Airflow in ac

[moondawn]

Hi Guys,

I am Currently going through the KOTZA course. I know the importance of airflow at both evap and condensor coils but i really dont know what i am looking for in testing the airflow.

in kotza it says i should be looking for a air on differntial to air off of 8 to 10 degrees c.

When i service ac units i put my probe to the return air and get a measurment and put my probe to the air off.
i seem to get readings in cooling mode over 10 degrees ie room at 21 air coming out the louvers at 10 degrees c or 9 degrees c. but is this okay.

Also when on heat mode i get air coming out at 33 degrees return air temp at 20 thats 13 degrees differential??

very unsure of what i am looking for could anyone help me out please.

cheers lee

[james10]

8-10c is a good deltaT
some manufacturers such as mitsi and daikin specify a 12c deltaT on their units i guess it depends on the fan speed and system design

[moondawn]

hi james its mitsi systems and daikin that i have been taking the readings so that could suggest my higher readings.

does heating demand more delta t as can have coil temps quite high at 47 degrees c?

[james10]

hi james its mitsi systems and daikin that i have been taking the readings so that could suggest my higher readings.

does heating demand more delta t as can have coil temps quite high at 47 degrees c?
i have seen off times this high on the odd occasion normally the higher 30's i wouldn't worry about it as long as the rest of the system is operating correctly

[moondawn]

thanks james just trying to find a medium that i can work to.

on kotza if for example the air on is 26 and air off 10 then it means there is restriction of airflow ie blocked filter or clogged coil. its trying to put this in practice out in the field doing it on the pc i finding it easy!

[Brian_UK]

Remember that when you are in heating mode your evaporator is now the condenser so the expected temperatures could change.

[james10]

thanks james just trying to find a medium that i can work to.

on kotza if for example the air on is 26 and air off 10 then it means there is restriction of airflow ie blocked filter or clogged coil. its trying to put this in practice out in the field doing it on the pc i finding it easy!
16C deltaT is something you'd come accross quite often in cooling mode on back to back splits i'm not familliar with KOTZA but it sounds to me they try to genralise all systems together if that makes sense, it is always best to treat each system individually

[moondawn]

yeah thats the problem i am finding james, i could prob find a system that is working well and giving a differential of 16 delta t air.

so really struglling with a rule of thumb to use as a starting basis for i think i have airflow problem.

Kotza is a training simulator seems good, first things first on diagnostics you put the gauges on then check for airflow at the evap, this is where i need to understand a general rule of thumbso i can say my airflow is good check superheat or my airflow is not good check coil, filter, belts etc

cheers lee

[TRASH101]

Moondawn

You might've opened a can of worms here because in this day and age with EEVs and such its all about the control scheme.

On cap and TXV you still got a fighting chance assuming you know what the design delta t was in the first place.

There is no substitute for direct air volume measurement but if you know the Hx design output at its rated air volume and you know the db and wb temperatures off you can work out if the air volume is somewhere near (assuming the system is under load and stable pressures).

[Gary]

Let's imagine that your target condition at the thermostat is 21C @ 50% RH.

Check this psych chart:

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

The air as it leaves the wet coil is at 100% RH.

Locate where 10C intersects with the 100% RH line. Draw a horizontal line to where it intersects with 21C. The RH is now 50%.

IOW, if the air leaving the coil is 10C@100%RH, then by the time it reaches the thermostat that air will be 21C@50%RH.

The temperature of the air leaving the coil should be 11K lower than the air at the thermostat in order to get 50%RH.

It can usually be assumed that the return temp will be about the same as the temp at the stat, therefore we can say that the delta-T (measured close to the coil)should be about 11K.

[DTLarca]

Interesting thoughts Gary. I imagine you are assuming some ideal coil arrangement? In practice we in our own applications don't come across coils with a by pass factor of 0. When we have a low cooling loads but high fresh air requirements then we actually want very high by pass factors.

We select a coil's dew point temperature for a design sensible heat ratio. Then the air velocity vs. coil rows give us a particular by pass factor. The specific humidity change and dry bulb change is a product of the by pass factor and the changes that would occur if we achieved 100% contact.

The concept you are drawing from sounds much more like the psychrometric processes over a refrigeration blower. But still thought provoking.

[moondawn]

hi all thanks for the comments but gonna be honest here i think i need to do my level 3 for me to understand and of these posts, not learnt anything to do with air movement wet bulb etc.

thanks gary for your detailed post just wish i could understand it all haha

cheer lee

[marc5180]

Take a look at this Lee;

http://www.vivoscuola.it/US/RSIGPP3202/umidita/copie/g626.htm

[moondawn]

cheers marc will look at that when i get back from working away.

speak soon mate and thanks again

[Gary]

Interesting thoughts Gary. I imagine you are assuming some ideal coil arrangement? In practice we in our own applications don't come across coils with a by pass factor of 0. When we have a low cooling loads but high fresh air requirements then we actually want very high by pass factors.


Yes... the bypass factor changes this... and it would be more accurate to use air off wetbulb temp, too.

On the other hand, I look for user friendly rules of thumb that get the system into the ballpark, as opposed to precision. It works. :)

[back2space]

i have seen off times this high on the odd occasion normally the higher 30's i wouldn't worry about it as long as the rest of the system is operating correctly

Yes sometimes my system gets upto 47C in heating but then other times about 37C even with same outdoor temp.

I dont quite understand it myself as even if I put upto 30C it then wont rise to 47C again.

[El Padre]

Hi Moondawn,

In addition to the advice allready posted, on Daikin hard wired controllers you can see the return air and two (sometimes one) coil thermistor values, code 41.

I am not familiar with Kotza but bear in mind that the control logic (fuzzy logic)in VRV/F's splits etc. as Trash explains can sometimes be misleading.

Cheers

[moondawn]

i have seen off times this high on the odd occasion normally the higher 30's i wouldn't worry about it as long as the rest of the system is operating correctly

hi el pardre, thanks for your input think i will check the units and use james advice for now.

cheers lee