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jonesy_87_
03-08-2015, 02:10 AM
Hi everyone,

I have probably what will be a stupid question. I've previously worked on mostly HVAC but in my new job I look after a few freezer rooms and cool rooms.

I repaired a gas leak on a freezer room (R404A) and had a bit of trouble getting my head around charging it.

How do people what out what pressure the unit should be running at when compared to the temp inside the room?

Say, I want a -20 degree freezer room, how do I work out what SST/pressure I want?
During the charge when the freezer hits -10 degrees, how do I know what my SST should be?

I charged the freezer to a full sight glass and a 6K superheat but I was still confused as to how other technicians over the phone knew what temp my freezer was just from me telling them the SST/pressures.

Also, does the answer to the above question also apply for cool rooms, etc?

Once again, sorry for what seems a straight forward question - I'm trying to wrap my head around refrigeration and learn a bit before my next service call.

Thanks

Grizzly
03-08-2015, 06:43 AM
Hi Jonesy.
I am amazed you have not come across a Temperature Comparator chart or as some call them "Comparators"!
Many use an app nowadays, try typing "refrigerant slider" into google.
Simply put if you know any 2 of the following 3 facts you can reference them against a chart or app.
To get the third.
1) what refrigerant?
2) What Temperature? (that refrigerant is being subjected to?)
3) It's Pressure?
Know any 2 of the above and the 3rd is read off the table, app or comparator.

If you are reading the superheat then you are using a comparator to tell you! Be it using compound gauges or digital display.

Totally fundamental to a refrigeration Engineer / Mech.

I am sure some of our trainer Guys will explain better!
Cheers Grizzly

jonesy_87_
03-08-2015, 08:01 AM
I do use a refrigerant slide but normally the trusty old pt chart.
This doesn't tell me how my room temp relates with what the SST or even the actual line temps should be (I'm not sure how well I am making sense)

I know the SST and actual line temp are going to be lower than the room temp but what is the general rule in calculating this.

For example should my -20 degree freezer room be running at a -30 degree SST so does that mean on a 3 degree cool room, will my SST be at around -7 degrees.
What is the normal range in the above regard?
Or does it come down to cool room size, design, etc.

Cheers for the reply Grizzly

RANGER1
03-08-2015, 09:24 PM
jonesy, you are on the right track.
In forums go to Fundamentals, then look for Refrigeration 101 by Gary.
Read everything, may answer a number of questions.

http://www.refrigeration-engineer.com/forums/showthread.php?19701-Refrigeration-101

Glenn Moore
03-08-2015, 10:54 PM
Hi Jonesy
Temperature difference (TD) between Evap temperature, and air of coil temperature is calculated during the design stage of the plant. With fresh meat etc the TD is something like 5-7K to keep the room humidity high to prevent product shrinkage, whereas something like a beer cellar where product shrinkage would not be an issue the TD could be higher maybe 10k. The system TD is often decided on by the cost and size of the evaporator.
Most of the large wholesalers have a free coldroom design software package that you can download for free. These walk you through coldroom design and product selections including the systems TD . You can learn a lot from these software packages Kr

jonesy_87_
04-08-2015, 08:05 AM
Thanks for the replies.

I had a read through the Refrigeration 101 thread and got some answers.

I read that a freezer room should have a SST of around -25 degrees Celsius and a coolroom of around -5 degrees Celsius (ballpark figures).

The more I look into it, the simpler it looks:
Charge to a full sight glass, ensure there is around a 6K superheat and if need be check the subcooling on the condenser coil.
As long as the SST is within an acceptable TD of the evaporator temp and the space is reaching temp then in my understanding the charge will be ok.

Any other advice or tips are welcomed but thanks again.

Cheers

RANGER1
04-08-2015, 10:11 AM
Thanks for the replies.

I had a read through the Refrigeration 101 thread and got some answers.

I read that a freezer room should have a SST of around -25 degrees Celsius and a coolroom of around -5 degrees Celsius (ballpark figures).

The more I look into it, the simpler it looks:
Charge to a full sight glass, ensure there is around a 6K superheat and if need be check the subcooling on the condenser coil.
As long as the SST is within an acceptable TD of the evaporator temp and the space is reaching temp then in my understanding the charge will be ok.

Any other advice or tips are welcomed but thanks again.

Cheers

Charging to a full sight glass not entirely true with most new refrigerants.
Subcooling & superheat will assist, otherwise overcharging possible, causing high head pressures.
Pipework layout could also effect things. Like flashing in liquid line

jonesy_87_
04-08-2015, 12:20 PM
Charging to a full sight glass not entirely true with most new refrigerants.
Subcooling & superheat will assist, otherwise overcharging possible, causing high head pressures.
Pipework layout could also effect things. Like flashing in liquid line

Well I read a full sight glass or 6-10K subcooling whichever came first.
Thanks though, something else for me to read into.

Cheers