Tímarit Verkfræðingafélags Íslands - 01.12.1967, Blaðsíða 100

98
TÍMARIT VFl 1967
heating plates is a medicated oil, as excessive
pressure in the plates as in the case of water can
thereby be avoided. The oil is recirculated but a
sufficient amount of hot oil is added to keep the
temperature, which at any moment is demanded
by the AFD-amatic system, following the optimal
temperature curve by cam control. The hot oil
tank is kept at the desired constant temperature
by means of an automatic oil fired heater.
Great emphasis has been laid on the design of
the condenser for the vapours from the products
- the so-called vapour trap.
This vapour trap has been built into the lower
sector of the cabinet and divided into two
separate halves. There is ample passage for the
vapours from the cabinet to the vapour taps but
a hydraulically operated door which can travel
from one side of the cabinet to the other can
alternatively seal either half of the vapour trap
from the cabinet. The movement of the door
is automatically controlled by a pre-set timer.
When one half of the vapour trap has been
in operation so that and adequate layer of ice
has formed on the ammonia-cooled tubes, the
door seals off this section, leaving the other half
free for operation.
The closed vapour trap is thereafter auto-
matically connected to the vapour part of a
water holding tank, whereby the water will
evaporate and melt the ice on the tubes of the
vapour trap. This melting takes place at low
pressure and temperature and very little
additional load is applied on the vacuum system
when the vapour trap is reconnected, as no air
is admitted during the melting. The necessary
heat for evaporation from the water tank is
supplied by hot ammonia gas, whereby a saving
in the operation of the refrigeration plant is
achieved.
As the defrosting of the vapour trap can take
place at any time during the operation, you will
see that we have here a continuously working,
self-cleaning, automatic vapour trap built into
the cabinet, whereby space requirements are
lowered and erection greatly facilitated as
compared to the former outside vapour traps.
Vacuum systems
Improvements in the vacuum systems have
also taken place. Whereas steam ejectors were
only a few years ago the reliable „working horse“
of the industry with oilsealed pumps as the
alternative, these are both being replaced today
by a system consisting of a Rootes pump and
water sealed pump, the latter being combined
with an air injector. The Rootes pump will com-
press from 0,5 Torr to abt. 6 Torr, whereafter
the other pump will compress to the atmosphere.
The use of this system has been facilitated by the
fact that modern cabinets are built so tight
that minimum air-leakage can easily be dealt
with by the air injector system.
Only for products like coffee, demanding a
pressure below 0,2 Torr, a system of two Rootes
pumps in combination with the air-injector
watering pump is required.
You will notice that in using this vacuum
system in connection with the oilheating of the
heating plates, we have completely done away
with the necessity of installing a steam boiler in
the plant. This piece of equipment is replaced
by an inexpensive, automatic oilheater, needing
practically no attention.
A central control panel contains all instruments
and switches for starting and operating the
plant at a human fingertip. When the operation
is finished the vacuum is broken by flushing with
nitrogen, whereafter the trolleys are conveyed to
the packing station for filling the products into
bulk containers, cans or pouches.
Yes. Freeze-drying is a new industry, but it
has developed at a breath-taking pace since its
break-through only six years ago. It is certainly
an industry which has come here to stay.
Útdráttur
Hagnýting frostþurrkunar í iðnaði er ekki eldri
en 6—7 ára, en hefur aukizt hratt og er 1965
áætluð 20.000 tonn af ýmis konar varningi.
Stutt skýring á vísindalegum grundvelli frost-
þurrkunar er eftirfarandi. Suðumark vatns lækk-
ar við lækkaðan þrýsting. Við 4,6 mm Hg-þrýst-
ing (4,6 Torr) sýður vatn við 0°C. Ef þrýsting-
urinn er lækkaður frekar, þá frýs vatnið. Ef hita
er veitt að ísnum við slíkar aðstæður, (þrýsting-
ur lægri en 4,6 mm Hg) þá bráðnar ísinn ekki,
heldur gufar upp.
Þegar matvæli eru frostþurrkuð, þá eru þau
fyrst fryst, og síðan sett í hólf, þar sem mögu-
legt er að halda þrýstingi lægri en 1 Torr (1
mm Hg). Þegar hita er síðan veitt til matvæl-
anna, gufar ísinn upp, og þar sem gufur bera ekki
með sér vatnsleysanleg efni, þá breytast mat-
væli ekki að öðru leyti en því, að vatnið hverfur.
Kostir frostþurrkunar eru þeir, að gæði matvæl-
anna eru meiri en við nokkra aðra þurrkunar-
aðferð, hvað útlit, bragð o.fl. snertir. Geymslu-
þolið er mörg ár og án kæligeymslu.