Fresh Air Handling Unit - Humidity Set point at 50% but air being supplied at 90%

[back2space]

Its turned very humid in the UK, our comfort cooling chilled water system at work (Lennox system) has had all the set points reduced to remove more humidity from the air whilst the building is cool the air feels damp and muggy.

The Fresh Air Handling system has the set point at 16C and humidity set at 50%. The air being supplied to the system is at 21C and the cooling coil is chilling the air down to 16.3C so this is working fine however the air being supplied back into the building is at 96% humidity so the building in most areas has humidity levels at 64% which is making it feel clammy and muggy.

The humidifier is off and in winter when the air is being reheated the air is always supplied wthin the set points of humidity and temperature. Obviously the cooling coil is working to bring the air to the right temperature but doesnt seem to be removing enough humidity. The reheat coil is not doing anything as the cooling coil is bringing the temperature down to the set point. This system has a pre heat coil, then a cooling coil then another heating coil then the humidifier then it is supplied to each floor.

Any help at understanding this would be greatfuly received.

Kind Regards
Richard.

[install monkey]

increase ur fresh air temperature setpoint to 21

[back2space]

Normally its set at 18C but problem is some areas get too hot then so they benefit from the cooler fresh air temp as it helps cool the building down.

[install monkey]

is the cooling via a dx coil or a chilled water coil?

[back2space]

Via a chilled water coil, the chilled water comes from the main chilled water circuit at 10.2C. Just changed the supply air set point to 21C the cooling coil closed and the dampers must have opened as it sstarted to recirculate more building air and the air on increased to 23C. The cooling coil then opened to 4% and then the air humidity went to 76% but doesnt seem to coming any lower than this.

[Brian_UK]

Follow a psychrometirc chart and you will see that you need to reduce the temperature to drop the moisture and then re-heat back up to the required supply air temperature.

[back2space]

Follow a psychrometirc chart and you will see that you need to reduce the temperature to drop the moisture and then re-heat back up to the required supply air temperature.

HI Brian

Yes understand this Brian but wondered why the AHU wasnt doing this... its dropping the temp to 16C. Problem is if I set a higher set point the system recirculates the building air then it has to do less tempering of the air but no reheating of the air happens.

This could be a problem with it being a chilled water cooling coil?

[The Viking]

The answers are out there...
The theory:
Cold air has less volume than warm air, cold air can therefore absorb less moisture before it get completely saturated. As we measure humidity as a % of the moisture required to fully saturate the air the same amount of moisture will give different readings depending on the temperature.
{To visualise this, imagine your van parked in a normal domestic single car garage - it will fill it to about 80%. Now, with the help of fairy dust, we heat the garage up and it magically expands to a footballer's 4 car garage - it will then only occupy ~20% of the available space}
(Cooling down will give the opposite effect)

If the air leaves your unit at 16.3ºC 96% and you say it entered at 21ºC, we can assume the humidity entering the unit was at least 60% but might have been higher depending on how much dehumidification took place (condensation) in the AHU.
These levels was taken from a psychometric chart and as muggy as it been here in UK lately this sounds about right.

In practice, what can be done?
The most effective way would be to first cool the air down and then reheat it, thus using the AHU as a dehumidifier.
But we already know that the building is ill equipped to cope with warm weather and the maximum temp difference across the AHU's cooling coil is about 5.7ºC.
A colder cooling coil would increase both the capacity and the condensation (therefore also the moisture removal), any chance the chilled water temperature could be dropped?
At the moment I assume it leaves the chiller at about 10ºC, returning at ~16ºC? If this could be dropped to even 6ºC leaving, 12ºC entering you might see an improvement on the AHU's performance.
Just be aware that this might cause issues elsewhere, not to mention that the chiller might not cope...
Another thing to look at is the flow through the AHU's coil, you said the water enters @ 10.2ºC - what is the temperature as it leaves the coil?

Happy hunting...

:cool:

.

[back2space]

Thanks for that wonderful reply.

Water returning back to the chiller is at approx 13C. The building is reaching set points via the fan coils. I dont think the chiller flow temp could be reduced any more as its quite an old chiller now and there are lots of problems with it at the moment such and PCBs have just been replaced but been told if it fails again parts are obsolete.

The set point of 21C has been running for the last hour. Humidity is at 71.3% and supply temp now at 22C doesnt seem to be dropping but I reckon it will be recirculating quite a lot of the building air now rather than bringing in lots of fresh air due to the higher set point.

[Magoo]

With BMS system do a total building air purge over night, full fresh air. Ok high RH over night but at low ambient. Then restrict fresh air from 8.00am and re-instate after say 10.00 am as sensible temp increases then that maintains a low RH in building, then introduce fresh air so people/ occupants do not nod off. The building structure will be cooler and aid / reduce chiller loadings

[back2space]

With BMS system do a total building air purge over night, full fresh air. Ok high RH over night but at low ambient. Then restrict fresh air from 8.00am and re-instate after say 10.00 am as sensible temp increases then that maintains a low RH in building, then introduce fresh air so people/ occupants do not nod off. The building structure will be cooler and aid / reduce chiller loadings

The fresh air FCU has been on all night usually set at 16C this has currently increased the humidity in the building to nearlly 70%.

If the outdoor temp is higher than the indoor set point then the system recirculates more and more internal air to save energy so technically speaking the system should start to recirculate a higher proportion of internal air as the morning goes on and outdoor temps increase.