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Section 1 Cooling Loads

Section 2 Air Conditioning Systems

 

Description:  The purpose of this unit is to enable you to understand how cooling requirements for a building are determined and to analyse the operational characteristics of an air-conditioning system.

Author:  Gates MacBain Associates


Section 1  Cooling Loads




Aims and Objectives


At the end of this section you should be able to:
  • Determine cooling loads and energy requirements for buildings and potential peak summertime temperatures for spaces without air conditioning.


Introduction 

A building gains heat from a number of sources. To keep internal environmental conditions at a comfortable level, we have to cool the building. To design a system, which will do so, require calculating so-called cooling loads. Such systems are referred to as air conditioning systems though incorrectly as these systems ‘condition’ the air to a comfortable level which can be either cooling or heating it. However, in this section reference to air conditioning system will mean cooling the air.  

In this section, we will describe how to determine the cooling loads. To do so, we will discuss how a building gains heat and what temperatures a building can reach during summer time if no cooling takes place.    


Heat gains in buildings 

The sources which contribute to a building’s heat gain include: solar radiations; internal heat gains; and ventilation and infiltration. We will discuss these below.  


Solar radiations 

When there are no clouds, we experience sunshine or heat through solar radiation. The amount of heat that reaches us is called the solar irradiance. This heat which strikes the buildings at a certain angle and intensity depending upon which part of the earth you live in. This heat is transferred inside through walls, roof and windows. As these elements of a building are made up of different materials, the heat which actually gets into the buildings through these is not the same.  

There are many ways to calculate the heat gain of a building through solar radiations. This includes some reference tables, guidelines and computer software.  


Internal heat gains 

This is the most important factor contributing to the heat gains. Consider a room full of people with lots of computers, printers, fax machines, etc. These all generate heat thereby increasing the internal temperature and heat gain of the building. The floor and wall finishes and the type of furnishings/furniture also contribute significantly.  

Hence, how much a building gain heat because of its environment depends upon number and age of people, activity they are doing, internal finishes and machinery and equipment.   


Ventilation and infiltration 

When you open a window or door in a room to have some fresh air, you are allowing the movement of air into and outside the building. This is what we can call as ventilation. On the other hand, air can also leak into the buildings through cracks and where doors and windows are not adequately sealed. This air leakage is called infiltration. 

Depending upon the difference between outside and inside temperatures, ventilation and/or infiltration can have a significant contribution to the overall heat gain by a building.  


Total heat gain 

Total heat gain or the cooling load of a building is the sum of all the individual heat gains. 

Total heat gain =         S Heat gains through [solar radiations (glass, walls, roof) + internal                                     + ventilation/infiltration]  


Peak Summertime Temperatures 

As discussed earlier, the internal temperature of a building can increase to point where it becomes uncomfortable. As human beings perceive comfort differently, it is difficult to fix a value of comfortable temperature. However, a temperature in the range of 24°C to 27°C is generally considered as comfortable. 

A building with heat gains in excess of these temperatures needs to be cooled. To ascertain whether a building requires cooling or not, we can calculate Peak Summertime Temperatures. 

To do so, we can monitor a building for 24 hours to find the mean temperature. This will include all the sources as described earlier. We also look into the variation around this mean temperature called a ‘swing’. We should also consider some heat loss from the building.  

The final temperature is calculated by balancing the heat gains and heat lost.     

The publications listed contain a more detailed explanation of heat gains and cooling loads. You are strongly 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. These include some useful calculators. 




Publications

  • McMullan, R. (2007) Environmental Science in Buildings. 6th edition: Palgrave Macmillan, New York (Chapter 4)
  • 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.




Websites



Videos / DVDs



Self-Assessment Task

  • Explain how a building gains heat during peak summertime and discuss the factors contributing to the buildings heat gain.





Section 2  Air Conditioning Systems




Aims and Objectives

At the end of this section you should be able to:
  • Analyse the operational characteristics of refrigerants, cooling plant and associated equipment for air conditioning systems.



Introduction 

An air conditioning system conditions the air to create a comfortable environment for the occupants of a building. The system does so by cooling the air, keeping it at the right humidity and by ventilation.   

To cool the air, we have to remove heat. This is the basic principle an air conditioning system follows. Heat removal is done with the help of a refrigeration cycle. 

In this section, we will discuss the characteristics of an air conditioning system.  


Refrigerants 

Refrigeration is required in an air conditioning system. We will discuss how it works. First, consider how your refrigerator at home gets rid of heat. It takes heat from your refrigerator and throws it out through pipe work at the back. We call this as discarding heat.  

To understand how this works, consider if by accident you get a little amount of petrol on your skin. After a while, the petrol evaporates and you feel cold on that area because petrol has removed heat from the skin. This is the principle of refrigeration. 

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.


Evaporator 

Consider a system in which we wish to let warm water in and get cold water out. We can do so if we pass a highly volatile liquid in the system which will take the heat from the warm water, we will be able to have cold water out of the system. The liquid after taking the heat will evaporate. Such systems generally fall under the category of Heat Exchangers. The heat exchanger in this example is called an Evaporator.   


Condenser 

In the previous example, we will have to replace the highly volatile liquid each time we require cold water. To let the cycle run continuously so that no replacement is required, we can condense the vapourised liquid back to the liquid state. In this case, we will let the vapours pass onto something cold so that it is condensed back to liquid. Such a system is called a Condenser.  


Refrigeration Cycle 

If we develop a system with a condenser and an evaporator where a refrigerant is passed through the system, we will have a simple and continuous refrigeration cycle. The highly volatile substance or refrigerant is passed through the system by using either a pump or a compressor.   


Air Conditioning Systems 

If we have one central source of conditioned air and supply it to the whole building through ducts, it is called the central plant system. We can also have systems for individual rooms as well as which can condition the air in that particular space.  

We have discussed condensers and evaporators. An air conditioning system keeps the humidity at the correct level. If it is required to add moisture to dry air, a humidifier is used. In UK, humidification is hardly used though it is required in dry regions throughout the year. In tropical climates, the system should be able to remove moisture for which a de-humidifier is used. 

In a central plant, Dampers are used to control the amount of air that goes into each duct. Filters are used to trap dust and pollution.   

The publications listed contain a more detailed explanation of refrigeration cycle and air conditioning components.  

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



Websites



Videos / DVDs



Publications

  • McMullan, R. (2007) Environmental Science in Buildings. 6th edition: Palgrave Macmillan, New York (Chapter 4)
  • 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.
 




Self-Assessment Task

  • Describe the properties of refrigerants
  • Illustrate the working of condensers and evaporators.
  • Describe a refrigeration cycle.
  • Produce a typical schematic diagram for central plant systems





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