Wet cooling towers: guide to reporting

Cooling towers are heat exchangers used to dissipate large heat loads to the atmosphere. They are important to many industrial and commercial processes.

This document provides guidance on how you can calculate particulate matter (PM) and volatile organic compound (VOC) releases from wet cooling towers to report to the National Pollutant Release Inventory (NPRI).

The NPRI has also developed a spreadsheet calculator to help you estimate the release of particulates from wet cooling towers.

Wet and dry cooling towers

A wet cooling tower evaporates water to exchange heat with air passing through the tower. The heat and humidity from the water stream raises the air temperature and relative humidity, and the warm, moist air is discharged from the tower.

Cooling towers can also operate without water, but a wet surface has much greater cooling potential than a dry one. The wet cooling towers described in this document are more commonly used in industrial and commercial processes.

What substances can wet cooling towers emit?

Cooling towers can use either process water or water drawn from a natural source for the sole purpose of cooling.  Regardless of the source, this water almost always contains entrained solids or particulates. When unevaporated droplets of the cooling water are emitted from the cooling tower, they often carry particulates with them.

Volatile organic compounds (VOCs) will need to be considered at facilities such as petroleum refineries and chemical plants.  At these facilities, cooling towers can be used to reduce the temperatures of hydrocarbons or VOCs in some process streams.   VOCs are released as part of the evaporated emissions from those cooling towers.

Cooling towers should be reported as individual stacks if they are greater or equal than 25 m above grade. Detailed information about each stack must also be reported. For multicell cooling towers, stack grouping maybe applicable if certain conditions are met, for more details please consult the Reporting guide for the NPRI. Cooling towers less than 25 m above grade should be reported under the stack/point source release category.


The spreadsheet calculator combines estimates of the total particulate matter (TPM) released based on the design characteristics of the cooling tower with experimental data to calculate release levels for particulate matter less than or equal to 2.5 microns in diameter (PM2.5) and particulate matter less than or equal to 10 microns in diameter (PM10). Test data is limited, so you will need to choose estimates of PM2.5 and PM10 based on the drift loss parameters of your cooling tower compared to those provided within the calculator.

The following sections explain how you can calculate your particulate releases.

How are particulates released from wet cooling towers?

Particulate matter is released from wet cooling towers when unevaporated water droplets containing particulates are part of the cooling tower’s exhaust air stream; refer to Figure 1. These droplets are referred to as Total Liquid Drift (TLD) and shown in the cooling tower diagram below as “W”.

Particulates in drift droplets crystalize to solid salt crystals when the droplets evaporate.  These solids are reported as PM2.5, PM10 and TPM.  Drift droplets carry the same impurities as the make-up water entering the tower and the water already in the tower (“M” and “C” in the cooling tower diagram).  Evaporation loss (shown as “E” in the cooling tower diagram) is pure water vapor. 

Drift loss

Drift loss (W) is usually expressed as a percentage of circulating water flow. This loss can be reduced by passing exhaust air through baffles, called drift eliminators, after it leaves the fill and spray zones of the tower.

Drift loss (W) is equal to the make-up water (M) less the evaporated water (E) and less the blow down water (D) used for the cooling process. (All values are in m3/hr.):

W = M - E - D

Drift loss (W) is usually expressed as a percentage of the circulating cooling water (C):

W [%] = W / C × 100%

Solving these two equations lets you use the known parameters of the cooling tower (M, E, D and C) to calculate the drift loss (W[%]) as a percentage of the circulating cooling water:

W [%] = (M - E - D) / C × 100%

Cooling system water

The amount of particulate matter in the cooling water is expressed as Total Dissolved Solids (TDS).  This is the sum of organic and inorganic material, generally non-volatile saline, and is expressed in ppm or mg/L. The particulate forms the drift droplet’s water evaporates.

If TDS data for your cooling tower water is not available, you can estimate source-specific content by multiplying the TDS for the make-up water by a concentration factor. Estimate the concentration factor as the ratio from a measured parameter (such as conductivity, calcium, chlorides or phosphates) for the cooling tower water to that same parameter for the make-up water.

Estimating atmospheric releases

Use this formula to calculate the total particulate matter (TPM) released to the atmosphere:

TPM [grams/hour] = TDS [ppmw] x Drift Loss [%] / 100 x Circulating Water Rate [m3/hr]

Multiply this value by a conversion factor of 10-6 and the number of hours the cooling tower operated during the year to obtain the TPM emissions in tonnes.

The calculator allows estimating the particulate distribution and the mass fraction of the solids in the drift droplets based on three different typical rates of drift loss for the cooling towers.

Using the computed percentage of emissions of PM10 in TPM and PM2.5 in TPM, the PM10 and PM2.5 emissions from the cooling tower can then be estimated using the above equation for TPM:

PM10 [grams/hour] =  (% in TPM) * TPM [grams/hour]

PM2.5  [grams/hour] =  (% in TPM) * TPM [grams/hour]

If the cooling tower is the only source of particulate matter at the facility, the calculated values can be compared to the mass release thresholds for PM2.5, PM10 and TPM to determine if a report is required.

All sources of particulates must be combined before comparing to the release threshold. 

Volatile organic compounds

VOC emissions from process cooling towers (or heat exchangers) typically result from leakage in systems that service hydrocarbon process streams, or from the treatment of circulating water with chemicals that include VOCs.  VOC emissions are expected from cooling towers at refineries and chemical plants, where water is circulated to cool the process stream.

There are two ways to sample, measure and estimate VOC emissions from wet cooling towers: mass balance and emission factors.  While the mass balance approach is preferred, the user can choose any method that is defensible and appropriate.

Mass balance approach

VOC emissions are usually estimated with a mass balance approach:  cooling water is sampled as it enters and leaves the cooling tower. The change in the measured concentrations of VOC in the cooling water is multiplied by the flow rate to estimate the atmospheric emissions.  The mass balance approach provides a more reliable accounting of emissions than estimating from AP-42 emission factors (discussed below).

As the determination of VOC emissions by measurement method involves a mass balance calculation, it would be more appropriate to use the NPRI emission estimate code “C” (mass balance) to represent this estimation method for calculating VOC emissions from the cooling tower.

The following formula can be used to estimate the VOC emissions from the cooling tower for the time between each measurement:

VOC[t] = (CVOC,In- CVOC,Out) [ppmw] x Dwater [t/m3] x Water Flow Rate [m3/hr] x H [hrs]


CVOC,In = Concentration of VOCs in the cooling water prior to entering the cooling tower

CVOC,Out = Concentration of VOCs in the cooling water after leaving the cooling tower

Dwater = Density of water at 1 kg/L or 106 g/m3 or 1 t/m3

H = Hours the cooling tower has operated since the last measurement

The recommended method for measuring VOCs in the cooling water is described in the sampling procedures manual of the Texas Commission on environmental quality.   

Emission factor approach

The U.S. Environmental Protection Agency’s compilation of air pollutant emission factors (AP-42) provides an alternative option to the mass balance approach.   

U.S. Environmental Protection Agency’s compilation of air pollutant emission factors


VOC emission factors

(kg/106 L cooling water)

Applicable control technology

Uncontrolled 0.7 n/a
Contolled 0.8 Minimization of hydrocarbon leaks into cooling water system; monitoring of cooling water for hydrocarbons

A facility where cooling water is directly monitored should have sufficient information to use the mass balance method described above. The controlled emission factor, which applies to refineries that directly monitor for hydrocarbons, should therefore not be required.

If you do not monitor VOC emissions, use the uncontrolled emission factor of 0.7 kg/106 L to estimate them.

Use this formula, based on the emission factor method, to calculate annual VOC emissions from a cooling tower:

VOC[t/y] = EF [kg/106 L] x Water Flow Rate [m3/hr] x H [hrs] x 10-3 [t/kg])


EF= VOC emission factor for cooling tower

H = Number of hours of operation of the cooling tower during the year



AP-42, CH 13.4: Wet Cooling Towers (epa.gov)

Cooling Technology Institute

J. Reisman and G. Frisbie, Calculating realistic PM 10 emissions from cooling towers.  AWMA, Proceedings Florida conference  2001, session No. AM-1b

J.Missimer, D.Wheeler, and K.Hennon, The relationship between SP and HGBIK drift measurement results. CTI paper TP98-16, 1998. 

Volatile Organic Compounds

Air stripping method for cooling tower monitoring of volatile organic compounds (Appendix P of the TCEQ sampling procedures manual).  

Emission estimation protocol for petroleum refineries (Version 2.1.1) - report done by RTI International for the U.S. EPA. May, 2011.

International workshop on VOC fugitive losses: New monitors, emission losses, and potential policy gaps. U.S. EPA. October, 2006.

National emission standards for hazardous air pollutants from petroleum refineries; Final rule. U.S. Federal Register, vol.74, no.207. October 28, 2009.

U.S. EPA's AP - 42 Chapter 5.1_petroleum_refining.pdf (epa.gov)



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