DESIGNING FOR EFFICIENCY AND ECONOMY IN AIR CONDITIONING SYSTEMS (Part 1)

WHAT IS AIR-CONDITIONING

The process of maintaining specific conditions of temperature, humidity (i.e. the moisture content in air), and dust level and odour control inside an enclosed space is termed air-conditioning.  The intended use of the space determines the specific levels of these properties to be maintained. Therefore air-conditioning requirements will vary from place to place: industrial production rooms, circulation places, equipment or material storage, public auditoriums, hospitals, churches, etc.

In addition to controlling the temperature and relative humidity of these spaces, it is of vital essence in comfort conditioning, to clean (filter) the air free of dust, germs, odour and maintain proper circulation keeping in mind, the avoidance of drought.

Human beings give off about 400 BTU (British thermal unit) of heat per person per hour due to metabolism. The human system has a temperature regulating mechanism which keeps the body temperature at about 98.6 deg F.  but the skin temperature varies according to the  surrounding temperature and relative humidity. Due to this difference in temperature, there if a continuous exchange and transfer of heat between the body and the surrounding.  Also the movement of air around the skin causes evaporation to take place. Therefore to maintain specific conditions in enclosed spaces as outlined above, close attention should be given to comfort design procedures.

DESIGN CONSIDERATIONS, CRITERIA AND METHODOLOGY

The design of air-conditioning systems begins with pre-design activities to establish the need for, feasibility of, and the proposed scope of a facility. Also one needs to consider the fact that our designs are very likely, over their lifetimes, to experience major changes in the way they are used and in their sources of energy supply. Therefore the designer is further faced with challenges such as designing for recycling, energy transition and designing for the information age (smart houses, intelligent buildings, smart appliances etc.)

To be successful, design teams should focus on achieving a solution that meets the expectations of a well- thought-out end explicitly defined design intent which might include the following:

·         Provision of well defined comfort conditions

·         Consideration and use of latest information technology

·         Focus on green technology (environmental quality, carbon neutrality, etc.)

·         Provision of a high degree of flexibility

It should be borne in mind that these design intents become progressively elaborated in the course of the design process.

Having considered the design intent, attention should then be given to the establishment of design criteria, which are the benchmarks against which success or failure in meeting design intent is measured.  Also the technical and philosophical issues underlying the design intent are taken into account. At this stage, clarification of keywords used in the design intent such as “green”, “flexibility” are made and translated into tangible requirements and deliverables. The process of developing design criteria may result in positions such as:

·         Thermal conditions will meet the requirements of ASHRAE Standard 55-2004

·         The power density of the lighting system will be no greater than 0.7 Watt per square meter,  etc.

Adopting the right design methods and tools is the next step and this is the means through which the design intent is accomplished. This will involve the use of calculators, charts such as psychometric chart, tables and so on to find:

·         The total heat loss of the building (ie. Loss through walls, roofs, door, window glass etc.)

·         Extra cooling load caused by exhausting air (through infiltration or ventilation)

·         Total cooling load from internal heat sources (electrical equipment, mechanical equipment, lighting etc)

·         Total room sensible and latent loads

·         Grand total refrigeration tonnage

·         Equipment selection

TYPICAL DESIGN PROCESS FOR AIR-CONDITIONING

The design process involves a minimum of two stages

1.       The preliminary design stage to:

·         List activity comfort needs

·         Development of activity schedule

·         Analysis of site energy resources

·         List climate design strategies

·         Consider alternative building forms

·         Consideration of passive and active systems

·         Design guidelines

This stage is usually done by the architect. For innovative and larger multi-zone buildings, the team is expanded to include the landscape architect and the engineers. The team focuses on assessing the strengths of various design alternatives. It is worth mentioning here that the architect and engineers may have different perspectives but when mutual goals are clearly agreed on early in the design process, it results in striking innovations whose benefits extend far beyond services to the clients of a particular building.

2.       The design development stage. At this point, the solution that offers the most promising combination of aesthetic, social and technical benefits has been chosen. The choice is then represented in drawings which are handed to the mechanical engineer to:

·         Establish design conditions

·         Determine the HVAC zones

·         Estimate the thermal loads of each zone

·         Select the most suitable HVAC system

·         Identify and locate the HVAC components

·         Size the components

·         Lay out the system.

It is clear therefore, that the first step to an efficient and economic design of an air-conditioning system should be to organize the design process the answer question “How should the building envelope respond to needs for cooling, day lighting, artificial lighting, ventilation, efficient use of energy and total cost constraints?” To answer these questions, codes and standards that typically prescribe relationships between floor area and fenestration area (residential buildings) or total wall area and fenestration area (non-residential buildings) are applied. Also the building form (tall, short, thick, thin) is considered.

In carefully designed buildings a proper combination of size, type, material and composition for the roofs, walls, windows and interior surfaces can maintain comfortable interior temperatures thereby reducing the complexity and cost of the air-conditioning system.

To be continued. Please join the discussion forum to post any questions or contributions. Full text of this article is available  on request (members only).

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