Returns” indicates at a Supply Pressure = 15 psig and a Return Pressure = 0 psig with a ¼” psi/100ft drop I can use a 4” retune line to handle up to 31,300 lbs/hr. That’s horizontal, no mention of vertical.

I went back to ASHRAE 2001 Fundamentals Handbook to check that 6” dry return (horizontal run only) and “Table 19 Vented Dry Condensate Return for Gravity Flow Based on Manning Equation” indicates that an IPS of 6” that is pitched at ½” per 10’ can handle 46,500 lbs/hr (Schedule 40 steel pipes). A 5” IPS can handle 28,500, but I’m using Schedule 80 so I went for the 6” line. Never heard any one complain of an oversized condensate return line (a horizontal one anyway).

I like the idea of placing a back-pressure regulator valve below the liquid level of the existing receiver in the ground floor MER. I can use a 4” line from the 5th floor penthouse MER down to 50 feet above the receiver water level and use a 3” flooded return from that point on. I can blame the thought process on “Value Engineering”.

Brad White_39

Thanks Bob

I am re-thinking years of stuff that "just was". Wonder why the Carrier tables are so conservative then?

One intersting factoid: Under gravity flow, a pipe that is half-full has the same velocity of flow as one that is full. Has to do with the friction and of sharing airflow being displaced. The velocity curve is a figure-8.

Keep in mind though that for condensate, Sch. 80 is recommended, not 40.

Good methodology- you have a good mind.

Brad

Christian Egli_2

Clouds of rain pouring on your rainbow

To follow Brad's train of thought - condensate lines are similar to sewer stacks - here is another analogy that will either give you reckless confidence or paralyzing fear.

Here goes...

Steamy clouds in the sky give off lots of rain during the storms. Right. That what umbrellas are for. This rain is technically nothing else but pure condensate, just not the miracle boiler kind but the kind that comes from higher above. Think act of God.

Back on earth - How do we deal with all this rain? We do have entire systems meant for removing and carrying away all this condensate. They're everywhere. They're very obvious. No one is ever afraid of them. No one ever thinks about them (except the crooks who steal copper and aluminum) Yet they can handle huge lb/hr load... Have you guessed?

The condensate removal from the fifth floor of your building is something the clunkiest and most lose fitted standard gutter and downspout piping could handle like the ordinary summer storm.

I'm not really suggesting you should go with aluminum downspouts (though I have no doubt they would work perfectly well) But I find it amusing how we wouldn't give any thought to a five story roof water removal system, while we'll beat our brains to a pulp on these condensate removal applications.

I hope this adds a little perspective to the numerous solutions you've already got (which I enjoyed thinking about) and, maybe I hope, extra confidence in your piping.

Call a roofer - ask them what they suggest for a "special condensate removal" application - confused stare? probably!

Brad White_9

Perfectly stated and erudite

as usual, Christian!

Philos Sophos

Love of Wisdom.

Thanks for enlightening me once again.

Brad

Christian Egli_2

Bright sun shine

No hope of rain either. I guess we'll have to wait for our boilers to act up; staring at the roof gutter sure isn't all that entertaining.

Thanks for reading and for setting the gears in motion.

Best regards