THE
USE OF COOLING TOWERS IN A DAIRY FOR PRE COOLING:
The
use of cooling towers for the removal of heat with water is not
a new technology; they have been around for approximately eighty
years or more. The application of a water-cooling tower in a dairy
for pre cooling of water is new. With stronger requirements on milk
quality from milk buyers and greater competition on a world market,
this has demanded that farmers source new advances in technologies
for higher quality milk with maximum return on capital. Hence the
application of the cooling tower for pre-cooling. |
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WHAT
IS A COOLING TOWER:
A
cooling tower transfers heat from water by bringing it in contact
with air. When the right mixture/ratio is used cooling of the evaporative
and sensible types takes place. A number of climatic conditions effect
the cooling performance of a cooling tower i.e.: wet bulb temperature,
dry bulb temperature and relative humidity.
WET BULB TEMPERATURE:
Is
measured with wetted wick on the bulb of a thermometer, (known as
psychrometer) The evaporation of water form the wick by creation of
air draft ("Slinging of the thermometer") provides us with
a wet bulb temperature. The drier the air temperature the greater
the evaporation of water from the wick and the lower the wet bulb.
Conversely the higher humidity in the air the less evaporation and
the higher the wet bulb temperature. Wet bulb temperature is also
known as the following: The thermo-dynamic wet bulb or the temperature
of adiabatic saturation. |
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DRY
BULB TEMPERATURE:
It
is the temperature measured by standard thermometer. Commonly given
in a weather report.
RELATIVE HUMIDITY:
Is
the ratio of the actual humidity of the air to the saturated humidity
at a given dry bulb temperature. Similarly humidity is also the
ratio of the existing vapour pressure to the vapour saturation pressure.
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RELATIVE
HUMIDITY=Ø= |
Ps |
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Pv |
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Where
Ps = |
Vapour pressure in the mixture |
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Pv
= |
Vapour
saturation pressure corresponding to the dry bulb temperature |
The
greater the percentage of moisture in the air the less heat we can
reject through evaporation.
What
is ambient wet bulb temperature?
It is
the temperature at which water vapour in the air condenses to form
fog or clouds.
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HOW
IS A COOLING TOWER USED IN A DAIRY:
Calculate
the volume of milk in a day milking to ratio to the volume of water
required to transfer heat. Most often this is a ratio of approximately
3:1 ratio of water to milk. Take the total water volume required into
storage tank. Total volume of the water in storage is then re-circulated
over the cooling tower. The more often this is performed the greater
the temperature drop of the water, therefore it is recommend a that
total volume should be circulated over the cooling tower minimum of
five times. This water is then pumped through a heat exchanger when
milking to remove heat from the milk before entering the vat. Therefore
the sizing of the heat exchanger is critical. Once the water has been
passed through the heat exchanger it is stored in the tank for further
use. Operation of the cooling tower is recommended at night in the
early AM, because of a number of advantages. |
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ADVANTAGES:
- Climatic
conditions improve greatly at night with lower air temperatures
hence the use of sensible cooling. Therefore reducing water temperatures
for the next days milking.
Example:
If a wet bulb temperature at night is 8oC and the dry bulb temperature
16oC It would be expected to receive cold water the next morning
at 8 - 9oC.
- Lower
cost on electricity with a cooling tower operating at night. The
fan motor on a cooling tower is normally smaller than the refrigeration
motor required to achieve the same temperature.
- Reduction
of power consumption for a refrigeration system.
- Reduction
of maintenance and servicing cost on the refrigeration system.
- Reduction
on water consumption.
- Milk
is cooled quicker therefore of a better quality to the buyer.
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How
is a cooling tower package selected:
When
selecting a cooling tower package for a dairy application, we usually
use a fundamental heat exchange transfer ratio formula of 3:1 of
water to milk.
Therefore
we calculate one days milking and multiply by three to find the
volume of water we require to take into storage. To achieve the
best performance results, efficiency and cost savings of this cooling
tower, we strongly recommend that the cooling process be performed
at night, when off peak power rates available and ambient wet temperature
bulb is likely to be at its lowest. Therefore if the ambient wet
bulb temperature is 6oC we would be able to supply water at 8oC.
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