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  1. #1
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    Volume Flow Rates



    Hi.

    I am trying to het a flow rate in a plate freezer. The total refrigeration capacity is 152720 kilocalories per hour (kcal/h).

    Accordingly to the specifications for Ammonia the flow rate is 135 Litres/hour/1000Kcals.

    Does Litres/hour/1000Kcals = Litre / (1000Kcals x Hour) ?

    How to calculate the required flow rate from the total refrigeration capacity is 152720 kilocalories per hour (kcal/h)?



  2. #2
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    Re: Volume Flow Rates

    How to calculate the required flow rate from the total refrigeration capacity is 152720 kilocalories per hour (kcal/h)?
    You have to give more information. Which rate: circulating rate or the overfeed rate and at what operating conditions?

    You have to respect the circulation rate informed by the manufacturer of the plates.

    You should check manufacturer’s literature or read older posts on this topic like:

    http://www.refrigeration-engineer.co...p/t-13313.html

    Now as to the math with no assurance these are correct numbers, look at it this way:

    In one hour you need to extract 152720 kcal and to do that you must circulate 132 litres per 1000 Kcal then you have to circulate (152720 x 132)/1000 = 20159 litres.

    As your calculations are in one hour you need 20159 liters/hour.
    Well, did anybody ever dream of calling Aramis a coward? No, certainly not!

  3. #3
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    Re: Volume Flow Rates

    You need to establish the internal volume of each plate, the circuiting inside the plate and if flow orifices are fitted in the plate or at the supply header. Then you can calculate the over feed rate, dependant on supply temperature of ammonia. Each plate has to be fully flooded for maximum performance and equal cross plate temps., the wet suction return and suction accumulator is where the liquid is boiled off and brought back down to design supply temperature, for pumping back to plates. Once you establish the circulation volume ammonia is lighter than water, and most pump manufacturers quote flow rates based on water, you then have apply the weight factors. Hermetiqe Pumpen ( spell check ) have a good web site to do calcs on line.
    The lower the supply temp then lower the overfeed rate, say 5 : 1 at -40'C.
    You state load at 152720 Kcals/hour, that is around 177.60 Kw refrig., but at what temperature. bare in mind that the outer plates horizontal or vertical are effective on one side only, that can stuff calcs up.

    magoo

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    Re: Volume Flow Rates

    Quote Originally Posted by Polisie View Post
    Hi.
    Does Litres/hour/1000Kcals = Litre / (1000Kcals x Hour) ?
    Somehow the first part of my previous post got lost, here it goes again:

    Both sides of the above equation are equal.

    But I think there is a mistake in the units for specifying volume rate which are Liters/kW or

    (Liter x Hour)/1000 kCal or Liter/(1000 kCal/h).

    This wouldn't be unusual in translations or technical papers written by marketing staff.

    Like Magoo says you need to give more information to check these calcs.
    Well, did anybody ever dream of calling Aramis a coward? No, certainly not!

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    Re: Volume Flow Rates

    Thank you.

    I work on the following:
    Mass to be frozen = 30.5 Metric Tons
    Product = fish Fillets
    Freezing time per day = 3hr + 15min for loading and unloading

    From this it worked out to be 8 cycles per day.

    The weight that will be frozen at each cycle will be 3813kg/cycle.

    So I will require 5 x 30 stations plate freezers (vertical plate freezers).

    My refrigerant will be Ammonia that will enter the plates at -18C and the fish is +10C.

    From what I calculated (which I know I made an error somewhere but do not know where) I get a total refrigerant capacity of 47.21kw. For 1 x 30 station.

    The plate freezers will have headers.

    According to the suppliers specifications for ammonia the pump circulation rate will be 67 litres / Hour per 1000 K Cals.

    How will I know if my refrigeration capacity is adequate enough?
    And how do I calculate the pump circulation rate as the litres / Hour per 1000 K Cals confuses me.
    , because i know how to convert kW to kCals/h.

  6. #6
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    Re: Volume Flow Rates

    Just want to correct something, the product Temp in is +10C and after the freezinf the core temp must be -18C. The evaporating Temp -38C.

  7. #7
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    Re: Volume Flow Rates

    work on the following:
    Mass to be frozen = 30.5 Metric Tons
    Product = fish Fillets
    I know it is difficult to catch only one species but the specific heat depends on the type of fish.

    Ill suppose 101 W/kg from +10 to -18C.

    Freezing time per day = 3hr + 15min for loading and unloading
    Time depends on many factors and you dont give any information to make assumptions. So freezing time is something you will have to determine.

    From this it worked out to be 8 cycles per day.

    The weight that will be frozen at each cycle will be 3813kg/cycle.

    So I will require 5 x 30 stations plate freezers (vertical plate freezers).

    My refrigerant will be Ammonia that will enter the plates at -18C and the fish is +10C.
    So far OK. This gives me 3813 kg/cycle x 101 W/kg = 385 kW
    for 30 stations => 12.8 kW/station.

    From what I calculated (which I know I made an error somewhere but do not know where) I get a total refrigerant capacity of 47.21kw. For 1 x 30 station.

    The plate freezers will have headers.

    According to the suppliers specifications for ammonia the pump circulation rate will be 67 litres / Hour per 1000 K Cals.
    I posted all my calculation so that all can help and check if they are correct.

    According to this 385 kW = 331 (1000 kcal/h) * 67 Liters/h (1000 kcal) = 22177 Liters/h =22.2 m3/h

    How will I know if my refrigeration capacity is adequate enough?
    Density of ammonia @ -38C saturated = 688 kg/m3
    Enthalpy of vaporization = 1383 kJ/kg
    1h = 3600s

    So 22.2 m3/h x 1/3600 h/s x 688 kg/m3 x 1383 kJ/kg = 5568 kJ/s = 5568 kW > 331 kW

    This means that you will evaporate only 1/16.8 th of the pumped liquid or 1318 Liters/h.

    You have more than enough capacity.

    And how do I calculate the pump circulation rate as the litres / Hour per 1000 K Cals confuses me.
    , because i know how to convert kW to kCals/h.
    1. Just want to correct something, the product Temp in is +10C and after the freezinf the core temp must be -18C. The evaporating Temp -38C.
    It confuses me too: the units are not correct and these numbers are not usual for a plate freezer.

    The only person that can assure how to do the calculations is the manufacturer of the plates or its rep.
    Well, did anybody ever dream of calling Aramis a coward? No, certainly not!

  8. #8
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    Re: Volume Flow Rates

    each freezer at 47.21 Kws by five plates freezers is nominal 237 Kw nomnal 200,000 Kcals divided by 1000 = 200 multiply by 67 litres per hour = 3.73 litres per second liquid supply @ -38'C or 0.75 l/s to each setup. If supplier has stipulated the flow rate at a particular supply temp they have already taken into account the overfeed rate, double check though with supplier.
    The 15 minute thaw/ hotgas and unload plus reload is a bit short. Remember a 30 station setup is 32 plates. You will need wet suction regulation at each setup for thawing/hotgasing.

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    Re: Volume Flow Rates

    Thank you so much! I really appreciate all your input.
    I will let you know if my boss agree with my excel plate freezer selection calculations.

  10. #10
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    Re: Volume Flow Rates

    Hello Polisie.
    the first challenge from your boss would be your option/selection of vertical plate freezer for fish fillets. He has probably given you a curved ball serve to check your application and general knowledge of the business.
    Horizontal plate freezers are preferred for fish fillets, either in formed trays or pre filled cardboard non corrigated boxes, perhaps you should rework the whole project. Generally vertical freezers are for block freezing offal and the like.
    Just a thought.
    magoo

  11. #11
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    Re: Volume Flow Rates

    Thank you magoo, Im busy revising my selection and calculations.

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