excessive airflow, effecting heat exchange

[chillyblue]

Hi

I was working on a air-con unit yesterday in a server room, and was experiencing the following.

Evap
Air on 23 deg C
air off 19 deg C

R407C
LP 45psi
suction superheat approx 7 deg temperature reading on suction line of -1deg C

The evaporator is a V block with the air entering the bottom of the v and exiting at the top of the V
The bottom of the V keeps icing up (just the bottom few runs about 12" up.
I defrosted the coil completely and within a hr or so the bottom starts freezing again.
It seems there is not a very good heat exchange taking place, i.e. low suction pressure and small td across coil, normal to low superheat.
I have replaced the filters and have checked the evap coil for cleanliness (it's very clean)
Could it be that the airflow across the evaporator is to fast/high and heat exchange is not taking place or is less than should be?? and the evaporator cannot abosrb the heat hence the low suction pressure and icing.

The discharge pressure is around 250psi at the moment i am maintaining it quite high to keep the suction pressure up, if i reduce the head pressure to 200psi the suction pressure falls too causing faster icing of the coil etc.

in my opion the indications all point to lack of load on the evaporator, or low air flow, there is definately not low airflow (hence why i'm assuming the same issues but with too much air flow)

Many thanks CB

[desA]

Do you have the correct refrigerant charge in the system?
How do you know it's correct, or not?

TXV? Is it blocked?

To my mind, with Ta,out=19'C & Te,sup (superheated vapour) exiting evap~-1'C, the evap is simply not getting enough refrigerant.

At Ta,out~19'C, I'd expect Te,sup around 17-18'C.

Set superheat: SH=0.65*TD (Magoo rule)

[nike123]

QUOTE]
Hi

I was working on a air-con unit yesterday in a server room, and was experiencing the following.

Evap
Air on 23 deg C
air off 19 deg C

What are wet bulb temperatures?


R407C
LP 45psi
suction superheat approx 7 deg temperature reading on suction line of -1deg C
45 psig is -4°C SST (dew point) and you have actualy 3K superheat.


The evaporator is a V block with the air entering the bottom of the v and exiting at the top of the V
The bottom of the V keeps icing up (just the bottom few runs about 12" up.With -4°C SST it is normal to have ice build up at bottom of evaporator.


What is expansion device?
If you have 3K superheat with capillary tube as expansion device than your evaporator is not short of refrigerant.
If you have TXV it could be that is factory adjusted at that superheat because of low humidity in server room applications
Also it could be measuring eror and TXV set at standard factory 4-5K.


I defrosted the coil completely and within a hr or so the bottom starts freezing again.
It seems there is not a very good heat exchange taking place, i.e. low suction pressure and small td across coil, normal to low superheat.Again, if SST is -4°C than ice formation is normal occurrence.




Could it be that the airflow across the evaporator is to fast/high and heat exchange is not taking place or is less than should be?? and the evaporator cannot abosrb the heat hence the low suction pressure and icing.High air flow doesn't degrade heat exchange.




in my opion the indications all point to lack of load on the evaporator, or low air flow, there is definitely not low airflow (hence why i'm assuming the same issues but with too much air flow)DB temperatures cannot tell you whole story, because in air conditioning, part of load is latent heat of condensation which have great influence on DB reading of delta t


You should measure subcooling, air flow and WB temperatures along with DB temperatures to get proper indications.
Also unit make and model # could help.

[Thermatech]

The indoor coil is frosting because the refrigerant temp is -1 deg C on the suction so air passing through the fins will form frost.
45 psi / 2 bar is very low suction pressure normally would expect 4 to 5 bar suction pressure.
On my comparitor 45 psi is -10 saturated vapor temp so with about 7 SSH that makes evaporating temp -3 deg C.

So the system may be SOG or refrigerant restriction as suggested by DesA.

The trick is to make the system run with evaporating temp just higher than frost conditions so about +1 or +2 deg C. Then the indoor unit does not stop on coil frost prevention or ice up the coil.You dont want to make the evaporating temp too high as then you will loose cooling performance but you dont want the evaporating temp too low otherwise you get into coil frost prevention or frosted coil problems.

To acheive this you will probably have to run with a discharge pressure arround 20 bar.

If the system is SOG then the discharge temp will be excessive with very high DSH so the compressor shell will be at overheat.

Suggest recover refrigerant charge to confirm correct charge.
When sure you have correct refrigerant charge for system adjust HP control for arround 20 bar discharge.
If the system has TXV then adjust to manufacturers specification for SSH.

At the end of this process the system should have correct refrigerant charge correct suction & discharge operating pressure & correct SSH.

With correct refrigerant charge & correct SSH setting I would expect arround 20 bar discharge 4 to 5 bar suction & discharge temp no higher than arround 80 deg C on a R407c system.

[nike123]

On my comparitor 45 psi is -10 saturated vapor temp so with about 7 SSH that makes evaporating temp -3 deg C.


Hi Thermatech! I think you made some error with saturation temperatures.

With R407, you should use dew point as Evaporation Saturation Temperature for R407C.
For 45 psig dew point is -4°C and -10°C is bubble point.

Also, I don't understand this part of sentence:
"-10 saturated vapor temp so with about 7 SSH that makes evaporating temp -3 deg C".
Isn't that evaporation temperature is same as saturated vapor temperature without any addition of SSH?

[chillyblue]

whats magoo rule??

CB

[desA]

^ Magoo is one of our senior members.

He introduced this rule for tuning evaporators to RE in an earlier thread. It is also used as standard tuning principle by Kueba (Kuba).

[chillyblue]

Have you got a link so i can read his rule or can you explain what the rule is??

CB

[Gary]

Is it possible that the air is bypassing the coil?

Another possibility is oil logging.

[chillyblue]

Is it possible that the air is bypassing the coil?

Thanks Gary, thats the first thing i checked

Another possibility is oil logging.

It's a possibility, how would i check for this??

CB

[Gary]

Is this a cap tube or TXV system?

Does the compressor have an oil sightglass?

Is there an oil separator?

Was the heat transfer improved after defrost?

[chillyblue]

Is this a cap tube or TXV system?

Does the compressor have an oil sightglass?

Is there an oil separator?

Was the heat transfer improved after defrost?

TXV

No

No

When the bottom of the coil ices you obviously get a lack of airflow through that section and then the suction superheat drops throttling the txv,starving the rest of the coil.
After defrosting the coil the suction pressures are higher but only for a short time then it starts icing again.

CB

[Gary]

Hmmm... I take it the coil is bottom fed?

[chillyblue]

No it's feed via a distributor.
I'm assuming the air flow would be at it's lowest at the bottom as it's a upside down v coil, with the fans at the top, so i assume most of the airflow would be through the top of the coil.
The fans pull through the coil, i.e the coil is on the suction side of the fans.
hope that all makes sense.

CB

[Gary]

Oil logging can be difficult to identify. Essentially the superheat tells us that the coil is not starved for refrigerant and yet the high approach temp (air off minus SST)tells us that the heat is not being transferred from the air to the refrigerant, because the inside of the coil has an oil coating acting as an insulator.

Assuming the coil feeds from the bottom up, you might try pumping down the system to slightly above zero psi, then remove the TXV to see if oil comes pouring out of the bottom of the coil.

[Gary]

No it's feed via a distributor.
I'm assuming the air flow would be at it's lowest at the bottom as it's a upside down v coil, with the fans at the top, so i assume most of the airflow would be through the top of the coil.
The fans pull through the coil, i.e the coil is on the suction side of the fans.
hope that all makes sense.

CB

Did you mean to say upside down A coil?

[Gary]

I assume the TXV is externally equalized and the bulb is mounted upstream from the equalizer line?

[Gary]

Does the refrigerant flow from the top down or the bottom up?

[chillyblue]

Did you mean to say upside down A coil?

No it's not a upside down A it's is a normal "A" coil, with the air coming through from the bottom to the top, the ice is forming at the base.


I assume the TXV is externally equalized and the bulb is mounted upstream from the equalizer line?

Yes externally equalized, didn't check line connection point.


Does the refrigerant flow from the top down or the bottom up?

The flow from the distributor (approx 12 lines) is quite odd it enters the coil in several different positions from front (air in) to back (air out), but flow is >sidewards not upwards.

Even after the coil was defrosted with air on temps of approx 27deg, when the system was started the suction pressure did'nt go above 65psi.

CB

[nike123]

Have you got a link so i can read his rule or can you explain what the rule is??

CB
http://www.kueba.de/en-us/Tools/K%C3%BCba-Expansion-Valve-Calculator/Pages/default.aspx

[chillyblue]

Thanks Nike

CB

[Abby Normal]

Server rooms will have next to no latent load so they need a high air flow rate, sometimes 50% more air flow than what is deemed typical.

The lack of load could be not enough air flow, could also be no latent load. What is the entering wet bub or RH, down below 40%?

[Gary]

Yes externally equalized, didn't check line connection point.


On rare occasions, the internal seal on the TXV can leak liquid into the equalizer line. If this liquid hits the TXV bulb (bulb mounted downstream) it will tend to close off the valve prematurely, thinking the coil is flooded. That's why the bulb must always be mounted upstream from the equalizer connection... just in case.

[chillyblue]

Thanks Gary

CB

[Gary]

The fans pull through the coil, i.e the coil is on the suction side of the fans.


Pull through is actually slightly more efficient than push through. The downside is that the drain pan is under vacuum. The drain line must be trapped or air will be sucked into the drain line preventing water from draining out.

[Magoo]

ChillyBlue
To simplify the Kueba super heat explanation.
Superheat at evaporator is/ should be aftre stabilized conditions, 65% of TD.
TD = deta T of air on and coil evap temperature.
magoo

[Abby Normal]

Pull through is actually slightly more efficient than push through. The downside is that the drain pan is under vacuum. The drain line must be trapped or air will be sucked into the drain line preventing water from draining out.
draw through is great when you need reheat

blow through is better when the load is mainly sensible

will need less total air flow and deal with less fan heat with blow through than draw through under a flat load that is almost all sensible

draw through forces you to over dehumidify the air to compensate for fan reheat

[Gary]

draw through is great when you need reheat

blow through is better when the load is mainly sensible

will need less total air flow and deal with less fan heat with blow through than draw through under a flat load that is almost all sensible

draw through forces you to over dehumidify the air to compensate for fan reheat

Hmmmm... good point. I hadn't considered the fan reheat effect.

[Abby Normal]

I did a telephone switch job once, blow through saved 9 tons off of the load

When it gets on the dry side, there is sort of less enthalpy per pound going into the coil. When the airflow is not exactly high, it tends to drive the suction pressure down, makes the coil run colder

Blow through adds some heat to the entering air

Easier for the coil to get the fan heat this way, than it is for the coil to over cool, and usually over dehumidify air, to compensate for the fan reheat of draw through

[chillyblue]

The indoor coil is frosting because the refrigerant temp is -1 deg C on the suction so air passing through the fins will form frost.
45 psi / 2 bar is very low suction pressure normally would expect 4 to 5 bar suction pressure.
On my comparitor 45 psi is -10 saturated vapor temp so with about 7 SSH that makes evaporating temp -3 deg C.

So the system may be SOG or refrigerant restriction as suggested by DesA.

The trick is to make the system run with evaporating temp just higher than frost conditions so about +1 or +2 deg C. Then the indoor unit does not stop on coil frost prevention or ice up the coil.You dont want to make the evaporating temp too high as then you will loose cooling performance but you dont want the evaporating temp too low otherwise you get into coil frost prevention or frosted coil problems.

To acheive this you will probably have to run with a discharge pressure arround 20 bar.

If the system is SOG then the discharge temp will be excessive with very high DSH so the compressor shell will be at overheat.

Suggest recover refrigerant charge to confirm correct charge.
When sure you have correct refrigerant charge for system adjust HP control for arround 20 bar discharge.
If the system has TXV then adjust to manufacturers specification for SSH.

At the end of this process the system should have correct refrigerant charge correct suction & discharge operating pressure & correct SSH.

With correct refrigerant charge & correct SSH setting I would expect arround 20 bar discharge 4 to 5 bar suction & discharge temp no higher than arround 80 deg C on a R407c system.

20 bar!! why so high?? i thought i should be aiming for a condensing temperature of around 40 deg C..

Thanks for the comments

CB

[chillyblue]

Just to confirm, the dew point and bubble point.

Which is the evaporting pressure??

And which is the condensing pressure??

CB

[chillyblue]

Just found my answer.. viewing Grizzlys thread, thanks Grizzly

Dew point is evap temp & bubble point is cond temp

CB

[nike123]

Try to remember it in this way:

On your PT chart, when you looking for evaporation temperature always use higher temperature to allow all components in refrigerant mixture to evaporate and only one drop is liquid => DEW POINT

When you looking for condensation temperature always use lower one on PT chart to allow all components in refrigerant mixtures to condensate and only one bubble of refrigerant is gas => BUBBLE POINT
You will then always get right picture.

[chillyblue]

Thanks nike123