Responding to the message of <001901c120ce$b74ccca0$7fe9b094@oemcomputer>
from phowgate@rsc.co.uk:
>
> Dear Kurt
>
> Question A. Typically regulations relating to frozen fish require that
> fishery products be held below -18ºC, but this is not cold enough for
> maintenance of good quality for more than 2-3 months. Fish products should
> be held at/below -30ºC if good quality is to be maintained. Product storage
> life, assuming good packaging to prevent oxidation and dehydration, is then
> more than 12 months. For a seasonal fisheries such as that for mackerel when
> products might want to be sold throughout the year, but caught over a short
> period, it is necessary to store at -30, particularly so in the case of oily
> fish such as mackerel. Common industrial practice in Britain, and from what
> I can gather, elsewhere in Europe, is to hold frozen fish at,
> nominally, -30ºC, though in actuality this is probably between -28
> and -30ºC. Andrew Strak drew attention to the effect of lowering of
> temperature on rate of deterioration, but the effect of lowering temperature
> might be greater than Andrew quotes - a halving of the rate of deterioration
> for each degree Celsius decrease in temperature. There is a not a great deal
> of good quantitative information on comparable rates of deterioration of
> fish in frozen storage at a range of temperatures in order to calculate the
> value of B in the Arrhenius equation, but my own analysis of what data there
> are suggests the halving rate is about 6ºC.
This fits in with the values I have of a Q10 of 3 to 3.5 (rate increase for a
10 C increase in temperature
>
> The cost of the extra refrigeration to maintain -30 rather than -18 is small
> given good store design and operation.
This is not true. According to refrigeration engineering texts and what I
remember from courses a 5F difference is a 10% energy cost so 12C is at least a
30 to 40% difference in running costs besides the insulation The insulation is a
small part of the cost. If the outside were at 25 C, in a freezer maintaining
-30, the driving force is 55 C while at -18 it is 43 C so there is a 20%
reduction in heat flow loss when running at the higher freezer temp. This would
translate into only a 20% needed increase in insulation thickness to run at the
lower temperature, this is a much smaller cost than the 30-40% energy costs.
The reason for the higher energy cost is thermodynamic. One need to look at the
COP (coefficient of performance) calcultions using the P-enthalpy or
Temp-entropy (Moillier) charts for the ammonia refrigerant.
There are extra costs of insulation
> for a -30 store compared with a -18 store, but these are negligible when
> amortized over the life of the store. The extra costs are recouped by the
> extra quality of product, if that can quantified.
I agree
> .
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