Recall Solved Problem 4.2, in which we analysed a Supercritical Steam Powr Plant with Reheat and an Open Feedwater Heater/De-aerator to service about 10,000 households in Athens, Ohio. The City Council was somewhat perturbed about the possible thermal pollution of the Hocking River and have requested that we evaluate the use of a Cooling Tower in order to cool the condenser fluid stream, rather than direct cooling by the Hocking River, with only the makeup water being supplied by the river. For purposes of this exercise we have chosen the additional parameters associated with the Cooling Tower as shown in the diagram below.
We found in Solved Problem 4.2 that the power required to cool the steam in the condenser is 12.9 MW. Furthermore since we are not returning the water to the Hocking River we no longer have the restriction of a 10°C maximum water temperature increase. We have chosen a temperature of 40°C at station (11) in order to ensure cooling since from the steam tables we find that the saturation temperature at 10 kPa is 45.8°C. In this solved problem we wish to evaluate the performance and suitability of this proposed design.
Solution approach:
a) Determine the volumetric flow rate of the cooling water through the condenser. You may neglect the power required to drive the circulating pump.
b) Draw the cooling tower process between stations (12) - (13) on the Psychrometric Chart. Notice that we have extended the moisture specific humidity range on this chart from 30 to 35 grams/kg-air in order to accomodate the extremely high humidity at station (13), Clearly mark and label the relevant values on the Psychrometric Chart, including the values of enthalpy (h), relative humdity (φ) and specific humidity (ω) of stations (12) and (13).
c) Starting with the energy equation and using values obtained from the Psychrometric Chart, determine the volumetric flow rate of the dry air required at station (12) in order to cool the water from 40°C to 25°C. Even though similar equations are developed in Chapter 10c, we will always require that you include the complete derivation. Determine also the mass flow rate of the makeup water required from the Hocking River
d) Discuss the proposed system with respect to its environmental impact and feasibility. In this discussion include a comparison of the flow rate of of the makeup water from the Hocking River with that required without the cooling tower.
Without even evaluating the amount of flow in the Hocking River (Hint- Google: Hocking River Flow) we find that the cooling tower has significantly reduced the water flow rate from 309 kg/s (refer: Solved Problem 4.2) to 4.8 kg/s. However we are not sure how large and tall the cooling tower will have to be in order to induce an air flow of 158 m3/s hence at this stage this can only be considered a partial evaluation. Furthermore we feel that the City Council have ignored a significant possible environmental problem: How much coal is required to fire this plant, and without rail or barge access how will the coal be delivered. We will need to wait until the following chapter on Combustion before we can begin to answer this question.
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