Purpose of Cooling Tower
If
you look at the of most high rise commercial buildings, or even around
industrial sites and power stations , you'll notice these large boxes with a
fan on top . these are cooling towers and their purpose is to reject the
unwanted heat form a building or process.
To help understand how Cooling Towers work, let's look at an
example of a typical , common Cooling Tower found on a city office type
building.
These Cooling Tower are known as ( open ) or ( wet ) Cooling
Towers because the water in the pipework leaves the pipe (system) when it
is sprayed inside the cooling tower, purposely soaking the inside of
the tower to cause evaporation and heat loss. Water will be able to
leave this circuit so it is referred to as (open) and the spray of
water to soak the inside of the tower is why it is referred to as ( wet ).
You can also find (closed) or (dry) cooling
towers. This type does not spray the
condenser water and the water stays within a sealed pipe at all times.
Therefore water can not leave the system and it is known as a ( closed )
type .
The
Cooling Tower receives warm water from the chiller at a temperature, for example, of 32°C (89.6°F). This
water is known as the condenser water because it picks up it’s heat in the
“condenser” of the chiller. The Cooling Tower must cool this water
down to around 27°C (80.6°F) so that it can return to the chiller and pickup
more heat.
How Cooling Towers work
In this topic, we will look at how a cooling tower operates. This
will cover the basic
workings of the most common type of cooling tower “wet” or “open”
which you will find on most commercial high rise and industrial sites .
Why are open cooling towers the most common ?
Open or wet cooling towers are the most
efficient way to reject heat from
the cooling system because the water evaporates to carry the heat away.
This results in a loss of water from the cooling system and so it is not
suitable for all locations and system designs .
How do they work ?
Consider the cooling tower on top of a typical
office type building. A centrifugal
pump moves water, known as “condenser water”, between the chiller in the
basement and the cooling tower on the roof. The chiller adds heat to the
condenser water and the cooling tower cools it down by rejecting this into
the atmosphere. The heat it rejects is all the unwanted heat from
the building caused by the people, computers, sunlight, lighting etc. It
must also reject the heat generated by the compressor of the chiller.
The condenser water leaves the condenser of
the chiller at around 32°C (89.6°F) and
the pump sends this up to the cooling tower. The system has been designed
so that the condenser water leaving the cooling tower and re-entering the
chiller condenser, must be around 27°C (80.6°F) in order to be
able to pickup enough heat on it's next cycle.
Parts of a cooling tower
Before we look at how a
cooling tower rejects heat. Let’s first look at the parts inside a cooling
tower.
Fan: The fan pulls cool ambient
air in through the filters at the base and pushes it out the top of the cooling tower, taking heat and moisture with
it.
Drift Eliminator: This causes the air to change direction and condense some of the moisture in the leaving air which reduce operating costs.
Condenser Water Inlet: This is where the warm condenser
water enters the cooling tower
Spray Nozzles: The warm condenser water is pushed
through these nozzles, causing it to spilt into a
spray of small droplets.
Fill Packaging: The condenser water spray droplets run
down this increasing the heat transfer surface
area and allowing the air (which flows in the opposite direction) to carry
some of the heat away as well as moisture from the evaporation.
Filter: This is where the fan pulls it’s air in from. The filter
limits the amount of dirt and leafs entering
the cooling tower. Air enters cooler and dryer here then when it leaves at
the top.
Condenser Water Outlet: This is where the condenser water will leave the cooling tower. It will leave at a lower
temperature than when it entered at the top.
Makeup Water: A minimum water level is held in the basin of
the cooling tower. Water is lost from
the cooling tower from evaporation but also when the cooling tower drains
to get rid of accumulated dirt and salt.
Overflow: If the water level in the basin gets too high,
it will flow through here and out to a drain.
Drain: Water will be drained from the cooling tower for
maintenance purposes but also periodically
during normal operation when the level of impurities in the water gets too
high. The impurities are from the accumulation of dirt and salts which
build up as water evaporates and leaves these behind. This is often reffed
to to “blowdown”.
How dose a cooling tower reject heat ?
The warm condenser water enters near the top
of the cooling tower. It passes through
some nozzles which spray the water in small droplets across the fill
packaging. The spray increases the waters surface area which allows it to
reject more heat .
Drive Belt and Drive Motor: The simplest method of spinning the
fan blades. It could also be direct
drive, chain driven or gear mounted.
The water is sprayed across the fill packaging
These droplets, from the
spray, run down the fill packaging causing a thin film of water on its
surface. The water will evaporate into the air and carry heat away with
it.
For greater cooling capacity, cooling towers use
a fan to create more evaporation. The fan pulls air through the filters at
the bottom of the unit and move it up and out through the top of the
cooling tower, passing through the fill packaging and drift eliminators. The
air flows in the opposite direction to the flow of condenser water.
Combining the spray of water,
the thin film of water on the fill packaging and the flow of cool air you
get a great amount of heat transfer.
Just to recap. Warm condenser
water enters the tower and is sprayed over the fill packaging. This spray
runs down the fill packaging creating a thin film of water over it,
the water will evaporate and cool down. Cool dry air enters through the
bottom of the tower and passes through the fill packaging in the opposite
direction to the flow of condenser water, this will pickup heat and
moisture from the thin film of water. The condenser water will leave the
bottom of the cooling tower at a lower temperature. The air will leave the top of
the cooling tower with that heat but it will also carry some of the water away
with it.
Water costs money. So in
order to keep operating costs and water consumption down, cooling towers
often use drift eliminators. These are located above the fill packaging
and spray nozzles but below the fan.
The drift eliminators
purposely change direction to cause the leaving air to condense against
the sides. This squeezes some of the moisture out of the air which then runs
back down to the fill packaging for further cooling and eventually
accumulates in the basin of the cooling tower to be sent back to the
chiller.
The cooler condenser water
accumulates in the basin of the tower. This returns to the chiller where
it will pickup more heat and the cycle starts over again. A filter is
usually present over the outlet to ensure no foreign objects enter the
pipe as this will enter the blades of the pump.
A float valve in the basin
operates the flow of water from the makeup water inlet and ensures
a minimum level of water is maintained. This water is topped up because
of evaporation losses and also during blow downs.
The overflow prevents too
much condenser water entering the cooling tower basin. Water entering here
will usually be sent to drain.
The drain usually has an
automated valve attached. Sensors located in the basin or the condenser
system pipework will detect the level of impurities (dirt and salts) and will
start to drain the water if the level is too high. This draining is known
as the (blowdown) and causes the float valve to drop which allows fresh
water to enter through the makeup inlet. The impurities are already in the
condenser water as well as the air, but as the water evaporates it leaves
these behind. If enough water evaporates it will cause a build up
of impurities which can damage the equipment as well as reduce the cooling
tower, chiller and pump effectiveness and efficiency.
In cooler climates, some
cooling tower basins will have an electric pan heater installed. This turns on
at a specified minimum air temperature (e.g. 5°C, 41°F) to prevent frost.
THANK YOU
Water costs money. So in
order to keep operating costs and water consumption down, cooling towers
often use drift eliminators. These are located above the fill packaging
and spray nozzles but below the fan.
The drift eliminators
purposely change direction to cause the leaving air to condense against
the sides. This squeezes some of the moisture out of the air which then runs
back down to the fill packaging for further cooling and eventually
accumulates in the basin of the cooling tower to be sent back to the
chiller.
The cooler condenser water
accumulates in the basin of the tower. This returns to the chiller where
it will pickup more heat and the cycle starts over again. A filter is
usually present over the outlet to ensure no foreign objects enter the
pipe as this will enter the blades of the pump.
A float valve in the basin
operates the flow of water from the makeup water inlet and ensures
a minimum level of water is maintained. This water is topped up because
of evaporation losses and also during blow downs.
The overflow prevents too
much condenser water entering the cooling tower basin. Water entering here
will usually be sent to drain.
The drain usually has an
automated valve attached. Sensors located in the basin or the condenser
system pipework will detect the level of impurities (dirt and salts) and will
start to drain the water if the level is too high. This draining is known
as the (blowdown) and causes the float valve to drop which allows fresh
water to enter through the makeup inlet. The impurities are already in the
condenser water as well as the air, but as the water evaporates it leaves
these behind. If enough water evaporates it will cause a build up
of impurities which can damage the equipment as well as reduce the cooling
tower, chiller and pump effectiveness and efficiency.
In cooler climates, some
cooling tower basins will have an electric pan heater installed. This turns on
at a specified minimum air temperature (e.g. 5°C, 41°F) to prevent frost.
THANK YOU
