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Hi to all
If we have a constant dimensions and load,say 2.4m*2.4m*4.7m walk in freezer ,with 8tons frozen chicken, what is the steps or changes that should be taken to reach -25 ,-30 ,-40 if the system with :
1.air cooled condenser
2.amb. temp. 45C
Notice:first with out using subcooler or two stage comp .
Your question is not quit clear fro me but I will make anyhow a try for you.
The final goal is reaching -40°C.
As you start the compressors, maintain an as low as possible outlet temperature. Try to remove as fast as possible the latent heat out of the meat so that the composition remains the same. If you do it to slow, then ice-crystals will form.
Maintain an as low as possible condensing temperature.
As you reach the -40°C, then you will need to have some sort of unloading device on you compressor because you will run with the 'classic' refrigerants close to atmospheric pressure. Especially because the heatload on the evaporators will decrease the more you reach -40°C.
But why gong that low -40°C for chicken meat?
I think that preserving meat at -40°C is almost the same as at -20°C but freezing to -40°C will cost you a lot of energy.
You can insert a probe in a bigger portion of the meat (in a breast just to the bone inside it) and let this probe control the freezing process. Eventually 2 probes in 2 duifferent pieces and both must trigegr the controller. First blowing air as cold as possible (-35°C blowing, evaporating at -43°C for R404a) and as soon the probe (or the 2 probes) senses -20°C, switch the compressor or pack to a higher evaporating temperature with a low DT (5 to 7K) at a much lower airspeed to maintain a stable roomtemperature of -20°C.
That's anyhow how we doing it.
Design very well teh air circulation and makes that there's enough air circulating inside the freezer, eventually with additional fans.
To attain -40oC without using a two stage compressor, is a bit tough job. You must have an additional cooling in the form of providing a fan over the head and a Discharge Temperature Controller with vapor injection. The air cooled condenser will have a high discharge pressure and as such it is better to use a oversized condenser.
We have just completed a project with air cooled condensers for maintaining Room Temperatures at -35oC and another at -40oC. It is also very important that you select the correct TEV.
Let me give you more information first:
1.the nominal capacity is 5 tons for the compressor,evaporator and condenser respectively.
2.the displacement for the compressor is 19 m3/h
3.R404a
4.at these considerations ,the freezer temp. reach round -21c. at amb temp of 45.
Now , to advance to -40 , are we going to do changes by:
1.using comp with bigger displacement.
2.with bigger condenser or not
3.with smaller TEV orifice or not
I hope its now less foggy.
If the capacity of the system is +/- 5 tons right now and you are reaching -21°C then changes are required to reach -40°C.
The colder temperatures in the freezer will increase the cooling load slightly.
The big issue is, if you need at least 5 tons of cooling (all of the time) the lower evaporating temperature will greatly reduce the capacity of the system.
This means the entire refrigeration system capacity should be increased to provide 5 tons of cooling at -40°C.
I do have to agree with the question Peter said. Why do you want -40°C for freezing chicken?
I think if you can maintain -25°C (air temperature) and provide a good airflow around the chicken pieces, it would freeze very well.
The good airflow is just as important as the temperature.;)
The total load must have been worked out for - 40oC freezer. Yes I do agree that for storage of frozen chicken you need -25oC. Maybe he is trying to install a Blast Freezer at - 40oC to freeze the chicken.
However if the load has been worked out, what do you mean by 5 Ton capacity. Is it 5TR at - 45oc Te and 54oC Tc. If that is the case then it is very important that the selection of TEV be made properly. What make of TEV is being used. The most important is that additional cooling should be there for the compressor to reduce the discharge temperature. The ideal system is to have a DTC with a fan on top of the compressor. As stated earlier we have just completed a similar project with air cooled condenser and Copeland Semi hermetic Compressor. The room is maintaining - 40oC temperature.
My Inquiry,in another way,comes like this:
1.If we are going to improve the system mentioned,to pull the temp from -21 down deeper. Is replacing the comp of 19 m3/h displacement with another of 30 m3/h can achieves this .
2. What are the suggested changes in:
a. Condenser(what is the benefits of TDC in 45 amb temp)
3.Are we going to use bigger or smaller orifice to the TEV?
any other changes or suggestions?
Hi Afak,
To offer you the answers, I would like to know what is the calculated load. Then what is the compressor capacity at -45oC Te as your room ltemperature has to be maintained at - 40oC. Once you state these datas and the compressor model and make, it will be easier to select the TEV. What TEV are you using.
The system has to be designed for bringing the Room Temperature to -40oC as desired by you. There is no system that you can bring the temperature to - 21oC and then change the controls to reach -40oC. The complete system has to be designed and components selected to attain the required temperature.
Thank You Samarjit Sen,US Iceman,Peter 1
Indeed I have slightly change my inquiry as in my last message:
For example, systems work on R22 hardly overtake ,here, the bariar of -18 for air cooled condenser : can we cross this for deeper temp(say -25) by change the compressor with another with fearly higer displacement?.Please read again the quote above carefully.<!-- / message -->Quote:
My Inquiry,in another way,comes like this:
1.If we are going to improve the system mentioned,to pull the temp from -21 down deeper. Is replacing the comp of 19 m3/h displacement with another of 30 m3/h can achieves this .
2. What are the suggested changes in:
a. Condenser(what is the benefits of TDC in 45 amb temp)
3.Are we going to use bigger or smaller orifice to the TEV?
any other changes or suggestions?
Dear Afak,
To answer your question, first it is very important to know the refrigeration capacity of the compressor. It is quite possible that the displacement of 19 m3/hr is suffecient. In case you are unable to attain the desired temperature there could be the reason of wrong capacity plant being used due to which you are not getting the temperature. A compressor with 30 m3/hr is definately a higher rated compressor. If you replace the 19 to 30 m3/hr compressor, you will need to change the complete system ie the Evaporator, Condenser and the TEV.
At present I am unable to understand as to what is the problem and what and why are you thinking in terms of changing the compressor.
To be able to answer your problem, it is required that more details be provided of the system.
Dear afak,
If I may let me try to state your requirements.
You need to operate at lower temperatures (maybe -25°C or colder).
What is the cooling load (chicken & freezer heat gain through walls, air infiltration, motors, lights, etc.) for the subject freezer?
Let's forget the compressor displacement for now and concentrate on other items.
If the cooling load for the freezer is 5 Tons of Refrigeration (that is a number I am using for my reply here as an example) when the freezer is at a higher inside temperature, the cooling load will increase if you lower the freezer temperatures. There will be an increased temperature difference across the freezer wall.
For the existing system in the current operating conditions the refrigeration system is selected to provide 5 Tons of Refrigeration capacity.
Since you want to operate at lower freezer temperatures temperatures the cooling load may increase to 6 Tons of Refrigeration (due to the colder temperatures in the freezer you want).
Now the entire refrigeration system has to be selected for 6 Tons of Refrigeration capacity at a much lower evaporating temperature.
Therefore, to go back to your question about increasing the compressor displacement the answer is YES. The compressor size will have to be increased to provide the higher cooling capacity needed for the lower air temperatures.
The TXV orifice will become slightly smaller because, with the lower evaporating temperature required for the lower air temperature the differential pressure across the orifice has increased (assuming the condensing pressure is the same as before).
Since the cooling capacity is now increased and a larger compressor is now needed the condenser also becomes larger as does the evaporator.
Does that help you?
Hi! Afak
I went through all the posts you have posted. Your data are incomplete to give you a stable solution. You want the forum members to calculate from a mid way just on assumptions. You can not calculate a cooling load for a particular quantity of a product with an available condensing unit. First you have to calculate the load & select the con unit or for given con unit you have to match the product load.
Taking into consideration of the above facts you have to furnish the following data to give a correct solution.
1. Inlet Temp of Chicken Not given
2. Weight of chicken 8 ton
3. Desired storing -40 deg C
4. Insulation Not given
5. Ambient Temp 45 deg C
6. Time period to pull down Not given
7. Size of the room Given<O:p</O:p
Further are you loading the 8 ton load at once or in stages if so how? Once you give the above data we all can let you know whether your condensing unit capacity is sufficient or not for 8 tons chicken. <O:p</O:p
Suny<O:p</O:p
Dear US Iceman,
Dear guys,
I try to quote from US Iceman and discusse on.
For the condenser, there is slightly higher load only in the very beginning starts of the system and will not last too much then will inter the normal range of the load when the suction decrease gradually.Quote:
Now the entire refrigeration system has to be selected for 6 Tons of Refrigeration capacity at a much lower evaporating temperature.
If we keep:1. the same orifice 2. with the same orifice adjusting,we sure need bigger evaporator.But as we gain higher capacity with lower suction pressure to the same evaporator we need to tighting the orifice or change it with smaller.Less evaporating pressure , with gain in capacity ,or at least no loose in it ,will leads to lower freezer temp. I am expecting that changing the comp. with another one bigger comp. (20- 25 %) in freezers would need to change the orifice and may be slightly change the ***** charge to help accurate adjusting the system to the new evaporating pressure and capacity . I think there would be no harm to the comp. because the modestly stable load of the freezers.Quote:
The TXV orifice will become slightly smaller because, with the lower evaporating temperature required for the lower Air temperature the differential pressure across the orifice has increased (assuming the Condensing Pressure is the same as before).
Hi Afak,
US Iceman has stated the facts in a very nice way. But it appears that you would prefer to keep changing your equipments and adjusting them as the temperature goes down. Yu have to select your equipments on the asis of your requirements and do not have to keep adjusting the controls.
Samarjit,
I received a very nice PM from afak which helped me to better understand his problems. I think the major problem is a lack of available supplies and equipment in Iraq. So... he is trying to make this work under less than ideal conditions.
afak, I must apologize for not responding to your PM yet. I have read it, but have been very busy with immediate problems of my own. I do hope you understand. I will try to answer you in the next day.
US Iceman