In today’s energy-conscious world, efficient heat rejection systems play a vital role in maintaining optimal temperatures for various industrial and commercial applications. With a wide range of system configurations available, choosing the right one for your needs can be a daunting task. This comprehensive guide delves into five key heat rejection system configurations, enabling you to make informed decisions and enhance the performance of your cooling infrastructure.
We will explore the ins and outs of the following configurations: Direct Air-Cooled System, Direct Water-Cooled System with Cooling Tower, Indirect Water-Cooled System with Cooling Tower, Direct Seawater-Cooled System, and Indirect Seawater-Cooled System with Heat Exchanger. By understanding the principles, advantages, and limitations of each configuration, you’ll be better equipped to select and implement the most effective heat rejection solution for your specific requirements. So let’s dive into the world of heat rejection systems and discover the secrets to optimizing their performance.
Heat Rejection Phenomena
The refrigerant inside a chiller gives off heat which is then transferred to an external cooling medium and taken away to a place where it can be safely disposed of – this could be the outdoor air, or if it’s lucky, a river, lake or the sea! An air-cooled chiller uses air while a water-cooled chiller uses water as its cooling medium.
Major equipment of a central chilled water plant
If air is not used as the direct cooling source, a heat transfer device, such as a cooling tower or radiator, must be employed to dispose of the heat to the atmosphere. Additionally, if the water source is not fed directly to the chiller condensers, a heat exchanger must be used to transfer the heat to the water.
Direct Air-Cooled System
A direct air-cooled system is a cooling system that does not require a coolant to dissipate heat. This type of cooling system is more efficient than a traditional water-cooled system, as it does not require the added components, such as a pump, radiator, and coolant tank. The system works by using a fan to circulate air over the heat source and dissipate the heat away from the system. Direct air-cooled systems can be used for a variety of applications, including cooling electronics, air conditioning, and even cooling the interior of a vehicle. This type of cooling system is often preferred over water-cooled systems due to its higher efficiency and the fact that it does not require the use of additional components.
The simplest way of rejecting the condenser heat is to use air to cool the condenser, which is the case when air-cooled chillers are used. An air-cooled chiller is equipped with finned-tube coils as its condenser and fans for drawing outdoor air to flow through the gaps between the fins of the condenser coils to cool the refrigerant being fed through the tubes. The condenser and the fans, as well as the compressor, evaporator, expansion device, a control panel and other accessories are typically assembled into a package unit, which can be used once fixed on site and with chilled water pipes and electricity supply connected. Air-cooled chillers are normally installed outdoors but, if needed, may be installed indoors with ducted exhaust.
Direct Water-Cooled System with Cooling Tower
A Direct Water-Cooled System with Cooling Tower is an effective cooling system for industrial applications. It uses a closed-loop system to transfer the heat generated by the machinery to the cooling tower. The water is then cooled by the heat exchange process, and returned to the machinery for reuse. This system is known for its energy efficiency and is often used in applications where large amounts of cooling are required, such as in power plants, chemical plants, and other industrial processes. Additionally, the cooling tower offers a more efficient way to dissipate heat, making it an ideal choice for many industries.
Cooling towers are heat transfer equipment that allow the condenser heat of watercooled chillers to be disposed of to the atmosphere. Detailed analysis on the operating principle and process of cooling towers will be given later in this chapter. In using this heat rejection method, water pumps are installed to circulate the condenser water between the chillers and the cooling towers. Water supply is required to replenish water losses in the process, and water treatment is needed to control water quality and prevent outbreak of legionnaire’s disease. A low limit temperature control system is typically installed to prevent condenser water temperature entering chiller from falling below a low limit (~16°C), which may happen in winter and cause unstable operation of the chiller.
Indirect Water-Cooled System with Cooling Tower
An indirect water-cooled system with cooling tower is an efficient way to cool down a building or industrial facility. This system includes a cooling tower that works by evaporating water and releasing the heat into the atmosphere. The water is then cooled as it passes through the tower and is then pumped back into the building or facility to provide cool air. This type of system is ideal for larger, more complex industrial facilities and can be easily adapted to fit any size or shape of building. It is also a cost effective way to keep a building or facility cool and is energy efficient, making it a great choice for those looking to reduce their energy costs.
The condenser water circuit in the abovementioned heat rejection system may be split into two that are thermally connected by a heat exchanger in between. This design may be used when seawater is used in the circuit through the cooling tower and fresh water is used in the circuit through the chiller, to protect the chiller condenser from the corrosion and erosion effects of seawater. Pumps are needed in each water circuit for heat transport.
Direct Seawater-Cooled System
Direct seawater-cooled systems utilize the natural environment to cool and condense steam from a power plant. This process is more efficient and environmentally friendly than traditional cooling systems. By using the ocean’s natural properties to transfer heat away from the power plant, direct seawater-cooled systems reduce energy consumption, decrease emissions, and require less maintenance than other cooling systems. Additionally, this system is cost-effective, as it does not require large amounts of energy to run, and does not require additional resources for cooling. Overall, direct seawater-cooled systems are an effective and sustainable alternative to traditional cooling systems.
For a building nearby a harbour, a pump station may be built to draw seawater for condenser cooling. The seawater drawn-in by the seawater pumps may be fed directly through chiller condensers, and the used seawater will be pumped back to the harbour for disposal. Filtration equipment and chemical / biocide dosing are needed to control water quality and prevent growth of micro-organisms
Indirect Seawater-Cooled System with Heat Exchanger
The Indirect Seawater-Cooled System with Heat Exchanger is an efficient and environmentally-friendly cooling system that uses seawater to cool and transfer heat. This system works by pumping seawater from the ocean into a heat exchanger, where the heat is transferred from the seawater to a freshwater loop. This freshwater loop is then circulated through a cooling tower, which helps to dissipate the heat, and then returned to the heat exchanger to be cooled. The cooled seawater can then be returned back to the ocean. This system is becoming increasingly popular due to its high efficiency, cost effectiveness, and its minimal environmental impact.
For protection of the chiller condensers, the condenser water circuit in the abovementioned system may be split into two that are thermally connected by heat exchangers between them. This allows fresh water to be used in the circuit through the chiller, although one more group of pumps is needed.
FREQUENTLY ASKED QUESTIONS
Indirect Water-Cooled heat rejection systems use a heat exchanger to transfer heat from the chiller to the cooling water, whereas Direct Water-Cooled systems circulate cooling water directly through the chiller. Indirect systems provide better protection against corrosion and fouling, and allow for more flexibility in terms of water quality and temperature. However, they may require additional pumps and piping, increasing their complexity and cost.
Seawater-Cooled heat rejection systems can provide significant energy savings and reduced water consumption in coastal applications. Seawater is a free and abundant resource, eliminating the need for cooling towers or condenser water treatment. However, seawater corrosion and fouling must be carefully managed through material selection and regular maintenance. Additionally, seawater intake and discharge regulations must be complied with to minimize environmental impacts.
To determine the most suitable heat rejection system configuration, consider factors such as heat rejection requirements, available water resources, ambient temperatures, and local regulations. Evaluate the pros and cons of each configuration, including upfront costs, operating expenses, maintenance requirements, and environmental impacts. It may be beneficial to consult with a qualified HVAC engineer or conduct a detailed feasibility study to ensure the selected configuration meets your specific needs and constraints.
Common design considerations for chiller heat rejection systems include chiller sizing, piping layout, and pump selection. It is essential to ensure that the chiller is properly sized for the application, and that the piping layout is optimized for minimal pressure drop and maximum heat transfer. Pump selection should be based on factors such as flow rate, pressure, and efficiency. Additionally, consideration should be given to noise levels, vibration, and accessibility for maintenance.
To optimize the performance of an existing heat rejection system, consider implementing measures such as regular cleaning and maintenance, optimizing chiller setpoints and control sequences, and upgrading to more efficient components. Analyze system data and trends to identify opportunities for improvement, and consider conducting an energy audit or retro-commissioning study to identify potential energy savings. Additionally, consider implementing water-saving measures, such as using grey water or rainwater for cooling, to reduce the system’s environmental impact.
