What is district cooling system?

The increasing demand for better indoor climate and comfort, especially in offices and commercial buildings, in combination with a growing usage of heat emitting technical equipment, is the driving force behind the interest in air conditioning. The idea is to use one central source instead of local systems for each building. This will create both economic and environmental benefits. District Cooling is an environmental friendly, energy saving and economical cooling method. The cooling is centrally produced and distributed as cold water to each building through a closed distribution net.

How district cooling system works

In district cooling one central source is used for several buildings. The district-cooling system offers operating flexibility and convenience, with the possibility of using the same supplier for electricity, while since each building can use as much or as little cooling as needed, without worrying about chiller size or capacity.

Energy efficiency means that the heat exchangers will transfer practically all cold to the top of the building with minimum loss. Therefore, Alfa Laval uses a “close approach” where there is minimal energy loss in the plate heat exchanger, that enables to achieve temperature exchange of no more than 0.5°C/<0.9°F.

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Why district cooling?

With district cooling being the most convenient method for production and distribution of cooling in commercial applications - delivering both environmental and economic benefits.

  • Environmental friendly 
The use of one cooling unit will benefit the environment more than the use of individual production. Due to regulations, many large chillers using Freon must be modified and replaced. The phaseout of Freon world-wide creates a powerful incentive to use district cooling service, as building owners will face high costs for conversion or replacement of their cooling equipment.
 
District cooling will therefore be uniquely suited to use where there will be less discharge of Freon due to the decrease of use of chillers that employs Freon at individual buildings.
  • Economical benefits
With centrally produced comfort cooling, end-users will enjoy benefits of space savings at their location as there is no need of a chiller as an investment cost and worrying about chiller size or capacity.
 
Operation is also flexible as each building can regulate as much or as little cooling as needed, as building owners can have the same supplier for electricity whilst offering quiet, vibration-free systems and cost-efficient as handling problem such as maintenance and equipment redundancy will be taken care of centrally.
 
Alfa Laval have the expertise and knowledge to provide the right equipment for providing solutions with both economical and environmental benefits.
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District cooling system

District cooling systems often depends on the different type of cooling source.

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Absorption chiller

If district heat or waste heat is available, for example from waste disposal, there is another possibility for comfort cooling with an absorption chiller. In this application, the Freon influencing the ozone are replaced with for example water and lithium bromide, both economical and environment-friendly.
 
In the evaporator the refrigerant (water) takes up heat/energy from the connected system, thus cooling the air conditioning circuit in a heat exchanger. The refrigerant enters the absorber as low pressure vapor, where the liquid solvent (lithium bromide) absorbs it.
 
The pump increases the pressure and the mixture continues to the interchanger where it is preheated in for example a plate heat exchanger. The high-pressure vapor is sent to the condenser, where heat is emitted during the refrigerant’s condensation.

 

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With water qualities deteriorating there is an increasing need to protect cooling systems from pollution in cooling tower water. An indirect system with an intermediate plate heat exchanger means material savings, less maintenance and a minimum of chemicals.

Cooling tower

Today water qualities are deteriorating because of different kinds of pollution. This increases the risk of chiller shutdowns due to operation problems of the condenser. The condenser is subject to attacks from either chlorides that will cause corrosion or impurities or biological activities in the water that will cause fouling. As the expectations of trouble-free cooling operations have increased, it has become more and more interesting to look at alternative solutions where these problems can be avoided.
 
One solution is an indirect system using a heat exchanger in combination with an open cooling tower. The advantages of this are:
  • Low system cost: Cost calculations show that the payback period of the heat exchanger is very short.
  • Material savings in the condenser: Less expensive materials can be used.
  • With an intermediate heat exchanger, chillers as well as cooling towers can be run at an optimal temperature.
  • An intermediate heat exchanger means that the use of water treatment chemicals, for example chromates used for the cooling tower water, can be minimized.
  • Less maintenance of the condenser.

 

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Thermal energy storage

Thermal energy storage facilitates via an ice accumulator or storage tank where ice can be accumulated during one period, stored and then thawed and used during another. There are two main reasons for using an ice accumulator/storage:

  • Where the cooling requirements vary during the day a smaller chiller can be used. As a result, the initial cost of cooling equipment can be reduced considerably.
  • Cooling energy can be purchased during the night or off-peak hours. In many countries this means that it can be obtained at a lower price.

There are two main types of ice accumulator systems:

  • Systems with internal melting consist of a polyethylene tank containing coils of the same material. The container is filled with water. When ice is accumulated, a –5°C/41°F a glycol solution is run through the coil. The water will gradually freeze to ice, first around the coils and then further and further out in the tank. When the extra cooling capacity is required, the glycol solution in the coils will be led through the system and returned to the tank at a higher temperature. The ice accumulated in the tank will then melt, and the glycol solution will be recooled until all the ice is consumed.
  • In systems with external melting the tank is made of steel or concrete. Here too are coils with glycol or a Freon coolant, and ice is accumulated to a thickness of 35 mm/1.4 inches around each coil. The rest of the tank will be filled with water. When there is a need for cooling energy, ice water is pumped out from the bottom of the tank to the system. When it returns to the ice accumulator it will be forced to circulate around the ice. In this system, the ice water that is pumped into the system will always retain the same temperature.
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Heat exchange for district cooling systems

In district cooling systems the distribution could be either direct or indirect.

 

Direct systems: 
The district cooling water goes directly into the internal piping system of each building.
 
Indirect system:
In an indirect system, a heat exchanger separates the internal from the external system. Today, this is the most common system, providing several benefits.
  • Leakage will be easier to detect, and if it does occur, will create minimum damage. There is no risk of one system contaminating another.
  • Responsibility line will be clearer and the regulation and sales are easier to monitor with clear borders.
  • With separate circuits the customers may experience fewer fluctuations and disturbances, should the central system expand or need maintenance.
  • Decrease of static pressure, thus working as a pressure interceptor. 

Installing Alfa Laval plate heat exchangers in an indirect cooling system ensures minimal energy loss throughout the system.

Why plate heat exchangers?

The heat-recovery plate heat exchanger will be installed between the condenser and the cooling source such as a chiller or a cooling tower, recouping part of the energy that would otherwise be let out in the air. While recovering heat for pre-heating tap water, for example, the cooling need decreases on the condenser side. Thus, the savings will not only be the energy recovered in the heating system, but also the energy not wasted in the cooling system.
 
Due to the extreme efficiency of the plate heat exchanger it is possible to recover up to 95 percent of the energy that would otherwise be wasted. This is often more than enough to offset the capital and operating costs of the plate heat exchanger. That is why it is important to demand new standards for your plate heat exchanger where it could bring you higher efficiency, better reliability and greater serviceability.
 
As Alfa Laval has been the frontrunner in the development of a wide variety of industrial plate heat exchangers, hence it is suitable for the demanding conditions in district cooling installations. Our heat exchangers are designed for and used all around the globe to provide cost-savings, energy efficient and comfortable cooling.
 
Benefits of using Alfa Laval plate heat exchanger are:
  • Water is chilled with almost no losses. 95% of the energy maybe recovered.
  • Plate heat exchanger is extremely efficient. The counter current and turbulent flow in the heat exchanger makes it possible to obtain a close temperature approach (<1° C).
  • Risk of scaling is reduced to a minimum by the high degree of turbulence.
  • Plate heat exchanger is flexible and can be opened and extended on site. 100% of the surface is accessible simply by removing the tightening bolts. It is easy to redesign for new requirements simply by adding or removing plates.
  • Plate heat exchanger is space-saving and compact. It uses only 30% of the space required by a shell & tube heat exchanger for a similar duty.
  • Installation is simple. The plate heat exchanger is delivered with feet for standing on a floor. The weight is low and the hold-up volume very small, which reduces the foundation to a minimum.

Learn more how plate heat exchanger works.

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