Cooling Tower Working Complete HVAC Heat Rejection Guide

 Cooling Tower Working Complete HVAC Heat Rejection Guide

A cooling tower is a heat-rejection device used mainly in water-cooled HVAC systems and industrial processes. Its purpose is to remove the heat absorbed by the condenser-water system and reject it to the outdoor atmosphere.

In a typical water-cooled chiller plant, the heat flow is:

• Building Heat
• Chilled Water 
• Chiller Refrigerant
•  Condenser Water
•  Cooling Tower 
•  Atmosphere

1. Why is a cooling tower required?

Inside a water-cooled chiller, the evaporator removes heat from the building's chilled-water circuit. The refrigerant carries this heat to the condenser.

The condenser transfers heat from the refrigerant into the condenser water. This warm condenser water then flows to the cooling tower, where its heat is rejected to the atmosphere.

Importantly, the cooling tower rejects:

Heat rejected = Building cooling load + Chiller compressor heat

Therefore, cooling-tower heat rejection is normally greater than the chiller's useful refrigeration capacity.

For example, a 1,000 TR chiller may require roughly 1,200–1,300 TR of heat rejection, depending on chiller efficiency and operating conditions.

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2. Basic cooling-tower working principle

A cooling tower primarily works through evaporative cooling.

Warm condenser water enters the cooling tower and is distributed over the tower fill. Air passes through the tower and comes into direct contact with the water.

A small portion of the water evaporates.

That evaporation requires latent heat, which is taken from the remaining water. As a result, the remaining condenser water becomes cooler.

The process is:

• Hot Water In
• Water Distribution 
• Fill → Air/Water Contact
• Partial Evaporation
•  Heat Rejection 
• Cold Water Basin

The cooled water collects in the basin and is pumped back to the chiller condenser.

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3. Complete condenser-water cycle

Step 1 — Chiller condenser receives heat

Refrigerant leaving the compressor enters the condenser as hot, high-pressure vapor.

Condenser water absorbs heat from the refrigerant.

As a result:

• Refrigerant condenses.
• Condenser-water temperature increases.

A common design condition is approximately:

Cooling Tower Leaving Water → 29.4°C / 85°F
Chiller Condenser Leaving Water → 35°C / 95°F

So the condenser-water temperature rise is approximately:

35 − 29.4 = 5.6°C

or

95 − 85 = 10°F

Actual design temperatures vary by project, climate, chiller selection, and tower design.

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4. Warm water enters the cooling tower

The condenser-water pump sends warm water from the chiller condenser to the top of the cooling tower.

Example:

35°C condenser water → Cooling tower

The water enters the distribution system.

Depending on tower design, distribution may use:

• Spray nozzles
• Distribution pipes
• Gravity distribution basins
• Pressurized headers

The objective is to spread the water uniformly over the fill.

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5. Water passes over the fill

The fill media is one of the most important components of a cooling tower.

Its purpose is to increase:

• Water surface area
• Air-water contact
• Water residence time
• Heat-transfer effectiveness

Two common types are:

Film fill

Water forms thin films over closely spaced sheets.

Advantages include high heat-transfer efficiency and compact tower size.

It is commonly used where water quality is reasonably good.

Splash fill

Water falls over bars or grids and breaks into droplets.

It is more tolerant of dirty or contaminated water but may require a larger tower volume for equivalent thermal performance.

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6. Air moves through the tower

At the same time, the fan moves outdoor air through the cooling tower.

Depending on tower design, airflow may be:

Counterflow

Air travels upward while water travels downward.

Crossflow

Air travels horizontally across the downward-flowing water.

Both arrangements provide intimate contact between air and water.

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7. Evaporation removes heat

This is the key cooling mechanism.

Only a small percentage of circulating water needs to evaporate to remove a significant amount of heat because the latent heat of vaporization of water is high.

When part of the water evaporates, heat is removed from the remaining water.

Therefore:

Evaporation → Heat removal → Lower condenser-water temperature

Cooling towers use both sensible and latent heat transfer, but evaporative/latent heat transfer is the dominant mechanism under typical operating conditions.

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8. Cooled water collects in the basin

After passing through the fill, the cooled water falls into the cold-water basin.

For example:

35°C entering tower → 29.4°C leaving tower

The condenser-water pump then returns this water to the chiller condenser.

The cycle repeats continuously:

Chiller Condenser → Cooling Tower → Cold-Water Basin → Condenser Pump → Chiller Condenser

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9. Cooling-tower Range

Range is the temperature difference between hot water entering the tower and cold water leaving it.

Formula

Range = Hot Water Temperature − Cold Water Temperature

Example:

Hot water = 35°C
Cold water = 29.4°C

Therefore:

Range = 35 − 29.4 = 5.6°C

A higher range means more temperature reduction across the tower, but it must always be considered together with water flow, load, and wet-bulb conditions.

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10. Cooling-tower Approach

Approach is one of the most important cooling-tower performance indicators.

Formula

Approach = Cold Water Temperature − Entering Air Wet-Bulb Temperature

Suppose:

Cold water = 29.4°C
Wet-bulb temperature = 26°C

Then:

Approach = 29.4 − 26 = 3.4°C

A smaller approach generally indicates a tower capable of producing water closer to the ambient wet-bulb temperature, but achieving a smaller approach typically requires greater tower size, airflow, fill area, or cost.

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11. Why wet-bulb temperature matters

Cooling-tower performance is governed mainly by the outdoor wet-bulb temperature, not simply dry-bulb temperature.

A cooling tower cannot normally cool circulating water below the entering-air wet-bulb temperature.

For example:

Outdoor DB = 40°C
Outdoor WB = 27°C

The cooling tower might produce:

Cold water = 30°C

Therefore:

Approach = 30 − 27 = 3°C

This is why cooling-tower performance can change significantly with outdoor humidity.

Hot, humid weather with a high wet-bulb temperature can substantially reduce the tower's ability to produce low condenser-water temperatures.

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12. Major cooling-tower components

A typical mechanical-draft cooling tower contains:

Component	Main Function
Fan	Moves air through the tower
Fan motor/gearbox	Drives the fan
Fill media	Increases air-water contact area
Spray nozzles	Distribute water over fill
Drift eliminators	Reduce water droplets leaving with exhaust air
Louvers	Control air entry and reduce splash-out
Cold-water basin	Collects cooled water
Make-up valve	Replaces water losses
Strainer	Prevents debris entering pumps/system
Overflow	Prevents excessive basin water level
Drain	Allows basin draining
Bleed/blowdown	Controls dissolved-solid concentration
Tower structure/casing	Supports and encloses tower components

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13. Cooling tower water losses

Cooling towers continuously lose some water.

There are three principal losses:

A. Evaporation loss

Water evaporated during heat rejection.

A commonly used preliminary estimate is:

Evaporation ≈ 0.0015 × Circulation Rate × Range (°F)

This is only an approximate engineering rule; actual evaporation depends on heat load and operating conditions.

B. Drift loss

Small water droplets carried out with the exhaust air.

Modern drift eliminators can reduce drift substantially.

C. Blowdown

Water intentionally discharged to prevent excessive concentration of dissolved minerals.

Therefore:

Make-up Water = Evaporation + Drift + Blowdown + other losses

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14. Cycles of Concentration — COC

As water evaporates, minerals remain behind.

Consequently, concentrations of:

• Calcium
• Magnesium
• Chlorides
• Silica
• Dissolved solids

increase.

Cycles of concentration indicate how concentrated the tower water has become compared with the make-up water.

A simplified relationship is:

COC ≈ Conductivity of circulating water / Conductivity of make-up water

Example:

Make-up conductivity = 500 µS/cm
Tower water conductivity = 2,500 µS/cm

Then:

COC ≈ 2500 / 500 = 5

Proper COC management helps balance water conservation against scaling and corrosion risks.

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15. Cooling-tower capacity

Cooling-tower heat rejection can be estimated from:

Q = ṁ × Cp × ΔT

where:

• Q = heat rejected
• ṁ = water mass-flow rate
• Cp = specific heat of water
• ΔT = cooling-tower range

For water:

Cp ≈ 4.186 kJ/kg·°C

Therefore, if water flow is 200 kg/s and range is 5.5°C:

Q = 200 × 4.186 × 5.5

Q ≈ 4,605 kW

Since:

1 TR ≈ 3.517 kW

Heat rejection is approximately:

4,605 / 3.517 ≈ 1,309 TR

Remember that this is heat-rejection tonnage, not necessarily the chiller's nominal refrigeration tonnage.

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16. Cooling tower vs chiller relationship

The complete HVAC heat path can be visualized as:

ROOM / BUILDING
↓ Heat absorbed
AHU / FCU Cooling Coil
↓
Chilled Water
↓
Chiller Evaporator
↓
Refrigerant
↓
Compressor
↓
Chiller Condenser
↓
Condenser Water
↓
Cooling Tower
↓
ATMOSPHERE

This means the cooling tower does not directly cool the building.

It cools the condenser water, allowing the chiller to reject heat efficiently.

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17. Why condenser-water temperature matters

Lower condenser-water temperature can reduce the refrigerant condensing pressure.

This can reduce:

• Compressor lift
• Compressor power
• Chiller kW
• Overall plant energy consumption

However, lower is not always better.

Every chiller has allowable condenser-water temperature and flow limits. Excessively low condenser-water temperatures can cause control or operational problems depending on chiller design.

Therefore, condenser-water temperature should normally be optimized according to:

• Chiller manufacturer's limits
• Cooling-tower capability
• Outdoor wet-bulb temperature
• Chiller load
• Tower fan power
• Condenser-water pump power
• Overall plant efficiency

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18. Cooling-tower fan control

Modern towers often use VFD-controlled fans.

The BMS may modulate tower fan speed based on:

Cooling Tower Leaving Water Temperature

or an optimized setpoint based on:

Outdoor Wet-Bulb Temperature + Required Approach

For example:

Wet bulb = 25°C
Target approach = 4°C

Possible condenser-water supply target:

25 + 4 = 29°C

The control system can increase or decrease fan speed to maintain the target.

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19. Common cooling-tower problems

High condenser-water temperature

Possible causes:

• Dirty fill
• Blocked nozzles
• Low airflow
• Fan failure
• Wrong fan rotation
• Loose/broken fan belt
• Gearbox problems
• Low water flow
• Poor water distribution
• Excessive chiller load
• High outdoor wet-bulb temperature
• Scale formation

Excessive water consumption

Check:

• Blowdown control
• Conductivity controller
• Float valve
• Basin overflow
• Pipe leakage
• Drift eliminators
• Cycles of concentration

Scaling

Usually associated with water chemistry and excessive concentration.

Effects can include:

• Reduced heat transfer
• Blocked fill
• Blocked nozzles
• Higher condenser-water temperature
• Higher chiller energy use

Biological growth

Cooling towers can support microbiological growth if water treatment is inadequate.

A proper water-management program therefore requires appropriate:

• Biocide treatment
• Scale control
• Corrosion control
• Monitoring
• Cleaning
• Inspection
• Blowdown control

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20. Important operating parameters for facility teams

For daily plant-room monitoring, operators should trend parameters such as:

Cooling Tower

• Entering water temperature
• Leaving water temperature
• Outdoor wet-bulb temperature
• Range
• Approach
• Basin water level
• Conductivity/TDS
• Fan status
• Fan VFD frequency/speed
• Vibration
• Make-up water consumption

Condenser System

• Condenser-water supply temperature
• Condenser-water return temperature
• Water flow
• Pump pressure
• Differential pressure
• Pump current
• Condenser approach
• Chiller head pressure

These values are much more useful when trended together rather than reviewed individually.

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21. Practical example

Consider a water-cooled chiller plant operating at:

• Chiller capacity: 1,000 TR
• Cooling tower entering water: 35°C
• Cooling tower leaving water: 29.5°C
• Outdoor wet bulb: 26°C

Range

35 − 29.5 = 5.5°C

Approach

29.5 − 26 = 3.5°C

Therefore:

Tower Range = 5.5°C
Tower Approach = 3.5°C

If the leaving-water temperature starts increasing from 29.5°C to 32°C while the outdoor wet bulb and plant load remain similar, the operations team should investigate tower performance rather than simply assuming weather is responsible.

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22. Cooling-tower efficiency

A commonly used simplified expression is:

Cooling Tower Effectiveness (%) = Range / (Range + Approach) × 100

Since:

Range + Approach = Hot Water Temperature − Wet-Bulb Temperature

the expression can also be written:

Effectiveness = (Tₕ − T𝚌) / (Tₕ − Twb) × 100

For:

Hot water = 35°C
Cold water = 29.5°C
Wet bulb = 26°C

Effectiveness = 5.5 / (35 − 26) × 100

≈ 61.1%

This is useful as a simplified field indicator, although formal tower performance evaluation requires design data and appropriate test procedures.

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Key takeaway for MEP/HVAC engineers

The most important relationship to remember is:

Cooling Tower Performance = Water Flow + Airflow + Heat Load + Wet-Bulb Temperature + Water Distribution + Clean Heat-Transfer Surfaces

And the three essential temperatures are:

Hot Water Temperature → Cold Water Temperature → Outdoor Wet-Bulb Temperature

From these three values, you immediately obtain:

Range = Hot Water − Cold Water

Approach = Cold Water − Wet Bulb

A cooling tower therefore should not be judged only by whether its fan is running or whether the leaving water “feels cold.” Range, approach, wet-bulb temperature, flow, load, water quality, and chiller performance must be evaluated together.