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Posted by Abby Normal on September 30, 2006, 9:12 am
New Directions In Building Services (Australia) wrote:
> > Guido wrote:
> >> Hi.
> >>
> >> Im commissioing a new installation on a project in wherby the
> >> repository / bookstores have to be kept at 16 DegC and 55% RH.
> >>
> >> Now then, the rooms are sealed and have a door mounted pressure relief.
> >> duct.
> >>
> >> We are supplying the rooms with 95% recirculatedair 5% is conditiond
> >> fresh air makeup. We are allowing for 5% losses through the relief duct
> >> in an effort to keep the space positvley pressurised.
> >>
> >> The fresh air make up runs through an ahu with the following (essential
> >> only) components, Frost battery, Cooling Battery, Heater battery, steam
> >> humidfier.
> >>
> >> External air comes in at this time of the year at around 22 DegC 78 %
> >> RH.
> >>
> >> This fresh air (5%) is supplied onto the recirc air handling units
> >> serving the repository.
> >>
> >> The repository units contain 4 coils and a humdifiers.
> >>
> >> cooling, heating, for sensible load and then cooling / heating for
> >> latent load.
> >>
> >> The units have been running for a month but i cant get the %RH level in
> >> the spaces below 60% it should be 55%.
> >>
> >> The return air coming back is at 16 DegC (therfore is at setpoint) and
> >> the humidity is at say 67%.
> >>
> >> We are controlling the coils on return air and therfore as you can see
> >> there is a demand for dehumidification.
> >>
> >> The secoond cooling coil actuator (latent load) has wound fully open in
> >> an effort to do this, this drops the temperature off the coil to 7
> >> DegC. Therfore the reheat coil comes into heat it back up to the
> >> required calculated setpoint this fluctuates depending on the extract
> >> air deviation from the room SP setpoint which has settled out at 15.7
> >> DegC
> >>
> >> Anyway the % RH off coil is around 95% and following the reheat it only
> >> drops to 70% it appers as though the air isnt any dryer.
> >>
> >> Ive worked it out pyschrometricaly and although the RH hasn't change
> >> the moisture conent has. But the consulatnt isnt intrested in moisture
> >> content only RH I have no idea where to go from here. it should be
> >> noted that the sensors we are using are specialist and have been pre
> >> calibrated. I have checked there positions and redings whilst point to
> >> point commissioning. Also the coils in the ahu have correct pipework
> >> portings and the coils are positioned in the correct way. Finally the
> >> steam humdifier remains isolated.
> >>
> >> Any suggestions will be appreciated.
> >
> >
> > Okay you are chilled water not DX. You need the water colder than the
> > dewpoint you want to maintain the space at. Right now it is roughly
> > equal.
> >
> > Paul already mentioned a desicannt, that is good advice.
> >
> > No real idea of how much air you have moving but why even put a relief
> > opening in? Is the place tight enough for FM200 gas and you got a smoke
> > damper on that relief opening? Otherwise construction be plenty loose
> > to let that air leak out.
> >
> Problem appears to be your relief air vent. 5% is a lot of air to discharge
> and vapour can readily flow against this airflow (check vapour pressures
> rather than air pressures).
> Try sealing it up to check. Also ensure all porous materials including
> masonry walls floors and ceilings are painted with an acrylic sealer as
> vapour can easily migrate through.
>
> Should also consider reducing the supply air fan speed, which will allow the
> air over the cooling coil to stay in contact with the coil longer (should be
> at least 4 row coils with fin spacing less than 8fpi for wet coil) other
> than that the desicant wheel or reduced chilled water temperature (or even
> better would be ethol glycol at 4degC) is the option.
>
> As for control - consider humidistat thermostat zone sensor combination with
> primary control on humidity and temperature sensor used to pull in
> sufficient reheat if zone too cold, etc.
>
> If the installation is installed as it was designed, then the problem needs
> to be referenced back to the Design consulting engineer for them to
> remediate. All too often installers cover up for designers by band-aiding
> and the designers getting the false impression that they've done a good
> design where they include this as a strategy in their future designs - they
> are accountable and especially in todays code regulated environment where
> their design has to achieve the required energy consumption performance
> constraints.
I was wondering about the relief vent. Concerning slowing the blower
down, I never really think of it from as contact and time making
contact, I think of it as reducing the percentage of air flow which
does not make contact.
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