De dumidfication problems.

[Guido]

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.

[Abby Normal]

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.

[New Directions In Building Ser]

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.

[Abby Normal]

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.

[Abby Normal]

Yes he will have to lower the chilled water temperature and get the coil colder than the dewpoint he is trying to maintain else it is hopeless.

[New Directions In Building Ser]

Agreed at his 16degC/55%RH zone conditions - the cooling coil needs to be less than 6degC to provide any effective dehum. Assuming 2degC temperature droop from the chiller (4degC ST) to the coil and the ethol glycol system turns from a maybe to a must have....

[~^Johnny^~]

Just for that, I am requesting a UDP.

[~^Johnny^~]

It's an ICMP flood.

The reason NOLA flooded was due to a bad Tom DeLay relay, and reneging form air.

This is representative of a maladjusted governer.