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hi-velocity question

Has anyone had any experience with high velocity ducting using a chilled water coil for cooling? I have an installation where the cooling capacities at the blower unit seem to be about half of what the manufacturer documents.



Here are some numbers just taken on a 4 ton Geocomfort water to water geothermal heat pump tied into a HE-100 air handler and WM-100 water coil (Hi-Velocity models) via HSS-40 buffer tank.

Temperatures were taken with electronic clamp-on thermometer and electronic psychrometer. Flow measurements were taken with WFT-17 flow meter tool by B&D manufacturing.

Outside air temperature was 73°F. Interior temperature at the thermostat was 74°F. System was running and measurements below were taken. Please confirm any mathematics that seem unreasonable.



Ground loop (20% methanol solution)

Source in ---- 61.9° F

Source out---71.4° F

Flow-----------12.0 gpm





71.4-61.9=9.5° x 12 gpm x 500 = 57kbtu





Load side of heat pump (100% water)

Load in ---41.7°F

Load out—35.4°F

Flow-------13.5 gpm

Buffer tank setup-----38°F to 45°F water temp. range (measurements taken at 44.5°F)





41.7-35.4=6.3°F x 13.5 gpm x 500 = 42.5kbtu







WM-100 air coil

Water in --- 44.5°F

Water out –48.6°F

Flow --------10 gpm



48.6-44.5=4.1°F x 10 gpm x 500 = 20.5kbtu





Supply air --- 53.4°F dry bulb, 47.1°F wet bulb

Return air --- 68.2°F dry bulb, 58.3°F wet bulb

Flow --------- 1030 cfm (dip switches set according HI-V design manual)



68.2-53.4=14.8°F x 1030 cfm x 1.085 = 16.5kbtu





HE-100 blower unit

HE outlets --- 17

2” outlets --- 10

10” 26 gauge trunkline arranged in racetrack style (84’ of total trunkline with 4 90° ells)

16”x 20” insulated ductboard return ducting (6x14, 8x24, and 16x20 grilles)







This building can maintain 75°F to 73°F temperatures during normal day to day activities. However, on near-design/design days or during increased occupancy, the inside temperature can be held at best to temperatures from 79°F to 80°F.





The manufacturer documents that the total capacity of the coil/air handler at 44°F water should be 49,500 btuh.



Please comment/question any or all of the above. Thank you.

Comments

  • Techman
    Techman Member Posts: 2,144
    Thats a lot of info

    ,what were your refrigerant pressurers/superheat/subcooling/metering device/and refrigerant used?
  • alwayslearning
    alwayslearning Member Posts: 15
    follow up

    Frankly speaking, my concern does not originate from within the refrigerant system. 

    The geo system is cooling water in the buffer tank water to the set point temperature effectively.  The problem (to me) seems to be delivering sufficient "cooling" at the fan coil from the tank.

    Both on the water and air side of the fan coil, the math shows us that we are not even close to the specs on the coil.  I am not sure that the following is the correct answer to ask, but:  Does capacity drop off significantly once the dry bulb is less than 72° and wet bulb less than 60°???
  • zacmobile
    zacmobile Member Posts: 211
    hi-v cooling

    how is the coil piped? water flow should be counterflow to the air. I believe you lose up to 60% cooling capacity if its piped the other way, the coil cannot absorb the heat in the airflow to stated capacity.  I've done quite a few geo/chilled water/hi-v systems and found out about this the hard way. it sounds likely that's whats happening as that's some mighty cold water you're making there.
  • Techman
    Techman Member Posts: 2,144
    edited July 2010
    heat/cool btu's

    I haven't done that math yet, but the refrigerant is the heat transfer device.For whatever cause,if the refrigerant pressure is lower than  what "should be normal "there is a loss of capicity and efficiency. Some times to a major degree O.K..I'll do the math!
  • Techman
    Techman Member Posts: 2,144
    Not done w/ the math.

    or other thinking.Why is the retunr air temp [68.28*f]so cold compared to the air temp @the tstat [74*f] ?
  • Techman
    Techman Member Posts: 2,144
    edited August 2010
    Q here?

           At the design  conditions of 72*db & 60*wb, that is pretty close to a "stabalized condition"    in the structure ,almost @ temp!To see the drop off in BTU's you need the "capacity curves chart" for the given mfrg and compressor . A Copelaweld 4t ac comp @ 45* evap temp is 54350 btu's ,@ 30* evap temp  it is 41050 btu's and @ 10* it is only 25000 btu's So capacity does drop, to a degree .
  • Tom Blackwell_2
    Tom Blackwell_2 Member Posts: 126
    Water temps

    Several comments. First, why is the air coil seeing 44 degree water when the heat pump is producing 36 degree water? Sounds like the buffer tank/secondary pump is flowing backwards. The HV system needs really cold air to transfer BTU's, since the airflow per room is so small. Second, the r/a sounds very cold, perhaps there is a stratification issue? I believe the main problem is the warmer entering chilled water to the coil. The coil should receive the coldest water; ie the leaving water from the heat pump.
  • Techman
    Techman Member Posts: 2,144
    Hi-V

    Right on Tom, some of my thoughts exactly!He did say water at 44.5*f, I agree too warm. I'm still waiting on an answer about that very cold return air temp! Let's hold our breath!And I still am courious about the refrigerant readings!Even IF the system was running fine it could run even finer,but , I'm a refrigeration guy at heart,who does other things.HVAC-ICE
  • bob_46
    bob_46 Member Posts: 813
    alwaysleaning

    The formula that you used to calculate the performance of the cooling coil is the formula for

    sensible heat. cfmx1.08x∆T the formula for total heat, sensible plus latent is cfmx4.5x∆h

    (change in enthalpy). If you plot the data you give on a psych chart you will see that each lb. of air across the coil picks up 6.6 btu. 3.6 btu's sensible and 3.0 btu's latent. When you plug those numbers into the formula you get 30,591 btu's. The result doesn't jibe with the 20.5kbtu the water is picking up. I suspect the 1030 cfm number is wrong. When a mfg.

    says a dip switch setting delivers a certain cfm that is at some specific total static pressure drop through the system.
    bob
  • Techman
    Techman Member Posts: 2,144
    bob

    does that formula hold true for hi-v systems? Being that the R.H. and room temp is slighty different vs. standard AC systems,or is this" close enough " ?
  • bob_46
    bob_46 Member Posts: 813
    Formula

    Yes
    bob
  • alwayslearning
    alwayslearning Member Posts: 15
    representative on sight

    We got a representative from High-Velocity from Edmonton on sight a few days ago.  He performed a number of performance tests on the entire system.  To make a long story short:  they are sending us a new chilled water coil.  It is simply unable to "deliver" the specified btus.  The new coil is significantly larger in area.  I hope this change is sufficient.

    Secondly, after reading some technical data from competing high pressure ducting company (Unico), there are correction factors that must be taken into consideration when one designs for target temperatures less than 80° db and 67° wb.  High-Velocity does not provide this data.  There is a capacity reduction factor that must be considered; or maybe I missed something along the way in the educational process.  I have attached the tech page so others are aware. 

    Thank you for your input and patience
This discussion has been closed.

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