July 09, 2006

Cooling Towers: Not the Coolest One

INTRODUCTION
Cooling tower is basically a heat exchanging device in which heat exchange takes place by virtue of mass transfer potential between process fluid (cooling water) and utility fluid (ambient air). Hence, it performs evaporative cooling of water.

Cooling tower is an equally critical section in a plant because the on-stream utilization of main plants and efficiency of the manufacturing process depends on the performance of cooling towers also. It’s the most neglected area of the plant where huge potential of savings lie within the existing tower volumes.

This article is covering certain practical aspects of cooling tower performance & its operation.

NEED
Very high cost of cooling water e.g. RS. 2 to 6 /m3 of treated water doesn’t permit plant operators to operate in an open loop cycle in the cooling water circuit. Hence, the closed loop cycle is selected in almost all process plants all over the world.

In a closed loop cycle, the power consumption required for cooling the water (in cooling tower fans only while considering pumping cost as almost same in both the cases) varies from 0.02 to 0.04 kWh/m3 of water so, the cost of cooling of water ranges from RS. 0.02 to 0.20/m3 in Indian conditions. This is almost 100 times lesser than the cost of cooling water in a once through cycle. That’s why it is customary to use cooling towers in any chemical process industry, especially in hot climatic conditions.

THEORY
Cooling towers work on the humidification principal, which is driven by the mass transfer potential between cooling water & ambient air. However, in actual operation it is a combination of different driving forces between water & air. Contrarily, the cooling tower design is considered based on the enthalpy difference as driving force, because this is the least driving force available throughout the tower at any point of time. This occurs when the passing air is completely saturated. In all other cases, more than one driving forces are available between air & water. So, the most popular merkel equation is only a conservative way of modeling of a cooling tower.

However, merkel equation is considered to be the most effective way of designing a cooling tower by many engineering institutions & academies. For details of merkel equation please refer Cooling Towers: Design & Operation on
www.cheresources.com

CONSTRUCTION
It’s a packed bed tower in which structured packings are used in a stacked loading pattern. Conventionally, these packings are called fills. These fills are means of providing larger surface area within a confined available volume as it is used in many conventional applications e.g. in distillation, absorption, saturators etc.

Construction-wise cooling towers are of two types mechanical draft & natural draft towers. Natural draft towers are very large stack type towers in which flow of air is through buoyancy force because of density difference. Mechanical draft towers are having induced/forced draft fans, which help in flowing air through the circulating water. These are sub classified based on flow pattern of air through the tower e.g. cross flow & counter flow towers.

OPERATION
Cooling towers are designed based on the worst condition of ambient air in terms of highest wet bulb in the area which prevails for one or two months only; during the year and, maximum cooling load from the process side as considered in the over design of process equipments. Also, the self, inherent design margin is always available in these towers because of their availability in standard size modules. Hence, every cooling tower practically has many margins in the existing tower volume for the desired cooling of water e.g.

  • Maximum Wet bulb during the year.
  • Maximum cooling load from process side.
  • Capacity margins from process side.
  • Self-inherent design margins due to modular construction.

Therefore, the cooling towers must give better results than anticipated in the design. The direct measurement of cooling towers performance is the approach of cooling at the outlet of tower. Hence, even if towers are operating at design value then too it is underutilized. We’ll see in the next paragraphs that how it is happening in actual operation.

  1. Effect of Water Temperature

    The first impact of design margins from the process side is that normally cooling water flow is kept same as envisaged in the original design & process side heat load may be lower (due to capacity over design or for any other reason). Therefore, the cooling tower operates at lower cooling water return temperature (the temperature of cooling water at the inlet of tower).

    The impact of lower return water temperature is that the cooling approach must reduce 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.
  2. Effect of Wet Bulb
    Similarly, since the tower is designed based on the wet bulb of 30°C, which prevails only for around two months during the year, it has to run at lower wet bulb temperature e.g. say 28°C for rest of the year. In this case, the approach of cooling will go up by ~50% of the difference between design & actual wet bulb resulting in actual approach of 5°C instead of 4°C. 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.
  3. 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.
  4. 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.
  5. 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.

PERFORMANCE MONITORING
Following are the general guidelines for evaluating the operational performance of the cooling towers.

  • Scheduled monitoring should be followed especially covering all seasonal changes.
  • Intermittent & need based monitoring at different loads.
  • Adjustment of water loading.
  • Regular inspection for air short circuiting, fills displacement, end side gaps in the walls.

According to Merkel Equation, one can go as per the following procedure for the performance evaluation of an existing cooling tower.

  1. Measure Air velocity through the tower in each cell.
  2. Consider Design Wet bulb of inlet air & L/G as given in data sheet.
  3. Exit wet bulb temperature of air by the heat balance on waterside.
  4. Given the exit density of wet air find out exit dry bulb, which should lie, in between its wet bulb & water inlet temperature.
  5. Now find out the inlet dry bulb temperature of air from the heat balance on airside. Of course, these two heat balances of airside & waterside must be same.
  6. Now calculate the equilibrium enthalpy of wet air at inlet & outlet corresponding to the water temperature of that side.
  7. Find the difference in equilibrium enthalpy & actual enthalpy of wet air & take reciprocal of it, for each increment in water temperature from the bottom to top of the cooling tower.
  8. Sum up the average of reciprocals between two intervals & thus reach to the top of cooling tower.
  9. This will give you the No of transfer units overall NTUG. This is the parameter, which is useful in predictions because the available numbers of transfer units are going to remain same in all cases.
  10. Now find KaV factor by multiplying NTUG with the airflow rate. This factor is the indicator of rate of mass transfer between water & air and is analogous to mass transfer coefficient in distillation/absorption unit operation.
  11. Repeat procedure from item 1 - 10 for actual operation & find out the cold-water temperature & expected ATE.
  12. Compare expected ATE with actual ATE to find out the performance of cooling tower.
  13. Establish reasons for the deviation found in item 12.
  14. Take corrective actions for improvement.

This Article is not yet complete, I will refine it & add more info later when I get some time.

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