Wet Cooling Tower Guidance For Particulate Matter

(Second Edition – April 2014)

Purpose: This document describes the National Pollutant Release Inventory (NPRI) reporting requirements for PM (particulate matter) from cooling towers and provides an example of how to calculate the PM emissions from them. This guide contains some improvements and replaces the previous guide.   

The old guide should no longer be used. The previous version used total suspended solids (TSS) to calculate PM emissions. The new model uses total dissolved solids (TDS). The dissolved solids in the drift from the cooling tower forms solid particulates as the water from the droplets evaporate in the atmosphere. The old version did not address this.


Background Information

What is a Cooling Tower?

Cooling towers are heat exchangers used to dissipate large heat loads to the atmosphere and are an important component in many industrial and commercial processes. When water is used, the type of heat dissipation in the cooling tower is termed "evaporative" because it relies on water evaporation to exchange heat between the process and the air passing through the cooling tower. The heat from the water stream transferred to the air stream raises the air's temperature and its relative humidity to 100%, and it is this moist air that is discharged outside. Cooling towers can also be dry; however, the cooling potential of a wet surface is much better than a dry one, which is why wet, or water, cooling towers are most commonly used in industrial processes. This document will deal with the release of particulate matter from wet cooling towers.

How is Particulate Matter Emitted from Wet Cooling Towers?

The circulating water used in cooling towers can either be process water or be drawn directly from a natural water source for use. Cooling tower drift loss will occur when the circulating water from the cooling tower is entrained in the exhaust air stream and emitted from the cooling tower. The liquid drift evaporates to a solid salt crystal when all the water in the water droplets in the drift evaporates. These solids emissions are classified as particulate matter less than or equal to 2.5 microns in diameter (PM2.5), particulate matter less than or equal to 10 microns in diameter (PM10), and total particulate matter (TPM).


Calculation of Particulate Matter Emissions

Definitions

Total Liquid Drift (TLD) - Water droplets that are carried out of a cooling tower cell with the exhaust air. Unlike the evaporation loss which consists of pure water vapor, drift droplets have the same concentration of impurities as the water entering the tower.  The rate of drift Loss (W), usually expressed in percentage of circulating water flow, is typically reduced by employing baffle-like devices, called drift eliminators, through which the air must travel after leaving the fill and spray zones of the tower.

Total Dissolved Solids (TDS) – Sum of organic and inorganic material dissolved in the cooling water and is usually expressed in ppmw or mg/L .  The dissolved solids are generally of saline nature (non-volatile), and therefore water droplets in the drift would evaporate leaving behind a particulate.

Make-Up Water (M) - Water added to the circulating water system to replace water loss from the system by evaporation, drift, blow-down, and leakage.

Blow-Down Water (D) – Water discharged from the cooling tower system to control concentration of dissolved solids (salts) or other impurities in the circulating water.

Evaporated Water (E) – Water evaporation loss from the circulating water into the atmosphere by the cooling process.

Parts per Million (ppm) - PPM is defined as one part of the pollutant in question to one million parts of solution or air. In other words, one ppm is equal to 1/1 000 000. Depending on the media, ppm can be on a volume or mass basis. For air pollutants, ppm is measured on a mass basis and is equivalent to mg/kg .

Theory

The amount of total particulate matter (TPM) released to the atmosphere can be calculated using the following formula:

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

This value then needs to be multiplied by the number of hours the cooling tower was operational during the calendar year and finally by 10-6 to obtain the annual mass emissions in tonnes for the tower.

The drift loss required in the above formula can be determined based on the mass balance of the water used for the cooling process as shown in the following process flow diagram[1]:

Process Flow Diagram.

[1]Cooling Towers.  http://upload.wikimedia.org/wikipedia/commons/8/87/CoolingTower.png

Figure 1. Diagram of Cooling-Tower-System

Details can be found below

M = E + D + W  

Therefore,   W = MED

The Drift Loss, W, usually expressed as a percentage of the circulating cooling water, C, and is varied depending on the types of drift eliminators used, can be determined by:

W [%] = W / C

Or W [%] = (MED ) / C

Where:

            M = Make-Up Water [m3/hr]

            E = Evaporated Water [m3/hr]

            D = Blow-Down Water [m3/hr]

            C = Circulating Cooling Water [m3/hr]

 

Example

Operating Parameters:

Cooling tower type: Wet with induced draft

Average water circulation rate of cooling tower: 25 000 m3/hr

Total Liquid Drift: 0.001%

Average total dissolved solids (TDS) concentration during the year: 3000 ppmw or 3000 mg/kg or 0.003 kg/l (after applying the density of water at 1 kg/l) or 3000 g/m3

Calculations:

The TPM emissions are calculated from the formula as follows:

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

TPM [g/h] = 3000 g/m3 x 0.001/100 x 25000 m3/hr = 750 g/hr

Annual hours of operation = 8 400 h/year

Therefore, annual TPM emissions are:

TPM [tonnes/year] = 750 [g/h] x 8400 [h/year] x 0.000001 [tonnes/g] = 6.3 tonnes/yr

If TDS data for water circulating in the cooling tower are not directly available, a source-specific TDS content can be estimated by obtaining the TDS for the make-up water and multiplying it by a concentration factor. The concentration factor can be estimated using the ratio of a measured parameter for the cooling tower water such as conductivity, calcium, chlorides, or phosphates to that same parameter for the make-up water. 

Three separate spreadsheet calculators have been developed to allow an estimate of the mass emissions of the solids in the drift droplets based on three different typical rates of drift loss for the cooling towers. Spreadsheet Calculators

The following Table presents the summary results of particulate emissions calculated from the Spreadsheet for the various concentrations of TDS in the circulating cooling water at the different rates of drift loss.

Summary Results of Particulate Emissions

Drift Loss

(%WF)

TDS

(ppmw)

TPM

(mg/m3H2O)

PM10

(%TPM)

PM2.5

(%TPM)

0.02300060011.40.3
0.00130003027.80.2
0.0005300015500.2
0.021200024004.10
0.0011200012040.1
0.000512000604.50.1
0.023300066001.70
0.001330003291.20
0.0005330001650.60

Note that the results for 0.02% drift loss are based on Table 1 of the Spreadsheet for the CTI 0.01% drift data, the results for 0.001% drift loss are based on Table 2 of the Spreadsheet for the interpolated drift data of 0.005%, and the results for 0.0005% drift loss are based on Table 3 of the Spreadsheet for the Electric Power Research Institute (EPRI) 0.0003% drift data.

Using the percentage of emissions of PM10 in TPM and PM2.5 in TPM as estimated for the solid particle size of 10 microns and 2.5 microns for the drift rate and the TDS pertaining to the calculation example above, the PM10and PM2.5 emissions from the cooling tower can then be computed as shown below.

PM10 [tonnes/year] = 750 [g/h] x 8400 [h/year] x 0.000001 [t/g] x 27.8% = 1.75 t/yr

PM2.5 [tonnes/year] = 750 [g/h] x 8400 [h/year] x 0.000001 [t/g] x 0.2% = 0.01 t/yr  

Assuming the cooling tower is the only source of particulate matter at the facility, the calculated values above can be compared to the mass release thresholds for PM2.5, PM10 and TPM to determine if a report is required for those substances. The mass release threshold for PM2.5 is 0.3 tonnes, and since the annual release from the cooling tower is 0.01 tonnes, a report would not be required.  The mass release threshold for PM10 is 0.5 tonnes, and since the annual release from the cooling tower is 1.75 tonnes, a report would be required for that value.  The mass release threshold for TPM is 20 tonnes per year which is significantly higher than the 6.3 tonnes released, therefore a report is not required for TPM.

If other sources of particulate matter are present at the facility then all sources must be combined before a comparison to the release threshold is made. For example all TPM releases must be added together before the total value is compared to the 20 tonne release threshold. If the resulting value is greater than or equal to 20 tonnes a report for the calculated value is required. This is the same procedure to be followed for PM10 and PM2.5.  

 


References

U.S. EPA's AP - 42 Chapter 13.4 Wet Cooling Towers

Cooling Technology Institute www.cti.org

J. Reisman and G. Frisbie, Calculating Realistic PM10 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. 

Draft Date: 2012-02-29