Cooling Tower Performance and Assessment

Now, after discussing about Components of Cooling Tower in the last Blog. Now, we will discuss about it's performance and performance assessment.

Cooling tower performance

The important parameters from point of determining the performance of cooling towers are illustrated in figure given below

                                                    Fig  Cooling tower performance chart

i)        “Range” is the difference between the cooling tower water inlet and outlet temperature.

ii)       “Approach” is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. Although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance. (see Figure ).

iii)     Cooling tower effectiveness (in percentage) is the ratio of range, to the ideal range, i.e., difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach).

iv)      Cooling capacity is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.

v)       Evaporation loss is the water quantity evaporated for cooling duty and, theoretically, for every 10,00,000 kCal heat rejected, evaporation quantity works out to 1.8 m³. An empirical relation used often is:

        Circulation Rate (m³/hr) * Temp. Difference in ^oC

Evaporation Loss = ----------------------------------------------------------- m³/hr

                                                   675

vi)       Cycle of concentration is the ratio of dissolved solid in circulating water to make up water

vii)     Blow down losses depend upon cycles of concentration and the evaporation losses and is given by relation:

Blow Down = Evaporation Loss / (C.O.C. – 1)

viii)     Liquid/Gas (L/G) ratio, of a cooling tower is the ratio between the water and the air mass flow rates. Against design values, seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness through measures like water box loading changes, blade angle adjustments.

Thermodynamics also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air:

where: L/G = liquid to gas mass flow ratio (kg/kg) T₁ = hot water temperature (⁰C) T₂ = cold water temperature (⁰C) h₂ = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature

(same units as above) h₁ = enthalpy of air-water vapor mixture at inlet wet-bulb temperature (same units as above)

Factors Affecting Cooling Tower Performance

There are some factors which affects the performance of cooling given as below.

Capacity

Heat dissipation (in kCal/hour) and circulated flow rate (m³/hr) are not sufficient to understand cooling tower performance. Other factors, which we will see, must be stated along with flow rate m³/hr. For example, a cooling tower sized to cool 4540 m³/hr through a 13.9^oC range might be larger than a cooling tower to cool 4540 m³/hr through 19.5^oC range.

Range

Range is determined not by the cooling tower, but by the process it is serving. The range at the exchanger is determined entirely by the heat load and the water circulation rate through the exchanger and on to the cooling water.

Range ^oC = Heat Load in kcals/hour / Water Circulation Rate in LPH

Thus, Range is a function of the heat load and the flow circulated through the system.

Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature. For example, the cooling tower

Cold Water Temperature 32.2^oC – Wet Bulb Temperature (26.7^oC) = Approach (5.5^oC)

As a generalization, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size. Usually a 2.8^oC approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. If flow rate, range, approach and wet bulb had to be ranked in the order of their importance in sizing a tower, approach would be first with flow rate closely following the range and wet bulb would be of lesser importance.

Performance Assessment of Cooling Towers

In operational performance assessment, the typical measurements and observations involved are:

·        Cooling tower design data and curves to be referred as the basis.

·        Intake air WBT and DBT at each cell at ground level using a whirling pyschrometer.

·        Exhaust air WBT and DBT at each cell using a whirling psychrometer

·        CW inlet temperature at risers or top of tower, using accurate mercury in glass or a digital thermometer.

·        CW outlet temperature at full bottom, using accurate mercury in glass or a digital thermometer.

·        CW outlet temperature at full bottom, using accurate mercury in glass or a digital thermometer.

·        Process data on heat exchangers, loads on line or power plant control room readings as relevant.

·        CW flow measurement, either direct or inferred from pump motor KW and pump head and flow characteristics.

·        CT fan motor amps, volts.

·        TDS of cooling water.

·        Rated cycles of concentration at the site conditions.

·        Observations on nozzle flows, drift eliminators, condition of fills, splash bars etc.

Control of tower air flow can be done by varying methods: starting and stopping (On-Off) of fans, use of two or three speed fan motors, use of automatically adjustable pitch fans, use of variable speed fans.

On-Off fan operation of single speed fans provides the least effective control. Two speed fans provide better control with further improvement shown with three speed fans. Automatic adjustable pitch fans and variable speed fans can provide even closer control of tower cold water temperature. In multi cell towers, fans in adjacent cells may be running at different speeds or some may be on and others off depending upon the tower load and required water temperature. Depending upon the method of air volume control selected, control strategies can be determined to minimize fan energy while achieving the desired control volume of the Cold water temperature.