Condensing unit too small?

[carlfoster]

Hi all, I just wonder what kind of symptoms you might expect to see if the whole condensing unit design is slightly too small to cope with the evaporating unit?

The system uses R407C and a capilliary tube for throttling.

I just wanted to see if I'm diagnosing this right. Thanks.

[The MG Pony]

I'd imagine long pull down times, high super heat, low evap deltas, thats as much as I can think of right now.

[mohamed khamis]

high discharge air temperature from the evaporator and high discharge pressure. low subcooling with high superheat.

[carlfoster]

Thanks for your answers, but those are not the symptoms I'm having.

I'm getting Low Discharge but High suction. Small air temp diff over evaporator and low superheat as if it's overcharged, but the condensing unit came pre-charged and I added no gas. The pipe run is only a couple of meters.

My guess is the condensing unit is slightly too small, its rated at 18.5KW cooling and the evap unit is rated at 20KW.

Any ideas?

[mohamed khamis]

U mentioned condensing unit and i understood as it is a condenser. so i mentioned to u if the condenser is undersized. However as these symptoms refer to lack of compressor capacity i.e. the compressor has low cooling capacity. The overcharge symptoms are similar to the lack of compressor except the discharge pressure in the case of the compressor inefficiency is low.

[chillyblue]

Hi

sounds to me like your not getting a good heat exchange at the evaporator, is the airflow across the evaporator OK?, are the evap fans going the right way? is the coil blocked?

CB

[lana]

Hi there,

Mohamed is completely right.

When a refrigeration fault occurs LP and HP increase or decrease together. This means is a fault makes the LP to increase so will the HP and vice versa.
There is only one exception which is lack of compressor capacity. LP and HP act opposite each other. LP increases and HP decreases.
This is why lack of compressor capacity is the easiest fault to diagnose.

Cheers

[old gas bottle]

suck the gas out and recharge it by weighing the correct amount in,it may be short,it may be that theres a system fault,may even be that the compressors not pumping, i would start there before riping it to bits.

[Abby Normal]

If the condensing unit was to small with respect to the evaporator coil, I would suspect that by the time the system was properly charged so that superheat and subcooling were normal,the suction pressure would be higher than typical.

[Abby Normal]

On second thought, I would say it would act like a fixed metering system under a big load. Normal subcooling, high suction pressure, high superheat

[carlfoster]

Wow, thanks for all the comments.

First, let me just clarify:

When I say condensing unit, I mean the whole package; Condenser, compressor, fan LP/HP etc as a pre-bought complete unit.

Second, I've reconsidered and I don't believe the condensing unit is too small and causing these unusual symptoms.

I believe the capilliary tube is the wrong size/length and is actually flooding the system.

A number of things lead me to this conclusion:

1) High suction pressure
2) Liquid return to compressor and evaporating "late" after leaving evaporator.
3) Low Discharge pressure. (can't be overcharge)
4) Warm evaporator.

I'm going to test this theory out and I will let you know the results.

Thanks you all again.

[TXiceman]

You should be able to get the data on the condensing up and cross plot the data wit the evaporator and see where ir should balance.

I have never trusted precharged units and you need to go back and tweak the charge.

If they are not grossly mis-matched, it should balance a a slightly higher ET and CT.

ken

[Chr1s]

Have you checked the txv?

[carlfoster]

Have you checked the txv?

There is no TXV on this system.

I've checked up on this and the capilliary tube is definately the wrong size. It also seems that there is no actual proven way of calculating the right capilliary size (not that I can find) it's a matter of trial and error.

So it looks like I'll be doing a lot of testing I might just take the easy route and fit an expansion valve instead.

[Abby Normal]

Wow, thanks for all the comments.

First, let me just clarify:

When I say condensing unit, I mean the whole package; Condenser, compressor, fan LP/HP etc as a pre-bought complete unit.

Second, I've reconsidered and I don't believe the condensing unit is too small and causing these unusual symptoms.

I believe the capilliary tube is the wrong size/length and is actually flooding the system.

A number of things lead me to this conclusion:

1) High suction pressure
2) Liquid return to compressor and evaporating "late" after leaving evaporator.
3) Low Discharge pressure. (can't be overcharge)
4) Warm evaporator.

I'm going to test this theory out and I will let you know the results.

Thanks you all again.

Concenring 3 above, don't use the pressure to determine if it is overcharged use the subcooling. I would say in this fixed system if you had in excess of 11C subcooling it is too much charge.

Low superheat, high subcooling is a fixed metering overcharge.

You could very well be correct on the wrong metering device, but don't assume it's not over charged based on pressure alone.