Performance Improvement of Natural Draft Cooling Tower

INTRODUCTION

 Cooling towers are heat removal devices used to transfer process waste heat to the atmosphere. Cooling towers may either use the evaporation of water  to  remove  process  heat  and  cool  the working fluid to near the wet-bulb air temperature or,  in  the  case  of  closed  circuit  dry  cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature. 

LITERATURE SURVEY

Performance of existing Natural Draft Cooling Tower is improved either by optimization of operation method or by employing some innovative ideas. Georgia F.Cortinovis, José L.

Paiva worked on optimization model for the operation of cooling tower system and concluded that.

  • In the most economical operation of the cooling water system, the temperature of water that leaves the tower must be maintained at the highest value possible, provided the thermal requirements are achieved.
  • In the most economical operation of the cooling water system, the temperature of water that leaves the tower must be maintained at the highest value possible, provided the thermal requirements are achieved.
  • In the most economical operation of the cooling water system, the temperature of water that leaves the tower must be maintained at the highest value possible, provided the thermal requirements are achieved.

M. Goodarzi, R. Keimanesh (2) experimented on radiator type wind breaker for enhancement of heat rejection through Natural Draft Cooling Tower. They concluded that Regardless of increasing the initial and fabrication costs, radiator type windbreakers improve the cooling efficiency more than solid windbreakers do. In fact, the radiator type windbreakers use the cooling potential of the blowing wind in addition to the velocity deceleration characteristic. They even improve cooling efficiency under normal condition.

The water particles that are carried away with saturated exit air are called as drift. This drift is a huge evaporation loss that needs to be eliminated. Manuel Lucas, Javier Ruiz (3) investigated the thermal performance of cooling tower with drift eliminator. The presence of an eliminator does not necessarily worsen the performance of cooling tower as expected by the additional pressure loss incorporated into the air flow. Further, the performance of Natural Draft Cooling Tower will be improved by using the munters media (4) of PVC as a cooling media.

But all above ideas are costly to execute and difficult also. Then we reached to next research paper that presents a good option.i.e. Improving the efficiency of Natural Draft Cooling Tower researched by J. Smrekar, J. Oman (5). J.Smerkar ,J.Oman (5) measured the velocity and temperature across the periphery of Natural Draft Cooling Tower. These provide a direct means of evaluating the extent to which the falling hot water droplets and the cooling air mix in the cooling tower. 

If the ambient differentiations and operational irregularities of the water distribution system, the CT packing and the nozzles are not taken into account, we can focus on improving the operation of the cooling tower. This kind of improvement involves determining the optimal water/air mass flow ratio on a local basis across the plane area of the cooling tower.

From the left diagram of Fig. 2.1, it is clear that there are three relatively distinctive areas with different air velocities. It is also clear that because of the specific hyperbolic shape of the cooling tower, the air velocity de- creases from the edge of the tower to the interior. Because of the uniform inlet water temperatures and uniform mass flow rates of water across the plane area and because of the different air mass flow rates, the outlet water temperatures are different, which results in increased entropy generation in the cooling tower. With a constant water/air mass flow ratio, the same amount of air per unit of water is ensured, which results in the same outlet water temperatures across the plane area of the cooling tower and smaller entropy generation.

To achieve a constant water/air mass flow ratio there has to be a variable mass flow rate of water that is adapted to the mass flow rate of the air across the plane area. Because of a lower total entropy generation, a lower outlet water temperature is achieved and, thus, a better overall cooling tower efficiency is ensured. A suitable distribution of water relative to the air flow can be achieved by regulation of the water distribution system or by different sizes of nozzles across the plane area of the cooling tower.

Conclusion

The analysis showed that we should distribute water from the cooling towers interior, where we have more than enough water relative to the air flow, to the cooling towers exterior. We will obtain the best possible results for the given operating conditions when we have the optimal distribution of water across the plane area of the cooling tower.

OPTIMAL WATER DISTRIBUTION SYSTEM

Measurement of exit air velocity & temperature from cooling tower

The Vane anemometer designed for operation in 100% humidity and Mercury in glass thermometer are used for measurement of velocity and temperature of exit air. Readings were taken for 1 hour at 12 locations.

Sr. No.LocationTemp(ºC)Velocity(m/s)
1  Avg. Of location A@120 m36.5  3.43  
2  Avg. Of location B@70 m39.9  2.43  
3  Avg. of location C@20 m40.5  1.90  
Temperature & Velocity

The analyzed cooling tower had the same type of nozzles across the plane area of the cooling tower. The water distribution system with flow channels is exposed to atmospheric pressure, which means that with the correct operation of the nozzles and channels, the water distribution across the plane area would Be uniform. On the basis of the air–velocity and air–temperature Table 3.1,

we can determine the position and the level of entropy generation. At the edge of the analyzed plane area, there is a relatively extensive region of high air velocities and low air temperatures. These represent the areas with a high entropy generation. The underlined areas in the velocity and temperature fields, shown in Table .3.1, that deviate from the expected values are called areas of anomalies and should, in any case, be additionally investigated.

If the ambient differentiations and operational irregularities of the water distribution system, the CT packing and the nozzles are not taken into account, we can focus on improving the operation of the cooling tower. This kind of improvement involves determining the optimal water/air mass flow ratio on a local basis across the plane area of the cooling tower. From the left diagram of Table 3.1, it is clear that there are three relatively distinctive areas with different air velocities.

It is also clear that because of the specific hyperbolic shape of the cooling tower, the air velocity decreases from the edge of the tower to the interior. Because of the uniform inlet water temperatures and uniform mass flow rates of water across the plane area and because of the different air mass flow rates, the outlet water temperatures are different, which results in increased entropy generation in the cooling tower.

With a constant water/air mass flow ratio, the same amount of air per unit of water is ensured, which results in the same outlet water temperatures across the plane area of the cooling tower and smaller entropy generation. To achieve a constant water/air mass flow ratio there has to be a variable mass flow rate of water that is adapted to the mass flow rate of the air across the plane area.

Because of a lower total entropy generation, a lower outlet water temperature is achieved and, thus, a better overall cooling tower efficiency is ensured. A suitable distribution of water relative to the air flow can be achieved by regulation of the water distribution system or by different sizes of nozzles across the plane area of the cooling tower. The analysis showed that we should distribute water from the cooling towers interior, where we have more than enough water relative to the air flow, to the cooling towers exterior. It means the water flow must be maximized at the edges of Natural Draft Cooling Tower and minimized near the interior of cooling tower.

Objective

 Above analysis shows that we should distribute water from the cooling towers interior, to the cooling towers exterior. It means the water flow must be maximized at the edges of Natural Draft Cooling Tower and minimized near the interior of cooling tower. It is decided to optimize the water distribution but with minimum possible alteration to minimize the expenditure. It is possible by using a larger diameter nozzles at the edges and shorter diameter nozzles at interior of natural draft cooling tower.          

Design of optimal water distribution system

Total water flow of cooling tower is 66500 cubic meter per hour and it is distributed through 25 NB nozzle in the existing water distribution system of cooling tower.

By using relation, q=A.V

Nozzle has flow of 6 cubic meters per hour,  Q=N.q

Total 11083 numbers of nozzles are used in existing water distribution system. This nozzles should be distributed such that, they should satisfy following condition.

  • Design requirement- Maximum flow at edges and minimum flow near the interior of cooling tower. 
  • Minimum Cost requirement-Only nozzles will be replaced i.e. same AC pipes with same number of holes are used. 
  • Same Discharge- Discharge should remain same to avoid head loss. 

We know that, for parallel pipes the total discharge is some of individual discharge. 

  • Q=Q1+Q2+Q3 ……… (1) 
  • N.q=n1.q1+n2.q2+n3.q3 ……. (2)  

After performing number of iterations, it is decided to replace 60% of 25 NB nozzles with 26.67% of 21 NB nozzle & 13.33% of 32 NB nozzles.   

Existing and proposed distribution of nozzle

EXISTING WATER        DISTRIBUTION SYSTEMPURPOSED WATER DISTRIBUTION SYSTEM
Dia. (mm)Flow (m3/hr)Qty. (Nos)Total flow (m3/hr)Dia (mm)Flow (m3/hr)Qty. (Nos)Total flow (m3/hr)
    214295611824
2561108366500256665139906
    3210147714770
Total flow1108366500Total flow1108366500
TABLE 2: Existing and proposed distribution

Now in equation (2),

n1=2956 ,   n2=39906 ,  n3=14770

q1=4cum/hr   q2=6 cum/hr   q3=10 cum/hr     In this way, we can use this nozzle by keeping same discharge and with same number of nozzles. Only thing we have to do is to replace the nozzles or nozzle diameter. But the nozzle used here are 3 stage spray type nozzle.

This nozzles diameter will be cut to the desire size, on site also. The minimum diameter of supplied nozzles is 18 mm and it is easily cut to 21 mm and 32 mm on site also.

Details of Nozzle 1
Details of Nozzle 1
Details of Nozzle 2
Details of Nozzle 2

EXPERIMENTAL INVESTIGATION AND RESULT

 DESIGN SPECIFICATION OF COOLING TOWER

Following are the design specifications of existing of cooling tower.

CWT: 32.5 ºC, Approach: 5.5 ºC, Range: 9.5 ºC,  

Flow: 66500 cum/hr,

WBT: 27 ºC, HWT: 42 ºC, RH: 40%

EXPERIMENTAL SETUP

The test is conducted for 4 hours with measurement of hot water temperature, cold water temperature, dry bulb temperature, wet bulb temperature and wind velocity taking readings every minute at the locations shown in fig. 4.1 below

Location for Measurement of Parameters in NDCT 1 1
Location for Measurement of Parameters in NDCT 1
Location for Measurement of Parameters in NDCT 2
Location for Measurement of Parameters in NDCT 2
  1. Hot Water Temperature measurement
  2. Cold Water Temperature measurement
  3. Water Flow Rate
  4. DBT&WBT
  5. Wind Velocity

The average values of each parameter for one hour duration s are obtained from these.

EXPERIMENTAL INSTRUMENTATION

All measurements for this test were carried out using calibrated instruments. Long Mercury-in-glass

thermometers (0.1ºCgraduations), Swirling Psychrometer with long mercury-in-glass thermometers (0.1ºC) are used for temperature measurements.

Vane anemometer with digital display was employed for wind velocity measurements. manometers, for measurement of water flow velocity, indicated as head in the manometers. Ultrasonic flow         meter was also used to measure the flow.

EXPERIMENT PROCEDURE

Hot Water Temperature:

Two locations Hot water duct inside the tower were chosen for HWT measurement, and the average of the readings for each one hour duration, is taken for each location. The average from the two locations is considered for calculations.

Cold Water Measurement:

Two locations were chosen for CWT measurements and from the average one hour duration for each location, the final average is obtained.

DBT/WBT:

At properly chosen three locations in the vicinity of the tower, both DBT and WBT were noted, taking care to wet the wick around the mercury bulb of the Wet Bulb Thermometer, and whirling the psychrometer every time a reading is taken, the average of the readings from three locations, for each hour is taken for evaluation purposes.

Wind Velocity:

The vane type anemometer is oriented to face the wind flow direction, and kept above the head level, every time a reading is taken.

Flow measurement:

Flow is measured by using ultrasonic flow meter. Readings obtained from ultrasonic flow meter are considered for further evaluation. The total flow into the tower is obtained as the sum of the two main flow risers and the two auxiliary flow risers.

Using the Performance curves evaluation is done from the average values for stable one hour for HWT and CWT, Range R is obtained, and likewise RH from DBT and WBT average values. Temperature readings of hot water and cold water between time periods 12.00 to 13.00 hr are more consistent. Load was constant besides fairly good values of range, WBT which are closer to design values.

SUMMERY OF READINGS 

ParameterAverage
HWT41.44
CWT32.15
Range9.29
DBT34.93
WBT27.82
RH58
Wind Velocity15

CALCULATION

The calculations are performed to compare the present performance of NDCT (with optimal water distribution) with previous performance of NDCT (with uniform water distribution). The past record of NDCT available with company is as follows. 

Previous Performance (with Uniform Water distribution)

Flow: 66250 cum/hr

WBT: 28 ºC

HWT: 41.44 ºC

RH: 60%

Wind velocity=15 km/h

CWT: 33.15 ºC      

Range = HWT-CWT 

= 8.290C

Approach = CWT-DBT

=5.350C

Efficiency = Approach / (Range + Approach)   = 60.77%

Current Performance (with optimal water distribution)

Flow: 65372 cum/hr              

WBT: 27.8 ºC            

HWT: 41.44 ºC                      

RH: 58                                  

Wind velocity=15 km/h               

CWT=33.15 ºC

Range = HWT-CWT

= 9.290C

Approach =CWT-DBT=4.350C                       

Efficiency = 68.10%

Sr. No.ParameterPreviousCurrent
1Range8.299.29
2Approach5.354.35
3Efficiency60.7768.10
TABLE II. COMPARISON OF ACTUAL AND PREDICTED PERFORMANCE OF NDCT

Present CWT is less than past cold water temperature. Thus we obtained a lower water outlet temperature from cooling tower which is 1ºC lower than with a uniform water distribution system. This results in improvement in efficiency by @7%.

GRAPHICAL REPRESENTATION OF READINGS

TABLE III. PSYCHROMETRIC CHART
TABLE III. PSYCHROMETRIC CHART
GRAPHICAL REPRESENTATION OF READINGS

ZILMS OPENION

Measurements of the temperature and velocity fields in a cooling tower were performed for the given power

plant parameters, cooling tower constructional characteristics and ambient air velocity conditions in the vicinity of the cooling tower. The last two parameters influence the homogeneity of the heat transfer, from which we can see the anomalies in the cooling towers operation. Homogeneity in the heat transfer could not only be achieved with fault free construction characteristics but also with a proper distribution of water across the plane area of the cooling tower.

In this study, we have analyzed the water distribution across the plane area of the cooling tower. We have adjusted the amount of water to suit the air flow conditions, which cannot be influenced with natural draft cooling towers. In this way, the optimal moistening of the cooling tower packing is ensured, which results in a more effective heat transfer.

With a optimal water distribution, a constant local water outlet temperature is obtained, which decreases the entropy generation and the exergy lost from the cooling tower. The result is lower outlet water temperature from the cooling tower and, thus, from the condenser, which results in greater efficiency of the power plant.

Leave a Comment

Crypto logo

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus cursus rutrum est nec suscipit. Ut et ultrices nisi. Vivamus id nisl ligula. Nulla sed iaculis ipsum.

Contact

Company Name

Address