Opinions on Freezer Room condition

[PaulYeates]

Recently got a service call to a large Freezer Room. The complaint was that the room took longer than expected to reach set point (-20*) after room was turned off for cleaning.
Readings for the last 5 days seem okay apart from the day in question.
The evaporator has had a history of partial freezing and had the txv and orifice replaced within the last month.
Today I checked the evaporator to find the partial freezing had returned.....the room was at -18.5*.
Evaporator coils were ice free as was the expansion valve, the ice formed on bottom left surface on the fins.

Anyway I'm wondering what you think of the condition of the system from the readings I took. Condensing Unit is on the building roof and the room has quite a large stock level but not overstocked.
R404a refrigerant
Ambient Temperature on roof 20*c
Saturation Temperature on gauge fitted to compressor suction port -30*c
Suction line temp at evaporator outlet -19*c
Temperature on gauge fitted to discharge port at compressor 34*c
Temperature of condenser outlet liquid line 17*
Air on condenser 17.5*
Air off condenser 19.5*

[cadwaladr]

When you say cleaned? was heat introduced steam cleaner how long was it off,the insulation may have warmed up so the pull down could take a while and if the stock has risen in temp then those factors should be taken into account,guess you have cleaned the condensor time is a great healer.

[monkey spanners]

Ice on the coil might be a defrosting or drain issue.

[Martin Davies]

Whereabouts is the air on probe located? Is it free from stock? I know these places like to pack the stock in and can sometimes distort the readings through lack of air circulation to the probes causing system to over work and cause ice build up

[PaulYeates]

Not sure how they clean it.....I think with just a trusty mop and disinfectant solution. The probe for the room temperature is located just behind the evaporator.....their is also a digitrak probe beside it that records daily temperatures.
Stock is loaded underneath evaporator to a level just under the drain outlet. What do you think of the superheat/subcooling readings?

[Magoo]

The system has gone flat and not doing too much with SST at compressor of -30'C at evap probably -28'C, air at -18.5'C is a TD of 9.5 suction line at outlet should be -21'C.
You have low discharge at compressor, and low liquid line temperatures
Sounds like a TXV problem, are MOP TXV valves fitted as the super low option vavs can restrict performance in pull down condition.
I have with R404a systems, because of the high density of liquid the discharge pressure should be up around 40' C SDT to punch liquid around system, possibly been the long term problem. then the TXVs can feed correctly and get system performance peculating.
magoo

[Martin Davies]

As I expected, it sounds like the stock is blocking air flow to the return air and A. confusing the air on probe and B. restricting the systems performance. Remember it's the air on probe that is controlling the solenoid and compressor and therefore if it is not reading correct room temperatures the compressor will keep running when it is not required and freeze up the coil. I used to remove the shelf directly underneath the drain pan and also tell the staff not to put stock there to help the freezer. May be wrong but I think it will certainly help.

[bjkmiami]

hi paul,
well i am really curious how do you manage to achieve subcooled liquid temp of 17(Temperature of condenser outlet liquid line 17*) when you have an ambient temp of 20C...are you running some sort of extra refrigeration system through yur condenser???(assuming air-cooled condenser)
it would be a wise thing to focus on TXV and orifice. make sure it is providing the desired capacity. To me not feeding enough liq. ref. through the evaporator may also cause partial frosting on it..Also see if this new TXV has a MOP future and temperature..this can also prolong the pull-down....
would be nice to hear if u come up with new findings.
good luck.............