Desending the Temperature in a Freezer

Dear Friends
In the coming weeks we have to start up a Freezer 60mx60mx12m which will work at room= -28ºC

It is conected to a two state ammonia system.
low SST =-35ºC, media SSt=-5ºC, Condensing at 35ºC

We will use the SST= -5ºC to keep the dock room at +5ºC.

How do you recomend to handle the start up?
Which commpressors should we start first?
How many steps we should use to low the temperature in the freezer?
What conditions will difine to go to the next step?(temperature or RH)
I appreciate your very kind comments
regards
Gwapa

If you have already pressure tested the system and confirmed safety/reliability, then start off in this general order:

1. Condensor
2. High Stage
3. Low stage
4. Circulation pumps and NH3 feed solenoid to evaps.
5. Evaporator units

Definitly the high stage first, to stabilize system before starting low stage.
magoo

As always, be ready for the unexpected. Oftentimes pressure changes can cause flanges to leak slightly... Tighten them carefully as to not damage the gasket.

I would ramp down at 5C per day on HT system, until you get to 3C, then 1C per day until -3C hold for 5 days, then ramp down again 1-3C per day

Hi, Gwapa :),

First run the high stage and cool down dock room at +5C

... when you have all under control - high stage is running in auto mode - then start low stage compressors

... keep in mind to not go too fast down to -28C follow suggestion of mad fridgie

Best regards, Josip :)

Thanks for your very kind advices

As long I start the low stage I should ajust the evaporator temperature at (let say) +10ºC in orther to have a room temperature +15ºC.
Then ajust 0ºC to get a room temperature at +5ºC

Then ajust at -5ºc to get a RT 0ºC

At this moment I would need to defrost the evaporators
and so on

Am I right???
need I to check the Relative Humidity?
Thanks again
gwaa

Hi Gwappa.
Hopefully you have designed with a PLC to control all systems, and not relying on manual ( human factor ) control.
magoo

Thanks Magoo
Yes we did design in a net.
All signal go to a main computer and all the logic is there.

However I am curious how will be the behavier of the low stage compressors runnig at to high succion pressure during the lowering temperature period
regards
gwapa

If you have a dual duty compressor use it for both chiller and freezer initially on chiller duty controls .

Bring temp down slowly and have doors cracked open so you don't accidentally implode cool room / freezer ( it has happened but don't let it happen to you ) .
Hold rooms at 5 deg C as long as you can , say min 4-5 days and measure floor temp . If down to temp then gradually bring temp down over another 3-4 days .
Concrete will hold a lot of heat especially if new .