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Section 1 Chilled Water Distribution Systems

Section 2 Refrigeration Systems

 
 

Description:  The purpose of this unit is to enable you to analyse and evaluate the operation and application of refrigeration and chilled water distribution systems within air conditioning applications.

Author:  Gates MacBain Associates


Section 1  Chilled Water Distribution Systems



Aims and Objectives

At the end of this section you should be able to:
  • Analyse and evaluate the operation and application of chilled water distribution systems within air conditioning applications.


For large buildings a central plant system is generally used. In such a plant, the conditioned air is kept at a central source and is distributed using a network of ducts. 

In larger and especially multi-storey buildings, the length of pipes and ducts becomes unmanageable. In such situations, we can consider using a chilled water distribution system. These systems are especially useful where control in individual rooms is required. These are commonly used in commercial and industrial buildings. 

In this section we will discuss the operation of such a system.  


The Basic Principle 

The basic principle is that chilled water from a source is sent through the pipe or duct work. This chilled water takes heat away from the spaces and return to the source with an increased temperature. It is like a loop starting with ‘cold’ or chilled water and returning with ‘warm’ water.  

What are the components of this system? The water gets chilled through Chillers, moves through pipes called chiller water supply system and enters into the a heat transfer device called a Heat Exchanger. Heat is transferred to the chilled water as it goes through the heat exchanger. Water is then returned to the chiller for re-cooling.  

Chilled water systems are also known as Hydronic systems.  We will discuss the components below.  


Chillers 

A chiller removes heat from a liquid. In its simple form, it consists of an evaporator, a condenser, a compressor and a device to control the flow. A refrigerant flows through evaporator and condenser with the help of a compressor resulting in chilled water at  temperatures between 4°C and 8°C. Chillers are of different types based upon the type of compressor used in them. These normally are packaged to lower cost and for ease of installation.   


Absorption Chillers 

Absorption chillers use a heat source to produce chilled water. The cooling effect occurs when refrigerant evaporates thereby removing heat. These use less electricity and have low noise.  

Absorption chillers are generally classified as ‘direct-fired’ or ‘indirect-fired’. In direct fired units, the heat source can be gas or some other fuel that is burned in the unit. Indirect-fired units use steam that brings in heat from a separate source, such as a boiler or heat recovered from combined heat and power (CHP).  


Air-cooled Chillers 

Air-cooled chillers utilise air to cool heat rejection coils. Compared to water cooled chillers, air-cooled chillers are easier to maintain, require more space, and do not require a dedicated water supply. They are, however, generally less energy efficient than water-cooled units.  Water-cooled chillers Water-cooled chillers are used where a high cooling demand exists, such as large commercial and industrial buildings.   Centrifugal pumps.It distributes the chilled water through the system. Centrifugal pumps can be of different types. Their basic principle of working is that they increase pressure of the liquid entering them through a rotating component called an impeller.   

Heat Exchangers 

In a heat exchanger the heat energy is transferred from one fluid to the other. This can be achieved by mixing the hot and cold fluids together although the heat transfer can still take place even if they are not ‘mixed’. Common types are flat plate, shell-and-tube and cross-flow.  


Other Components 

Other important components include: Strainers which trap particles from fluid flow; valves which regulate flow and help to isolate the network for maintenance; and pipes.  

The publications listed contain a more detailed explanation of chilled water distribution systems. You are advised to read these before attempting the tasks. 

Some useful websites and video resources are listed with self explanatory titles which will help you understand the concepts.   



Websites


Publications

  • Brumbaugh J E. (2004). HVAC Fundamentals, Vol 1: Heating Systems, Furnaces and Boilers, 4th edition: Wiley Publishing, Indiana (Chapter 8)
  • Chadderton D V. (2007). Building Services Engineering. 5th edition: Taylor & Francis, England (Chapter 5)
  • Oughton D R. and Hodkinsons S. (2002). Heating and Air-conditioning of Buildings, 9th edition. Elsevier Science Ltd. (Chapters 18 & 19)


Video / DVDs


Self-Assessment Task

  • With the aid of a drawing explain the working of a chilled water distribution system.
  • Select and describe  three different types of chillers
  • Explain the working of a typical heat exchanger.




Section 2  Refrigeration Systems



Aims and Objectives

At the end of this section you should be able to:
  • Analyse and evaluate the operation and application of refrigeration systems within air conditioning applications.


Have you ever thought why the cup of tea you left on the table to do something ‘urgent’ gets cold by the time you came back? A simple explanation is that your tea has transferred its heat to the cup and the surrounding air. This is the basic principle of refrigeration and is applied in air conditioning systems to keep the areas cool and comfortable. Heat removal is done with the help of a refrigeration cycle. 

In this section, we will discuss the components of a refrigeration system as applied in air conditioning.  


Refrigeration System  

In a typical system, five components are required: Evaporator; Condenser; Compressor; Expansion valve; and a Refrigerant. We will discuss how the system works as well as working of these components.  


Refrigeration Cycle 

The process begins with the refrigerant in the evaporator. Refrigerant is a highly volatile substance which can give off heat when compressed as well as take heat in when expanded. In an evaporator, the refrigerant absorbs heat while in liquid form and hence is converted to vapours.  

A compressor draws it from the evaporator and compresses it which results in a high temperature as well as the pressure. The refrigerant is then moved to a condenser where it is cooled again and becomes a liquid. It is ready to go back to the evaporator again for another cycle. An expansion valve regulates this flow.   


Evaporator  

Evaporator removes heat with the help of a refrigerant. As the refrigerant absorbs heat, it starts boiling though at a low temperature. That means it is taking heat away. Once completed, refrigerant is drawn from the evaporator. At this stage it is in a vapour form.  

The refrigerant is at a low pressure within the evaporator. The pressure level depends upon how much heat is being absorbed and how quickly the refrigerant in its vapour form is removed.     


Compressor  

It helps to push the refrigerant through the system. It draws vapours from the evaporator. Vapours are at low pressure and low temperature. In a way, it is extracting the heat out of evaporator contained in the refrigerant. It compresses them which increases their temperature and pressure.   


Condenser 

It takes heat from the refrigerant and throws it off or discards it to the outside air. The refrigerant at high temperature and pressure is received and cooled so that it changes to liquid form again ready for the next cycle. The condensers are normally placed at point where they can easily loose heat to the outside air such as roof top of a building.  


Expansion Valve 

It is used to control the flow. When liquid at a high pressure reaches the valve, it helps to reduce the pressure before it is pumped into the evaporator.  


Refrigerants 

Any substance which evaporates quickly, such as petrol, is called volatile. We use a highly volatile liquid in refrigerators to take the heat way. This highly volatile liquid is called a Refrigerant. Common examples are ammonia, methane and carbon dioxide. 

The selection will depend upon the desired temperatures to be achieved and how much pressure system is designed to take.  

The publications listed contain a more detailed explanation of refrigeration system components.  

Some useful websites and video resources are listed with self explanatory titles which will help you understand the concepts and their application.   


Websites


Publications

  • Brumbaugh J E. (2004). HVAC Fundamentals, Vol 1: Heating Systems, Furnaces and Boilers, 4th edition: Wiley Publishing, Indiana (Chapter 8)
  • Chadderton D V. (2007). Building Services Engineering. 5th edition: Taylor & Francis, England (Chapter 5)
  • Oughton D R. and Hodkinsons S. (2002). Heating and Air-conditioning of Buildings, 9th edition. Elsevier Science Ltd. (chapters 18 & 19)


Video / DVDs


Self-Assessment Task

With the aid of a drawing explain the refrigeration cycle.
  • Explain vapour compression refrigeration.
  • Explain the working of a compressor.





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