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Engineering Help - DF

Brad White_171
Brad White_171 Member Posts: 22
Dan, the first thing I see is using 2-pipe in a humid climate.

The only people that love 2-pipe systems are owners at budget time. Even Jesus hates 2-pipe I am told. :)


There are significant days when it is cool enough to require heat in the AM and later in the day it is time to cool/dehumidify but not enough time for the system to respond. This affects both the fan coil units (FCU's) but especially the Air Handling Unit (AHU).

I would recommend creating a 4-pipe system -at the very least for the AHU. That I see as the largest contributor but also the largest potential agent to help you on the humidity side.

If you extend the piping and replace FCU's or add HW coils, at least the HW portion you would install is the smaller of the two, CHW sizing being inherent in the system now.

Keep in mind that the OA system is likely the largest agent of dehumidification. FCU's I consider mostly sensible coolers. If the AHU is not in a ready-to-cool mode, humidity control is an almost immediate casualty. Depending on the psychrometrics, you may have to cool that air to 52 degrees or more to give you some margin to absorb space gains.

(Mind you I know nothing about this building or function.)

Another detriment I have found is pipe corrosion, especially if the operators want to save a few dollars by raising the CHW temperature. Dehumidification is lost but the piping, even if insulated, is still way below the now increased dewpoint. Peel back some insulation and see. I am now replacing all iron pipe in a 1970-vintage school with similar circumstances. Even 1994-95 vintage replaced pipe is being replaces. Go to the cause.



What is the big picture if you could say? 100% OA, if for make-up air for small offices served by fan coil rooms is one thing. A wholesale repair shop with lots of exhaust is another. Heat recovery, specifically energy recovery is the simple possibility if the return air is not too filthy.

I wish I was close-by. I would love to walk it through with you.

I know Fort Belvoir. "Through These Doors Pass the Best Damn Engineers in the World."

OK, they shopped this one out. :) I did not say that.

Comments

  • Dan Foley
    Dan Foley Member Posts: 1,258
    Engineering

    Attached is a sketch of a building I looked at yesterday at Ft. Belvoir, an Army Post just south of Alexandria. The solid masonry building was built in the 1930's and my best guess is that it was originally heated with convectors supplied by district steam.

    The building was remodeled in 2002 and is now heated and cooled by a 2-pipe system feeding perimeter wall mounted console fancoils and a central station air handler with ductwork down the main corridor. A gas boiler and electric chiller power the system .

    100% of the return air is exhausted outside and 100% of the supply/make-up air also comes from outside. This strikes me as extremely wasteful as well as creating a host of other problems.

    I was called in by a building science expert I work with to help solve some of these problems:

    - high fuel bills

    - temperature control problems

    - noise

    - sweating ducts, pipes and walls

    - Mold everywhere

    I have some ideas on ways to improve this system but I would like to see if there are any other ideas. TIA. - DF


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  • Dave Yates (GrandPAH)_2
    Dave Yates (GrandPAH)_2 Member Posts: 377
    dang

    Except for the noise issue, the 100% fresh air condition will either be the primary cause or contributor to the other problems! Lots of humid air outdoors.

    A fresh-air heat-recovery unit coupled up with a system utilizing enthalpy to regulate FA would be a much better approach and the HR unit might qualify for the required 10% min FA.
  • Weezbo
    Weezbo Member Posts: 6,232
    seems that they could have some secondary cooling&heating

    at a glance.minimum...
  • The Wire Nut
    The Wire Nut Member Posts: 420
    Good News!

    At least the intake and the exhaust are in the same area... that way, you can play with bypasses, ERVs, etc.

    If 100% exchange is required, I'd consider something like the Stirling @ UltimateAir.com With an efficiency of 97% you have a lot more options re: humidity control. However, I would first determine how much fresh air they really need and then tailor the air exchange accordingly.

    I would also ping Geoff M. and see what he thinks.... this is his bread and butter.

    Lastly, I wonder to what extent the environmental conditions inside the building are comparable from fan-coil to fan coil. If certain areas require heating, others cooling at the same time, a two-pipe system as shown cannot work. Even a four-pipe system might require localized mixing to get the temperature right. It all depends on the envelope, insolation, etc.
    "Let me control you"

    Lost in SOHO NYC and Balmy Whites Valley PA
  • Edward A. (Ed) Carey_3
    Edward A. (Ed) Carey_3 Member Posts: 236
    WHY 100% OA

    Hi Dan,

    Why do they have 100 make up OA. Was that 100% OA something that was needed in the past and was never changed in later years, or is there some reason it is still actually needed now.


    Ed Carey
  • hvacfreak
    hvacfreak Member Posts: 439
    de-humid and economize ( as said in the prev. posts )

    My first thought is to change the ahu piping / coil configuration to a chilled only coil and a hw coil in the re-heat position ( valves and controls to complete a de-humid sequence ).

    Economize that outside air / exhaust " bastardization ". Also thinking vfd's on these fans if none are there now.

    VAV boxes or at least control dampers in the space areas might be a good idea.

    Sorry I'm not an engineer , I'm in the area.( I think we worked on the same job a couple of years ago ). Good luck - Mike
  • Dan Foley
    Dan Foley Member Posts: 1,258
    OA

    Ed,

    The 100% OA design was added when the building was remodeled/renovated in 2002. It is now being used as an office building. I consider it (100% OA) wasteful, excessive and unnecessary. -DF

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  • Dan Foley
    Dan Foley Member Posts: 1,258
    Ideas

    Mike,

    I like your thinking. It would be fairly easy to convert the AHU coil to a four-pipe as the boiler room is fairly close.

    VAV boxes are out as it would require complete demo of a building only recently renovated. Ceiling height may also be an issue.

    Thanks for your input. Where are you located? Which job did we work on together? -DF

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  • Dan Foley
    Dan Foley Member Posts: 1,258
    Fresh Air

    Constantin,

    I was thinking along similar lines - re-direct the return air back to the AHU and control the fresh air requirements through an ERV such as the Renewaire or similar. I am not familiarwith UltimateAir products but I will check them out. - DF

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  • Dan Foley
    Dan Foley Member Posts: 1,258
    Brad White

    Hello All,

    Thank you for your interest and ideas on this project that has me stumped. I especially want to thank Brad White who has emailed his thoughts and ideas offline. In return I sent him photos and documents which were in files too large to post on the Wall. I will assimilate Brad's coments and post them here so that others can follow and post their ideas. Thank you for your help. -DF

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  • Dan Foley
    Dan Foley Member Posts: 1,258
    Comments

    Here are some of the comments and thoughts Brad White emailed to me regarding this project:

    "Their recommendations went to the symptoms not the cause. I did not see
    a mention of reducing the OA temperature. Any idea of the CFM?
    Population?

    I see replacing the unit coil with a good 8-10 row coil. May and
    probably will require more chiller OR reduce the OA to a net minimum so
    that the chiller can handle it. Backwards engineering, I know.

    My gut says that the unit is the cause the way it is set up now.

    Later!

    Brad"

    And:

    "As you saw from my prior note, the AHU is key. Everything else they are recommending dances around the problem without correcting it at least directly.

    Now, you as the contractor, are you working in response to the engineer's report (to implement the measures)? Or are you to come up with your own recommendations?

    Granted some of what I am suggesting is critical of the report. Not meant to say it is a bad report, but it misses the mark in my opinion. Does this expose you to some conflict? Curious.

    The UV lamps have to run nearly year round as they maintain a level of bio-contaminants. Once they get a toe-hold and form a bio-film, all bets are off. Point being, just running them in the cooling mode will not give you the benefits over time.

    If 20 CFM per person, you could handle at 7,000 CFM, 350 people. 80 SF per person seems logical with some conference rooms thrown in for higher density here and there.

    Not knowing the population, the report does say the AHU supplies about 8200 CFM. This is almost 500 FPM which is pushing it for 100% OA. Carryover and all that. If you could get the velocity down to 400 FPM (7000 CFM more or less) it would take about 42 -45 tons. How big is the chiller?

    For a 28,000 SF building, if an office type, I would expect to see 80-100 tons depending on the usual variables. So half of that is ventilation air. If replacing the chiller is not in the cards, can a "re-cool" DX coil be installed in the ductwork? Used as needed for the dog days.

    Anyway, see how far that takes you.

    Let me know your thoughts. We can also post on the Wall if this helps others. Maybe in Prof. Silberstein's room. Either way, I leave that up to you.


    Have a great day!

    Brad"


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  • Edward A. (Ed) Carey_3
    Edward A. (Ed) Carey_3 Member Posts: 236
    MOLD

    Dan,

    I heard an attorney once say that MOLD is their Asbestos of the 21 century.

    If you take on this project, the building owners are obviously expecting you to solve their ongoing HVAC problems, including the MOLD!!!!.

    That, in itself could be a very touchy issue. If there is MOLD in the building, then there is mold inside the duct system that you will be modifying to solve the problem.

    If you do not solve THEIR MOLD problem, YOU could inherit THEIR MOLD problem.

    I suggest that you plan your course of action very carefully, and be VERY sure that all mold in the building is removed before you start modifying that duct system.

    If it is not, the work that you do to correct the duct problems could release much more MOLD into the building, and you could be held accountable for exacerbating THEIR problem.

    Just a thought,

    Ed Carey
  • hvacfreak
    hvacfreak Member Posts: 439
    Medimmune ( Gaithersburg )

    I think you did the radiant heat portion on 3 floors in the front common areas. I was working for the mechanical contractor doing start-up and commissioning of other equipment. If I remember correctly , the commercial framers / drywallers shot some Hilti nails in the pex , lol. Your stuff looked and worked great.

    As for the vav...I was thinking of smaller " Envirotech " ( providing the ceiling is typical suspended ). If not , perhaps the areas where the diffusers penetrate the drywall could be worked to allow motorized dampers controlled by space thermostats. Anything to create a pressure increase ( main duct ) to allow ahu ( and relief / exhaust ) fans to ramp down ( via vfd's ). There my be a way to do this say on space temperature alone , but I have not seen this done.

    Economizer would be motorized dampers on that " exhaust " duct ( requires a bypass duct to the return of the ahu ). Typically based on space pressure , but could also be controlled by say a CO2 sensor of some sort. Outside air inlet ( linked to return damper ) controlled based on outside enthalpy with a min position.

    Thats my 2 cents on the air side...improved IAQ , operating costs down ( vfd payback could be very fast on this job depending on the motor sizes ). I wonder if it would be better to run the chiller at a lower temperature for the ahu , and send warmer water to the fcu's if you decide to go this way ( Brad ). Peace All - Mike
  • Kal Row
    Kal Row Member Posts: 1,520
    two cents: erv's with ultraviolet lights in them....

    and suplimental discharge dehumidifires on their indoor discharges, you are going to have to break some financial "eggs" to make this omlet for sure!!
  • mtfallsmikey
    mtfallsmikey Member Posts: 765
    Doing this for the Army, Dan?

    There will be LOTS of new construction there in the coming years, thanks to BRAC. That stuff there is OLD!
  • Brent_2
    Brent_2 Member Posts: 81


    > Here are some of the comments and thoughts Brad

    > White emailed to me regarding this

    > project:

    >

    > "Their recommendations went to the

    > symptoms not the cause. I did not see a mention

    > of reducing the OA temperature. Any idea of the

    > CFM? Population?

    >

    > I see replacing the unit

    > coil with a good 8-10 row coil. May and probably

    > will require more chiller OR reduce the OA to a

    > net minimum so that the chiller can handle it.

    > Backwards engineering, I know.

    >

    > My gut says

    > that the unit is the cause the way it is set up

    > now.

    >

    > Later!

    >

    > Brad"

    >

    > And:

    >

    > "As you saw

    > from my prior note, the AHU is key. Everything

    > else they are recommending dances around the

    > problem without correcting it at least directly.

    > Now, you as the contractor, are you working in

    > response to the engineer's report (to implement

    > the measures)? Or are you to come up with your

    > own recommendations?

    >

    > Granted some of what I

    > am suggesting is critical of the report. Not

    > meant to say it is a bad report, but it misses

    > the mark in my opinion. Does this expose you to

    > some conflict? Curious.

    >

    > The UV lamps have to

    > run nearly year round as they maintain a level of

    > bio-contaminants. Once they get a toe-hold and

    > form a bio-film, all bets are off. Point being,

    > just running them in the cooling mode will not

    > give you the benefits over time.

    >

    > If 20 CFM per

    > person, you could handle at 7,000 CFM, 350

    > people. 80 SF per person seems logical with some

    > conference rooms thrown in for higher density

    > here and there.

    >

    > Not knowing the population,

    > the report does say the AHU supplies about 8200

    > CFM. This is almost 500 FPM which is pushing it

    > for 100% OA. Carryover and all that. If you could

    > get the velocity down to 400 FPM (7000 CFM more

    > or less) it would take about 42 -45 tons. How big

    > is the chiller?

    >

    > For a 28,000 SF building, if

    > an office type, I would expect to see 80-100 tons

    > depending on the usual variables. So half of that

    > is ventilation air. If replacing the chiller is

    > not in the cards, can a "re-cool" DX coil be

    > installed in the ductwork? Used as needed for the

    > dog days.

    >

    > Anyway, see how far that takes you.

    > Let me know your thoughts. We can also post on

    > the Wall if this helps others. Maybe in Prof.

    > Silberstein's room. Either way, I leave that up

    > to you.

    >

    > Have a great day!

    >

    > Brad"

    >

    > _A

    > HREF="http://www.heatinghelp.com/getListed.cfm?id=

    > 262&Step=30"_To Learn More About This

    > Professional, Click Here to Visit Their Ad in

    > "Find A Professional"_/A_



    The purpose of the 100% OA unit is to provide the amount of OA that is required by code. The individual fan coils provide the space heating and cooling. Now for this system to work correctly the OA unit needs to supply room neutral air to the space. whic is around 70°. With a 2 pipe system I don't know how this is supposed to be accomplished. You should be cooling the air to 55° or lower and then heating the air to 70°.
    This means you will need to run the boiler all year.
    The first thing I would do is determine how much OA is required and balance the air handler to that number. Hopefully it is lower than what is currently being supplied.
  • Brent_2
    Brent_2 Member Posts: 81




    The purpose of the 100% OA unit is to provide the amount of OA that is required by code. The individual fan coils provide the space heating and cooling. Now for this system to work correctly the OA unit needs to supply room neutral air to the space. whic is around 70°. With a 2 pipe system I don't know how this is supposed to be accomplished. You should be cooling the air to 55° or lower and then heating the air to 70°.
    This means you will need to run the boiler all year.
    The first thing I would do is determine how much OA is required and balance the air handler to that number. Hopefully it is lower than what is currently being supplied.
  • Brad White_9
    Brad White_9 Member Posts: 2,440
    You are onto it, Brent.

    I will let Mr. Foley expand on his end, but that is pretty much my conclusion, the inability of the AHU to dehumidify which positively adds humidity to the space. The design called for "neutral" air to be supplied but without wringing it out, there is no hope. I agree with you, the AHU should be "cooling ready" any time it needs it. That means 4-pipe at least on the AHU.

    I have a copy of the report and the humidity added is about 75 lbs. per hour.

    Reheat is an OK thing but the pre-cooled air can contribute significant cooling to the space while serving as an agent of humidification.
This discussion has been closed.