The Component Wet Cooling Tower
The Wet Cooling Tower component represents a general model for the provision of cooling by a cooling tower that evaporates water into the surrounding air.
Component Template
The Component template Wet_Cooling_Tower.e-ctpl is located in the component templates folder Cooling Supply.
Integration Into a Scheme
How the Component Works
The Wet Cooling Tower component represents the removal of heat from another process by evaporation of water into the ambient air as the main cooling principle. The coolants are water and air. It is an open circulating cooling system in which the major part of the thermal energy is transferred to the air by evaporation of the water. Due to evaporation, water must be added regularly.
Technical Input Data
The following Design parameters must be specified in the Technical input data (see following figure):
Water
Under the heading Makeup water, the Cycles of concentration and the Drift loss factor must be entered.
The Cycles of concentration refer to the accumulation of dissolved minerals in the recirculating cooling water. In order to balance the increasing concentration of solids in the cooling system the accumulated minerals must be regularly removed. The cooling water is partially drained (blow down) and replaced by makeup water. The Cycles of concentration \( COC \) are defined as quotient of salt concentration in the blow-down (dividend) and the salt concentration in the makeup water (divisor). A value between 3 and 7 must be entered.
The Drift loss factor \( DLF \) indicates the proportion of water that is released into the air as water droplets from the cooling tower and which must be replaced by Makeup water.
The following Technical input data are required for the Heat transfer medium water:
Enter the Estimated inlet temperature as estimatetd input value.
The nominal flow rate of the pump \( \dot{m}_{H_{2}O\ nominal} \) and fan \( \dot{m}_{air\ max} \) is calculated from the temperatures of the heat transfer medium in the nominal state.
From the temperatures of the heat transfer medium in the operating state (Estimated inlet temperature and outlet temperature) the actual Pump power consumption \( P_{el\ pump} \) and Fan power consumption \( P_{el\ fan} \) are calculated.
The Approach is the temperature difference between the cooling water Outlet Temperature and the Wet Bulb Ambient Temperature. The cooling tower cannot cool below a temperature lower than the sum of the Wet bulb ambient temperature and the Approach.
The Wet Bulb Ambient Temperature is calculated from the Ambient temperature and either the Reference specific or Relative humidity. The Wet bulb ambient temperature is given in the Technical output data under the heading Transfer medium (see figure below).
Air
The following data are required for the Heat transfer medium air:
These data are used to calculate the required enthalpy for the energy balance.
The ambient temperature, the ambient pressure, and either the relative humidity or the water vapor content (the specific humidity) must be specified under the heading State of the inlet air (environment).
Calculation
The following formulas are used to calculate the mass flows and the energy:
\( \begin{equation} \begin{aligned} \dot{Q}_{cool\ nominal} = \dot{m}_{H_{2}O\ nominal} \cdot c_{p\ H_{2}O} \cdot \Delta T_{H_{2}O} \end{aligned}\end{equation}\), where \( c_{p\ H_{2}O} \) is the heat capacity of water, and
\( \begin{equation} \begin{aligned} \dot{Q}_{cool\ nominal} = \dot{m}_{air\ max} \cdot \Delta h_{air} \end{aligned}\end{equation}\), where \( \Delta h_{air} \) is the enthalpy difference of the air.
The following empirical formulas are used to calculate the amount of makeup water required:
\( \begin{equation} \begin{aligned} Evaporation = \dot{V}_{H_{2}O} \cdot 0,00085 \cdot 1,8 \cdot \Delta T_{H_{2}O} \end{aligned}\end{equation}\),
\( \begin{equation} \begin{aligned} Blow\ down = evaporation / (COC-1) \end{aligned}\end{equation}\),
\( \begin{equation} \begin{aligned} Drift\ loss = DLF \cdot \dot{V}_{H_{2}O} \end{aligned}\end{equation}\), and
\( \begin{equation} \begin{aligned} \dot{V}_{makeup\ water} = evaporation + blow\ down + drift\ loss \end{aligned}\end{equation}\).
Part Load Behavior
The Part load behavior can be specified by the Constant efficiency in part-load (proportional electrical power consumption) or Characteristic curves (relative to nominal point).
The following equations show the relationship between the flow rates and the electrical power consumption when selecting Constant efficiency in part-load:
\( \begin{equation} \begin{aligned} \frac{ \dot{V}_{air}} {\dot{V}_{air\ max}} = \frac{P_{el\ fan}} {P_{el\ fan\ nominal}} \end{aligned}\end{equation}\),
\( \begin{equation} \begin{aligned} \frac{ \dot{V}_{H_{2}O}} {\dot{V}_{H_{2}O\ nominal}} = \frac{P_{el\ pump}} {P_{el\ pump\ nominal}} \end{aligned}\end{equation}\).
When selecting Enter characteristic curve (relative to nominal point), two characteristic curves must be entered.
The first characteristic curve \( f_{el\ Vent\ rel} \) establishes the relationship between the volume flow of air \( \dot{V}_{air} \) and the electrical power of the fan \( P_{el\ fan} \).
The second characteristic curve \( f_{el\ pump\ rel} \) shows the relationship between the volume flow of the cooling water \( \dot{V}_{H_{2}O} \) and the electrical power of the pump \( P_{el\ pump} \) (see following figure).
The following formulas are used to calculate the part load behavior:
\( \begin{equation} \begin{aligned} f_{el\ fan\ rel} \left ( \frac{ \dot{V}_{air}} {\dot{V}_{air\ max}} \right) = \frac{P_{el\ fan}} {P_{el\ fan\ nominal}} \end{aligned}\end{equation} \),
\( \begin{equation} \begin{aligned} f_{el\ pump\ rel} \left ( \frac{ \dot{V}_{H_{2}O}} {\dot{V}_{H_{2}O\ nominal}} \right) = \frac{P_{el\ pump}} {P_{el\ pump\ nominal}} \end{aligned}\end{equation} \).
Economic Data
In addition to the other Operating costs and the capital-related costs (Investment costs and Reinvestment costs), the Costs for operating resources (fresh water and sewage water) are also taken into account in the economic evaluation. In order to map the water consumption and wastewater generation described above, the Water costs and Sewage costs are included in the economic balance of the Variant analysis as Operating costs if the Water price and the Sewage price were previously specified in the Economic input data (see following figure).
Technical Output Data
In the Technical output data (see following figure), the Cooling Efficiency is displayed. The Cooling Efficiency indicates the ratio of actually emitted heat to potentially maximum dissipated heat. It is calculated with the following formula:
\( \begin{equation} \begin{aligned} \eta = \frac{ T_{coolant\ in} – T_{coolant\ out}} {T_{coolant\ in} – T_{wet\ bulb}} \end{aligned}\end{equation}\).
In the section Electric power consumption, the Total calculated electric power consumption and its components Fan power consumption and Pump power consumption are output (see following figure).
For the heat Transfer medium (water) the Range (difference between Coolant inlet temperature and outlet temperature), the Water volume flow rate, Air volume flow rate, and the Wet bulb ambient temperature are output (see following figure).
Finally, the volume flow of the Makeup water and its parts (Evaporation loss water, Drift loss water, and Blow-down water) and the State of the inlet air (environment) with the Wet bulb temperature, Steam content, Saturated steam content, and the Saturated steam pressure are displayed (see following figure).
