The open loop evaporative cooling tower

The open loop evaporative cooling tower consists of a shell containing packing/fill material with a large surface area. Nozzles arranged above the packing, spray and distribute heated cooling water from the condenser evenly onto the packing. The water trickles through the packing into a pond from which it is pumped back to the condenser. The water is cooled by air, drawn or blown through the packing by means of a fan. The air flow, which is either in counter-flow or cross-flow to the water flow, causes some of the water to evaporate. The evaporated water is continuously replenished by make-up water. Evaporation however also increases the concentration of TDS (Total Dissolved Solids) in the cooling water. Blow-down of the cooling water is therefore required which is replenished by additional make­up water to dilute and to maintain concentration levels within acceptable limits. It is possible to cool the water below the ambient dry bulb temperature using a cooling tower, as the wet-bulb temperature determines the degree of cooling. An approach of 4°C can still be achieved economically, which is the difference between the water outlet temperature and the ambient wet-bulb temperature [4].

The cooling tower was simulated with the TRNSYS Type 51 b, which models the performance of a multiple-cell counter-flow or cross-flow cooling tower and sump. To employ this Type the user has to enter two coefficients, used in the mass transfer correlation [10]. Although these coefficients are usually difficult to obtain, in the present case they were provided by a cooling tower manufacturer.

The physical parameters used in the cooling tower Type are:

• Maximum cell flow rate = 3800 m3/h

• Fan power at maximum flow rate = 0.33 kW

• Mass transfer constant, c = 2,28

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Подпись:Fig. 3. Open loop wet cooling

The selected TRNSYS Type is particular useful and simplifies the method of solution when compared to more conventional numerical procedures. As the Merkel method, this approach is based on the following simplifying assumptions:

• The Lewis factor relating heat and mass transfer is equal to 1;

• The air exiting the tower is saturated with water vapor and it is characterized only by its enthalpy;

• The reduction of water flow rate by evaporation is neglected in the energy balance.

Because of these assumptions, however, the Merkel method does not accurately represent the physics of the heat and mass transfer process in the cooling tower fill.

The method of Poppe, developed in the 1970s, does not make the simplifying assumptions of Merkel. Predictions from the Poppe formulation result in values of evaporated water flow rate that are in good agreement with full scale cooling tower test results. In addition, the Poppe method predicts the water content of the exiting air accurately [11]. An EES code exploiting the Poppe method for a wet cooling tower is being developed and will be employed in future works.