Designing a chilled water system with a cooling tower involves several critical steps and considerations. This comprehensive guide provides a detailed overview of the system configuration, chiller selection, cooling tower selection, piping design, control system, and technical specifications, along with a DIY approach for those with the necessary knowledge and experience.
System Configuration
When designing a chilled water system with a cooling tower, the system configuration plays a crucial role. The chilled water system should be designed with variable primary chilled water pumping with a controlled bypass to maintain minimum chilled water flow at the chiller evaporator. This approach ensures efficient operation and reduces energy consumption.
For the condenser water arrangement, constant volume condenser water pumping with variable speed cooling tower fans is recommended. This approach provides better control and reduces energy consumption. The variable speed cooling tower fans can adjust their speed based on the cooling load, allowing for optimal energy efficiency.
Chiller Selection
Choosing the right chiller is essential for the efficient operation of the chilled water system. Electric drive water-cooled chillers are commonly used due to their high efficiency and reliability. The chiller should be selected based on the cooling load and the ambient temperature of the location.
When selecting a chiller, consider the following factors:
- Cooling capacity (tons): The chiller should be sized to meet the peak cooling load of the building or facility.
- Chiller efficiency (kW/ton): Look for chillers with high efficiency, typically in the range of 0.5-0.7 kW per ton of cooling capacity.
- Refrigerant type: Opt for environmentally friendly refrigerants, such as R-134a or R-410A, which have a lower global warming potential.
- Compressor type: Scroll, screw, or centrifugal compressors are commonly used in water-cooled chillers.
- Condenser water temperature range: The chiller should be able to operate efficiently within the expected condenser water temperature range.
Cooling Tower Selection
The cooling tower is a critical component of the chilled water system. The cooling tower should be selected based on the cooling load, the wet bulb temperature of the location, and the available space. Cooling towers with variable speed fans are recommended for better energy efficiency.
When selecting a cooling tower, consider the following factors:
- Cooling capacity (tons): The cooling tower should be sized to match the chiller’s capacity.
- Cooling tower efficiency (gpm/ton): Aim for a cooling tower efficiency of 1-2 gallons of water per minute per ton of cooling capacity.
- Wet bulb temperature: The cooling tower’s performance is heavily influenced by the local wet bulb temperature, which should be considered in the selection process.
- Cooling tower type: Cooling towers can be either induced draft or forced draft, with induced draft towers generally being more energy-efficient.
- Cooling tower materials: Choose materials that are corrosion-resistant and suitable for the local water quality.
Piping Design
Piping design is crucial for the efficient operation of the chilled water system. The piping should be designed to minimize pressure drops and ensure proper flow rates. The piping should be insulated to reduce heat gain or loss.
For multiple cooling tower systems, there are two common approaches:
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Independent Circuit: Each chiller has its own pump, cooling tower, and set of pipes. This approach is the simplest, but it lacks flexibility and has higher installation costs due to the extra piping.
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Header Supply and Return: A common header is used in the supply and return piping. This approach is more flexible and cost-effective, allowing for the switching of chillers to different cooling towers.
When designing the piping system, consider the following:
- Pipe size: Determine the appropriate pipe size based on the flow rate and pressure drop calculations.
- Pipe material: Choose corrosion-resistant materials, such as copper, stainless steel, or PVC, depending on the water quality and system requirements.
- Insulation thickness: Determine the appropriate insulation thickness based on the temperature difference and ambient temperature to minimize heat gain or loss.
- Expansion tanks and air separators: Include these components to manage system expansion and remove air from the chilled water and condenser water loops.
Control System
A control system is essential for the efficient operation of the chilled water system. The control system should be designed to control the flow rate, temperature, and pressure of the chilled water and condenser water. The control system should also monitor the operation of the chillers, cooling towers, and pumps.
The control system should include the following components:
- Temperature sensors: Monitor the chilled water and condenser water temperatures.
- Flow meters: Measure the chilled water and condenser water flow rates.
- Pressure sensors: Monitor the system pressures.
- Variable frequency drives (VFDs): Control the speed of the chilled water and condenser water pumps, as well as the cooling tower fans.
- Programmable logic controller (PLC) or building automation system (BAS): Integrate and coordinate the operation of the various system components.
Technical Specifications
Here are some technical specifications for designing a chilled water system with a cooling tower:
| Specification | Range |
|---|---|
| Chilled water flow rate | 2-5 gallons per minute per ton of cooling capacity |
| Chilled water temperature difference | 10-15°F |
| Condenser water flow rate | 3-5 gallons per minute per ton of cooling capacity |
| Condenser water temperature difference | 10-15°F |
| Chiller efficiency | 0.5-0.7 kW per ton of cooling capacity |
| Cooling tower efficiency | 1-2 gallons of water per minute per ton of cooling capacity |
| Piping size | Based on the flow rate and pressure drop |
| Insulation thickness | Based on the temperature difference and ambient temperature |
DIY Approach
Designing a chilled water system with a cooling tower can be a complex task, and it is recommended to hire a professional engineer for the design. However, if you have the necessary knowledge and experience, you can design the system yourself.
Here are the steps for a DIY approach:
- Calculate the Cooling Load: Determine the building’s cooling load based on factors such as size, occupancy, and equipment.
- Select the Chiller: Choose the chiller based on the cooling load and ambient temperature of the location.
- Select the Cooling Tower: Select the cooling tower based on the cooling load, wet bulb temperature, and available space.
- Design the Piping System: Design the piping system based on the flow rate and pressure drop calculations.
- Select the Control System: Choose the control system components based on the system requirements.
- Verify the Design: Ensure that the design calculations and specifications comply with industry standards and regulations.
Remember to consult with local authorities and follow all applicable building codes and regulations when designing a chilled water system with a cooling tower.
References:
– U-M Design Guide: Chilled Water Systems, July 2019, https://umaec.umich.edu/desguide/tech/23/DG230050.pdf
– Designing Chilled Water Systems, Consulting-Specifying Engineer, September 16, 2014, https://www.csemag.com/articles/designing-chilled-water-systems/
– Cooling Tower and Condenser Water Design Part 6, JMP, https://www.jmpventures.com/cooling-tower-and-condenser-water-design-part-6/