The three cooling tower components Dry Cooling Tower, Wet Cooling Tower, and Hybrid Cooling Tower are characterized by special features such as the Approach and the Design state relative humidity. These features and the associated behavior are demonstrated in the tutorial in three cases:
Cooling Water Inlet and Outlet Temperature
If the actual difference between the inlet and outlet temperature of the cooling water is smaller than the difference between the Nominal inlet temperature and Nominal outlet temperature, the maximum Cooling capacity of the cooling tower is also smaller. The following formula illustrates the cause:
\( \begin{equation} \begin{aligned} \dot{Q} = \dot{m}_{H_2O} ⋅ c_{p\ H_2O} ⋅ \Delta T_{coolant}\end{aligned}\end{equation}\).
If at a constant mass flow of the cooling water and a constant heat capacity of the water \( c_{p\ H_{2}O} \) the temperature difference is smaller than the nominal temperature difference, the Cooling supply is smaller than the Nominal cooling capacity. Because the cooling capacity is limited by the volume flow of the air, the maximum cooling capacity of the wet cooling tower or the wet section of the hybrid cooling tower cannot exceed the respective nominal cooling capacity.
Air Temperature
The Cooling supply and the Rated power of the fan of the Wet Cooling Tower and Hybrid Cooling Tower components depend on the Ambient temperature.
The Ambient temperature must be specified in the form of the Simulation node under Settings → Reference condition.
In the Technical input data of the Wet Cooling Tower and Hybrid Cooling Tower components, the relative or specific humidity, and the Design state inlet air temperature must also be specified.
The lowest temperature to which the cooling tower can cool the cooling water is determined by the Ambient temperature, Wet bulb temperature, and the Approach.
Case 1 – Approach and Ambient Temperature
In this case, two Wet Cooling Towers connected in parallel have a Nominal cooling capacity of 1,000 kW each and a Design state inlet air temperature of 25 °C. The two cooling towers are intended to cover a Cooling Demand of 700 kW (see following scheme).
The cooling water is cooled down from 55 °C to 40 °C. The Nominal outlet temperature and the Outlet temperature coincide with 45 °C and the Nominal inlet temperature and the Estimated inlet temperature with 55 °C. A logarithmic function from 10 °C to 39.43 °C was specified for the Ambient temperature.
The pump and fan of the main cooling tower are more efficient than those of the auxiliary cooling tower, but the Approach of the main cooling tower, at 12 °C, is greater than that of the auxiliary cooling tower, which is only 6 °C.
With an Approach of 12 °C, the main cooling tower can only cool the cooling water to 40 °C at a maximum Wet bulb temperature of 28 °C. A Relative humidity of 50 % and a Wet bulb temperature of 28 °C correspond to an air temperature of approximately 37.3 °C. When the air temperature reaches 37.3 °C, the main cooling tower stops and the auxiliary cooling tower starts working. The auxiliary cooling tower can cool down to an air temperature of 44.4 °C (wet bulb temperature of 34 °C and humidity of 50 %). If the Wet bulb ambient temperature rises above 34 °C, the system can no longer cool the cooling water to the desired temperature.
The following figure shows the time series of the Ambient temperature (air temperature), the Wet bulb ambient temperature, the Cooling supply of the main cooling tower and the Cooling supply of the auxiliary cooling tower. This view is stored in the Time Series Interface component.
Case 2 – Cooling Portion and Switched Off Dry Part
According to the following scheme, a Hybrid Cooling Tower with Parallel and forced air mass flow, a Nominal cooling capacity of the dry part of 300 kW, and a Nominal cooling capacity of the wet part of 700 kW at a Design state inlet air temperature of 25° C shall cover a Cooling Demand of 350 kW.
The Nominal cooling capacity of the dry part is 15 kW, the Nominal cooling capacity of the wet part is 5 kW, and the Rated power of the pump is 8 kW. The cooling water is cooled down from 55 °C to 45 °C. The Nominal outlet temperature and the Outlet temperature coincide with 45 °C and the Nominal inlet temperature and the Estimated inlet temperature with 55 °C.
The Ambient temperature increases steadily from 0 °C to 35 °C.
The following figure shows the cooling portion of the dry section. Up to the first marked point, the dry section alone can cover the Cooling Demand. The wet and dry sections then work simultaneously. After the second marked point, the dry section does not cool any more, because due to the price of electricity, water, and power it is cheaper to operate only the wet section. The power consumption is significantly lower when the fan of the dry section is switched off. The sum of the capacities of the wet section fan and the pump is less than the Rated power of the fan (dry part).
Case 3 – Stepped Versus Variable Speeds
The third case compares the power consumption of two Wet Cooling Towers with pumps and fans once with stepped speeds and once with variable speeds. The stepped speeds of the pumps and fans can be created, for example, by combining pumps and fans with different constant speeds into groups and operating them one after the other.
The Cooling Demand is a time series with a continuously increasing value from 10 kW to 790 kW. The Ambient temperature is 10 °C and the Relative humidity is 50 %. Both cooling towers are connected in parallel (see following figure).
The following figure shows the Technical input data of the Wet Cooling Tower With Variable speeds of pump and fan.
The Technical input data of the Wet Cooling Tower With stepped speeds of fan and pump are shown in the following figure.
The relationship between Cooling Demand, Power consumption of the Cooling Tower With variable speed, and Power consumption of the Cooling Tower With stepped speed can be seen in the following figure. At first, only the Cooling Tower With stepped speed operates, only on the first stage. Then both cooling towers work simultaneously. When a higher pump and fan stage of the Cooling Tower With Stepped Speed is turned on, the Power consumption of the Cooling Tower With variable speed decreases.
