This technical article compares the predicted energy consumption of different cooling tower control strategies when the building cooling load is known, and the chiller(s) design has been established.
The available heat-rejection devices in ApacheHVAC include a closed-circuit fluid coolers or an open-circuit cooling towers. However, the closed-circuit towers are not used as often1, due to higher cost and reduced efficiency owing to the added approach of a heat exchanger, this comparison will focus on open circuit cooling tower. In commercial buildings, most of the waste heat, being rejected via a cooling tower, comes from the condenser side of the chiller(s).
Figure 1: Multiple cooling tower control strategies
Cooling Tower Control Strategies:
In IESVE Software (Version 2019 FP2) three different types of multiple cooling tower operation/control strategies are compared. In all cases, the chillers are rejecting the same amount of heat to the CW Loop.
Note: The energy codes (ASHRAE 90.1, IECC, and Title 24) require the use of maximum number of cells as possible. This can be modeled in ApacheHVAC using the “Maximize cell operation” control strategy. 2
Comparison of Cooling Tower Fan Energy:
The total fan energy for each of the cooling towers design options are compared in Figure 2. For this predictive analysis, all options assume that the cooling tower fans are on VSD control, minimum fan flow fraction of 0.20 and the approach temperature of the cooling towers is 10°F. In this particular design scenario, the most efficient control strategy for cooling tower fan energy is the “Maximize cell operation” control strategy with 5 cooling towers, followed by “N+1 Cooling tower per chiller” control strategy with 5 cooling towers. The least efficient control strategy is “Interlocked with chillers” which has four cooling towers.
Figure 2: Fan energy Consumption of the three cooling tower Operation/control strategies
Comparison of Cooling Tower Airflow:
The total fan airflow of each cooling towers is compared in Figure 3. The cooling tower airflow is calculated by APACHE and are shown in the VistaPro. The cooling tower fan airflow follows the same hourly trend as the cooling tower heat rejection plot.
Figure 3: Cooling tower Fan Airflow
Figure 4: Wet-Bulb Depression at 5°F, comparing San Francisco and Sacramento
Cover photo courtesy of Baltimore Aircoil Company
1 Taylor, Steven T. “Optimizing Design & Control of Chilled Water Plants” ASHRAE Journal, Mar. 2012, pp. 60-66
2 Morrison Frank “Saving Energy with Cooling Towers” ASHRAE Journal, Feb. 2014, pp. 34-40