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HTRefrig
16-07-2010, 01:24 PM
Hello All,

I am a Ammonia tech at a large -10 freezer. I have been having debates with our QA dept (Quality Assurance) about the temps in our freezer. They are stuck on taking Ambient air temps in the freezer and using them to judge the cooling capacities of my system. I have told them they need to stick their ambient air sensor in a wet sponge and use that as a better tool to correctly measure the temp inside the different zones of the freezer. I have explaned that being in an air stream would change their readings a minimum of 4 degrees. They are very hard headed as u can see.

So my question to you all is... Does anyone have any studies or info I can back up my arguement to them with? It would truely help.

Thanks

Segei
16-07-2010, 10:34 PM
Hello All,

I am a Ammonia tech at a large -10 freezer. I have been having debates with our QA dept (Quality Assurance) about the temps in our freezer. They are stuck on taking Ambient air temps in the freezer and using them to judge the cooling capacities of my system. I have told them they need to stick their ambient air sensor in a wet sponge and use that as a better tool to correctly measure the temp inside the different zones of the freezer. I have explaned that being in an air stream would change their readings a minimum of 4 degrees. They are very hard headed as u can see.

So my question to you all is... Does anyone have any studies or info I can back up my arguement to them with? It would truely help.

Thanks
I'm just wandering how they are going to judge the cooling capacity of your system based on air temperature in the freezer.:eek: I think that your wet sponge will freeze at -10F:)
Typically, there are 2 air temperatures in the freezer. Evaporator air inlet temp -10F and evaporator air outlet temp -14F. Air go through the evaporator and it cooled by 4F. Evaporator air inlet temp is very close to the product temperature. This is the reason that this temp assumed as freezer temperature.

Brian_UK
16-07-2010, 10:51 PM
Came across this report which might be of interest to you. If anything it shows that just a few temperature readings are not enough for thorough examination.

http://www.grimsby.ac.uk/documents/defra/stor-coldstoreenergy&opti.pdf

Segei
16-07-2010, 11:51 PM
Came across this report which might be of interest to you. If anything it shows that just a few temperature readings are not enough for thorough examination.

http://www.grimsby.ac.uk/documents/defra/stor-coldstoreenergy&opti.pdf
I found little useful information. Sensors should be calibrated. Compressors and valves should be repaired. Heat load should be optimized. I always thought that heat load should be minimized.:)

Peter_1
17-07-2010, 03:32 PM
I disagree using a wet sponge for measuring air temperatures.
This is not the ambient bit the wet bulb ambient.
And how does a wet bulb temperatures relates to the different zones in the freezer'? I don't see the relation.
This is more related to a good air distribution in these different zones.
Besides this, how can you judge capacity by measuring air temperatures, be it now wet or dry bulb? You need a lot more information to measure or judge capacities and wet bulb isn't one of it.

mikeref
19-07-2010, 08:34 AM
HI HTRefrig, a part of your question was about capacity but air temperature reading seems to me to be related to efficiency. capacity issues should have been dealt with in the freezer-room planning stage. I'd set up a datalogger near the air onto the evaporator(s) for the various zones and let it run over several days. Going back to capacity, measuring compressor hours runtime in a 24 hour period would be a good start,cheers...

NoNickName
19-07-2010, 09:29 AM
Cooling capacity can be directly measured by multiplying the delta T across the coil times the airflow crossing it times the specific heat of air.
This value must be compared to the cooling capacity declared for the rack of compressors at those evaporating and condensing temperature.
If the two values vary considerably, then one of the two (or both) is wrong.

Peter_1
19-07-2010, 09:37 AM
Which specific heat NNN, that of the air in or the air out?
I know you know ;)

NoNickName
19-07-2010, 09:45 AM
Doesn't matter. between +40 and -40, specific heat of air is 1.005 kJ/Kg K. I personally never multiply by it, as it would only make a 0.5% difference (well below the precision of instruments).

That does not apply to IP system (because specific heat of air is not unitary in that system: 0.2375 Btu/(Lb °F))

norseman
19-07-2010, 10:31 AM
If the quality assurance is in the bottom line subjected to the product, it would be wise to use product temperature sensors. I handle some rapid freezers for blood products and the units use 16 sensors and the cycling off does not occur before the last of the product reach the wanted/demanded temperature. Such a way is at the outer end of controlling, but for sure a good assurance.
As we know, the air sensing by a single sensor would not give a full idea of all the space. (a room?) as this item is? I have been into quite some testchambers with a high airflow and a low area to handle still gives several degrees difference seen all over the space.
I had one +220 to -80c. At -80c we got 4c higher in the front end at this small unit. The air was forced to pass through a return air channel in the lower front too The conditioned air was given by a huge fan in the front of an evaporator in the rear end. It is not easy to create it exact all the way whatever many sensors added to this one. A honeycomb attached to the evaporator front later on made the air to stay in a better stream to the front instead of the "normal" hurricane who make a lot of the air to be sucked back to the evapoartor "too early".
If the goal is the assure that every products get/holds a certain temperature you have to at least measure the highest temperature inside the area and put your controlling sensor at this spot to achive that. You will find it in the upper area with the smallest airflow probably.
The most different temperatureI am into is LN2 storage tanks. You get -196c in the liquid at the bottom and just 1 meter up scary high as -120c and even higher. Not easy to make a confirm temperature on those ones:rolleyes:

Related to capacity test, I know that several of our suppliers use a heating source inside the units and calculate how much input used to rise the temperature to give that facts pretty exact.

HTRefrig
19-07-2010, 01:50 PM
Thanks all for your input...

With our day to day dealings with people with no true understanding of an eviroment inside a freezer its tough to change their thinking. Brian_UK thanks for the link that info is very usefull in the presentation im going to give ot QA.

In response to some questions:

Product temp sensors are on order for various locations throught my facility. Which hopefully will be added to the QA report.

My evaporator outlet temps are well below the ambient air temp throught my zones. Thus product temps sensors are my answer for the time being.


:D