Commissioning an AHU

[marc5180]

I have been installing an AHU this week with 2 Qualtair 14kw external condensors serving the cooling coil for a data hall.

The problem i had was on commissionig.

I ran one system up and the controls guy put a set point in of 16DegC and gave the unit some load.

I calculated the system should have 4KG++ of R407c but to be on the safe side i dump charged 3kg and ran it up then tried to trim the charge.

I got the subcooling to around 5k but the SH was 18K and the Suction Pressure was 2.5b with a Delta T on evap of 5k.

I added more refrigerant and put in 4.7Kg in total the subcooling increased to 10k but the suction pressure stayed at just above 2.5b and superheat was still high at 15+.

The evap Delta t increased to 6K and the suction was sweating nicely but the compressor discharge was around 90C and the compressor was getting very hot indicating not enough refrigerant returning to the compressor.

However when i spoke to my boss he did a calculation, and worked out the following and said that it was ok;

Density of air (1.102??) + the air volume (1.81) + Delta T (6) equals the KW, which was around 12Kw.

I was told it could be a problem with the air volume on the evap as some dampers weren't fully set yet and the inverter was set at 50hz and this may also need to be alterered.

The units i work on always have the correct air flow and it isn't something that i have ever come across which is why i'm looking for some pointers.

The TEV is TEZ2 with a No4 orrifice.

[multisync]

Did you try opening the TEV out as the suction is way too low for 407c

[marc5180]

Yes i held the bulb and the suction pressure rose.

I also tried adjusting the superheat screw half a turn but it seemed to flood the coil too much because i was getting 1DegC SH but the compressor did cool down considerably and the comp discharge dropped to 75C but...the head pressure was at 21-22b with the condensor fan running flat out. and the evap delta T stayed at 6k.

[Brian_UK]

On the airflow side Marc you need to get the fans performing at design duty as the first item.

Any flow/duty tests for heating/cooling coils need the correct airflow otherwise you could be looking at problems that shouldn't be there.

[Gingerair]

Need more info, but you sure you've selected the correct size orifice ??

You got a No.6 in the back of that Ferrari anywhere ?? :D

[marc5180]

Brian: There are 2 other identical AHU's in the same area and they both run at 50hz on the inverter with what i'd expect to be the same air volume.

If there wasn't enough airflow then i would expect the Delta T of the evap to increase but 6k isnt excessive so if anything could it be too much airflow?


Gingerair;

The expansion device was sent to us by the manufacturers as an add on a few days ago.

I was thinking maybe the wrong size orifice at first. If a smaller orifice is used then would these be the symptoms you would expect.

[yinmorrison]

Marc as Brian says you need to have the correct airflow proved first otherwise its a waste of time. The fan motor may be running at 50hz , 100% ,but that does not mean that the airflow is correct.If the air flow side is not balanced yet you could check what the fan motor design run current is and ramp down the invertor to simulate the correct air flow using ammeter.

[Gingerair]

Whats the design Delta T for your coil ?
You need to check the orifice size against the equipment you've got.
Have you got any documentation re how the manufacturer has sized a No.4 orifice ?

How long are you're pipe runs ? With a 4-5Kg charge i'd presume they're not long..

Not able to look at a orifice sizing table for you were i am at the mo...

Can anyone else help ??

Just re-read the post, if you think there's any airflow problem, as the others said you need to make sure it's as per design first..

[marc5180]

ok when i go back to site then i will be sure the airflow is correct, have any of you used the formula and is it correct?

Density of air (1.102??) + the air volume (1.81) + Delta T (6) equals the KW, which was around 12Kw

[Tesla]

Hi marc5180
At a quick check on psychometric chart it looks about right. As most above wrote the system needs to be balanced and the correct air flow rate IE dampers stroked and in return with min fresh, dampers at 100% (if vav), vsd = 100% and a good load (R.air 21+degC). Then you can trim your charge, set SH and check SC along with other parameters.

[frank]

ok when i go back to site then i will be sure the airflow is correct, have any of you used the formula and is it correct?

Density of air (1.102??) + the air volume (1.81) + Delta T (6) equals the KW, which was around 12Kw
The formula is nearly correct Marc - you just need to add the specific heat capacity of the air

Q = m x Cp x p x dt

For air at 20C
Cp = 1.005 kj/kgK
p = 1.205 kg/m3


m = mass flow

For air, volume flow can be substituted for mass flow.

[1torr]

Hi, you opened the tev and the sh dropped to 1k, so its capable. With high head pressure it could be overcharged. Try recovering some refrigerant and adjust tev to 6k sh. Charge to full sight glass or sub cooling.

[marc5180]

Tesla; I assumed that the airflow was correct but maybe i was wrong. I do know that the return air was 27DegC and the dampers were at 100%.

Frank; thanks for that,the density of air that he used then was of air at 35Deg++ and not at 20DegC.

1torr; The high side does look overcharged due to the high SC even though i only added refrigerant at 200g per time.
I've always read not to alter the TEV since it comes preset to maintain superheat which is why i turned it back to it's origional setting.

[1torr]

It may be, but valve or oriface may be too small.

[Scramjetman]

Marc, if your air volume is too high, the AHU is going to appear oversized to your condenser. Setting up your refrigerant controls before this is sorted out is probably going to mean you have to go back and do it again after it is straightened out. Get the air balance done firsty that set up your machine otherwise we put the cart before the horse.

With TX's, you are correct - the manufacturers usually send them out preset at about 8°C superheat. They do this to err on the side of caution. You can generally safely adjust this down to 6°C SH without penalty. Any lower and we are getting into the danger zone unless you have a means to capture any slop over like an accumulator or the like.

[marc5180]

Update;

Went back to the system today, the airflow had been commisioned.

The DT of the coil should be 9k but still i was only getting 7k.

The suction was still at 3bar but however SH was 6k with subcooling at 9k.

Even turning the TEV SH screw 1/4 of a turn, it flooded the evap coil and since there is no acumulator i turned it back to how it was.

The comp discharge was at 65DegC and the compressor wasn't too hot to touch. Everything now seems ok apart from the DT of the coil which is still slightly low.

[TRASH101]

Hello Marc

If the TD across the coil is only 7k (is this dry bulb?) and you are evaporating at say -11 celsius (3 barg) then I would suggest you consider the amount of condensate you are creating through latent cooling.

Does the coil manufacturer state 9k at design air volume as 100% sensible load ?

If a quarter turn on the superheat screw is flooding the evap and your sub cooling is 9k then I think you may have over charged.

Did you double check the txv phial charge for yor application?

[wilks]

So Frank marcs boss forgot to put the specific heat capacity of the air in his formula q= m x CP x p x dT where cp is the specific heat capacity ( 1.005) you say this is for air at 20oC , how does this figure change in relation to air temp .eg if I was working in a cold room when the air was 10oC ?

[TRASH101]

Sorry for butting in Wilks

The specific heat capacity is taken as 1.005 down to about -50c and up to above +40c

The density however changes more noticeably

[wilks]

Thanks trash ,

[frank]

So Frank marcs boss forgot to put the specific heat capacity of the air in his formula q= m x CP x p x dT where cp is the specific heat capacity ( 1.005) you say this is for air at 20oC , how does this figure change in relation to air temp .eg if I was working in a cold room when the air was 10oC ?
Google Specific Heat of Air and there are numerous tables which show the details you ask for