Cooling Tower - Audit & Efficiency

I have written three posts & one audit report on this topic of cooling towers calculations. Recently one of our reader suggested that I should go for posting an article on how to identify different components of cooling tower performance. Therefore, I decided to provide the details on how to establish different factors contributing to the inefficiencies in case of a cooling tower.

The article & report I have earlier published are here.

• Cooling Towers: Not The Coolest One


• Cooling Towers - Performance Calculation - I


• Cooling Towers - Performance Calculation - II

Factors affecting cooling tower & how to find out the impact of each of them.

Effect of Water Temperature
The impact of return water temperature is that the cooling approach must change by ~50% of the difference in the design & actual return water temperature. For example: If a tower is designed for 4°C approach for the cooling of water from 44°C to 34°C and actual return water temperature is 42°C then you must get an approach of 3°C instead of 4°C. The range of cooling will be 9°C against design value of 10°C. Thus, if your return water temperature is 42°C against design value of 44°C and you are getting design approach of 4°C that means your cooling tower is operating inefficiently.

Effect of Wet Bulb
Similarly, In this case, the approach of cooling will go up by ~50% of the difference between design & actual wet bulb, if wet bulb is lower than design value. However, total range of cooling will also increase in this case by ~50% of the difference in actual & design wet bulb i.e. 44°C to 33°C means range of cooling will increase from 10°C to 11°C.

Yes, one should not infer from these two examples that the thumb rules expressed here are valid for any changes in the temperatures. Instead, it is always governed by the equilibrium conditions & for larger changes one should go for proper evaluation procedure as described later in this article.

Effect of Water Loading
Cooling towers are normally supplied in standard modules called tower cells. Therefore, the cooling water is distributed equally on each cell in parallel configuration. However, in actual operation it deviates from what it should be (Average water flow/cell). This causes for example 80% water on one cell & total water flow being the same it will be 120% on the other. Even the same cell might have different water loading on both sides of distribution deck. This much deviation may result in ~5-10% rise in cooling approach.

Effect of Air Distribution
Similarly, the imbalance in air loading in each side of every cell may cause 1.5 times more negative impact as compared to effect of disturbed water loading. Thus, 20% deviation in air distribution may cause ~10-15% rise in cooling approach.

Effect of Air Short Circuiting
By now it is clear that operators has to regularly monitor the performance of their cooling tower especially at different loads & at different ambient conditions. More appropriately, the comparison of actual approach with the design approach is not a good & actual indicator of its performance

NTU Calculation
For the identification of actual differences in the performance calculation of a cooling tower & for identifying the impact of each factor, NTU calculation method is the most useful & recommended one based on my experience.

As Promised earlier in the performance calculation - II, I am giving here the method for using this NTU for prediction of performance.

Step - 1
Start with how much is approximate difference in inlet & exit temperature from cooling tower & divide them in equal parts with say an increment of 0.2°C or so.

Start from first row & column A with some approx t temperature & make diff in column G as zero. (This all is explained in above linked article on this Blog)

Since all other rows are linked with same increment of 0.2 till last row, finally you should get your inlet temperature in last row & column A. If it is less or more change it in first row to get same.

With each change all rows in column G should have zero value as difference.

Step - 2
Once you get all zero in column G and inlet temperature in last row of Column A, the first row in column A should give you the exit temperature, but wait.

Now you need to check the sum of NTUL in column K or NTUG in column N. This value should be equal to your design NTU. (Yes, design NTU is also found in similar manner)

Example, so if you want to evaluate the impact of change in wet bulb compared to design, first consider all data of design & find out NTUG in design.

Now change wet bulb & you will see that ha in column F is changed, so you need to change t in column A to make difference in column G = 0.

Finally you will find the changed temperature figure at the exit by keeping NTUG same.

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Ultimate Analysis of Biomass

Heating value & ultimate analysis of any fuel be it biomass or fossil fuel is correlated long back by Du-Long in 19th Century.

Most recently a Mr. S. A. Channiwala 1992 thesis, The Indian Institute of Technology, Bombay) collected data on over 200 species of biomass and fitted the following equation to the data:

HHV (in kJ/g) = 0.3491C + 1.1783 H - 0.1034 O - 0.0211 A + 0.1005 S -0.0151 N

Where C is the weight fraction of carbon; H of hydrogen; O of oxygen; A of ash; S of sulfur and N of nitrogen appearing in the sltimate analysis.

This equation fitted the experimental data with an average error of 1.45%, typical of the error of most measurements. This equation permits using heat values in calculations and models of biomass processes.

However, I am giving here the table for fuels indicating their ultimate analysis & HHV.










Source: - BioMass Energy Website

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Steam Properties - I

I have been giving few posts on different properties estimation methods for various uses for process engineers. However, the most useful & most frequently used property is the steam property for which earlier I suggested using an Excel Add-In called water_97.xla.

But, sometimes if you are using different programs other than Excel, in that case, the add-in will not work & you need to know the correlations for different properties to use in other calculators.

So I thought it would be better to share these correlations for the benefit of all readers. We will cover these properties in few parts of this post. This is the first one for P & T correlation only.


1. Saturation Pressure at Given Temperature

Log(e) (2256500/P) = [7.21379 + (alpha + beta * T + gamma * T^n) (T-483.16)^2] x [647.31 / T - 1]

T = Temperature in Deg K
alpha, beta & gamma are given as below,

Case-1
alpha = 1.152 x 10^-5
beta = -4.787 x 10^-9
gamma = 0 & n = 0 for t = 0 to 210 °C

Case-2
alpha = 1.0071 x 10^-6
beta = 1.9312 x 10^-8
gamma = 8.913 x 10^-96 & n = 32 for t = 210 to 374.15 °C

Alternatively more simpler formula is

T = (P ^ 0.25) * 100

Or

P = ( T / 100) ^ 4

Where T is in °C.

One more equation developed by me
This equation is valid from t = 0 to 374 °C


Log10 (Pv) = A t^5 + B t^4 + C t^3 + D t^2 + E t + F

Where
A = 3.482223 x 10^-13
B = - 4.890675 x 10^-10
C = 3.038026 x 10^-7
D = -1.1351158 x 10^-4
E = 0.03090855
F = -2.2016923


List of other property estimation methods on this Blog.

• Calculate Diffusion Coefficient in Gases


• Calculate Diffusion Coefficient in Liquids


• Heat Capacities of dissolved solids & organic solutions - Quickest methods


• Heat capacities with dissolved solids


• Kinematic viscosity of air Vs Temperature


• Thermal Conductivity of air Vs Temperature


• How to calculate viscosity of liquid mixtures


• Surface Tension


• Viscosities of Pure Gases at Low Pressure - 1


• Viscosities of Pure Gases at Low Pressure - 2


• Viscosities of Pure Gases at High Pressure


• Viscosity of Gaseous Mixture

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HAZOP - Basic Understanding

Hazard and operability studies are a methodology for identifying and dealing with potential processes, particularly those which would create a hazardous situation or severe impairment of the process. It is commonly known as HAZOP.

Definitions
HAZARD any operation that could possibly cause a catastrophic release of toxic, flammable or explosive chemicals or any action that could result in injury to personnel.

OPERABILITY any operation inside the design envelope that would cause a shutdown that could possible lead to a violation of environmental, health or safety regulations or negatively impact profitability.


HAZOP Process
The HAZOP focuses on specific portions of the process called “nodes”. Generally these are identified from the P&ID of the process. A process parameter is identified, say flow, and an intention is created for the node under consideration.

Then a series of guidewords is combined with the parameter “flow” to create deviations. For Example, the guideword “no” is combined with the parameter flow to give the deviation “no flow”. Then focus is on listing all the credible causes of a “no flow” deviation beginning with the cause that can result in the worst possible consequence one can think of.

Once the causes are recorded, a list is made of the consequences, safeguards and any recommendations deemed appropriate. The process is repeated for the next deviation and so on until completion of the node.



Guidewords, Selection of Parameters and Deviations
The HAZOP process creates deviations from the process design intent by combining guide words (no, more, less, etc) with process parameters resulting in a possible deviation from design intent.



SPECIFIC PARAMETERS
These include Flow, Pressure, Temperature, Level, Time, Agitation, Reaction, Start-Up/Shut-Down, Draining/Venting, Utility Failure(instrument air, power), Maintenance, Vibrations etc.

Concept of Point Of Reference
When defining nodes and performing a HAZOP on a particular node it is useful to use the concept of point of reference (POR) when evaluating deviations.

Screening for Causes of Deviations
It is necessary to be thorough in listing causes of deviations. A deviation is considered realistic if there are sufficient causes to believe the deviation can occur. However, only credible causes should be listed.

There are three basic types of causes. They are:

• Human error which are acts of omission or commission by an operator, designer, constructor or other person creating a hazard that could possibly result in a release of hazardous or flammable material.


• Equipment failure in which a mechanical, structural or operating failure results in the release of hazardous or flammable material.


• External Events in which items outside the unit being reviewed affect the operation of the unit to the extent that the release of hazardous or flammable material is possible. External events include upsets on adjacent units affecting the safe operation of the unit (or node) being studied, loss of utilities, and exposure from weather and seismic activity.

By Associate Writer : Ms Nidhi Gupta

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