Cooling Tower efficency... 1

[Jebb]

I’m looking for some formulas to determine the efficency of a dry type cooling tower. I’m trying to do some cost justification to determine when towers should be cleaned to remove pollutants, dirt, etc. from the fins. The cleaning process is very time consuming / labor intensive with the amount of pollution fouling the fins varying from year to year so it’s not a “clean every year” type of thing.

The cooling tower arrangement has 50 cells with a fan for each cell operated by a control system. A 50/50 glycol/water mix is circulated through a closed loop system with the control system using an algorithm to regulate the return temp to 25C (within reason) by turning on or off the required number fans to control the water temp. I have the following information from the control system:

Flow: (l/s)
Supply Temp (to towers): (C)
Return Temp (from towers): (C)
Ambient Temp: (C)

We just cleaned the towers so I would like to baseline the data I have now. Any help would be greatly appreciated…

Thanks, Jebb

 

[davefitz]

It s been a while ( 10 yrs) since I had available the correlations. The primary correlations were developed over 40 yrs ago.

The cooling tower institute CTI may have available some procedures for determining tower efficiency. A large factor is the fill effectiveness- you will need to get this from the fill vendor directly, not the tower manufacturer- many towers buyilt in the period 1998-2005 were undersized and the tower mfr data apparently was fraudulent. Other site factors inlcude shadowing by adjacent buildings, and prevailing wind direction vs tower axis.

As I recall, a great menas for cleanign the fill once per year involed using hydrogen peroxide, but the envirnomnetal issues need to be adressed.

 

[CH5OH]

heat removed by tower=flow(m3/s) x delta t(degC) x specific heat glycol/water(kJ/m3 degC) in kJ/s or kW

energy used to remove heat=kW fan + kW circulating pump

basecase efficiency (clean)=100%
kW energy/kW heat removed= current efficiency

kW fan increases with fouling

common sence would probably tell to clean towers before high cooling load (summer) to get the best return value

to determine best cleaning cycle time:
maximum value=sustain heat load with n-1 recource=49 fans on line
min value=regained profit (reduction in energy cost)=maintenance cost
best value= some where in between

 

[rmw]

For me the controlling factor would be the approach to ambient. After all, it is cooling you want. What is the design approach to ambient and how far off of that are you willing to go before cleaning?

rmw

 

[Jebb]

Thanks for the information. I'm an EE so I don't spend much time doing thermo stuff.

The design max ambient is 38C with the with the max cooler supply temp at 55C and the max cooler return temp at 44C. The total cooling design capacity is about 4100kW. So would the design approach to ambient be approx 6C based on the above info?

The system typically regulates to 20-25C when the ambient temp is lower (15C or lower). I'll have to look at the control software to figure out how the ambient fits into the regulation equation.

Thanks again,

 

[CH5OH]

the ambient temperature isn't much of a controlling factor,since a cooling tower doesn't work on the principal of temperature difference (dump the heat into a lower temperature sink) but on the principal of removing the heat through evaporisation.if the ambient temperature would be higher than the tube temperature, and the suck in air through the fan has a relativ humidity < 100 %, the cooling tower would still remove the heat

 

[davefitz]

Its a dry type cooling tower, per the posted question. There is no evaporation at the tower fins, and there is no "fill" per se. Just a finned tube array with a fan.

While I have not worked with dry type towers, I would expect the one would calculate the heat exchanger effectiveness "e" as per compact heat exchanger theory, sometimes labeled the e- NTU method.

First find the fan curves and determine the cooling air flow for the current ambient air temp and your location's elevation. The air flowrate times the air Cp ( W*Cp,a)is the first value to calculate. Next, calculate the W*Cp,g for the water glycol mixture . Whichever fluid has the min value of W*Cp is normally chosen as the governing fluid for heat exchanger effectiveness calculations.

Suppose the min value of W*Cp is for the water glyclol mixture. Then the effectiveness of the tower at the current time is e= (Ti-To)/ (Ti-Ti,a), where Ti,a is the ambient inlet air temp.

Next , compare the current "e" to the original design clean "e" on the tower's spec sheet. Fin fouling, and also loss of airflow due to fan blade reconfiguration, likely has lowered the "e" below the spec sheet value.

My original comments on hydrogen peroxide only applies to wet towers- it does not apply to a dry tower.

 

[rmw]

I doubt that 38C is your design temperature, just the max ambient expected based on the other numbers that you gave.

If the difference is 20C return vs a 15C day, I'd call that good performance. I'd also call 6C on a 38C day not too bad-not the best, but not too bad.

25C on a 15C day isn't good at all to me.

rmw

 

[Jebb]

38C is the design max ambient temp. I took a look at the temp regulation software and it appears to try and regulate to 6C above ambient it the ambient temp is above 25C. If the ambient is below 25C then it trys to keep the return temp around 25C because the equipment to be cooled doesn't like the temp to be below 20C.

After spending a long time decoding the control software, I think I'm going to use the calculation in the software which determines the cooler output temp and number of fans which are suposed to be running and compare it to the actual values to come up with an efficeny factor....

Thanks for all your help... This forum was a good learning experience.