AHU investigation

[mechman]

I need too investigate an exist ducted air con system that is not conforming (cooling capacity insufecient)
the duct system is served by means of a chilled water AHU.
Any tips on where to start my investigation?
see attached scada image. please note that it was taken not on a hot day therfor the temperatures don't seem to high.
my concerns is:
-high delta T (10C) over coil

what is the recomended / typical supply air temperature (range) if the required room temperature is 23C?

[mechman]

see attachment

[Brian_UK]

First question: Has the system worked to specification in the past?

If not, what has changed?

Check water flow rates, temperatures etc.

[Gary]

The key temperature is the air leaving the cooling coil. This needs to be 11K lower than the target room temp in order to maintain 50% humidity in the room. Since your target room temp is 23C, the cooling coil air off temp should be 23-11=12C.

[Gary]

Assuming the 7C and 17C are the entering and leaving water temps, it looks like a water flow problem.

[Tesla]

Hi mechman
Making a few assumptions here - It looks like (from the SAT GUI) that the Supply Air Temp is 16.34°C and the Supply Air Temp Set Point is 16.38°C, so the cooling is doing what the controller is commanding. My first check would be some of the GUIs show oper (operator) while others show (none) - is this the control priority? in a Siemens system operator is the highest and none is the lowest priority. This would indicate these operator values are overriding the normal program. Has some one been playing with the controls here? I dial into BMSs every day and interrogate them - as Gary wrote above it could be lack of cooling. If you right click (or double left click) on the graphics page (with the operators logg on) on the supply air temp another window may pop up where you could click on none then command and see if this solves the problem.:)

[NoNickName]

Assuming the 7C and 17C are the entering and leaving water temps, it looks like a water flow problem.

Gotcha, but only half of it. The control signal is 5.67 V, that is 56% of opening. It looks like there is a control problem, that causes half of water flow across of the valve.
But ahead of that, why a two way valve? Is there an inverter pump? Or is it just a misrepresentation?

[Tesla]

Miss-interpretation - the majority of chilled water systems with AHUs use a two way valve using a chilled water byspass or decouple pipe bypass to maintain flow across the chiller.

[RSTC]

Agree with Gary, insufficient water flow to coil, check the valve.

Failing that look at the rest of the system, is pump ok? ... is something else on system starving this coil of water (commissioning issue)?

Flowing in at 7 you want to be coming off at 12 (in design conditions).

[TRASH101]

I think it may well be an airflow problem as all areas are above setpoint except the cooling which is "bang on".

Do the six offices have individual heaters?

Is there seperate extracts for the "bulk cash" and "note sorting" rooms ?

As there is no humidification system note well Garys' advice as paper is noticabley hygroscopic (assuming that is an issue)

More importantly I refer to Brians question



Has the system worked to specification in the past?



Know what the system was built to do before saying what it can or can't do.

[Gary]

The insufficient water flow through the coil could simply mean the controls have reduced the flow. It could be just a matter of adjusting the settings.

[NoNickName]

The insufficient water flow through the coil could simply mean the controls have reduced the flow. It could be just a matter of adjusting the settings.

I second that. :D

[Gary]

Consider this psychrometric chart:

http://www.uigi.com/UIGI_SI.PDF

Find where 12C intersects the 100%RH line. Follow horizontally to the right where it intersects the 23C line. The RH at this intersection is slightly over 50%.

IOW, if the air leaving the coil (12C@100%RH) is warmed to 23C, the RH will be about 50%.

If the air leaving the coil is 16C@100%RH, then at 23C the RH will be about 65%.

[mechman]

thks chaps
how can I verify the water flow, no installed metering device.
what is the recomended meter to install?

[NoNickName]

http://www.automation.siemens.com/w1/automation-technology-flow-measurement-18626.htm

[Brian_UK]

Try an ultrasonic flow meter.

[Gary]

I would monitor the coil air off temp and adjust the SA control until the air off temp is 12C.

[shaaf]

Hello All Dear,
i am new user & first post here.i have a question & hope.you can solve it.suppose 11k what is mean of ` k ` ?

[mechman]

what is recommended allowance of fresh air in the basement 7l/s per person with a minimum numer of air changes as there is very litlle occupancy in the basement

how many air changes than , 0.5?

[NoNickName]

It depends on the activity done in the room. If it's a gym, at least 20 vol/h, if it's a kitchen 30 vol/h.
If it's all office space I would say 6 to 8 vol/h with 20% fresh air.

[RSTC]

Shaaf, k is degrees kelvin. Like degrees centigrade but starts at absolute zero. Often used when comparing temperature or measuring a differential. example: air on temp 32 oC air off temp 37 oC; temperature difference is 5 K.

This is not to confuse with 5oC (which is a 'cold' temperature)

[shaaf]

thanks my dear