help!! getting beat up by a walk in freezer

[avalanche]

having trouble with a walk in freezer getting fustrated. im having trouble maintaining proper superheat.

hi there new to this site.

I have a low temp freezer running at -25 F box temp.
heatcraft condensing unit with a copeland discus.
rated at 16400 btu @ -25 sst. matched with a 25500 btu evap. have a 2 ton txv on the system.
installed new condensing unit, evap is 2 years old.
system had a scroll condensing unit lasted only a year then wouldnt pull temp. matched btu output and decided to change to a discus. unit ran great for 8 months now having trouble. cant seem to set txv superheat. they put 3000 lbs of processed meat dog food in a 9ft by 13ft by 8ft freezer try to freeze to -25 f in about 18 hours. unit now is flooding back to the compressor when temp is around -18f so i adjust back to 6-10 f superheat then pulls to -25.

next day they load with fresh meat and the system seems to starve the evap. tried everything, new txv,
seems to work okay when i adjust the txv manually but will only work 1 or 2 days. system seems undersized but worked fine for 8 months. unit running 404a refrig. wondering if such low temp is causing not enough suction cooling and possible valve damage. looking for any suggestions or feedback.

thanks in advance

[Toolman]

Unit rated at 16400 btu @ -25 sst. matched with a 25500 btu evap.

I know that Kilowatts of Condensing Unit and Evap should match . You say 25500 btu's & 16400 btu's , maybe btu's are different but I cant see how ?

[abet_meneses]

avalanche.

1.Based on the given capacity of the equipment the selection is not matched.
2.check defrost cycle,should be 4 defrost cycle on 24 hours
maybe 20 to 30 minutes defrosting time.
3.check if there insulation leak or door left open.

hope that helps,

regards,

[Gary]

We would need a lot more information to help you with this.

Low side:

Evap air in temp
Evap air out temp
Saturated suction temp (SST) converted from pressure
Suction line temp at evap outlet

High side:

Cond air in temp
Cond air out temp
Saturated condensing temp (SCT) converted from pressure
Liquid line temp at receiver outlet

[Gary]

The superheat should be 6-8F ONLY when the refrigerated space is down to design temperature. When the space temp is higher than design, the superheat should be high.

[Gary]

next day they load with fresh meat and the system seems to starve the evap

This is normal. Under heavy load, the superheat is supposed to be high.

You cannot properly adjust a TXV until the refrigerated space is down to design temperature.

[tonyelian]

hi avalanche
ur room size needs 28500 btu @-32 evaporation temp it seems that ur comp is too small for the task fresh food freezing needs too much power than frozen to frozen ur fan size is correct but the compressor is not
to get -25 ur evaporation temp should be -32

[Sledge]

I am not understanding how the unit can be too small if in fact it worked for 8 months.

regardless of whether we measure heat in BTU's or KW, if you have properly matched the capacity of what you have taken out with the equipment you have put back in, then the capapcity should be the same.

I am not sure if I completely agree with the statement that superheat should be high, when the system is under load. That holds true in a cap tube system, and if the TXV system is subject to extremely large load conditions, then the SH will be high, but as the temperature starts to pull down the SH must balance out. If it doesnt, then the TXV cannot be doing the job.

I am concerned about the TXV flooding back to the compressor. I see that as the root cause. There has to be an issue with the TXV. I dont have the sizing charts with me, but I would confirm that everything is ok with that. Perhaps the TXV bulb is not attached properly, or there is a blockage in the external equalizer line.

As a fellow Canuk, and a contractor, if you want some help send me a note.

[Gary]

I am not sure if I completely agree with the statement that superheat should be high, when the system is under load. That holds true in a cap tube system, and if the TXV system is subject to extremely large load conditions, then the SH will be high, but as the temperature starts to pull down the SH must balance out. If it doesnt, then the TXV cannot be doing the job.

The advantage to using a TXV is that it can provide increased refrigerant flow when the heat load is high. High superheat is what causes the increased flow.

When the superheat is normal, the flow is normal. When the superheat is high, the flow is high. You don't get the increased flow without the high superheat.

Heavy load = high refrigerant flow = high superheat

The system is flooding because the TXV is wide open as a result of trying to adjust the superheat under high load conditions. It can only be accurately adjusted under normal load conditions.

[Sledge]

The advantage to using a TXV is that it can provide increased refrigerant flow when the heat load is high. High superheat is what causes the increased flow.

When the superheat is normal, the flow is normal. When the superheat is high, the flow is high. You don't get the increased flow without the high superheat.

Heavy load = high refrigerant flow = high superheat

The system is flooding because the TXV is wide open as a result of trying to adjust the superheat under high load conditions. It can only be accurately adjusted under normal load conditions.

Gary, I am sorry, but I understand it differently.

Heavy load=high refrigerant flow...I agree completely. This means that the TXV is doing its job.
If you have high SH, the evap is being starved, which means that the TXV is not doing its job, it is not keeping up.
If you are flooding, then once again the TXV is not doing its job, it is overfeeding.
If it is alternately flooding and starving, then your TXV is hunting or the evap is big enough to cause a significant time delay between when the conditions are sensed by the TXV bulb and the reaction of the TXV to the changed conditions.
In this case I would be thinking that either the TXV is the fault, or the exterrnal equalizer line has problems, (or was not connected).

I would re-examine the TXV specs, the evap specs, and make sure that they are correctly matched. If they are all good, I would be looking at the TXV for being faulty, improperly installed, etc.

[Gary]

Keep in mind that the orifice is sized for a normal load.

Given a heavy load, the superheat goes high and stays high. The TXV is wide open and the flow is limited by the orifice size. It is acting just like a fixed orifice system.

And just like a fixed orifice system, as the load diminishes the superheat starts to drop, until the load comes within the range of the orifice, then the TXV begins to modulate... and not until.

If you increase the orifice size so that it will modulate under heavy load, then it will hunt under normal load.

[Sledge]

Keep in mind that the orifice is sized for a normal load.

Given a heavy load, the superheat goes high and stays high. The TXV is wide open and the flow is limited by the orifice size. It is acting just like a fixed orifice system.

And just like a fixed orifice system, as the load diminishes the superheat starts to drop, until the load comes within the range of the orifice, then the TXV begins to modulate... and not until.

If you increase the orifice size so that it will modulate under heavy load, then it will hunt under normal load.

I agree.

Soooo

The question remains: why does it not consistently perform, and cool the meat?
A problem with compressor valves doesnt come and go. So it cannot be that.

If the equipment cools even one load properly, then the equipment must be properly sized, unless load conditions are changing.

Do load conditions change? Do they sometimes put more meat in freezer, or is the meat sometimes warmer? What kind of condensor do they have and what are the conditions it is exposed to.

[Gary]

The original poster seems to have disappeared, so let's examine what he said:

cant seem to set txv superheat. they put 3000 lbs of processed meat dog food in a 9ft by 13ft by 8ft freezer try to freeze to -25 f in about 18 hours. unit now is flooding back to the compressor when temp is around -18f so i adjust back to 6-10 f superheat then pulls to -25.

next day they load with fresh meat and the system seems to starve the evap.

At this point the TXV was adjusted properly. He should have left it alone and waited 18 hours for it to pull down to temp, but instead he...

tried everything, new txv, seems to work okay when i adjust the txv manually but will only work 1 or 2 days. system seems undersized but worked fine for 8 months.

If you want to call 18 hour pulldown "worked fine".

Personally, I would have put in a larger condensing unit to match the coil and TXV, but the question was about superheat adjustment.