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Cooling Tower Basic Calculations

The recirculation fee and the temperature drop across the cooling tower are the 2 items of information needed to calculate the amount of water misplaced from the open recirculating cooling system (attributable to evaporation).

Following objects might be mentioned and calculated in this article :

  • – Evaporation
  • – Temperature Drop
  • – Recirculation Price
  • – Focus Ratio or Cycles of Concentration
  • – Make Up Water
  • – Holding Capability or System Quantity
  • – Blowdown Rate


– Evaporation losses will vary relying upon temperature and humidity, however a general rule is that for every one hundred F. (60 C.) temperature drop throughout the tower, approximately 0.85% of the recirculation price will probably be evaporated.

  • – Evaporation (Estimation: See Value Added Troubleshooting Information – Cooling Section for precise method )
  • – E = ∆T × R × 0.00085 when T measured in Fahrenheit
  • – E = ∆T × R × 0.00153 when T measured in Centigrade

This figure can be used for estimating purposes, however should not be used when more actual information is required (e.g., in a proposal, problem solving, etc.). Particulars for calculating evaporation fee based mostly upon temperature and humidity conditions are provided in the PAC-3 section of the value Added Troubleshooting Information.

Temperature Drop :

The temperature drop ( ∆T ) for a cooling tower might be measured by taking the temperature of the tower return water (TR) and subtracting the temperature of the basin supply water (TS). This distinction can be used to calculate the approximate quantity of evaporation that has occurred in the cooling tower:

∆T = TR TS Recirculation Price

To maintain a move of water by means of the heat switch tools, water should be pumped or recirculated. The recirculation price could be determined from information on pump efficiency, tower hydraulics, etc. An in depth description of how to determine recirculating rate is given within the PAC-3 section of the worth Added Troubleshooting Guide .

It can be grossly deceptive to easily use the pump title plate data to determine recirculating fee. Many times throttling valves, pipe restrictions, and head stress restrictions interfere and can produce deviations as great as 50-seventy five% from the title plate values.

Concentration Ratio or Cycles of Focus

The concentration ratio of an ion carried in a recirculating system is merely the concentration of that ion in the recirculating water divided by the focus of the ion within the make-up water. Focus ratio can also be referred to because the cycles of concentration.

C R = Particular Ion Focus within the Recirculation Water Specific Ion

Focus within the Make Up Water

Theoretically, evaporation from a cooling tower is pure water. All of the dissolved ions are left behind to concentrate in the system. If the one system water loss was by evaporation, the dissolved ions within the recirculating water would proceed to concentrate (from the ions left after evaporation) till the solubility of every ion in the water was exceeded and large scale/deposition resulted. Most methods can not tolerate any scale; subsequently, the level or focus of crucial scaling-prone ions within the water is usually controlled by a mix of bleeding off a certain portion of the recirculation water and adding anti-scaling compounds. The rate at which water is bled from a system (in gpm; m3/hr) in contrast with the quantity of fresh water being launched within the system (in gpm; m3/hr) may also determine the focus ratio.


To test the concentration ratio in a system, select and monitor a soluble ion (akin to silica or magnesium) that is current in sufficient amount, stable, and simply tested. Evaluate its focus in the make-up water to its focus in the recirculating water by dividing the tower content material by the make-up content material.

Repeating this same testing for scaling species (e.g., calcium) will provide a sign if scaling is occurring or if the system is in chemical steadiness. If the cycles of calcium focus are persistently lower than the cycles of magnesium focus, for instance, the calcium can be assumed to be precipitating within the system. (There could also be scale forming in the heat switch tools, thereby impeding production.) Entry of ions from sources aside from the makeup water can invalidate any ratio being developed. These sources embrace chlorination, chemical additives, process leaks, acid additions, and airborne gases.

Make Up Water :

Water that should be added to change water lost from the recirculating system by evaporation and bleed-off (or blowdown) is known as makeup water (MU). The amount of water entering the system must be equal to the amount leaving the system.

BD = Blowdown Rate. gpm (m3/hr) includes drift, leakage, filter wastage and export

If the temperature drop throughout the tower and the recirculation charges are recognized, the quantities of water loss by evaporation will be calculated. If the focus ratio is also known then the makeup water necessities could be calculated as follows.

The expression was developed from the next fundamental cooling tower water balance relationships.

MU = E + BD


Substituting BD = MU/CR in the first equation. MU = E + MU/CR

(MU)(CR) = (E)(CR) + MU

(MU)(CR) – MU = (E)(CR)

MU = E × CR CR

The blowdown (bleed-off) rate is generally defined because the water lost from the system for all reasons besides evaporation. In very tight (low water loss) open recirculating methods, the 2 primary areas for system water loss are evaporation and water blowdown. In practice, nevertheless, quite a lot of water might even be lost by system water leaks, by water combining with the product or process, or by tower drift. For calculation functions, all of those water losses, apart from evaporation, are usually thought-about together and called tower water blowdown. The blowdown charge is generally measured in gallons pen minute (m3/hr).

System blowdown (BD) charge might be calculated from the following expression:

BD= E x CR

The place: BD = blowdown price, gpm (m3/hr)

E = tower evaporation price, gpm (m3/hr)

CR = concentration ratio or cycles

This expression was derived from the following cooling tower water balance relationship: MU = BD + E

Substituting MU = (CR)(BD) in MU = BD + E : (CR)(BD) = BD + E

(CR)(BD) – BD = E

(BD)(CR-1) = E


Non-Blowdown Water Losses Included in Blowdown [ Drift, Leaks, Filter Wastage, Export ]

If cooling system is operated under supreme conditions all water faraway from the system would be as a result of evaporation or blowdown. Unfortunately the perfect cooling system solely exist in concept and in working systems we find different water losses that need to be understood and factored into the overall cooling system materials stability equation.

Drift – Tiny droplets of water that develop into entrained within the airstream and carried out of the unit in the leaving airstream. In contrast to evaporation drift is a droplet of water and incorporates solids and micro organism. Drift is the first mechanism for transmission of pathogens from a cooling system to a host. Drift is often estimated based mostly on a percentage of recirculation. Estimates fluctuate from 0.002 to 0.01% of recirculation.

Splash fill towers are likely to have greater drift rates then movie fill towers. Drift eliminator design, unit upkeep and air movement even have an influence on the amount of drift that’s launched from a cooling system.

Leaks – Uncontrolled water lost from a system. Leaks must be identified, quantified and corrected where attainable. Leak identification and administration is a useful service to any client operating a cooling system. Potential sources: pump seals, valves that don’t seal, overflow, tower containment or splash out, exchanger failures . . .

Filter / Separator Wastage – Water wasted from a system as a result of separator flush or filter back wash.

Export – Water deliberately faraway from the system and used in one other system.

Holding Capacity or System Quantity :

The holding capability of a system is the quantity of water within the system expressed in gallons (cubic meters). Normally a lot of the capacity of a system is contained in the cooling tower basin; the exact quantity, nonetheless, may be determined solely by conducting a TRASAR analysis or an ion focus examine. This system is described in detail in the value Added Troubleshooting Guide, PAC-3 . Assumptions about holding capacities might be harmful and may lead to incorrect dosages for biocides, including biological management packages which are ineffective or too costly.

Holding Time Index or Half-Life

The holding time index (HTI) is a calculated determine that indicates the time required to scale back the chemical or make-up water added to a system to 50% of its authentic concentration. It is basically the half-life of a chemical added to the system. The basic technique of calculating the holding time index is as follows:

HTI = zero.693 ×HC BD

Expressed within the time models used for blowdown BD. Usually reported in hours.

Where: BD = Blowdown Fee. gpm (m3/hr) consists of leakage

HC = Holding Capability or Quantity, gal (m3)

The holding time index is vital in choosing a chemical therapy program. Very long holding time indexes might preclude using sure chemicals, equivalent to polyphosphates, because of excessive reversion of the polyphosphate species to orthophosphate and subsequent precipitation as tricalcium phosphate (a compound that has a really low solubility in water). A short holding time index could restrict using some chemicals because of the higher amount of chemical required to take care of the required treatment ranges (and the accompanying elevated costs). Additional, not all chemical inhibitors will stop scale, corrosion, and fouling for a similar length of time. Subsequently, the particular chemical program chosen and the level at which the chemicals are utilized are influenced by the holding time index.

Finally, the holding time index is used to determine the required amount of some biocides to achieve proper control of microorganisms. This is especially true when slug feeding slower-acting biocides. Brief holding time indexes might not permit sufficient time to keep up crucial biocide focus for kill and can lead to growing resistance. We will manage holding time index to some degree by pre- blowing down previous to biocide dosing to extend the holding time.


The time per cycle is outlined as the time it takes all the water in a system to make one journey around the recirculating loop (from the discharge aspect of the recirculation pump back to the suction aspect of the pump).

Time per Cycle = HC R

recirculation charge R.

Expressed in the time models used for

The place: BD = Blowdown Price. gpm (m3/hr) contains drift and leakage

CR = Focus Ratio

E = Evaporation Rate, gpm (m3/hr)

HC = Holding Capacity or Quantity, gal (m3) HTI = Holding Time Index

MU = Makeup Charge, gpm (m3/hr)

R = Recirculation Rate, gpm (m3/hr)

Aldo Zaffalon


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July 20, 2017

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Model : CMB 30
Circulate : 396 L / Min
Heat Load : 117000 k/cal
inlet & outlet : 2.5 B
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