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Dairy Processing Handbook/Chapter 6.9 173
Tanks
Tanks in a dairy factory are used for a number of purposes. The
sizes rangefrom 150 000 litres for the silo tanks in the reception
department down toapproximately 100 litres for the smallest
tanks.
Tanks can generally be divided into two main categories
according tofunction:
Storage tanks Process tanks
Storage tanksSilo tanksSilo tanks for milk reception belong to
the storage category and have beendescribed in Chapter 5,
Collection and reception of milk. They vary in sizefrom 25 000 to
about 150 000 litres and the wetted surfaces are made of stainless
steel. They are often placed outdoors to save on building
costs.
In these cases, the tanks are insulated. They have a double
shell with a
minimum of 70 mm mineral-wool insulation in between. The outer
shell canbe of stainless steel, but for economic reasons, it is
usually made of mildsteel and coated with anti-corrosion paint.
To make complete drainage easy, the bottom of the tank
slopesdownwards with an inclination of about 6 % towards the
outlet. This is astatutory requirement in some countries.
Silo tanks are fitted with various types of agitators and
monitoring andcontrol equipment.
The number and size of the silo tanks are determined by such
factors asthe milk intake per day, the number of days per working
week, the number
Fig. 6.9.2 Silo tank alcove with manhole and motor for propeller
agitator.
Fig. 6.9.1 Layout of outdoor silo tanks with their manholes in
alcoves in the walls of a covered control station.
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Dairy Processing Handbook/Chapter 6.9 175
eccentrically-pivoted roller that operates the inlet valve on
the tank. As thefloat moves downwards or upwards with the liquid
level, the valve is openedand closed respectively.
If the pump draws more from the tank than flows in at the inlet,
the leveldrops and the float with it. The valve opens and lets in
more liquid. In thisway, the liquid in the tank is kept at a
constant level.
The inlet is located at the bottom of the tank so that the
liquid entersbelow the surface. Consequently, there is no splashing
and, above all, noaeration. Any air already present in the product
on entry will rise in the tank.Some deaeration takes place. This
has a favourable effect on the operationof the pump, and the
product is treated more gently.
The balance tank is often included in a recirculating system
where liquidis returned for recycling, e.g. as a result of
insufficient heat treatment. In thiscase, a temperature indicator
actuates a flow diversion valve, which directsthe product back to
the balance tank. This causes a quick increase in theliquid level
and an equally quick movement of the float mechanism to closethe
inlet valve. The product then circulates until the fault has been
repairedor the plant is shut down for adjustment. A similar
procedure is employedfor circulating cleaning solution when the
line is cleaned.
Fig. 6.9.6 Balance tank for constant inlet pressure to the
pump.
Fig. 6.9.7 Balance tanks are available in different sizes.
