The cooling tower has been around since the early days of HVAC systems and continues to serve as a reliable method of heat rejection in commercial buildings. A typical cooling tower consists of two basic components – a water tank or reservoir and a heat exchanger. Cooling towers work by allowing hot air to pass through the heat exchanger before being ejected into the atmosphere. These towers are typically designed to handle a certain amount of energy output, based on the size of the building in that they are located.
For large-scale data centers, multiple units are often installed together side-by-side to provide several times the cooling capacity of a single unit. For smaller applications, a single tower may suffice. Most people think of industrial-scale cooling towers when they hear the word “tower” but there are many different types of cooling towers that exist today, including evaporative (or swamp), open-loop, closed-loop, and hybrid designs.
Evaporative cooling towers are still used quite commonly and are a cost-effective option for large-scale cooling needs. They’re also highly mobile, making them ideal for use at construction sites, outdoor concerts, and other events where mobility is necessary. Evaporative cooling towers simply require a constant supply of outside air to move across the cooling surface.
Design Formula for Cooling tower
Q = mcPΔT
where, m - mass flow rate / discharge in lbs/hr
cP - specific heat of water in Btu / lb °F
Q - Heat flow rate / discharge (GPM) gallons per minute/ Btu / hr [1 gpm = 3.78 liters]
ΔT - Temperature difference
Formula 1 :
Imperial system or English system or FPS system
Q = mcPΔT
where, m - mass flow rate i.e; 8.33 x GPM
cP - specific heat i.e; 1 Btu/ lb °F
Q - Cooling Capacity in Btu/min
ΔT - Temperature difference in °C
Formula 2 :
MKS system or Metric system
Q = mcPΔT
where, Q - Cooling Capacity in Kcal/min
m - mass flow rate i.e; 0.003785 x GPM x 1000. Here GPM = (Tr x 24/ ΔT)
ΔT - Temperature difference in °C
Formula 3 :
In SI system,
Q = mcPΔT
where, Q - Cooling Capacity in Kcal/min
m - mass flow rate i.e; 0.003785 x GPM x 1000.
ΔT - Temperature difference in °C
cP - specific heat = 4.187 Kj/Kg °C
Water Evaporation Losses = 0.00085 x 1.8 x water circulate rate(WR) x ΔT
(It is done with respect to gpm of project of mass flow rate)
Definitions
Range:
The difference between Entering water temperature or Hot Water supply and leaving water temperature is known as Range.
Therefore, ΔT = 7 °F to 10 °F but usually we take 10 °F.
Approach:
The difference between Leaving water temperature and Entering Wet Bulb temperature is known as the Approach.
Evaporation:
The method by which a cooling tower cools water is known as Evaporation.
Drift:
Water droplets carried off by the cooling tower by the process of Evaporation is known as Drift.
Drift Range:
0.0006 to 0.0012 GPM/Ton and recommended drift is 0.0002 GPM/Ton.
Blowdown / Bleed off:
Water intentionally discharge from the cooling tower, to maintain water quality is known as Bleed off.
Blowdown = Evaporative loss (meter cube per hour) / (c.o.c - 1)
Plume:
Hot moisture discharge from the cooling towers in the form of dense fog is known as a Plume.
Condenser Water:
Most common entering water temperature (EWT) + or - 85 °F, common leaving water temperature (LWT) 10 °F
Types of condensers used in a system with respect to cooling towers :
* Centrifugal
* Reciprocating
* Screw
* Scroll
For all these compressors Flowrate is 40 GPM/ 1000 Tons.
Absorption Compressors:
For this flowrate will be 80 GPM/ 1000 Tons
Drift Range - 0.0006 to 0.0012 GPM/ Ton and recommended drift is 0.0002 GPM/ Ton.
Installation location
A cooling tower should be located at least 100 ft from the building when located on the ground to reduce noise and prevent moisture from condensing on the building during the intermediate seasons.
Cooling towers should also be 100 ft away from Parking lots.
Drift loss
A small amount of water in the form of small droplets is in turn carried by air passing through water and water gets lost in this manner which is called drift loss. Generally drift loss is 0.02%. With the development in technology, drift loss is reduced to 0.001% to 0.003%.
Blowdown / Bleedoff loss
Water is taken out continuously or intermittently as a waste of a certain percentage of circulating water.
Blowdown = (Evaporative loss / (coc -1)) m³/hr
Solved Problem :
If coc is 3, find out the Blowdown / Bleedoff factor when the evaporative factor is 0.047.
Blowdown = (Evaporative loss / (coc -1)) m³/hr
= ( 0.047 / (3-1))
Blowdown = 0.0235
The selection of cooling towers depends upon :
* Capacity in ton
* design flow rate
* design wet bulb temperature
* Entering and Leaving Water Temperature
Cooling Tower Size
The size of the cooling tower should match the output requirements of the equipment. Larger cooling towers allow more space for air movement, which results in higher air velocities. High velocity air flows help ensure that the hot exhaust air has enough time to dissipate before reaching the environment outside of the cabinet. However, the larger fan needs to be strong enough to move large volumes of air at high speeds.
Fan Type and Speed
Fan type and speed refer to the method used to force air through the cooling tower. Fans typically fall into two categories: electric, where motors directly drive fans, and mechanical, where fans are connected to compressors that generate forced air. Electric fans offer greater control over the direction and amount of air flowing through the system. Mechanical fans are more commonly used than electric fans because they are quieter and cheaper. Also, fans have different diameters to regulate airflow.
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