Indoor air quality is one of the most important aspects of occupant comfort in site accommodation for construction villages. In order to provide a safe, healthy and comfortable indoor environment, minimum ventilation standard need to be provided depending on the pattern of occupancy and functionality of the accommodation. Ventilation is required for one or more of the following purposes:

• provision of fresh air for breathing

• dilution and removal of airborne pollutants including odours

• control of excess humidity (arising from water vapour in the indoor air)

• provision of air for fuel burning appliances

• provides a mean to control thermal comfort.

Ventilation system is needed in an enclosed building to satisfy health, comfort and cooling needs. It will be best provided with a good degree of local control. According to the CIBSE Building Guide, the minimum required fresh air is 5 l/s per person (or 0.5 l/(s m²) while the recommended rate is 8 l/s per person (or 0.8 l/(s m²). Unsatisfactory internal air quality in a cabin will happen with too little fresh air. However, with too much ventilation, unnecessary heat losses will occur with a waste of energy.

An enclosed space can be provided with natural or mechanical ventilation, depending on the condition and the environment. Natural ventilation which uses the pressure differential of the air can be provided with very low capital and operating costs. It has no mechanical or running parts therefore less maintenance as well as noise pollution. However, external environment conditions may limit or prevent the use of natural ventilation. For example, a high level of noise or pollutants especially in a construction site.

This will then require the use of mechanical ventilation system. It requires smaller openings, provides controllability and security as compared with natural ventilation. The disadvantage of mechanical ventilation is that it consumes electricity and cools the air. Therefore, mechanical ventilation should be kept to a minimum of the acceptable levels of indoor air quality.

Providing more ventilation than is really required increases costs. This is because internal ‘conditioned’ (heated or cooled) air that is removed from a building has to be replaced with the same amount of air from outside. This also needs to be heated or cooled to match the temperature of the building. Besides, moving air around mechanically, using fans uses electricity. Therefore, it is important to determine the actual requirements and to design a ventilation system that suits the specific functionality of the accommodations in construction villages.  

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The objective of this part of the project is to propose a methodology to identify the minimum requirements and standards for ventilation for different types of cabin accommodation to improve the indoor air quality and the comfort level for the occupants. In addition to that, further analysis is conducted to recommend low energy solutions for each accommodation type.

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i)          Evaluation of the required ventilation rate

In order to determine the level of ventilation required, it is important to identify the conditions of the surrounding, the kind of activity being carried out and the level of occupancy in the cabin accommodation. There are five factors that will influence the design of the ventilation system requirement in a cabin accommodation as shown in the figure above. The five factors described in detail are:

      1)       External Air Quality

The external environment is an important factor to be considered when designing a ventilation system. Noise level, ambient air temperature and pollution from dust and smoke will determine if natural ventilation would be possible.

      2)       Occupancy Levels

Occupancy levels will determine the amount of air change that is required for a cabin accommodation depending on the standard minimum requirements to satisfy health, comfort and cooling needs of occupants. According to the CIBSE Building Guide, the minimum required fresh air is 5 l/s per person (or 0.5 l/(s m²) while the recommended rate is 8 l/s per person (or 0.8 l/(s m²).   

      3)       Occupancy Activity

The type of activity people does in the cabin accommodation will also determine the required air change rates and the ventilation system required. People moving around require more air than those who are stationary.

      4)       Relative Humidity Level

Some specific accommodation such as the drying room in construction villages need ventilation to control the humidity level and for creating air flow to dry clothes.  

      5)       Indoor Pollutants

The types of equipments and materials used indoor will also emit pollutants such as from synthetic carpets, photocopying machines, fresh paints, etc. which will need a higher fresh air to remove those pollutants that poses health threat to workers. 

ii)         Selection of best options of ventilation system and control solutions

Low energy design shall be considered when designing the ventilation system to the requirements determined from the first part of the methodology. By designing the system as close to the required air change rate as possible will prevent excess of ventilation which will cause wastage of energy. The following are the design strategies that can be used when designing ventilation system for cabin accommodations.

      b.      Control with Carbon Dioxide sensor

The levels of CO₂ are monitored and ventilation rates increase as CO₂ levels rise. The level of CO₂ is a good indicator of the per-person ventilation rate, since its presence depends on the number of people in a space and the rate at which the air in the space is being diluted with outdoor air. This system controls the outside air volume by regulating CO₂ levels. When the CO₂ level in the area reaches a predetermined level, an electronic controller adjusts the ventilation rates accordingly. This type of system is more precise than occupancy sensors, as it can adjust for different rates of occupancy demand, based on levels of physical activity, age and health. Ventilation control based on CO₂ levels is an important tool that can help control occupant-related pollutants and satisfy occupant-based ventilation standards. This control type is very effective in meeting rooms and canteen of construction villages with varying occupancy pattern all the time. The most common application measurement of indoor air for CO2 concentration is typically less than 1,000 PPM in properly ventilated buildings.

Note: Air Composition

Fresh air contains 21% oxygen and 0.03% carbon dioxide.

Expired air is typically 16% oxygen and 4% carbon dioxide.

CO2 levels > 5% (50000 ppm), acute danger to health (also if O₂ < 12%)

CO2 levels > 0.35% (3500 ppm), long term health implications

CO2 levels > 0.1% (1000 ppm), poor comfort

 

      c.    Control with humidity sensor

Humidity control increase ventilation rates when internal humidity levels rise. The system uses a humidistat that can be set to the required humidity level to be maintained as required. This control is effective in the drying room of construction villages by maintaining the required humidity level so as to prevent air saturation which deters clothes from drying.

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This case study is conducted to determine the minimum ventilation requirement of a standard cabin accommodation design that serves different functions based on the building standards mentioned in the introduction. Knowing the requirements, ventilation systems and control options are proposed for the different cabin functions.

i)          Evaluation of ventilation rate requirements

 Assumptions

• A typical standard cabin with a size of 9.5m x 3.0m x 2.5m high is used for analysis.

• The minimum required fresh air is 5 litres per second per person while the recommended rate is 8 litres per second per person.

• The external air quality depends on the location of the accommodation in the construction site and the amount of noise and pollution level at site. In typical construction sites, pollution from dust, airborne particles and noise are common. Therefore, it is common to minimize the use of uncontrolled natural ventilation for site accommodations. 

Drying Room Ventilation Requirements

This study is conducted to determine the minimum ventilation requirement for a standard drying room.

Assumptions

            i)          Every working day, 0.25 litres of water on each set of clothes are brought in to the drying room and there are 40 sets of clothes to be dried in the drying room of standard size.

ii)          The clothes need to be dried overnight from 1700 to 0730 the next day.

Analysis

i)          Total amount of water to be evaporated = 0.25 litres x 40 sets = 10 litres

ii)          Total time required = 14.5 hours

iii)         The evaporation/dehumification rate required = 10/14.5 = 0.69 litres/hour

Therefore, in order for the clothing to dry between 1700 and 0730 the next day, the temperature and relative humidity of the room is to be maintained at 20ºC and 30% respectively with an air extraction rate of 157 m3/hour or more.

ii)         Selection of Ventilation System and Control Options

 The selection methods are based on the methodology as described in the methodology section.

Suppliers of the recommended ventilation system and control options

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 Different types of cabin accommodation requires different ventilation rate for the purpose of satisfying the minimum safety and health requirements as well as providing the sufficient level of comfort to the occupants in construction villages. The ventilation requirements in a cabin accommodation at construction villages can be evaluated using the methodology above and low energy ventilation systems could be selected based on the requirements and functional considerations of each accommodation type. Although providing ventilation to the accommodation requires energy, the indoor air quality and comfort level could be enhanced and indirectly will contribute to the increase in productivity and good health of the workers in construction villages. 

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- Kreith, Frank, West, Ronald E., CRC Handbook of Energy Efficiency, CRC Press, 1997, pg 285 – 291

- McMullan, Randall, Environmental Science in Building, Fifth Edition, 2002

- Office of the Deputy Prime Minister, Document F - Ventilation, The Building Regulations 2000, pg 6.

- The Carbon Trust, Energy Saving Fact Sheet - Ventilation, May 2005.

- http://www.admsystems.co.uk

- http://www.dealec.co.uk

- http://www.silavent.co.uk

- http://www.vortice.co.uk

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