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Cooling systems are significant consumers of energy
Cooling water is used extensively in a number of industries for the lowering of process temperatures. Cooling systems are significant consumers of energy. The energy is needed to pump water to heat exchangers and to operate fans in the cooling towers. Efficiency of the cooling system has a big impact on the productivity of some industrial plants. In many cases, there is a direct relation between the cold water temperature and process efficiency.
DHI examined the possibilities of improving the cooling system energy efficiency with our Industrial Cooling Water Solution, which applies new advanced algorithms based on online water distribution models. The study was executed based on historical data and operational measurements from a major petrochemical production site in a moderate climate.
Optimisation of pressures and flows
Our study revealed substantial energy-saving potential for the client, especially in cold and moderate climates. Applying just one of the identified options in the studied system would potentially provide 11% of energy savings annually.
To reach for further savings in the studied system, some significant challenges need to be handled with caution. The highest saving potential is directly linked to optimisation of cooling water flow and/or pressure. Such operations without the proper approach may however have a negative impact on the production processes. Thus, a deep understanding of the system’s hydraulics and thermal conditions is required.
An online water distribution model
To safely reduce cooling water flow and/or pressure, it is necessary to continuously monitor the conditions on the water aspects of all significant heat exchangers. Such monitoring is possible by application of an online water distribution model which can calculate the flows and pressures in the whole system.
The results of these calculations, in combination with basic process data, can be then used to monitor critical water-side heat exchange parameters like heat transfer coefficients, flow speeds and water temperatures. The control system would then compare the calculated values against predefined threshold values to trigger alarms when necessary, and ensure that any optimisation step will not adversely affect the production process.
Advanced monitoring and control
The described mechanism can also be by itself a valuable extension of existing process monitoring and control systems. Continuous monitoring of critical water-side heat exchanger parameters is a solution which can increase process safety and additionally enhance productivity in plants where cooling capacity is a limiting factor (for example, in warm climates) by providing answers to difficult questions like:
- When and where is the risk of scaling significant?
- When should I increase the water flow to improve process safety?
- Which installations have the greatest ‘cooling comfort’?
- What will be the impact of reduction of cooling water flow on various process nodes?
- How to distribute the cooling water to particular process nodes in order to provide the highest productivity of the whole plant?
- How to adjust the cooling system to diurnal and seasonal changes of air parameters?
The study identified these promising optimisation measures for the client:
- Improve hydraulics
This will make it easier for the water to flow through the pipes, remove bottlenecks (replace old pipes, increase diameters where appropriate), open half-closed valves, and so on. A hydraulic model greatly facilitates finding and evaluating the required actions.
- Improve system layout
This is especially if you have heat exchangers on different levels. It might be worth fully or partially to separate pressure zones or add local pressure boosting pumps.
- Optimise water distribution
This optimisation measure ensures that the required amount of cooling water reaches the right heat exchangers in the right time. Advanced algorithms should be used to optimise water distribution in real time.
- Optimise total flow and temperature
Reducing air flow through the cooling tower on a cold day is often much less energy-effective than reducing water flow and achieving lower water temperatures.
- Improve water distribution over the cooling towers
Cooling tower performance may be strongly reduced when the water distribution is uneven.
DHI’s Industrial Cooling Water Solution has been developed as a tool which enables users to access these identified gains. Depending on the local technical conditions and climate, the resulting improvements can be substantial: cooling capacity increase up to 20%; pumping energy savings up to 40%.