(372h) Optimal Synthesis of Cooling Water Systems

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Cooling water systems are basically present in every plant since many processes have to reject heat. In the plant design phase, a design cooling water demand has to be set in order to size the cooling water pump system. This demand can vary significantly if the plant operates with batch or semi-batch processes or if the plant production capacity is not constant. Even varying climate conditions can impact the plant cooling water demand [1]. Therefore, a cooling water pump system can have a significant slack for some considerable periods, resulting in high capital and operating costs [2], besides operating challenges. Since pumping operating costs can reach as high as 50% of the energy consumption in industries [3], the optimal synthesis of a pumping system can yield a significant cost reduction. During low cooling water demand periods, cooling water pumps can operate with significant reduced flow rates work far from the best efficiency point (BEP) or even below the manufacturer minimum flow rate [4]. Therefore, the pumping system has to be designed to attain a high turndown by using either variable frequency drives (VFDs) or multiple pumps.

VFDs have become more used in industries to increase the turndown of motor-driven rotating equipment [5, 6]. Although VFD costs have decreased in recent years, purchasing and installation costs [7], as well as reluctance to use VFDs [8], are factors that make some industries still prefer the use of control valves. For large industrial plants, the cooling water demand can significantly be above 1,000 m3.h-1 [9], hence it is recommended that the cooling water system uses multiple pumps for the service due to difficulties of purchasing large pumps and motors. As the number of pumps increases, both pumping system turndown and capital expenses increase.

This work presents a methodology for determining the optimal number of centrifugal pumps in a cooling water system given that the required system head is previously established. Using the Pump Selection Systems software (Release 1.70.0.0) from Goulds Pumps, pump data, such as BEP, consumed pump power, pump head and the required net pressure suction head (NPSHr), is obtained for different pump and impeller sizes. This data is used to determine the flow rate for individual pumps, and to estimate capital and operating costs. Comparing the analyzed alternatives, the optimal alternative is obtained to determine the number of pumps, if a VFD is used in order to minimize the overall (capital and operating) costs.

The operating point of a centrifugal pump is defined as the intersection of the centrifugal pump characteristic curve with the system characteristic curve [10]. Project engineers select a centrifugal pump model that yields an operating point that complies with the required design flow rate and design head. Centrifugal pump manufacturers often have more than one pump model that complies with these selection criteria. However, sometimes only few models, or even none, operate near the pump’s BEP, which is the point in the pump characteristic curve where efficiency reaches a maximum value. If the operating point of the pump is too far from BEP, not only the pump efficiency is low, hence increasing electrical energy consumption, but also the pump may be difficult to operate. It is recommended that the pump operating point is situated at the 70-120% range of the BEP [10], known as the optimal range.

For an operation with a constant flow rate, pump selection is relatively straightforward [10]. But if there is a wide range of operating flow rates, pump selection has to account for multiple operating points, where the probability that one of these points is located outside of the pump optimal range is high. The use of VFDs can mitigate this problem, but if the operating range is very wide, the use of more than one pump associated in parallel can be required. For design flow rates above 1,000 m3.h-1, not only suitable pump models are hard to find, but also required motor sizes increase, which have high delivery times and are more expensive. As the number of pumps increases, the pumping system turndown also increases since the operating flow rate can vary significantly by turning on and off the pumps. This work investigates the performance and costs of pumping systems with no VFD installed compared to pumping system with at least one VFD installed.

As a case study, this paper considers a paper mill that requires a maximum cooling water flow rate of 5,500 m3/h and a minimum flow rate of 2,200 m3/h. It is assumed that the system head is maintained constant at 45 m inside this range by the action of control valves. Different scenarios are considered for this case study, each with different and variable cooling water demands within the maximum and minimum flow rates range.

For the optimal synthesis of the cooling water pumping system, two costs are considered in this work: capital and operating. The capital costs consider the purchase of the equipment (pumps, motors and VFDs), and indirect costs related to the purchase and installation of those. For purchase cost estimation, Brazilian suppliers were consulted for selected models and sizes price quotes. The operating costs consider the electrical energy consumption, and this work considers electrical energy prices practiced in Brazil.

The pump model selected is the Goulds Model 3196 ANSI Standard Dimension Process Pump, which is an overhung centrifugal pump used in several types of industries [11]. The PSS software is used to select pump size and impeller size and, in this case, the PSS software is unable to find a pump size capable of pumping the maximum flow rate, therefore an association of pumps of at least five pumps in parallel is required. For each operating scenario, the overall costs were evaluated for alternatives with five to eight pumps.

The costs are estimated for systems without VFD, and with one VFD installed. Due to the use of multiple pumps, the pumping system has the necessary turndown to comply with the varying cooling water demand. Therefore, the pumps operate in most of the scenarios within the 70-120% range of the BEP. For systems where the pumps operate near their design operating point, the use of VFD does not yield significant operating costs reduction. However, if the pumps operate at points farther from the design operating points, the use of VFD can yield savings as high as 17% for larger pump sizes.

The greatest cost difference for the alternatives evaluated was for capital costs. The alternative that yields the lowest capital cost is the one with a pumping system composed of five pumps, which is 17% least expensive that the second best alternative, a pumping system with 6 pumps. This occurs since the pump size selection for all alternatives resulted in similar pump sizes and therefore similar purchase cost estimates for all these alternatives.

Therefore, for the case study, the established methodology by this work for the optimal synthesis of a pumping system shows that although operating costs can account for more than 70% of the overall costs, the capital costs is determinant to the optimal solution.


LITERATURE CITED

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[7] Siemens. Cost Considerations when Selecting Variable Frequency Drive Solution [Internet]. [cited 2019 Jan 29]:[9p.]. Available from: https://www.industry.usa.siemens.com/drives/us/en/electric-drives/medium-voltage-drives/products/perfect-harmony/gh180/Pages/new-perfect-harmony-white-paper.aspx

[8] Van Der Merwe R.G., Hoogendoorn C. VSD Advantages, Disadvantages, Selection Criteria and Installation Tips. Energize. 2005 May: 44-54.

[9] Rubio-Castro E., Serna-González M., Ponce-Ortega J.M., El-Halwagi M.M. Synthesis of Cooling Water Systems with Multiple Cooling Towers. Appl. Therm. Eng. 2013; 50:957-974.

[10] Girdhar P., Moniz O. Practical Centrifugal Pumps: Design, Operation and Maintenance. 1st Ed. Netherlands: Elsevier; 2005. 260p.

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