Quick freezing of fresh products

To everyone who is proficient in quick freezing rooms

I am working as an engineer in russian refrigeration company.

I have some questions on quick freezing rooms. At present it is necessary for me to design freezer room for quick freezing of ravioli. I am not proficient in designing such kind of rooms, so it is interesting for me:

1) What are your recommendations about selection of main components of refrigeration plant:
compressor, air cooler, air condenser?

Dimensions of room 3 x 2,5 x 2 > volume is about 15 m3
Product inlet temperature +20 o C (68F)
Product outlet temperature –12 o C (10F)

100 kg of ravioli
Necessary time for freezing 1 hour
Temperature in cold room: -35 o C (-31F)

I am going to use 2stage compressor (DORIN). Air cooler and air condenser Lu-Ve.

2)What should be the meaning of freezer room temperature?

3)What should be the difference between room temperature and evaporating temperature in such kind of freezer?

4)What is the temperature I should choose compressor capacity at?

5)What are the criteria of choosing of evaporator (air cooler)?

6)What should be fin spacing of air cooler?


Russia, Moscow September 13 2002

Hi, Maxim:)
What info do you require, a plant capacity?
Why do you need -35 deg C room temp, capacity and proper air flow/ducted air flow has a bigger effect on product temp than low room temps.

In answer to some of you questions.
1/ Main components, this rather depends on refrigerant selection, R404a would be suitable and in selecting R404a a single stage compressor could be used.
2/please re-phrase
3/Return air and refrigerant evaporation 6-8K td.
4/Dependant on pressure drop in the suction line, usually 2-4K lower than evaporation.
5/ Required air off velocity;Cleanable specification if the product is un-wrapped;physical size, will it fit in the room(you should design the room to fit the cooler/false ceiling and racking not the other way arround).
6/Dependant on the product, whether it is wrapped or un-wrapped, usually 6mm upwards.

I hope this helps you, post again.
Regards. Andy:)

Hi, Andy Thank you for reply

The main idea is following:
the lower temperature is maintained in the cold room the quicker freezing process occurs. And the quicker freezing process occurs the better product quality .

This fact forces engineers to use more powerful equipment, that leads to decreasing temperature inside the cold room to low values (-35 ..-40 deg C) ("shock" freezing).

1) What are your reccomendations about temperature in the freezing room ?

Next question

2) For instance: product inlet temperature +20 deg C, product outlet temperature is -12 deg C (inside the body).

Calculated load is 10 kW.

At what temperature should the compressor capacity be chosen?

I mean - suction temperature = room temperature - td, but ...

What is the correct room temperature in case of freezing?

3) And if room dimensions are too small and it is impossible to place air cooler of required size inside, then we have to reduce air cooler size that results in increasing td.

Is it necessary in this case to use compressor with less capacity
(and with increased time of freezing therefor) or it is possible to set air cooler with insufficient capacity, that results in increasing td


Moscow September 15, 2002

Regards Max

Gday Maxim,
I have designed and installed snap freezers for the fish processing industry for the last 10 yrs ie: fresh fillets,
sardines,shellfish etc etc.I am interested in your product load calculations ie:specific heat above and below freezing(kj/kg/-K) latent heat kj/kg approx freezing point of product etc etc.
Also room panels material , thickness etc.

Ambient temp? Any infilration loads? Product packaging?etc etc.

I can only suggest more information be posted.

Regards, Dave

Hi, Maxim:)
dave is right about breakdown your loading down a little. The following would be useful.

1/ Product load
2/ Fabric/wall load
3/ Air infiltration
4/ Personell, lights and fans.

Your highest load is your product, but the air infiltration can be high on some application, where the process is continuous, where product is added and removed from the freezer as required. Don't forget your fan load, this will be a good % in a blast freezer.
As for the room temp I would design around -32 to -34 deg c with the evaporation 8K below, 6K for the td and 2K for suction loss, dependant on the suction pipe run ofcourse.
Even that room temp may be over kill, remember I said air distrubution is the key, direct your air through the product otherwise the air will short circulate, rendering the freezer ineffective.
Also with regards to the cooler selection a smaller cooler will force the evaporation down reducing the compressor capacity for the given room capacity, this will lead to a large compressor having to be employed, possibly a two stage compressor for the lower evaporation. After all this you will have a system which is less effecient all because the system is being manipulated to meet the room size. I would design the system, then build a room to suit.
Having said that it may be possible to fit two smaller coolers in the room back to back, this would also increase the air velocity in the room which would be desirable.
Regards. Andy.

Hi, Andy. Thank you for your reply.


Dave,

Here are the conditions:

1) Dimensions Length 8.2 ft (2.5 m)
Width 7.2 ft (2.2 m)
Height 6.6 ft (2 m)
Volume 388 ft3 (11 m3)

2) Ambient temperature 95 deg F ( 35 deg C)
relative humidity 60 %

3) Room temperature - 30 deg F (-35 deg C)

4) Insulation Walls, floor and ceiling Polyurethane panels 4.7 in ( 0.12 m)

5) Doors
- number of doors : 1
- average door width : 3 ft (1 m)
- average door height: 6 ft (2 m)
-hours open per 24 hours : 1 hour

6) Number of air cooler motors : 4 x 175 W

7) Product ravioli

Freezes at 28 deg F
Specific heat above freezing : 3.15 kJ/(kg K)
Latent heat : 239 kJ/kg
Specific heat below freezing : 1.68 kJ/(kg K)

Weight 220 lb (100 kg)
Product inlet temperature: 68 deg F (+20 deg C)
Product outlet temperature: 10 deg F (-12 deg C)
Time of freezing : 1 hour

Product packaging small cart

Heat calculation:

1) Wall load -> 1716 BTU/h (= 502 W) calculated by computer program (Calc - Rite)

2) Infiltration load -> 8066 BTU/h (= 2363 W) calculated by computer program (Calc-Rite)

3) Product load -> 9029 W = 30815 BTU/h

100 kg * 3.15 kJ/(kg K) * 22 K/ 3600 sek = 1.925 kW = 1925 W = 6570 BTU/h

100 kg * 239 kJ/kg / 3600 sec = 6.638 kW = 6638 W = 22655 BTU/h

100 kg * 1.68 kJ/(kg K) * 10 K/ 3600 sec = 0.466 kW = 466 W = 1590 BTU/h

4) Miscellaneous load

4 air cooler motors x 170 W = 680 W = 2320 BTU/h

TOTAL LOAD REQUIREMENT 42917 BTU/h = 12 574 W = 12.5 kW


with regards this freezing room
I have following compressors:

compressor A : ref. capacity 12.5 kW (evaporating temperature -10 deg C (14 deg F))
compressor B : ref. capacity 12.5 kW (evaporating temperature -15 deg C (5 deg F))
compressor C : ref. capacity 12.5 kW (evaporating temperature -20 deg C (-4 deg F))
compressor D : ref. capacity 12.5 kW (evaporating temperature -25 deg C (-13 deg F))
compressor E : ref. capacity 12.5 kW (evaporating temperature -30 deg C (-22 deg F))
compressor F : ref. capacity 12.5 kW (evaporating temperature -35 deg C (-31 deg F))
compressor G : ref. capacity 12.5 kW (evaporating temperature -40 deg C (-40 deg F))
compressor H : ref. capacity 12.5 kW (evaporating temperature -45 deg C (-49 deg F))

What compressor should I choose for this freezing room ?




as regards the temperature inside the freezing room I’d like to cite some quotations:

<<The typical temperatures for a quick commercial freezing, nowadays, are surely located between -31 deg F ( -35 deg C ) and -40 deg F ( -40 deg C ), depending on product , thickness and packing or the required freezing velocity for that product.

In plate freezers, by contact, the typical temperature is normally -35 deg F ( - 37 deg C ).
>>
THE VERY BASICS OF FOOD PRESERVATIONS by Walter Gameiro
<< Freezing time is defined here as the time taken for the temperature of the warmest part of the fish, usually the centre, to be reduced to -20°C.
The recommended storage temperature for frozen fish in the UK is -30°C and, to ensure that the fish is frozen quickly, the temperature of the freezer must be lower than this. Thus when the surface of the fish is at freezer temperature and the warmest part is down to -20°C, the average temperature of the fish on removal from the freezer will be close to the required storage temperature of -30°C.>>

<<Freezing time depends mainly on
freezer type freezer
operating temperature
air speed in a blast freezer
product temperature
product thickness
product shape
product contact area and density
product packaging
species of fish
Operating temperature
The colder the freezer the faster the fish will freeze, but the cost of freezing increases when the freezer temperature is reduced, and in practice freezers are usually designed to operate a few degrees below the required storage temperature of the product. For example, plate freezers usually operate at about -40°C and blast freezers at about -35°C to freeze products for storage at -30°C. >>

Hi Maxim,
The reason for my asking about the specific heat loads etc for ravioli was that i had no idea what that would be. As you know If you stuff this figure up it will have a large inpact on your final heat load.As regards to SST temp for condenser unit I normally calc @ -35c (this figure depends on application).I try to apply the KISS PRINCIBLE( Keep It Simple Stupid) for myself when selecting refrigerant,compressors etc on low temp.ie single stage semi hermetic-R404a.On calculating heat loads I do have software to do this but always do it the old fashion way first to check and double check my final calc.Does your software give 10% safety figure? Any way Maxim good luck and if you need any advice etc Just ask.

Regards,
Dave

Hi Maxim,

WHO THE HELL IS WALTER GAMEIRO


Bye Dave

Hi, Maxim:)
what type of product is ravioli I assumed it was a pasta, are you telling me that it is fish based or a mix?
Bread dishes should not need to be refrigerated to -35 deg c.
I would chose my compressor baised on a -40 capacity, with a room temp between -30 and -32 deg c.
Regards. Andy.:)

Good day, Andy and Dave.

Is freezing 1000 kg of fish / 10 hours equal to freezing 100 kg of fish / 1 hour? Because calculation gives the same load in both cases.

Or is it possible in freezer, that designed to freeze 1000 kg /10hours, to freeze 100 kg /1hour.

Hi Maxim,:)
Sounds like your still having fun with your heat load calcs.Your question in regards to 1000kg product pull down in 10 hrs same as 100kg in 1hr is the same.Unfortunatly no it does not work that easy, i wish it did. A crude calc would be about 20% more capacity required for the 1hr pull down time.
I have found over the years it is best to sit down with your client and work out a realistic pull down time for a product and at what running cost .Is the product value high or low for the size of unit running over a period of time.Many of my clients eyes light up when explained the cost savings in regards to unit size and pull down time.My last client wanted 1000kg of fish product pulled down from 3c to-18c in 6hrs .when explained the $1000s saved to do it in 12 hrs he agreed that 12 hrs was better than 6hrs.If your heatload software has been downloaded from internet i would email them and ask if the software is ok for blast- snap freezing calcs or read any info that comes with it.

Regards dave:)

Hi vishal here,
Does the latent heat of fish or any other product change according to the capacity?

Questions,
Are you freezing as a batch only (no storage) 100kg in freeze 100Kg out.
Very important!
How are you going place the product on side the freeze.
Best is many shelves with very thin layers, and an air gap between.
Air velocity of product needs to be in excess of 5M/S
Allow for floating SST (freezing point likely to be -2 to -5C)
Condenser size most critical, either size for SST -10C or install CPR
Defrost, hand intiated during unloading (temp terminated)
Rate duty at -28C, but allow system to run to-35C