Air Conditioning

A Practical Introduction

David Chadderton

• Publisher: Routledge
• Year: 2014
• ISBN:
  9781317743385
• Edition: 3
• Topic: Architecture
• Subtopic: Architecture Methods & Materials

David Chadderton's Air Conditioning is the complete introduction and reference guide for students and practitioners of air conditioning design, installation and maintenance. The scientific principles involved are introduced with the help of case studies and exercises, and downloadable spreadsheets help you work through important calculations.

New chapters on peak summertime air temperature in buildings without cooling systems, air duct acoustic calculations and air conditioning system cost enhance the usefulness to design engineers. Case studies are created from real life data, including PROBE post-occupancy reports, relating all of the theoretical explanations to current practice. Trends and recent applications in lowering energy use by air conditioning are also addressed, keeping the reader informed of the latest sustainable air conditioning technologies. Over 75 multiple choice questions will help the reader check on their progress.

Covering both tropical and temperate climates, this is the ideal book for those learning about the basic principles of air conditioning, seeking to understand the latest technological developments, or maintaining a successful HVAC practice anywhere in the world.

1 Uncooled low energy design

Learning objectives

Study of this chapter will enable the reader to:

1. calculate peak summertime temperature in uncooled buildings;

2. analyse case study buildings;

3. assess a floor of The Shard if not cooled;

4. use CIBSE Post Occupancy Review of Building Engineering (PROBE) case studies;

5. assess the London 2012 Olympic Velodrome;

6. compare White Tower with modern buildings;

7. assess lightweight habitable spaces;

8. understand conditions in traditional old and new UK houses.

Key terms and concepts

Air conditioning 2; air temperature 2; building air leakage 2; discomfort 2; environmental temperature 2; evaporative cooling 2; internal heat gains 2; low energy building 2; mean 24 h heat gain 5; mean radiant temperature 8; overheating 8; Passivhaus 2; peak summertime temperature 2; PROBE 3; Simple (cyclic) Model 2; swing in heat gain 2; thermal analysis software 2; ventilation 2; windows 2.

Introduction

This chapter calculates the hourly predicted peak summertime temperature in uncooled buildings with a workbook using the Simple (cyclic) Model. Case studies are calculated and discussed. Examples analysed include The Shard, London 2012 Olympic Velodrome, a university office and a small home. It uses CIBSE Post Occupancy Review of Building Engineering (PROBE) reports as case studies. It also helps in the decision on the need for air conditioning.

Peak summertime temperature

The internal environmental temperature created in a room or building that does not have refrigeration or an evaporative cooling system, is calculated using a workbook and checked against measurements taken in a sample office. Low energy buildings are likely to have natural ventilation with operable windows and ventilators. These rely on architecture to limit solar radiation, convection and conduction heat gains from the warmer external environment. Ventilation air may be from any combination of natural, mixed mode or entirely mechanical systems. A really low energy building design might rely on natural, or assisted natural ventilation, make use of solar heat gains, have controllable shading devices, maximise the use of internal heat gains from people, lighting, computers and machinery, and have a minimal system providing top-up space heating; for example, the Passivhaus design. What quality of comfort conditions are likely to be found in UK buildings of this type? Most of us know the answer because we live in such a building, travel by car, train or bus in a mobile equivalent of such a building, and work in one as well. Motor cyclists and cyclists are more tolerant of discomfort while travelling.

When we have experimented with all the possible combinations of solar shading, operable windows and ventilators, some summertime overheating is experienced by most people. Then we resort to cool drinks, adjusting clothing, switching on portable fans, taking breaks from work and perspiring a lot. Thunderstorms and heavy rain invariably follow a series of uncomfortably hot days, unavoidably raising humidity and discomfort. We tend to adapt to a series of uncomfortable days, knowing it will not last long (CIBSE TM52, 2013). Designers should have access to a simple method of predicting whether a building, typical module or a room, is expected to overheat unless it has air conditioning. Thermal analysis software models real-time conditions and calculates what internal air temperatures will be on an hourly basis. These cost thousands of pounds, require extensive training to use them and have ongoing maintenance and upgrade costs for the design office. The spreadsheet file provided here, gives a suitably accurate hourly assessment using the Simple (cyclic) Model (CIBSE Guide A, 2006, Example 5.2, pages 5–19 to 5–21). We know how airtight constructed buildings really are, as distinct from design load calculations and computer modelling, from the PROBE reports. Surprisingly perhaps, measured building air leakage rates are higher than some might expect. Air leakage standards for the buildings are: leaky 36 m³ h m², meaning typically 12 air changes/h; average 18 m³/h m², meaning 6 air changes/h; and tight 9 m³/h m², meaning 3 air changes/h for a 3 m high ceiling height (Chadderton, 2013, page 77, Chapter 5). We will use these standards of measured infiltration rates as a starting point for assessment of peak summertime temperature. We know these are real values from audited buildings that were constructed and maintained to standards of good practice. There may be other ways of establishing air flow rates through a building, such as when it has a mechanical ventilation system running, but an idealised zero leakage does not happen in the real world when a building is closed up.

During warm sunny weather, buildings without any method of lowering the internal temperature may become overheated and uncomfortable for normal work or habitation. The upper limit of acceptability may be as high as an internal environmental temperature of 27°C. Such a choice is arbitrary and does not take account of the glaring effect of direct solar irradiance upon the person, their activity level, their clothing’s thermal insulation or the temperature and speed of the air around them. Increased air velocity aimed at a sedentary person reduces discomfort in excessively hot conditions. Athletes on running or cycling machines in a gymnasium extend their performance time in the presence of high air flows similar to being outdoors. An assessment of the peak summertime temperature within a building ought to be made before the decision to design a cooling system is made. The provision of low cost cooling systems can be investigated.

Internal environmental temperature will be combinations of the 24 hour mean heat gains, cyclic gains producing temperature swings about the mean, and heat loss from the internal environment mainly due to external air ventilation. Some of this heat loss might be accomplished using some form of mechanical cooling. The final temperature reached is a balance between gains and losses, some of which are potentially under the control of the occupier or engineer. Painting the exterior of the roof with white paint or spraying water onto the roof can reduce the heat gains. Some large areas of glass that were built during the 1950s and 1960s when there was little thought given to energy economy are known to have been painted white. Additional mechanical ventilation with outdoor air that is already at 25°C to 30°C or more, may not produce human comfort conditions. It may be sufficient to avoid the overheating of hardware such as computer servers, stored goods and operational electric motors when personnel are not at risk. The increased air velocity around personnel will alleviate discomfort and may produce tolerable conditions. Ideally there needs to be manual control over the direction and velocity of increased air circulation as weather conditions vary quickly.

The method of assessing the peak environmental temperature is to calculate the:

1. 24 hour mean solar heat gains;

2. 24 hour mean internal heat gains;

3. Mean internal environmental temperature from the known gains and the 24 hour mean external air temperature;

4. Peak swing in heat gains above the 24 hour mean;

5. Swing in environmental tem...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• List of figures
• List of tables
• Preface
• Acknowledgements
• Introduction
• List of units and constants
• List of symbols
• 1 Uncooled low energy design
• 2 Air conditioning systems
• 3 Heating and cooling loads
• 4 Psychrometric design
• 5 System design
• 6 Ductwork design
• 7 Controls
• 8 Commissioning and maintenance
• 9 Fans
• 10 Fluid flow
• 11 Air duct acoustics
• 12 Air conditioning system cost
• 13 Question bank
• 14 Understanding units
• 15 Answers to questions
• References and further reading
• Index